Abstract

Naturally occurring microfractures, called cleats, provide the permeability essential for bulk fluid flow in coalbed reservoirs. The bulk fluid flow, in turn, is controlled by the cleat properties, specifically their orientation, spacing, compressibility, and effective porosity. A two parameter, geologic process-based model for predicting the effective cleat porosity of coalbed reservoirs was developed from an analysis of cleat property data for San Juan Basin Fruitland Formation coal. The cleat property data indicate that thermal maturation and effective stress act in opposite directions on the effective cleat porosity of Fruitland Formation coal. These two variables are the basis for the geologic process-based model for calculating effective cleat porosity values.

Introduction

Coalbed reservoirs in the U.S. contain an estimated 703 Tcf (19.9 × 1012 m3) of natural gas resources, hold 11.7% (141.4 Tcf; 4.0 × 1012 m3) of the estimated total recoverable U.S. natural gas resource base, and in 1997 accounted for 6% (1.13 Tcf; 32 × 109 m3) of total U.S. natural gas production.1–4 Coal deposits function as self-sourced natural gas reservoirs wherein the three crucial petroleum system elements of source rock, reservoir and trap are located together in a single geologic unit. Thus, coal deposits represent a relatively simple, low risk exploration situation with respect to locating natural gas accumulations.

The major risk in most coalbed methane plays is generally not the drilling of a dry hole; rather it is not being able to produce commercial quantities of natural gas from the reservoir. Naturally occurring microfractures, called cleats, provide the permeability essential for bulk fluid flow in coalbed reservoirs.5–7 The bulk fluid flow, in turn, is controlled by the cleat physical properties, specifically their orientation, spacing, compressibility, and effective porosity.6–12 This paper presents results from analysis of the effects of geologic variables on the cleat properties of Upper Cretaceous Fruitland Formation coalbed reservoirs in the San Juan Basin, Colorado and New Mexico.

Natural Fracture Origin and Types

Natural fractures in rocks have varied origins and are formed when the applied stress exceeds the yield stress of the bulk rock matrix material. The applied stress may be the result of either a physical or chemical process and it may originate either external or internal to the rock body. Natural fracture formation in coalbed reservoirs results from stresses generated by such varied geologic processes as structural deformation, differential compaction and volume contraction.7,13–17

Five types of natural fractures are distinguishable in coalbed reservoirs. The two most commonly observed types of natural fracture are orthogonal sets of closely spaced subparallel microfractures called face and butt cleats that are oriented essentially perpendicular to the bedding plane. Coal cleats are extension (opening-mode) fractures that form as a result of the stress generated by the volume contraction or shrinkage of the coal matrix as a result of desiccation during thermal maturation.5,7,17

The face and butt cleats are also referred to as primary and secondary cleats, respectively. By convention, the oldest and generally most prominent of these two microfracture sets is called the face cleat. The butt cleats terminate against face cleats, which is interpreted as indicating that they were formed later in geologic time.7,17

Three other fracture systems, referred to as tertiary cleats, joints and faults, may also be present in coalbed reservoirs. Tertiary cleats are microfractures whose orientations are different than those of either the face or butt cleats. The tertiary cleats terminate against either face or butt cleats, which is interpreted as indicating that they were formed later in geologic time. Joints and faults are larger-scale fractures that typically cut across coalbeds and non-coal interbeds.7,15–17

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