Natural Gas Industry B (Dec 2020)
A new method for evaluating the unstable deliverability of gas wells in gas formation testing phase
Abstract
Predicting the deliverability change laws of a gas well in the early stage is one of the technical difficulties in natural gas development. The commonly used steady seepage analysis methods have relatively large errors. And this problem cannot be solved effectively by the classical methods of production decline analysis and pressure transient well test analysis. To solve this problem, this paper did calculation based on the well test model of constant-pressure production, changed the previous approximate method of simplifying the calculation of the exponential integral function, and accurately calculated the analytical solution of absolute open flow potential of a gas well. In addition, the deliverability instability characteristics of different types of gas wells were quantitatively described by taking vertical wells in homogeneous reservoirs as the reference benchmark. Then, combined with the deliverability evaluation needs of a new well, a new practical method focusing on solving the problems of gas formation testing analysis was researched and developed, and also applied on site at some key wells in the hot spots of natural gas development in the Sichuan Basin. And the following research results were obtained. First, the accurate formula for the analytical solution of well test model significantly enhances the adaptability to short-term test conditions. Second, the newly established chart briefly reveals the quantitative relationship between the unstable deliverability characteristics of a gas well and the main influencing factors. Third, if the characteristic parameter of turbulence effect is unknown, it is necessary to perform iterative calculation of the variable skin factor when the well test model of constant-pressure production is used to analyze the change trend of the absolute open flow potential of a gas well. Fourth, by conducting comparative analysis on the calculation results of typical cases, the decline laws of the absolute open flow potential of different types of gas wells can be understood further. It is concluded that this new method is capable of improving the prediction accuracy of the unstable deliverability of gas wells. Therefore, it can be widely applied to the deliverability evaluation of gas wells in the exploration stage, the evaluation stage of early development and the commissioning stage of new development and production wells, which is conducive to the formulation of natural gas exploration and development decision.
