Multimineral Modeling and Estimation of Brittleness Index of Shaly Sandstone in Upper Assam and Mizoram Areas, India

Summary The brittleness index (BI) has major implications for hydraulic fracture studies and production toward optimized recovery in unconventional reservoirs. The paucity of brittleness studies in Mizoram and Upper Assam, located in northeastern India, motivates us to take up multimineral modeling and estimation of BI. Two commonly used BI estimation approaches, mineralogical and geomechanical, have been implemented to characterize the shaly sandstone in the study area. Laboratory analyses of the available drill-cutting samples and crossplots from well log data along with previous literature confirm the types of minerals present in the study area. With this mineralogical information, a new approach of BI log estimation from multimineral modeling is suggested here using conventional log data in the absence of core/drill cutting samples. A multimineral model for Mizoram and Upper Assam is developed by using bulk density (ρ), compressional sonic velocity (Vp), shear sonic velocity (Vs), lithodensity, and acoustic impedance (AI) logs to calculate volumetric percentage of minerals. Estimated mineralogical BI from well log data using four established models are compared and calibrated with X-ray diffraction (XRD)-derived BI to validate the proposed procedure. Most brittle zones having a BI ≥ 66% are demarcated for high Young’s modulus (Y ≥ 60 GPa) and low Poisson’s ratio (ν ≤ 0.25) values in the Y vs. ν crossplot for the study area. The presence of brittle minerals estimated from both XRD and the multimineral model suffices the reason for the high brittleness of shaly sandstone in Mizoram compared with Upper Assam.