Pore pressure and stress at the Macondo well, Mississippi Canyon, Gulf of Mexico

At the Macondo (MC 252-1) well, the overpressure (fluid pressure greater than hydrostatic) in the main reservoir is nearly identical to that within a stratigraphically equivalent sandstone at the Galapagos Field development 21 miles (34 km) to the south; we interpret that these reservoirs share a permeable, laterally extensive, and hydraulically connected aquifer. At Macondo, pore pressure and least principal stress approximately parallel the overburden stress to a depth of 17,640 ft zₛₛ (5,377 m) subsea and thereafter decreases abruptly by 1,200 psi (8.3 MPa) over 370 ft (113 m) as the main sandstone reservoir is approached. In contrast, at Galapagos Field, pore pressure increases with the overburden stress for the entire well depth. We infer that lateral flow through the permeable sandstone controls the reservoir pressure. By modeling the least principal stress with an effective stress ratio, we show that the pore pressure regression at Macondo was responsible for a reduction in fracture pressure across the reservoir interval. This, in combination with the extreme pore pressures above, drastically narrowed the range of safe operational borehole pressures. These geologic factors led to drilling, casing, and cementing decisions that ultimately contributed to the Deepwater Horizon blowout.