Pore Pressure Estimation From Velocity Data: Accounting for Overpressure Mechanisms Besides Undercompaction Glenn L Bowers,” Exxon Production Research Co Summary A new method for estimating pore pressure from formation sonic velocity data is presented Unlike previous techniques, this method accounts for excess pressure generated by both undercompaction, and fluid expansion mechanisms such as aquathermal pressuring, hydrocarbon maturation, clay diagenesis, and charging from other zones The method is an effective stress approach; the effective stress is computed from the velocity, and the result is subtracted from the overburden stress to obtain pore pressure To include multiple sources of overpressure, a pair of velocity-vs.-effective-stress relations are introduced One relation accounts for normal pressure and overpressure caused by undercompaction The second is applied inside velocity reversal zones caused by fluid expansion mechanisms Example applications of the method are presented from the U.S gulf coast, the Gulf of Mexico, and the Central North Sea Some other pore pressure estimation approaches are also examined to demonstrate how these techniques have unknowingly accounted for overpressure mechanisms other than undercompaction It is also explained how velocity-vs.-effective-stress data can be used to identify the general cause of overpressure in an area For instance, the empirical correlation of Hottman and Johnson indicates that overpressure along the U.S gulf coast cannot be due only to undercompaction Introduction Numerous methods have been developed for estimating pore fluid pressure from geophysical data, and the list continues to grow Empirical approaches equate departures from the normal trend line of some porosity-dependent measurement to an equivalent pore pressure gradient Recent methods have followed the more fundamental effective stress approach pioneered by Foster and Whalen,! Ham,’ and Eaton.3 All current pore pressure estimation methods fail to formally take into account the cause of overpressure It will be demonstrated that this can lead to significant errors For a given velocity at a given depth, the pore pressure can vary by !bm/gal or more, depending upon how the excess pressure was generated This paper presents a method for estimating pore pressure from sonic velocity data that systematically accounts for the cause of pressure When applied to wireline sonic logs, it is preferable to only use shale data to minimize the effects of lithology changes However, the method is also applicable for pre-drill predictions from seismic interval velocities The paper begins with a review of fundamental aspects of shale compaction behavior that form the foundation of this method This is followed by a discussion of how different causes of overpressure affect the sonic velocity Some current pore pressure estimation methods are then examined in light of these concepts The new method is then described, and example applications are presented and discussed Compaction Behavior Non-Decreasing Effective Stress States, Under increasing effec- tive pressure, sediments compact, and their sonic velocity goes up At the limit, their porosity approaches zero, and their sonic velocity approaches the value for the sediment grains Borrowing terminolo*Now with Applied Mechanics Technologies Copyright 1995 Society of Petroleum Engineers Original SPE manuscript received for review March 10, 1994 Revised manuscript received Nov 30, 1994 Paper accepted for publication Feb 27, 1995 Paper (SPE 27488) first presented at the 1994 IADC/SPE Drilling Conference heid in Dallas Feb 15-18 SPE Drilling & Completion, June 1995 gy from soil mechanics, the velocity-effective stress relation for non-decreasing effective stress states will be referred to as the virgin curve Fig 1a plots shale virgin curve data derived from well log and RFT measurements from the Gulf of Mexico slope, and shows an estimate of the complete virgin curve Effective Stress Reductions Much (but not all) of the porosity loss/ velocity gain that occurs during compaction is permanent As a result, the sonic velocity will not go down the virgin curve when the effective stress is reduced (unloading) The velocity will track a different, faster velocity-vs.