Lessons Learned from a Prematurely Ended High-Pressure Air Injection Test in a Light Oil Naturally Fractured Reservoir

Abstract

Sixty billion barrels of oil still reside in the matrix of mature onshore and offshore Mexican reservoirs located in the southeast basins after primary and secondary recovery. Capillary and viscous forces are responsible for this amount of oil retained within the pore structure of the matrix (immobile oil). Gravitational forces are not enough to counterattack these forces due to the high fracturing intensity. On the other hand, laboratory testing demonstrates that oil residing in the matrix could be mobilized by the exothermic reaction that takes place with air injection. Air injection in homogeneous heavy and light oil sandstones and nonfractured limestones, at small or large scales during short and long periods of time, is feasible for producing resources technically and economically nonrecoverable by other means. However, to the best of our knowledge, the published literature does not report any application of an air injection project in naturally fractured reservoirs. During 2015, an air injection pilot test was performed in a light oil naturally fractured reservoir in Mexico, referred to as “A” field. The implementation of the pilot test was preceded by its corresponding laboratory study, which consisted of five accelerating rate calorimeter (ARC) tests and two combustion tube (CT) experiments. The analysis of the aforementioned experimental work led us to corroborate that air and oil react at reservoir conditions. Based on the above finding, the pilot test was conducted by injecting air at a rate of 10 MMscf/D with a wellhead pressure of 4,500 psia for 1.5 years, which was followed by a 1.5-year production period giving a total of 3 years for the pilot test. The results indicate that combustion was successfully applied in the reservoir. However, no oil was produced. This paper discusses the results of a prematurely ended air injection pilot test in “A” field and the main lessons learned from it, which could help in the design and its subsequent implementation in other naturally fractured reservoirs.