Evaluation of Surfactant Formulation for EOR in Some Depleted Oil Fields of Upper Assam

Abstract

Enhanced Oil Recovery (EOR) techniques are becoming the thrust area to recover the residual oil in most of the matured depleted reservoirs. The microscopic displacement efficiency (E_D) of chemical Enhanced Oil Recovery (CEOR) particularly surfactant and/or alkali are dependent on the interfacial properties between the reservoir fluids, mainly crude oil and formation water. This paper discusses the details of all the results of the laboratory experiments carried out to develop an alkali-surfactant (A-S) EOR slug for two depleted oil fields of Barail formation of Upper Assam. A series of experiments such as IFT, adsorption study, compatibility study and thermal stability have been carried out to optimize an A-S formulation. Depending on the anionic nature of most reservoirs of Upper Assam, an anionic surfactant system comprising of a natural surfactant ‘black liquor (BL)’ and a synthetic surfactant ‘Sodium Dodecyl Sulfate (SDS)’ was adopted. This system at its critical micellar concentration (CMC) was mixed with alkalis: Na₂CO₃ and NaHCO₃ and corresponding CMC values at the lowest IFT were recorded. IFT experiments conducted separately with formation water (containing K⁺, Ca²⁺, Mg²⁺) and with laboratory prepared brine (containing Na⁺ only) established that IFT values are lower with formation water than that with the later. This study conducted in the temperature range from 30 °C to 80 °C showed that IFTs increase slightly with temperature, but within the range under study, temperature did not appear to have significant effect on IFT. Based on IFT reducing capabilities, a slug comprising of BL, SDS and Na₂CO₃ was selected for the EOR application in the considered depleted oil fields.

Thermal stability tests were carried out to evaluate the thermal decomposition tendency of the components. Both the surfactant system and alkali did not undergo any thermal degradation upto 90 days. The core flood experiments were carried out on core samples of the selected oil field with the optimized AS formulation following Buckley Leverette theory and calculations were done by Johnson, Bossler and Neumann (JBN) method. AS formulation thus developed in the laboratory was capable of lowering the oil water IFT to the range 10⁻²–10⁻³ mN/m and the components were very much compatible with each other at reservoir temperature. Thermal stability study also produced expected results. The results of the core flood experiments were encouraging as 50–76% additional recovery has been achieved.