-effective-stress relation that will be called the unloading curve If the effective stress is subsequently increased, the velocity will follow the unloading curve back to the virgin curve Fig 1b illustrates unloading behavior with laboratory velocityeffective stress data for Cotton Valley shale (Tosaya*), The veloci- ties measured at effective stresses below the maximum in-situ stress state must be on an unloading curve For-cémparison, the virgin curve for the Gulf of Mexico sediments is replotted in Fig Ib Overpressure Causes/Effects Normal Pressure During burial under normal pressure conditions, the effective stress continually increases with depth Consequently, normal trend velocity-vs.-effective-stress data follow a virgin curve Undercompaction Overpressure most commonly occurs when pore fluid trapped by low permeability is squeezed by the weight of newly deposited sediments This overpressuring process is referred to as undercompaction or compaction disequilibrium Undercompaction cannot cause the effective stress to decrease Therefore, the virgin curve also applies for formations overpressured by undercompaction The most undercompaction can is “freeze” the effective stress in time, which would cause the velocity to become fixed on the virgin curve On a velocity/depth plot, this would appear as a velocity plateau Fig illustrates undercompaction overpressure in the Gulf of Mexico Pore pressure, sonic velocity, and stress information are shown in Figs 2a, 2b, and 2c, respectively Fig 2d plots velocityvs.-effective- stress data determined at RFT locations It can be seen that all of data in Fig 2d appear to lie on a virgin curve, and that the points below 7200 ft are approaching a fixed point Fluid Expansion Overpressure can also be generated by fluid ex- pansion mechanisms such as heating,>-â hydrocarbon maturation, charging from other zones,đ and expulsion/expansion of intergranular water during clay diagenesis.! Here, excess pressure results from the rock matrix constraining the pore fluid as the fluid tries to increase in volume Unlike undercompaction, fluid expansion can cause the pore pressure to increase at a faster rate than the overburden stress This forces the effective stress to decrease as burial continues, which produces a velocity reversal (see Fig 3b) Velocities inside the reversal will track an unloading curve(s), while velocities outside the reversal will remain on a virgin curve Fig illustrates this with data from an Indonesian well Pore pressure, velocity, and stress data are displayed in Fig 3a, 3b, and 3c, respectively, while Fig 3d compares velocity-vs-effectivestress data from inside (open circles) and outside (solid circles) the velocity reversal The start of the reversal coincides with the top of overpressure at approximately 6350 ft It can be seen in Fig 3d that the velocities from inside the reversal track a much faster trend This suggests that fluid expansion mecha89 17 F Unloading Curve 15Ƒ \ 157 = = ze - ity Estimated eL 92 13 fF 137 s eu 8 ° L Virgin Curve Virgin Curve tor Gulf of Mexico Data - â Tosaya đ Gulfí of Mexico In Situ Data L L al 10 i L 12 (1982) Cotton Valley Shale Lab Data L 14 16 18 L i 1 i 10 12 14 16 18 -_Effective Stress (ksi) Effective Stress (ksi) b) a) Fig 1—Shale compaction behavior: (a) virgin curve and (b) unloading curve otra 10 tT Pore Pressure (ppg) 12 14 16 18 T T T a T 20 § 75 T Velocity (ktt/s) 10 125 T 15 T T Stress (ksi) T T T 10 12 @ RFT's Effective Stress (ksi) T T T 11 er Estimated Pore Pressure F Depth (Wdt) 4F F 10 | Virgin Curve _ = Mudweight Used et = = St 8£ 8773' - 8870' > | 7216' -7509' aL r BF " 3058" â 10 Đ a) b) c) d) Fig 2^—Undercompaction overpressure—Gulf of Mexico B 10 Pore Pressure (ppg) 12 14 1E 16 18 FT đ RFTs' 20 Đ Velocity (ktt/s) 11 13 T—T—T—T—T 15 r 17 09 r TT Stress (ksi) TT TT 10 Overburden NM Blrone a 11 Stress Depth (kt) L \ Velocity Reversal | 9r r |a0ag;.