ABSTRACT

Subsidence has been extensive in the coastal area of southeast Texas. Despite enormous hydrocarbon production in the area, however, most subsidence has been attributed more to regional shallow groundwater withdrawal than to hydrocarbon production. To quantify the impact of hydrocarbon production on subsidence, we studied Port Neches field in Orange County, Texas, where the effects of groundwater withdrawal are minimal. Subsidence in the area has been inferred from active surface faults and an expansion in wetland area. Hydrocarbon production, pressure, and reservoir data were collected. The degree of subsurface subsidence was analyzed using a simple reservoir compaction model and compared with surface subsidence data.

More than 40 MMSTB of oil and 500 Bcf of gas has been produced from the Port Neches field area. Pressure in the Hackberry reservoir declined from an original 4,000 psi to less than 1,000 psi by the 1970's, then to less than 200 psi by the 1980's. The pressure drop from 4,000 to 1,000 psi produced an estimated reduction in pore volume (h) of 6%. Assuming an average gas column of 150 ft (46 m), the estimated compaction of the Hackberry reservoir is 3 ft (0.91 m), which is consistent with reported surface subsidence of 2 ft (0.6 m) at the east edge of the Port Neches fault block. These data strongly suggest that subsidence in the Port Neches field area be induced by hydrocarbon production and reservoir depressurization.