Analysis of Crossdipole Sonic Data for an Intelligent Targeting of Stack Pay in the Permian Basin

Abstract

Abstract Horizontal placement of a producer well plays an important role in the efficient production of hydrocarbons from unconventional reservoirs. The preferred placement of a producer well is based on the available information about the presence of natural fractures and formation stresses. Identifying the depth intervals with natural fractures and the relative magnitudes of the maximum and minimum horizontal stresses enables us to intelligently place a producer well that will support effective hydraulic fractures for enhanced hydrocarbon production. Crossdipole sonic data can be processed to output the fast- and slow-dipole flexural dispersions. The fast dipole flexural dispersion can be inverted to obtain C55, C66, C33, and C13, whereas the slow dipole flexural dispersion can be inverted to obtain C44 (<C55) and C23. Relative magnitudes of the three shear moduli C44, C55, and C66 provide orientations of the natural fractures primarily_oriented along the axial or cross-sectional planes of the vertical pilot well superposed on the transversely isotropic (TI) constants C44 = C55, and C66 for an organic shale reservoir. Relative magnitudes of C23 and C13 can help to identify the ratio of the maximum and minimum horizontal stresses as a function of depth. This workflow has been applied to crossdipole sonic data acquired in a vertical pilot well drilled in the Permian Basin. This well traverses several identifiable lithologies with varying amounts of illite, chlorite, quartz, calcite, dolomite, kerogen, oil saturation, and unbounded water. The upper portion of the logged interval exhibits crossdipole slowness anisotropy, implying C55 > C44. The presence of crossdipole shear slowness anisotropy indicates a dominance of vertically aligned fractures in this logged interval. The lower portion of the logged interval displays TI constants typically associated with an organic shale reservoir characterized by C66 > C55 = C44. In contrast, the upper portion of the logged interval is characterized by C66 nearly equal to C55 or even less than C55, caused by the presence of vertical fractures dominating the TI anisotropy effects on the shear moduli. Moreover, differences between C13 and C23 are more pronounced in the upper fractured interval, implying differences between the maximum and minimum horizontal stresses. More detailed 3D finite-element stress analysis of horizontal stress distributions in an upper depth interval confirms the presence of interspersed stiff and compliant layers. Significant variations of maximum and minimum horizontal stresses in these layers are potential barriers to the hydraulic fracture propagation in the vertical direction. These observations suggest a need to account for such variations in the hydraulic fracture models and might require multiple laterals for an efficient drainage of hydrocarbons from this reservoir.