pressure concepts overburden pore fracture

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Pressure ConceptsPetroforma - Luanda #1Loss of Primary Well ControlIf primary well control is lost, a kick (unwanted intrusion of fluids into the wellbore) may occur. A kick can turn into an uncontrolled blowout. In such a case, secondary well control measures come into effect. This primarily involves the use of surface well control equipment.#4Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified

We are fully certified in N.Sea, US & Canada, Venezuela, West Africa District, Australasia, China, South East Asia,Indonesia.

Other areas such as Brazil, N.Africa will be completed in 2001

1. As direct measurement in different units (psi, bar, kg/cm2, etc.)2. EMD (Equivalent mud density) 3. Gradient (psi/ft, kg/cm2 per 10 m)4. PotentialDifferent Expressions of Pressure#5Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



A pressure is a force divided by the surface upon which this force applies. Pressure Pascal = Force Newton / Surface m2 The official pressure unit is the Pascal It is a very small unit: 1 Pascal = 1 Newton/m2 1 bar = 105 Pascal 1 atm = 1,013*105 Pascal A practical unit on the rig is the kgf/cm2: 1 kgf/cm2 = 0.981 bar In API, the unit is the pound per square inch (psi): 1 bar = 14.4988 psi

#6Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Exercise 1 Unit Conversions Convert the following values to the requested units: 25 kgf/cm2= bars 15 Bars= psi 155 Psi = bars1 kgf/cm2 = 0.981 bar1 bar = 14.4988 psi

217.4810.6924.53#7Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Hydrostatic Pressure is the pressure exerted by the weight of a static column of fluid.Function of: - Height of the column - Fluid density only.

Hydrostatic PressureThe geometry and dimensions of the fluid column have no effect on hydrostatic pressure.#8Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Hydrostatic Pressure




The scientist Pascal bet he could destroy a barrel with just a pint of water: He fixed a long and thin tube on the barrel and poured in the water.

Despite the tiny volume of the water, the height was enough to make the barrel explode*!

*Also credited to the Flemish sicentist Simon StevinusPascals Demonstration




Ph = d * g * H With Ph = hydrostatic pressure (Pascal)d = Fluid specific gravity (kg/m3)H = Vertical height of fluid (m) Hydrostatic Pressure Formula:#11Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Hydrostatic Pressure FormulaAPI units: Ph = 0.052*H*d

Where: Ph = hydrostatic pressure (psi) d = Fluid density (ppg) H = Vertical height of fluid (ft)Metric units: H*d 10

Where: Ph= hydrostatic pressure ( kg/cm2) d = Fluid specific gravity (kg/l) H = Vertical height of fluid (m)NB: you must use 10.2 with pressure in barsPh =#12Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Exercise 2 Hydrostatic PressureCalculate Ph in the following examples: Height of fluid (m): 1000 Fluid density (kg/l): 1.5 Ph (kgf/cm2):Height of fluid (ft): 5000 Fluid density (ppg): 10 Ph psi:API units: Ph = 0.052*H*d

Metric units: H*d 10

Ph =1502600#13Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



If the mud weights in the pipes and annulus are different, a U-tube effect occurs due to the difference of Ph, as the system seeks equilibrium. The slug of weighted mud pumped just prior to a trip takes advantage of this effect to keep the inside of the pipe dry at drill floor level.

The U-tube Effect

PipeAnnulusPipeAnnulusWeightedMud#14Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Pressure vs. Depth Plot1.031.201.30Pf & Ph (due to MW) in kgf/cm2Depth (m)The pressure vs. depth plot provides a convenient means to show changes in pressure gradient with depth.#15Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Pressure vs. Depth PlotDepth(m)Pressure from RFT (kgf/cm2)Gas/oil contact(m)Oil/water contact(m)Another purpose of this type of plot is to determine the contacts between fluids in a reservoir by tracing the trend lines of the RFT pressure data. #16Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Pressure vs. Depth PlotThe pressure vs. depth plot also allows easy comparison of pressure parameters whose interaction may result in drilling problems, such as formation pressure and fracture gradient.




Formation (Pore) PressureFormation pressure is the pressure of the fluid contained in the pore spaces of the sediments. 0 500 1000 1500Overburden pressured = 2.31Hydrostatic pressured = 1.08Hydrostatic pressured = 1.00NegativepressureanomalyPositive pressure anomaly05000DepthPressureNegative pressure anomalyPressure less than hydrostaticpressure.

Hydrostatic zonePressure remains in normal hydrostatic regime.

