The objective of this paper is to demonstrate methods of testing and predicting the behavior of very tight gas wells that must be tested under nonideal conditions and that result in data that does not conform to the ideal radial flow concepts. Isochronal testing using constant production rates and bottom hole pressure guages is well understood by most engineers. Interpretation of radial flow isochronal data to obtain the gas well performance when it reaches pseudo steady state flow is also well defined. However, it appears that the flow of gas from a very tight reservoir with a "large" artificial fracture at the well has not been treated in a practical manner.
It will be shown that such a well tends to flow under linear unsteady state infinite acting conditions for relatively short periods of time. Then the well gradually changes to a radial flow geometry. An understanding of these flow conditions can lead to an approximate prediction of the gas well behavior with the accuracy depending upon the length of the flow tests and the accuracy of the "guesstimate" of the drainage area of the well. The treatment of the subject is based on a flow test of a gas well producing through the annulus while water production is being pumped through the tubing. Only surface pressures were measured during the flow tests.
Very tight low permeability reservoirs are generally completed by hydraulically fracturing the well during completion. A long vertical fracture and very low permeabilities permit the well to exhibit linear flow characteristics for very short periods of time. This is demonstrated in Fig. 1. The very low permeability means that the disturbances in the reservoir caused by initiating production will move at a relatively slow rate through the reservoir. Since the effect of the disturbance will move outwardly from the long fracture it means that except for the relatively small amount of the reservoir near the ends of the fracture the flow will be linear. Of course the longer the fracture is the less will the flow into the ends of the fracture cause the overall behavior to deviate from a linear flow geometry.
As the affected volume of the reservoir increases with the production time, the fixed length of the fracture becomes smaller in relationship to the affected volume of the reservoir until substantially radial flow will exist throughout most of the drainage area. Thus, for small producing times, t, we would expect the pressure behavior of a well being produced at a constant rate to be governed by an equation for the constant rate infinite acting linear flow behavior in a reservoir.