A clear understanding of mechanisms impacting on performance of water floods in the field is critical to identifying optimal operating strategies. We made some empirical observations from certain ongoing water floods in the field suggesting that varying of injection and production rates can lead to improvement in production performance.
We speculate that variable rate water floods involving frequent flow ‘perturbations’ can lead to redistribution of water and oil flow triggered by pressure transient effects. The latter results in changes in capillary ‘holdup’, typically involving short periods (hours) whereas the former (saturation and flow redistribution) results in changes to flow stability (viscous fingering) and involves relatively long periods (weeks or months) due to hystereses of relative permeability and capillary pressures.
It is contended that verification/ further development of these insights would be useful in optimizing performance of water floods/ improved oil recovery projects.
Significant variations in performance of various water floods observed in the field are often attributed to differences in geological/ reservoir features, and to operating practices. To improve the latter, surveillance and optimization of water floods are undertaken by the operators. These usually involve changes in water quality and handling aspects, testing of reservoir continuity via pressure pulses and tracers, infill drilling, altering rates of injection and production (‘pattern balancing’), controlled fracturing, selective exclusion of intervals/wells producing excess water, etc. Oftentimes these measures result in the desired improvements, but it would be fair to say that common controllable causes of poor water flood performance are not well understood.
A clear understanding of mechanisms impacting on performance of water floods in the field is obviously critical to identifying optimal operating strategies. We made some empirical observations from certain ongoing water floods1 in heavy oil reservoirs, suggesting that varying of injection and production rates can potentially lead to significant improvement in production performance. Here we extend the analysis to light oil reservoirs.
The basic premise here is that we can derive useful insights on variations in transient distribution of oil and water within a given reservoir by studying selected field projects. These insights can then be the bases for identifying optimal operation of water floods/ EOR.
Our basic premise is we disturb the prevailing quasi-equilibrium in the reservoir via cyclically varying injection rates and by causing transient changes to the fluid and flow distributions. Our hypothesis is based on the following postulates:
High rates, in addition to accelerating the oil drainage, enhance chances of mobilizing oil from parts of a heterogeneous reservoir that might otherwise be bypassed.
High flow rates promote frontal instability that is further aggravated by adverse mobility ratios. Under these conditions, water bypasses some of the oil and results in an early water breakthrough leading to excessive water production thereafter. This often results in inefficient displacement of oil in terms of large processing requirements for source and produced water.
