Reservoir Shear Failure Analysis: A Produced Water Disposal Case Study

Abstract

Injection of water, gas, or steam, into subsurface reservoirs causes an increase in the reservoir pore pressure, with a corresponding impact on the in-situ reservoir stress state. This can lead to tensile fracturing, shear failure or a combination of complex failure mechanisms, depending on factors such as rock mechanical strength, elastic properties, pore pressure & magnitude of total stresses. Reservoir shear failure due to water injection/disposal can result in faulting or fault re-activation, which may present a containment risk if propagated into the caprock or under burden. A robust assessment of shear failure is usually challenged due to limited or unavailable geomechanical core measurements that fully assess lifecycle impact of stress and pressure changes. This paper demonstrates a deterministic and probabilistic methodology in the assessment of formation shear failure due to water disposal, that manages the uncertainty and constraints posed by limited geomechanical data. This approach employs the Mohr-Coulomb failure criterion in evaluating the tendency for shear failure in two water-bearing reservoirs A1 and A2, identified as Produced Water Disposal/Re-injection reservoirs in X-field, a brown field Onshore Nigeria. It integrates offset LOT data, log data, formation pressure data and empirical relationships between geomechanical and petrophysical rock properties, to establish rock strength parameters, the associated uncertainties, and combining them probabilistically through Monte Carlo simulation to determine the probability of shear failure in reservoirs selected for produced water disposal. Results indicated the key uncertain parameters impacting this assessment and also showed minimal probability of shear failure in both reservoirs; however, recommendations were provided to maintain a safe operating window and why an integrated approach to optimization is key to success.