Abstract

Hydraulically-fractured vertical and horizontal wells completed in shale gas and some tight gas plays are known to exhibit long periods of linear flow. Recently, techniques for analyzing this flow period using (normalized) production data have been put forth, but there are known errors associated with the analysis. In this paper, linear flow from fractured wells completed in tight/shale gas reservoirs, subject to a constant production rate constraint, is studied. It is shown analytically that the square root-time plot, a plot of rate-normalized pressure versus square root of time which is commonly used to interpret linear flow, depends on the production rate. It is also shown that depending on production rate, the square root-time plot may not be a straight line during linear flow; the higher the production rate the earlier in time the plot deviates from the expected straight line. This deviation creates error in the analysis. To address this issue, a new analytical method is developed for analyzing linear flow data for the constant gas rate production constraint. The method is then validated using a number of numerically-simulated cases. As expected, based on the analytical derivation, the square root-time plots for these cases depend on gas production rate and for some cases it does not appear as a straight line during linear flow. Finally, it is found that there is excellent agreement between the fracture half-lengths obtained using this method and the input fracture half-lengths to numerical simulation.

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