spe-116364-entrance pressure of oil based mud into shale effect of shale, water

8/10/2019 SPE-116364-Entrance Pressure of Oil Based Mud Into Shale Effect of Shale, Water

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SPE 116364

Entrance Pressure of Oil Based Mud into Shale: Effect of Shale, Water

Activity, and Mud PropertiesAndres Oleas, SPE, Collins E. Osuji, SPE, Martin E. Chenevert, SPE, and Mukul M. Sharma, SPE,Universityof Texas at Austin

Copyright 2008, Society of Petroleum Engineers

This paper was prepared for presentation at the 2008 SPE Annual Technical Conference and Exhibition held in Denver, Colorado, USA, 2124 September 2008.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; il lustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

AbstractOil-based muds (OBMs) have been developed to combat drilling problems often caused by shale hydration. Therefore, it is of

utmost importance to understand the interaction of OBMs as they contact shales. Past research (Chenevert 1970) has shownhow the movement of water from OBMs can be controlled by the addition of salt. This paper deals with the movement of the

oil phase of the OBM, as described by its hydraulic Entrance Pressure. Although the oil filtrate of the OBM does not

hydrate the shale, it can penetrate and flow into the shale at a certain entrance pressure. Such flow into shale can be as

damaging as water flow, because it increases the pore pressure of the shale, which can cause wellbore failure. It is of primary

interest to understand this when using OBMs in shales. It is also important to understand how factors such as the emulsifier

concentration in the OBM and the porosity of the shale affect this entrance pressure.The objective of this study is to perform laboratory tests in order to determine and quantify the factors that control the entry

pressures for shales. For this purpose, five OBM samples with different emulsifiers and oil and water concentrations were

prepared. The study was also intended to derive an understanding of the effects of shale porosity on entrance pressure. Inorder to vary the porosity of the test samples, it was convenient to place them in various controlled environments with relative

humidity (i.e. water activities). Data are reported for Arco China shale samples having porosities of 1.8% (native), 3.9%, and

4.9%, at water activities (aw) of 0.72, 0.86, and 0.96, respectively.It was observed that as the emulsifier concentration increased, the oil breakthrough pressure increased. It was also observed

that as the porosity of the shale is reduced (smaller pore throats), the entrance pressure for the mud increases.

IntroductionShales are low-permeability sedimentary rocks with small pore radii that have medium to high clay content, in addition toother minerals, such as quartz, feldspar, and calcite. The distinguishing features of shale are its clay content and low-

permeability, which results in poor connectivity through narrow pore throats. Shales are also fairly porous and are normally

saturated with formation water, with several factors affecting their properties, such as burial depth, water activity, and theamount and type of minerals present.

Considering the fact that shales account for 70 75% of the formations drilled around the world(Manohar, 1999), it is

important to understand and minimize shale-related problems while drilling. Drilling performance has exhibited the

effectiveness of OBMs in combating drilling problems caused by shale hydration, differential pressure sticking, corrosion, andhigh formation temperatures (Simpson 1978). However, when using a water-based mud, water movement from the mud to the

shale results in swelling stresses and pore pressure increases that lead to wellbore failure.

OBMs are water-in-oil emulsions that contain water, emulsifiers, organophilic clay, and a weighting material (Growcock,

et, al. 1994). The water phase is usually a calcium chloride (CaCl2) salt solution, with a water activity (aw) that resembles theaw of the formation. This eliminates water transfer to or from the water-sensitive zones and thereby maintains a stable

wellbore. The water in the oil is stabilized with a primary emulsifier (often a fatty acid salt), while the weighting material and

the drilled solids are made oil-wet and dispersed in the mud with a secondary emulsifier. It is thought that both emulsifiers

have dual roles, with the primary emulsifier also acting to some extent as a wetting agent, and the secondary emulsifier actingas a true emulsifier.

Ions that are added to a water-based mud (WBM) reduce the water activity (aw) of the fluid, and consequently, water

movement into the shale is reduced due to osmotic effects (Chenevert, M. E. and Sharma, A. K. 1991). This effect is not long


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2 SPE 116364

lasting because the hydrated ions are not very restricted and they thereby invade the low-salinity shale. However, for OBMs,

an efficient membrane exists around each water droplet, and very little (if any) ion transfer occurs.

Background of the ProblemEven though the osmotic pressure generated by the OBM prevents the flow of water into the shale, this membrane might not

be completely efficient. Data from different tests using inverse emulsion drilling fluids do not yield a perfect osmotic

membrane (Schlemmer, F. J et. al. 2002). The nature of the emulsified salt solution and the emulsifier package may affect theinteraction of invert emulsion fluids with shales, including the transport of fluids into or out of the shale.

The shale sealing characteristic is a result of the small water-wet pore sizes (100 nm or less), making it difficult for a non-wetting hydrocarbon fluid to penetrate a shale. In order to overcome the opposition of shale to fluid invasion, the hydraulic

pressure differential between the invading fluid and the water present in the shale needs to be higher than the capillary entrypressure of the shale. When using an OBM, exceeding this capillary entry pressure will result in the oil filtrate of the mud

displacing the free water of the shale.

It is expected that the entry pressure of OBM filtrate (oil-only) will be higher than the entry pressure of a water-based mud;this is because it is difficult to separate the emulsified water droplets from the emulsifier coating, and even after separation

occurs, the filtrate is immiscible with the pore fluid. Therefore, the OBM filtrate needs to displace the pore fluid in order to

invade the shale. This could explain the ability of an OBM to maintain a stable wellbore during drilling operations. The

capillary entry pressure can be calculated by Equation 1.

rPc

cos2= Eq.1

where is the interfacial tension between the mud filtrate and native pore fluid, is the contact angle between the drilling

fluid and the rock surface, and r is the pore radius. This equation suggests that increasing the interfacial tension and contactangle can increase the capillary pressure for a given pore size.

Even though a high entrance pressure is an important component in understanding the interaction between OBMs and shale

formations, it is not the only aspect to be considered. It is known that shale stability is a time-dependent problem, meaning

that the state of stress and strength of a shale varies when the shale is invaded over a period of time. Stresses in the shale at thewellbore wall are altered as the drilling fluid filtrate pressures up the pore-fluid in the shale.

Scope of ResearchThis work was initiated to achieve the following goals:

1. Establish a testing procedure and equipment suitable for measuring entrance pressures of shale samples.2. Determine the effect of OBM composition on the entrance pressure.3. Measure the entrance pressure of various OBMs into shale of varying porosities.

Arco China Shale Properties and Preparation

The Arco China shale core used during the laboratory testing was cored at a depth of 11,812 ft. and was preserved at the rigsite in polyethylene bags covered with heavy-duty plastic. After opening the plastic covering in the research lab, the core was

immediately immersed in mineral oil so as to prevent any contact with air prior to sample encapsulation, cutting, and slicing ofthe tests samples.

