TAMU - PemexWell Control Lesson 7 Pore Pressure Prediction Contents Porosity Shale Compaction Equivalent Depth Method Ratio Method Drilling Rate dC-Exponent Moore’s Technique Comb’s Method 2 Pore pressure prediction methods Most pore pressure prediction techniques rely on measured or inferred porosity. The shale compaction theory is the basis for these predictions. 3 then abnormal pressures are suspected to be present. 4 .Pore pressure prediction methods Measure the porosity indicator (e. density) in normally pressured. When the indicator suggests porosity values that are higher than the trend. The magnitude of the deviation from the normal trend line is used to quantify the abnormal pressure.g. clean shales to establish a normal trend line. Extrapolate normal trend line 3.Porosity should decrease with depth in normally pressured shales 1. Establish “Normal” Trend Line in good “clean” shale Transitio n 2. Determine the magnitude of the deviation 5 . so porosities would tend to be lower (at a particular depth).Older shales have had more time to compact. Use the trend line closest to the transition. 6 . Lines may or may not be parallel. Equivalent Depth Method De The normally compacted shale at depth De has the same compaction as the abnormally pressured shale at D. V = Ve i..e.pp = obe .obe) D ob = V + pp 7 . Thus. ob .pne pp = pne + (ob . At 9.200’.6 Estimate the pore pressure at 10.433 psi/ft. obe = 1.100 = 9.100 psig At 10.100’. The normal pore pressure gradient is 0.100 = 3.200 = 10.100’. ob = 1.433 * 9. pne = 0.0 psi/ft. The overburden gradient is 1.200’ if the equivalent depth is 9.200 psig 8 .Example 2.100’.00 * 9.940 psig At 9.00*10. 040 psig The pressure gradient.100) pp = 5.940 + (10.200 – 9. gp = 5. (2.13) = 3.obe) …………….5 ppg 9 .052 = 9.200 = 0.494/0.Solution pp = pne + (ob .494 psi/ft EMW = 0.040/10. The Ratio Method uses (Xo/Xn) to predict the magnitude of the abnormal pressure We can use: Depth • drilling rate • resistivities • conductivities Xn Xo • sonic speeds 10 Shale Porosity Indicator . After drilling 11 .Pore pressures can be predicted: Before drilling (planning) During drilling. Before drilling the well (planning) Information from nearby wells Analogy to known characteristics of the geologic basin Seismic data 12 . 13 . 6 – Cont’d 14 .Table 2. Faults. as used in conventional geophysical prospecting. diapirs. may indicate possible locations of abnormal pressures 15 . can yield much information about underground structures. etc.Seismic Surveys. and depths to those structures. Typical Seismic Section 16 . For this reason the interval velocity also increases with depth. so travel time decreases t = tma(1-) + tf 17 .Under normal compaction. density increases with depth. 600 ft/sec In low density.100 ft/sec In distilled water.Sound moves faster in more dense medium In air at sea level.000 ft/sec In dense dolomites. Vsound = 6.000 ft/sec 18 . high porosity rocks. Vsound = 4. Vsound = 20. Vsound = 1. 000’ and 11.Example 2. Assume Eaton’s Gulf Coast overburden gradient. and estimate the pore pressure at 19.000’.7 to determine the top of the transition zone. 19 .000’ using the equivalent depth method using Pennebaker’s empirical correlation Ignore the data between 9.7 Use the data in Table 2. at 19. gob = 0.995 psi/ft (ob)19. 6.000- From Fig.905 psig 20 . depth on semilog paper (Fig.000 = 0.000 data.000 = 18.995 * 19.20. 2.31) Plot normal trend line using the 9.000’. 2.Solution Plot interval travel time vs. 905-1.000 =1.465 * 2. De = 2.085 psig 21 .750 (Fig.000 = 930 psig tn to pp = 930 + (18.Equivalent Depth Method: Use Ignore From the vertical line. pne = 0. 