Detecting Fluid Movement and Isolation in Reservoir Core With Medical NMR Imaging Techniques

Abstract

Summary A medical nuclear magnetic resonance (NMR) imaging instrument has been modified to image water and oil in reservoir rocks by the construction of a new receiving coil. Both oil and water inside the core produced readily detectable hydrogen proton NMR signals. while the rock matrix produced no signal. Because of similar NMR half-lives, the water was dope with a paramagnetic ion, Mn + 2 to reduce its relaxation time. This procedure enhanced the separation between the oil and water phases in the resulting images. Sequential measurements, as water imibibed into one end and oil was expelled from the other end of a core plug, produced a series of images that showed the location of the fluids as a function of flow time. For water-wet Berea sandstone, a flood front was readily observed, but some of the oil apparently was left behind in small, isolated pockets that were larger than individual pores. After several additional PV's of water flowed through the plug, the NMR image indicated a homogeneous distribution of oil. The amount of residual oil determined from the ratio of NMR intensities closely approximated the residual oil saturation of fully flooded Berea samples measured by the Dean-Stark method. A Berea sandstone core treated to make it partially oil-wet did not show a definitive flood front but appeared to channel the water around the perimeter of the core plug. The relative ease with which these images were made indicates that NMR imaging can be a useful technique to follow the flow of oil and water through a core plug for a variety of production processes. Introduction The petroleum industry has used average fluid saturations to determine the amount of oil originally in place (OOIP) and potentially producible by various production processes from the beginning of scientific reservoir analysis. These average saturation measurements do not define the distribution of the fluids inside the core plug or reservoir. To maximize the efficiency of oil recovery and/or to increase the accuracy of calculating recoverable reserves. one should know how the fluids are distributed inside the reservoir rock. For the design and evaluation of alternative production methods. the change in distribution with various treatments is important. Simple mathematical and experimental models can be used to predict that fluid flow through a reservoir can occur in either a heterogeneous or homoizeneous fashion. The former would leave multiple pores of oil surrounded by water, while the latter would leave oil as microscopic globules inside individual pores. 1.2 Channeling between injection and production wells would appear to represent the heterogeneous cases however, inside the channel, the oil may be homogeneously distributed. It is probable that the mechanism changes with time and from one reservoir to another. In the early 1950's, attempts were made to locate water movenient through a core by use of radioactive tracers and gamma ray absorption. Recent improvements, primarily in data acquisition, have caused a re-examination of these techniques The flood front can be located by detecting rapid changes in saturations, but because of resolution and averaging over the core cross section, residual pockets or channels of the original fluid cannot be located with these methods. A number of ingenious laboratory tests have been devised to follow fluid movement, but these require artificial systems. such as glass-bead packs and glass plates, and cannot be applied to real reservoir rocks. By placing a dye in the flooding fluid, one can determine a three-dimensional image of the flood front. Because the core must be cut for analysis, however. this technique precludes further testing of an individual core. X-ray computer-aided tomographys 11 has been able to resolve oil and water phases in cores, but compensation for the rock matrix contribution must be made. A considerable amount of work to measure average NMR properties of porous materials was performed in the 1950's and 1960's. This work addressed important properties, such as average pore geometry bound vs- mobilwater and wettability, but did not produce images of fluid distributions in porous media. Moving a core through the analyzing volume of an NMR spectrometer made it possible to obtain an average saturation along the core axis. 16 This technique, however, was tedious and one-dimensional. Recent studies with NMR imaging have been able to locate oil and water in artificial and sandstone cores. This study used a medical imager to follow the flow of oil and water in a stepwise imbibition waterflood. NMR imager or magnetic resonance imaging (MR[) has burst into the arena of modern medical diagnostic tools. It is used primarily to detect tumors and abnormalities in the soft tissue of patients. The technique measures the intensity of the proton NMR signal from water and hydrocarbons (lipids) in the individual examined. The image is forced by developing a magnetic field gradient across the patient, causing resonance to occur only at one specific spatial location. The resultant signal is stored in a dedicated computer that can recon-strict a two-dimensional image on a monitor. The various body tissues have different proton NMR half-lives, allowing them to be separated by observing relative intensities as a function of time after excitation. For example, the water in tumors is less tightly bound, giving a longer half-life than the healthy tissue. Thus, the image of a tumor stands out as brighter than the surrounding healthy tissue at longer pulse-delay times. Experimental Fig. 1 shows the Picker Intl. Model MR Vista TM used in these experiments, The consoles are in the foreground, and the measurement area is located in the background. The test samples and receiving coil are placed inside the large cavity. The superconducting magnet surrounds the cavity and is obscured by the wall. The electronics and computer are located in adjacent rooms. Fig. 2 shows a diagram of the coil and core holder. The excitation coils are located in the large body cavity, with the receiving coil located in the center of the cavity. The sample is placed in the center of the receiving coil. All materials other than the coils must be nonmetallic and nonmagnetic. The sample containers for the preliminary experiments were polyethylene, glass, or styrofoam. all of which ar invisible in the final image. For the flow experiments, a sample container (see Fig. 3) was constructed with polymethylmethacry-late platens at either end of the core. The assembly was constrained inside a Viton Tki sleeve by teflon shrink tubing. Water was introduced through vinyl tubing. A water-wet ceramic frit was included on the water inlet to ensure proper mixing at the face of the core and to inhibit oil flow back into the water line. The amount of water imbibed was measured by the change in the water- column height. The maximum pressure exerted by the water column was 10 cm 14 in.] of water. Thus, the experiments described here are largely imbibition waterfloods. The hydrogen protons of the fluids inside the magnetic field are excited by a radio-frequency (RF) pulse from the transmitting coil in the NMR body coil housing. SPERE P. 207^