Formation Evaluation of Coalbed Methane Formations
Authors: Aguilera R. 
Published: March 2010

A new method is presented for estimating fracture porosity in coalbed methane formations from the Dual Laterolog. Historically, well logs have been useful for providing qualitative information of coal seams. However, very little has been published regarding quantitative evaluation of fracture porosity from logs.

A theoretical model indicates that for coalbed formations, the value of the fracture porosity exponent, mr, should be very small and close to 1.0. Actual log data from the San Juan Basin in New Mexico tends to corroborate this finding.

For evaluation purposes, a model has been used that assumes matrix and fractures are connected in parallel. Based on this model, the Dual Laterolog provides an ideal tool for evaluating porosity of vertical and sub vertical fractures.

In addition to the mathematical solutions, charts have been developed for estimating fracture porosity in those cases in which the fractures contain either zero or 100% water saturation at original conditions.

For illustration purposes, the quantitative estimate of fracture porosity in a well of the San Juan Basin (New Mexico) is presented in detail. In addition, methods are presented for estimating fracture aperture, fracture spacing, and fracture permeability for the same well.

The methods should prove valuable to Alberta companies in the future as more than 45% of the province is underlain by coal bearing formations. It should also prove valuable internationally in other countries such as the U.S.A., China and Hungary.

Introduction

Evaluation of coalbed methane reservoirs is becoming more important every day. This investigation was undertaken to try to find reliable means of detecting and evaluating fracture porosity from the Dual Laterolog. It appears from various cases analyzed to date that the resistivity from the shallow laterolog should be smaller than the resistivity from the deep laterolog, provided that a highly conductive mud is in place while logging the well. At the same time it appears that the separation of the two resistivity curves provides a reliable way of estimating fracture porosity.

An excellent paper by Hoyer(1) presented estimates of fracture aperture in coalbed seams of the San Juan Basin based on knowledge of the mud conductivity, and deep and shallow laterolog responses. The estimates utilized an equation developed originally by Faivre and Sibbit(2).

This paper presents a different approach that allows direct estimates of fracture porosity. Under favorable conditions the method permits calculation of fracture spacing and fracture permeability.

FIGURE 1: Cleat system determined for coal seam in the Fruitland formation, San Juan Basin (after Jones et al.(8)). (Available in full paper)

Physical Principles

Figure 1 shows a schematic of micro fractures in a 8.9 cm (3.5 inch) diameter core. The fractures are approximately vertical and perpendicular to bedding.

The coalbed methane industry follows the tradition of British mining engineers and calls the micro fractures in coal seams cleats. The larger dominant micro fractures are called face cleats. The smaller micro fractures that go perpendicular to face cleats are called bun cleats. They are very short and usually are interrupted at the face cleats.