A New Practical Method for Predicting Equivalent Drainage Area of Well in Tight Gas Reservoirs

Abstract

Abstract The tight gas is one of the main types of the unconventional gas. Typically the tight gas reservoirs consist of highly heterogeneous low permeability reservoir. The economic evaluation for the production from tight gas production is very challenging task because of prevailing uncertainties associated with key reservoir properties, such as porosity, permeability as well as drainage boundary. However one of the important parameters requiring in this economic evaluation is the equivalent drainage area of the well, which relates the actual volume of fluids (e.g gas) produced or withdrawn from the reservoir at a certain moment that changes with time. It is difficult to predict this equivalent drainage area of well in tight gas reservoir as it takes utterly long time for reservoir pressure to reach to the impermeable physical boundary of the reservoir. The effective drainage area, which grows with time during the transient period; and consequently it is much smaller than the physical drainage arear over the transient flow period in case of tight gas reservoir because of the low permeability. Consequently the production forecasting using physical drainage area (as generally considered for conventional reservoir) can results not only significant error in estimation but also mislead the decision making process. In this paper however, a practical method for predicting the equivalent drainage area of a fractured well in tight gas reservoir is proposed. This method is based upon combined gas material balance equation and decline curve analysis. The developed method is validated against reservoir simulation results, which demonstrates that the proposed method is accurate enough to predict the equivalent drainage area, and may be considered as a practical tool for production forecasting for tight gas reservoir. Sensitivity analyses are carried out to investigate various factors, such as porosity, permeability, facture length on equivalent drainage area for fractured vertical well in tight gas reservoir. Based on the sensitivity study it is observed that the fracture half-length and the porosity have strong impact on the equivalent drainage area, and propagation of equivalent drainage area with time.