Development of a New Theoretical Model for Three-Layered Reservoirs With Unequal Initial Pressures

Abstract

Abstract In this paper we present a new theoretical model for three-layer reservoirs with unequal initial pressures. We developed a semi-analytical simulator based on the developed model. The computational model proved to be much faster than a three-dimensional, finite-difference commercial simulator used to validate the analytical model. Also, the amount of information (spatial gridding) needed to run the model is much reduced. The analytical model allows each layer to have different layer properties, different boundary conditions, and different initial pressures. We used the semi-analytical simulator to perform a detailed study of the behavior of three-layered reservoirs during the "pre-production period." The pre-production period occurs early in the life of the reservoir, after perforation but before any surface production and is caused by crossflow in the wellbore. The layer information obtained is very important for scheduling production and making economic decisions concerning the future of the wells. The pre-production well test requires no production on the surface during the test; thus the impact on the environment is negligible. The main objective of this work is to develop a qualitative approach to extract more information about the reservoir from pre-production wellbore pressure, dimensionless pressure and dimensionless pressure derivative curves. More specifically, we studied the pre-production behavior of three-layered reservoirs. Effects of the ratio of flow capacity and storage in different layers were investigated. We found that the layers with highest and lowest permeabilities can be characterized with the logarithmic time derivative of pre-production pressures. A positive derivative indicates high permeability in the layer with the highest initial pressure, and a negative derivative indicates high permeability in the layer with low initial pressure. Also, the layer with the highest initial pressure will always flow into the wellbore whereas flow will always be from the wellbore into the layer with minimum initial pressure. As the sandface rates reach steady state, the three-layer system behaves like an equivalent single layer system. Thus, we can apply a single-layer model to analyze the late transient in high permeability reservoirs.