Investigation of Suitable Slug Models for Polymer Alternating Gas Deployment (Niger Delta Case Study)

Abstract

Abstract Core Flood analysis is one of the processes that can be deployed towards investigating the property changes associated with the reservoir post polymer flooding. The core flooding system is a test to determine rock permeability and how a core sample will interact with various fluids. The core sample is placed in a rock core holder. Once the core is inserted in the core holder, the outer surface is pressurized to simulate the same reservoir pressure created by the overburden in the reservoir. A test fluid is subsequently passed through the core and the flow rates and pressure drops across the system is measured. From this data, various parameters are measured including unsteady state liquid relative permeability and single-phase permeability, EOR tests, Residual Resistance Factor (RRF) and Resistance Factor (RF). The mobility of water to the mobility of a polymer solution under the same conditions is defined as the Resistance Factor (RF) (Jennings et al. 1971)1. This is a good index in the determination of the success of a good polymer flood (see the equation below). Kw and Kp are water and polymer relative permeabilities and µw and µp are water and polymer viscosities, respectively. RF = KwKpμpμw Residual Resistance Factor (RRF) is another useful parameter which is defined as the ratio of the initial water mobility to the water solution mobility after polymer flooding (Jennings et al. 1971)1: where wi indicates the water permeability or viscosity at initial condition (before polymer injection) and wa is the water solution permeability and viscosity after polymer flooding. The RRF is a quantitative indication of the reduction of water mobility which controls the water fingering process due to water injection after polymer flooding (Jennings et al. 1971)1.RRF = KwiKwaμpaμwi This study investigated the RF and RRF associated with a Niger Delta reservoir post Polymer Alternating Gas (PAG) flood which was subsequently instrumental in the selection of an optimal (PAG) slug model. Biopolymer (Xanthan) and Synthetic polymer (Polyacrylamide) were used as the base fluids. Polyacrylamide which is a copolymer of acrylic acid/acrylamide and Xanthan has been used extensively in enhanced oil recovery (EOR) processes. High molecular weight polyacrylamide strongly increases the water viscosity and thus decreases the water–oil mobility ratio in water-flooding. The Resistance Factor and Residual Resistance Factor for single phase saturations and slug deployments investigated gave interesting outcomes for each polymer type. PAA gave a more favorable outcome as indications from the flooding suggests the existence of high injectivity associated with the Xanthan flooding. Results from the different Slug scenarios suggest that using smaller slug model would not be favorable for the deployment. Larger slug models yield better RRFs. A combination of larger slugs of CO2 + Polymer gives better results and enables greater injectivity.