Chemical injection has been emerged to be one of the processes that can improve oil recovery from major Malaysian offshore oilfield which is currently under waterflooding. Alkali-surfactant (AS) process was identified to be an optimized chemical system for this application. The chemical recipe and formulation for the field trial tests were obtained from comprehensive laboratory experiments studies. Pilot tests were designed and executed to evaluate the effectiveness of this chemical injection prior full field-scale implementation.
Single well chemical tracer (SWCT) technique has been utilized to determine the residual oil saturation (Sor) before and after chemical injection in a one-spot pilot and cost-effective manner approach. Two wells and two different chemical formulations were investigated for AS injection responses and four tests were conducted in sequence. The objectives were to validate the laboratory results, assess the critical chemical process parameters such as Sor reduction, adsorption, injectivity, and obtain an operating experience at a harsh offshore environment with high reservoir temperature. Sea water treatment and softening process was needed to protect chemical slug from high-salinity and high-hardness environment.
Favorable results achieve where successfully mobilized substantial amounts of Sor, chemicals easily mixed with no injection problems encountered, and no measurable dilution effects that indicated fluids travelling outside of test zones. The initial Sor waterflooding observe to be 0.16 and 0.27 for well 1 and well 2 respectively, while, interestingly, both wells show a 0.04 increase in Sor after first pilots were completed. It is most likely due to a shift in rock wettability toward more water wet. Competing reactions of alkaline was a major concern. Pre-flush and post-flush buffer of soft water was designed to minimize these reactions and allow surfactant to work in more favorable lower salinity water. Fortunately, Sor results show that the competing reactions were not severe enough to prevent the AS systems from working. The lowest Sor were 0.06 and 0.08 in the case of 1.5 PV and 1.0 PV soft-water buffer, respectively. It shows that Sor was not significantly reduced with additional 50% PV buffer. Even in the case of 0.15 PV buffer and 23,000 ppm salinity, Sor decreases only by 25%. It demonstrates that some degree of success will be gained even in the worst case where the salinity was reduced by 60%.
This paper presents the principals of SWCT pilot application for a chemical EOR (CEOR) project case study and share the best practices and lessons learnt were achieved from these field trail tests. This paper can be used as a technical reference and guideline for upcoming CEOR projects and promote a detailed development plan which can potentially address various challenges that are often encountered in implementing chemical flooding, particularly at offshore oilfields.