Effectiveness of Hot-Water Stimulation of Heavy-Oil Formations
Authors: Diaz-Munoz J. , Ali S.M. Farouq 
Published: March 2010

Abstract
In Venezuela and California, heavy-oil producing well are often stimulated by hot-water injection into the formation. Because of the resulting decrease in oil "Viscosity, wellbore cleanup and other minor thermal effects, the oil production rates usually show an increase over the prestimulation rates.
The present investigation involved numerical simulation of the hot-water stimulation process. Heat and mass transport equations for a two-dimensional radial flow system were formulated and solved by two different methods. Tile stability and the sensitivity of the simulator developed were examined. In particular, the variation of the results obtained with changes in thermal conductivity, and also with relative perm abilities, assuming these to be temperature-dependent, were examined. Following this, the simulator was used to investigate the relative benefits and effectiveness of the stimulation of wells in heavy oil formations by hot-water injection.
The principal findings of the simulation studies, under a wide variety of conditions, were as follows.For a given heat input and injection rate, the production rate and the oil cut increase with an increase in water temperature.The total oil recovery increases with an increase in the soak time. Although the peak production rate does not change appreciably, the rate decline is slower for the longer soak times, thus showing the importance of conductive heat transfer.More oil is recovered for lower injection rates, all other factors being equal. This again shows the importance of conduction-heating.Hot-water stimulation is more effective for heavier oils. The effectiveness of the stimulation process decreases markedly for oils of moderate viscosities, and it is nil for light oils.
The simulator results were relatively insensitive to the thermal conductivity of the formation and the surrounding media, in view of the short injection times involved. Similarly, variations of relative-permeability characteristics with temperature Were found to have a negligible effect, for the data tested.
The results of this study indicate that the increases in both rate and recovery obtained from hot-water stimulation are considerably lower than one would expect on the are of some of the reported field data, strongly suggesting that wellbore cleanup is the key factor in the success of a hot-water stimulation program, with oil viscosity reduction playing a secondary role. This is substantiated by the negative results obtained for moderately viscous oils, where the small gain in oil recovery may be offset by the loss of production during the injection period. As a result, it u; concluded that hot-water stimulation should be employed only in viscous oil formations, and its benefits should be carefully examined prior to field implementation.

INTRODUCTION
A THERMAL STIMULATION that has been employed in some instances, principally in Venezuela and California involves alternate injection of hot water into heavy oil producing formations, much along the lines of cyclic steam stimulation. Such treatment may be more advisable than steam injection if the mechanical condition of the wells is not suited for steam or if the wells are very deep, and high pressures are required, together with relatively low temperatures.