How abnormal pressures affect hydrocarbon exploration, exploitation Social Media Tools Tools Share
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01/12/1998 Gordon Holm Hydrocarbon Management Management International Ltd. Ltd. Edinburgh, Scotland Scotland
Abnormal pressures, principally overpressure but also underpressure, underpressure, have been discussed in numerous papers. These have concentrated on overpressure generating mechanisms and t he ability of abnormal pressure prediction. This article will examine the information that can be deduced by the knowledge that a basin is abnormally pressured and how this can affect exploration risking, r isking, reserves estimation, and hydrocarbon production.
Background Abnormal pressures consist of both underpressure and overpressures. An underpressured system occurs when the formation pore pressures are less t han the hydrostatic gradient, 0.45 psi/ft (Fig. 1 [110,114 bytes]). bytes]) . Underpressures are less common, although they are widely recognized in 1 gas reservoirs in the foothills to the Rockies and have been studied in detail in Elmworth field. Overpressures are widespread throughout throughout the world and present in a wide variety of basin types. They can be defined as moderate overpressures, varying between 0.5-0.75 psi/ft, and hard overpressures, overpressures, between 0.75 psi/ft and 1.0 psi/ft. The upper limit of overpressure is the fracture gradient, equivalent to the minimum horizontal stress. In uniaxial stress conditions, the fr acture gradient can approach and equal the overburden gradient. 2
Many authors have discussed the origin of overpressure, more recently, Mouchet and Mitchell and 3 Osbourne and Swarbrick. A variety of processes pr ocesses have been proposed for the generation of overpressure; the most commonly quoted are porosity reduction due to compactional disequilibrium or lateral stresses, and fluid expansion. In the
latter case, the excess fluid associated with hydrocarbon generation is considered to be a major contributor 4 to overpressure generation, particularly at depths greater than 12,000 ft. While it is probable that overpressure is the result of a number of processes, it is important to recognize the dominant mechanism. The dominant process may change both in spatial and temporal position within the basin, and it is critical to recognize which process is currently active, as this may influence the interpretation of the hydrocarbon potential of a basin.
Prospect risking Exploration prospect risking is quantified by multiplication of the risking factors: trap, seal, hydrocarbon 5 generation and timing, and migration. In an overpressured basin there are implications for the estimation of all of these factors with the exception of the trapping mechanism.
Seal Every overpressured system has a seal. There is always a top seal, which in overpressured terms is the transition zone. Side seals can be present, either as a result of faults, facies variations, or diagenetic alterations. The side seal may not be recognizable from seismic sections, but is most often inferred from lateral pressure variations. The presence of top, side, and bottom seals results in a compartment. The Anadarko basin is an example of a 6 mega-compartment and contains numerous compartments within it (Fig. 2 [109,425 bytes]). The significance of the pressure seals in abnormally pressured compartments is that stacked reservoirs may be present. Thus each pressure compartment has the potential for hydrocarbon trapping, and a number of plays may be stacked vertically on top of each other.
Source and timing Overpressures produced by organic matter transformation may be responsible for generating pressures that exceed the fracture gradient. This assists in the expulsion of hydrocarbons from the source rock. The Kimmeridge claystone in the Central Graben of the North Sea is an extremely rich source rock that is currently gas generative at depths greater than 10,000 ft, and large amounts of gas are released when drilling through the Kimmeridge claystone. The presence of this gas indicates that overpressured source rock should also be regarded as a potential reservoir. Source rock claystones have good porosity but poor permeability. In areas where natural fracturing has occurred, it may be possible to tap into the natural fracture system and produce hydrocarbons directly from the overpressured source rock. The timing of the onset of gas and oil generation from the source rock is one of the most important factors 7 when risking individual or basin prospectivity. A number of papers have indicated that in the closed environment of an overpressured system, the timing of oil and gas generation is delayed. Thus oil and gas are generated at depths greater than predicted in conventional geochemical modeling. This can have critical importance, particularly in areas where oil is economically advantaged compared with gas. In the overpressured basin the oil will continue to be generated at depths where gas would normally be expected.