Positive pressure anomalyPressure more than hydrostatic pressure. Usually limited byoverburden stress.Normal pressure regime#18Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Equivalent Mud Weight (EMW)Depending on drilling conditions, the equivalent mud weight may be greater or less than the actual mud weight. Equivalent MW < actual MWEquivalent MW > actual MWMud lossesSwabbingFluid Influx (kick)Leak Off Test/Formation Integrity TestSurgeCirculation (Hydrostatic + annular pressure loss)#19Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Depth 2000mDepth 0 mDepth 1000 mFm. Press. 200 kgf/cm2Well - AWell - BEMW for well No 1 = 1 SG (200 X 10) / 2000(Mud weight required to balance formation pressure)EMW for well No 2 = 2 SG (200 X 10) / 1000(Mud weight required to balance formation pressure )Knowledge that the formation pressure is 200 is not enough. We have to see it in relation to the balancing mud column.Equilibrium Mud Weight#20Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Equivalent Circulating Density (ECD)This is the effective density of the circulating fluid when pressuredrop in the annulus is considered: Metric:ECD = ((Ph + APL in kg/cm2) X 10) / Depth in mAPI:ECD = ((Ph + APL in psi) / Depth in ft) / .052Ph + annular pressure loss, converted to mud weightECD thus implies that the pressure at any point in the annulus while circulating will be higher than when the fluid is at rest (static). StaticCirculating#21Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Swab/SurgeThe drill string can act like a piston when moved vertically, and this movement can affect the wellbore pressure.Swab: a decrease of effective hydrostatic caused by mechanical movement upwards Surge: an increase of effective hydrostatic caused by mechanical movement downwards #22Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Swab/SurgeExcessive swabbing, for example by pulling the pipe too fast during a trip out of the hole, can result in a kick.

Excessive surge, usually from running in too fast, can result in formation breakdown and lost circulation.

As these effects can be subtle, we must monitor trips carefully, by keeping a trip sheet and by observing the differential volume (volume +/-) parameter on the real-time data acquisition system.#23Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Exercise 5 Equivalent Mud DensitiesCalculate the EMW for the following cases: Well total depth (m)2000Mud weight in hole (kg/l)1.20A. During a trip, the driller forgot to fill the hole and the mud level is lower than normal; its distance from flow line is 100m; EMW = kg/lB. During a Leak Off Test, the pressure reached A maximum of 10 kgf/cm2; EMW =kg/lC. Circulation starts; the pressure losses in the annulus is 4 kg/cm2; ECD = kg/l1.141.251.22#24Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Plotting Pressure GradientsMW and pressure gradients may be plotted on the same graph, allowing a comparison between MW, Formation pressure gradient, Fracture gradient and Overburden gradient. Pressure (kg/cm2)Depth(m) GPf MW FRAC S #25Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Overburden Stress: OVBPName convention : OVBP for overburden Pressure (often called S in the literature)OVBg for local gradientFor intervals with constant density

OVBG for averaged Gradient, referenced to flow line,expressed in EMW.




Overburden Stress: OVBPOverburden stress is the pressure exerted by the weight of the overlying sediments.The term pressure is usually applied for fluid hence the word stress. OVBP = P + s OVBP: Overburden Stress (total pressure) P: Pore Pressures: Pressure supported by rock matrix (effective stress) Cumulative weight of rocks + fluidMatrix: Rocks + Cement (Solids) Contribution to total weight will depend on matrix density.Fluid: Fluid assumed as water; gas will affect the density.#27Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Matrix Stress Components

S3 S1 (overburden) S2 Unlike liquids, solids can withstand different loads in various directions:Imagine a cube of porous rock somewhere in the subsurfaceWe can divide the stresses into 3 resulting forces according to the 3 directions of space: S1 can be considered the Overburden, S2 and S3 the tectonic forces. Open hole ovalization can give an idea of the difference between S2 and S3. #28Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Terzaghi and Peck EquationIn a porous rock, the fluid may support part of the stress (due to undercompaction) and the total stress will have 2 components:OVBP = P + s (Terzaghi equation)With OVB = Total stress (kgf/cm2)P = Pore pressure ( or formation pressure) (kgf/cm2)s = Effective stress (on the grains of the rock) (kgf/cm2)Consequently:S1 = P + s1 (OVBP)S2 = P + s2S3 = P + s3So, in theory, the formation pressure is limited by the overburden!

S3 S1 S2 #29Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Overburden Stress FormulaMetric units: H*rb 10Where: Ph = hydrostatic pressure (kg/cm2) rb = Average formation bulk density (no unit) H = Vertical thickness of overlying sediments (m)OVBP =API units:OVBP = H*rb*0.433 Where: Ph = hydrostatic pressure (psi) rb = Average formation bulk density (no unit) H = Vertical thickness of overlying sediments (ft)#30Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Bulk Density FormulaThe bulk density of a sediment is a function of the matrix density, the porosity and the density of the fluid in the pores. rb = (f * rf) + (1-f) * rm Where:rb = Bulk density (no unit) rf = Formation fluid density (no unit) f = Porosity (from 0 to 1) rm = Matrix density (no unit)#31Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Overburden GradientSediments laid down in the basin are buried deeper and deeper with the continuation of the sedimentary process.BURIAL INREASES THE OVERBURDENINCREASE OF OVERBURDEN LEADS TO COMPACTIONCOMPACTION DECREASES POROSITYSandstoneClay / ShalePorosityDepthClay / Shale behaviourPorosity decrease is rapid in the upper part of the curve - shallow depth and unconsolidated section.The curve levels off in compacted clay whichprogressively changes to claystone and shale.

