A Practical Model for Production Forecast of Fractured Vertical Well in Coalbed Methane Reservoirs: Dynamic-Drainage-Area Concept

A great deal of attention has been attracted to the exploitation as well as the development of unconventional gas reservoirs, which expect to act as an essential role in counterpoising the daily increasing energy demand around the world. In this research, with the intent of contributing to the successful development of coalbed methane (CBM) reservoirs, which is an indispensable member of the family of unconventional gas reservoirs, a novel production prediction model is proposed for fractured vertical CBM wells. The main difference of the research compared with previous excellent documents is taking the effect of pressure propagation behavior on production performance of CBM wells into account. In general, CBM reservoirs possess the low-permeability (<1 mD) physical property, which results in the slow pressure propagation speed during the whole production life. More importantly, because of the unique gas desorption effect inside coal matrix system, more and more adsorption gas will enter into coal cleat system with the production proceed, which will accumulate the formation energy and further mitigate the pressure propagation speed. As a result, it is a relatively time-lengthy period for the pressure propagation process with regard to CBM reservoirs, which currently has not been detailed and comprehensively analyzed. Additionally, it should be noted that the formation pressure is a key sensitive parameter affecting production performance of CBM wells, resulting from the fact that gas production rate takes place only when formation pressure is lower than critical desorption pressure. In this view, the pressure propagation behavior shows a close relationship with production performance of CBM wells, which however fails to receive due attention up to date. In these regards, research content in this paper attempts to shed light on the effect of pressure propagation behavior on production performance of CBM wells from both theoretical and application scopes. With the capacity of capturing the pressure propagation behavior, a novel production prediction model is proposed for fractured vertical CBM wells, the reliability and accuracy of which has been well verified by numerical simulator. Also, the pressure propagation details during production process can be characterized by the proposed model, which is supposed to be highlighted as the main novel point when comparing with previous models. The proposed model is able to generate sensible production performance with less input parameters and calculation time than that of a full-calibrated numerical simulator. Furthermore, details of formation pressure variation regularity are clearly presented by the proposed model, which provides a completely new pathway to evaluate and optimize production performance of fractured vertical CBM wells.