Postbuildup Drawdown Analysis of Tight Gas Wells in the Rocky Mountains

Abstract

Summary Through the use of recently available daily volume data on Northwest Pipeline Corp.'s gathering systems, drawdowns following pressure buildups can be analyzed to predict flush production behavior of tight gas wells that have been shut in, perhaps owing to market restrictions. The "flush" rate as a percentage of the final flowing rate is a function of flowing time after the buildup. The magnitude of the initial flush rate is related to the length of time the well is shut in, to the final flow rate before shut-in, and to the age of the well. The amount of flush production increases as the well downtime increases. Daily production histories of the wells on the Douglas Creek Arch and the Green River, San Juan, Piceance, and Uinta basins were studied. Wells within each area were grouped by geologic formation, by flow rate, and by similar periods of shut-in time. When plotted on a log-log scale, the ratio of the postbuildup rate to the stable rate shows a straight-line relationship to the drawdown time. Theoretical curves to predict flow rates after a buildup are presented that are based on the superposition of constant-pressure solutions to the flow of a real gas in a porous medium and that show a nearly straight-line rate/time relationship. In light of the current gas surplus and the evolving spot market, the prediction of accurate short-term performance is useful in scheduling gas sales and gas transportation.