Study on High-Concentration Polymer Flooding To Further Enhance Oil Recovery

Abstract Initially the field operator considered injection of high-concentration polymer to be high-risk and marginally economic. But the results of the core flood experiments show that an incremental recovery over water flooding of more than 20% OOIP (original oil in place) can be obtained by early time injection of high molecular weight, high concentration polymer solutions, and the earlier the better. The incremental recovery is comparable to surfactant flooding but at a lower cost. A high concentration (concentration 1,500 ppm ∼ 2,000 ppm, viscosity 400 mPa.S ∼ 500 mPa.S) polymer flood pilot test has been started in May 2003. The pilot is located in the West Central area of Daqing Oil Field and consists of 9 injectors and 16 producers. Before high concentration polymer flooding, 0.247 pore volumes of low concentration (1,000 ppm, viscosity 44mPa.S∼51 mPa.S) polymer solution had been injected and the water cut at the central area had reached its lowest point (65.5%). By the end of April 2006, the water cut at the central area was 78.9%. After injecting high concentration polymer solution, the rate of the water cut increase slowed down and then the water cut decreased from a maximum of 79.6% to a minimum of 73.1% (a decrease of 7.5%) again. The current recovery over that of water flooding at the central area is already 16.7% OOIP. According to numerical simulation, when the water cut reaches 98%, 19.8%∼22.9% OOIP incremental recovery over that of water flooding will be attained. 6 dispersed wells located in the West Central area of Daqing Oil Field started to inject a higher concentration of polymer fluid also rein March and August, respectively. The 6 wells were first injectedby low concentration polymer and then increased the concentration of the injected polymer solution from 1,000ppm to 1,300ppm. At present, good results have been obtained. Another pilot is located in the northwest of Daqing Oil Field and consists of 39 injectors and 44 producers. The pilot increased the concentration of the injected polymer solution from 1,250ppm to 1,800 and the molecular weight of polymer from 25×106 to 35×106. According to numerical simulation, when the water cut reaches 98%, 19.4OOIP incremental recovery over that of water flooding will be attained. The above results show that this technology can enhance oil recovery economically. Introduction In many cases, about 60–70% of the oil remains in the reservoir after waterflooding. This remaining oil is the target for various enhanced oil recovery methods. According to theliterature,1 2 polymer flooding increases capillary numbers by only about a dozen times, which is less than the increment needed to recover residual oil. This would indicate that polymer flooding can only improve the swept volume and not the sweep efficiency. But in recent years, many petroleum engineers consider that polymer flooding also can improve microscopic-oil displacement efficiency because of the viscoelastic properties of polymer solutions.3 4Core flood analysis indicates that injection of a high-concentration polymer solution can increase by 20% the incremental oil recovery after waterflooding.5 Daqing undertook its high-concentration polymer flood pilots after completing successful pilot injectivity tests of the high-concentration (1,500–2,000 ppm and 400–500 mPa-sec viscosity) polymer fluids. The field pilots attest to the feasibility of high-concentration polymer flooding and provides technical and practical experiences for expanding this technique to other areas of Daqing. Only a few fields have had such high concentrations of polymer fluids injected and this pilot should become a valuable reference for designing and implementing future chemical floods, especially polymer projects.