Natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low-porosity and low-permeability shale host rock. Many shale outcrops, cores, and image logs contain fractures or fracture traces, and microseismic event patterns associated with hydraulic-fracture stimulation have been ascribed to natural fracture reactivation. Here we review previous work, and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization, orientation, and size patterns. A wide range of shales have a common suite of types and configurations of fractures: those at high angle to bedding, faults, bed-parallel fractures, early compacted fractures, and fractures associated with concretions. These fractures differ markedly in their prevalence and arrangement within each shale play, however, constituting different fracture stratigraphies—differences that depend on interface and mechanical properties governed by depositional, diagenetic, and structural setting. Several mechanisms may act independently or in combination to cause fracture growth, including differential compaction, local and regional stress changes associated with tectonic events, strain accommodation around large structures, catagenesis, and uplift. Fracture systems in shales are heterogeneous; they can enhance or detract from producibility, augment or reduce rock strength and the propensity to interact with hydraulic-fracture stimulation. Burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated. The role of microfractures in production from shale is currently poorly understood yet potentially critical; we identify a need for further work in this field and on the role of natural fractures generally.

Natural fractures in shale: A review and new observations

Identification of risk, the potential for occurrence of an event and impact of that event, is the first step in improving a process by ranking risk elements and controlling potential harm from occurrence of a detrimental event. Hydraulic Fracturing has become a hot environmental discussion topic and a target of media articles and University studies during development of gas shales near populated areas. The furor over fracturing and frac waste disposal was largely driven by lack of chemical disclosure and the pre2008 laws of some states. The spectacular increase in North American natural gas reserves created by shale gas development makes shale gas a disruptive technology, threatening profitability and continued development of other energy sources. Introduction of such a disruptive force as shale gas will invariably draw resistance, both monetary and political, to attack the disruptive source, or its enabler; hydraulic fracturing. Some ―anti-frack‖ charges in media articles and university studies are based in fact and require a stateby-state focused improvement of well design specific for geology of the area and oversight of overall well development. Other articles have demonstrated either a severe misunderstanding or an intentional misstatement of well development processes, apparently to attack the disruptive source. Transparency requires cooperation from all sides in the debate. To enable more transparency on the oil and gas side, both to assist in the understanding of oil and gas activities and to set a foundation for rational discussion of fracturing risks, a detailed explanation of well development activities is offered in this paper, from well construction to production, written at a level of general public understanding, along with an initial estimation of frac risk and alternatives to reduce the risk, documented by literature and case histories. This discussion is a starting point for the well development descriptions and risk evaluation discussions, not an ending point. Introduction to Risk There are no human endeavors without risk. ―Risk management is the identification, assessment and prioritization of risks followed by coordinated and economical application of resources to minimize, monitor and control probability and/or impact of unfortunate effects‖ (Wikipedia). Managing these risks and communicating both risks and changes to reduce risks are part of an often repeated approach termed ―license to operate‖ (Liroff, 2011). At a minimum, basic risk concerns are: People, Economic Loss (to all concerned), Environmental Damage and Reputation Loss. Figure 1, a standard loss matrix used by Apache Canada in the Horn River development (DeMong, 2010), is a good starting place for the discussion. Consequences run from slight and practically unavoidable to severe and avoidable at all costs. This paper, for purposes of brevity, will focus solely on risk to the environment from hydraulic fracturing operations, starting with transport of materials and ending when the well is routed to the production facilities and gas sales begin. The form of Figure 1 will be expanded and comments and

Hydraulic Fracturing 101: What Every Representative, Environmentalist, Regulator, Reporter, Investor, University Researcher, Neighbor and Engineer Should Know About Estimating Frac Risk and Improving Frac Performance in Unconventional Gas and Oil Wells

Reservoir Modeling in Shale-Gas Reservoirs

Practical Solutions for Pressure-Transient Responses of Fractured Horizontal Wells in Unconventional Shale Reservoirs

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Development of an Efficient Embedded Discrete Fracture Model for 3D Compositional Reservoir Simulation in Fractured Reservoirs

Comparison of Fractured-Horizontal-Well Performance in Tight Sand and Shale Reservoirs

In this paper point-source solutions are derived in the Laplace-transform domain and an extensive library of solutions is documented to obtain pressure distributions and well responses for a wide variety of wellbore configurations: partially penetrating vertical wells, horizontal wells, and fractured wells (complete or limited entry). Wells may be located in infinite or bounded systems (rectangular or circular reservoirs). Several combinations of closed and/or constant-pressure boundary conditions are considered at the vertical and lateral reservoir boundaries. These solutions may be used to examine homogeneous or naturally fractured reservoirs.

Erdal Ozkan

Papers

Practical Solutions for Pressure-Transient Responses of Fractured Horizontal Wells in Unconventional Shale Reservoirs

Comparison of Fractured-Horizontal-Well Performance in Tight Sand and Shale Reservoirs

New solutions for well-test analysis problems. Part 1; Analytical considerations

Pressure-Dependent Natural-Fracture Permeability in Shale and Its Effect on Shale-Gas Well Production

Modeling of Fluid Transfer From Shale Matrix to Fracture Network