Offshore Surfactant Injectivity Trials: From Core to Successful Field Trials in the Al-Shaheen Field

摘要

Surfactant injection as an EOR method is considered for the Al-Shaheen Field, offshore Qatar. The field is characterized by thin oil column, low permeability reservoirs, and large lateral variations in fluid properties. Currently, Al-Shaheen produces 45% of Qatar's total oil production. Kharaib, the most prolific reservoir of Al-Shaheen is an oil wet carbonate with an average porosity and permeability of 30% and 5mD respectively. Even after an extensive water-flood program, there are substantial amounts of oil left behind in the reservoir due to the highly oil-wet nature of the Kharaib rock. An extensive surfactant based EOR screening process was conducted to single out the best surfactant for improving the oil recovery. The selected surfactant was then injected in the field to de-risk future long term trials and measure injectivity improvement. A detailed EOR screening program helped select surfactants that were suitable under the field conditions. The selected surfactants were then screened further with contact angle, spontaneous imbibition and core flood experiments, which led to final selection of a wettability altering surfactant. Core flooding results were history matched to obtain necessary parameters for field scale modelling. The next step was to inject this surfactant in the field. Considering the low permeability of the Kharaib reservoir, it was decided to conduct injectivity trials for a short period to de-risk injectivity impairment. Additional objectives from these trials were to measure injectivity gains, demonstrate ability to inject surfactant in challenging offshore environment and to ensure repeatability. The candidate well for the trial was selected after screening for short reservoir section, with no pre-mature water breakthrough due to conformance issues, and logistical/ operational availability. The injection trials showed improvements in injectivity that is indicative of the changes in wettability. More importantly, there was no evidence of any injectivity impairment, which has paved the way for long term surfactant injection in Kharaib wells. Currently, a long term surfactant injection trial is being planned. Options such as single well tracer test, inter-well tracer test, micro-pilot tests, time lapse saturation logging, and drilling infill wells have been evaluated to quantify incremental oil. Various reservoir, fluid and geological parameters, logistical considerations, field applicability, drilling schedule and cost implications have been considered for determining the most optimum solution to help de-risk for field scale implementation of a long term trial. This paper presents a phased approach to scale up an EOR project in a highly complex offshore carbonate field. The approach taken, from screening the surfactants to short term injectivity trial and then continuing to a long term trial, is unique for an offshore field and in the current oil price environment. The cost effective means of executing the trials at different stages of the project are testament to the fact that EOR projects are possible even at low oil prices and in tough offshore environments.