The evolution of well completions in the Barnett Shale Reservoir of North Texas has moved into the realm of horizontal wells. This move was clearly evidenced by an increase from 10 permits in 2002 to more than 100 permits in 2003 to drill Barnett Shale horizontals. In late 2003 and into 2004, success with horizontal completions has possibly proliferated the most in the more economically challenging areas of the field (outside of Denton and Wise Counties) where the Viola "bottom" is poor to absent, making Ellenburger water a concern.
Conventional fracturing processes that were well defined in the vertical well stimulation arena have helped to shape the completion strategies of horizontal wellbores, such that operators generally assume that a large-volume, light-sand waterfrac stimulation will be a requirement. However, many are concerned that fracturing of uncemented liners may not provide adequate effective reservoir drainage in horizontal wells. At times, these concerns have pointed to a need for complicated and expensive multistage fracture treatments using cemented liners to get complete lateral zone coverage.
At least one Barnett Shale operator has made extensive use of microseismic fracture mapping during fracture stimulation of horizontal wells for both cemented and noncemented liner completions. Additionally, wells with cemented and noncemented casing have been completed using the relatively new hydrajet-fracturing technology and were microseismic mapped. This paper presents a comparison of varied completion techniques and what has been learned from these mapping efforts.
The Barnett Shale reservoir of North Texas is an ultra low-permeability reservoir that must be effectively fracture stimulated to obtain commercial production. This reservoir has sometimes been described as a "spent oil-prone source rock." Most of its porosity and permeability developed upon the thermal transformation of its organic matter from liquid to gas with resulting maturation-induced microfractures, with reported matrix pore radius values in the range of 0.005 microns. Much of the gas is stored in these microfractures, as well as having gas absorbed in the solid organic matter (kerogen) in the range of 40 to 60%, with TOC averaging 2.5%. The microfractures were possibly created when oil cracked to gas, with up to a 10-fold volume increase resulting from this reaction. The Barnett is the only known source rock on the east shelf of the Fort Worth Basin that is a source, reservoir, trap, and seal.
While the Barnett is classified as shale, it is more complex and not homogeneous. Where the shale is more dolomitic, there is less organic matter, and traditional log analysis is further complicated by the presence of minerals such as calcite, dolomite, and pyrite. Natural fractures are sometimes sealed with calcium carbonate. Some operators have reported that wells that show natural fractures in EMI logs tend to produce better. Gas shows on a mud log are sometimes a good indicator of permeability and gas storage. Additionally, many believe that Vitrinite reflectance may have a correlation to production potential of this Barnett Shale.
Through most of the North Texas area, NE to SW faults (or karsts) are predominate, and sometimes are believed to cause poor stimulation results, possibly by "stealing" much of the energy of a fracture stimulation treatment. Induced hydraulic fractures from stimulation will also mostly trend NE-SW, but the deviatoric stresses are not thought to be very high, resulting in significant cross-fracturing, which occurs during fracture stimulations. Denton and Wise Counties were home to nearly all successful Barnett Shale completions until the past few years, largely because of the presence of the Viola separating the Barnett and the Ellenburger, which is usually wet and often an aquifer. Success in outlying counties has been greatly enhanced by increased use of horizontal completions. With the very active drilling activity, in many cases horizontal completions are necessitated by urban considerations in most every county.