Relative Permeability Measurements Using Reservoir Fluids

Water-oil relative permeabilities measured using reservoir fluids and fresh, preserved cores are shown to differ considerably from those obtained routinely using relined fluids and extracted cores. Upon saturating the extracted cores with the reservoir fluids and allowing them to come to equilibrium, in this case for 6 days, the original relative permeability curves were reestablished. Introduction Ordinarily, relative permeabilities are measured using refined fluids and restored-state cores. On occasion, fresh core samples that had been flushed with a refined oil, without any other cleaning or extraction, have been used. To date, no relative permeability measurements have been reported permeability measurements have been reported using reservoir fluids and fresh preserved cores, although a number of published studies have dealt with the influence of core handling and other laboratory experimental factors on results of displacement tests. This is not surprising. Special procedures have to be followed for obtaining, procedures have to be followed for obtaining, preserving, and handling native-state cores; and preserving, and handling native-state cores; and companies are generally reluctant to permit any procedures that may increase the expense or the procedures that may increase the expense or the time required to complete a well. Consequently, suitable cores often are simply not available. Furthermore, only a very few of the petroleum production research laboratories have the facilities production research laboratories have the facilities for measuring interfacial tension, contact angle and relative permeabilities at the elevated temperatures and pressures normally encountered in reservoirs. The following study provides, for the first time, relative permeability data obtained with fresh, preserved cores and reservoir fluids at reservoir preserved cores and reservoir fluids at reservoir pressure and temperature. Measurements were also pressure and temperature. Measurements were also made routinely with refined fluids and extracted cores to afford comparisons. Advancing and receding contact angles were measured as a function of time with the actual reservoir fluids on a solid surface representative of the reservoir rock to characterize the wetting equilibrium during the tests. Finally, a procedure was devised which, for the extracted procedure was devised which, for the extracted cores, yielded the original set of relative permeability curves. permeability curves. EXPERIMENTAL THE CORE The core was cut using lease crude oil in a Pennsylvanian sandstone reservoir. Crude oh was Pennsylvanian sandstone reservoir. Crude oh was considered as the best coring fluid to preserve both reservoir wettability and the interstitial water. The core was obtained from a pumping well that was to be deepened 50 ft for improving the productivity index. The production string was 4 1/2-in. casing and had been cemented and perforated with the total depth at 5,200 ft. Initially, a three-cone, hard-rock rotary bit was run on 2-3/8-in. drill pipe to drill out the cement plug and the casing flow shoe. Drilling was continued for an additional 10 ft to cut new formation and clean out the hole. For the coring operation, the rotary bit was replaced by a diamond core barrel having 3 1/2-in. OD. De-gassed crude oil containing no additives was used as the drilling fluid during both drilling and coring. This crude had an API gravity of 35.5 degrees (i.e., a specific gravity of 0.8473 gm/cc) and provided sufficient pressure against the formation. The intake pipe for pressure against the formation. The intake pipe for the mud pump was positioned several feet above the bottom of a production-storage tank. This was done to avoid picking up any sediments or asphalts that may have accumulated at the tank bottom. The mud return line from the well was diverted into a pit, and the return crude was not recirculated to avoid introduction of any oxygen into the well. The total quantity of the crude oil used was approximately 400 bbl during the entire operation. When brought to the surface, the core was quickly canned in crude oil to minimize exposure to air. In the laboratory, the core container was placed inside an airtight lucite box, which was fitted with sealed gloves for sample manipulation from the outside. A nitrogen atmosphere was maintained inside the box. An approximately 1-ft-long core piece was chosen which, by visual examination, appeared to be uniform. From each end, a 1-in. piece was cut for the x-ray diffraction and mercury injection studies. SPEJ P. 398