Low permeability reservoirs contain a significant and growing portion of the world oil reserves, but their exploitation is often associated with poor recovery even after waterflood. Miscible or immiscible gas injection is usually the first choice in terms of EOR methods but it is not always feasible for instance due to lack of adequate supply. In such cases chemical EOR is often considered.
In this paper we propose to examine the specific challenges of chemical EOR in low permeability reservoirs by reviewing the well documented chemical EOR field operations that were implemented in reservoirs ranging from conventional low permeability (around 100 mD) to so-called tight reservoirs (few mD). Shale plays where permeability is in the µD range and which only produce when simulated by hydraulic fractures are not considered in our investigation.
We show that what works at the lab scale in low permeability plugs cannot be automatically transposed to the field scale. In particular low permeability can lead to injectivity issues and uncontrolled fracturing due to near wellbore plugging or simply to the high pressures required to propagate the injected chemical over large distances. Another challenging aspect of chemical EOR in low permeability reservoirs is the high chemical adsorption due to important surface to volume ratio and specific mineralogy, as in the case of carbonates (fractured or not). Success and failures of chemical EOR pilots in such challenging reservoirs, including innovative approaches such as wettability alteration, are reviewed.
Overall, this review will provide the reader with an updated view of past and on-going developments in chemical EOR applied to low permeability reservoirs. It should help operators determine whether a given low permeability reservoir is eligible to such processes or not.