Cleat Characterization in CBM Wells for Completion Optimization

Abstract

Abstract Fracture systems comprise the primary flow path within coal bed methane (CBM) reservoirs. These fractures, also called as cleats, define the reservoir character and fluid flow potential. Cleats are commonly mutually orthogonal and occur perpendicular or at very high angles to the bedding. The standard suites of logs, such as density/neutron, gamma ray and resistivity, define some of the petrophysical properties of the coal layers, but the nature and extent of cleating often remains poorly defined from these logs and by using standard log evaluation methods. A CBM well may often penetrate multiple reservoir zones (seams) and properly characterizing the cleats will help in determining which of these seams should be completed to optimize the production. In addition, through better seam characterization, a technical basis for a preferred completion method (horizontal well, hydraulic fracture, open hole or cavity) can be ascertained. High cleat density in coal seams is an essential requirement for better fluid flow in CBM reservoirs. The primary cleat direction and its relation with the in-situ horizontal stress directions define the fluid flow potential through the cleats and such information can be used to select the completion method. In this study, full waveform sonic log with monopole and flexural waveform and high resolution electrical image log data from CBM wells in Jharkhand, India, have been integrated, in a bid to identify the ideal candidates for completion. Coal seams for best production potential are identified through cleat density characterization. In this paper, we show how compressional and shear slowness variation and Stoneley waveform transmission coefficient analysis are used to interpret the variation in cleat density. The cleat density is further validated from the fracture analysis using micro-resistivity image logs. Cleat orientation can be determined from the detailed structural evaluation of the fractures seen in the high resolution resistivity image. Maximum horizontal stress direction has been computed from acoustic anisotropy evaluation. This maximum stress direction and cleat orientation have been integrated to identify seams that will have better deliverability of fluids. In this paper we provide a guideline for selecting a completion methodology in coal bed methane wells based on cleat density, stress direction, cleat orientation and wellbore stability. Introduction CBM reservoirs have dual permeability system, characterized by low permeability matrix part connected by high permeability orthogonal and sub vertical fractures (with respect to bedding) called cleats. The extended, continuous fractures are termed as face cleats and subsidiary shorter length fractures are classified as butt cleats. As a result of geometry and connectivity variation, significant face and butt cleat permeability anisotropy is prevalent in coal seams ((McCulloh et al. 1974, Mavor at al., 1991,). Figure 1 displays a schematic of cleat network in a coal bed. The permeability of the cleat system is a reservoir property of primary importance because commercial levels of production cannot be obtained unless a well-developed natural fracture system is in communication with the wellbore (Mavor et. al. 1994) The degrees of cleating, nature of cleat network and relative connectivity of the cleat system varies from one coal seam to those of the other and have a significant bearing in their production characteristics even in the commingled production scenario.