Abstract

A linear composite reservoir is defined as one consisting of two regions of different rock and fluidd properties and in which the fluids flow in one direction away from the well bore. Many steam or insitu combustion pilotsdevelop communication paths between wells where fluids flow basically in one direction away from the injection well. This type of reservoir should be more appropriately represented by a linear rather than radial composite reservoir system. An analytical model has been developed and used to investigate the effects of well bore storage, skin damage, mobility ratio, swept zone volume and various reservoir boundary conditions on the transient pressure behaviour in a linear composite reservoir. The results appear to be consistent with the pressure buildup data obtained from a three ell insitu combustion pilot where a communication path between these wells is known to exist. Work is being done to apply this model for determining the transmissibility (kh/ μ) of a communication path by matching the pressure history obtained at either the injection or the observation wells.

Introduction

Due to the poor injectivity in most oil sand deposits, it is believed that the formation must be fractured to allow steam injection at a reasonably high rate. Initially, the steam zone surrounding the fracture is probably elliptical in shape. As the steam injection continues or after the well has been operating for several injection or production cycles, the steam zone may become more and more circular in shape. Surrounding this steam zone is a bank of heated oil which results in a radial composite reservoir configuration. A similar radial composite reservoir system may exist for the insitu combustion process in which the burnt zone is surrounded by a bank of heated oil.

Recently many papers have been published on the transient pressure analysis of a radial composite reservoir system. Eggenschwiler et al1,2 proposed that the idealized reservoir configuration for the steam injection or insitu combustion processes can be represented by two concentric circular regions. The inner region is the swept zone which can be either the steam zone or the burnt zone depending on the thermal process involved. The outer region is the oil bank.. Each region has different mobility and diffusivity properties. These authors found that, due to the large mobility difference between the swept zone and the oil bank, the latter acts like a no flow boundary and causes pseudosteady state flow in the swept zone after the well has been operating for a relatively long time. Hence, a linear plot of the bottomhole pressure with time results in a cartesian straight line which slope can be used to estimate the volume of the swept zone. Walsh3. Messner4, Am bastha5 and Fassishi6 verified this pseudosteady state method and applied it to both steam injection and insitu combustion processes. In addition, Stanislav7 proposed an elliptical composite reservoir model for studying the effects of a vertical fracture during the early stage of steam injection.

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