Numerical Simulation Results in the Carthage Cotton Valley Field
Authors: Meehan D. Nathan , Pennington Ben F. 
Published: December 2007

Summary
Numerical simulation results for two Cotton Valley wells are presented. A three-dimensional, radial, dry-gas model was used to simulate multiple-layer, very low permeability reservoirs. Examples of inputs and modifications to the model from various tests and techniques are presented. These include the use of radioactive sand tracers during fracturing to determine fracture height, pressure buildup analysis, production logging, well log analysis, core analysis, and detailed geological mapping.
The final permeabilities used in the model were much lower than those estimated from core analysis. Both conventional methods and more recent type-curve methods were used to analyze long-term pressure buildups and drawdowns to estimate fracture length, fracture conductivity, and formation permeability. Indicated reserves from the simulation study are higher than presimulation estimates. Infill drilling will be required to drain this very low permeability reservoir adequately and efficiently. Information derived from individual well simulations is being used to optimize well spacing and fracture length.

Introduction
The Cotton Valley group of producing formations covers many counties in east Texas. These low permeability sandstones and limes were classified as tight gas by the U.S. Federal Energy Regulatory Commission Oct. 24, 1980, significantly increasing the maximum legal price that can be paid for the gas. Improved hydraulic fracturing technology and higher prices have resulted in dramatically increased production from these formations. For example, the Carthage (Cotton Valley) field produced 70,892 MMcf in 1980, up from 2,193 MMcf in 1976.
Champlin Petroleum Co. operates 34 wells in the Carthage (Cotton Valley) field in Panola County, TX. These wells have been massively fractured in multiple-stage jobs. treating essentially the entire Cotton Valley interval. All the wells completed to date are commercial, and are generally more productive than the typical well in the field. The reservoir under Champlin's acreage near the crest of the Carthage Cotton Valley structure is better quality than much of the field. Champlin also opens the entire Cotton Valley interval as opposed to fairly limited intervals perforated by other operators. A summary of Champlin's typical stimulation treatment is given in the Appendix.
Champlin began engineering studies of the Cotton Valley formation early in the development of the field. The basic goals of these studies includedimproved forecasts of Cotton Valley reserves and long-term deliverability,optimizing well spacing and fracture length, anddeveloping a better method of analyzing these wells.
To this end, Champlin conducted pressure buildups, performed a variety of pressure transient analyses, cored several wells, analyzed a variety of open hole and production logs, and performed three-dimensional numerical simulations. This paper presents some of the results of these studies.

History and Geology
Carthage field is located in northeastern Texas. approximately 25 miles from Louisiana, and is one of the largest gas condensate fields in the U.S, The field covers most of Panola County and is approximately 45 by 20 miles on the northwest-southeast axis. The field was discovered in 1936. The two main producing zones are the Upper and Lower Pettit, with significant Travis Peak production.
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