Stimulating Unconventional Reservoirs: Maximizing Network Growth While Optimizing Fracture Conductivity

Abstract

Abstract Unconventional reservoirs such as gas shales and tight gas sands require technology-based solutions for optimum development. The successful exploitation of these reservoirs has relied on some combination of horizontal drilling, multi-stage completions, innovative fracturing, and fracture mapping to engineer economic completions. However, the requirements for economic production all hinge on the matrix permeability of these reservoirs, supplemented by the conductivity that can be generated in hydraulic fractures and network fracture systems. Simulations demonstrate that ultra-low shale permeabilities require an interconnected fracture network of moderate conductivity with a relatively small spacing between fractures to obtain reasonable recovery factors. Microseismic mapping demonstrates that such networks are achievable and the subsequent production from these reservoirs support both the modeling and the mapping. Tight gas sands, having orders of magnitude greater permeability than the gas shales, may be successfully depleted without inducing complex fracture networks, but other issues of damage and zonal coverage complicate recovery in these reservoirs. As with the shales, mapping has proved itself to be valuable in assessing the fracturing results. Introduction Unconventional reservoirs provide a significant fraction of gas production in North America and increasing amounts in some other regions of the world. Such reservoirs include tight gas sands, coalbed methane (CBM), and gas shales; in 2006 these reservoirs provided 43% of the US production of natural gas (Kuuskra 1). Because of their limited permeability, which is foremost among many other complexities, some type of stimulation process (and/or dewatering in the case of CBM) is required to engender economic recovery from wells drilled into these formations. The focus of this paper is on gas shales, with particular emphasis on how these reservoirs perform relative to tight gas sands. The important role of natural fractures in both the stimulation and production processes, the importance of conductivity in the developed fracture or fracture system, and the critical influence of the matrix permeability are investigated using both mapping and modeling results. Gas shales, such as the Barnett, Fayettville, and Woodford in North America, are relatively recent plays, but gas production from shales has occurred since the early 1900's from the Devonian shales of eastern North America and more recently from the Antrim shale and others. These shales 2 typically contain a relatively high total organic content (e.g., the Barnett has a total organic content of 4–5%) and are apparently the source rock as well as the reservoir. The gas is stored in the limited pore space of these rocks (a few per cent, including both matrix and natural fractures) and a sizable fraction of the gas in place may be adsorbed on the organic material. Matrix permeabilities of these shales are extremely difficult to measure because they are so low, but various approaches to determine their value have yielded permeabilities on the order of 1–100 nanodarcies. Clearly, economic production cannot be achieved without an enormous conductive surface area in contact with this matrix, either through existing natural fractures or the development of a fracture "network" during stimulation. Economic production would then also rely on the existence or development of sufficient conductivity within this network.