A review of EOR techniques for carbonate reservoirs in challenging geological settings

A large portion of the remaining oil world-wide is located in tight formations, often of carbonate origin, with substantial heterogeneity. In this review article, we aim to study EOR opportunities from a reservoir characterisation point of view. The main categories chosen include heavily-fractured reservoirs, communicating reservoir layers with large permeability contrast, low-permeability reservoirs and finally reservoirs with large oil-water transition zones. The literature sources, which are critically reviewed, have highlighted suitable methods in each of the four categories. In many cases, field implementation suggests that EOR is indeed economically feasible and a number of key learnings can be extracted from these successful projects. In general, the earlier EOR can be implemented, the better. This reduces the incremental cost required for EOR and provides an earlier pay-back in terms of higher production. The increment due to EOR may become more difficult to quantify since the baseline case would have to be entirely based on numerical simulation. A low baseline recovery without EOR shifts the economics in favour of EOR. Use of the existing well stock is not always sufficient or suitable; in most cases EOR must be complemented by drilling of new wells with appropriate well type, well placement, sufficient monitoring and surveillance. Large fields offer more EOR opportunities because injectant breakthrough can be better managed by redirecting injectant to poorly swept areas. EOR can also be taken offshore; industry experience shows that if the size of the prize is big enough, EOR projects are being sanctioned. Gas injection has the broadest range of applicability for a number of reasons. Occasional lack of market to export produced gas from remotely-located fields provides a strong incentive for gas re-injection. Furthermore, gas offers better injectivity compared to water or chemical EOR methods. Gas injection does not require development of miscibility to be successful, although miscibility helps. Swelling of highly under-saturated fluids leads to oil viscosity reduction and oil saturation increase and delays premature gas breakthrough. However, gas based EOR may suffer from poor sweep efficiency due to geological heterogeneity, gravity override and viscous fingering leading to unfavourable mobility ratio and premature gas breakthrough. Hence, gas-based EOR will require mobility control to improve sweep efficiency. Appetite for risk taking varies among operators. Unique problems require unique solutions. Operators who demand firm proof before field testing may have to adjust their mind-set and pilot a new concept in a statistically significant number of wells to fully evaluate the potential.