TAMU - PemexWell ControlLesson 7Pore Pressure Prediction

*Contents Porosity Shale Compaction Equivalent Depth Method Ratio Method Drilling Rate dC-Exponent Moores Technique Combs Method

*Pore pressure prediction methodsMost pore pressure prediction techniques rely on measured or inferred porosity.

The shale compaction theory is the basis for these predictions.

*Pore pressure prediction methodsMeasure the porosity indicator (e.g. density) in normally pressured, clean shales to establish a normal trend line.When the indicator suggests porosity values that are higher than the trend, then abnormal pressures are suspected to be present.The magnitude of the deviation from the normal trend line is used to quantify the abnormal pressure.

*2. Extrapolate normal trend line1. Establish Normal Trend Line in good clean shaleTransitionPorosity should decrease with depth in normally pressured shales3. Determine the magnitude of the deviation

*Older shales have had more time to compact, so porosities would tend to be lower (at a particular depth).

Use the trend line closest to the transition.

Lines may or may not be parallel.

*DDeEquivalent Depth MethodThe normally compacted shale at depth De has the same compaction as the abnormally pressured shale at D. Thus,sV = sVei.e., sob - pp = sobe - pnepp = pne + (sob - sobe)

sob = sV + pp

*Example 2.6Estimate the pore pressure at 10,200 if the equivalent depth is 9,100. The normal pore pressure gradient is 0.433 psi/ft. The overburden gradient is 1.0 psi/ft.At 9,100, pne = 0.433 * 9,100 = 3,940 psigAt 9,100, sobe = 1.00 * 9,100 = 9,100 psigAt 10,200, sob = 1.00*10,200 = 10,200 psig

*Solution pp = pne + (sob - sobe) . (2.13) = 3,940 + (10,200 9,100) pp = 5,040 psig

The pressure gradient, gp = 5,040/10,200 = 0.494 psi/ft EMW = 0.494/0.052 = 9.5 ppg

*XnXoThe Ratio Methoduses (Xo/Xn) to predict the magnitude of the abnormal pressureWe can use: drilling rate resistivities conductivities sonic speedsShale Porosity IndicatorDepth

*Pore pressures can be predicted: Before drilling (planning) During drilling. After drilling

*Before drilling the well (planning) Information from nearby wells Analogy to known characteristics of the geologic basin Seismic data

*

*Table 2.6 Contd

*Seismic Surveys, as used in conventional geophysical prospecting, can yield much information about underground structures, and depths to those structures. Faults, diapirs, etc. may indicate possible locations of abnormal pressures

*Typical Seismic Section

*Under normal compaction, density increases with depth. For this reason the interval velocity also increases with depth, so travel time decreasesDt = Dtma(1-f) + Dtf f

*Sound moves faster in more dense mediumIn air at sea level, Vsound = 1,100 ft/secIn distilled water, Vsound = 4,600 ft/secIn low density, high porosity rocks, Vsound = 6,000 ft/secIn dense dolomites, Vsound = 20,000 ft/sec

*Example 2.7Use the data in Table 2.7 to determine the top of the transition zone, and estimate the pore pressure at 19,000using the equivalent depth methodusing Pennebakers empirical correlationIgnore the data between 9,000 and 11,000. Assume Eatons Gulf Coast overburden gradient.

*SolutionPlot interval travel time vs. depth on semilog paper (Fig. 2.31)Plot normal trend line using the 6,000-9,000 data.From Fig. 2.20, at 19,000, gob = 0.995 psi/ft(sob)19,000 = 0.995 * 19,000 = 18,905 psig

*UseIgnoreEquivalent Depth Method:From the vertical line, De = 2,000sobe = 0.875 * 2,000 =1,750 (Fig. 2.20)But,pne = 0.465 * 2,000 = 930 psigpp = 930 + (18,905-1,750) pp = 18,085 psigDtnDto

*Pennebakers correlation for Gulf Coast sedimentsHigher travel time means more porosity and higher pore pressure gradient

Example 2.7 (Table 2.7)

Dto = 95 msec/ft @ 19,000Dtn = 65 msec/ft @ 19,000Dto/ Dtn = 95/65 = 1.46pp = 0.95 * 19,000= 18,050 psig0.95Fig. 2.30

*Comparison Pore Pressure at a depth of 19,000 ft: Pennebaker: 18,050 psi or 0.950 psi/ft or 18.3 ppg Equivalent Depth Method: 18,085 psi or 0.952 psi/ft or 18.3 ppg

*While Drilling dc-exponent MWD & LWD Kicks Other drilling rate factors (Table 2.5)

*TABLE 2.5 -

*Penetration rate and abnormal pressureBits drill through overpressured rock faster than through normally pressured rock (if everything else remains the same).When drilling in clean shales this fact can be utilized to detect the presence of abnormal pressure, and even to estimate the magnitude of the overpressure.

*Note, that many factors can influence the drilling rate, and some of these factors are outside the control of the operator.TABLE 2.8 -

*Effect of bit weight and hydraulics on penetration rateInadequate hydraulics or excessive imbedding of the bit teeth in the rockDrilling rate increases more or less linearly with increasing bit weight.A significant deviation from this trend may be caused by poor bottom hole cleaning0

*Effect of Differential Pressure on Drilling RateDifferential pressure is the difference between wellbore pressure and pore fluid pressure

Decrease can be due to: The chip hold down effect The effect of wellbore pressure on rock strength

*Drilling underbalanced can further increase the drilling rate.

