Brittleness estimation and mineralogical analysis for evaluation of fracturability: Application to the Cambay Shale, Gujarat, India

Abstract:

Brittleness index (BI) has been considered as a key geomechanical parameter in evaluating fracturability of the shale formation for optimal extraction of hydrocarbon. BI is commonly used to quantitatively classify the brittle-ductile zones in thick shale reservoir and is considered as completion quality index. There are different methods for determining BI. In the present study, mineralogy based BI estimation method has been followed to identify brittle zones of Cambay Shale of Jambusar-Broach block, Cambay basin, India. Mineralogical composition is having a very significant role in controlling brittleness of a rock. Advanced geochemical logs like the Elemental Capture Spectroscopy (ECS) / Litho Scanner were utilized to get the continuous estimation of mineralogical composition of the Cambay Shale formation. XRD and SEM-EDS techniques are also used for mineralogical analysis of selected core samples taken from different depths. Analysis from XRD and SEM-EDS techniques reveals that the shale is composed of kaolinite, chlorite, quartz, siderite, pyrite and other minerals. Good match has been observed between the BI estimated using mineralogical composition from geochemical log and XRD analysis of core samples.

Along with BI, another parameter namely fracability index (FI) has been also used for hydro-fracture evaluation. FI has been estimated continuously, combining normalized brittleness index and normalized Young’s modulus. In addition to BI and FI, type of clay minerals present has some controlling role during hydro-fracture. Mineralogical analysis reveals that for hydro-fracture operation, Cambay Shale has the advantage of having kaolinite and chlorite as dominant clay minerals which are comparatively less susceptible to swelling and plastic behaviour than other clay minerals like illite. An integrated analysis of BI, FI, mineralogy, petrophysical and geochemical properties will be helpful for identification of potential target zones within thick shale.

Acknowledgement
The authors would like to thank Oil and Natural Gas Corporation Limited, India for providing the requisite dataset and core samples to carry out the present study.