Estimating the Static Bottom-Hole Pressure of Gas Wells by Top Node Calculation Using Apparent Molecular Weight Profiling

A reservoir’s static bottom-hole pressures (SBHPs) are an integral component of many reservoir evaluation disciplines. The SBHP is normally acquired through gauge measurements; however, this method has disadvantages such as cost and risk. Accordingly, the ability to accurately estimate the SBHP would provide a cost-effective and safe alternative to well intervention. In this work, a new calculation method is introduced to predict the SBHP of a natural gas well. This method differs from existing methods by utilizing the apparent molecular weight profiling concept. Based on the inputs of pressure and temperature gradient data, an iterative calculation scheme is applied to produce a well-specific molecular weight profile. This profile is used along with a modified form of the equation of state to perform top node pressure calculations and ultimately predict the SBHP for gas wells. The top node method was tested rigorously using 75 case studies from five different fields and reservoirs and the prediction results were compared with actual field measurements. Also, the prediction performance of the top node method was compared with those of four previous methods (Rawlins and Schellhardt, Rzasa and Katz, Sukkar and Cornell, and Cullender and Smith). The results of this work showed that the top node calculation method was accurate in predicting the SBHP and has outperformed the four previous methods.