A mineralogic analysis of the shale was performed by Newpark Laboratories and the results are given in Tables 1 and 2.Based on the clay analysis, illite is the main clay present. It can be inferred that the Arco China shale does not have a high

affinity to absorb water, due to the low surface area of this mineral (80m2/gm) as compared to the surface area of smectite (750

m2/gm) (Manohar 1999). A Cation Exchange Capacity (CEC) value of 9.3 was reported, suggesting that the shale is not very

reactive, and the petrophysical data confirms that the shale is very tight, with a porosity of 1.8% and a permeability value of

0.039 microdarcies.

XRD - X- Ray Difraction Weight %

Quartz 42.0

Feldspar 16.0

Calcite 19.0Total Clay 24.0

Kaolinite 15.0

Chlorite 12.0

Illite 42.0

Smectite 22.0

Mixed layer (Illite/Smectite) 10.0 (13/87)

Density

(gr/cc)

Porosity

(%)

Median Pore

Aperture diameter

(microns)

Air

Permeability

(md)

2.69 1.8 0.0158 0.000039 Table 1 - Arco China shale mineralogical composition Table2 - Arco China petrophysical properties from mercury injection


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SPE 116364 3

The water activity of the shale (aw) is defined as the vapor pressure of the shale divided by the vapor pressure of pure water

at the same temperature. Therefore, following the procedure developed earlier (Chenevert, 1969), an adsorption isotherm for

the Arco China shale was measured. The procedure for determining the water activity was done at atmospheric conditions,

using five desiccators with five different saturated salts of known activities. Table 3 describes the salts used and theircorresponding theoretical and measured water activities.

Salt aw theoretical aw m easured

CaCl2 0.3 0.34

Ca(NO3)2 0.5 0.544NaCl 0.76 0.777

KCl 0.86 0.873

KH2PO4 0.96 0.963 Table 3 - awof salts used to develop Arco China adsorption isotherm

The procedure used to determine the native moisture content of the shale starts by cutting 0.5 cubic samples of the shale.

After cutting the samples, their initial weight was recorded (wi). They were then dehydrated in an oven at 220oF for about 48

hours. The samples were then weighed (wd), and the weight loss was recorded. Separate cubic samples were placed in the

desiccators which had controlled activity atmospheres. The air in the desiccators was extracted using a vacuum pump, so as to

speed up moisture absorption or desorption. The samples were weighed every day until no further changes were observed, atwhich time the weights were recorded as the final weights (w f) of the shale samples. At this point, the shale samples wereconsidered to be in equilibrium with the controlled atmospheres in the desiccators, and to, therefore, have known water

activities. The native moisture content and the percentage variation of weight of the samples were calculated using Equations 2

and 3, respectively. The results are summarized in Table 4 and Fig. 1.

Native moisture content100%

=

d

di

w

ww ....... (2)

Percentage water adsorption 100%

=i

if

w

ww (3)

Native Moisture Content (%) = ((wi-wd)/wd)*100

SAMPLE wi (gr) wd (gr)weight after

hydration (gr)

water absorbed

from dry sample

(gr)

Native Moisture

Content (%)

Water Absorbed

from original

sample (%)

Water Absorbed

from dry sample

(%)

1 6.617 6.553 6.578 0.025 0.977% -0.589% 0.38%

2 7.911 7.862 7.887 0.025 0.623% -0.303% 0.32%

3 6.565 6.525 6.568 0.043 0.613% 0.046% 0.66%

4 6.202 6.163 6.215 0.052 0.633% 0.210% 0.84%

5 6.333 6.287 6.361 0.074 0.732% 0.442% 1.18%

Average 0.715%

Table 4: Native Moisture Content and Water Absorbed for Arco China shale

Adsorption isotherm curve for Arco China shale

R2= 1

-0.65%

-0.40%

-0.15%

0.10%

0.35%

0.60%

0 0.25 0.5 0.75 1

Water Activity

WaterAbsorbed(%)

Shale altered

to this aw

Fig. 1 Adsorption Isotherm for the Arco China shale

As demonstrated in Figure1, the native water activity of the Arco china shale is 0.72, the point at which there was no

water loss or gain.


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4 SPE 116364

Shale Sample Preparation

The original core that was preserved at the well site was 4 inches in diameter and 6.5 inches long. The pressurized testing

cell used in this study requires the shale test specimen to be 2.5 in diameter and 0.260 0.020 inches thick. Consequently, the

core needed to be cut and sliced to satisfy the required cell measurements. Using a circular saw, the core was cut along its axisinto two half-moon pieces. These two pieces were then cut obtaining two core columns of dimensions 1.4 x 1.4 x 6.5

each. The two core columns were placed in a can filled with mineral oil for preservation.

Material s and Sample I ncapsulation Procedure

The materials used to prepare the shale samples include the following: a plastic tube of 2.5 OD x 2.25 ID x 11 length;an acrylic base; epoxy curing agent; and resin.

The procedure to obtain circular samples of 0.260 0.020 inches consisted of sealing the 1.4 x 1.4 shale column inside

the center of the plastic tube using a 2-part epoxy. After curing, the tube-epoxy-shale unit was sliced into test specimens 0.26

thick using an oil-cooled diamond saw. A typical specimen is shown in Fig.2.

Fig. 2 - Shale sample after slicing

Characterization and Preparation of Oil-Based MudsThe three major components of an OBM are synthetic oil, the aqueous phase, and the emulsifiers. The aqueous phase is

normally a calcium chloride solution (CaCl2) in the range of 25 to 40% by weight. The presence of CaCl2 provides theosmotic pressure needed to prevent the movement of water within the emulsified droplets (applied?) to the shale. Normally

two emulsifiers are used in an OBM formulation. The first emulsifier lowers the interfacial tension of the aqueous phase to

allow small-sized droplets to be formed, but will not form a physical barrier around them. The barrier is obtained by the use of

a second emulsifier that provides high emulsion stability, high temperature tolerance, lower filtrate loss, and better solidswetting.

Shale stability using OBMs is accomplished by eliminating the wellbore hydration caused by water-based muds. Adjusting

the CaCl2concentration to match the water activity of the troublesome shales provides a stable wellbore, since the transfer ofwater to or from the water-sensitive zones is minimized (Growcock, et, al. 1994). Laboratory swelling-tests confirmed the

balanced activity concept by showing that water adsorption does not take place when the activity of the water in the shale is

equal to the activity of the water in the mud (Chenevert, 1970).

A good OBM must satisfy some guidelines to be considered acceptable, including an oil-only filtrate of no more than 5 to

10 ml, and an electrical stability (ES) of 500 Volts or higher (Simpson, 1978).The ES value is considered an emulsion stability parameter (Growcock, et, al., 1994). High values of ES voltage are

directly related to more stable emulsions. On a given well, the lowering of values of ES voltage indicate the need to add

emulsifiers to the system in order to obtain stability. In general, the ES and the HTHP fluid loss are considered the mainparameters used to evaluate OBM stability. They are both expected to move in opposite directions. In this study, the HTHP

fluid loss equipment was not available, so the low temperature low pressure API cell was used in its place.

