Microscopic Rock Characterization and Influence of Slug Size on Oil Recovery by CO2 Miscible Flooding in Carbonate Oil Reservoir

Abstract

ABSTRACT Carbon Dioxide (CO2) has been recognized as a preferred miscible solvent for Enhanced Oil Recovery (EOR) methods. Miscible flooding by CO2 has proven its positive effectiveness through numerous laboratory and field projects. The main goals of this study are to provide microscopic reservoir rock characterization, and optimize the selection and the effect of CO2 slug size to be applied for carbonate oil field, U. A. E., on oil recovery by CO2 miscible flooding. Scanning Electron Microscope (SEM) is used to provide accurate description of the reservoir rock surface topography. The SEM is used to attain the pore size distribution of the rock. Energy Dispersive X-Ray Spectrometer (EDS) and Inductively Coupled Plasma (ICP) are applied for detailed accurate analysis of elements constituting the rock. Five flood experiments were performed to achieve the goals of this study. The first experiment was carried out using continuous CO2 flooding. Other three experiments used different CO2 slug sizes of 0.15, 0.30, and 0.45 hydrocarbon pore volume and chased by actual reservoir brine.The fourth experiment was carried out to evaluate the performance of waterflooding. All experiments were performed using actual crude oil injected into actual reservoir rock samples of almost 23 cm length and 10 cm diameter. These samples were saturated at initial-oil-in-place condition and then subjected to water and/orCO2 floodings. The minimum miscibility pressure (MMP) was calculated to be 3,700 psia using current correlations from literature. To secure good miscibility condition, all experiments were conducted under injection pressure of 4,200 psia and at 250 oF similar to the reservoir condition. The results indicated that water flooding recovered 66 % IOIP and a continuous CO2 miscible flood produced 96 % IOIP. Furthermore, the increase of slug size increases oil recovery by CO2 miscible flooding, but the slug size of 0.15 HCPV is recommended since the increment in oil recovery using larger slugs does not represent significant addition to the oil recovery in comparison to applied smaller slug sizes. 1. INTRODUCTION AND LITERATURE REVIEW The increase of CO2 in the atmosphere may cause climate changes. Therefore, required measures are necessary to regulate the emission of green house gases. Carbon dioxide may be implemented in miscible, near miscible or immiscible forms after water flooding as an enhanced oil recovery (EOR) method or as a secondary displacement method. Most CO2 floods are of the miscible type where CO2 is either continuously injected or intermittently injected with alternating injections of water. The application of CO2 injection into an oil reservoir in Norwegian continental Shelf indicated that CO2 injection produced 63 % IOIP while water injection recovered 43 % IOIP (Lindeberg and Holt, 1994). The increment in oil recovery by miscible flooding was attributed to improvement in displacement efficiency and reduction in oil viscosity. Grigg and Schechter (1997) reviewed 25 CO2 projects as a maturing EOR process. They concluded that the application of CO2 floods provided good increments in oil recovery. On the other side, there were always concerns about high mobility of CO2 and its early breakthrough (BT). Other problems were identified about the source and cost of CO2. The study also indicated that most of the newer floods and some older floods increased CO2 slug size, injected CO2 continuously till CO2 breakthrough, and then convert to water-alternating-gas (WAG) process.