Computational Modeling of Multi-Layered Reservoirs With Unequal Initial Pressures: Development of a New Pre-Production Well Test

Abstract

Abstract The objective of this paper is to present a description and verification of a new semi-analytical simulator used to model the wellbore performance of multi-layered reservoirs with unequal initial pressures. The semi-analytical simulator proved to be faster than a conventional three-dimensional, finite-difference commercial simulator. Also the amount of information needed to run the model is much reduced. The semi-analytical simulator allows each layer to have different layer properties, different boundary conditions, and different initial pressures. The semi-analytical simulator was used to model the reservoir performance for two, three and five layered reservoir cases from the literature. The results were verified by comparing them to the results generated using a finite-difference simulator. The agreement was excellent for all the tested cases. Also, we developed a new well test, the Pre-Production Well-test or PPWT. The PPWT is performed early in the life of a reservoir when the information is most needed for planning production schedules and making economic decisions concerning the life of the wells. Pre-production is the period after completion but before production of the well. Immediately after perforation, we position a pressure gauge above the top perforation to measure the pressure performance from the total system (in two or three layer systems). Cross flow in the wellbore from one layer to another will cause the pressure signal. The cross flow is due to the differential pressure between the layers. One important advantage of the pre-production well test is that there is no production on surface during the test. Thus, the environmental impact caused by flaring oil or gas during a conventional well test is alleviated. We developed the "Derivative Extreme Method" (DEM) for the analysis of wellbore pressures measured during the pre-production well test. The DEM determines layer properties form single pressure profile; no rate measurements are required. The DEM requires that the wellbore pressure be measured until one boundary is felt (i.e., the late-transient region must be reached).