“ ` _v\ Pore Pressure ° =9 $ \ F 6543' - 6649), 0-— — §~ | \ T Vmax 10 fo -” 6950' Vmax - Curve Effective \ 12 | Unloading L be Etfective Stress (ksi) 13 [TT ® 4780 990° 57a9' » 3649 > ` 7T ` - ag Virgn Cure đ Outside Reversal â Inside Reversal 10 a) b) c) d) Fig 3—-Fluid expansion overpressure offshore indonesia 90 SPE Drilling & Completion, June 1995 12 15 11 ° a s9 s11 > = i, > > œ F, e ° Pore Pressure kh 1< L i 4L i Effective Stress (ksi) L j i Effective Stress (ksi) a) b) Fig 3d also demonstrates the importance of accounting for the cause of overpressure when estimating pore pressures The virgin curve would overestimate the effective stress at 6950 ft by 1700 psi, which means the pore pressure would be underestimared by the same amount This would correspond to a 4.7 lbm/gal! error in the equivalent mudweight prediction Fig present further evidence of unloading Fig 4a shows velocity-effective stress data derived from RFT and well log measurements along the Gulf of Mexico slope The data are divided into two groups according to the magnitude of overpressure Solid circles are in the to 15 Ibm/gal equivalent mudweight range, while the open circles are in “hard” overpressure Contrary to what might be expected, the higher pressure data tend to have faster velocities As Fig 4b indicates, a possible explanation is that most of these data are from formations that have undergone some unloading Similar trends are evident in the Central North Sea velocity-vs.-effectivestress data plotted in Fig § Others have also reported evidence of unloading caused by high pressure Magara!! found that the Equivalent Depth method underpredicted the pore pressure data of Hottman and Johnson.!2 The equivalent depth method? equates the effective stress in an overpressured zone to that in a normally pressure interval with the same velocity (see Fig 6) This assumes that the overpressure data are on a virgin curve If fluid expansion has driven the data onto an unloading curve, as in Fig 3, the effective stress will be overestimated, and the pore pressure will be underestimated, as Magara observed ` | ‘ *Equvialent Depth’ i ` I \ | + | 10 ‘ 7.5 Trend mm 10 Velocity (ktV/s) Fig 6—Case Mexico \ Determining the Cause of Overpressure In Velocity Reversals A reliable way has not been found to determine from velocity data alone whether a reversal was caused by undercompaction or fluid expansion However, some general guidelines can be offered on the geologic conditions that are conducive to each of these causes of pressure (Miller and Luk®) The amount of overpressure generated by undercompaction depends upon the relative compressibility of the rock matrix and the pore fluid They act like two springs in parallel If the rock matrix is much more compressible, increases in overburden stress will be carried primarily by the pore fluid If the rock matrix is much less compressible, then it will bear most of the overburden Therefore, undercompaction will typically generate the greatest overpressure at shallower depths, where formations are soft A ` ê Stress (ksi) Velocity Reversal Without Unloading Not all velocity reversals are caused by fluid expansion mechanisms The velocity will also drop across the transition from a normally pressured sand/shale sequence to a massive undercompacted shale If fluid expansion is not the cause, the velocity-vs.-effective-stress data from the reversal will follow a virgin curve Overburden Svess F \ Effective Stress Normal Pore Pressure = € Estimated Pore Pressure st { + \ ` Pore Pressure at 6950 tt Actual (RFT) = 16.2 ppg Estimated = 11.7 ppg 10 where equivalent depth method works—Gulf of SPE Drilling & Completion, June 1995 L_ lar conclusion using density log data from a south Texas well Estimated = 15.7 ppg L [ Estimated Pore Pressure pe 12.6 had undergone unloading Berg and Habeck!4 came up with a simi- _ Actual (RFT) = 15.9 ppg Normal Log L Pore Pressure at 7200 ft \ Sonic SF | § Effective Stress (ksi) r \ I' \ \ ' 6} 1 \ overburden Stess ' F © 14-18 ppg ` & “Ƒ & @ 8-13 ppg Curve sured interval having the same effective stresses The porosity in the overpressured zone was half the value in normal pressure (17.6% vs 38%) Piumley concluded this was because the high pressure zone al Pore Pressure _ Virgin Plumley!? discussed unloading caused by fluid expansion overpressure, and presented a U.S gulf coast example of its occurrence He compared porosities from an overpressured and normally pres- Effective Stress \ = Vmax for Well #1 Fig 5—Fluid expansion overpressure, central North Sea nisms contributed to the overpressure inside the reversal Note the similarity between the unloading curve in Fig 3d and the trend followed by the Cotton Valley shale data in Fig 1b L Fig 4—Fiuid expansion overpressure, Gulf of Mexico at 46 e ?7† © 15-17 ppg #5 Pore Pressure @ 9-15 ppg #4 ely e > a Curve for Each Vmax 137 10 Unloading ƑỊ 10 + 75 10 Velocity (ktt/s) 12.5 oh va 4s i 10 Stress (ksi) Fig 7—-Case where equivalent depth method fails—offshore Indonesia 91 Velocity (kf/4) 75 —r | + 4Ƒ \ =% 15 