Drilling a 2,000-ft Horizontal Well in the Devonian Shale

Abstract

This paper discusses drilling a 2,000-foot horizontal well in the Devonian Shale, Wayne County, West Virginia, to test the concept that multiple hydraulic fractures from a horizontal wellbore can increase gas recovery efficiency over vertical stimulated boreholes. Discussion focuses on the air/mist drilling, wireline probes, and bottomhole assemblies that were used to drill the well, The target was a 50-foot zone located at a depth of 3,400 feet. Total hole length was 6,020 feet. The angle-building section was achieved using a 4.25 degrees/100-foot design to reduce the risks associated with casing installation in the horizontal section. Directional control methods proved to be unreliable in an air-drilling environment. Bottomhole assembly performance was heavily dependent on motor life and lithologic type. The completion program for the well consisted of installation of 4 1/2-inch casing with external casing packers and port collars. This completion string was installed in 2,000 feet of open hole section that was air drilled. Both geophysical well logs and a borehole television camera survey were used to design the casing string so that shale intervals could be isolated for testing and evaluation before and after stimulation. Background The drilling of directional wells and even horizontal wells to augment oil and gas production goes back to at least 1944 in the Appalachian Basin wherein a horizontal well was drilled from a 500-foot deep shaft in the Franklin Heavy Oil Field in Venango County, Pennsylvania, to improve oil recovery. Several hundred feet of horizontal core was taken during the drilling operations in the Venango Sand to characterize the reservoir. The Federal Government has been investigating the application of high-angle drilling to improve reservoir access in tight formations for more than 20 years. The focus of much of this research was to determine the properties of earth fracture systems in productive Devonian shale reservoirs and to develop improved techniques for recovering hydrocarbons with increased efficiency. The value of high-angle drilling perpendicular to natural fracture systems for maximizing production from fractured reservoirs was recognized early. Several field tests have been conducted to investigate the concept. In 1972, a high-angle borehole was drilled in Mingo County, West Virginia, to a measured depth of 4,678 feet with an average deviation of 41" from vertical through the Devonian shale section. After establishing the feasibility of drilling high-angle wellbore using air, a subsequent well was drilled to 53 degrees in the Cottageville Field, Jackson County, West Virginia. Total measured depth of that well was 4,736 feet. Experience from these two directional wells using mud motors on air identified many limitations of economic directional drilling. In DOE's latest experience, an oriented core was obtained from a Meigs County, Ohio, Devonian shale directional well to determine natural fracture spacings. High-angle air-drilling experience using downhole motors is not well documented in the petroleum literature. The most recent experience in air-drilled high-angle wells is the Grand Canyon Directional Drilling and Waterline Project which was accomplished using air-driven motors and wireline steering probes. Conventional wellbore sizes and tools were used to achieve a 71 degrees hole angle. Recent reservoir modelling studies used to estimate recovery efficiency of a Devonian shale horizontal well show a two- to three-fold increase in gas reserves per unit volume of reservoir. P. 291

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