Use Of Seal, Structural, and Centroid Information in Pore Pressure Prediction Philip D. Heppard and Martin O. Traugott Amoco Exploration and Production Technology Group, Houston, Texas INTRODUCTION
This report describes the effect of geologic structure and lateral continuity on pore pressure in overpressured environments. It sets out three main points: 1) pore pressure in shales, in general, is not equal to pore pressure in adjacent sandstones, sandstones, 2) pore pressure in sandstones can be sharply higher than that in the bounding shale - for structures with large vertical amplitude, and 3) a well drilleddirectly at the crest of a large overpressured structure is at considerable risk of mechanical failure because there is no kick tolerance i.e. because the pore pressure in the reservoir is nearly equal to the fracture pressure in the shale. CENTROID CONCEPT
The centroid is the depth where the pore pressure in a reservoir and the bounding shale are in equilibrium, as illustrated in Figure 1. Above the centroid, the pore pressure in a reservoir is higher than that in the bounding shale. Below the centroid, the reservoir pressure is less than in the shal e. The centroid concept, developed by Amoco (Traugott and Heppard, 1994), is an extension of work by Shell (Dickinson, 1953) and is sometimes referred to as a lateral pressure transfer effect (Swarbrick, 1998). The centroid effect effect has been modeled at the Gas Research Institute (Stump et al., 1998) and has been applied to exploration problems by Arco (Kan (Kan and Kilsdonk, Kilsdonk, 1998). As shown in Figure 1, the pore pressure in an overpressured cell or compartment decreases at a hydrostatic rate with decreasing depth while that in the shale pressure decreases at a overburden rate. The net effect is that the pressure in shales is about 50 psi lower than the pressure in the juxtaposed reservoir for each each 100 feet above the centroid. The effect is larger if hydrocarbons are present because of buoyancy.
feet sea floor
1000
Examp xample le .Rft .Rft
2000 3000 4000
hydrostatic pressure shale pressure fracture pressure
5000 6000
overburden pressure
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Cell pressure 15 ppg at crest 13 ppg at base
9000 10000 11000
centroid
12000 13000
9
0
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4000 6000 8000 10000 Pressure, psig 1000/div
Figure 1 - A pressure plot of depth versus pressure illustrating terminology.
17 ppg
12000
14000
Another effect illustrated in the figure is that the equivalent mud weightincreases toward the crest in each cell while it decreases in the bounding shales which therefore also has a corresponding decrease in hydraul ic fracture pressure. It is not uncommon to have a pore pressure in the reservoir at the crest equal to the fracture pressure of the overlying shale. A well drilled directly at the structural crest of a trap can lose returns into the seal with the mud pumps on and have flow from the reservoirs with the pumps off (Traugott, 1997). FIELD EXAMPLES
Two examples illustrate some of the implications of the centroid concept in conjunction with log and seismic derived calculations of pore pressure for successful exploration and drilling. The first example is from Samaan field, offshore Trinidad. Samaan field is a large, northeast-southwest northeast-southwest trending anticlinal structure 22 miles off the southeast coast of the island of Trinidad (Heppard et al., 1998). The field has produced over 200 million barrels of oil and 636 billion 3 ft of natural gas from normally normally pressured and mildly overpressured, overpressured, Pliocene sandstone sandstone reservoirs. reservoirs. Very high overpressure is present in the deepest sandstones in the field below 12,000 feet (Figure 2). The deepest sandstones within Samaan field have very high pore pressures which are essentially at the hydraulic fracture gradient of the overlying shale at the structural crest of the field. These sandstone pressure compartments are at their maximum pore pressure which is limited by the hydraulic fracture gradient of the overlying shale seal which acts as a pressure release valve. Under these conditions only very small hydrocarbon columns can exist at the crest of the trap. Also, from log calculations and from evidence of overpressure noted during drilling, the interstratified shale beds are significantly lower in pressure than these sandstone beds; on the order of 3 PPG less in the shale above the 13 Sand pressure compartment at about 12,000 feet (Figure 2).