Normal \ \ ` 10Ƒ Vv Vmax \ ý mo, e, 14Ƒ 14 T Effective Stress (ksi) 16 T 18 © Pressure Tests i > T T ⁄ e ° z/ ⁄Z %$ O° © Inside Reversal “ et ‘Assumed (Pressure Tests) @ Outside Reversal Normal f Trend 16 Equw.Daph quw Dept Solution inside Reversai ⁄ >> “a F s == T T Tiệp ` T Vmax =>8 Hottman & Johnson F Euwalent alen X $ s r \ r ` 12Ƒ 12 T % ` 10 T F Trend at Pore Pressure (ppg) 12.5 Y Assumed =6Ƒ $ 10 T {Normal Trend} § a) b) c) Fig 8—U.S gulf coast application of the Hottman & Johnson and equivaient depth methods On the other hand, the activity of many fluid expansion mechahisms increases with temperature, and therefore depth To be a strong source of overpressure, fluid expansion also requires a fairly rigid, well-compacted rock matrix that can adequately constrain the pore fluid Consequently, fluid expansion is more likely to be an im- portant source of overpressure at deeper depths, in stiffer rocks The only sure way to determine the cause of overpressure in a velocity reversal is with measured pore pressures One way is to plot velocity-vs.-effective-stress data from inside and outside the reversal, as in Fig 3d The reversal data will track a much faster trend if fluid expansion mechanisms were active Another approach is to compare measured pore pressures with those computed with the equivalent depth method The equivalent depth method will underestimate pore pressure caused by fluid expansion Figs and present cases where the equivalent depth method works and fails Cementation Unloading may not be the only reason that velocity reversal data deviate from the virgin curve; cementation could also be a factor From the standpoint of pore pressure estimation, this is inconsequential What counts is that the separate trend tracked by the reversal data be recognized and accounted for However, cementation does complicate diagnosing the cause of overpressure within a reversal Even with petrographic analyses, it can be difficult to sort out the relative effects of unloading and cementation What can be said is that cementation is conducive to fluid expansion overpressure, because it increases the rock matrix’s constraint of the pore fluid Consequently, while local geologic conditions must be considered, there is reason to believe that undercompaction is generally not the sole source of overpressure when velocity reversal data diverge significantly from the virgin curve Velocity (ktt/s) § Y \ 7.5 10 T Y Eaton i >= Ỏ o ? T Eaton sane T T T Vmax 2i : Original Eaton Equation inside the Reversa! T= Revised Eaton © or sf ve oa a / i / / Pnorm( Vv ) norm ye oe voy s T >8Ƒ Ộ Revised F 10 F r V =564 16 Hottman and Johnson The Hottman and Johnson (H&J) method empirically correlates departures from the velocity normal trend line to an equivalent pore pressure gradient.!2 Empirical correlations have no inherent bias towards one particular overpressure mechanism They simply reflect whatever the dominant cause of overpressure is in the area in which they were developed For their U.S gulf coast correlation, H&J assumed this was undercompaction If this is true, then the Equivalent Depth and H&J methods should give similar results As a test, both approaches were applied to shale sonic log data from H&J’s original paper!? (Well “R”) Fig 8a shows the velocities, while Fig 8b compares the estimated pore pressures with bottomhole pressure measurements It can be seen that the equivalent depth method underpredicts the pressure, while the H&J correlation performs well This has two possible implications First, because the Equivalent Depth method failed, this suggests that fluid expansion is an important source of overpressure Second, because the H&J method worked, H&J’s Gulf Coast correlation appears to have fluid expansion “built” into it As further evidence, Fig 8¢ plots velocity-vs.-effective-stress Trend 10F 14Ƒ 15 T 4Ƒ _ Most pore pressure estimation methods claim to only be applicable for overpressure caused by undercompaction However, it turns out that many current pore pressure estimation methods are (unknowingly) predicting fluid expansion overpressure To illustrate this, two popular techniques are examined Pore Pressure (ppg) 12.5 Assumed Nomal eo Current Pore Pressure Estimation Methods ‘ © vzse 0! Revised Eaton Equation inside Reversal (Pressure Tests) ® Outside Reversal {Normal Trend) C= Cnorm( Vv norm ) b) c) Fig 9—U.S gulf coast application of the Eaton and equivalent depth methods SPE Drilling & Completion, June 1995 the velocity reversal The faster trend tracked by the reversal data 1s characteristic of unloading The solid line in Fig 8c is the velocity-vs.