Effect of Hydrocarbon Production and Reservoir Depressurization on Subsidence

By

Wang, Fred P., and Nance, H.S.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Land subsidence has been severe in the Houston/Galveston area of Texas. However, despite enormous oil and gas production in the area, most land subsidence and surface faulting have been attributed more to regional shallow groundwater withdrawal than to hydrocarbon production. To quantify the impact of hydrocarbon production on subsidence, we studied two areas where the effects of groundwater withdrawal are minimal-Port Neches field in Orange County and Caplen field in Galveston County. Subsidence in both areas has been inferred from active surface faults and an increase in wetland area. The degree of subsurface subsidence was analyzed using a simple reservoir compaction model with reservoir, pressure and coring data.

More than 40 MMSTB of oil and 600 BCF of gas have been produced from the Port Neches area. The pressure of Hackberry reservoir has declined from original 4,000 psi to less than 1,000 psi by 1970's, and to less than 200 psi by the 1980's. Pressure drop from 4,000 to 1,000 psi produces an estimated reduction in pore volume, (fh) of 6 percent. Assuming an averaged gas column of 150-ft, the estimated compaction of Hackberry reservoir is 3 ft (1 m), which is consistent with reported land subsidence of 2 ft (0.6 m) at the east edge of the Port Neches fault block.

Caplen is a small hydrocarbon-producing field with a cumulative production of 26 MMSTB of oil and 26 BCF of gas. Although highly faulted, the degrees of reservoir depressurization, compaction and subsidence are small because many reservoirs are under water drive.