Formulation of Chemical Slug for EOR Application of Moran Oil Field of Upper Assam Basin, India

Abstract

Generally, most of the depleting reservoirs of Upper Assam Basin, India are treated with chemical enhanced oil recovery (CEOR) processes which are one of the tertiary oil recoveries to recover about 30–60% of oil which cannot be produced by means of primary or secondary recovery methods. CEOR processes are mainly emphasise on injecting surfactants (S), alkalis (A) and polymers (P) or a combination S+A or A+P or S+P or A+S+P to the aqueous phase of the depleting reservoirs. The S reduces interfacial tension (IFT) between oil and aqueous phase and increases displacement efficiency (ED) of the reservoirs. While the A tend to reduce the wettability from oil wet or less water wet to more water wet. A also decrease the adsorption of S into the porous media. Water floods are often very inefficient in naturally sandstone oil reservoirs because many of these reservoirs are mixed-wet or oil-wet towards the end of secondary brine flooding as well as extremely heterogeneous. These reservoirs are challenging targets for chemical flooding because they typically have a high permeability contrast between the fractures and the matrix with low matrix permeability. Some of the world’s largest oil reservoirs are fractured sandstone with high reservoir temperature and high salinity formation brine. Some of them also have low APIo gravity oils, which increases the difficulty of recovering the oil. A slug comprising of surfactant and alkali has been developed that shows promising results for such difficult reservoirs. Ultra-low interfacial tension (IFT) and good aqueous stability were achieved with this surfactant slug in high salinity brine at a high reservoir temperature of 78 ℃. Both static and dynamic imbibition experiments were conducted using a sandstone core of Moran Oil Field. Oil recovery was higher in only SDS+NaOH slug flooding compared to flooding of SDS, NaOH and SDS+NaOH sequentially. The oil recovery is good taking into account that the temperature and salinity conditions were harsh, the core was less permeable and fractured, no mobility control was used, and only a small surfactant slug was injected. The unsteady state coreflood results were interpreted using Johnson-Bossler-Naumann (JBN) method. It showed that both the mechanisms of IFT reduction and wettability alteration were important for oil recovery. Neither IFT reduction nor wettability alteration alone recovered oil as high as the combined contributions from both. So, this study focuses on effect of S, A followed by S+A slug flooding (F1) or flooding of S+A only (F2) to determine the efficiency of residual oil recovery from the depleted Moran Oil Field. The residual oil recovery was calculated by JBN method based on Buckley Leveret assumptions for unsteady state process.