Stimulation of Gas Shales: They're All the Same—Right?

Abstract

Abstract In recent years, the upswing in drilling activity and the higher price of natural gas has caused the industry to focus more on non-conventional gas reservoirs. Heading up this list are gas shales. This peaked interest was brought on not only by an increase in natural gas prices, but also the apparent abundance of potential gas shales present in the continental United States and the well publicized success of the Barnett Shale of North Texas. In order to obtain commercial production from these low permeability shale reservoirs, fracture stimulation is required. Numerous technical papers have been published on the merits and success of stimulation in this well documented reservoir. While slickwater treatments have become the stimulation method of choice, it is important to note that several other stimulation and completion techniques have been employed that have resulted in economic and commercially viable Barnett Shale producers over the last 20 years. Although there has been tremendous success with slickwater fracture treatments, many believe this technology can be universally applied to all gas shale reservoirs. While this may be the case in the Barnett Shale it is not necessarily true for other prospective shale basins being explored. This paper will include a brief history of Barnett Shale stimulation practices and go on to address shale reservoirs and characteristics that make each one unique from a stimulation perspective. It is not the attempt of the authors to provide a stimulation checklist, but to identify a number of shale reservoir characteristics and their effect on the process of stimulation optimization. Case histories in the Ft. Worth Barnett and Permian Basin Barnett and Woodford Shales will be used to infer key differences in reservoir attributes and how these differences influence stimulation decisions. Introduction to gas Shales Natural gas production from shale reservoirs is commonly thought to be a recent phenomenon. However, the earliest known production occurred in the Devonian shale of Fredonia, New York in 1821 and more substantial gas production was recorded in the late 1880's in the Appalachian region again in Devonian gas shale. The Barnett Shale of the Fort Worth Basin has received the most attention in recent years as its explosive production growth has brought it into prominence as the largest gas producing field in Texas with current production in excess of 1.6 Bcf/d. The success of the Barnett Shale and strong gas prices have spurred exploration in many new gas shale plays across the United States. Figure 1 shows the gas shale basins and plays across the continental United States. As knowledge of the Barnett shale has matured over the first 25 years of its productive life, it has provided many insights and benchmarks by which to judge other basins for their prospective gas shale reservoirs. The Barnett demonstrates the significance of shale hydrocarbon content, frac barriers and the structural environment. Variations in these three major geological and geochemical variables across the Fort Worth Basin greatly impact the well plan and specific engineering designs required to maximize its productive potential. East Texas Barnett Shale The Barnett Shale of North Texas is a Mississippian-age marine shelf deposit that exhibits variations in mineralogical and geochemical properties. It is described as a black, organically rich, fine-grained shale1. The depth and thickness range from 3500 ft and 150 ft in Erath County to over 8000 ft and 1000 ft in Denton county, respectively. As with all gas shales, the quality of the reservoir is dependent on many factors such as kerogen type, total organic content (TOC), thermal maturity (measured by vitrinite reflectance), gas content, mineral content and traditional petrophysical properties. The North Texas Barnett has provided excellent data resources to define the favorable geochemical attributes for gas productive shale as well as the transition area into oil productive shales that are not as thermally mature. The silica-rich Barnett has proven to be conducive to hydraulic fracturing due to its mechanical properties and mineral composition. Early field development focused on an area that was very environmentally friendly in that the shale was thick and well bounded by hard, dense limestone, both above and below, and the structural setting was relatively calm with very minor faults, if any, and no karsts. Once its productive capability was firmly established and the limits of the "core area" were developed with vertical wellbores, the more challenging environments of the Barnett were explored where fracture boundaries were weak and faults/karsts communicated with a wet Ellenberger (Dolomite) below. With proven economic success in these challenging environments, horizontal completions have become the standard throughout the basin.