Stimulating Unconventional Reservoirs: Maximizing Network Growth While Optimizing Fracture Conductivity

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 supports both the modelling 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 [Kuuskraa(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.

A critical evaluation of unconventional gas recovery from the marcellus shale, northeastern United States

“…This schedule was designed for a vertical well; horizontal wells typically use between four and eight times more water. Hydraulic fracture stimulation in gas shales typically increases fracture conductivity to between 0.5 and 10 md-ft, which represents an overall increase in formation permeability between 1 and 7 orders of magnitude, depending on the width of the fractures; larger permeability values lead to greater gas recovery (Warpinski et al, 2008).…”

Numerical study of the effect of uneven proppant distribution between multiple fractures on shale gas well performance

“…Proppants such as sand and ceramic with small size are injected with the fluid into the fractures, which can hold fractures open to provide a conductive path for fluid flow from reservoir to wellbore. Multiple long hydraulic fractures with uniform proppant distribution and sufficient fracture conductivity play an important role in achieving effective well stimulation and economic production of shale reservoirs [1][2][3][4][5][6][7]. Cipolla et al [8] studied the effect of proppant distribution in the fracture network on well performance and showed that proppant distribution significantly affects the fracture network conductivity and treatment design.…”

Review: Mathematical expressions for estimating equivalent permeability of rock fracture networks

“…Hydraulic fracturing has been widely implemented for the exploitation of oil, gas, and geothermal resources (Hubbert and Willis 1957;Warpinski et al 2009;Beckwith 2010). The current numerical simulations mainly focused on the propagation of single fractures (Paris and Erdogan 1963;Hoek and Bieniawski 1965;Germanovich et al 1994;Olson 2004;Yang et al 2013) and several fractures (Bobet and Einstein 1998;Sagong and Bobet 2002;Yang et al 2012;Li J et al 2013), yet investigating the hydraulic fracturing behavior of complex fracture networks is still a great challenge because of the complex rock-fluid mechanisms involved and the time-consuming computation (Zhao et al 2014b).…”