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Abstract

The effect of depressed oil and gas prices on investors' confidence has been reflected by the low activity levels in the Permian Basin as elsewhere. Yet, even during the 1986-87 period viable projects were available to the industrious and projects were available to the industrious and imaginative. One example is the Wentz (Clearfork) Field where a small independent operator recognized potential in what appeared to be a very marginal potential in what appeared to be a very marginal waterflood prospect. Through data gathering and engineering, the potential was documented to the point that investor funding became available. A point that investor funding became available. A cooperative waterflood plan was agreed to with the field's other operator. The project was installed with an emphasis on practicality, cost savings and attention to details. After a year of injection, production response is exceeding expectations. production response is exceeding expectations

Introduction

Most of the major carbonate oil reservoirs on the Central Basin Platform of the Permian Basin were discovered and developed during the 1940's and 1950's before modern logging tools at a time when reservoirs were usually visualized as "tanks". This lack of reservoir characterization led to inefficient well spacing and completion practices that became evident only after failure of some of the earlier waterflood projects. The problems of slow response, low recovery, early water breakthrough and injection out of zone led to the realization that a gross pay section consisted of many separate reservoirs in the form of stringers that had little or no communication except through wellbores. A successful waterflood requires that the main pay stringers contain sufficient input and output points spaced to efficiently sweep the productive area. As this concept gained productive area. As this concept gained acceptance, nearly all major secondary projects in the Permian Basin's carbonate trends were infill Permian Basin's carbonate trends were infill drilled from 40-acre to 20-acre spacing and to 10-acre on many EOR projects.

Reservoir characterization requires the identification of net pay by facies type and the determination of each facies' spatial distribution within the reservoir. The evolution of reservoir characterization in the Permian Basin is well documented in the major fields. There are hundreds of smaller carbonate reservoirs also created by the complex cyclic depositional and diagenetic processes that form highly stratified, heterogeneous producing intervals. Often each field has numerous independent operators, and consequently, has not received the indepth study that the majors are able to apply to the larger fields. One such field is the Wentz (Clearfork) Field in northeastern Pecos County, Texas, which produces from the Glorieta and Clearfork produces from the Glorieta and Clearfork formations of the Lower Permian between 2200' and 2550'. The field was discovered in 1953 and developed with 85 wells over 2000 acres (Figure 1). Nominal well density varies from 20 to 40 acres per well. Total production has been 2.7 MMBBL, which is only 31 MBBL per well recovery.

A majority of the leases at Wentz were put up for sale in 1986. The reservoir had never been waterflooded although large volumes of produced water had been disposed of into the pay zone with no significant effect. Many years of stripper operations had depleted the reservoir energy and maximized voidage. Not pay is scattered over 350' of a lithologically complex section. There were limited core data and no modern logs for quantitative analysis.

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