borehole instability by zerdasht haydari (soran university)

Soran University

Faculty of Engineering

Department of Petroleum Engineering

Third Stage

Borehole

Instability

Prepared by: Zerdasht Jamshid Haydar

Supervisors: Mr. Jagar Abdulazez

Mr. Nazir Mafakheri

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Borehole Instability – Zerdasht J. Haydari 2015-2016

2015-2016

Table of Contents

Abstract:................................................................................................................................................................2

Introduction:..........................................................................................................................................................2

Important Terminology..........................................................................................................................................3

Elasticity............................................................................................................................................................3

Stress.............................................................................................................................................................3

Anisotropy and Isotropy................................................................................................................................4

In-Situ Stresses..................................................................................................................................................4

Determining the overburden (vertical) stress σv:..........................................................................................5Determining the maximum and minimum horizontal stresses (σHmax and σHmin):............................................6

Far-Field Stresses..............................................................................................................................................6

Normal Faulting:...........................................................................................................................................6

Reverse Faulting (Thrust Faulting):...............................................................................................................6

Strike-Slip Faulting:......................................................................................................................................7

Factors leading to borehole instability:..................................................................................................................8

Impact of mud on borehole instability:..................................................................................................................9

Choosing the appropriate mud weight:..............................................................................................................9

Choosing the proper mud type:........................................................................................................................10

Discussion...........................................................................................................................................................10

References...........................................................................................................................................................11

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Keywords: borehole, instability, geomechanics

Abstract:

Borehole instability is the unwanted condition in which an uncased, non-

cemented wellbore (open hole) does not maintain its desired size and/or

shape. This report discusses the factors that cause it, whether it be

uncontrollable (natural) or controllable factors, some theories and methods of

determining stresses that lead to the instability of a borehole. Even though

borehole stability problems are much more common in deviated or horizontal

wells, the focus of this report will mainly be about vertical wells. Possible

prevention methods and techniques will also be discussed.

Introduction:

Borehole instability is one of the major technical problems encountered

during drilling. Starting in the exploration phase and continues through the

development and production phases. Predicting the location of the vulnerable

region(s) of the underground formations, although not so accurate and

usually unexpected, is a fundamental step in reducing the risks associated

with borehole instability. Drilling a hole causes the pre-existing stresses in a

rock to be released, which causes elastic deformation of the rocks The risks

associated with wellbore instability include: loss of time, waste of money on

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numerous operations such as side-tracking, fishing operations …etc. It may

also lead to permanent well abandonment in certain situations. (PetroWiki,

2015) (Pašić, et al., 2007). The total money spent on borehole stability

related problems costs the oil and gas industry more than 1 billion U.S dollars

each year (Zeynali, 2012). The next section will briefly introduce some of the

basic laws and theoretical subjects for a better comprehension of borehole

stability problems.

Important Terminology

Before how borehole instability can be possibly prevented or its effects minimized, one should be

familiar with certain terms related to stress determination of rocks.

Elasticity

Most materials possess the ability to withstand and recover deformations caused by forces, this

property is called is elasticity. It is the fundamental basis of all rock mechanics. When it comes to

petroleum related rock mechanics, the focus is mainly on rocks with a significant amount of

porosity and permeability rather than solid materials. Thus, the concept of poroelasticity must be

taken into account. (Fjær, et al., 2008)

Stress : is the force acting per unit area and can be formulated as shown below :

σ= FA

The SI unit for Stress (σ) is the Pascal (Pa= N/m2) indicating that if the area decreases while

having a constant force acting on an object, the stress will increase, which leads to eventual

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failure of any object. Stresses and the determination of stress orientations will be discussed in

more detail in the later sections of this paper.

Anisotropy and Isotropy

i. Isotropic materials are those types of materials whose response does not depend on the

orientation of the stress being applied.

ii. Anisotropic materials : “If the elastic response of a material is not independent of the

material’s orientation for a given stress configuration, the material is said to be

anisotropic.” (Fjær, et al., 2008)

In the case of formations in the earth, rocks are considered to be anisotropic and extremely

complex in nature. Meaning they are strongly dependent on the orientation of the stress. But for

simplified calculations, they are assumed to be isotropic in most cases.