Migration
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The conventional view of migration is the "Fill and Spill" model of Gussow. In this case the earlier generated oil fills the deepest trap full and then leaks updip from the spill point to the next trap (Fig. 3 [55,905 bytes]). This process continues, and when the source becomes gas generative, the gas selectively displaces the oil, starting from the lower traps. In the dynamic overpressure system, leakage can also occur by intermittent fracturing of the top seal. As the overpressure approaches the fracture gradient, micro-fractures in the top seal will intermittently open, releasing fluids. The fluid loss results in a reduction of the pore pressure. When this becomes less than the fracture gradient, the microfractures anneal. This process is c yclical with the microfractures re-opening as the pore pressure again increases. This process is significant because in addition to creating separate migration routes compared with the "Fill and Spill" model (Fig. 4 [121,636 bytes]), there is also a tendency for gas to preferentially leak from the top of structures. Thus the top seal acts like a safety valve on a pressure cooker, and the gas preferentially escapes through the top seal. This process may be recognized by the presence of a gas cloud overlying the prospect. Thus there may be "gas bypass" in the structure where the oil is not displaced at the spill point by the later generated gas.
Reserves estimation Reserves estimation is critical in both the exploration and evaluation phases of a prospect/discovery. The factors used in estimating reserves are: structural area, hydrocarbon column, porosity, hydrocarbon saturation, and the gas expansion factor/oil shrinkage. When dealing with an abnormally pressured system, there are implications affecting three of these factors; the hydrocarbon column, porosity, and gas expansion factor.
Hydrocarbon column An estimate of the maximum potential height of the hydrocarbon column can be made with knowledge of the fluid density and the difference between the pore pressure and the fracture gradient. Knowledge of the maximum potential height may be important in discriminating between differing structural models and also may assist in deciding if a prospect is below an economic threshold (Fig. 5 [102,023 bytes]).
Porosity There is a strong association between enhanced porosity and overpressure. In the compactional disequilibrium model of overpressure generation, the presence of excess claystone porosity, compared to a normal compaction curve, has been extensively used to predict overpressure. In the deep Central Graben of the UKCS, sandstone porosities in excess of 30% have been encountered at 9 depths greater than 15,000 ft. Wilkinson et al. believe that this is a result of secondary porosity, while 10 Osbourne concluded that the excess porosity resulted from the overpressure inhibiting pressure solution and quartz overgrowths. While the relation between overpressure and abnormally high porosity is not proven, the coincidence of excess porosity associated with overpressure is very common. Thus, when calculating reserves within an overpressured environment, higher than normal porosities should be assumed.
Gas expansion factor Gas expansion factors (GEFs) are much higher in an overpressured situation.
The GEF in the deep Central Graben play is often greater than 300. If this is compared to a more normal GEF of 100, it is apparent that for a constant rock volume the overpressured prospect will contain three times the gas reserves compared to a normally pressured prospect.
HC exploitation The exploitation of hydrocarbons is a data rich environment for pressure information, and this can be used to assist in correlation, fault strength estimation, and detecting hydrocarbons overlain by water.
Correlation Pressures can provide definitive evidence on the correlation of sands at the appraisal stage, but especially during production when preferential production can produce differing pressures in sandstone bodies. In the example (Fig. 6 [92,404 bytes]) a number of similar sandstone can be observed in the two wells. As these sands were deposited in similar environments, there is little to distinguish them on conventional wireline logs. It is only when the pressures are compared that a ready and accurate correlation can be made.
Fault strength The sealing capacity of faults is currently of great interest. Despite computer methods for fault modeling, it is only by examining the pressure differential across two sides of a fault compartment that the sealing strength of the fault can be measured. This is undertaken by comparing the difference in heights between oil-water, oil-gas, or gas-water contacts and multiplying this by the fluid density. The resultant pressure is a measure of the fault seal strength. One of the most important parameters for input into a reservoir model is whether a fault is sealing or leaking. In fact, a fault can change from a seal to a fluid conduit during the producing life of a field. This can occasionally be inferred by observation of pressure and production from isolated fault blocks. Initially the faults are sealing, but as the pressure inside the fault block declines in comparison to the surrounding reservoir, the P becomes great enough that the fault starts to leak. Subsequently production from within the fault block is enhanced by inflow across the fault. Evidence of this can be obtained by detailed examination of the changes in pressure during production and the mass balance for the fault block.
