The Low-Tension Polymer Flood Approach to Cost-Effective Chemical EOR

Abstract

Abstract The Low Tension Polymer Flood approach addresses the two EOR problems of cost and control. Coinjection of low concentration surfactant and a biopolymer, followed by a further mobility control buffer, leads to much reduced overall chemical consumption, even in relatively high clay content rock. Though the optimum chemical injection period is more prolonged, the oil recovery timescale is not unduly prolonged. The LTPF process moves chemical flooding surfactant a long way toward being a simple, low-level waterflood additive process, attractive for field operations. It uses surfactant-polymer interactions as an advantage rather than coping with them as a problem. The chemical system described is based on a scleroglucan biopolymer (500 – 750 ppm) plus surfactant mixture (3000 ppm total surfactant). No cosolvent is used. This system was developed specifically for use with high salinity injection water (seawater) in a high temperature reservoir. The essential phase behavior description is given, together with other physical parameters and detailed core flood performance. Finally, we discuss the computer simulation approach used to predict reservoir performance and the results obtained. predict reservoir performance and the results obtained Introduction Many surfactant-polymer field tests have been conducted over the past three decades. This activity stemmed from the promise of highly effective oil recovery by surfactants in a period of increasing oil prices. Results of these tests and critical reviews published on them prices. Results of these tests and critical reviews published on them identified the main parameters important to the technical success of a surfactant-polymer project. One of the key findings is that preflushes to remove excessive salinity and hardness have been preflushes to remove excessive salinity and hardness have been ineffective and chemical systems capable of being effective in the presence of both injection brine and formation water are presence of both injection brine and formation water are needed. A second important conclusion is that effective and timely oil recovery from surfactant-polymer field tests correlates well with maintaining good mobility control of both the surfactant slug and the polymer buffer. Further, as the pattern size increases so does the polymer buffer. Further, as the pattern size increases so does the need for having a salinity tolerant system with favorable mobility control. However, this is not meant to suggest that other design parameters such as low interfacial tension and high injectivity are unimportant. Rather, we should focus on robustness and control as also being mandatory. In this context, waterflood modification with viscous polymer alone is usually reported as being technically and economically successful in field operations addressing mobility control problems. However the process effectiveness is generally low. It successful, oil recoveries are process effectiveness is generally low. It successful, oil recoveries are in the range of 2-12% of the residual oils. Since the oil price fall of 1986, concerns over the risks, costs, ineffectiveness and test failures considered to be associated with chemical flooding have led to widespread abandonment of chemical EOR research by the scientific community. In addressing these concerns we decided to explore ways of combining the effectiveness of surfactants with the control of polymers. The properties of the combined system, which we refer to as Low Tension Polymer Flood (LTPF), can confer some significant advantages. The aim of LTPF is to increase the oil recovery of a relatively ineffective but low chemical cost polymer flood by the addition of limited amounts of polymer compatible surfactants. The basic LTPF injection scheme described here involves the injection of up to 0.7 swept pore volume (PV) of LTPF with additives comprising about 3000 ppm surfactant(s) and 500–750 ppm polymer in a brine. This is ppm surfactant(s) and 500–750 ppm polymer in a brine. This is followed by 0.5 PV of graded viscosity polymer buffer. However, optimum flood design, including presence or absence of a polymer preflood and length/level of chemical additions, depend on reservoir description. If operationally attainable, the salinity of the make-up water used for the polymer buffer may be made lower than that used during LTPF injection to take advantage of the salinity gradient effect. Thus LTPF is technically a bridge between the surfactant and polymer flooding processes. processes. P. 475