This paper discusses the results of an experimental study. using a scaled modelfor investigating the steamflood process under bottom water conditions. Themain emphasis was on the use of small amount of solvent with steam. Otherstrategies for improving steamflood efficiency were also examined. Theprototype modeled is a heavy oil reservoir in Canada, where a steamflood iscurrently in progress.
In the experiments conducted, the emphasis was on a thin heavy oil formation, with underlying water, and in some cases, a gas cap. Such reservoirs areconsidered to be uneconomical for a conventional steamflood for reasonsdiscussed. The purpose of using solvents in combination with steam is toincrease steam injectivity in the oil zone, and create an initial flow path inthe oil layer in order to divert the steam partially from the underlyingwater.
The experiments simulated bottom water thicknesses from one-tenth of the oilzone to one-fourth. Solvent slugs of the order of 10% pv were injected prior tosteamflooding. The injection- production intervals were among the variablesinvestigated. In several runs, the model was waterflooded prior to thesolvent-steamflood, while in other runs, the initial oil saturationcorresponded to irreducible water. It was found that, depending on theinjection parameters (solvent slug size, rate, injection interval) and bottomwater-to-oil zone thickness ratio, oil recovery varied from 11% to 34%. Theexperimental results were compared with those predicted by classical steaminjection theories, with fair to good agreement in most cases.
Many heavy oil reservoirs in Alberta and Saskatchewan contain a characteristicbottom water zone, which creates a serious field problem when a steamfloodprocess is implemented. The presence of the underlying water zone causes alarge amount of the injected steam to be diverted away from the target oil zoneand into the bottom water layer. This lowers the steamflood thermal efficiencyand therefore a steamflood is uneconomical in such reservoirs.
The main focus of this research was to incorporate solvent injection with asteamflood process in an attempt to improve oil recovery from thin heavy oilformations with underlying water zones. The purpose of the solvent sluginjection prior to steamflooding was to create highly conductive flow channelsin the oil zone for the steam to penetrate. The solvent channels formed in meoil region would aid in partially diverting the steam away from the detrimentalbottom water zone, resulting in a greater volume of oil being heated, mobilizedand consequently produced. The objective of the solventsteamflood method was t0guide the steam into the upper portion (oil layer) of the bottom waterformation and keep the steam there as long as possible.
Other recovery schemes were investigated which also attempted to divert thesteam away from the underlying water and into the oil region. The alternatemethods considered were gas injection prior to a steamflood, or asolvent-steamflood, and steamflooding with a limited barrier at the oil/watercontact, extending radially outward from the injection well.