HC overlain by water The phenomenon of hydrocarbon, particularly gas, overlain by water is known from the western U.S. and Canada, where underpressured gas is found underneath water bearing reservoir.1 To date this phenomenon has not been encountered in exploration wells in the North Sea, but it can occur in productive fields. In mature fields, the pressure may locally decline below the hydrostatic gradient, and this mimics the natural phenomenon observed elsewhere. This is important, as hydrocarbons can be present vertically below or downdip from a water filled section of a reservoir. Pressure data are vital in locating such zones, which may often contain significant quantities of bypassed hydrocarbons.
Conclusions In this article, an attempt is made to demonstrate many of the facets that can be encountered when dealing with abnormally pressured sediments. A great deal of information can be inferred from recognition of the t ype and origin of the abnormally pressured basin. We conclude with an appeal to geoscientists to recognize that abnormal pressure is not just
a phenomenon responsible for drilling difficulties but an important tool that can provide added value to a data set, whether in prospect exploration, reserves estimation, or hydrocarbon exploitation.
References 1. Masters, J.A., Elmworth-Case study of a deep basin gas field, AAPG Memoir 61, AAPG, Tulsa, 1984, 316 p. 2. Mouchet, J.P., and Mitchell, A., Abnormal pressures while drill ing: Origins-Predictions-DetectionEvaluation, Manuels Technique Elf Aquitaine No. 2, Elf Aquitaine Editions, Boussens, 1989, 264 p. 3. Osbourne, M., and Swarbrick, R.E., Mechanisms for generating overpressure in sedimentary basins: A reevaluation, AAPG Bull., Vol. 81, 1997, pp. 1,023-41. 4. Holm, G.M., The Central Graben-A dynamic overpressure system, in AD 1995: NW Europe's Hydrocarbon Industry, Glennie, K., and Hurst, A. eds., Geological Society of London Special Publication, 1996, pp. 107-122. 5. Otis, R.M., and Schneiderman, N., A process for evaluating exploration prospects, AAPG Bull., Vol. 81, 1997, pp. 1,087-1,109. 6. Al-Shaieb, Z., Puckette, A. Abdalla, A., and Ely, P.B., Mega-compartment complex in the Anadarko basin: A completely sealed overpressure phenomenon, in Ortoleva, P.J., ed., Basin compartments and seals, AAPG Memoir 61, AAPG, Tulsa, 1994, pp. 55-68. 7. Neruchev, S.G., and Gildeva, I.M., Abnormally high reservoir pressure and t he principal phase of oil generation, abs. from AAPG Hedberg Research Conference-Abnormal Pressures in Hydrocarbon Environments, Golden, Colo., June 8-10, 1994. 8. Gussow, W.C., Differential entrapment of oil and gas: a fundamental principle, AAPG Bull., Vol. 38, 1954, pp. 816-853. 9. Wilkinson, M., Darby, D., Haszeldine, R.S., and Couples, G.D., Secondary porosity generation during deep burial associated with overpressure leak-off: Fulmar formation, U.K. Central Graben, AAPG Bull., Vol. 81, 1997, pp. 803-813. 10. Osbourne, M.J., Preservation of high porosity in the Central North Sea HPHT clastic reservoirs: Influence of overpressure and quartz cementation, abs. from Compaction and Overpressure, IFP, Reuil-Malmaison, France, Dec. 9-10, 1996
Gordon Holm became a founding director of Edinburgh based consultancy group Hydrocarbon Management International Ltd. in 1991. Since then he has worked in a variety of consultancy positions. He has a special interest in overpressure and has written and presented papers in Europe and the U.S. He was graduated with an honors degree in geology from the University of Aberdeen. After graduation he worked as an exploration and mining geologist in Africa. On returning to the U.K. he joined BNOC (later to become Britoil), where he worked as a petroleum geologist in the exploration, production, and operations departments.
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