Use Of Pressure Buildup Tests For Describing Heterogeneous Reservoirs
Authors: Perez-Rosales Candelario 
Published: December 2007

A method for obtaining the areal distribution of flow properties of heterogeneous reservoirs is presented. The proposed method makes use of pressure presented. The proposed method makes use of pressure buildup tests and is based on the assumption that an oil reservoir is a heterogeneous and random system. Accordingly, a statistical technique for interpreting pressure buildup curves is proposed. The use of this pressure buildup curves is proposed. The use of this technique, in conjunction with contouring procedures, gives the desired reservoir description. For illustration of the method, a field example is presented. presented

Introduction

There are different methods for the study of heterogeneous reservoir. One of the techniques frequently used is pulse-testing. This technique has been applied by McKinley et al. and by Rijnders in the areal description of reservoirs. Likewise, Pierce has used the same technique for studying Pierce has used the same technique for studying reservoir connectivity across faults.

In this paper a method for the determination of the areal distribution of flow properties is presented. The proposed method makes use of pressure presented. The proposed method makes use of pressure buildup tests, and is based on the following conceptual considerations.

For an observer on the surface of the earth, an oil reservoir is like a black box to which no direct access is possible, with the exception of very small portions, namely, the wellbores. However, in spite portions, namely, the wellbores. However, in spite of this condition of inaccessibility, it is possible to make some general considerations about its structured characteristics. Thus, it can be postulated that an oil reservoir is a heterogeneous system postulated that an oil reservoir is a heterogeneous system that is the final result of a random process of sedimentation and accumulation of fluids. The heterogeneous character is evident from observations made on formation outcrops, and the random nature is confirmed by analysis made on rock samples. It is this property that allows the determination of petrophysical properties by statistical techniques. petrophysical properties by statistical techniques. Contrary to this point of view, it might be argued that sometimes, during transient pressure tests, some reservoirs behave as if they were homogeneous systems. This, of course, is true. However, this fact does not imply necessarily that the system in itself is homogeneous. What really happens is that a reservoir under dynamic conditions operates as a filter that tends to smooth the response to a pressure stimulus. In other words, in a transient pressure test the effect of heterogeneities tend to disappear due to filtering or averaging phenomena. phenomena. When a reservoir is considered as a black box, the best way to determine its flow properties consists in applying pressure stimuli at different points of the reservoir and then observing the points of the reservoir and then observing the natural responses of the system. An adequate analysis of these responses should give an indication of the reservoir characteristics.

There are well known techniques for applying pressure disturbances (or stimuli) to a reservoir, pressure disturbances (or stimuli) to a reservoir, and they are in an advanced stage of development. Transient pressure tests are the best known examples. However, when a reservoir is treated a heterogeneous system, the conventional methodology for analyzing and interpreting pressure data should be modified.

Theory

As a point of departure for the theoretical development, the case of a homogeneous infinite reservoir will be considered first. When a well has been producing at a rate q during a time t, the pressure behavior can be expressed, according to the pressure behavior can be expressed, according to the line-source approximation, as

(1)

If the well is closed in at time t= t1, a pressure disturbance that propagates radially is pressure disturbance that propagates radially is produced. produced.