Fracture Detection In West Coast Reservoirs Using Well Logs
Authors: Heflin J.D. 
Published: December 2007

Out of necessity, the search for hydrocarbons has been expanded into the harder-to-evaluate formations. At present, this often involves locating and evaluating fractured reservoirs. In areas of interest to oil companies operating in California this could involve detecting and evaluating several types of fracture systems located and differing lithologies. These include the so-called fractured shales, fractured schists, fractured sands, and even fractured volcanics.

Commercial surveys available can require refined interpretation techniques or can be as simple as identifying predictable responses caused by fractures on measurements designed to determine other parameters. parameters. It has been found that no single method will give all the answers all the time. Therefore, when we discuss fracture detection, we usually imply a method that has been successful in identifying a specific type fracture in a specific member of a formation.

This paper will review the well log methods available for fracture detection and will discuss the merits and applications of each method.

The intent of this paper is to provide an aid in planning logging programs which offer the best planning logging programs which offer the best opportunity to detect an expected type fracture system.

Introduction

In the past many methods have been used to detect fractured porosity in reservoirs. Many published case histories have illustrated an effective method of detection. However, these methods are usually confined to a specific type fracture system in a specific member of a formation.

When evaluating a method for a particular situation, in order to determine the probability of a method detecting fractures, it is important to understand the resolution and volume surveyed.

Often, after a fracture has been conclusively identified by one method, a review of other measurements confirms that an anomaly was also indicated by them.

Another important factor in the ability to detect fractures is the frequency or density of the fractures in a system. Usually, increasing fracture density improves the chance of detection.

When discussing a method of fracture detection, some consideration must be given to the resolution of the instruments involved. The descriptive term of thin or thick beds can have varying dimensions. However, for the purposes of this paper, this dimension will be a function of the vertical resolution of each measurement. For this reason an approximate vertical resolution of each measurement is presented in Table 1. presented in Table 1. Much interest has been placed on the detection of high angle or vertical fractures. However, the presence of low angle or bedding plane fractures, or presence of low angle or bedding plane fractures, or the absence of any fracturing, is useful information. That is because artificial fracturing methods are available to make some tight hydrocarbon-bearing reservoirs productive.

One of the major problems however is that in most cases fracturing does not increase the total porosity drastically. Such a result would make the identification of productive zones easier.

A brief review of most of the wireline methods in use today follows, with the inclusion of a chart which might be helpful in selecting a method best for specific situations.

SPONTANEOUS POTENTIAL

The most basic method to locate fractures would be through the use of the Spontaneous Potential.