Relative Permeabilities at Simulated Reservoir Conditions

Abstract

Summary. Relative permeability was determined at conditions from ambient to those simulating a depth of 1200 m [3,940 ft] from laboratory displacement data. The data were analyzed with the semianalytical interpretation method of Civan and Donaldson, which includes capillary pressure. The unsteady-state displacement method requires that the data be collected at high flow rates to diminish the influence of capillary end effects. Including capillary pressure removes this constraint but results in a more complex mathematical model. The results show the capillary end effects and the error introduced by not including them in the calculation of relative permeability. When capillary pressure was ignored in our model, it reproduced the literature values. Relative permeability measurements at elevated temperature show the characteristic temperature effect of higher oil relative permeability and lower residual oil saturation (ROS). These effects result from a change of wettability to a more water-wet condition at the higher temperatures. The equipment used for relative permeability measurement can easily be constructed; in fact, some commercial designs can be used without modification. The results represent data at reservoir conditions; the data are taken on cores that are recompressed, eliminating the adverse influence of microfractures and changes of pore-size distribution frequently present in field cores run an ambient conditions. The displacement data are not restricted to high-flow-rate experimental conditions; removal of this restriction allows the analysis of low-permeability cores when flow rates are necessarily low. Introduction Civan and Donaldson presented a semianalytic method for calculation of relative permeabilities from unsteady-state displacement data with inclusion of capillary pressure. The method was tested and compared with data presented by Odeh and Dotson. The relative permeabilities computed by the Civan and Donaldson method matched the predictions of Odeh and Dotson's graphical method for correcting the predictions of the Jones and Roszelle method for the capillary end effects. In the present study, laboratory waterfloods starting at the irreducible water saturation were conducted with three cores of outcrop rocks whose mineralogy is described by Crocker et al. The equipment and procedures used to conduct the waterfloods at simulated reservoir conditions also are presented. This work was conducted to furnish further evaluation of the mathematical procedure and to investigate the difference between measurements of relative permeabilities at ambient conditions and simulated reservoir conditions. The difference observed was an increase of the relative permeabilities of the oil phase and a decrease of the ROS with increase of temperature. These effects have been observed by numerous researchers, many of whom attributed them to a change of wettability to a more water-wet condition, which is confirmed by this work. The changes of relative permeability and endpoint saturations are great enough to justify the expense, time, and technical difficulty of conducting waterfloods with reservoir samples at conditions of temperature and pressure representing those of the specific reservoir for which the data are required. A correlation of the change of these properties with respect to temperature, and the difference between overburden and fluid pressure, may be possible after a considerable amount of data is accumulated, but until that is possible, it is recommended that fluid-flow studies be conducted with reservoir fluids at simulated reservoir conditions. In 1965, Edmondson presented results of an investigation of temperature effects on waterfloods with two refined oils and two crude oils. He observed an increase of k, and Si, and a decrease of S, and showed that the effects were more pronounced for the crude oils than for the refined oils. He also stated that the temperature effects were not predictable from evaluation of the change of fluid viscosities. Furthermore, the refined oils exhibited piston-like displacement with very little subordinate production after water breakthrough, but the crude oil floods had early water breakthrough with significant subordinate production. Poston et al. measured the effect of temperature on relative permeability with unconsolidated sands. They observed the same effects of temperature increase reported by Edmondson and attributed them to a change of system wettability to a more water-wet condition at increased temperature. Sinnokrot et al. studied the effect of temperature increase on capillary pressure curves with a refined oil and cores that were fired at 550 degrees C [1,022 degrees F]. They observed an increase of Siw, and a shift to higher capillary pressure as the temperature was increased and stated that the data implied a change of wettability to a more water-wet condition at higher temperature. Weinbrandt et al. reported the same results for temperature in-crease when sandstone cores were used; i.e., increase of kro and krw increase of Siw and decrease of Sor. They compared their results to those obtained by Owens and Archers and concluded that the observed temperature effects resulted from a change of system wettability to a more water-wet condition. The same results were reported by Lo and Mungan, who worked with initially water-wet and oil-wet systems. In addition, they reported more pronounced effects when more viscous oils were used, which indicated that the change of mobility with increasing temperature added to the observed changes of relative permeability and endpoint saturations. Sigmund and McCaffery presented recovery curves at various mobility ratios that show greater recovery at lower mobility ratios. They also showed that capillary-pressure forces affect the pressure and recovery response of waterflood data. Morrow et al. working with crude oils, observed very low relative permeabilities to water at Sor and offered the possible explanation that this was a result of capillary end effects involving the retention of oil at the outflow face of the core and oil-wetting of the pore throats. Equipment and Procedures A diagram of the equipment is shown in Fig. 1. SPERE P. 1323^