Permeability Controls on Aquathermal Pressuring

Aquathermal pressuring refers to fluid pressures in excess of the hydrostatic which have been created by fluid thermal expansion against a less expansive sediment matrix. Two commonly stated objections to this mechanism are: (1) hydraulic gradients and the finite, but very low, permeabilities now existing in the Gulf of Mexico are sufficient to dissipate aquathermal pressures, and (2) the increasing temperatures decrease fluid viscosity at a rate so that hydraulic conductivity increases fast enough to dissipate these pressures. Evaluation of the momentum transport equations in the Eulerian frame of reference indicates the first objection is not applicable because it treats the problem as steady flow and not transient flow. Furthermore, existing data indicate that under normal diagenetic conditions the decrease in intrinsic permeability caused by consolidation is greater than the viscosity effect. Therefore, the two commonly stated objections to aquathermal pressuring do not by themselves prove the mechanism is ineffective in settings such as the Gulf of Mexico. Permeability increases by fracturing and faulting may, however, be sufficient to bleed off aquathermally produced excess pressures. The measured, very low permeabilities of Gulf Coast fine-grained sediments indicate that aquathermal pressuring is a potentially important mechanism if the fine-grained sediments do control the hydraulic response.