An Improved Recovery and Subsidence Mitigation Plan for the Lost Hills Field, California

Abstract

Abstract Maximizing oil recovery in the Lost Hills Diatomite Reservoir presents interesting technical challenges due to this reservoir’s low matrix permeability (less than 2 md) and thick net pay interval (600 to 800 feet). The high porosity (averaging 50%) and high oil saturation (up to 60%) provides significant incentive to improve and/or accelerate recovery considering the tremendous asset base of more than 2 billion barrels of oil-in-place. Massive hydraulic fracturing, that placed as much as 3000 lbs/ft of sand, has been employed since the mid-1980s to accelerate recovery. Although these treatments have proven profitable, the accelerated fluid withdrawal and associated pressure depletion has increased compaction of the highly compressive diatomite. The compacting reservoir is causing a redistribution of stresses in rocks from the base of the productive interval to the surface. As a result, surface subsidence is occurring in the area of greatest development at a current rate of approximately 1 ft/year and wellbore integrity is at risk for a large number of wells. A waterflood project was initiated in January, 1991 to improve recovery and mitigate subsidence.