Mud Preparation Procedure

The base fluid used to form every mud sample of this study consisted of 600 ml of Escaid 110 oil, 24 grams of emulsifier

1 (EZ-Mul) and 8 grams of emulsifier 2 (X-Vis). All components were stirred at 5000 RPM for 5 minutes. Later an additional200 ml of Escaid were added to the mixer and stirred for 2 more minutes. This was the first step prior to the preparation of the

five different muds to be tested with the Arco China shale samples. This base fluid was later altered by the addition of CaCl2,lime and mineral oil, at different concentrations so as to obtain five different oil-base muds (Table 6). The emulsifiers used in

the preparation of the base fluid are Baroid products.

During the preparation of the five mud samples, the API filtration and ES tests were performed at different stages in orderto monitor the stability of the mud. In the initial test, the mud was hot-rolled for 15 hours at 260

oF, after it was mixed in the

constant speed mixer so as to make the emulsions more stable. The five samples are described in Tables 5 and 6.

The five muds that were tested with the Arco China shale were the result of adding different volumes of calcium chloridesolution (CaCl2 30% w/w, Eq. 3) and Escaid 110 to the base fluid.


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SPE 116364 5

)(22

2

2%

deonized

CaClOHCaCl

CaClweight

+= Eq. 3

Mud Base fluid (ml) Escaid (ml)CaCl2 - 30% w/w

(ml)Lime (gr)

Emulsifier

Concentration/Total

Mud Weight (%)

CaCl2 / Escaid w/w Vol Oil (%) Vol Emulsion (%)

Mud 1 100 100 45 2.5 3.1% 0.37 80.0% 20.0%

Mud 2 100 100 90 2.5 2.5% 0.74 67.6% 32.4%

Mud 3 100 100 180 2.5 1.8% 1.47 51.6% 48.4%

Mud 4 100 0 45 2.5 4.6% 0.75 66.2% 33.8%

Mud 5 100 50 45 2.5 3.7% 0.49 74.9% 25.1%

SG Base Fluid = 0.78

SG Escaid 110 = 0.78 Escaid EZ-Mul X-Vis

SG CaCl2 = 1.25 78 gr 3 gr 1 gr

Base Fluid (100 ml)

Table 5 - Mud matrix and relevant ratios

MudBase Fluid

(ml)

Escaid

(ml)

CaCl2 (30%)

(ml)Lime (gr)

Emulsifier

Concentration / Total

Mud Weight (%)

CaCl2 / Escaid

w/w (%)

API filtrate

(ml)

ES

(Volts)Mud aw

Mud 1 100 100 45 2.5 2.9% 0.37 5.2 1130 0.57

Mud 2 100 100 90 2.5 2.3% 0.74 4.6 380 0.61

Mud 3 100 100 180 2.5 1.6% 1.47 8.1 200 0.66

Mud 4 100 0 45 2.5 4.6% 0.75 0.9 795 0.66

Mud 5 100 50 45 2.5 3.6% 0.49 3 830 0.57 Table 6 - Oil-base muds properties and representative ratios

The mud preparation procedure was developed while mixing the first mud (mud 3). Both filtration and ES values were

measured. The initial mud was mixed using the multimixer. The next step was to monitor the ES and temperature whilemixing the mud at different speeds using the constant speed mixer until the maximum speed of 19000 rpm was reached. Fig. 4

describes the sequence of mixing that was followed and shows the response of the ES and temperature. The temperature

reached with the constant speed blender will depend on the friction between the mud and the blade inside the blender. After

the mud was mixed, it was hot-rolled overnight and then poured into a beaker and the ES was measured. The final

measurement of ES was used to show the benefits of the hot-rolling procedure. The same procedure was followed when

preparing the four other mud samples.The general procedure for preparing laboratory OBMs started by mixing 100 ml of the base fluid with a defined volume of

CaCl2solution and Escaid. Then, 2.5 grams of lime were added, after which the mixture was stirred using the multimixer andrecording an ES measurement every 5 minutes.

The speed of the mixer was increased gradually, using steps of 4000 RPM to the maximum allowable (the maximum speed

will depend on the friction between the mud and the rotating blade). The mud was stirred until the temperature of the mud

increased to a final temperature of 220oF. When the temperature was reached, a second ES measurement was taken. The

OBM was then cooled to ambient temperature and another ES measurement was taken.

The API filtration test that followed took filtration volume measurements after 1, 2, 3, 5, 7.5, 15 and 30 minutes. The hot-

roll oven was pre-heated to 260oF. The OBM was then hot-rolled for not less than 15 hours, after which a measurement of the

ES of the mud was taken. The mud was then cooled to ambient temperature and a final ES reading obtained. The filtration

test that followed used the same time intervals as the pre-hot-roll test. In total, five main ES data points were taken for each of

the mud samples prepared (Fig.3).

Electrical Stability

0 Volts

200 Volts

400 Volts

600 Volts

800 Volts

1000 Volts

1200 Volts

1400 Volts

1600 Volts

In multimixer for 5

minutes @ 71 F

In blender at maximum

rpm @ 220 F

ES before filtration @

71 F

After hot-roll @ 190 F ES before filtration @

71 F

Mud 1 Mud 2 Mud 3 Mud 4 Mix 5

Initial Mud (Mud 3)

50

75

100

125

150

175

200

225

250

275

0 10 20 30 40 50 60 70 80 90

Time (min)

Temp(F)&ES(Volts)

Temp (F) ES (Volts)

ES before mixing ES after mixing before hot-roll

ES after hot-roll

10000 RPM 15000 RPM

19000 RPM

Fig. 3 - ES data taken during OBM preparation Fig. 4 - Sequence of preparation for Mud 3


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6 SPE 116364

A comparison of the filtration data before and after hot-rolling is shown in Fig. 5 and 6. From the filtration tests, it can be

seen that the reduction in filtrate volume is significant.

Filtration at 71 F (before hot roll)

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

Time (minutes)

Filtrate(ml)

Mud 1 Mud 2 Mud 3 Mud 4 Mud 5

Filtration at 71 F (after hot roll)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

Time (minutes)

Filtrate(m

l)

Mud 1 Mud 2 Mud 3 Mud 4 Mud 5

Fig. 5 - API filtration before hot-roll Fig. 6 - API filtration after hot-roll

As shown in Fig. 4, after all the oil-based mud preparation sequence had been completed, the final ES measurement had

improved by 100% for muds 2 and 3, and by more than 800% for the other muds. Muds 2 and 3 are the emulsions with alower emulsifier/CaCl2water ratio. Therefore, it is believed that in a drilling situation, should formation water enter these

muds, the stability of these muds will decrease. For such muds, it is normal to add extra emulsifier to the muds to compensate

for the additional water entering the well, so as to maintain a stable emulsion. It is also noted in Fig. 3 that the ES for mud 4

decreased from 1300 to 550 V after it was hot-rolled. The ES then increased again to 800 Volts after it cooled down toambient temperature. This irregular behavior might indicate that for this particular mud, the hot-rolling is not beneficial, in

terms of improving the ES. Mud 4 has the highest emulsifier/CaCl2ratio among the five muds.

