It is quite common for production/injection wells to install permanent or temporary downhole pressure gauges. When the well is shut in, the pressure gauge will record the falloff/buildup (BU) transient data lasting from hours to several days with high frequency. The pressure transient analysis (PTA) can be carried out using the recorded data. When the well is producing, surface pressure and rate will be recorded daily or monthly based on which production data analysis (PDA) can be performed. For a fractured tight gas well, we may get accurate permeability from PTA if a radial flow is reached; however, seldom is the case that we can obtain boundary information from PTA because the permeability is too low. On the contrary, we may easily get boundary and volume from PDA if the well is producing long enough, but sometimes it's hard to get permeability estimation. If we can integrate PTA and PDA together, we may get both permeability and boundary, and increase the interpretation's confidence by letting PTA and PDA result consistent with each other.
This study illustrates one fractured vertical well in a typical tight gas reservoir in China. The well contained one pre-fracture buildup test and annual pressure buildup tests in 6 subsequent years. From the pre-fracture buildup test, we get the accurate permeability with value of 0.086md; the annual buildup tests showed continuous changes in the fracture morphology with fracture conductivity decreasing from 3000 to 300 md.ft and fracture half-length increasing from 226m to 322m.
Such successive Buildups recordings are rare and the observed changes in fracture conductivity and length over time were unanticipated. It reveals that variation of fracture half-length and conductivity may be a typical factor we should consider in pressure transient analysis. The PDA shows the difficulties in determining formation and fracture parameters when the transient response lacks radial flow.