Summary The evolution of fracturing technology has provided the industry with numerous advances, ranging from sophisticated fluid systems to tip-screenout designs to propagation modeling. Interestingly, these advances typically have focused on conventional designs that use a crosslinked-fluid system. However, as the development of unconventional (e.g., tight gas, shales, coalbed methane) or underpressured reservoirs has increased, so has the demand for innovative hydraulic-fracture designs. The most recent of these design changes has been the popular method of placing proppant with slickwater, linear gel, or hybrid treatments. Although our industry has significant expertise in fracture design, most of our experience has been in conventional crosslinked-fluid systems. However, there are many aspects of crosslinked-fluid design that either do not apply to slickwater treatments or, in some cases, are contrary to the requirements of slickwater treatments. This paper will begin by reviewing the motivation, benefits, and concerns with slickwater fracturing and discuss why this seemingly old method has regained popularity over conventional crosslinked designs in many reservoirs. In addition, the authors will detail some of the important theories related to slickwater fracturing, including fracture width and complexity, proppant transport and settling, and conductivity requirements. In each scenario, emphasis will be placed on the different strategy employed compared to crosslinked-fluid designs, and the mistakes or misunderstandings that are frequently made will be highlighted. Where appropriate, laboratory testing, field measurements, reference material, and other resources are presented to support the observations made by the authors. This paper will serve as a resource to any engineer or technician who is designing/pumping slickwater fracs, or who is considering this technology for potential application. By applying the concepts presented in this paper, engineers will be able to appropriately evaluate the potential benefits of using this technique in their completions, as well as draw on the experiences of others to take full advantage of this technology.

In many reservoirs fracture growth may be complex due to the interaction of the hydraulic fracture with natural fractures, fissures, and other geologic heterogeneities. The decision whether to control or exploit fracture complexity has significant impact on fracture design and well performance. This paper investigates fracture treatment design issues as they relate to various degrees and types of fracture complexity (i.e., simple planar fractures, complex planar fractures, and network fracture behavior), including the effect of fracture fluid viscosity on fracture complexity, proppant distribution in complex fractures, and fracture conductivity requirements for complex fractures. The impact of reservoir properties (including permeability, stress and modulus) on treatment design is also evaluated. The paper includes general guidelines for treatment design when fracture growth is complex. This includes criteria for the application of water-fracs, hybrid fracs, and crosslinked fluids. The paper begins with an evaluation of microseismic fracture mapping data that illustrates how fracture complexity can be maximized using low viscosity fluids, which includes an example of how microseismic data can be used to estimate the permeability and spacing of secondary or network fractures. The effect of proppant distribution on gas well performance is also examined for cases when fracture growth is complex, assuming that proppant was either concentrated in a primary planar fracture or evenly distributed in a fracture network. Examples are presented that show when fracture growth is complex the average proppant concentration will likely be too low to materially impact well performance if proppant is evenly distributed in the fracture network and un-propped fracture conductivity will control gas production. This paper also extends published conductivity data for un-propped fractures and embedment predictions for partially propped fractures to lower modulus rock to provide insights into fracture design decisions. Exploiting fracture complexity may not possible when Young's modulus is 2 x 106 psi or lower due to insufficient network conductivity resulting from asperity deformation and proppant embedment. Fracture conductivity requirements are examined for a wide range of reservoir permeability and fracture complexity. Reservoir simulations illustrate that the network fracture conductivity required to maximize production is proportional to the square-root of fracture spacing, indicating that increasing fracture complexity will reduce conductivity requirements. The reservoir simulations show that fracture conductivity requirements are proportional k1/2 for small networks and k1/4 for large networks, indicating much higher conductivity requirements for low permeability reservoirs than would be predicted using classical dimensionless conductivity calculations (Fcd) where conductivity requirements are proportionate to reservoir permeability (k). The results show that when fracture growth is complex, proppant distribution will have a significant impact on network conductivity requirement and well performance. If an infinite conductivity primary fracture can be created, network fracture conductivity requirements are reduced by a factor of 10 to 100 depending on the size of the network. The decision to exploit or control fracture complexity depends on reservoir permeability, the degree of fracture complexity, and un-propped fracture conductivity. The paper also examines the effect of fluid leakoff on maximum fracture area, illustrating potential limits for fracture complexity as reservoir permeability increases. Although the expected range of un-propped fracture conductivity is controlled by Young's modulus and closure stress, in many reservoirs it can be beneficial to exploit fracture complexity when the permeability is on order 0.0001 mD by generating large fracture networks using low viscosity fluids (water-fracs). As reservoir permeability approaches 0.01 mD, fluid efficiency decreases and fracture conductivity requirements increase, fracture designs can be tailored to generate small networks with improved conductivity using medium viscosity or multiple fluids (hybrid fracs). Fracture complexity should be controlled using high viscosity fluids and fracture conductivity optimized for moderate permeability reservoirs, on order 1 mD.

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.

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.

Summary In many reservoirs, fracture growth may be complex because of the interaction of the hydraulic fracture with natural fractures, fissures, and other geologic heterogeneities. The decision whether to control or exploit fracture complexity has significant impact on fracture design and well performance. This paper investigates fracture-treatment-design issues as they relate to various degrees and types of fracture complexity (i.e., complex planar fractures and network fracture behavior), focusing on fracture-conductivity requirements for complex fractures. The paper includes general guidelines for treatment design when fracture growth is complex, including criteria for the application of water-fracs, hybrid fracs, and crosslinked fluids. The effect of proppant distribution on gas-well performance is examined for cases when fracture growth is complex, assuming that proppant was either concentrated in a primary planar fracture or evenly distributed in a fracture network. Examples are presented that show that when fracture growth is complex, the average proppant concentration will likely be too low to materially impact well performance if proppant is evenly distributed in the fracture network and unpropped-fracture conductivity will control gas production. Reservoir simulations illustrate that the network-fracture conductivity required to maximize production is proportional to the square root of fracture spacing, indicating that increasing fracture complexity will reduce conductivity requirements. The reservoir simulations show that fracture-conductivity requirements are proportional k1/2 for small networks and k1/4 for large networks, indicating much higher conductivity requirements for low-permeability reservoirs than would be predicted using classical dimensionless conductivity calculations (FCD) where conductivity requirements are proportionate to reservoir permeability (k). The results show that when fracture growth is complex, proppant distribution will have a significant impact on network-conductivity requirement and well performance. If an infinite-conductivity primary fracture can be created, network-fracture-conductivity requirements are reduced by a factor of 10 to 100, depending on the size of the network. The decision to exploit or control fracture complexity depends on reservoir permeability, the degree of fracture complexity, and unpropped-fracture conductivity. It can be beneficial to exploit fracture complexity when the permeability is on the order of 0.0001 md by generating large fracture networks using low-viscosity fluids (water fracs). As reservoir permeability approaches 0.01 md, fluid efficiency decreases, and fracture-conductivity requirements increase, fracture designs can be tailored to generate small networks with improved conductivity using medium-viscosity or multiple fluids (hybrid fracs). Fracture complexity should be controlled using high-viscosity fluids, and fracture conductivity should be optimized for moderate-permeability reservoirs, on the order of 1 md.