-effective-stress path defined by H&J’s pore pressure estimates inside the reversal This curve clearly deviates from the normal pressure data, which again suggests the H&J Gulf Coast correlation is biased towards fluid expansion overpressure This also means that this correlation will overestimate the pore pressure at wells where undercompaction tru- ly is the dominant cause of overpressure, as in Fig Eaton Original Eaton Method Eaton’s method? is an effective stress approach, with the effective stress, 0, computed from the equation: — O= (7) Overview The new method is an effective stress approach that em- Here v is the measured velocity, and Oporm and Vnorm are the values the effective stress and velocity should have under normal pressure conditions Eq implies that normally pressured and overpressured formations both follow a virgin curve relation of the form: VEC O/B cece eee een enna (2) Consequently, Eaton’s method should underestimate fluid expansion overpressure The velocity normal trend line is usually assumed to be a straight line on a plot of log(Vporm) vs depth However, according to Eq I, V and Vnorm Should both satisfy Eq Therefore, to be consistent, Vnorm Should actually be calculated from Eq using Onorm values determined from the overburden stress and normal pore pressure profiles If this were done, the normal trend line would not be a semilog straight line Fig 9a compares H&J’s semi-log normal trend for Well R (dot- dashed curve) with one analytically computed from Eq (solid line) The parameter C = 564 was obtained by fitting Eq through the normal trend velocity-effective stress data in Fig 9c It can be seen that the semi-log normal trend line is faster than the analytical solution below the top of overpressure By overestimating Vporm, the semi-log normal trend will make Eq predict lower effective Stresses, and therefore higher pore pressures than Eq This will generally be true However, unless unreasonably large values are assumed for Vporm, Eq will still underestimate fluid expansion over- pressure This is the case with the Eaton pore pressure estimates for Well R, which are plotted as a solid line in Fig 9b The dot-dashed line is the equivalent depth solution It can be seen that both methods fall well short of the measured pressures in the velocity reversal The velocity-effective stress data in Fig 9c explain why Even with the semi-log normal trend line, the Eaton solution inside the reversal remains close to the virgin curve defined by Eq (Us 00) Z Vinax Velocity (U=1.0) V=s5000+A Unloading Curve The unloading curve is defined by the empirical relation: v=5000 + A[0max (0/Ømax)(/UJIB where A and B are as before, U is a third parameter, and oe)5000\ (4 ° /" Vmax~ Here, Omax and Vmax are estimates of the effective stress and velocity at the onset of unloading In the absence of major lithology changes, Vmax is usually set equal to the velocity at the start of the velocity reversal This assumes that all formations within the reversal at one time passed through the same maximum stress state While this generally may not be true, using the velocity at the start of the reversal for Vmax has been found to work satisfactorily Qo F ® #P — g He V=5000+A [m»«.(0/z.) mm (max) Fig 10—The unloading parameter “U.” > (-\nsax) U Virgin Curve Unloading 04 > Vmax} \ Curve % Wella 0.2 v[-z- Weil #3 Well #4 ‘ : O Weias oc T, Tron D> Welle L Effective Stress Unloading Curve cữ ® Normalized * 06E ¢ Unloading Curve: SPE Drilling & Completion, June 1995 QB) where v is velocity (ft/s), is effective stress (psi), and A and B are parameters calibrated with offset velocity-vs-effective-stress data O Wete2 Virgin Curve: Cuwe v=5000 + A OB cà gmax (U = to 8) ⁄ Virgin found that the virgin curve for shale, as shown in Fig 1a, can be adequately represented by the following equation: 0.8 Unloading Curve Elastic Virgin Curve Over stress ranges of practical interest, it has been Typical Perfectly ploys virgin and unloading curve relations to account for both undercompaction and fluid expansion overpressure Effective stresses outside of velocity reversal zones are computed from the virgin curve Inside a velocity reversal, offset well data are used to decide