2.20) But.875 * 2.000’ obe = 0.750) pp = 18. 46 0.000’ tn = 65 sec/ft @ 19.050 psig 22 .95 * 19.Fig.000 = 18.000’ to/ tn = 95/65 = 1. 2.7 (Table 2.95 pp = 0.7) to = 95 sec/ft @ 19.30 Pennebaker’s correlation for Gulf Coast sediments Higher travel time means more porosity and higher pore pressure gradient Example 2. 952 psi/ft or 18.000 ft: Pennebaker: 18.085 psi or 0.Comparison Pore Pressure at a depth of 19.950 psi/ft or 18.3 ppg 23 .050 psi or 0.3 ppg Equivalent Depth Method: 18. 5) 24 .While Drilling dc-exponent MWD & LWD Kicks Other drilling rate factors (Table 2. 5 - 25 .TABLE 2. and even to estimate the magnitude of the overpressure. When drilling in clean shales this fact can be utilized to detect the presence of abnormal pressure. 26 .Penetration rate and abnormal pressure Bits drill through overpressured rock faster than through normally pressured rock (if everything else remains the same). 8 - Note. and some of these factors are outside the control of the operator.TABLE 2. 27 . that many factors can influence the drilling rate. A significant deviation from this trend may be caused by poor bottom hole cleaning 0 28 .Effect of bit weight and hydraulics on penetration rate Inadequate hydraulics or excessive imbedding of the bit teeth in the rock Drilling rate increases more or less linearly with increasing bit weight. Effect of Differential Pressure on Drilling Rate Decrease can be due to: • The chip hold down effect Differential pressure is the difference between wellbore pressure and pore fluid pressure • The effect of wellbore pressure on rock strength 29 . 30 .Drilling underbalanced can further increase the drilling rate. The chip hold-down effect The mud pressure acting on the bottom of the hole tends to hold the rock chips in place Important hold-down parameters: Overbalance Drilling fluid filtration rate Permeability Method of breaking rock (shear or crushing) 31 . 32 .TABLE 2.9 - • Drilling rates are influenced by rock strengths. • Only drilling rates in relatively clean shales are useful for predicting abnormal pore pressures. ob is generally the maximum in situ principal stress in undisturbed rock 33 . Since the confining stresses H1 and H2 increase with depth. H1. H2 and p all tend to increase with depth ob is in general the maximum in situ principal stress. rock strength increases.Stresses on Subsurface Rocks ob. 34 . Mohr-Coulomb behavior is controlled by the the effective stresses (matrix). ob is replaced by dynamic drilling fluid pressure. 35 . cannot produce shear in the rock. and cannot deform the rock.Stresses on Subsurface Rocks The pore pressure. When drilling occurs the stresses change. p. 36 .The degree of overbalance now controls the strength of the rock ahead of the bit. 37 . o Formation fracture is resisted by the shear stress. o. This friction depends on o. The differential pressure from above provides the normal stress. which is a function of the rock cohesion and the friction between the plates.Rock failure caused by roller cone bit. Fig.000 psi = 4. Vertical Stress Horizontal Stress Pore Pressure Wellbore Pressure = 10. FEM Study) When ob is replaced by phyd (lower) the rock immediately below the bit will undergo an increase in pore volume. In sandstone this pressure is increased by fluid loss from the mud.700 psi (Induced Differential Pressure in Impermeable rock.41 . associated with a reduction in pore pressure.1 in below the bit.700 psi = 4.Differential Pressure 0.000 psi = 7. 2. 