Although high gas flow rates from shales are a relatively recent phenomenon, the knowledge bases of shale-specific well completions, fracturing and shale well operations have actually been growing for more than three decades and shale gas production reaches back almost one hundred ninety years. During the last decade of gas shale development, projected recovery of shale gas-in-place has increased from about 2% to estimates of about 50%; mainly through the development and adaptation of technologies to fit shale gas developments. Adapting technologies, including multi-stage fracturing of horizontal wells, slickwater fluids with minimum viscosity and simultaneous fracturing, have evolved to increase formation-face contact of the fracture system into the range of 9.2 million m2 (100 million ft2) in a very localized area of the reservoir by opening natural fractures. These technologies have made possible development of enormous gas reserves that were completely unavailable only a few years ago. Current and next generation technologies promise even more energy availability with advances in hybrid fracs, fracture complexity, fracture flow stability and methods of re-using water used in fracturing. This work surveyed over 350 shale completion, fracturing and operations publications, linking geosciences and engineering information together to relay learnings that will identify both intriguing information on selective opening and stabilizing of micro-fracture systems within the shales and new fields of endeavor needed to achieve the next level of shale development advancement.

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 pre-2008 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 state-by-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.

Do oil and gas wells leak to the environment? The great majority of wells do not pollute. The purpose of this paper is to explain basic concepts of well construction and illustrate differences between single barrier failure in multiple barrier well design and outright well integrity failure that could lead to pollution, using published investigations and reviews from data sets of over 600,000 wells worldwide. For US wells, while individual barrier failures (containment maintained and no pollution indicated) in a specific well group may range from very low to several percent (depending on geographical area, operator, era, well type and maintenance quality), actual well integrity failures are very rare. Well integrity failure is where all barriers fail and a leak is possible. True well integrity failure rates are two to three orders of magnitude lower than single barrier failure rates. When a series of barriers fail and a leak path is formed, gas is the most common fluid lost. Common leak points are failed gaskets or valves at the surface and are easily and quickly repaired. If the failure is subsurface, an outward leak is uncommon due to lower pressure gradient in the well than in outside formations. Subsurface leaks in oil wells are rare and are routinely exterior formation salt water leaking into the well towards the lower pressure in the well. Failure frequency numbers are estimated for wells in several specific sets of environmental conditions (location, geologic strata, produced fluid composition, soils, etc.). Accuracy of these numbers depends on a sufficient database of wells with documented failures, divided into: 1) barrier failures in a multiple barrier system that do not create pollution, and 2) well integrity failures that create a leak path, whether or not pollution is created. Estimated failure frequency is only for a specific set of wells operating under the same conditions with similar design and construction quality. Well age and era of construction are variables. There is absolutely no one-size-fits-all well failure frequency.

Summary Do oil and gas wells leak to the environment? This paper will show the great majority of wells do not pollute. The purpose of this paper is to explain basic concepts of well construction and illustrate differences between single-barrier failure in multiple-barrier well design and outright well-integrity failure that could lead to pollution by use of published investigations and reviews from data sets of more than 600,000 wells worldwide. For US wells, while individual-barrier failures (containment maintained and no pollution indicated) in a specific well group may range from very low to several percent (depending on geographical area, operator, era, well type, and maintenance quality), actual well-integrity failures are very rare. Well-integrity failure occurs when all barriers fail and a leak is possible. True well-integrity-failure rates are two to three orders of magnitude lower than single-barrier-failure rates. When one of these rare total-well-integrity failures occurs, gas is the most common fluid lost. Common final-barrier leak points are failed gaskets or valves at the surface and are easily and quickly repaired. If the failure is in the subsurface, an outward leak is uncommon because of a lower pressure gradient in the well than in outside formations. Subsurface leaks in oil and gas wells are rare, and routinely comprise exterior-formation salt water leaking into the well toward the lower pressure in the well. Failure frequencies are estimated for wells in several specific sets of environmental conditions (i.e., location, geologic strata, produced-fluid composition, and soils). Estimate accuracy depends on a sufficient database of wells with documented failures, divided into (1) barrier failures in a multiple-barrier system that did not create pollution, and (2) well-integrity failures that created a leak path, whether or not pollution was created. Estimated failure-frequency comparisons are valid only for a specific set of wells operating under the same conditions with similar design and construction quality. Well age and construction era are important variables. There is absolutely no universal definition for well-failure frequency.

Creating an optimum hydraulic fracture to produce gas from nano-darcy shale uses much different technology than that commonly used in low permeability sandstone or carbonate reservoirs. Potentially productive natural fracture pathways are often present but thought to be seldom open in most shales. Effective stimulation in shales requires that the fracture creates an extensive, interconnected and stable flowing network of these natural fractures within the shale, without penetrating into water bearing zones below the shale. Shale wells capable of producing gas at several million scf/d, require development of a fracture pathway with shale face contact of an estimated five to ten million square feet (about one million square meters). Use of downhole microseismic during fracturing has defined sound patterns that confirm the "fracture flow path" in these stimulated shale wells is actually linking intersecting natural fractures into a network of fractures with a flow path width of several hundred feet. These flow paths often have patterns of fracture growth at right angles to the primary fracture direction. Examples of these right angle secondary fractures and the growth cloud of microseismic events that define the complex fracture network are shown in the paper. During extensive fracturing, downward fracture growth can penetrate the carbonates that are found beneath nearly all gas shales. Fracturing into water-wet lower reservoirs can reduce fracture efficiency and severely limit production by flooding the well with water. Effective methods of curtailing this downward frac growth are absolutely essential. This paper reviews some of the prior work on network or complex fracture work in shale and discusses some of the science behind creating the complex fracture network development. The paper also describes the follow-up work using both RA and chemical tracers that helped optimize the flow paths. Taken together, the development of a very large contact area in the gas-bearing shale (increased shale complexity) and the control of downward growth of the fractures can lead the effort to economically develop and extend the present edges of the Barnett Shale play. Introduction Although the title of the paper involving developing fracture complexity and preventing detrimental downward fracture growth might seem an odd combination, the subjects are often intimately related. Most shales contain hydrocarbons, but few shales are unconventional gas reservoir candidates. Total organic content, hydrocarbon generation maturity, water saturation, shale thickness and depth of burial (from a thermal maturity, gas pore volume and pressure standpoint) are among the major geological and geochemical gas generation and preservation influences ; however, creation of interconnecting natural fracture pathways while maintaining the overall seal of the reservoir enclosure are the keys to creating candidates for unconventional reservoirs.