Comparison of elastic anisotropy in the Middle and Upper Wolfcamp Shale, Midland Basin

Abstract

Organic-rich shales contain large amounts of oil and gas and are anisotropic because of fine-scale layering and the partial alignment of organic matter and anisotropic clay minerals with the bedding. An example is the Wolfcamp Shale in the Permian Basin. Elastic anisotropy needs to be accounted for in the characterization of such formations using seismic data and plays a role in hydraulic fracturing and in the evaluation of stress changes and geomechanical effects resulting from production. Using extensive well log data acquired in the Midland Basin, the eastern sub-basin of the Permian Basin, we estimate and compare the elastic anisotropy in the Middle and Upper Wolfcamp Shale by combining data from a vertical pilot well with two lateral wells, one (6SM) drilled in the Middle Wolfcamp and one (6SU) drilled in the Upper Wolfcamp. The data used were acquired at the Hydraulic Fracture Test Site 1, located in the eastern part of the Midland Basin. Thomsen's anisotropy parameter $\gamma$ calculated from the fast and slow shear sonic is higher on average for the 6SM lateral than for 6SU, consistent with there being less carbonate content in 6SM than in 6SU. However, the anisotropy parameter $\gamma$ in some regions with higher carbonate content in well 6SU is higher than in well 6SM. This may indicate the influence of natural fractures. The primary set of steeply dipping fractures observed in the lateral wells at Hydraulic Fracture Test Site 1 acts to increase $\gamma$ if the ratio of the normal-to-shear fracture compliance is less than about 0.5. Sub-horizontal fractures may also increase $\gamma$ and could affect the vertical extent of hydraulic fractures. Relations between elastic moduli C₃₃ and C₅₅ in the Upper and Lower Wolfcamp in a vertical pilot well allow C₃₃ to be predicted in a lateral well using measurements of C₅₅ in that well. Comparison of Thomsen's anisotropy parameters $\gamma$ and $\epsilon$, with $\gamma$ calculated from the measured values of C₅₅ and C₆₆ and $\epsilon$ calculated from the measured values of C₁₁ and predicted values of C₃₃, show that $\epsilon$ is mostly greater than $\gamma$.