A New Approach to Estimate CBM Gas Content from Well Logs

Abstract

Abstract Determination of the initial gas content in a coal section is the critical step in the evaluation of CBM resource. Gas content of coals is perceived to have a functional relationship with the relative abundance of two sole components namely organic and inorganic. The organic components remain fairly constant in a particular field, whereas the inorganic component may change considerably, both laterally and vertically within the field. Thus the log responses are logically expected to respond to the variation in inorganic components and offer a scope to effectively estimate the gas content of coals through log responses. However, the sole dependence on density log instead of using it in tandem with other logs like sonic, photoelectric absorption, gamma ray etc, which also respond to the variation in the inorganic components of the coal render the present in vogue methods less effective. As a sequel in this proposed approach above mentioned logs have been used along with density log to generate a composite number C for each cored sample point. Then the depth wise plot of composite numbers C has been matched with the actual gas content data curve. Interestingly, it has been found that the signature of curve C overlays nicely with the core derived gas content curve with a constant shift (K). This shift appears to be unique and will vary from field to field depending upon the characteristics of coal. Estimation of gas content at a particular depth can be obtained by simply adding the K value with the value of C. This methodology has been applied in two completely different coalfields namely Jharia (Bituminous) and Barmer-Sanchore (Lignite) of India and the results are consistently encouraging and show reasonably good accuracy. This technique offers a scope to be used in various types of field having different coal characteristics. Introduction: Presently available methods of estimating gas content mainly uses density log to determine ash content and other proximate parameters for the determination of gas content. Density log has a reasonably good relationship with gas content especially in case of good ranking coals, but the relationship is not always linear and simple. For this reason usual practice is to relate density to inorganic content first, and then relates the inorganic content to gas content. Every available method takes the advantage of the relationship of density Vs ash content and dependencies to calculate gas content of coal. But these methods are not always applicable to various types of coal field, mainly because of over dependency on a single log like density, which usually yields subjective estimation of gas content due to uncertainty in every step of proximate parameters determination compound into cumulative error. Gas content variations in coals depend a lot more on relative inorganic/ organic content rather than on individual components. As a matter of fact, variation of gas content will mainly be the result of variation in inorganic content and the reservoir conditions. Although inorganic component has no gas generation or storage capacity, the changes in relative properties of inorganic and organic material in coal section will have a strong and direct impact on gas content, even when organic properties are held steady. Analogous to signal to noise ratio in case of sound quality variation, the inorganic matter, which acts as noise, also has a considerable effect on overall gas content variation. A change in relative inorganic percentage in a given sample or reservoir volumes means a direct change in the amount of material actually storing the gas. In other way, any variation in inorganic components and hence variation in gas content also has a considerable effect on different log responses.