Recovery Improvement Study of the Gray Sand Reservoir
Authors: Chen T.K. , Kennedy J.H. , McElhiney J.E. 
Published: December 2007

A study was performed to determine the optimum economic number and location of new wells necessary to maximize wet gas production subject to a 17-year estimated remaining gas plant life. The study uses an unsteady-state, single-phase, two-dimensional gas model, coupled with transient test analysis techniques, to provide a basis for the recovery improvement projections. projections. Introduction
This study was performed to determine the optimum economic number and location of new wells necessary to maximize wet gas production subject to a 17-year estimated remaining gas plant life. The produced gas contains approximately 60 bbl of liquid per MMscf; this liquid currently provides incremental income over and above that derived from the dry gas contract. Since field separation is considerably less efficient (9 bbl/MMscf) than that which the plant provides, it is desirable to improve or at least sustain provides, it is desirable to improve or at least sustain production while the plant is still available. production while the plant is still available. The saturation pressure for the gas in this reservoir is 1,900 psia. Over the range of pressures encountered in this study, liquid condensation is considered to be a wellbore phenomenon. Due to this fact, the reservoir study utilizes an unsteady-state, single-phase, two-dimensional gas model. Coupled with the modeling approach, transient test analysis techniques are used to provide a basis for the recovery improvement projections. The development of the pressure history projections. The development of the pressure history match, production well controls, and prediction rules are discussed. Utilizing economic indicators, the comparison of various alternative development cases is considered. The resulting direction taken following the study is described and conclusions as to its effectiveness are drawn. The model is similar to those described by Burns et al. and Henderson et al. This model is a single-phase, unsteady-state, two-dimensional gas model for multiwell systems. The pressure variable has been converted to real gas potential to linearize partially the equation of flow. After the model was pressure history matched using reservoir production data, a trial and error approach was used to provide new well locations. Analysis of buildup tests became an integral part of the history match and the production-prediction phase of the reservoir modeling production-prediction phase of the reservoir modeling study. After several prediction runs were made with various new well locations, economic calculations were made to determine which case was best. Although this is not an optimization in the truest sense of the word, the method did provide a "best case" situation.

Field History
The Gray sand reservoir is located in the Cotton Valley field, Webster Parish, La. It is operated by the Cotton Valley Operating Committee (CVOC). Major owners of this field are Hunt Oil Co., Ceja Corp., Bodcaw Co., Mobil Oil Corp., and Marathon Oil Co. The Gray sand, which is a member of the Lou-Ark group of upper Jurassic age, is encountered at depths ranging from 10,379 to 10,816 ft subsea. The Gray sand was discovered in 1944 with the drilling of the CVOC Gray No. 1. Eleven wells have been drilled through the Gray sand, resulting in eight producers and three dry holes. There are presently producers and three dry holes. There are presently seven wells producing from the Gray sand. To date, the cumulative production has been 87.6 Bscf.
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