CO2-EOR: A feasibility study of an Indian oil field

Near the west coast of India there is an oil field producing since last 51 years. The oil field consists of multi-layered Eocene sandstone reservoirs. The main reservoir layer was put on water flooding since 1966. The field has produced about 50% of the initial oil in place. The oil production has substantially declined in the field and now the water cut is more than 90%. The field has light oil with varying API between 45 to 470 API from bottom to top layers of the reservoir. Estimation of EOR potential in brown oil fields is an important input to the company in order to decide if large scale CO2-EOR is being planned. A good EOR potential (tertiary recovery) can extend the life of oil field for many years (typically 15-20 years). This paper shows the preliminary results from simulations of CO2 injection into a representative reservoir model of an onshore Indian oil field. Enhanced oil recovery (EOR) by CO2 injection is an attractive option because it has the potential to increase the oil, gas and condensate recovery of producing fields. The oil reservoir under study was under massive strategic and infill water flooding resulting in high recovery from the field. However, now the plan is to use more advanced tertiary recovery methods so as to increase the recovery. Using CO2 as injection fluid has several advantages. In case CO2 develops miscibility in the displacement front with the oil in-place when propagating through the reservoir at the reservoir conditions, then it enables miscible displacement of the targeting capillary trapped residual oil after water flooding (Per and Idar, 2010). The density of CO2 at reservoir conditions is in most cases lesser than the injected water and it may therefore reach other parts of the reservoir and consequently improve the sweep efficiency. The oil reservoir under study is Eocene sandstone reservoir primarily due to fluvial deposits. Representative live reservoir oil is composed to represent fluid properties of the oil. The Soave-Redlich-Kwong (SRK) equation-of-state EOS is used to characterize the crude oil and gas, to recombine the two and to calculate the phase behavior, the composition of the gas and liquid phases and the physical properties of the phases at relevant pressure and temperature conditions. The EOS model is based on compositional analysis of the stock-tank oil and gas samples from the field as well as fluid properties from laboratory analysis. A commercial software program PVTSim was used to build the EOS model, to tune the parameters and analyse the fluid properties. The oil field under study has 11 main producing layers, some of the good reservoir sands are further divided into sub-layers. Based on the laboratory and log data, representative petrophysical properties and fluid compositions have been used in the models. Different injection schemes including CO2 injection with and without recirculation of CO2 breakthrough gas and CO2− water-alternating-gas (WAG) have been evaluated.