The variation of ES with time for a mud is a direct indication of its stability while the mud is not being used. Fig. 7 showsthe ES data for the five OBMs. It can be seen that these muds maintain, or even increase, their ES properties with time. Muds

4 and 4b are the same mud; the only difference is that ES measurements were taken before and after Mud 4 was used in an

actual test, while Mud 4b was prepared without being tested. It is noted that the ES of the mud did not decrease after being

exposed to a pressurized test-environment for two weeks.

ES Variation with time for Oil-Based Muds

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

1300.0

1400.0

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

time (days)

ES

(Volts)

Mud 1 Mud 2 Mud 3 Mud 4 Mud 5 Mud 4b

Fig. 7 - ES measurement for OBMs

Shale Capillary Entrance Laboratory Pressure TestEntrance pressure tests performed by different authors ave shown that it is possible to flow fluid through shales ay high

enough overbalance pressures(Al-Bazali, et. al. 2005). This study focused on the use of laboratory OBMs only, so as to

understand the behavior of shale while it is exposed to various oil-base mud emulsions. The capillary entry pressure

determines the sealing capacity of the rock, and the hydrostatic pressure necessary to overcome the capillary properties of the

shale. It is dependant on the following factors: interfacial tension between the water-wet shale and the non-wetting fluid, thecontact angle between them, and the pore throat radius (Eq. 4), where hH is the hydraulic head, Pc is the capillary entry

pressure, mis the density of the mud, and g is the acceleration due to gravity.


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SPE 116364 7

cH

m

Ph

g= Eq.4

A large capillary pressure prevents entry of the oil into the shale, since the pressure differential is lower than the minimumcapillary entry pressure (Manohar 1999). Capillary pressures for low permeability water-wet shales can be as high as 2200 psi

for a pore radius of 10 nm.

Entrance Pressure Test Description

The entrance pressure test and equipment used was very similar to the one used by Al-Bazali in 2005. A shale sample of

0.260.010 inches thick by 2.5 inches in diameter was placed in a metallic chamber that had top and bottom ports that allowed

the shale sample to be subjected to different OBMs and pore fluids (Fig. 8 and 9).

Fig. 8 - Main cell used to measure theOil breakthrough pressure

Fig. 9 Test Equipment schematic

As mentioned above, the shale sample was exposed to a top oil fluid (OBM) and bottom NaCl fluid of 35,000 ppm salinity(simulated sea water) or a CaCl2solution at 30% w/w. During the test, the pressure differential across the shale was initiated

by first setting the upstream (top of sample) and downstream (bottom of sample) pressure to 50 psi, then gradually increasing

the upstream pressure. The entrance pressure was detected at the moment an increase in the pressure of the bottom side of theshale was observed. Once the downstream pressure stabilized, the upstream pressure was again increased and the behavior of

the downstream pressure was observed. During the tests, time between pressure-increase steps (100 to 250 psi at a time) was

at least eight hours. This extended period allowed the entire system to stabilize its internal volume of fluid and temperature.The duration of each of the tests was as long as one week.

Temperature-Pressure Sensitivity

Because the top and bottom fluid chambers were very small, it was necessary to determine how the pressure

measurements of the equipment were affected by temperature variations. This was done by increasing the temperature of theentire test unit and measuring the response, as shown in Fig. 10. A temperature increase of one degree Fahrenheit can produce

a 15 psi change in pressure, thus all experiments were run under tightly controlled temperature conditions. As will be shown

below, cell temperature values are often reported along with entrance pressure data.

Temperature Calibration - Test Cell with Solid disc

20.0

70.0

120.0

170.0

220.0

270.0

320.0

370.0

420.0

0 4 8 12 16 20 24

Time (hours)

Pressure(psi)

65

70

75

80

85

90

95

100

105

Temperature(F)

Upstream Pressure (psi) Downstream Pressure (psi) Temperature (F)

Downstream = NaCl 35000 ppm

Upstream = NaCl 35000 ppm

Fig. 10 - Pressure response (inside cell) to temperature changes

N2

Vacuum PumpVacuum Pump

Hand PumpHand Pump

Downstream Fluid

Upstream Fluid

V1a

V2

V3

V4

V5

V6V6

V8

T1

N2

C1C2

N2

C1C2V7

T6

T2

VV

Vacuum PumpVacuum Pump

Shale sample

Cell Chamber

V1a

Piston reaction chamber

VB2

VB1

VB3


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8 SPE 116364

Entrance Pressure Tests

Arco China shale at 0.72 Activi ty

After the system was successfully tested for temperature effects and leaks, five tests were performed using the native 0.72

activity Arco China shale. These fluids consisted of fresh water, oil, and emulsion muds. As shown in Figures 11 to 14, overthe time period shown, none of these fluids were able to penetrate this shale, even after applying about 1500 psi differential

pressure.

The first test consisted of attempting to obtain breakthrough utilizing fresh water. For this purpose, both the top and thebottom lines, and consequently the cell, were filled with fresh water. The outcome of this Initial Test 1 is shown in Fig. 11.

Arco China Shale (aw=0.72) - Initial Test 1

Date: 14-Sep-2005

No Breakthrough

0

200

400

600

800

1000

1200

1400

1600

0 24 48 72

Time (hours)

Pressure(psig)

Upstream Pressure (psi) Downstream Pressure (psi)

Water (aw=1.0)

Water (aw=1.0)

Fig. 11 - Initial Test 1 using fresh water in the top and bottom

As shown, no pressure increase was registered on the bottom side after 60 hours, even though the upstream pressure was

increased to 1500 psig for 8 hours. From this observation, it is concluded that the permeability of the shale was too low and

the size of the pore throats very small. Due to pressure limitations of the equipment, the pressure could not be increased above

1600 psig. Three additional tests using Mud 4, Escaid 110, and Mud 3 produced similar results (Figs. 12, 13 and 14).


Date: 26-Sep-2005

No Breakthrough

0

200

400

600

800

1000

1200

1400

1600

1800

0 12 24 36 48

Hours

Pressure

(psig)


Mud 4 (aw=0.661 - EC = 4.6%))

NaCl (aw=0.985)


Date: 4-Oct-2005

No Breakthrough

0

200

400

600

800

1000

1200

1400

1600

0 5 10 15 20 25

Time (Hours)

Pressur

e(pi


Escaid 110

Air at Patm

Fig. 12 Initial Test 2 using Mud 4 in the Fig. 13 Initial Test 3 using Escaid 110 in thetop and NaCl in the bottom top and Air in the bottom


9/19

SPE 116364 9

Arco China Shale (aw = 0.72) - Initial Test 4

Date: 7-Nov-2005

No Breakthrough

0100200300400500600700800900

1000110012001300

0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360

Time (hours)

Pressure(psig)


NaCl 35000 ppm (aw =

Mud 3 (aw = 0.655 - EC = 1.8%)

Fig.14 Initial test 4 using Mud 3 in the top and NaCl in the bottom

Alteration of Shale PorosityGiven the lack of fluid penetration over the time allotted, it was decided to open up the pores of the 0.72 aw shale samples in an

attempt to obtain breakthrough. Therefore, samples were partially hydrated by placing them in desiccators with controlledhumidities of 0.86 and 0.96.