Figure 2 - A pressure profile of Samaan field illustrates that most of the sandstone pressure compartments at the crest are higher in pressure than the bounding shales, indicative of laterally and vertically continuous pressure compartments in a large structure (after Heppard et al., 1998). The deepest, highly overpressured sandstone beds at Samaan field are good examples of the results of this proposed origin of overpressure and the centroid or structural affect of overpressure within sandstones relative to shale. At Samaan the Pliocene shallow marine sandstone beds are continuous over miles extending to the nearest fields such as Teak 12 miles to the south. The top of overpressure is at a relatively consistent depth in the Samaan field area at about 10,000 feet stepping up stratigraphy away from from the field (Figure 3). The centroid depth of the overpressured sandstones is very deep off down the flank of the anticline and the pore pressure exerted by the shale over the entire surface of the
sandstone body has been redistributed redistributed along a water gradient throughout the sandstone sandstone bed. At the crest therefore the pore pressure in the sandstones can be very much greater than in the bounding shale which has only built up pressure from taking on the overburden stress from much much less burial. As illustrated in Figure 2 the overpressure in the shale is much less than that in the sandstone beds at the crest of Samaan field. We would also emphasize that we do not believe that this is a systematic error in our estimation of the pore pressure of the shale beds. In highly overpressured wells more to the center of the basin the same techniques used to estimate shale pore pressure show consistently similar or higher pressure in shale relative to measured pressures in the sandstone. Well
Samaan field
Well
Well
Well
E S a a n nd d T S a an d n d
5000’
Overpressure
2 S a an d n d
E Sand
Top
Overpressure
F Sand
10,000’ 9 S a n nd
1 3 3 S a n nd d
15,000’
T S a an d n d
Top 17 PPG
17 PPG
1 3 S a n d
Figure 3 - A geologic cross section from Samaan field and to wells to the northeast illustrates the complex faulting and structure of the area. The top of overpressure at about 10,000 feet steps up stratigraphy away from the field and becomes greater in the shale section with depth such that normally pressured and overpressured sands within the field dip into a very overpressured section. Modified from T. Pish and J. Sharp, Amoco Trinidad Oil Company (1996). feet
MAINTAINING KICK TOLERANCE 13 Sand of Samaan field
12000 Hydraulic Fracture Gradient of shale seal
SHALE
12500
13 Sand Pressure Compartment Water Grad. 0.45 psi/ft
13000
600 ft. Off Crest 0.5 PPG
19
17
16 P P G 11000
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SHALE
12000 Pressure, psig 500/ div
18 12500
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Figure 4 - A pressure depth plot illustrates that by drilling 600 feet off the crest a 0.5 PPG kick tolerance can be achieved. An implications of this concept is that very highly pressured compartments can not be safely or efficiently drilled at the crest, but can be drilled safely a few hundreds of feet off the crest. Figure 4 illustrates that at the crest of the Samaan
field the very highly pressured 13 Sand is within 200 psi of the calculated hydraulic fracture gradient of the overlying shale seal. Therefore at the crest only a static mud weight is stable which almost exactly balances balances the pore pressure of the sandstone and the hydraulic fracture gradient of the overlying shale. If the mud pumps are turned on to establish circulation for drilling ahead then the additional pressure will fracture the shale and circulation will be lost. Three early attempts to drill through the 13 Sand near the crest failed. In 1993 we recommended drilling at least 300 feet off the crest of the structure to increase the kick tolerance while drilling this sand and the well was drilled successfully (Figure 4). A second example this time from the Nile Delta, offshore Egypt, illustrates several other implications of the centroid concept. Recent exploration drilling has been largely successful at discovering gas and condensate reserves in Pliocene and Miocene sandstones in the Nile Delta. Significant volumes of hydrocarbons are trapped within overpressure. overpressure. Overpressure begins within the Pliocene Kafr el Sheikh formation at about 1500 m and increases in steps to about 16 PPG at about 3600 m (Figure 5). Like at Samaan field, at the crest of structures the pore pressure of the sands is higher than the bounding shales. Within the Miocene section the level of overpressure within the shale is remarkably consistent across the eastern Nile Delta increasing from about 15 PPG to over 16 PPG at about 4200m. However the principal sandstone reservoir within the Serravallian age section is often lower lower in pore pressure than the bounding shale. Rarely a much higher pressure sand is found within the Miocene Serravallian section. We interpret these pressure differences as due to the origin of the overpressure which is similar to that described for the Samaan field example but is also affected by leak points.
