Atoka Sands (Pregnant Shale or Davis Sand) Play
Barnett Shale Analogy
The Barnett Shale play was slow to develop. Mitchell Energy held much of the area with shallower Boonsville field production and noticed that some of the deeper tests had strong gas shows while drilling through the Barnett. The initial attempts at completion were all economic failures (AAPG Explorer, July, 2002), but over time a solution for extracting natural gas with nitrogen (N2) or carbon dioxide (CO2) gas-assisted hydraulic fracturing treatment was implemented. Early play developments yielded an average estimated ultimate recovery (EUR) of 400 million cubic feet (MMCFG or 0.4 BCFG). Fear of reacting water with shale hindered the idea of using slick-water fracture treatments (using a surfactant agent and some sand as proppant), but in 1999, due to low-price economic conditions, a light sand slick-water frac (LSF) was tried with impressive results. Subsequent wells showed good results, and the new wells were projecting recoverable EURs of 1,000 MMCFG (1.0 BCFG). Re-fracs in older wells also yielded impressive results, and today some wells have seen as many as four stimulations with various frac jobs. Today, the average EUR is estimated at 1,400 MMCFG (1.4 BCFG).
Most of these new LSF-style frac jobs show increasing rates over the course of a few months, implying that the frac fluids are actually cleaning up over time, with production. There are various camps on the theory of why LSFs work. Irregardless, the Pregnant Shale appears to share many of the characteristics of the Barnett Shale. The only discernable difference with the Barnett is the presence of some thin laminations of fine grain silt and sand intermingled with the dense black shale. The worst rock in the Pregnant Shale interval looks similar to the typical Barnett.
Rather than investigate the recoverable reserves from a volumetric view point, this study takes a more conservative statistical view point to project future economic returns.