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Upscaling anisotropic geomechanical properties using Backus averaging and petrophysical clusters in the Vaca Muerta formation
Willis, Maxwell
Willis, Maxwell
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Tutuncu, Azra
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Date
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2014
Date Submitted
2014
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2015-02-01
Abstract
Hydraulic fracturing, a necessity for economic production in most unconventional reservoirs, requires planning and accurate knowledge of geomechanical properties. Most unconventional reservoirs are anisotropic and highly heterogeneous, both vertically and laterally, and due to the prohibitive cost of recovering cores, wireline acoustic logging tools are the primary source of the downhole measurements of the formation acoustic and geomechanical properties. In addition, few wells have shear slowness data. In order to aid the hydraulic fracturing design and modeling effort, as well as the efficiency of horizontal well placement, it is desirable to upscale geomechanical properties for creation of a three-dimensional (3D) mechanical earth model (MEM). A new approach for upscaling and modeling of the geomechanical properties using cluster analysis has been introduced for the Vaca Muerta shale in the Neuquén Basin of Argentina. Using wells with core and cross-dipole logging tools, a core calibrated anisotropic model of the formation has been developed. Clusters were determined from a logging suite comprising only gamma ray, compressional slowness, and bulk density in a key exploration well, and this cluster group was implemented to several more wells in the study area. Backus averaging was used to upscale the elastic stiffness tensor for each cluster group and in the wells with cross-dipole logging tools providing full anisotropic stiffness tensor without shear. A comparison between these wells yielded the average stiffness tensor representative of each cluster group. Using microseismic data obtained from three of the wells in the study area with two fracturing stages each, the vertical extent of microseismic events was determined, and the clusters obtained through our analysis have been upscaled over this interval. All four wells show similar upscaling results by cluster for elastic stiffness coefficients and Young's moduli, with a very tight range of values. Poisson's ratio has higher variation and no trend with the clusters for Poisson's ratio was noticed. When compared to the core data, similar trends are observed in the stiffness coefficients and Young's moduli. Microseismic events tend to occur in the moderate to stiffer clusters, while production comes primarily from the more compliant ones. This knowledge may help understand the limitations of microseismic evaluation of unconventional shale reservoirs. These clusters have been used as geomechanical facies to populate a 3D MEM which can be used to couple the petrophysical model for the study area, regional stress model, regional structure, and natural fracture network in order to combine the fully coupled geomechanics and flow effects in hydraulic fracturing treatments. The method we have developed allows for anisotropic properties to be applied over a wide area with limited available logging data and is anticipated to be used in upscaling input parameters for realistic representation of formation characteristics in well placement and stimulation design and planning effort for unconventional reservoirs.
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