This paper presents the methodology and results of a reservoir characterization study of Clear Fork carbonates in the TXL South Unit Field located in Ector County, Texas. The principal objective of our study was to evaluate a targeted infill drilling strategy for future field development. Our study incorporated an integrated approach for which the primary evaluation tool was decline type curve analysis of well production data. The well performance analysis was both supplemented and complemented with petrophysical and geological studies, each representing different reservoir scales. On the basis of our study, we identified areas of the field with the highest reservoir quality and largest oil-in-place volume, thus identifying the areas of the field best suited for infill drilling.
Like most Permian-age carbonate reservoirs in the Permian Basin, Clear Fork carbonates in the TXL South Unit Field are characterized by very thick, heterogeneous pay intervals with significant discontinuities, both laterally and vertically. Low reservoir energies, consistent with solution-gas-drive oil reservoirs, as well as low effective permeabilities to oil are manifested by primary production recovery efficiencies typically ranging from 8 to 12 percent on 40-acre well spacing.1–4 Consequently, infill drilling is required not only to increase recoveries from primary production, but also to enhance sweep efficiencies and improve recovery from secondary and tertiary enhanced oil recovery operations. Even at reduced well spacing, however, many operators observe low oil recoveries, poor sweep efficiencies, and early water breakthrough.1–4 Poor performance at a denser well spacing is indicative of the significant reservoir discontinuity. Accordingly, a better understanding of the reservoir heterogeneity will help to design and implement enhanced oil recovery operations more successfully.
Moreover, operators in the Permian Basin have historically implemented "blanket" infill drilling strategies in which wells are drilled on uniform patterns and spacing with little consideration of reservoir quality. Development at non-optimum well spacing may result in poor economic returns, even under favorable oil pricing scenarios similar to current conditions. In fact, several previous studies5–7 have shown that "targeted" infill drilling programs are required to optimize field development by reducing capital expenditures and maximizing economic returns. Targeted infill drilling, however, requires a reservoir characterization program to identify areas of the field with the best quality rock and the largest volume of oil-in-place. Because of the significant volume of original oil-in-place remaining in Permian-age carbonates in West Texas,8,9 there is an economic incentive for optimizing field development with infill drilling programs, both for primary depletion and enhanced oil recovery operations.
The purpose of this paper is to present the methodology and results of a reservoir characterization study of the Clear Fork carbonates in the TXL South Unit Field located in Ector County, Texas. Similar to a study conducted by Doublet, et al.7 for the North Robertson Unit in Gaines County, we incorporated an integrated approach in which we combined results from geological, petrophysical, and reservoir performance analyses, each representing different reservoir scales. Furthermore, rather than initiating a cost-prohibitive data acquisition program, we conducted our study using existing field data typically available to most operators.
Historical Field Background
Located in the center of the Central Basin Platform in the Permian Basin, the TXL South Unit encompasses approximately 10,200 acres in the western half of Ector County, Texas. Wells in the TXL South Unit produce from both the Upper Clear Fork (5600 Reservoir) and the Lower Clear Fork (Tubb Reservoir). As shown by Figure 1, current field production is about 1,000 STB/day and 3,000 Mscf/day from approximately 400 active wells.