A Tight Gas Field Study: Carthage (Cotton Valley) Field

Abstract

SPE Members Introduction Evaluation of the need for infill drilling in complex, low-Permeability, gas reservoirs requires the application of advanced technology. Use of typical engineering tools such as conventional decline curve analysis, conventional pressure transient analysis, and single layer reservoir descriptions often create inaccurate results and misleading conclusions. Despite the accuracy of advanced analysis techniques, these techniques are typically not performed on a well-by-well basis in field-wide studies because the cost cannot be justified. As a result, accurate conclusions, valid for all areas of a field, are not obtained. In this paper we present a practical means of applying advanced analysis techniques to an entire field. We describe a powerful statistical method for dividing the reservoir into areas of like productive behavior. This method provides an unbiased means of comparing well performance, selecting areas for advanced analysis, and defining the areal locations where specific conclusions apply. Emphasis is placed on incorporating a sound geological and petrophysical description, and evaluating the consistency of reservoir descriptions developed through independent geological and reservoir engineering techniques. This analysis technique was used to evaluate the infill drilling potential of the Carthage (Cotton Valley) field in east Texas. The Carthage field, discovered in 1968, is a thick, layered, low-permeability gas reservoir underlying 250,000 acres in Panola County, Texas. See Fig. 1. At the time of this study, there were approximately 900 wells producing 400 million cubic feet and 300 barrels of condensate per day. The Carthage Cotton Valley sand has been classified as "tight gas" by the FERC. Prior to this study, there were three periods of intense drilling activity corresponding to authorizations of 640-, 320-, and 160-acre drilling densities. Carthage is currently experiencing a fourth active drilling period, in which numerous wells are being drilled on 80-acre spacing in selected areas of the field. GEOLOGY The geological study provided a basis for the understanding of field and well performance. Because the objective of this study was to evaluate current well performance and extrapolate our analysis to infill locations, it was critical to understand the internal structure of the reservoir and those factors which control thickness, permeability, porosity and continuity within the reservoir. The geologic study concluded that the depositional environment and subsequent diagenesis resulted in extensive compartmentalization throughout the field. While some intervals could be expected to be more continuous than others, permeability barriers and baffles exist throughout the reservoir. Sand packages which can be correlated across significant distances within the field are comprised of thin sand/shale sequences and very fine laminations which are impossible to detect on conventional logs. Severe heterogeneity is apparent both vertically and laterally. P. 35^