SandstoneFor the sandstone compaction is due to realignment of grains and effects of diagenesis.#32Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Overburden GradientOverburden gradient is calculated by averaging density from surface to the depth of interest.

Knowledge of overburden gradient is necessary: For evaluation of formation pressure For calculation of fracture gradient

Bulk density increases with depth and also varies depending on fluid and lithology. Hence averaging is necessary.

(Metric)(API)OVBGOVBG#33Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Overburden GradientConsider 3 layers of different densities:

Interval Thickness app. density interval ovb M M rb ovb pressure gradient

0 - 100 100 2.1 21 2.1100 - 200 100 2.3 23 2.2200 - 400 200 2.5 50 2.35 Gr = Press x 10 / Int

1) 100 x 2.1 / 10 = 21 kg / cm2(21) x 10 / 100 = 2.12) 100 x 2.3 / 10 = 23 kg / cm2(21+23) x 10 / 200 = 2.23) 200 x 2.5 / 10 = 50 kg / cm2(21+23+50) x 10 / 400 = 2.35#34Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Overburden GradientExample overburden density curves(Mouchet and Mitchell, 1989)

Some literature quotes average bulk density as 2.31 kg/l, or 1 psi/ftThis is an approximate value only. For accurate interpretation it is necessary to calculate actual values.




Overburden GradientNote the difference: Onshore overburdenexceeds 2.31.

Off-shore overburdenremains below 2.31. Onshore curve Offshore curve Density 2.31 Density 2.31Sea BedDepthDepthOffshore wells have a water column that affects bulk density. This effect can be very important in moderate- to deep-water areas#36Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Estimating Fracture Gradient It is essential to increase mud weight during drilling of an abnormally pressured section. There is a limit to which MW can be increased. Shallower parts of the well are weaker than deeper, more compacted strata. Weakest point in the well is just below casing. Increase in MW beyond limit may open fractures, possibly leading to mud loss and kick. Mud loss may also occur at the limit of the pressure in porous rocks such as SST and LST. MW is also limited by the casing and BOP.Why do we need to know the Fracture Gradient?#37Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Fracture GradientFracture gradient is in part controlled by the overburden gradient. In general, the lower the overburden gradient, the lower the fracture gradient. In deepwater drilling, the range of usable mud densities may be very narrow due to the low fracture gradients involved.The Waterdepther spreadsheet shows this relationship graphically. Note how increased water depth and/or formation pressure tends to increase the number of casing strings required.




Fracture GradientIn exploratory drilling, the limit or fracture gradient is determined by Leak-off Test (LOT) or Formation Integrity test (FIT).LOT/FIT results also help define:1. Adjustments to Casing Program and MW2. Maximum pressure permissible during kick control to avoid internal blow out.3. Hydraulic fracture pressure required for stimulation.A LOT tests the formation to the fracture-opening pressure, while a FIT merely tests to a pre-determined value assumed to be slightly less than the maximum pressure allowable. FIT is preferred when formations are already known to be weak.




Leak-off TestPump

in a well with closed BOPs

until the pressure in the well reaches fracturation pressure of the formation.It is usually performed after drilling a few meters below the last intermediate casing shoe.Normally, the cement pump is used, to better control the volume and pressure pumped.




Leak-off Test A : Start PumpingAB : Elastic behaviour of formation.B : Leak off - fracture BC : Mud penetrates formationC : Pumping Stopped.CD : Fracture propagation ceases Pressure falls to a stabilised pressure (D) which is less than or equal to BE : Bleed-off. Mud recovered should be equal to the volume pumped. If less, the fractures remained open; pressure at D in this case will also be less than pressure at B.LOT pressureBCDPumping Volume l Time AEPressure#41Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Formation Integrity TestVolumePressureTimeFITPressureBleed offPump off#42Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Calculating Maximum Allowable Mud DensityPFRAC = PLOT + Mud Hydrostatic Pressure in the well First calculate bottom hole pressure:Then determine EMW:Metric:EMW = (Ph kg/cm2 X 10) / Depth in mAPI:EMW = (Ph psi / Depth in ft) / .052#43Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



Exercise 6 -- Fracture Gradient Calculate Fracture - LOT- EMW from the data given below.Drilled Depth: 1912 m. Casing shoe at 1900 m.MW: 1.2 SGLOT press.: 56 kg/cm2 Hydrostatic Pressure =LOT pressure = Total leak off pressure =Frac. EMW =1900 X 1.2 / 10 = 228 kg/ cm256 kg/cm2284 kg/cm2284 X 10/1900 = 1.49#44Geoservices is one of the few companies to have its Service Certified, most companies only have the manufacturing, design etc certified



pressure concepts overburden pore fracture

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