Increasing Fracture Path Complexity and Controlling Downward Fracture Growth in the Barnett Shale

Abstract

Abstract 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.