Stress state, subsidence, and faulting in the Wilmington Oil Field, California: a multiphase flow-geomechanics modeling assessment (1936-2020)

TL;DR

This study uses multiphase flow-geomechanics modeling to analyze how long-term oil extraction in the Wilmington Oil Field has altered stress, caused subsidence, and influenced fault stability, with implications for seismicity.

Contribution

It provides a detailed numerical assessment of the long-term geomechanical impacts of reservoir operations, challenging previous assumptions about the stress regime in the field.

Findings

Best fit to deformation data with low initial stress in sedimentary section

Significant variation in stress state with depth and potential regime change

Minor fault destabilization with larger effects on bedding planes

Abstract

Nearly a century of oil production in the Wilmington Oil Field, Los Angeles Basin, California, has modified the stress state, caused nearly 9 m of ground surface subsidence, and been associated with earthquakes that sheared wells. This offers a unique opportunity to elucidate the processes that govern these phenomena: Since the 1930s, approximately 2.5 billion barrels of oil have been produced, accompanied by water injection volumes roughly an order of magnitude larger. Combined with extensive structural and geophysical constraints, this history allows us to interrogate the long-term geomechanical impacts of reservoir operations. Here, we assess (i) how the initial stress state, typically uncertain in the shallow crust ($<5$ km depth), influences subsidence and uplift, and (ii) how production and injection operations affect fault stability. Our numerical model, calibrated with published measurements of reservoir pressures and surface displacements, incorporates a detailed representation of fault surfaces within and around the field, well-level production and injection schedules, and an elastoplastic constitutive framework. Model results show that the previously assumed stress regime in the field (reverse faulting) needs to be reassessed$\unicode{x2014}$the best match to the ground deformation data is achieved when the sedimentary section is initialized with low deviatoric stress (i.e., not critically stressed). This suggests significant variation in the stress state with depth, including a likely change in the stress regime. DCFF values suggest minor destabilization on reservoir faults and larger changes on sub-horizontal bedding planes; both could explain the faulting that led to sheared wells and seismicity between 1947 and 1961.