Assessment of World First Two Polymer Injectivity Tests Performed in Two Giant High Temperature/High Salinity Carbonate Reservoirs Using Single Well Simulation Models and PFO’s Analysis

Abstract

Abstract Polymer flooding is a well-known enhanced oil recovery (EOR) technology and has been widely implemented in all around the world for more than 60 years, and mostly targeted medium-to-high permeability sandstone reservoirs with moderate salinity, hardness and temperatures. The envelope of polymer flooding has been expanded to harsher reservoir and field conditions where few years back were considered not feasible and applications tailored towards HSHT (High Temperature and High Salinity) carbonate reservoirs were uncommon. Development of novel polymers and innovative field application concepts allowed to re-consider polymer-based EOR as a promising technology to improve sweep efficient for these challenging reservoirs. Likewise, polymer injectivity is one of the key parameters to de-risk within the polymer flood projects and requires rigorous assessment of the polymer rheology in porous media and related injectivity losses in order to improve program design and polymer project economics. In that regard, this paper presents the interpretation of two polymer injectivity tests (PIT) performed in two giant light oil HTHS carbonate reservoirs in Onshore Abu Dhabi. The detailed data acquired in both tests was used to evaluate the polymer injectivity at representative field conditions and in-depth mobility reduction. The polymer injectivity tests together with extensive laboratory studies are part of a thorough de-risking program for upcoming world’s first innovative hybrid CO2/polymer-based (SIMGAP) and Water/polymer-based (SIWAP) EOR interwell pilots. The two PIT were performed in different light oil (~ 0.3 cP) carbonate reservoirs under harsh conditions of salinity (>250 g/L), hardness (>20 g/L), temperature (>250 °F) and H2S concentrations up to (40 ppm). In addition, the polymer used during the PITs is a new generation of EOR polymer with high 2-acrylamido-tertiary-butyl sulfonic acid (ATBS) content that was specifically developed to tolerate the harsh conditions and qualified based on extensive laboratory experiments. Subsequently, field trials were performed through PIT#1 in 2019 and PIT # 2 in 2021, respectively. Both PITs are composed of 3 stages; a multi-rate water flood baseline, polymer injection using different rates and polymer concentrations followed by extended chase water flooding. In addition, a sequence of multiple PFOs (Pressure Fall Off) were acquired during the PIT executions and analyzed to obtain the required uncertainty parameter ranges for the history matching exercise. Polymer pre-shearing was also considered as part of both PIT programs with the aim to homogenize the polymer molecular weight distribution, reduce possible shear-thickening effects near the wellbore as per laboratory measurements. Two mechanistic 3D simulation models were built to incorporate the information from polymer laboratory studies and to interpret the large field datasets during the PIT. Lessons learned from the PIT#1 allowed to optimise PIT#2 design program and achieve better understanding of polymer characteristics. The interpretation of the pressure transient analysis of the PFOs and the mechanistic 3D simulation models of the two PIT confirmed the generation of polymer banks and demonstrated effective propagation of the polymer into the reservoirs at target concentrations and representative rates of the future SIWAP and SIMGAP interwell pilots.