Analysis of unsteady responses of natural gas reservoirs via a universal natural gas type-curve formulation

Gas decline curve analysis has traditionally relied on the use of liquid (oil) type curves along with the concepts of pseudo-pressure and pseudo-time and/or empirical curve fitting of rate-time production data using Fetkovich type curves for gas well performance and reserve estimations. In this work, we show that a single-line, universal type curve can instead be derived for the analysis of unsteady state of natural gas wells under boundary dominated flow (BDF). The proposed decline equation - presented in its analytical, closed functional form - models production from any gas well producing at constant bottomhole pressure under BDF. On the basis of this decline model, a universal type-curve is formulated for type curve matching of natural gas well producing under BDF, for any drawdown condition, and regardless of intrinsic reservoir and fluid properties. New-generation analytical procedures for gas well performance analysis are presented, which does not necessitate the calculation of pseudo-pressure or pseudo-time. Explicit OGIP predictions are thus enabled from the proposed universal type curve matching. The importance of conducting unambiguous reserve estimations based on rate-time data and reliably predicting future well performance cannot be overstated. The proposed single-line type curve is demonstrated to do so by successfully matching rate-time production BDF data, predicting future performance, and reliably estimating fluid in place for a large number of numerical simulations and field cases. It is finally shown that the proposed formulation can be alternatively implemented in terms of straight-line analysis of l/q_sc^b versus time plots that enable future well performance and explicit reserves estimations.