Elementary Mechanisms of Oil Recovery by Chemical Methods

Abstract

Summary Basic mechanisms of oil recovery by introducing chemical agents act either by altering fractional flow relations, as by lowering interfacial tension or raising viscosity, or by changing phase behavior, as by making oil partially soluble in an aqueous phase (solubilization) or water soluble in the oleic phase (swelling). These mechanisms are cataloged. Simplified idealizations of them are analyzed with graphical construction of Buckley-Leverett type for saturation and concentration profiles doting representative linear displacements starting with waterflood residual. Emulsification mechanisms with nonequilibrium fluid states are shown to be amenable to similar analysis under certain conditions. Introduction The oil remaining after the energy sources of primary production are exhausted is the target of secondary recovery, which is most often by waterflooding, except for heavy crudes. At the end of the economically productive life of a conventional waterflood. some 40 to 60% of the oil originally in place typically remains as residual. This residual is, of course, the target for tertiary recovery processes. The amount of residual depends on three factors: oil in place at the start of waterflooding, reservoir sweep efficiency, and microscopic displacement efficiency. These factors in waterflooding now are understood comparatively well, although the relative importance of the disconnection and film-deposition mechanisms that lead to entrapment of residual oil is not clear. Following a tertiary recovery process, the oil remaining again depends on the same three factors, but there is a fourth - process performance efficiency (Fig. 1). Coming between sweep efficiency and microscopic displacement efficiency, this additional factor in chemical and thermal processes is the efficiency with which the process performs in those parts of a reservoir that it actually sweeps - i.e., its effectiveness in the "stream tubes" it travels. Generally, to be effective a tertiary process must generate a traveling oil bank in which the fraction of pore space occupied by oil (i.e., the oil saturation) is high. Oil in place ahead of the bank is incorporated into it as it advances. The saturation in the traveling bank and the speed with which it moves are crucial to process economics. So also are the mechanisms that control microscopic displacement efficiency, the factor that describes how much of the oil is mobilized and prevented from being retrapped again as residual even in those portions of the reservoir thoroughly swept by chemical. This paper examines the mechanisms responsible for performance efficiency and microscopic displacement efficiency in chemical flooding processes - surfactant, micellar, alkaline, carbon dioxide, hydrocarbon, alcohol, and sundry combinations. Despite the complexity of the chemical flooding processes, the number of mechanisms able to mobilize residual oil lodged in pores and to prevent its retrapment is limited. Here we identify them in elementary forms that bring out their essentials. By first focusing on the distinct basic mechanisms and then treating each process as a combination of these basic mechanisms. one perceives more readily what is important, appreciates better the relative strength and weaknesses of processes, transfers more effectively concepts developed for one process to another and is better able to come up with technological improvements and innovations. The same elementary mechanisms can appear in thermal processes, which usually are aimed at heavy oil, but there are a number of others that we do not consider here. JPT P. 243^