The samples weight increase by water absorption was monitored every day, and it was observed that weight equilibrium

was essentially reached after two weeks.

It was of interest to calculate the change in porosity of the shale; this was done by placing several shale samples that hadbeen altered to 0.86 and 0.96 awback into the 0.72 awdesiccator and recording their weight loss. A sample calculation of the

new porosity is shown below:

- gwi 148.5= (aw= 0.72)

- gw 055.52 = after dehydrating the sample in the oven

- gw 186.53 = after hydrating the sample in a 0.76 water activity desiccator

Knowing the dimensions of the sample (1.728 x 1.610 x 1.857 cm) and assuming that the pore fluid (water) has a density

of 1 g/cc, the porosity calculation gave the results shown below:

( )

018.0

857.1610.1728.1

/1

055.5148.5

3=

==

cm

ccgr

gr

V

V

total

pores

iThis is the initial porosity of the rock

( )026.0

857.1610.1728.1

/1

055.5186.5

3=

==cm

ccgr

gr

V

V

total

pores

fThis is the final porosity of the rock after hydration.

Fig. 15 shows the porosity vs. water activity results obtained for the Arco China shale using water activity desiccators, set

at 0.76, 0.86 and 0.96 activity.

Shale Porosity change due to Water Activity Alteration

4.9%

3.9%

1.8%

2.6%

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

0.7 0.75 0.8 0.85 0.9 0.95 1

Water Activity (fraction)

Porosity

(%)

Fig. 15: Porosity Increase due to water activity alteration for the Arco China shale


10/19

10 SPE 116364

Breakthrough Pressure Tests using Altered Shale SamplesWith shale samples of altered porosity now available, pressure breakthrough was achieved with fresh water, Escaid, and the

five OBMs. Tables 7 and 8, and Figs. 16 through 25 show the results.

The first test, Fig. 16, used fresh water in the top (upstream) and a sodium chloride solution was used to simulate reservoirfluid in the bottom (bottom do you mean downstream?). The shale sample used in this initial test had an altered awof 0.96. The

breakthrough pressure is proof that the pores of the shale had been enlarged in diameter and fresh water had invaded the

sample.After pressure was increased several steps, and a final stable downstream pressure was observed, a reverse flow test was

performed. The reverse flow test consisted of reducing the upstream pressure below the last stable pressure observed on thebottom side; the purpose of this test was to determine whether the downstream pressure follows the trend which was observed

while the upstream pressure was increased in steps. This assured that there was backflow from the bottom to the top,confirming the communication between both sides of the sample.

It was also noted that after increasing the upstream pressure and achieving breakthough, the downstream pressure never

completely equalized the upstream pressure. This could indicate that there is an osmotic effect between the top fresh waterfluid and the saline pore fluid.

The results for the five initial tests (Fig 1116) are given in Table 7. Notice in Table 7 how increasing the porosity from

1.8% to 4.9% lowered the entrance pressure of oil from 1540psi to 40psi.

Arco China Shale (aw = 0.96) - Initial test 5

Date: 21-Nov-2005

0

100

200

300

400

500

600

700

800

0 12 24 36 48 60

Time (hours)

Pressure(psig)


NaCl 35000 ppm (aw = 0.985)

Fresh water (aw = 1.0)

Fig. 16: Test 5 using fresh water in the top and NaCl in the bottom with altered shale (aw = 0.96)

Test # Top Fluid EC (%) Shale aw

( ) Result1 Water 0.00 0.72 1.8 No Breakthrough

2 Mud 4 4.60 0.72 1.8 No Breakthrough

3 Escaid 110 0.00 0.72 1.8 No Breakthrough

4 Mud 3 1.80 0.72 1.8 No Breakthrough

5 Water 0.00 0.96 4.9 Breakthrough

1250

40

Initial Test Sequesnce

Final Differential

Pressure (psig)1450

1520

1450

Table 7 - Initial Test Sequence for the Arco China shale

It was of primary interest to measure and compare the oil breakthrough pressures obtained using (a) muds of different

compositions and (b) the same mud with shale samples altered to different water activities.


11/19

SPE 116364 11

Arco China Shale Tests at 0.96 Activity ( = 4.9%)

Arco China Shale (aw = 0.96) - Breakthrough Pressure

Test 1 - date: 30- Nov - 05

0

100

200

300

400

500

600

700

0 24 48 72 96 120 144 168

Time (hours)

Pressure(psig)

Upstream Pressure (psi) Downstream Pressure (psi) Pressure Differential

NaCl 35000 ppm (aw = 0.985)

Mud 3 (aw = 0.655)

EC = 1.6%

BTP = 194 psig

Osmotic HydraulicMud 3, aw = 0.66

NaCl, aw = 0.98

Upstream

Downstream

Shale aw = 0.96


Test 2 - date: 7-Dec-05

0

200

400

600

800

1000

1200

0 24 48 72 96 120 144

Time (hours)

Pressure(psig)

Upstream Pressure (psi) Downstream Pressure (psi) Pressure Differential

NaCl 35000 ppm (aw=0.985)

Mud 1 (aw=0.565)

EC = 2.9%

BTP = 735 psig

Osmotic Hydraulic

Upstream

Mud 1, aw = 0.57

Shale aw = 0.96

NaCl, aw = 0.98

Downstream

Fig. 17 Fig.18

Arco China Shale (aw=0.96) - Breakthrough Pressure

Test 3 - date: 14-Dec-05

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

0 24 48 72 96

Time (hours)

Pressure(psig)

60

80

100

120

Temperature(F)

Upstream Pressure Downstream Pressure

Pressure differential Temperature (F)

NaCl 35000 ppm (aw=0.985)

Mud 2 (aw=0.606

EC = 2.3%

BTP = 475 psig

Osmotic Hydraulic

Upstream

Mud 2, aw = 0.61

Shale aw = 0.96

NaCl, aw = 0.98

Downstream


Test 5 - date: 2-Jan-06

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

0 2 4 4 8 7 2 9 6 1 20 1 4 4 1 6 8 1 92 2 1 6 2 4 0 26 4 2 8 8 3 12 3 3 6 3 6 0 3 84 4 0 8 4 3 2 4 56 4 8 0 5 0 4 5 28 5 5 2

Time (hours)

Pressure(psig)

60

70

80

90

100

110

120

130

Temperature(F)



NaCl 35000 ppm (aw=0.985)

Mud 4 (aw=0.661)

