7.1 Introduction

Naturally fractured reservoirs are different from conventional (unfrac-tured) reservoirs. They are heterogeneous in type and consist of matrix blocks separated from one another by the fracture system as shown in Figure 7-1. The matrix blocks are made of the original rock that was present before fracturing took place. The matrix is characterized by its permeability k _m and porosity ? _m. The fracture system is characterized by its permeability k _f and porosity ? _f. It means a naturally fractured reservoir is a double-porosity and double-permeability reservoir. This chapter presents identification of fractures, main characteristics, and brief review of the most widely used well testing models.

Figure 7-1: Idealization of a naturally fractured heterogeneous porous media

7.2 Identification of Natural Fractures

It is essential to identify fractures during exploration and drilling. Well logs are useful tools in identifying these natural fractures. A program for fracture detection based on logging techniques consists of using different logs.

• Borehole televiewer logs can be used to identify induced and natural fractures.

• Acoustic, receptivity, and temperature logs can be used to obtain a realistic picture of fractures and their width and orientation in the vicinity of wellbore.

• Variable-density logs can define fracture intervals.

Both core analysis and logging are valuable techniques for explored wells in detecting fracture porosity ? _f, permeability k _f, and nature of the matrix or intergranular porosity. However, there are many wells drilled where no core was taken and the logs do not...