The Effect of Noncommunicating Layers on Interference Test Data
Authors: Chu Wei C. , Raghavan Rajagopal 
Published: December 2007

The characteristics of the pressure vs. time curve at an observation well in a layered reservoir are investigated. The pressure response at the well is dominated by the crossflow effect that results due to interlayer communication and is governed by the properties of the individual layers and the skin regions surrounding flowing and observation wells. It appears that the skin regions may be the dominant influence on the pressure response. Any viable method of analysis should include the effect of the skin regions.

Introduction

A reliable description of the variations in reservoir properties is needed to design and to ensure the success of an enhanced recovery process. Interference tests commonly are used to obtain information on the character of the porous medium. of the several types of reservoirs that exist, one of the simplest is a reservoir consisting of a number of layers that have distinct values of permeability, porosity, thickness, etc., stacked on top of each other and that are in communication only at the wellbore. A review of the literature indicates that a comprehensive examination of interference test data in a layered reservoir with communication only at the wellbores is yet to appear.The objective of this study is to examine the effect of reservoir parameters (permeability, porosity, thickness, and compressibility) on the pressure response at an observation well in a layered reservoir. The layers are separated by impermeable barriers and are in communication only at the wellbores. Results for two- and three-layer systems are discussed in detail. Ten- and 20-layer systems are examined briefly. The effect of the flow capacity and the diffusivity of the layers on interference data is discussed. The influence of the skin region around the observation and flowing wells is documented. We also document the consequences of communication between the layers at the wellbore and show that the communication at the wellbore has a dominant effect on the observation-well response. In this work, the term "crossflow" is used to refer to the flow between the layers at the observation well.It is emphasized that the intention of this study is to provide a broad, comprehensive understanding of the pressure behavior in reservoirs subject to the assumptions incorporated in our model. This, in turn, should allow a better analysis of data influenced by crossflow and enable the designing of tests to obtain reliable information on the characteristics of a layered reservoir. Procedures used to analyze data are not discussed.

Theory and Assumptions

The model considered consists of several horizontal, homogeneous layers of differing physical properties, separated by impermeable barriers (see Fig. 1). The top and bottom of the reservoir are sealed by an impermeable boundary. The lateral extent of the reservoir is assumed to be infinite, and each layer is filled with a slightly compressible fluid of constant viscosity. The initial reservoir pressure pi of each layer is the same, and gravity effects are assumed to be negligible. All wells in the reservoir completely penetrate all the layers, and there is communication between the layers at the wellbores. Each layer, denoted by the subscript j, has distinct values of flow capacity kjhj and diffusivity eta j.

JPT

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