Geologic Problems Related to Characterization of Clastic Reservoirs for EOR

Abstract

Summary. Geologic problems encountered in four U.S. DOE-sponsored EOR projects in clastic reservoirs were analyzed. The reservoir heterogeneities were grouped into four categories: depositional, diagenetic, structural, and formation-fluid composition/distribution. Each category had a variable effect on the performance of the EOR projects. Requirements for developing one static geologic model for each category of heterogeneity are proposed, The interrelationship of geologic factors and the effect of heterogeneities resulting from the origin and timing of geologic events are described. Besides heterogeneities associated with the depositional environment, which has a significant effect on EOR performance, diagenetic effects resulting from geochemical alterations and different types and polymorphs of clays are also important but are often neglected. The structural component of the model needs to be improved through the application of new methodology or improvements in existing technology (e.g., remote sensing, high-resolution seismic). A realistic prediction of fluid movement in clastic reservoirs subjected to EOR requires the integration of the four models that define all the heterogeneities. Introduction The purpose of this continuing research is to develop a generic approach to reservoir characterization for EOR rather than to provide a technique for constructing individual reservoir models. Results of this work will bring awareness to many problems tharecur in EOR projects within clastic reservoirs and that indicate the need for further refinement. Small-scale heterogeneities. not always critical to primary and secondary recovery, may affect sweep and displacement efficiencies in EOR operations significantly. In addition to the log and core data available, new methodology is needed to improve reservoir characterization in the interwell areas. The importance of reconstructing the paleohistory of formations is emphasized to provide information on the variability of diagenetic, structural, and formation-fluid heterogeneties. Improvement in reservoir characterization for EOR requires an understanding of the origin, timing, trend, and magnitude of geologic events that affected the reservoir anatomy (internal structure and composition of the rock). A biased or partial approach to reservoir characterization may result in complete failure or suboptimal results, In 1979, Dickey described several EOR projects that failed because the actual geology had not been considered. In 1986, Dickey further stated that the most common cause of EOR project failure was heterogeneity of the reservoir. Reservoir characterization for EOR purposes should include data from all available sources (analogous reservoirs. outcrops, aquifers, etc.). The multimodel system developed from these data would include a subsequent ranking and integration of the various factors responsible for specific flow behavior. The basic framework for a clastic reservoir description is derived from the geometry and anatomy of the oil-producing formation at the time of deposition. The span of time from deposition to the present introduces a variety of changes in internal structure and composition in the formation. Reconstruction of postdepositional conditions should includeinterpretations of maximum depth of burial, residence time at different depths. and tectonic history;the extent of paleoexposure, erosion, weathering, and reburial conditions; andknowledge of the time of hydrocarbon entry into the reservoir. During the evolutionary history of reservoir rock, mineral assemblages continue to equilibrate chemically with formation fluids, and intermediate byproducts are produced that contribute to the overall heterogeneity of the rocks. To determine the amount of residual oil and to define the most cost-effective recovery method, reservoir parameters altered during primary, secondary, and/or tertiary recovery must be identified and quantified. In practice, however. comprehensive geologic reservoir characterization is not always included in designing EOR projects. Geologic data from four DOE-sponsored EOR pilot projects were analyzed to determine the effect of various geologic heterogeneities on EOR performance. Results of the four case history analyses as well as our experience with similar projects indicate that the various heterogeneities assigned to four categories can be studied best through development of four static geologic models, one corresponding to each category. The requirements for optimum geologic modeling of clastic reservoirs for EOR are proposed subsequent to the review of the geologic problems encountered in the EOR pilot projects. Geologic factors that affect fluid movement in reservoirs are defined. and attention is given to certain aspects of reservoir characterization-geochronology, geochemistry, and man-induced alterations. A final hydrodynamic model derived from the integration Of four partial geologic models will provide the means for a more accurate prediction of fluid movement in the reservoir. Pilot Area Studies The historical background, performance, and geologic problems encountered in four EOR pilot projects (Big Muddy, WY, North Burbank, OK. Bell Creek, MT, and El Dorado, KS) reported in the literature were studied. Information from these studies contributed to the development of the proposed modeling for improving characterization of clastic reservoirs for EOR. Big Muddy Field. Production in Big Muddy field is primarily from the Second Wall Creek sandstone member of the Upper Cretaceous Frontier formation. There are three Wall Creek sandstones in the field. The upper and lower sandstones are unproductive and are separated from the middle Second Wall Creek sandstone by 20 and 30 ft 16.1 and 9.1 mi, respectively. Structural entrapment is within an east-plunging anticline characterized by more than 300 ft 191.4 m] of closure above the oil/water contact (OWC). The reservoir characteristics for the Second Wall Creek sand in Big Muddy field are summarized in Table 1. Several interpretations of the depositional environment have been given (deltaic, barrier bar, and blanket sand). SPEFE P. 449^