Pre-stack HTI Velocity Variation with Azimuth (VVAZ) and Amplitude Variation with Azimuth (AVAZ) are employed to locate open fracture networks at the Wolfcamp by detecting changes in azimuthal moveout and amplitudes caused by long, narrow bands of fractures associated with seismically mapped shallow graben features (Cook, McKee and Bjorlie 2019). Vertical fracturing within the Wolfcamp located beneath graben features mapped at the Lamar limestone horizon have demonstrated a fracture stimulation connection to excessive extraneous water and H2S in Wolfcamp horizontal wells within the Reeves County portion of the Delaware Basin. The working model assumes the shallow grabens are the result of middle-late Tertiary tectonic extension with faulting terminating at various depths: often directly beneath the Lamar limestone, but sometimes into the Bone Spring and even the Wolfcamp.
Frac gradient data was spatially referenced by frac stage for 31 horizontal Wolfcamp wells located within a 290 square mile 3D dataset. Low frac gradients are observed for 11 of the 31 wells. Low frac gradients are defined as greater than −0.1 psi/ft departure from the individual well frac gradient trend. All of the 11 low frac gradient wells produced elevated levels of H2S and excessive water in quantities that had severe negative economic impact and, in several cases, significantly reduced hydrocarbon deliverability. The low frac gradient stages of all low frac gradient wells is aligned beneath the shallow graben features.
There are 9 horizontal Wolfcamp wells that crossed beneath the shallow graben features that did not record low frac gradients and did not produce extraneous water or elevated levels of H2S. Another 11 wells were drilled between the shallow graben features and did not record low frac gradients or excessive water or elevated H2S.
Seismic attributes were used to map the lateral and vertical extents beneath the shallow graben features. However, there remains a great deal of uncertainty in determining the degree of vertical faulting or fracturing beneath the shallow graben features.
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