A Centrifuge Method To Predict Matrix-Block Recovery in Fractured Reservoirs

Previous laboratory investigations to predict recovery from matrix blocks in a fractured reservoir have been primarily concerned with the imbibition oil recovery process. Gravity segregation can sometimes be more important than imbibition as a mechanism for oil recovery from matrix blocks. This paper describes a new method to predict oil recovery behavior for matrix blocks in a fractured-matrix reservoir. Centrifuge tests are performed with preserved reservoir core samples to scale the effects of both gravity and capillarity. The tests are easy to perform. Yet, The results should be more reliable than results obtained by other available methods for predicting matrix-block recoveries. Centrifuge tests were performed on different-sized reservoir rock samples to confirm the scaling theory. These tests were performed to simulate a water-oil displacement in a weakly water-wet reservoir. The method is equally applicable to other reservoir wettabilities and gas-oil displacements. In the weakly water-wet system, oil recovered by imbibition was found to be almost negligible as compared with the amount of oil recovered by gravity segregation. Introduction In many fractured-matrix reservoirs, most of the oil is contained in matrix blocks that are surrounded by a high-permeability fracture system. The flowing pressure gradient in the fractures is small, and pressure gradient in the fractures is small, and local capillary and gravity forces will dominate the recovery process for each matrix block. The unit matrix-block behavior then becomes an important factor in predicting reservoir oil recovery and assessing secondary or tertiary recovery prospects. Other investigators have used synthetic models and reservoir core samples to study the imbibition oil recovery process in fractured-matrix reservoirs. The methods they used to predict matrix-block recoveries are applicable when the reservoir rock is strongly wet by water and imbibition is the dominant mechanism for oil recovery. Wettability studies show that some reservoir rocks are not strongly wet by water in the presence of crude oil so that imbibition oil recovery will be low. Under these circumstances, gravity segregation can be an important mechanism for oil recovery, and its influence should be included in predicting matrix-block recovery. Marx has previously used scaling principles to show that the gas-oil gravity drainage characteristics of long, linear columns can be predicted from centrifuge tests on reconstituted core samples. In the current work, application of the scaling theory is extended to include three-dimensional systems and water-oil displacements. In addition, procedures are suggested to either duplicate or scale reservoir wettability and interfacial tension. The objectives of the present work are to present a centrifuge method that can be used to predict the effects of both gravity and capillarity on matrix-block oil recovery and to demonstrate the importance of gravity in recovering oil from matrix blocks in fractured reservoirs. In this paper, discussion of the centrifuge method will be limited, for the most part, to water-oil displacements. Application of the method to gas-oil displacements is discussed briefly near the end of the paper. PROCEDURE FOR PREDICTING PROCEDURE FOR PREDICTING MATRIX-BLOCK RECOVERY The laboratory procedure developed for predicting matric-block recovery consists of the predicting matric-block recovery consists of the following.Select a preserved core believed to have the same wettability, porosity and permeability as the prototype reservoir matrix block. prototype reservoir matrix block. A preserved core is defined as one cut and stored under conditions designed to preserve the original reservoir wettability.Cut a sample (or model) from the core so that it will simulate the reservoir matrix-block geometry in the centrifuge tests. SPEJ P. 164