Types of Stresses:

In-Situ StressesFormations deep under the ground are under the influence of numerous

stresses caused by the weight of overlaying formations. Although the number

of stress vectors on a certain point might be infinite if all three axes are

taken into consideration, only three of the stresses are significant enough to

be used in calculations for estimation of stress on a single point or defined

area. These three stresses are (σHmax, σHmin and σv) which stand for maximum

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horizontal stress, minimum horizontal

stress and the vertical stress (also

called the overburden stress)

respectively. These stresses are

illustrated below:

Determining the overburden (vertical) stress σv:

Usually, the formations that are underlying carry the weight of the

formations that are overlaying them. If a homogenous (same material in

each and every point) height column is taken into consideration , the

Figure 1 ; Shows the major horizontal stresses acting around the borehole. In the case of vertical wells, the vertical stress (σv) is parallel to the axis into the page.courtesy of (Zoback, 2007)

Figure 2 Wellbore stresses in and around the borehole (Pašić, et al., 2007)

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vertical stress at the bottom will be calculated as follows : σv = ρgz , where ρ is the density of the material (the

formation) , g is the gravitational acceleration , and z is the depth. Meaning the stress will increase as depth and

density increase. However, if the density changes with depth, the vertical stress at a given depth (D) will

become: σ V=∫0

D

ρ ( z ) gdz

Notice here that the z-axis is pointing vertically downwards. And z = 0 at the surface of the Earth. However,

to find the pore pressure, the previous equation must be modified even further. See the equation below :

Pfn=∫0

D

ρ f ( z ) gdz

Where Pfn is the normal pore pressure. (Fjær, et al., 2008)

Determining the maximum and minimum horizontal stresses (σHmax and σHmin):

The most simplified version of determining these stresses is using “Kirsch equations”

Minimum Horizontal Stress:

σ Hmin=v

1−v ( σ v−α pp )+α p p

Where : v is Poisson’s ratio

α is the Biot coefficient for the specific type of rock.

pp is the pore pressure

Maximum Horizontal Stress:

σ Hmax=3σ Hmin−2 pp+0.1(σ Hmin− pp)

The variables are defined above. (Fjær, et al., 2008) (Zoback, 2007)

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Far-Field StressesThese are the types of stresses formed in the earth naturally by the activities of the tectonic plates.

These stresses are most significant in regions near plate boundaries, they are mainly faults and

are of three types which are listed below.

Normal Faulting:

Is a fracture in a rock volume, in which a rock layer or (strata) has been significantly displaced

because of tectonic movement. This type of faulting happens when the layers above slip down.

(see figure 3)

Reverse Faulting (Thrust Faulting):

This type of fault is similar to a normal fault but the direction is opposite. Thus, the layers above

move up the “dip” (see figure 3)

Strike-Slip Faulting:

Horizontal displacement parallel to fault trace are the dominant feature of this type of fault.

Figure 3: Types of faults . Courtesy of pixshark.com

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By incorporating the knowledge obtained from the equations above and knowing what type of

tectonic activity is happening or has happened in the region, borehole instability problems and

fracture initiation can be predicted. The figure below shows the tendency of breakouts and

fractures in the three different kinds of faulting scenarios.

Factors leading to borehole instability:

Generally, a combination of factors cause borehole instability, these factors may be roughly

categorized as being either uncontrollable (natural) or controllable in origin. The main causes

divided into the main two categories can be seen in Table1 below:

Causes of borehole instability

Uncontrollable (Natural) Factors Controllable Factors

Naturally Fractured or Faulted Formations Bottom Hole Pressure (Mud Density)

Figure 4: Tendency for the initiation of tensile fractures (left) and wellbore breakouts (right) (Jia, et al., 2014)

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Tectonically Stressed Formations Well Inclination and Azimuth

High In-situ Stresses Transient Pore Pressures

Mobile Formations Physico / chemical Rock-Fluid Interaction

Unconsolidated Formations Drill String Vibrations

Naturally Over-Pressured Shale Collapse Erosion

Induced Over-Pressured Shale Collapse Temperature

Table 1 ; Causes of Borehole Instability (Pašić, et al., 2007)

As it can be seen in table 1, there are numerous factors that lead to borehole instability, however

the focus of this report will mainly be the pressure differential of the bottom hole pressure (which

refers to the density of the mud) and pore pressure, and hydration of clay materials in shale.