38 . Drilling Rate as a Pore Pressure Predictor Penetration rate depends on a number of different parameters. R = K(P1)a1 (P2)a2 (P3)a3… (Pn)an A modified version of this equation is: W R K 3 N db d 39 Drilling Rate as a Pore Pressure Predictor Or, in its most used form: d R log 60 N 12W log 6 10 d b W R K 3 N db R ft/hr N rpm d d exponent W Bit Weight , lbf d b Bit Diameter, in 40 d d-exponent The d-exponent normalizes R for any variations in W, db and N Under 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. 41 An adjustment to d may be made: dc = d (n /c) where dc = exponent corrected for mud density n = normal pore pressure gradient c = effective mud density in use 42 ..d-exponent Mud weight also affects R…. 1 ppg (Equivalent Circulating Density) db = 8.Example While drilling in a Gulf Coast shale.5 in Calculate d and dc 43 . R = 50 ft/hr W = 20.000 lbf N = 100 RPM ECD = 10. 465 dc 1.Solution 50 log 2.000 log 6 10 * 8.1 dc 1.34 0.19 log R 60N d 12 W log 10 6 d b n d c d c 44 .052 * 10.554 12 * 20.34 0.5 d 1.079 60 * 100 d 1. 9 Predict pore pressure at 6.Example 2.050 ft (ppg): from data in Table 2.10 using: Rhem and McClendon’s correlation Zamora’s correlation The equivalent depth method 45 . TABLE 2.10 d-EXPONENT AND MUD DENSITY DATA FOR A WELL LOCATED OFFSHORE LOUISIANA 46 . 47 .Step 1 is to plot the data on Cartesian paper (Fig.700 ft. 2.700 ft? …or is it a fault? Seismic data and geological indicators suggest a possible transition at 5. Transition at 4.43). Fig. 2.000038 ft-1 48 .43 Slope of 0. 606 / 0.7 ppg 49 .86 = 0.95) + 0.606 psi/ft p = 0.18 .052 = 11.398 log (dcn-dco) + 0.86 gp = 0.Rehm and McClendon gp = 0.398 log (1.0. Zamora From Fig. 2.95 1.052 p = 11.578 psi/ft 0.465 * (1.95) gp = 0.1 ppg 50 .578/0.18 p = 0.18/.44 gp = gn (dcn/dco) = 0. 20. at 6.915 psi/ft ob = 0.050 = 5.Equivalent Depth Method From Fig.536 psi 51 .915 * 6.050 ft. gob = 0. 2. 2.86 * 750 = 645 psi obe pne = 0. = 0.Equivalent Depth Method From Fig.43. Equivalent Depth = 750 ft At 750 ft.465 * 750 = 349 psig 52 . 645) = 5.7 ppg Perhaps the equivalent depth method is not always suitable for pp prediction using dc !! 53 .Equivalent Depth Method From Eq.050 ft pp = pne + (ob . 2.050) = 16.240 psig p = 19.240 / 6.25 * (5.13.obe) pp = 349 + (5. at 6.536 . Overlays such as this can be handy. 54 . the slope is correct for normal trends. the correct overlay for the formation is utilized. but be careful that the scale is correct for the graph paper being used. 55 . (Frictional drag in directional wells can cause large errors) Add geological interpretation when possible.To improve pore pressure predictions using variations in drilling rate: Try to keep bit weight and rpm relatively constant when making measurements Use downhole (MWD) bit weights when these are available. MWD can help here also. Use common sense and engineering judgment.Improved pore pressure predictions Keep in mind that tooth wear can greatly influence penetration rates. 56 . Use several techniques and compare results. 45 Moore proposed a practical method for maintaining a pore-pressure overbalance while drilling into a transition. 2. 57 . Drilling parameters must be kept constant for this technique to work.Moore’s Technique Fig. and N 58 .Comb’s 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. 6 for offshore Louisiana aq = flow rate exponent = 0.500 db aW N 200 aN q 96 db dn aq f pd f tN q = circulating rate dn = diameter of one bit nozzle f(pd) = function related to the differential pressure f(tN) = function related to bit wear aW = bit weight exponent = 1.3 for offshore Louisiana 59 .Comb’s Method W R R d 3.0 for offshore Louisiana aN = rotating speed exponent = 0. rock hardness.Tooth wear factor Correction would depend upon bit type. and abrasiveness 60 . Differential pressure factor Method is too complicated and too site specific. 61 .