Application of 3D Geomechanics Modeling to Enhance Reservoir Simulation Model Forecast in Terrigenous Fractured Reservoir
Authors: Balin Daniil Valeryevich , Alekhin Ilya Georgievich , Brovko Vyacheslav Igorevich , Naimyshin Anton Georgievich 
Published: September 2020

Executive Summary

Conductive tectonic faults introduce significant uncertainty in reservoir simulation models during the history-matching stage, and, as a result, affect the forecast of development performance. Modeling such fields is complicated by the presence of gas migration from the gas cap, breakthrough of auto-frac fractures toward bottomhole producers, and poor performance of the water injection system. Based on the foregoing, a three-dimensional (3D) geomechanical model of the study reservoir was created and the data obtained was used to monitor the current development status and forecast.

The properties of multiple faults were estimated and the model was calibrated based on the results of more than 100 mini-fracs, mud losses, and equivalent circulating density (ECD) data while drilling horizontal sections. Each fault intersects several well trajectories drilled at different times; therefore, the criterion for the stiffness of the faults must be selected in such a way as to satisfy all calibration data at different times, taking into account changes in reservoir pressure.

To evaluate fault conductivity, multiple geomechanical simulations were run, and the results were further integrated into the reservoir simulation model. Numerical calculations confirmed a reorientation of the direction of the minimum horizontal stress in the immediate vicinity of tectonic faults.

The initial reservoir simulation model contained 15 conductive faults generated from the history-matching results of the reservoir simulation model. The results of geomechanical modeling showed that a significant number of faults are critically loaded and therefore conductive. An increase in the number of faults in the reservoir explains the high gas-oil ratio (GOR) values in individual zones and increases the reliability of the predicted development performance. Based on the analysis of the injection system performance, it was concluded that injection failure into the target wells was caused by the uncontrolled propagation of auto-frac fractures in the gas-cap interval and by the deviation of significant volumes of injected water into natural highly conductive faults. The analysis of the G-function of the pressure decline curve, based on the well-test data, shows a good match with the results of the geomechanical modeling. The 3D and four-dimensional (4D) geomechanical modeling made it possible to account for an extensive range of data for the analysis of auto-frac in injection wells and to optimize the target pressures and injection volumes. In addition, according to the results of this work, the conversion of a number of wells to injection well stock was canceled due to the significant risk of water breakthrough. The results obtained were then integrated into the reservoir simulation calculations.

It is worth noting that currently, a comprehensive analysis of the results of drilling production wells, data related to fracturing, well logging, well testing, geomechanics core testing, and tracer studies is rarely used. Such an approach would allow for a good-quality calibration of the geomechanical model, enhance understanding of the reservoir mechanism, improve the quality of the reservoir simulation model, and suggest further field development activities.