Gas Technology Symposium, 17–18 April, Shreveport, Louisiana
It is often desirable to predict gas well performance during a relatively short period after opening the well. The conventional methods will become unreliable in the cases of tight or altered formations. This paper explains the mechanics of one of several solutions of performance predictions under unsteady state conditions. The solution, devised by Katz and Cornell, centers around a graphical presentation of the pressure disturbances in a reservoir created by constant flow rates. The variables in the graphical presentation are dimensionless and can be applied to a variety of problems. The procedures in the solution of two problems determining reservoir limits and predicting maximum injection pressure are discussed here, including numericalexamples.
The reservoir engineer is called upon to predict different aspects of performance of a gas well with time. Often new completions are involved with no production histories. The engineer then relies on core analyses, and backpressure or production tests using Darcy's law or the back pressure equation. This becomes unreliable when the formation has a low permeability or if it is sufficiently altered from its original state. In these cases performance prediction by applying solutions of the unsteady state equation are useful as well as in other situations where prediction of pressure change with time are needed.
When a gas reservoir is in an unsteady state, it can be considered as unstable. The flowing pressure with a constant rate is continually changing with time. For practical purposes, the well becomes stabilized when gas in place at its ultimate drainage radius begins moving toward the well bore. After this time further pressure decline under constant rate is caused by depletion. In tight formations pressure change with distance from the well bore is rapid. Draw-down at the well bore is large, while a few feet away, the reservoir will be in an undisturbed condition. This results in steep pressure sinks, which move slowly toward the ultimate drainage radius. During this time, in which a pressure disturbance is being created outward from the wellbore to the ultimate drainage radius, the well is unstabilized and is regarded as being in a condition of unsteady state.