EC = 4.6%

BTP = 944 psig

Osmotic Hydraulic

Upstream

Mud 4, aw = 0.66

Shale aw = 0.96

NaCl, aw = 0.98

Downstream

Fig. 19 Fig. 20


Test 6 - date: 26-Jan-06

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

0 2 4 4 8 7 2 9 6 1 20 1 44 1 68 1 9 2 21 6 24 0 2 64 2 88 3 12 3 3 6 36 0

Time (hours)

Pressure

(psig)

69.6

70

70.4

70.8

71.2

71.6

Temperature(F)


Pressure dif ferent ial Temperature (F)

CaCl2 (30%)

Mud 4 (aw=0.661)

EC = 4.6%

BTP = 299 psig

Osmotic Hydraulic

Osmotic

Upstream

Mud 4, aw = 0.66

Shale aw = 0.96

CaCl2, aw = 0.64

Downstream


Test 7 - date: 15-Feb-06

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

0 24 48 72 96 120 144 168

Time (hours)

Pressure

(psig)

69.5

70

70.5

71

71.5

Temperature(F)



NaCl (35000 ppm) aw=0.985

Mud 3 (aw=0.655)

EC = 1.6%

BTP = 677 psig

Osmotic Hydraulic

Upstream

Mud 3, aw = 0.66

Shale aw = 0.86

NaCl, aw = 0.98

Downstream

Fig. 21 Fig. 22


12/19

12 SPE 116364

Arco China Shale Tests at 0.86 Activity ( = 3.9%)


Test 9 - date: 9-Mar-06

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 432

Time (hours)

Pressure(psig)

70

70.5

71

71.5

72

Temperature(F)



NaCl (35000 ppm) aw=0.985

Mud 5 (aw=0.567)EC = 3.6%

BTP = 886 psig

Osmotic Hydraulic

Upstream

Mud 5, aw = 0.57

Shale aw = 0.86

NaCl, aw = 0.98

Downstream

Arco China Shale (aw = 0.86) - Breakthru Pressure

Test 13 - date: 19-Apr-06

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

0 24 48 72 96 120 144 168 192

Time (hours)

Pressure

(psig)

70

70.25

70.5

70.75

71

71.25

71.5

Tem

perature

(F)

Upstream Pressure (psi) Downstream Pressure (psi) Pressure differential Temperature (F)

NaCl (35000 ppm) aw=0.985

Base Fluid

BTP = 855 psig

Fig. 23 Fig. 24


Test 11 - date: 3-Apr-06

0.0

100.0

200.0

300.0

400.0

500.0

600.0

0 24 48 72 96 120 144 168

Time (hours)

Pressure(psig)

70

70.5

71

71.5

Temperature(F)



NaCl (35000 ppm) aw=0.985

Escaid 110

EC = 0%

BTP = 321 psig

Osmotic Hydraulic

Upstream

Escaid, aw = 0

Shale aw = 0.86

NaCl, aw = 0.98

Downstream

Fig. 25

MudBase Fluid

(ml)Escaid (ml) CaCl2 (30%) (ml) Lime (gr)

Emulsifier

Concentration /

CaCl2 (%)

CaCl2 / Escaid

w/w

API filtrate

(ml)

ES

(Volts)

Breakthrough

Pressure

(Psig)Mud 1 100 100 45 2.5 6.76 0.37 5.2 1130 890

Mud 2 100 100 90 2.5 3.38 0.74 4.6 380 601

Mud 3 100 100 180 2.5 1.69 1.47 8.1 200 194

Mud 4 100 0 45 2.5 6.76 0.75 0.9 795 1023

Mud 5 100 50 45 2.5 6.76 0.49 3 830 414

Mud 3 100 100 180 2.5 1.78 1.47 8.1 200 796

Shale aw = 0.96

Shale aw = 0.86


13/19

SPE 116364 13

MudBase Fluid


Emulsifier

Concentration /

CaCl2 (%)

CaCl2 / Escaid

w/w

API filtrate

(ml)

ES

(Volts)

Breakthrough

Pressure

(Psig)

Mud 1 100 100 45 2.5 6.76 0.37 5.2 1130 890

Mud 2 100 100 90 2.5 3.38 0.74 4.6 380 601

Mud 3 100 100 180 2.5 1.69 1.47 8.1 200 194

Mud 3 100 100 180 2.5 1.78 1.47 8.1 200 796

Shale aw = 0.96 (same volume of Escaid in all muds)

Shale aw = 0.86

MudBase Fluid


Emulsifier

Concentration /

CaCl2 (%)

CaCl2 / Escaid

w/w

API filtrate

(ml)

ES

(Volts)

Breakthrough

Pressure

(Psig)Mud 1 100 100 45 2.5 6.76 0.37 5.2 1130 890

Mud 5 100 50 45 2.5 6.76 0.49 3 830 414

Mud 4 100 0 45 2.5 6.76 0.75 0.9 795 1023

Shale aw = 0.96 (Same Emulsifier/CaCl2 Concentration in all muds)

Table 8 - Test Results for the Arco China shale using altered shales

Computation of Pore Entrance RadiusIt is possible to calculate the pore entry radius (r) of the shale, assuming that the sodium chloride solution on the bottom

(downstream side) is similar to the pore fluid in the shale. For this purpose, the interfacial tension between the top fluid filtrateand the sodium chloride was measured using a tensiometer (see Table 9). The estimated pore entry diameter (radius) was

calculated using Equation 1.

Downstream Fluid Upstream Fluid Interfacial Tension (D/cm)

NaCl (35000 ppm) Mud Filtrate 1 15.04




CaCl2 (30% w/w) Mud Filtrate 4 7.80


NaCl (35000 ppm) Escaid 21.06

NaCl (35000 ppm) Base fluid filtrate 4.32

Interfacial Tension between upstream and downstream fluids

Pore Entry Diameters for shale water activities of 0.86 and 0.96

0.00000

0.00500

0.01000

0.01500

0.02000

0.02500

0.03000

0.03500

0.04000

MF3 (EC=1.6%) MF4 (EC=4.6%) MF5 (EC=3.6%)

Mud Filtrate

PoreEntryDiameter(microns)

0.86 aw shale 0.96 aw shale

Table 9: Interfacial Tension measurements Fig. 26: Pore entry diameters for water activities of 0.86 and

0.96

Altered Shale PermeabilityIt was also of interest to observe the relation between permeability and oil breakthrough pressure. The Arco China shale

samples with altered water activity were exposed to KCl fluids, so as to measure their vertical permeability. Permeabilities forboth 0.96 and 0.86 awwere determined using the Darcy equation (Eq. 6). In Eq. 6, k represents the permeability of the sample,

q is the flowrate obtained from the slope in Fig. 29, is the viscosity of the flushing fluid (KCl), L is the thickness of the shale

sample, A is the cross sectional area of the sample, and P is the difference between the upstream and the downstream

pressure.

PA

Lqk

=

. Eq. 6

The equipment and procedure used to measure permeability are the same as was utilized in the oil breakthrough test. The

only difference consists of the steps followed after the sample has been flushed with the test fluid.