which equation is appropriate For rank wildcats, the pore pressure can be computed both ways to establish lower and upper bounds on the pressure The unloading curve will always yield higher pore pressure estimates Z Perfectly Plastic ⁄ For example, by raising the exponent from to 5, the revised Ea- ton solution (dotted line in Fig 9b) is able to closely match the Well R pressure data As Fig 9c shows, this is because the higher exponent has allowed Eaton’s equation to simulate an unloading curve New Method Vv nom Modified Eaton Method If the Eaton pore pressure estimates are too low, Eq | can be adjusted to yield lower effective stresses One way is to increase Vaorm by shifting and rotating the normal trend line (Weakley!®) Another, simpler alternative, is to raise the exponent in Eq Either way, the net effect is the same: it allows the original Eaton virgin curve relation to be transformed into an unloading curve 0.2 L 0.4 0.6 Effective Stress L 0.8 (0a) Fig 11—Normalized unloading curve 93 Veiocty (kf/4) 75 ot 10 T r 6} *£ 10 T 12 + Se 14Ƒ Etfective Siress (ksi) 16 x 18 +1 T T 10 —_ s , Estimated g ‘ore Pressure L = J # Ber g ¿ > ; Z a oe Œ 16 a) Virgin e inside Reversal {Pressure Tests} @ Outside Reversal Cure b) a ° § ° © oe °% T T Vmax New Method HÀ [ Vmax a 34 + bE F ° 10F 12 © Pressure Tesis eo L 3° a 15 Trand ar =Ss Pore Pressure (ppg) 126 T {Normal Trend) e) Fig 12—U.S gulf coast application of new method The unloading parameter U is a measure of how plastic the sediment is (see Fig 10) U = implies no permanent deformation, because the unloading curve reduces to the virgin curve U = © corresponds to completely irreversible deformation, since v = Vmax for all values of effective stress less than Omax In practice, U typically ranges between and Pore pressures inside the velocity reversal were computed from the unloading curve relation, with U =3.13 This is a regional unloading parameter determined from U.S gulf coast and Gulf of Mexico data Fig 12b compares the pressure estimates with the measured values The accuracy achieved is similar to that for the Solving for U While virgin curve data track a single curve, unloading data from multiple wells will generally lie on multipte unloading curves (see Fig 5) However, by substituting Eq into Eq 4, the unloading curve can be recast into a form that normalizes multiple well unloading data onto a single curve: with velocity-effective stress data from inside and outside the reversal Also shown is the velocity-vs.-effective-stress path defined by (G/Omax)= (OyelOmane, cece cece cece e teens (6) where 1/B Oye = (=#) ¬—— b eee teteeteneeneenes (7) As the insert in Fig 11 illustrates, o,, is the stress at which the cur- rent velocity v intersects the virgin curve Fig 11 shows the normal- ized version of the unloading data in Fig to Hottman and Johnson’s Well R The virgin curve parameters (A = 4.4567, B =0.8168) were determined by fitting velocity-vs.effective-stress data from the normal pressure interval above 10000 ft (see Fig 12c) A normal pressure gradient of 0.465 psi/ft, and Eaton’s U.S gulf coast overburden stress curve were used to estimate effective stresses along the normal trend The normal trend line in Fig 12a was calculated from the virgin curve relation Velocity (ktt/s) Deepwater Gulf of Mexico The next example is from a well drilled in nearly 1400 ft of water in the Gulf of Mexico Fig 13a plots the sonic log data, and a norma! trend line analytically computed from Eq There are a number of smail velocity reversals above 9700 ft, and one major reversal between 9700 ft and TD Because all of the shallower reversals are very weak, they were not considered to be due to fluid expansion Within the large reversal, the velocity at first drops at arate similar Example Applications H&J’s pore pressure estimates inside the reversal As can be seen, the new method’s unloading curve is very close to H&J’s stress path Their divergence is due to the Hottman and Johnson normal trend line exponentially increasing with depth to that in the smaller reversals However, at 10200 ft, the slope steep- U.S Gulf Coast Fig 12 shows an application of the new method Ệ Hottman & Johnson U.S gulf coast correlation Fig 12c compares the virgin curve and unloading curve relations ens significantly This slope break was interpreted to be the onset of fluid expansion overpressure Therefore, in the unloading relation, Vmax