A Laboratory and Field Evaluation of the CO2 Huff 'n' Puff Process for Light-Oil Recovery

Abstract

Summary. Cyclic CO2 injection for enhanced recovery of light crude oil is investigated. Results from watered-out Berea corefloods and 14 field tests demonstrate that first and second cycles recover waterflood residual oil. Factors that may improve performance include larger reservoir slug volume, soak period, thicker interval, and lower prior water cut. Introduction This paper is a laboratory and field investigation of the CO2 huff ‘n’ puff process for enhanced recovery of light crude oil. The results of continuous and cyclic CO2 displacements with a 32 degrees API [0.87-g/cm3] stock-tank oil in watered-out Berea cores are presented. Fourteen single-well cyclic CO2 field tests in south presented. Fourteen single-well cyclic CO2 field tests in south Louisiana sands are examined. Laboratory results demonstrate that the CO2 huff ‘n’ puff process recovers waterflood residual oil. Incremental oil recovery process recovers waterflood residual oil. Incremental oil recovery increased with the amount of CO2 injected and was not benefited by operating at the minimum miscibility pressure (MMP). Maximum ultimate incremental oil recovery required a soak period and additional water influx. Incremental oil recovery continued with a second cycle of CO2, but a third cycle showed significant decline. Utilization factors averaging less than 2 Mscf [less than 57 std m3] of CO2 per barrel of incremental oil were achieved in 9 out of 14 field tests. Field results suggest that in the absence of mechanical problems, initial response improved with larger reservoir space occupied by CO2, with thicker net pay or perforation interval, and with lower CO2 reservoir viscosity. Lifetime response, however, improved with lower prior water cut. Neither reservoir pressure nor soak duration significantly influenced field performance. Field results confirm that the CO2 huff ‘n’ puff process recovers waterflood residual oil and that a second cycle can be successful. The laboratory corefloods and south Louisiana field data presented suggest that the cyclic CO2 injection process initially developed for heavy crudes is a viable and timely approach to greater ultimate recovery of light oils. Furthermore, the risk associated with implementation of CO2 huff ‘n’ puff is considerably less than that associated with other EOR methods, which typically have much longer project lives. project lives. The literature provides limited information on the applicability of the CO2 huff ‘n’ puff process to the enhanced recovery of light crude oil. The process is usually examined for very viscous oils, and computer-simulated results are more available than actual laboratory or field data. One report considers how light-oil reservoir properties might influence process performance. It seems likely properties might influence process performance. It seems likely that some reservoir and operational aspects of the light-oil/CO2 huff ‘n’ puff process will imitate the heavy-oil process, while others will require re-evaluation. This paper highlights these similarities and differences, provides experimental data on the light-oil process, and interrelates laboratory and field results. process, and interrelates laboratory and field results. This paper addresses six questions with laboratory corefloods to model the light-oil/CO2 huff ‘n’ puff process physically.Does cyclic CO2 injection recover waterflood residual oil?What is the role of MMP?Does water influx influence oil recovery?Is a soak period necessary?Can repeated cycles of CO2 yield additional oil?Does oil recovery depend on the amount of CO2 injected?Numerous variables were addressed in the evaluation of field results, including those reservoir rock and fluid properties that are typically used as screening parameters in full-scale CO2 miscible flooding. While the laboratory and field evaluations are not completely parallel, the combined results formulate a coherent picture of which factors contribute most significantly to a favorable field performance. performance. Materials and Methods The CO2 used for the laboratory corefloods was 99.6 mol% pure. The stock-tank oil used came from the Timbalier Bay field located in Lafourche Parish in south Louisiana. This 32 degrees API [0.87-g/cm3] crude was selected for the huff ‘n’ puff experiments because large quantities were available, and the oil did not show a tendency to form asphaltenes during CO2 sandpack displacements. Table 1 lists the physical properties of this crude. A schematic of the apparatus used for the continuous and cyclic CO2 corefloods is shown in Fig. 1. Although this apparatus includes a core-rotating device, the core was not rotated because gravitational overriding or underrunning effects were desired. The corefloods were performed in one 5.77-ft [1.76-m]-long, 2-in. [50.8-mm]-diameter Berea sandstone consolidated core. The core had a PV of 756 cm3 and a porosity of 21.2%, measured at room temperature and at about 1,000-psi [6895-kPa] pressure. For the cyclic CO2 displacements, the core was manifolded at the inlet and outlet to floating-piston transfer vessels of greater than 2,000-cm3 capacity. A positive-displacement pump filled with hydraulic oil was used to displace fluids from the transfer vessels into the core. The outlet end of the core was also manifolded to a production panel. Other features of the consolidated-core apparatus are described elsewhere. The use of consolidated cores is essential to delineate the effects of high water saturations. The water used in the corefloods was 50,000 ppm NaCl brine to minimize clay swelling. In addition, 20,000 ppm NaCl brine, isopropyl alcohol, toluene, and sometimes xylene were used to clean the core between runs. The permeability of the core was checked routinely, and when damage by hydrocarbon deposition was suspected, cleaning was performed in both flow directions. The direction of flow during the continuous CO2 displacements and during the huff portion of the cyclic CO2 tests was the flow direction used in routine permeability checks and in initial cleaning procedures. After cleaning, the core was saturated with brine at room temperature and about 1,000 psi [6895 kpa]. Then, about 1 PV of filtered crude was metered into the core. Irreducible water saturation was not established for economic reasons. Next, the core was flushed to residual oil saturation (ROS) with brine at a rate three to four times the CO2 displacement rate, yielding pressure drops at least thrice those prevalent during the coreflood experiments. SPERE P. 1168