14/19


15/19

SPE 116364 15

These results (Fig. 28) show a large difference in entrance pressures between the tests that used the base fluid as compared

to the tests that used only Escaid 110. A lower entry pressure is observed when oil is used as the top fluid, even though the

interfacial tension between the oil and the pore fluid (21 dynes/cm) is much larger than that measured between the base fluid

and the same pore fluid (4.3 dynes/cm). In addition, wettability alteration of the shale to oil wetting conditions should reducethe capillary pressure needed to invade the shale pores. Clearly, the experimental trends do not show this behavior.

Pore Entry Diameter

Escaid and Base Fluid

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

Escaid Base Fluid

Filtrate

Entry

Pore

diam

eter

(m

icrons)

Shale (aw = 0.96) Shale (aw = 0.86)

Breakthrough Pressure

Escaid and Base Fluid

0

100

200

300

400

500

600

700

800

900

Escaid Base Fluid

Filtrate

Pressure

(psig)

Shale (aw = 0.96) Shale (aw = 0.86)

Fig. 28: Oil breakthrough Pressure and Pore Entry Diameter for Escaid and Base Fluid

Therefore, based on capillary entrance pressure calculations, changes in the interfacial tension and wettability should lead

to smaller pore entry pressures with the base fluid. Using the capillary pressure equation (Eq. 1), an effective pore entry

diameter was calculated for both the base fluid and the pure mineral oil (Escaid 110). Fig. 30 shows that the pore diameterscalculated in this manner are significantly larger when the Escaid is used, as compared to when the base fluid is used. Because

this difference in the estimated pore entry diameter is so large (a factor of 10), it suggests that the mechanism of pore

penetration by the oil is controlled by both capillary pressure and other factors, which will be discussed in the followingparagraphs.

Fig. 29 shows invasion of the oil into the pore space of the shale when pure mineral oil is used. This process of invasion

must be controlled by capillary pressure, as the interface between the brine in the pore space and the mineral oil does not

contain any surfactants. However, the interface between the base fluid containing surfactants and water must behave in a moreelastic manner. This conjecture is based upon our experimental observations, which suggest that despite the interfacial tension

being lower, the pressure needed to invade the pore space is significantly higher. The additional pressure drop needed toinvade the pore space may be related to the energy required to elastically deform the interface between the base fluid and the

brine saturating the shale pore space. The surfactants used in the base fluid (EZ-Mul [a quarter amine] and X-Vis [a

Polymerized Fatty Acid]) are both known to stabilize emulsions by adsorbing on the oil-water interface and forming a rigid

interfacial film.

It is known that the force required to deform the elastic surface film is 1) proportional to the rigidity of the film (Adkins1951), and 2) inversely related to the pore throat size. Our hypothesis, which states that the pore entry pressure is controlled by

the elasticity of the interface, is supported by the experimental observations, which show that the entry pressure increases as

the concentration of emulsifier is increased (higher concentrations of emulsifier make the film more rigid). Our hypothesis isalso consistent with our observation that the pore entry pressure increases as the awof the shale is reduced.

Hydraulic Gradient

OIL Pore Throat

Interfacial Tension21.04 D/cm

POREFLUID

OIL

Hydraulic Gradient

Interfacial Tension4.32 D/cm Pore Throat

POREFLUID

BASE FLUID

Fig. 29: Oil (Escaid 110) pore invasion Fig. 30: Base fluid pore invasion


16/19

16 SPE 116364

Effect of Water Droplets and Emulsifier Concentration in Base-Oil

In the results reported above, no emulsified water droplets were present in the liquid fluid. In this section, we discuss

results in which water droplets are present in the oil-based mud. The addition of water droplets results in an additional

resistance to flow provided by the mud cake, which is formed by the water droplets coating the surface of the shale.As illustrated in Fig. 31, increasing the emulsifier concentration increases the oil breakthrough pressure for the different

muds containing water droplets. Increasing the concentration of CaCl2 in the mud results in larger water droplets, and

therefore, higher permeability of the filter cake and lower oil breakthrough pressures (Fig. 32).

Upstream

Fluid

Base Fluid

(ml)

Escaid

(ml)

CaCl2

(ml)

% Emulsifier

Concentration/Total

Mud weight (gr/gr)

Water

Content (%)

Breakthrough

Pressure (psig)

Mud 1 100 100 45 2.9% 20% 735

Mud 2 100 100 90 2.3% 32% 475

Mud 3 100 100 180 1.6% 48% 194 Table 12: Oil breakthrough Pressure for muds with different CaCl2concentration (awshale= 0.96)

Breakthrough Pressure vs. Emulsifier Concentration (muds with

different CaCl2 content - muds 1, 2, 3)

0

100

200

300

400

500

600

700

800

1.5% 1.7% 1.9% 2.1% 2.3% 2.5% 2.7% 2.9% 3.1%

Emulsifier Concentration (%)

BreakthroughPre

ssure(psig)

Breakthrough Pressure vs . Emulsifier Concentration (muds

with different CaCl2 content - muds 1,2, 3)

0

100

200

300

400

500

600

700

800

15 20 25 30 35 40 45 50 55

% of CaCl2 brine in emulsion

Breakthrough

Pressure

(psig

)

Fig. 31: Oil breakthrough pressures for muds with Fig. 32: Oil breakthrough pressures vs. CaCl2content (aw shale = 0.96)different CaCl2concentration (awshale = 0.96)

At high concentrations of emulsifier and low water content, small droplets of water with thick layers of emulsifier provide

a low-permeability cake that resists the flow of oil filtrate into the pore space.Results were also obtained by diluting the base fluid with different quantities of pure Escaid oil (muds 1, 4 and 5). Table

13 shows the pore entry pressures for these muds. Figs. 33 and 34 show the effect of emulsifier concentration as well as of

Escaid concentration on the capillary entry pressure. No clear trends are observed in this case. This is because as the base oilis diluted with the Escaid mineral oil, both the emulsifier concentration and the water-to-oil ratio in the mud are changing.

Since both the water content and the emulsifier concentration affect the pore entry pressure, it is difficult to make substantial

conclusions based on these plots.

