This paper represents a study of reservoir pore modification accompanying diagenetic secondary porosity development within a deep (13,400 ft) overpressured Anahuac Formation sandstone in southern Louisiana. Secondary porosity formed by dissolution of carbonate cement, detrital grains, and other soluble minerals comprises a significant portion of porosity formed in U.S. Gulf Coast Tertiary reservoir sands. The primary pore system within this reservoir is believed to have been significantly enlarged (by up to 32% porosity) by acidic fluids generated during hydrocarbon maturation and dewatering of adjacent shales. Subsurface secondary porosity development within sandstones is significant in influencing the development of potential reservoir porosity after much of the primary porosity has been destroyed by mechanical and chemical compaction. Properties of the reservoir pore system that affect fluid flow and mechanical resistance of the reservoir to compaction accompanying production will also be influenced.
Characteristics of the reservoir pore system were established by study of whole core samples using scanning electron microscopy, petrographic examination, mercury injection, and simulated in-situ reservoir condition core testing. Secondary pore size and distribution was found to be influenced by sandstone mineralogy, grain size, sorting and angularity, the pore matrix content, and by sedimentary structures and resulting textural components that may hinder fluid flow.
Changes in the mechanical resistance to compaction caused by the development of secondary porosity in sandstone reservoirs is important when considering reservoir stress sensitivity. Keystone bridging relationships between grains can be established during the initial phases of compaction so that when leaching of cement and soluble grains occurs, a less soluble quartz grain matrix is left to support porosity development. Special core tests were performed at simulated in-situ reservoir conditions of pressure and temperature to examine porosity and permeability reduction as a function of effective stress generated by pore pressure reduction (simulated fluid production). Observed volumetric strain to uniaxial compaction at reservoir conditions was determined within portions of the sand containing high (25–30%) porosity. Test results exhibited less than 1% reduction in total bulk volumes accompanying a 60% reduction in pore pressure. Permeabilities measured at in-situ conditions were commonly an order of magnitude less than those measured at ambient conditions. However, with increased effective stress applied to the rock fabric, data suggest that permeabilities decrease at a much slower rate, reflecting constriction of pore throats rather than constriction of stress-induced microfactures thought to exist in core samples at ambient conditions.