eters
Osiri-e1 Osirs-e1 Osirs-e1
OSIRIS EAST #1, ORIG. HOLE AEOC/IEOC, CMP: 12/1996 SERRAVALLIAN SAND PRESSURE
2000
.Mud .Mdt 2000
Kafr El Sheikh A30
Overburden
2400
Normal Trend
2400
Pressure Profile
MDT's
A40?
2800
Hyd. Frac. Gradient
2800
A50?
LOT NN6 Sand pressure higher than shale, updip to
3200
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Serravallian NN6 NN 6 NN5 NN 5
NN6 Centroid
4000 Casing NN5 Sand pressure lower than bounding shale, downdip from centroid or 'leaked'.
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Normal Water Pressure
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0. 2 20 Resistivity
Figure 5 - A pressure versus versus depth plot of the Osiris East #1 #1 well from the Nile Nile Delta of Egypt. Overpressure begins begins in the Pliocene Kafr el Sheikh formation at 1780 m and builds in a step-wise fashion through the Miocene section to total depth. Through most of the section pore pressure in the sandstone is higher than the bounding shale as is expected at the crest of large structures. The example well shown in Figure 5, the Osiris East #1 encountered two overpressured sands within the Miocene section below 3500 m. The NN6 sand at about 3800 m is at 5578 psi above normal near and over 17 PPG. A sand just a few 10’s of meters deeper, the NN5, is much lower lower in overpressure at about 3800 psi above normal. The very high pressure of the NN6 Sand is greater than the bounding shale at the Osiris East #1 well indicating that the sand was penetrated at a depth above the centroid. This indicates that the sand extends well down down into increasingly overpressured shale. At the crest of the structure just above the Osiris East #1 penetration the sand pressure approaches the hydraulic fracture
gradient of the overlying shale suggesting that gas may have leaked out of the sandstone due to hydraulic fracture explaining why it is wet. The pore pressure of the deeper, gas filled filled NN5 Sand however is much much lower than the bounding shale suggesting that the well penetrated the sandstone well below the centroid, that is well down from the highest point of the vertical extent of the sandstones distribution. Applying the centroid principle then the NN5 Sand has at least 800 m of vertical relief being about 400 m below the it’s centroid or greater relief while the NN6 may have a much less vertical relief. However an alternative interpretation for the relatively lower pressure of the NN5 Sand seems more viable. An interpretation of available seismic lines in the area as illustrated in Figure 6 suggests that the NN5 sandstone is truncated by a major unconformity at a depth below the implied crest. Pliocene beds on lap the unconformable surface and likely provide a fluid leak point allowing overpressure fluids to escape. Pressure prediction for deep, Miocene exploration wells in the eastern Nile Delta is conducted using interval velocity derived from surface seismic and geologic interpretations. Within the area of the Osiris East #1 well seismic interval velocity reflects the overall velocity of the shale dominated section but does not reflect velocity variations from the relatively thin sandstone beds that might suggest their pressure state. Therefore we can only estimate the average shale pressure state and must use structure maps and geologic interpretation to predict overpressure in sandstone beds. We can not indirectly measure measure some property such as velocity to predict overpressure in non-shale formations. formations. In the eastern Nile Delta and elsewhere we use the structural relief and to some degree the shape of sa ndstone beds to correct for the redistribution of overpressure within within the unit using the centroid concept. This must be modified where well data and geologic interpretations suggest that the system may be ‘leaky’.
