Infill Drilling Enhances Waterflood Recovery
Authors: Wu Ching H. , Laughlin B.A. , Jardon Michel 
Published: December 2007

Summary
Two sets of west Texas carbonate reservoir and waterflood data were studied to evaluate the impact of infill drilling on waterflood recovery. Results showed that infill drilling enhanced the current and projected waterflood recovery from most of the reservoirs. The estimated ultimate and incremental infill drilling waterflood recovery was correlated with well spacing and other reservoir and process parameters. Results of the correlation indicated that reducing well spacing from 40 to 20 acres [16 to 8 ha] per well would increase the oil recovery by 8 to 90% of the original oil in place (OOIP). Because of the limited data base and regressional nature of the correlation models, the infill-drilling recovery estimate must be used with caution.

Introduction
The concept of optimal well spacing for oil recovery has been am important and controversial subject for more than 50 years. Before 1960, ultimate recovery by primary mechanisms was considered to be independent of well spacing. In 1969, Davis and Shepler reported that by reducing well spacing from 40 to 20 acres [16 to 8 ha], primary oil recovery from the San Miguel Unit of the Sacatosa field in southwest Texas was increased by at least 14% OOIP. The relationship between primary ultimate recovery and well spacing was not well established, possibly because reservoir heterogeneity was not considered. Waterflood technology began developing in the early 1920's and became popular in the 1950's. Mainly for economic reasons, the use of existing wells with some additional infill wells was common for waterflood projects, but the impact of well spacing on optimal waterflood recovery was not seriously considered. In 1971, Emmett et al reported that reducing well spacing from 40 to 20 acres [16 to 8 ha] economically accelerated the producing rate and increased ultimate recovery by gas/water injection in Wyoming's Tensleep reservoir. In 1973, Thomas and Driscoll reported that infill drilling in chickenwire patterns in the Slaughter field, TX, increased oil recovery by an average of 3.6% OOIP and was profitable. Infill drilling for improving waterflood recovery was initiated in the early 1970's in the carbonate reservoirs in the Permian Basin of west Texas. Results in the literature indicated that infill drilling can improve ultimate recovery from heterogeneous reservoirs; however, a consistent set of field data was not available for developing a correlation between waterflood recovery and well spacing. The objective of this study was to acquire a set of consistently evaluated field data from west Texas carbonate reservoirs to determine the impact of infill drilling on waterflood recovery and to develop linear regression models correlating waterflood recovery with respect to well spacing and other reservoir/process parameters.

Effect of Well Spacing on Waterflood Recovery
Reservoirs Studied. For this Phase 1 study, 24 reservoirs were selected from Railroad Commission of Texas Bulletin 82. The purpose of this study was to use a publicly available data base to evaluate statistically the effect of well spacing on waterflood recovery. Table 1 lists the reservoir units and properties. Reservoirs developed on a five-spot pattern only were selected, to avoid the effect of different flood patterns on oil recovery efficiency and on the correlation. In this study, the well spacing was of primary concern; the effect of infill drilling on incremental recovery was not considered. The reservoir and process data were obtained mainly from Bulletin 82. The data were adjusted and updated with additional data gathered from Railroad Commission of Texas dockets and from the literature. The reservoirs studied are located in the region on the north end of the central basin platform and Midland basin and south of the Matador Arch, as shown in Fig. 1. The pays of these reservoirs are in the lower part of the San Andres formation. The lithology is composed of dolomite, anhydrite, siltstone, and salts. The depositional sequences are cyclic in nature. The component facies of each cycle are thin and laterally discontinuous. The heterogeneity of the reservoir rocks and the discontinuity of the pay Sections are very favorable for infill-drilling operations to improve waterflood recovery.
Correlation of Waterflood Recovery With Well Spacing. Table 2 shows the oil recovery and the well spacing of the 24 units studied. A series of least-squares fittings was made to correlate waterflood recovery with each reservoir and process parameter, which included productive area, net pay, porosity, permeability, gravity, flow capacity, and well spacing. Results showed that the correlation with all parameters except well spacings was very poor. The waterflood recovery showed a correlation trend with well spacing. Two correlation equations were developed with a least-squares fitting program.
JPT
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