Impact of mud on borehole instability:Choosing the appropriate mud weight: Choosing the right mud density, or (mud weight as usually referred to in the oil and gas industry)

is crucial to keep the borehole in a safe and stable state, numerous additives are added to mud to

increase its weight/density (bentonite, barite, haematite …etc.). When choosing the right mud

weight, one must consider the lifting capacity (the ability of the mud to lift cuttings to the

surface) , the gel strength “The ability of drilling mud to suspend drill cuttings” (Rabia, 2002)

and several other properties. Mud weight should be in a safe window, one where it is higher than

the pore pressure, but at the same time does not exceed the least principle stress at that certain

point. See the figure below :

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Choosing the proper mud type:Appropriate mud type must be carefully chosen when drilling to avoid a condition called shale

sloughing (swelling). This condition happens when WBM (water based mud) is used during

drilling, water particles react with clay minerals in shale formations, causing them to swell. Thus,

reducing the borehole radius, leading to several consequences, such as stuck pipe. OBMs (Oil

based muds) are more reliable for such situations, because water particles are emulsified within

the oil phase, thus almost completely reducing (shale-water) contact. (Rabia, 2002)

Figure 5 Effect of depth on borehole stability. Courtesy of (Pašić, et al., 2007) . Notice that as depth increases, the mud weight should increase gradually with it to resist the pore pressure around the borehole.

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DiscussionBorehole stability issues strongly depend on the difference between the principal horizontal

stresses and mud weight differential in the case of break outs. So if we take as an example, a mud

pressure of 30MPa, and a pore pressure of 25MPa, this will induce accidental hydraulic

fracturing. If the mud weight is 27 MPa and pore pressure is 30 MPa, then the probability of

kicks will increase significantly. The difference, between this and a break out is that, breakouts

happen because of the difference in the principal stresses in the earth, SHmax and SHmin. When

drilling is in process and parts of the formations are removed, pressure is released and this extra

stress to be put on the zone where horizontal stress is max, this will support that half of the

borehole arc, and the remaining arc will break out.

References

1. Fjær, E. et al., 2008. Petroleum Related Rock Mechanics. 2nd ed. Amsterdam: Elsevier.

2. Jia, Q., Schmitt, D., moeck, I. & Kofma, R., 2014. Improving Borehole Instability Analysis by Investigating the Impacts of Stress and Rock Anisotropy. GeoConvention2014 :Focus, p. 7.

3. Pašić, B., Gaurina-Međimurec, N. & Matanović, D., 2007. WELLBORE INSTABILITY: CAUSES AND CONSEQUENCES. Volume 19, p. 12.

4. PetroWiki, 2015. Borehole Instability. [Online] Available at: http://petrowiki.org/Borehole_instability[Accessed 13 November 2015].

5. Rabia, H., 2002. Well Engineering and Construction. 1st ed. s.l.:Entrac Consulting .

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6. Schlumberger, 2015. slb.com. [Online] Available at: http://www.slb.com/services/technical_challenges/geomechanics/drilling_management/wellbore_stability.aspx[Accessed 20 November 2015].

7. Zeynali, M. E., 2012. Mechanical and physico-chemical aspects of wellbore stability during drilling operations. Journal of Petroleum Science and Engineering, Volume 82-83, pp. 120-124.

8. Zoback, M. D., 2007. Reservoir Geomechanics. 1st ed. New York: Cambridge University Press.

borehole instability by zerdasht haydari (soran university)

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controllable factors