Natural Fractures in Coal

The literature that describes natural fractures (cleats) in coal can be divided into two areas of activity, including an older mining industry viewpoint and a much newer petroleum industry viewpoint. Cleats in coal have been of interest to the coal mining communit y for more than 100 years. Knowledge of cleat geometr y is important in designing coal mines for maximum extraction efficiency and for safety considerations . One of the earliest references on cleats in coal from the mining perspective is by Mammat (1834, cited in Kendal and Briggs, 1933), who noted the uniform strike of cleats in coal fields of the United Kingdom. Since that time a large number of papers on cleats have been published. Because of the relatively recent interest in commercial production of coal gas resources , many contemporar y research efforts have been directed toward the development of a better understandin g of factors that influence the production of gas from coal reservoirs. Among the more important of these factors is the cleat system. The importance of cleat development stems from the fact that the principal natural fracture permeabilit y pathways for the flow of gas and water are through the cleat systems . Cleat permeabilit y is often the reservoi r characteristi c that has the greates t influence on the economi c succes or failure of coal gas exploration and development programs (Dhir et al., 1991a, b). A better understandin g of cleat genesi s is therefore useful for reasons besides academi c interest , which include the following: (1) One can better asses why certain coal gas plays are characterize d by significant and highly variable cleat permeability; (2) developmen t drilling and completion decisions can be made for areas where offset commercial production has been established ; and (3) more efficient exploration for coal gas resources in frontier areas can be undertaken. The estimation of reservoi r quality prior to performing quantitative formation evaluation is a problem that wil l always be associate d with the exploration and production of oil and gas resources . Coal gas reservoirs are in this sense no different, in that the reliable prediction of the presence of commercial cleat permeabilit y before drilling is difficul t at present . One of the many reasons for this problem is that a multitude of cleat genesi s hypotheses have been postulated, which makes the formulation of predictive model s more complex. Given the importance of cleat permeabilit y development to the economi c production of coal gas resources , the objectives of this paper are to: (1) discuss the relationships between cleats and cleat permeability; (2) characteriz e the geometr y of cleats; (3) review previous hypotheses on cleat genesis ; and (4) provide an opinion concerning which hypothesi s or combination of hypotheses for the genesi s of cleats satisfies the availabl e data on cleat geometry. The key conclusion of this chapter is that cleat genesi s was commonl y caused by the interdependen t influences of desiccation, lithification, coalification, and paleotectoni c stress. Several interlinked constraint s on the selection of a tenabl e cleat genesi s hypothesi s have been formulated by many workers that support this conclusion and are present ed herein.