Pressure Seal Permeability and Two-Phase Flow
Pressure compartment seals all have permeability to single-phase flow. Complete sealing can occur only in a multiphase fluid environment. For physical properties typical of the Powder River basin, Wyoming, Darcy flow allows single-phase leak rates such that observed pressure compartments would leak off in about 1 million years. Pressure compartments can be held indefinitely, however, under multiphase flow. Muddy sandstones of anomalously high threshold displacement pressure, about 2000 psi, appear to contain gas reservoirs at high pressure. Such high displacement pressures correlate well with those of classic carbonate and shale seals. The Muddy, however, contains sandstones capable of sealing adjacent reservoir sandstones. Sealing sandstones correlate with zones of unconformities between sandstones of good reservoir quality. Capillary sealing, as observed here, is certainly a worldwide phenomenon but is not the only mechanism of holding a pressure compartment. Other pressure compartments might be actively leaking (e.g., Gulf Coast type) and geologically temporary. Conversely, the capillary seal is permanent up to the threshold displacement pressure, which is the observed pressure in Muddy pressure compartments.
Figures & Tables
Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.