Well test analysis, application to thermal recovery processes for reservoir characterization

Abstract

Thermal recovery by steam injection is considered to be a promising method for achieving a high ultimate recovery. A composite reservoir may occur during any enhanced oil recovery (EOR) project like steam injection into an oil reservoir. Thermal falloff test analysis offers a quick way to obtain an estimate of the swept volume and steam zone properties. Most of the models used for the analysis assume two regioncomposite reservoirs with different but uniform properties separated by a sharp vertical interface which is not very realistic. Numerical simulation study of steam injection in both vertical and horizontal wells (SAGD well pairs) was done to evaluate the applicability and accuracy of thermal well test analysis method and effects of several parameters on the results. Primary results showed that quite reasonable estimates were obtained. Some trends seen on the pressure plots, however, cannot be explained using the existing models and there are errors associated with the volume estimates that could be related to the simplifying assumptions of the conventional models. Therefore, the main objective of this PhD project is to develop a new analytical model for pressure transient analysis of composite reservoirs to improve previous models with inclusion of some parameters and more realistic assumptions. Two models are presented. In the models proposed, the interface separating the two regions is not sharp. Instead, there is an intermediate region between the inner and the outer region in which mobility and storativity decrease smoothly, as power-law functions of the radial distance from the first interface. The interfaces are considered to be tilted due to gravity effects. A single-layer model with continuous tilted front is assumed in the first model. In addition, tilted front is modeled using the conventional way of treating the gas override by assuming a multi-layer reservoir model in the second model of this study. Inclusion of steam condensation in the form of heat loss from the steam zone to the surroundings was first studied numerically with the conclusion that in the case of huge heat loss, pressure data are significantly affected and method of analysis should change. Heat loss effect is therefore included in the analytical models of this work. In another effort, a conventional multi-region model is improved to include the effect of gravity. This model with some modifications can finally match the models presented in this thesis within reasonable accuracy. The new sets of type curves for thermal well test interpretation are generated and verified against some conventional models. Effects of several parameters included in the models on the shape of type curves are discussed. The models developed can be used to predict the pressure behavior in thermal recovery processes and estimate reservoir properties using type curve matching instead of the conventional pseudo steady state method. The method is general and can be applied to other types of composite reservoirs.