Abstract

A recent study has been conducted on the "Clinton" sandstone of Ohio by the U.S. Department of Energy. The purpose of the study was to identify production mechanisms as related to the geometry of the "Clinton" sandstone. The theory behind the study was that a log-log plot of cumulative production versus time is indicative of the flow behavior in the reservoir. A 0.5 slope would indicate linear flow exists while a slope of about 0.90 would indicate radial flow exists.

Data from 284 producing "Clinton" gas wells from six counties in eastern Ohio were used in this study. Log-log plots of cumulative production versus time were generated and then the slope values were contoured so that a comparison could be made with regional geological parameters. The results of the study showed that the production mechanisms in the four regions are causing a linear to "intermediate" (between linear and radial) flow. Radial flow was exhibited in wells where the "Red Clinton" was better defined on the gamma ray log, suggested an offshore bar or shoal. Linear flow was exhibited in wells where the "White Clinton" was more well developed suggested channel or distributary bars.

The intent of this paper is to make available to industry a practical approach to analyze the performance of past production. This knowledge can performance of past production. This knowledge can be used to gain insight to the factors affecting recovery so that an approach to improvements can be made.

Introduction

Production of natural gas from many "Clinton" sandstone wells has often been disappointing in the past. The chances of drilling an economic well, past. The chances of drilling an economic well, one that would pay back within 3 to 4 years, was fairly poor (assuming a 30 percent rate of return). Today, with increased costs of drilling and hydraulic stimulation, the risk of drilling an uneconomic well is further increased.

Although thousands of wells have been drilled in the "Clinton" sandstone and many studies have been conducted, reservoir characteristics and production mechanism are still not clearly understood. One noteworthy study involved the collection and establishment of a 328-well data set from the East Ohio Gas Company by researchers at the Morgantown Energy Technology Center (METC). An independent statistical analysis of this data set by TRW, under contract to the Gas Research Institute, produced an empirical equation to predict 5 -year production. This equation, however, was strictly statistical and did not consider reservoir characteristics nor production mechanisms. Also, there was an apparent discrepancy as to the use of the equation as a predictor. The entire data set was used to derive the empirical equation which was then checked with a portion of the same data set; obviously a high correlation resulted.

In an attempt to resolve the question of "Clinton" gas production, the Department of Energy re-examined the 328-well data set, gas production data and completion records from 284 Clinton gas wells in eastern Ohio were used. This recent study considered the relationship between production behavior and reservoir flow geometry (linear or radial) as suggested by Dr. C.F. Knutson at the Department of Energy's Unconventional Gas Recovery Simulation Workshop in October 1982.

Geology of the Clinton

The "Clinton" sandstone is a basal Silurian unit that has produced natural gas and oil in the Northern Appalachian Basin for over 90 years. The "Clinton" sandstone is the driller's name referring to the Thorold (" Stray Clinton"), Grimsby (" Red Clinton"), and Cabot Head (" White Clinton"), sequences of interbedded sandstones, siltstones, and shales in eastern Ohio (Figure 1). These sandstones are composed primarily of quartz with feldspar, calcite, and clays primarily of quartz with feldspar, calcite, and clays as secondary minerals. Most "Clinton" sandstone reservoirs are well cemented.

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