14.1 Introduction

This chapter discusses the effects of some common reservoir heterogeneities on pressure transient behavior. It is difficult to delineate specific heterogeneities from well tests. The difficulty occurs because many different conditions can cause the same or similar well test response. If we have an idea of the type of heterogeneity, it may be possible to determine some of the properties involved by pressure transient testing. Some knowledge of geological, seismic, fluid flow, and performance data is necessary before hypotheses are formed about the type and location of the heterogeneities. It may be possible to design a specific transient test to investigate the possibility of a particular type of heterogeneity. This chapter discusses and classifies reservoir heterogeneities, permeability, and anisotropy and describes how these heterogeneities affect transient testing. Several types of heterogeneities can cause similar transient test pressure response, but results should be supported by other data. The next section will illustrate a variety of situations such as faults, lateral changes in the hydraulic diffusivity such as occur at fluid contacts, and man-made heterogeneities.

14.2 Causes of Heterogeneities

Heterogeneities may have the following causes:

• Postdepositional changes in reservoir lithology

• Folding and faulting

• Changes in fluid type or properties

• Variations in rock and fluid properties from one location to another

• Physical barriers, gas water contacts, thickness changes, lithology changes

• Different properties in each layer, etc.

• Man-made heterogeneities, including changes near the wellbore from hydraulic fracturing, acidzing, or gas injection

14.3 Pressure-Dependent Properties

It is...