Delaware Basin Horizontal Wolfcamp Case History: H2S and excessive extraneous water linked to shallow seismically mapped features
Shallow Lamar limestone graben features are seismically mapped across a 290 square mile 3D seismic area. The grabens are 1000-4000 feet in width and can extend more than 25 miles across this area. The orientation of these linear features rotates from N104E in the northern mapped area to N136E in the southern mapped area. Uncertainty remains in determining the vertical fracturing extents below these graben features. In some cases, vertical faulting appears to be contained within the Delaware Mountain Group while other cases extend the vertical faulting into the Bone Spring intervals and even into the Wolfcamp. Frac gradient data from more than 31 horizontal Wolfcamp wells was spatially located within the 3D seismic dataset. Low frac gradients correlate to and align beneath the graben features for 11 wells that produced elevated levels of H2S and extraneous water in quantities that had severe negative economic impact. There are 20 wells that did not produce extraneous water or elevated levels of H2S and did not record low frac gradients, 9 of which crossed under the graben features and another 11 were drilled between the graben features. Oil and water chemical tracers were pumped in select frac stages of 4 low frac gradient horizontal Wolfcamp wells. Recovered tracers were either absent or in very low proportions for the stages that recorded low frac gradients. These 4 wells used heel side bridge plugs set adjacent to the low frac gradient intervals to successfully eliminate or greatly reduce H2S and extraneous water. The working theory is that the Lamar limestone grabens are extensional features created in response to early-middle Tertiary tectonics that uplifted the western Delaware Basin. The shallow extensional faults created migration pathways for Delaware Mountain Group sands to deliver water and methane into Upper Permian evaporites and initiate halite and anhydrite dissolution. As the dissolution of evaporites advanced eastward across the basin, the resulting dense brines flowed downward via the more permeable routes of faults and open fracture networks beneath the Lamar grabens. The shallow graben features do not appear to correlate with older Paleozoic faulting. Fast RMS azimuth maps at the Wolfcamp from pre-stack HTI Velocity Variation with Azimuth (VVAZ) 3D seismic data is showing some promise to locating these fracture corridors at the Wolfcamp. The azimuth data indicates present day stress in those areas between shallow graben features is generally aligned parallel to the shallow graben features. This observation agrees with observed microseismic data located nearby but outside of the 3D seismic data area.
AAPG Datapages/Search and Discovery Article #90343 ©2019 AAPG Southwest Section Annual Convention, Dallas, Texas, April 6-9, 2019