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Geologists, working in close cooperation with engineers, have an important role to play in maximizing the recovery of oil and gas from producing fields and especially in implementing enhanced oil recovery (E.O.R.) projects. In order for this contribution by geologists to be useful, however, geologic descriptions of reservoir rocks must be quantified for inclusion in numerical models of fluid flow within the porous interval. In particular, research is needed in characterizing both large and small scale heterogeneities within porous reservoir rocks. In addition, research is needed in geophysical monitoring of E.O.R. processes in heterogeneous reservoirs.
At one scale are heterogeneities in the stratification of porous sedimentary facies. Thin shale beds, evaporite layers, cemented zones, and other features that affect the movement of fluids within a formation should be noted and made as important a part of reservoir description as is the nature of the porous rock itself. Numerical models for forecasting production commonly lack this kind of geologic input until development drilling is completed; so models of discontinuities in various clastic and carbonate facies are essential for accurately predicting reservoir heterogeneity with only a minimum of well control.
At a different scale are heterogeneities within the pore systems of reservoir rocks. Recent work by a few investigators has shown the importance of geometry of the pore system to entrapment and retention of nonwetting fluids. Observations by engineers and petrophysicists of differences in the capability of certain rock types to produce the fluids they contain have long been a basis for subdividing reservoir intervals for numerical modeling, but only recently has an understanding of the causes of these differences been gained through work with models and casts of actual pore networks. Further research is needed in this microscopic realm to link the description of rocks and their pores quantitatively with anticipated reservoir behavior.
Yet another field for future research is the chemical interaction of reservoir rocks with various non-native fluids to which they are exposed. Most petrographic studies stop with simple descriptions of pore-lining components of rocks, and only a few published studies provide empirical data on potential chemical reactions between these components and various acidic, caustic, and organic solutions in the subsurface environment.
Finally, another area of development that would be highly beneficial to E.O.R. projects is in our ability to monitor indirectly the progress of various fluids through the reservoir. Remote sensing of fluids of different compositions, through surface or borehole geophysics, without the need for numerous monitor wells between injectors and producers, would be desirable for control of the progress of an E.O.R. project and for reducing the cost of evaluating studies of pilot-areas.
Research of the kind mentioned is, of necessity, a multi-disciplinary effort. Geologists or geophysicists working alone tend to stop short of seeing that the reservoir analysis they provide is adequate for answering the questions at hand; and engineers, without geologic guidance, tend to have an oversimplified concept of a reservoir. Either extreme is less than the desired result of which we are capable as a team, if all of the kinds of pertinent information are integrated and maximum use is made of them.
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