Effects of Transient Conditions in Gas Reservoirs

Abstract

Abstract A simple disturbance in a gas reservoir travels with a finite velocity whichis nearly independent of the amplitude of the disturbance. As a result verycomplex transients may be set up which seriously affect observation at a well.The time of transient is much longer than commonly supposed. Introduction A change in pressure at a gas well is not propagated through the reservoirinstantaneously but with a definite finite velocity. Knowledge of the period ofthis transient is of value because:Observations of pressure and flow-rate during the transient may affectinterpretation of back-pressure tests.Interference effects between wells may be used to determine continuity offormation.Additional information concerning the physical characteristics may possiblybe deduced from the behavior of a well during the transient. Mathematical Development It will be assumed that the well is initially closed in and that thepressure is constant throughout the reservoir. As the initial disturbancepasses outward from the well it is further assumed that a logarithmic pressuredistribution is established between the most distant point reached and thewell. Conclusions The great length of time which may be involved in transients may becalculated from the appropriate constants. Thus the Monroe gas field containsareas having transient time of the order of years. This produced the anomaly ofa field effectively depleted in one portion but with virgin pressure in anothereven though both were connected. Some attempts have been made to establish or disprove continuity offormation by the observance of interference effects between wells. Althoughthis is theoretically possible and such interference effects have been observedthe observational difficulties are formidable and the probable existence ofcomplex transients further decreases the likelihood of an extensive applicationof this method. The same conclusions, even more forcibly, apply to attempts to measurereservoir constants by transient observation. T.P. 2547