A Steam-Soak Model for Depletion-Type Reservoirs

Abstract

A mathematical model for predicting first-cycle performance of steam stimulation in depletion-type reservoirs agrees reasonably well with field observations. It can be applied to both stratified and nonstratified reservoirs. Introduction The widespread application of the steam-soak process has made more essential an understanding of process has made more essential an understanding of the basic mechanism of the process. As presently applied, it consists of injecting an arbitrary quantity of steam into a formation, stopping injection and closing in the well for some "soak" time, and then producing oil from the injection well. Recent reports producing oil from the injection well. Recent reports on field applications have been given by Bowman and Gilbert, Adams and Khan and de Haan and van Lookeren. Theories to describe the steam-soak process have been presented by Boberg and Lantz Davidson et al., Martin, Seba and Perry, and Kuo et al. None of these theories has attempted to include the detailed distribution of the steam or the oil viscosity distribution. The present method is applicable to depletion-type reservoirs and includes the specific interval of steam penetration as well as the viscosity distribution resulting from heating. The method assumes that during the injection phase oil is displaced from the steam zone until some residual value of oil saturation is attained. During the production phase oil is snowed to flow back across the outer radius of the steam zone. The time to resaturate this zone is calculated. Heating of oil in adjoining strata results in a greatly increased flow of oil through the heated layers into the well during backflow. To estimate this effect, it is necessary to use the viscosity-temperature curve for the particular oil being considered. We hope that this method will be useful to operating personnel and that it will provide insight into some of the essential factors of the steamsoak process. Two types of formation are treated in the present method. For the first case, zero vertical permeability is assumed, and oil flows only horizontally. This calculation should be applicable to cases of horizontally stratified reservoirs. For the second case, isotropic permeability is assumed, and crossflow into the permeability is assumed, and crossflow into the depleted steam zone is estimated by means of crossflow factors developed for a range of formation thicknesses, steam-zone radii, and viscosity distributions. In many practical cases the vertical permeability will be significant but still less than the horizontal permeability. Results for this situation will then be inter permeability. Results for this situation will then be inter mediate between the two extremes calculated by this model. However, it should also be possible to compute crossflow for these cases as well. The model should apply to both light and heavy oil reservoirs. The relevant data used, such as steam-zone thickness, residual oil saturation in the steam zone, and oil viscosity, should be chosen accordingly. The effect of steam distillation is not taken into account explicitly but could affect some of the data chosen. JPT P. 757