The Feasibility Study of Reservoir Geomechanics from Brittleness Evaluation

Abstract

A detailed understanding regarding the rock Brittleness Index is useful in oil and gas exploration as upfront information to determine the rock fracture gradient. Researchers have proposed several methods to estimate the rock Brittleness Index.  However, different methods may yield different results and, hence, lead to varying interpretations regarding the Brittleness Index classification. This paper estimates the Brittleness Index of an Indonesian gas well using three approaches based on the log data and Rock Physics Modeling and to assess the consistency of the methods. In this study, the rock’s brittleness is estimated based on the elastic parameters obtained from the log data as well as the rock physics method and mineralogical data. In the case that the acoustic data is unavailable, the acoustic log data was first estimated using Rock Physics Modeling using Voigt-Reuss-Hill, Kuster-Toksӧz, and Biot-Gassman methods. These elastic property-based methods were then compared with the Brittleness Index obtained from the mineralogical method. The results obtained in this study suggest that the elastic property-based and the mineralogical-based methods produced a consistent Brittleness Index. However, they are different in terms of their vertical resolution. It is observed that the Brittleness Index estimated using the actual log data produced a higher resolution index as compared to the one estimated based on the mineralogical data. The Brittleness Index estimation based on the elastic parameters from the log data suggests that the rocks can be classified as less ductile to less brittle, while the Rock Physics modeling suggests a generally less ductile type of rocks. To optimize the hydraulic fracturing design and planning, it is advised that the TOC data be combined with the Brittleness Index to identify the most suitable depth for an effective and optimum hydraulic fracturing. For further investigation in the future, it is necessary to log geomechanical and direct sample tests in the laboratory from the sample/core to obtain the best geomechanical model of the hydrocarbon shale formation in the study area.