Permeability Considerations in Generation of Abnormal Pressures

Abstract

Abstract Abnormal fluid pressures in shale-sandstone sequences commonly occur in the relatively deeper parts of the sequences. The causes of these parts of the sequences. The causes of these anomalous pressures may be explained by the shale permeability and the fluid-expulsion mechanisms permeability and the fluid-expulsion mechanisms related to compaction. The volume of fluids that should be expelled from shales in unit time for the compaction equilibrium (or the hydrostatic pressure) condition to be maintained would increase with increases both in the rate of sedimentation and in the total thickness of the sequence. The minimum permeability necessary for maintenance of the hydrostatic pressure condition would, therefore, increase with increases in the sedimentation rate and the total thickness. When the actual permeability in the subsurface is less than this estimated minimum permeability /or the equilibrium condition, some fluids will remain in the shale, in such a case, the compaction equilibrium condition cannot be attained and abnormal pressures will result. Results obtained from an analysis of data on the Gull Coast area tend to support this theory. Introduction If the pore water in a shale or mudstone can escape easily, the rate of compaction is rapid and a hydrostatic pressure condition prevails. However, if the escape of pore water is hindered by low permeability, the rate of compaction is slow and the permeability, the rate of compaction is slow and the pore water must support a part of the weight of pore water must support a part of the weight of overburden. In other words, an abnormal pressure occurs. According to Thomeer and Bottema, favorable conditions for abnormal pressures below shale columns may be found in younger sedimentary basins where thick shales were relatively rapidly deposited over considerable areas-time thus being too short for hydrostatic equilibrium condition to be reached. With respect to the common occurrence of abnormal pressure in more deeply buried shale successions, pressure in more deeply buried shale successions, Rubey and Hubbert advanced the following explanation: "When the rate of sedimentation is somewhat greater, pore water may still escape rapidly enough to maintain an essentially hydrostatic pressure in the relatively porous mudstone at shallow pressure in the relatively porous mudstone at shallow and intermediate depths but not in the more compacted and therefore less permeable rock at greater depths." In a significant recent contribution, Powers advanced a new interpretation of the origin of abnormal fluid pressure in the deep subsurface, based on the application of current knowledge of clay colloid chemistry and mineralogy. According to Powers, alteration of montmorillonite to illite begins Powers, alteration of montmorillonite to illite begins at a depth of about 6,000 ft and continues at an increasing rate to a depth, usually about 9,000 to 10,000 ft, where there is no montmorillonite left. The alteration offers a mechanism for desorbing the last few layers of bound water in clay and transferring it as free water to interparticle locations. The last few layers of bound water have a considerably greater density than free water, and water increases its volume as it is desorbed from between unit layers. As the water expands, A increases the pore fluid pressure to abnormally high levels. (See also Burst.) pressure to abnormally high levels. (See also Burst.) In this paper, we discuss qualitatively and quantitatively an alternative explanation of abnormal fluid pressure in the relatively deeper part of sedimentary sequence. This alternative explanation is based on shale permeability and fluid-expulsion mechanisms related to compaction. EXPULSION OF FLUIDS FROM SHALES DURING SUBSIDENCE Suppose a clay or shale sequence in which the clay or shale has reached a compaction equilibrium and within which the fluid pressure is hydrostatic (Stage A of Fig. 1). Additional sediments are added above this sequence in the marine condition and the sequence subsides F in time interval t. If the entire slide makes a new equilibrium condition of compaction after the subsidence of, such porosity distribution as shown by Stage B in Fig. 1 would be established. An exponential function between shale porosity -and depth would be established at Stages porosity -and depth would be established at Stages A and B (compaction equilibrium conditions). SPEJ P. 236