Thermally Induced Fines Mobilization: Its Relationship to Wettability and Formation Damage

Abstract

Abstract Reservoir rocks are thought to become more water wet during thermal recovery. To date, no plausible mechanism describes this shift toward water wetness. Fines mobilization presents one pathway to create fresh water wet surfaces. Likewise, fines movement is a mode of formation damage. In order to understand the mechanisms of fines mobilization, we review and use DLVO (Derjaguin, Landau, Verway and Overbeek) theory. It explains the stability of colloidal systems as a result of the balance of attractive and repulsive forces. DLVO calculations predict a profound effect of temperature on fines stability. Release of fine clay material is predicted at conditions (pH, salinity, temperature) obtained during steamflood. Predictions are presented in the form of isotherm maps. They are confirmed experimentally by core waterfloods performed in Berea sandstone at temperatures ranging from 20 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 (SEM) was used to analyze composition of the effluent samples collected during experiments. This study shows that temperature is a decisive factor in fines release and the temperature at which the detachment of fines occurs is well predicted by DLVO theory. Introduction Thermal recovery research and field operations indicate that reservoir rock exposed to high temperature undergoes a systematic shift in wettability toward water wetness1,2. The consequence of a shift toward water wetness is that steam and steam condensate become more effective displacement agents and the rock matrix imbibes water more effectively. It is well understood that wettability influences the flow and relative permeability characteristics of porous media1,3. Examination of temperature as a variable on relative permeability has been conducted since the 1960's4,5,6. The effects of fluid properties, such as pH and salinity, on fines migration and permeability are discussed extensively in the formation damage literature7–11. Yet, controversy exists regarding the origin of temperature effects and their extent. This paper reexamines the role of temperature on rock-fluid interactions and rock properties from a surface forces perspective. We find experimentally and theoretically that fine particles are released from pore walls under conditions of elevated temperature, high pH, and moderate aqueous-phase salinity. The mechanism of fines release resolves the controversy, in part, and provides a new mode of wettability alteration as a function of temperature. This paper is organized in the following fashion. First, a synopsis of the literature is presented. Second, corefloods with hot water are used to show that temperature mobilizes fines in Berea sandstone. Third, surface forces, so called DLVO theory, are used to explain experimental observations, Discussion and conclusions round out the paper. Literature Sypnosis There is disagreement about whether temperature alone causes permeability changes, and whether this change of absolute permeability is related to fines migration and subsequent clogging of the pores. A review of studies performed on the effect of temperature on permeability and relative permeability is challenging due to the variety of rock and fluid properties, conditions, and interpretation methods used for the experiments.