Was assumed to be the velocity at 10200 ft, not the peak velocity at 9700 ft Where the velocities near TD are above the value at 10200 ft, the virgin curve was used to compute effective stresses As in the U.S gulf coast example, a value of 3.13 was assumed for the unloading parameter U Regional virgin curve parameters, determined from wells in water depths between 600 and 1500 ft were used for A and B, with A = 28.3711, B=0.6207 Pore Pressure (ppg) 10 11 10 42 T 14 T T— ` Velocity (kit/s) 16 18 ORFTS § 7.5 + 10 125 + + 15 + Pore Pressure (ppg) 175 + 20 10 Y 12 14 T T ob #16 18 T 20 T ORFTS ~~ L \ VO Trend(s) L ' MN Mudweight Used NO Depth (kit) Depth (kt) o Norma! ee 16F \ ==" Now Method —-—.—-:—-ain \ Vmax r a ệ X-Unconlormity a) b) Fig 13~-Deepwater Gulf of Mexico example 94 a) b) Fig 14—Central North Sea example SPE Drilling & Completion, June 1995 Fig 13b plots the pore pressure data Open circles are pore pressures determined from RFT measurements The dotted line shows the mudweights that were used, while the solid line is the estimated pore pressure It can be seen that the new method is able to track both the rise in pressure, and the pressure regression Central North Sea The final example is from a high pressure/high temperature (HPHT) well in the Central North Sea Fig 14a shows the sonic log data, and three separate normal trends; one for the Tertiary shales above 9000 ft (A = 2.8746, B =0.9037), another for the chalk (A = 802.1, B= 0.3215), and a third for all other formations above and below the chalk (A = 8.116, B = 0.8002) Overpressure above the chalk is primarily due to undercompac- tion, while below the chalk, fluid expansion mechanisms appear to be important This is evident from the velocity-effective stress data in Fig The to 13 Ibm/gal group (solid circles) includes formations from above and below the chalk All of the data in the 14 to 18 lbm/gal range (open circles) are from Jurassic formations below the X-unconformity A fit of the normalized Central North Sea unloading data in Fig 11 yielded U = 4.48 There are essentially no normally pressured shale intervals along this well Consequently, it would be very difficult to apply pore pressure estimation methods that rely on a normal trend line This would include empirical correlations, the equivalent depth, and Eaton methods Near the top and bottom of the chalk, velocity changes due to pore pressure are obscured by those due to lithology Significant variations in sonic properties also occur within the chalk Consequently, pore pressure estimates in and near the chalk are not considered reliable To compensate for lithology effects, the following approach was used Above the chalk, pore pressures estimated down to the onset of the rapid velocity rise at 9800 ft were honored Pore pressures computed below the X-unconformity were also assumed to be valid The estimates on either side of the chalk were then connected with straight lines to the pore pressure calculated at the first major veloc- ity peak within the chalk Lithology effects also made it necessary to use a different criterion for picking Vmax for the clastic formations below the X-unconformity The velocity at the start of the reversal could not be used, because this point is in the chalk From pore pressure hindcasts at other Central North Sea wells, it was decided to set Vmax equal to the nor- mal trend velocity at the X-unconformity Fig 14b compares the estimated pressures with mudweights used during drilling, and RFT data It can be seen that outside the chalk, the pore pressure estimates are in good agreement with the measured values However, within the chalk, as was discussed above, the predictions are essentially a guess Conclusions Based on the literature, there appear to be some misconceptions about fluid expansion as a source of overpressure First, it occurs more frequently than is generally assumed For instance, undercompaction is often cited as the cause of overpressure along the U.S gulf coast However, velocity-vs.-effective-stress data from this area indicate that fluid expansion mechanisms are an important factor The second misconception is that fluid expansion overpressure cannot be estimated from geophysical data It can In fact, a number of current pore pressure estimation methods have been doing so without realizing it Failure to account for the absence or presence of fluid expansion overpressure can lead to large errors in the estimated pore pressure Therefore, it is important to have a systematic approach for estimating pore pressure due to both undercompaction and fluid expansion Such an approach has been presented It consists of two key elements: 1) a pair of velocity-vs.