The choice of an Improved Oil Recovery (IOR) process for use in a particular reservoir depends on several factors; the habitat of residual oil, the properties of the reservoir fluids, reservoir conditions and reservoir heterogeneity.
Reservoir heterogeneity exists at all scales, from the micro to the mega-scopic. Previous workers have studied the effects of micro and meso-scale heterogeneities on IOR processes in detail, as many processes are designed to act at those scales, but have ignored macro-scale heterogeneities such as facies variations. These can have a large effect on an IOR process; controlling the magnitude and nature of the connectivity between wells, compartmentalising the reservoir and influencing the balance of capillary, viscous and gravity forces.
A database of 499 IOR projects in clastic reservoirs was collated. The macro-scale heterogeneity present in each reservoir was categorised by depositional environment using the Tyler and Finley Heterogeneity Matrix. The results show that successful IOR projects using a particular process cluster at certain combinations of lateral and vertical heterogeneity.
To investigate the distributions, a quantitative method of evaluating macro-scale heterogeneity was devised. These Lateral and Vertical Heterogeneity Indices (LHI and VHI) provide a simple method of summarising and communicating geological information between different people and disciplines.
Reservoirs with known levels of LHI and VHI were modelled, in which various IOR processes were simulated. Over 350 simulations of steam, polymer and Water Alternating Gas (WAG) injection processes were run and used to identify the processes that worked best under different levels of heterogeneity, dip and net to gross. The results showed that the Heterogeneity Indices can be used to predict the effect of macro-scale reservoir heterogeneity on these three processes and that objective, geologically based screening criteria could be derived. Using these criteria, it is demonstrated that in the high cost and low well density environment of the North Sea, WAG injection is the most viable IOR process, as the efficiency of the process is relatively unaffected by macro-scale heterogeneity.