The Effect of Small, Discontinuous Shales on Oil Recovery

Abstract

A mathematical model is presented for analyzing the effect of small, discontinuous shales on oil recovery. The model was validated by calculations with a fine-grid computer model. The study illustrates the importance of teamwork between geologists and engineers when describing a reservoir and when predicting its performance. Introduction A complex problem when predicting reservoir performance is assessment of permeability of a formation performance is assessment of permeability of a formation perpendicular to bedding planes. Vertical permeabilities, perpendicular to bedding planes. Vertical permeabilities, which may be less than horizontal permeabilities because of orientation of rock particles and cementing material, can be measured on core samples. It also is necessary to determine distribution of nonpay intervals because small amounts of impermeable rock can profoundly affect vertical permeabilities, even if the rock is discontinuous and randomly distributed. Lateral dimensions of shale laminae, shale bodies, or other impervious materials and their distribution are difficult to describe solely from data on cores and logs. Proper description requires knowledge of depositional environment and detailed information from geologic studies of outcrops and recent sediments. Correct assessment of vertical permeability is particularly important in gas drive operations because high oil particularly important in gas drive operations because high oil recovery depends on effective oil drainage by gravity. For example, shale bodies may be large enough laterally in some reservoirs to prevent coning and yet be few enough to permit good vertical oil drainage. A laminate barrier in a pinnacle reef (carbonate reservoir) was extensive enough to cause loss of miscibility by collecting oil and part of an LPG bank above it. Recent studies indicate the presence of frequent, small, randomly distributed shale bodies in the upper formation of a large sandstone reservoir, which reduce vertical permeability in that interval. Fortunately, conceptual models based on geologic studies of recent sediments, outcrops, cores, and logs are available for describing the distribution and dimensions of pay and nonpay in both sandstone and carbonate reservoirs. Also, reservoir simulators are available for solving increasingly complex problems in practical times at reasonable costs. Construction of simulators is relatively straightforward when a reservoir is divided into separate zones by continuous barriers. Usually, at least three layers are provided in a computer model for each zone. Thin layers provided in a computer model for each zone. Thin layers are used above impervious boundaries to represent flow from gas-invaded regions. An alternative procedure is to generate pseudorelative permeabilities for layers above boundaries. Layers beneath impervious boundaries in water-invaded regions require similar treatment for accurate simulation of reservoir performance. However, no standard treatment is available for modeling reservoirs containing many small, discontinuous barriers. Adequate simulation of performance in large regions of a reservoir could requite many more small computer blocks than the user could afford. This paper illustrates how geologists and engineers can cooperate in describing and predicting oil recovery from complex distributions of permeable and impermeable intervals. A brief overview is given on geologic modeling of different types of deposits. A simple mathematical model is presented for assessing the impact of a given sized impervious body on oil recovery. JPT P. 1531