-effective-stress relations that account for overpressure mechanisms besides undercompaction, and 2) a procedure for determining when each relation should be used Both elements are equally important Omax = effective vertical stress at the onset of unloading, psi, m/Lt? Vmax = A,B= U= Oye= sonic velocity at the onset of unloading, ft/sec L/t virgin curve parameters unloading curve parameter effective vertical stress at which the sonic velocity intersects the virgin curve, psi, m/Lt? Snorm = effective vertical stress for normal pore pressure, psi, m/LtVnorm= sonic velocity for normal pore pressure, ft/sec, L/t C= parameter in Eaton’s implied virgin curve relation Acknowledgments The author thanks Exxon Production Research Co for permission to publish this paper Data for the deepwater Gulf of Mexico example were provided by Exxon Exploration Co., while the Central North Sea well data were received from Shell E&P U.K., and Esso E&P U.K The interpretations of these data are those of the author and not of the organizations furnishing the data References Foster, J B, and Whalen, J E.: “Estimation of Formation Pressures From Ham, Coast Eaton, Electrical Surveys Offshore Louisiana,” JPT (Feb 1966), 165 H H.: “A Method of Estimating Formation Pressures From Gulf Well Logs,” Trans., Gulf Coast Assn of Geol Soc., 16, 185-197 B A.: “The Equation for Geopressure Prediction from Well Logs,” SPE 5544 Tosaya, C A.: “ Acoustical Properties of Clay Bearing Rocks ” Ph.D Dissertation, Stanford U., Palo Alto, CA (1982) Barker C.: “Aquathermal Pressuring—Role of Temperature in Devel- opment of Abnormal Pressure Zones,” AAPG, 56, 2068-2071 Miller, T W and Luk, C H.: “Contributions of Compaction and Aqua- thermal Pressuring to Geopressure and the Influence of Environmental Conditions: Discussion,” AAPG Bull (Nov 1993) 77, No 1], 2006-2010 Spencer, C W.: “Hydrocarbon Generation as a Mechanism for Overpressuring in Rocky Mountain Region,” AAPG Bull (April 1987) 71, No 4, 368-388 Fertl, W H.: Abnormal Formation Pressures, Elsevier Scientific Publishing Company, New York City (1976) 21 Powers, M.C.: “Fluid-Release Mechanisms in Compacting Marine Mudrocks and Their Importance in Oil Exploration,” AAPG Bull (July 1967) 51, No 7, 1240-1254 10 Bruce, C H.: “Smectite Dehydration-Its Relation to Structural Development and Hydrocarbon Accumulation in Northern Gulf of Mexico Ba- sin,” AAPG Bull June 1984) 68, No 6, 673-683 11 Magara K.: “Importance of Aquathermal Pressuring Effect in Guif Coast.” AAPG Bull, (Oct 1975) 59, No 10, 2037-2045 12 Hottman, C E and Johnson, R K.: “Estimation of Formation Pressures from Log-Derived Shale Properties,” JPT (June 1965) 717 13 Plumley, W J.: “Abnormally High Fluid Pressure: Survey of Some Basic Principles,” AAPG Bull (March, 1980) 64, No 3, 414-430 14 Berg, R R and Habeck, M F.:: “Abnormal Pressures in the Lower Vicksburg, McAllen Ranch Field, South Texas,” Trans., Gulf Coast Assoc Geol Soc (1982) 32, 247-253 15 Eaton, B A.: “The Effect of Overburden Stress on Geopressure Predic- tion from Well Logs,” JPT (Aug 1972, 929 16 Weakley, R R.: “Use of Surface Seismic Data To Predict Formation Pore Pressure (Sand Shale Depositional Environments),” SPE 18713 SPEDC Glenn L Bowers is president of Applied Mechanics Technologies in Houston, which provides consulting services in abnormal pore pressure, wellbore stability, wellbore fracturing, and general rock mechanics He previously spent 12 years at Exxon Production Research Co., where he performed research in these same areas He holds BS and MS degrees in mechanical engineering from the U of Akron, and a PhD degree in theoretical and applied mechanics from the U of Illinois Nomenclature G= effective vertical stress, psi, m/Lt? v= sonic velocity, ft/sec, L/t SPE Drilling & Completion, June 1995 95 ... 14Ƒ 15 T 4Ƒ _ Most pore pressure estimation methods claim to only be applicable for overpressure caused by undercompaction However, it turns out that many current pore pressure estimation methods... source of overpressure when velocity reversal data diverge significantly from the virgin curve Velocity (ktt/s) § Y \ 7.5 10 T Y