Figure 6 - A geologic model of the distribution of overpressure and hydrocarbon migration in the Ha’py Graben where the Osiris East #1 well was drilled. The NN6 Sand is very overpressured approaching approaching the hydraulic fracture gradient of the overlying shale suggesting that gas has been released from the crest of the structure through pressure induced fracture feeding into shallower Pliocene sandstones. sandstones. The relatively lower pressure of the NN5 Sand however however indicates that laterally there is a pressure and fluid le ak point which is suggested to be acros s the Messinian unconformity into onlapping Pliocene sandstone beds (after Heppard and Albertin, 1998).
CONCLUSIONS
Pore pressure prediction should take into account that most pressure estimation and prediction methods whether log or seismic velocity based estimate the pore pressure in the shale only. The pore pressure of shale can be much different from the adjacent sandstone due to the relative impermeability of shale and the redistribution of overpressure within the pore system of porous and permeable rocks. The centroid principle can be used to predict overpressure in nonshale units such as sandstone. At the crest of large structures we should expect the pressure in the sandstone beds to be significantly higher relative to the bounding shale due to the redistribution of overpressure throughout the entire volume of the sandstone along a water gradient. Where long hydrocarbon columns are present the pressure gradient can be sharply increased at the crest due also to the buoyancy of the gas or oil. Where very high pore pressure is expected a well should be located at least 300 feet off the crest of the structure to allow a manageable kick tolerance to exist. Directly at the crest of structures the pore pressure in sandstone beds may be very close to the hydraulic fracture gradient of the overlying shale which is a safety hazard, a pressure condition where economic hydrocarbon columns are unlikely to exist, and often leads to the abandonment of the well since there is too little kick tolerance. REFERENCES
Dickinson, G., 1953, Geological Aspects of Abnormal Reservoir Pressures in Gulf Coast Louisiana, AAPG Bulletin,37 , 410-432. Heppard, P. D., Cander, H. S., and Eggertson, E. B., 1998, Abnormal pressure and the occurrence of hydrocarbons in offshore eastern Trinidad, West Indies, in Law, B. E., Ulmishek, G. F., and Slavin, V. I., eds., Abnormal pressures in hydrocarbon environments: environments: AAPG Memoir Memoir 70 , 215-246. Heppard, P. D., and Albertin, M. L., 1998, Abnormal pressure evaluation of the recent Pliocene and Miocene gas discoveries from the eastern Nile Delta, Egypt, using 2D and 3D seismic data: 1998 AAPG Annual Convention, Salt Lake City, Utah, expanded abstract. Kan, T.K., and Kilsdonk, Kilsdonk, W., 1998, Geopresure prediction fro m 3d data: case studies fro m the Gulf of Mexico, Workshop on Overpressures in Petroleum Exploration, April 7 and 8, Pau, France . Stump, B.T., B.T., Flemings Flemings,, P.B., Finkbeiner, Finkbeiner, T., Zoback, M.D., 1998, Pressures Pressur es differences diffe rences between b etween sands sa nds and bounding shales for fluid flow induced by sediment loading, Workshop on Overpressures in Petroleum Exploration, Exploration, April 7 and 8, Pau, France. Swarbrick, R.E., Osborne, M.J., Yardley, G.S., Macleod, G., Bigge, A, Grunberger, D., Aplin, A., and Larter, S., 1998, Bean Field - Integrated study of an overpressured Central North Sea oil and gas field, Workshop on Overpressures in Petroleum Exploration, April 7 and 8, Pau, F rance. Traugott, M. O., and Heppard, P. D., 1994, Prediction of pore pressure before and after drilling - taking the risk out of drilling overpressured prospects: AAPG Hedberg Research Conference Conference Abnormal Pressures in Hydrocarbon Environments, Golden Colorado, June 8-10, 1994, abstract. Traugott, M.O., 1997, Pore/fracture pressure determinations in deep water, Deepwater Supplement toWorld Oil, August, 1997.
