Dynamic and Static Forces Required To Remove a Discontinuous Oil Phase from Porous Media Containing Both Oil and Water

Abstract A number of experiments were carried out on samples of reservoir rocks to determine the conditions required to remove the oil (often termed residual oil) that remains as a discontinuous phase after a complete water flood. In every case it was found that the oil removed was a unique function of the ratio delta P/L sigma, where delta P is the pressure drop across the distance L and sigma is pressure drop across the distance L and sigma is the interfacial tension between the oil and water. It was found that no residual oil can be removed from a porous rock until a critical value of delta P/L sigma is exceeded. The critical ratio appears P/L sigma is exceeded. The critical ratio appears to be a fundamental property of the rock. For Berea sandstone samples, the critical ratio has a value of about 5 (psi/ft)/(dyne/cm) in common field units. If this critical value is exceeded by applying more pressure or by reducing the oil-water interfacial pressure or by reducing the oil-water interfacial tension (or both), some additional oil is invariably produced. The exact amount of additional oil is produced. The exact amount of additional oil is always a monotonically increasing function of the increase in the value of the ratio delta P/L sigma. To recover economically attractive amounts of oil, the critical delta P/L sigma value should be exceeded by at least one order of magnitude. In the laboratory with short core samples (L is very small), the required high values of delta P/L sigma can be achieved readily by adjusting the flow rates or by reducing the oil-water interfacial tension or both. By utilizing various combinations of surface-active agents and pressure gradients, excellent recoveries of close to 100 percent were obtained in the laboratory experiments. percent were obtained in the laboratory experiments. However, in translating the laboratory results to the reservoir, it becomes apparent that extremely low values of the oil-water interfacial tension must be used to avoid excessively high waterflooding pressures that would fracture the formation. pressures that would fracture the formation. Methods for obtaining the low values of the oil-water interfacial tension are described along with techniques for altering the typical flooding patterns to achieve the required delta P/L sigma patterns to achieve the required delta P/L sigma value in the reservoir. Introduction Forty years have passed since one of the early patents was issued to cover the use of surface-active patents was issued to cover the use of surface-active materials as an aid to the waterflooding of petroleum reservoirs. The intervening years have petroleum reservoirs. The intervening years have been replete with volumes of technical papers and patents on the same subject. Authors have patents on the same subject. Authors have discussed numerous aspects of the inherent difficulties of using a surface-active material in the reservoir with all of the conditions adjusted in exactly the right fashion to recover the oil that is lost in an ordinary waterflood. Many investigators have carried out laboratory experiments to examine various facets of the problem. The investigators have centered most of their concern on the effect of changes in the oil-water interfacial tension, the oil-water-rock contact angle, and the adsorption of the surface-active agent on the rock surfaces. Others have included the effect of flooding rate (or pressure gradient) along with variations in the pressure gradient) along with variations in the interfacial tension. The work of Wagner and Leach is especially noteworthy because excellent oil recoveries were achieved when the interfacial tension between the phases was reduced to extremely low values (about phases was reduced to extremely low values (about 0.07 dynes/cm or less). In addition, the wettability of their system (gas-oil) was well defined and Wagner and Leach conducted many of their experiments at slow flooding rates that were comparable to the rates possible in waterflooding operations. Thus, their conclusions on the low values of interfacial tension required for additional oil recovery seem to be valid. In spite of the careful work performed in many different laboratories, there is still some disagreement on the precise mechanism by which surface-active materials actually remove more oil from the porous rock. SPEJ p. 3