16.1 Introduction

This chapter discusses variations of heterogeneities in rock and fluid properties including causes and effect of pressure-dependent properties. It also presents how to analyze and interpret pressure behavior in heterogeneous reservoirs near fault or other barriers and lateral changes in the hydraulic diffusivity such as occur at fluid contacts. Pressure behavior analysis methods are presented in brief to obtain adequate reservoir description for isotropic, anisotropic, and heterogeneous systems. Numerical solutions must be used to analyze pressure transient data from heterogeneous systems.

16.2 Effect of Pressure on Rock Properties

It is well known from laboratory studies as well as from observed pressure behavior in some wells that both porosity and permeability decrease as reservoir pressure declines. For reservoir rocks, which are "normally" compacted, these effects are usually less than for those which have unusually high pore pressure, i.e., geopressured reservoirs. Carbonate rocks are more heterogeneous. Sandstone rocks are less complex than carbonate rocks. However, a quantitative evaluation of the porosity resulting from the interaction of the various factors is possible only by laboratory measurements. Sandstone and other classic rocks tend to be more elastic in their behavior than carbonate rocks. Limestone often is somewhat plastic in its behavior.

In general, it is expected to observe a decline in calculated permeability from successive transient pressure tests run throughout the life of a well in depleted reservoirs, declines of 10% or so may be observed, but because of variations of other kinds such as two-phase flow effects, etc., quantitative evaluation becomes...