Due to economic considerations, some operations involve commingled Improved Oil Recovery (IOR) such as water flooding of more than one overlapping reservoirs via the same wells traversing them. When different reservoirs have dissimilar reservoir quality (e.g. permeability) and different extents of oil saturation depletion/ pressurization, they might offer different levels of resistance during an IOR operation. Consequently, the more prolific reservoirs may accept the bulk of injection and contribute disproportionately to total fluid production, bypassing oil in the lower quality or lower pressure intervals. This could lead to serious technical and economic challenges in sustaining acceptable performance of commingled operations, especially when injection costs are high.
Potential problems besides oil bypassing, include inadequate injection into some intervals, cross-flow (due to significant pressure differentials across intervals) especially during production/ injection interruptions. Although during early periods we may see benefits of increased oil rates (the main motivation for commingling), very often the combined oil recovery may show only modest improvement over individual or sequential exploitation. Commingling thus makes sense only if some of the contributing intervals are otherwise uneconomic to exploit.
Some of the conditions favorable to commingled IOR are:
Well trajectory/ completions that permit interventions
Well completions allowing zonal isolation
Surface facilities that enable oil production at high water-cut and gas-oil ratios
Ability to obtain information on individual reservoirs via measurements of pressures, individual fluid contributions (PLT, spinner, etc) and fluid sampling
Maintaining sand-face pressure in production wells below pressures in all contributing intervals; and in the injection wells, above all of them
Sufficient injectivity into each pay interval at the proposed injection pressure
Fluids from different intervals being compatible with each other and with the reservoir rocks (no clay swelling, damage, fines migration, precipitates or mixing of sweet and sour fluids)
Minimal interruptions in injection-production operations
Periodic reviews of performance to ensure that operation is optimally managed.
Due to economic considerations, many future operations might involve commingled IOR from more than one reservoir via the same wells traversing them, as we attempt to exploit resources of progressively lower quality. Such situations are encountered in fields containing many different vertically overlapping reservoirs when some reservoirs are otherwise uneconomic to exploit by flooding. If only two reservoirs are involved, one could consider their individual exploitation via separate wells or the same wells with dual completions. But when multiple reservoirs are involved, IOR operations may have to be conducted in a commingled fashion. As an illustration, suppose wells # 1, 2 and 3 traverse partially overlapping reservoirs A, B and C (Figure 1). Well # 2 could be converted to injection to sweep oil from these reservoirs towards wells # 1 and 3. First, we evaluate pros and cons of sequential exploitation of the reservoirs, from the best to the worst (assumed in order of ‘quality’: B, A and C) as the ‘Base Case’. Next, we consider the option of exploiting all the three simultaneously or ‘concurrently’ (Option 1, Figure 3). Alternately, we could also consider the option of flooding the best reservoir ‘B’ alone, follo
