The long test times required to attain radial flow in stimulated Devonian shale gas wells are impractical. However, experience shows that a short test period (about two weeks) is sufficient to evaluate period (about two weeks) is sufficient to evaluate reservoir and fracture properties using conventional techniques followed by mathematical simulation of the observed pressures. History matches, whether unique or not (insufficient production data are available at present to establish uniqueness), give clear insight present to establish uniqueness), give clear insight into reservoir properties, the effectiveness of frac treatment, and well deliverability.
Estimates of recoverable gas reserves in eastern Devonian shales range from a few trillion cubic feet to several hundred trillion cubic feet, depending on whether (1) the bulk of gas is contained within natural fracture system networks and the matrix contributes very little to production (single porosity system), or (2) the bulk of gas is contained in the matrix, and fractures merely provide flow channels to the wellbore (dual porosity system). Each school of thought is supported by production data and history matching of production decline curves. Both have been supported production decline curves. Both have been supported by numerical models and can account for total gas production. In other words, at present the exact production. In other words, at present the exact mechanism of production from Devonian shales is not fully understood.
In 1978 the U.S. Department of Energy initiated a gas well testing program whose principal objective was to "develop and conduct a program of gas well testing procedures and to analyze the results in support of the Eastern Gas Shales Program. The well tests and analyses are intended to permit evaluation of the relative effectiveness of various hydraulic and explosive stimulation techniques as conducted under this program."
To accomplish this objective, a two-phase project was planned. Phase I consisted of tests on three Devonian shale wells to develop (1) a practical field procedure for testing and (2) a comprehensive analytical procedure for testing and (2) a comprehensive analytical technique for evaluating reservoir rock and fracture properties and the results of subsequent stimulation properties and the results of subsequent stimulation treatment. Phase II involves the application of the techniques developed in Phase I to routine testing and analysis of Devonian shale wells for a contract period of about one year. So far, two wells have been tested in Phase II. of the five wells tested during Phases I and II, four were stimulated by cryogenic frac treatments and the fifth was foam fraced. No well which had been stimulated with a chemical explosive was tested; therefore, the procedure developed may not apply to wells completed by this method.
It has long been recognized that the pressure behavior of a reservoir following a rate change directly reflects the geometry and flow properties of the reservoir fluids. However, when a well is fractured, the pressure behavior can no longer be described by conventional radial flow theory. Instead, pressures exhibit linear flow behavior in the early stages of testing. Later, pressure waves of semielliptical shape move away from the fracture. As the pressure disturbance continues to be propagated away from the fracture, the streamlines approach radial geometry, assuming that boundary effects do not first come into play. The duration of linear flow, which is a function play. The duration of linear flow, which is a function of reservoir and fracture properties, is extremely long in Devonian shale wells, and radial flow does not occur during tests of practical duration. Hence, only the linear flow regime is available for analysis to estimate reservoir rock and fracture properties.
The surface facilities shown in Fig. 1 were designed for testing Devonian shale wells and performed exceptionally well.
Bottomhole pressures were recorded by a quartz crystal Hewlett-Packard gauge with a surface readout. Surface pressures were recorded by an Amerada RPG-6 recorder. However, surface measurements are of little value in the analysis except for those cases where fluid remains in the wellbore and fracture and the fluid level must be computed as it changes with time, or where bottomhole pressures cannot be recorded because of mechanical problems.