SPE 169942 Using Polymer-Alternating-Gas to Maximize CO2 Flooding Performance for Light Oils

Carbon dioxide has been used commercially to recover oil from reservoir by enhanced oil recovery technologies for over 40 years. Currently, the CO2 flood is the second most applied enhanced oil recovery (EOR) processes in the world behind steam flood. Water alternating gas (WAG) injection has been a popular method to control mobility and improve volumetric sweep efficiency for CO2 flooding. The average improved recovery is about 9.7% with range from 6% to 20% for miscible WAG injection. Despite all of the success of WAG injection, sweep efficiency during CO2 flooding is a typically challenge to reach higher oil recovery and better application of the technology. The paper proposes a new combination method, named as polymer-alternating-gas (PAG), to improve the volumetric sweep efficiency of WAG process. The feature of this new method is that polymer is added to water during WAG process to improve mobility ratio. In a PAG process, polymer flooding and immiscible/ miscible CO2 injection are combined. To analyze the feasibility of PAG, couples model considering both miscible and polymer flooding process are built to study the performance of PAG. In this paper, sensitivity of polymer adsorption and concentration are studied. Feasibility of PAG in reservoirs with different permeability, different Dykstra-Parsons permeability variation coefficient (VDP), and different fluid are also studied. A reservoir model from typical section of North Burbank Unit is used to compare the performance between PAG, WAG and polymer flooding. This study demonstrates that PWAG can significantly improve recovery for immiscible/miscible flooding in homogeneity or heterogeneity of reservoir. Introduction Although CO2 flooding is a well-established EOR technique, its density and viscosity nature is a challenge for CO2 projects. Low density (0.5 to 0.8 g/cm 3 ) causes gas to rise upward in reservoirs and bypass many lower portions of the reservoir. Low viscosity (0.02 to 0.08 cp) leads to poor volumetric sweep efficiency. In heterogeneous reservoirs with high-permeability zones and natural fractures, the condition is even worse (Zhang et al. 2010). Almost all commercial miscible gas injection projects use WAG to control mobility of gas and alleviate fingering problems. Recovery of WAG is better than gas injection alone, and 80% of commercial WAG projects in the US are economic (Christensen et al. 1998). However, recent studies show that most of the fields could not reach the excepted recovery factor from the WAG process, especially for reservoirs with high-permeability zones or there are naturally fractured (Christensen et al. 2001). Gel application is considered the most aggressive type of conformance control. Gel acts as a blocking agent to reduce channeling through fractures or high-permeability zones of reservoirs (Ali and Schechter 2013). The most applied gel system in the oil industry for conformance control is hydrolyzed polyacrylamide (HPAM) with Cr (щ) acetate. Woods et al. (1986) presented one of the earliest successful gel treatments for Lick Creek field in Arkansas. Hild and Wackowski (1999) reported a successful gel treatment at the Rangely Weber Sand Unit in northwestern Colorado. In this treatment, a large-volume (10,000 bbl) chromic-acetate acrylamide polymer gel was applied to improve CO2 flooding performance. The cost of the gel treatment was estimated to be around USD 6 to 8/bbl, and the project return rate was 365%. Karaoguz et al. (2007) and Topguder (2010) reported several field applications of gel in Bati Raman field. In General, gels can treat water coning successfully in reservoir with vertical fracture. However, water coning through matrix reservoir is very difficult to be treated successfully with gels. On the other hand, conventional foams are considered effectively in matrix rock and are not applicable in reservoir fracture channels with aperture widths on the order of greater than 0.5 mm (Robert 2007). Bond and Holbrook (1958) first proposed the idea of using foam for mobility control. Since then, CO2 foam with surfactant