Effects of Contamination by Geothermal Drilling Mud on Laboratory Determinations of Sandstone Pore Properties: an Evaluation

Numerous studies have been presented, mainly in the petroleum literature, about permeability impairment of reservoir formations due to rock/drilling fluid interactions. For example, Glenn et a1;(1957) showed that mud particles can invade to an appreciable depth into the pores of a porous medium, form a "filter cake" in the pores, and cause a substantial reduction in the permeability of the invaded zone. Drilling muds contain water and dissolved salts that can cause reductions in permeability ay tributed to clay minerals which expand or disperse upon contact with water (Alexander and Johnson, 1949) that is less saline than the connate water (Jones, 1964; Atwood, 1964, Elonaghan et al; 1959). Other studies showed that permeability reduction due to salinity changes, can also occur in formations containing only nonexpandable clays, such as illite or kaolinite, and that permeability impairerent can be cause3 by changes in PH (Mungan, 1965). Experl, ments designed for geothermal conditions indicated that formation damage depends on the composition of the drilling mud, temperature and stagnation time (Ennis et al;1979). These and other studies (e.q. Abrms, 1977; Nicholson, 1978) demostrated that permeabilityreduction is site-specific,in the sense that it depends strongly on the compatibility of the rock/drilling fluid system.