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The pore and grain surface of reservoir rocks often has clay and other fine material attached onto pore walls. It has been long recognized that brine salinity and pH are key factors affecting the attractive forces between pore surfaces and fines. If mobilized particles are assembled in sufficient quantities, they obstruct pore throats and reduce the permeability of the formation. There is anecdotal evidence of substantial fines migration during steam injection enhanced oil recovery operations. As of yet, the mechanism of fines release with temperature is unexplained. The Derjaguin, Landau, Verwey, and Overbeek theory of colloidal stability is used in conjunction with laboratory, core-scale experiments to demonstrate that high temperature, alkaline pH, and low salinity (typical characteristics of steam condensate) are sufficient to induce fines mobilization. Temperature is a key variable in calculations of fines stability. Hot-water floods are performed in Berea sandstone at temperatures ranging from 20°C to 200°C. Permeability reduction is observed with temperature increase and fines mobilization occurs repeatably at a particular temperature that varies with solution pH and ionic strength. A scanning electron microscope is used to analyze composition of the effluent samples collected during experiments. It confirms the production of fine clay material. On the practical side, this study provides design criteria for steam injection operations so as to control fines production.

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Direct laboratory measurements of in situ water-phase saturation history are used to estimate relative permeability and capillary pressure functions. The magnitude of so-called nonequilibrium effects during spontaneous imbibition is quantified and, if significant, these effects are incorporated within the estimation technique. The primary constraint employed is that curves must increase or decrease monotonically; otherwise, no predetermined functionality is assumed. The technique is demonstrated using water saturation profile histories obtained for diatomite (a low-permeability and high-porosity rock). Results indicate that nonequilibrium effects detected at laboratory scale in low-permeability rocks influence the estimation of unsteady-state relative permeability and capillary pressure.

Summary The evaluation of thermal-recovery processes requires relative permeability functions, as well as information about the effects of temperature on these functions. There are significant challenges encountered when estimating relative permeability from laboratory data, such as the accuracy of measurements and generalized assumptions in the interpretation techniques. A novel method is used here to estimate relative permeability and capillary pressure from in-situ aqueous-phase saturation profiles obtained from X-ray computerized tomography (CT) scanning during high-temperature imbibition experiments. Relative permeability and capillary pressure functions are interpreted simultaneously, including possible nonequilibrium effects. Results obtained show a systematic shift toward increased water-wettability with increasing temperature for diatomite reservoir core. The measured changes in relative permeability are linked to the effect of temperature on the adhesion of oil-coated fines to rock surfaces and, ultimately, to rock/fluid interactions. Introduction An understanding of the effects of temperature on wettability and relative permeability functions is essential to optimize and forecast the results of diatomite thermal-recovery projects. Most of the controversy regarding the effect of temperature on relative permeability is caused by the mechanisms involved in rock-wettability change that are dependent on both fluid and rock characteristics. A secondary, and equally important, problem is the technique used to process the data, such as oil recovery, phase saturation, or pressure, as well as data interpretation in the form of relative permeability curves. This paper re-examines the influence of temperature on rock/fluid interactions and heavy-oil relative permeability of diatomite from a core-level experimental and a pore-level perspective. We find experimentally and theoretically that fine particles are released from pore walls under conditions of elevated temperature, high pH, and moderate to low aqueous-phase salinity. The release of fines correlates with changes in relative permeability curves toward greater water-wetness. The mechanism of fines release provides new understanding of a mode of wettability alteration at elevated temperature. This paper is organized as follows. First, a synopsis of the literature is presented, followed by a discussion of recent developments in the understanding of wettability alteration. Second, the experimental method and the relative permeability interpretative methodology are outlined. Third, relative permeability results interpreted from field core samples at temperature are presented. Discussion and conclusions round out the paper.

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