Upstream

Fluid

Base Fluid

(ml)

Escaid

(ml)

CaCl2

(ml)

% Emulsifier

Concentration/Total

Mud weight (gr/gr)

Oil Content

(%)

Breakthrough

Pressure (psig)

Mud 1 100 100 45 2.9% 80% 735

Mud 4 100 50 45 4.6% 66% 944

Mud 5 100 0 45 3.6% 75% 320 Table 13: Oil breakthrough Pressure for muds with different Escaid 110 concentration (awshale= 0.96)


17/19

SPE 116364 17

Breakthrough Pressure vs. Emulsifier Concentration (muds

with different Escaid 110 content - muds 1, 4, 5)

0

100

200

300400

500

600

700

800

900

1000

2.5% 3.0% 3.5% 4.0% 4.5% 5.0%


Breakthrou

ghPressure(psig)

Breakthrough Pressure vs. Escaid 110 concentration (muds

with different Escaid 110 content - muds 1, 4, 5)

0

100

200

300

400

500

600

700

800

900

1000

65% 70% 75% 80% 85%

Escaid 110 Concentration (%)

Breakthrou

ghPressure(psig)

Fig. 33: Oil breakthrough pressures for muds with Fig. 34: Oil breakthrough pressures vs. Escaiddifferent Escaid concentration (aw shale = 0.96) content (aw shale = 0.96)

Effect of Osmotic Pressure

To test the importance of osmotic pressure in the pore entry pressures, mud 4 (containing 0.985 awemulsified NaCl water)

was used on the top and 0.64 awcalcium chloride water was used as the bottom fluid. It is observed that the oil breakthroughpressure is significantly larger when a higher awfluid is placed on the downstream end (Fig. 35). This result clearly shows that

osmotic pressure differences across the shale can play an important role in controlling the entry pressures into the shale.Indeed it appears as if a combination of the hydraulic gradient and the osmotic gradient will control the entry pressure into theshale.

Breakthrough Pressure

Mud 4 with different downstream fluids

0

100

200

300

400

500

600

700

800

900

1000

MF4 (EC=4.6%) NaCl MF4 (EC=4.6%) CaCl2

Filtrate

Pressure(psig)

Shale (aw = 0.96)

Fig. 35: Entrance Pressure Effect for mud 4 with different bottom fluids

This experimental result shows the importance of maintaining a significant osmotic pressure differential between the shale

and the OBM, so that the invasion of oil filtrate into the shale can be minimized. To the best of our knowledge, this is the firstclear experimental evidence provided for the relative magnitude of the osmotic contribution to pore entry pressures for oil-base

muds.

Effect of Shale Water ActivityThe results reported thus far have focused on the shale with aw= 0.96. Similar results were obtained with the shale with

the water activity of 0.86. Fig. 36 shows the pore entry pressures for both the aw of 0.86 and 0.96. As might be expected, the

oil breakthrough pressure for the shale with lower awis significantly higher than for the higher awshale, because the pore sizesare smaller.

An increasing trend in oil breakthrough pressure with emulsifier concentration is observed in both shales. Smaller pore

entry diameters are calculated for the shale with aw= 0.86, as compared to the shale with aw= 0.96, as shown in Fig. 38. Thisis consistent with the loss in water content and the shrinkage of the shale when the shale was partially dehydrated in

desiccators.


18/19

18 SPE 116364

Breakthrough Pressure vs. Emulsifier Concentration

Comparison between shale aw = 0.96 and shale aw = 0.86

Muds 3,4 and 5

0

100

200

300

400

500

600700

800

900

1000

1.5% 2.0% 2.5% 3.0% 3.5% 4.0% 4.5% 5.0%


Breakthrough

Pressure

(psig)

0.86 aw shale 0.96 aw shale

Fig. 36: Entrance Pressure comparison for shales altered to different water activity

Conclusions

The following conclusions are based on results obtained in this study for the Arco China shale:

1. The pore entry pressures measured for shales are controlled both by capillary pressure (pore size, interfacial tension)and the rigidity of the oil-water interface. The results obtained for surfactant laden interfaces show that a more rigid

oil-water interface, created by the addition of surfactant, results in higher oil breakthrough pressures, even when the

interfacial tension is lower.

2. Higher emulsifier concentrations in the base oil result in larger entry pressures, as a result of the increase in rigidity ofthe oil-water interface.

3. The presence of water droplets in the OBM increases the pore entry pressures because the water droplets form a filtercake and provide an additional resistance to the entry of oil filtrate into the shale.

4. Increasing water content and decreasing emulsifier concentration result in lower pore entry pressures.5. The Arco China shale samples that have a smaller pore size have substantially higher pore entry pressures.6. The osmotic pressure generated between the brine droplets in the oil-based mud and the pore fluid in the shale plays a

significant role in controlling the pore entry pressures.

7. OBMs should clearly contain 1) sufficient emulsifier to provide small droplet size and mechanical rigidity of the oil-water interface, and 2) a high concentration of calcium chloride in the water phase in order to take advantage of

osmotic gradients. These factors will cumulatively result in high pore entry pressures, resulting in minimum invasion

of oil and water filtrate into the shale.


19/19

SPE 116364 19

Nomenclatureaw = water activity

= contact angle

= interfacial tension, dynes/cm

= porosity

CEC = cation exchange capacityHTHP = high temperature high pressure

ES = electrical stability (volts)

RPM = revolutions per minutePPM =parts per million

Pc = capaillary pressure (dynes)

EC = emulsifier concentration

References

Adkins, J.E. and Rivlin, R.S., Large Elastic Deformations of Isotropic Materials, Faraday Laboratory of the Royal Institution, September

18, 1951.

Al-Bazali, T.M., Zhang, J., Chenevert, M.E., and Sharma, M.M., Measurement of the Sealing Capacity of Shale Cap-rocks, SPE 96100,

Presented at the SPE Annual Technical Conference and Exhibition held in Dallas, TX, 9 12 October, 2005.

Al-Bazali, T.M., Experimental Study of the Membrane Behavior of Shale during interaction with Water-Based and Oil-Based Muds,

Dissertation presented to the Faculty of the Graduate School of The University of Texas at Austin, Austin, Texas, May, 2005.

Chenevert, M.E., and Sharma, A.K., Permeability and Effective Pore Pressure of Shales, SPE 21928, Presented at the SPE/IADC Drilling

Conference in Amsterdam, 11 14, 1991.

Chenevert, M.E., Shale Alteration by Water Adsorption, SPE 2401, Presented at the SPE Fourth Conference on Drilling and Rock

Mechanics held in Austin, TX, 14 15 January, 1969.

Chenevert, M.E., Shale Control with Balanced-Activity Oil-Continuous Muds, SPE 2559, Journal of Petroleum Technology, 1970.

Growcock, F.B., Ellis, C.F., and Schmidt, D.D., Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds,SPE20435, SPE Drilling and Completion, March, 1994.

Manohar, L., Shale Stability: Drilling Fluid Interaction and Shale Strength, SPE 54356, Presented at the SPE Latin American and

Caribbean Petroleum Engineering Conference held in Caracas, Venezuela, 21 23 April, 1999.

Schlemmer, R., Friedheim, J.E., and Growcock, F.B., Membrane Efficiency in Shale An Empirical Evaluation of Drilling Fluid

Chemistries and Implications for Fluid Design, SPE/IADC 74557, Presented at the SPE/IADC Drilling Conference held in Dallas, TX,

26 28 February, 2002.

Simpson, J.P., A New Approach to Oil-Base Muds for Lower-Cost Drilling, SPE 7500, Presented at the SPE/AIME 53 rd Annual Fall

Technical Conference and Exhibition held in Houston, TX, 1 3 October, 1978.

spe-116364-entrance pressure of oil based mud into shale effect of shale, water

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