Land subsidence along the Texas Gulf Coast due to oil and gas withdrawal
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Land subsidence caused by groundwater withdrawal in the Houston-Galveston region is a well-documented phenomenon. Subsidence of up to 3m (10ft) has been calculated in the region since 1905. Hydrocarbon withdrawal is a plausible cause of subsidence where groundwater withdrawal has diminished and significant petroleum production has occurred for over 70 years. Sixteen fields were investigated by acquiring reservoir bottom hole pressure data (BHP) near bore-hole extensometers set up by the Harris-Galveston Coastal Subsidence District (HGCSD). All reservoirs were found to be well below hydrostatic pressure; a few of them were underpressured even before production began showing a possible hydraulic connection between fields. BHP data was used in a reservoir model and a boundary clay reservoir model to calculate subsidence. Subsidence under these fields is predicted to be as high as 0.30 m (~1.00 ft) in a nine-year period or 33.33 mm yr⁻¹ (1.3 in yr⁻¹) at the Goose Creek field and as low as 0.007 m (0.023 ft) in a twenty-year period or 0.33 mm yr⁻¹ (0.01 in yr⁻¹) at the Gillock field. Elevation benchmark data used in producing a cross section of a line of oil fields show connection of subsidence bowls above fields on two scales: A smaller local scale subsidence bowl on the order of 5 km (3.1 mi) and a more regional subsidence bowl on the order of 40 km (24.9 mi). Point estimation of the contribution to total subsidence from petroleum production and groundwater withdrawal show that the majority of the subsidence occurring presently in Harris and Galveston counties is from oil and gas withdrawal. Effects from clay equilibration caused by previous groundwater pumping were interpreted to be minimal. Implications of this study are: 1) hydrocarbon production, although not the major contributor to most land surface subsidence in this area, is significant and 2) depressurization and subsequently subsidence from oil and gas fields may be regional, connecting neighboring fields. This is inferred from the fact that some fields were already underpressured before production began in addition to benchmark data showing connection of subsidence bowls.