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Abstract

This study is part of the Department of Energy's Eastern Gas Shales research whose primary objective is to increase the gas reserves for the Devonian shales in the Appalachian, Illinois, and Michigan Basins. The study simulated the effect of using horizontal wells to increase the recovery efficiency of shale gas from two specific sites: Wayne County, West Virginia, where vertical well gas production has been historically high and no permeability anisotropy is thought to exist, and Meigs County, Ohio, an area with a history of moderate gas production and a calculated permeability anisotropy of approximately 8:1.

In this study, a three-dimensional, dual-porosity reservoir simulator was used to characterize the study areas after sensitivity analyses were made to determine those parameters significant in determining gas production profiles. Once the study areas were characterized, the 20-year production profile for a 2,000-foot (610-meter) horizontal well was simulated for three well locations in Wayne County and one location in Meigs County. The performances for several vertical wells were also simulated and compared with those for their corresponding horizontal wells in Wayne County. Results of the simulation showed that a horizontal well could produce seven to ten times more gas than a vertical well placed at the same location for the Wayne County site. In the Meigs County area, the study showed that permeability anisotropy is an important factor in determining the orientation that a horizontal well should be drilled. Furthermore, the study concluded that horizontal wells are more efficient than vertical wells in producing Devonian shale gas from a fixed volume of rock.

Introduction

The U.S. Department of Energy (DOE) has been conducting research to determine economical ways of producing natural gas from Devonian shale for over 10 years. In support of this research, the United States Geological Survey (USGS) estimated the in-place gas of the Appalachian Basin Devonian shales to be 577-1,100 Tcf (1) (1.6 x 10-3.1 x 10m) with 85-160 Tcf (2.4 x 10-4.5 x 10M) located in areas of historical shale gas production. As of 1976, only about 3 Tcf (8.5 x 10m) of this large resource had been produced by about 10,000 vertical wells (2). Gas production from the shales has not increased substantially since this time although the number of shale wells is now in excess of 12,000.

Earlier efforts by DOE (3) showed that only 10-20 percent of the available shale gas was being produced with stimulated vertical wells. Analysis of shale gas production mechanisms indicated that an increase in the amount of surface area connected to the borehole by fractures could cause more of the adsorbed gas to be released and produced over the entire life of the well. This potential increase in recovery efficiency was thought to be achievable using a directionally drilled horizontal well to cross natural fractures orthogonally. Afterwards the horizontal well would be stimulated to increase the surface area in contact with the borehole. This study was undertaken to investigate the feasibility of using horizontal wells in Devonian shales.

Candidate Sites for Horizontal Wells

Two areas having different geologic properties were selected from a site selection screening survey that was completed in August 1986. More details on the Wayne County horizontal well site selection activity are described in SPE 16410 (SPE/DOE Low Permeability Reservoirs Symposium, Denver, Colorado, May 1987). Within the geologically most favorable area, reservoir simulation was used to determine where in the Wayne County area (Figure 1, Wilsondale and Webb Quadrangles) a 2,000-foot (610-meter) horizontal shale well should be drilled to measure the effect of horizontal drilling on recovery efficiency.

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