*The chip hold-down effectThe mud pressure acting on the bottom of the hole tends to hold the rock chips in placeImportant hold-down parameters:OverbalanceDrilling fluid filtration ratePermeabilityMethod of breaking rock (shear or crushing)

* Drilling rates are influenced by rock strengths. Only drilling rates in relatively clean shales are useful for predicting abnormal pore pressures.TABLE 2.9 -

*sob is generally the maximum in situ principal stress in undisturbed rock

*Stresses on Subsurface Rockssob, sH1, sH2 and p all tend to increase with depth

sob is in general the maximum in situ principal stress.

Since the confining stresses sH1 and sH2 increase with depth, rock strength increases.

*Stresses on Subsurface RocksThe pore pressure, p, cannot produce shear in the rock, and cannot deform the rock.Mohr-Coulomb behavior is controlled by the the effective stresses (matrix).When drilling occurs the stresses change.sob is replaced by dynamic drilling fluid pressure.

*The degree of overbalance now controls the strength of the rock ahead of the bit.

*Rock failure caused by roller cone bit.The differential pressure from above provides the normal stress, soFormation fracture is resisted by the shear stress, to, which is a function of the rock cohesion and the friction between the plates. This friction depends on so.

*Fig. 2.41 - Differential Pressure 0.1 in below the bit.When sob is replaced by phyd (lower) the rock immediately below the bit will undergo an increase in pore volume, associated with a reduction in pore pressure.In sandstone this pressure is increased by fluid loss from the mud.(Induced Differential Pressure in Impermeable rock. FEM Study)Vertical Stress = 10,000 psiHorizontal Stress = 7,000 psiPore Pressure = 4,700 psiWellbore Pressure = 4,700 psi

*Drilling Rate as a Pore Pressure PredictorPenetration rate depends on a number of different parameters.R = K(P1)a1 (P2)a2 (P3)a3 (Pn)anA modified version of this equation is:

*Drilling Rate as a Pore Pressure PredictorOr, in its most used form:

*d-exponentThe d-exponent normalizes R for any variations in W, db and NUnder normal compaction, R should decrease with depth. This would cause d to increase with depth.Any deviation from the trend could be caused by abnormal pressure.

*d-exponentMud weight also affects R..An adjustment to d may be made:dc = d (rn /rc)wheredc = exponent corrected for mud densityrn = normal pore pressure gradientrc = effective mud density in use

*Example While drilling in a Gulf Coast shale,R = 50 ft/hrW = 20,000 lbfN = 100 RPMECD = 10.1 ppg (Equivalent Circulating Density) db = 8.5 in Calculate d and dc

*Solution

*Example 2.9 Predict pore pressure at 6,050 ft (ppg): from data in Table 2.10 using: Rhem and McClendons correlation Zamoras correlation The equivalent depth method

*TABLE 2.10

d-EXPONENT AND MUD DENSITY DATA FOR A WELL LOCATED OFFSHORE LOUISIANA

*Step 1 is to plot the data on Cartesian paper (Fig. 2.43).Transition at 4,700 ft?or is it a fault?Seismic data and geological indicators suggest a possible transition at 5,700 ft.

*Fig. 2.43Slope of 0.000038 ft-1

*Rehm and McClendon gp = 0.398 log (dcn-dco) + 0.86= 0.398 log (1.18 - 0.95) + 0.86 gp = 0.606 psi/ft

rp = 0.606 / 0.052 = 11.7 ppg

*ZamoraFrom Fig. 2.44 gp = gn (dcn/dco)= 0.465 * (1.18/.95) gp = 0.578 psi/ft

rp = 0.578/0.052 rp = 11.1 ppg1.180.95

*Equivalent Depth MethodFrom Fig. 2.20, at 6,050 ft, gob = 0.915 psi/ftsob = 0.915 * 6,050 = 5,536 psi

*Equivalent Depth Method From Fig. 2.43, Equivalent Depth = 750 ft At 750 ft,sobe = 0.86 * 750 = 645 psipne = 0.465 * 750 = 349 psig

*Equivalent Depth Method From Eq. 2.13, at 6,050 ft pp = pne + (sob - sobe) pp = 349 + (5,536 - 645) = 5,240 psig rp = 19.25 * (5,240 / 6,050) = 16.7 ppg

Perhaps the equivalent depth method is not always suitable for pp prediction using dc !!

*Overlays such as this can be handy, butbe careful that the scale is correct for the graph paper being used;the slope is correct for normal trends;the correct overlay for the formation is utilized.

*To improve pore pressure predictions using variations in drilling rate:Try to keep bit weight and rpm relatively constant when making measurementsUse downhole (MWD) bit weights when these are available. (Frictional drag in directional wells can cause large errors)Add geological interpretation when possible. MWD can help here also.

*Improved pore pressure predictionsKeep in mind that tooth wear can greatly influence penetration rates.

Use common sense and engineering judgment.

Use several techniques and compare results.

*Moores TechniqueFig. 2.45Moore proposed a practical method for maintaining a pore-pressure overbalance while drilling into a transition.Drilling parameters must be kept constant for this technique to work.

*Combs Method Combs attempted to improve on the use of drilling rate for pore pressure by correcting for: hydraulics differential pressure bit wear in addition to W, db, and N

*Combs Methodq = circulating ratedn = diameter of one bit nozzlef(pd) = function related to the differential pressuref(tN) = function related to bit wearaW = bit weight exponent = 1.0 for offshore LouisianaaN = rotating speed exponent = 0.6 for offshore Louisianaaq = flow rate exponent = 0.3 for offshore Louisiana

*Tooth wear factorCorrection would depend upon bit type, rock hardness, and abrasiveness

*Differential pressure factorMethod is too complicated and too site specific.