Elastic mechanical properties from SEM imaging in tight formations

Elastic properties of unconventional rock, including gas/oil shale and tight gas sand (TGS), are crucial in hydraulic fracture modeling. The two most important rock elastic properties are Young's modulus and Poisson's ratio. These properties can be determined from sonic well logs, but the required logs (compressional and shear velocity) are not always available. These properties can be measured from plug samples using a triaxial load frame, but these tests are slow, expensive, and require an intact cylindrical sample. An alternative is to use rock physics modeling applied to mineralogy and porosity computed from ion-milled scanning electron microscope (SEM) images to compute elastic constants from small rock fragments. This method can also be applied to data from whole core computed tomography (CT) scans. This approach was used to develop a digital rock workflow to compute elastic properties from rotary sidewalls cores, drill cuttings, and core CT data. The new approach combines quantitative information obtained from 2D ion-milled SEM images with rock physics effective-medium models, the latter used to relate volume properties to elastic properties. These models can be obtained from wireline and/or laboratory measurements of bulk rock volumetrics together with elastic rock properties. This process of finding a rock physics model is called rock physics diagnostics. The SEM images provide porosity, organic matter volume, and pore structure. The mineralogy of the sample obtained through quantitative X-ray diffraction (XRD) is added to those inputs. Well log data relevant to the local area are then used to establish a rock physics model linking the elastic properties to porosity, organic matter content, and mineralogy. These models are established for each basin and formation, based on available wireline log data. High quality wireline data is key to successful rock physics diagnostics (RPD). In this study, wireline logs and core samples were obtained from a well in Culberson Co, TX. The zone of interest in this case was the Wolfcamp A formation. After establishing the appropriate rock physics effective medium models, the elastic properties were computed, including Young's modulus, Poisson's ratio, compressional wave velocity, and shear wave velocity from SEM images and XRD mineral data. The computed, upscaled elastic properties closely matched the log variability. This method can be used to obtain the required elastic properties from wells that lack compressional and dipole shear wave data. This mechanical properties data can be used to compute horizontal stress, unconfined compressive strength, and other critical properties that control hydraulic fracture growth. In many cases, drill cuttings can be used for the SEM analysis. This new approach does not require cores, and so can be especially valuable in quantifying elastic and mechanical properties along the lateral wellbore where wireline logs are seldom available.