Little Buffalo Basin Tensleep Heterogeneity - Its Influence on Drilling and Secondary Recovery

Abstract

Infill development drilling in this field has more than doubled oil production and has increased ultimate recovery. Changes in injection production and has increased ultimate recovery. Changes in injection patterns and changing from gas injection to a combination of gas and water patterns and changing from gas injection to a combination of gas and water injection are expected to improve sweep and displacement efficiency. Introduction The Little Buffalo Basin field is located in northwestern Wyoming on the southwest side of the Big Horn basin (Fig. 1). The structure is a north-south trending asymmetrical anticline that encompasses 1,500 productive surface acres (Fig. 2). Five reservoirs on the productive surface acres (Fig. 2). Five reservoirs on the structure produce hydrocarbons. The Pennsylvania Tensleep reservoir discussed in this paper produces 20 degrees API gravity, 42-cp viscosity crude from an average depth of 4,600 ft. Reservoir energy is primarily supplied by an active edge-water drive. Because of unfavorable mobility ratios, it is quite common for moderately viscous crude reservoirs of this type to perform less efficiently than reservoirs exhibiting mobility ratios of 1 or less, especially if the formation is thick and heterogeneous. For example, when the driving phase prematurely breaks through open fractures or fingers through high-permeability strata the result generally is poor sweep efficiency. Even though well logs might indicate that a reservoir is continuous and relatively homogeneous from well well, cross-bedding, sealed vertical fractures, siltstone and pore filling can cause horizontal permeability to vary considerably. This permeability permeability to vary considerably. This permeability variation can drastically influence both primary and secondary recovery operations. Therefore even a generalized description of reservoir heterogeneity can be helpful, and in fact often results in operation changes that improve recovery. A geologic and engineering analysis of the Little Buffalo Basin Tensleep was undertaken to define the reservoir better and to explain producing characteristics. Oriented cores, using lease crude as the drilling fluid, were taken during the study, the purpose being to develop a better understanding of reservoir fluid saturations, cross-bedding, directional permeability, permeability variation, lenticularity and fracturing. permeability variation, lenticularity and fracturing. Knowledge gained from the core studies was then used to interpret how lithology influences flow of fluids in the reservoir and how field operations could be improved. Development History The Tensleep reservoir of the Little Buffalo Basin field was discovered in 1943 when Trigood Oil Co. completed T. A. Pedley Well 1 (now Unit Well 39). A unit was formed in the same year for the purpose of development and operation. Pan American Petroleum Corp., the unit operator, completed the confirming Well 2 in June, 1944. Development was slow at first because of the limited demand for the sour, viscous, asphalt-base crude. During the first 14 years, maximum production was 2,600 BOPD. Edge wells that completed development on 40-acre spacing within the productive limits were not drilled until 1958. Fig. 3 shows performance of the Tensleep reservoir since 1958 and the results of development drilling, which established a peak producing rate of 6,500 BOPD during mid-1958. To date, 68 Tensleep wells have been drilled. JPT P. 161