Transient Pressure Analysis for a Vertical Gas Well Intersected by a Finite-Conductivity Fracture

Abstract

Abstract Pressure-transient analysis for gas wells has a great importance in the oil and gas Industry. Transient pressure responses of unfractured and hydraulically fractured gas wells may be affected by non-Darcy flow near the wellbore, such important effect needs to be taken into account when estimating reservoir and well parameters. An additional problem encountered in gas wells test analysis is the presence of the rate-dependent skin, which requires a search for another parameter (non-Darcy flow coefficient). Several tests are performed in order to measure the deliverability of gas wells and to describe reservoir performance. Such specific tests as flow after flow, isochronal and modified isochronal were initially designed to obtain the absolute open flow potential of a well, however, the use of these tests has been extended to obtain additional information from the reservoir. Drawdown tests are focused in obtaining such data as wellbore storage, reservoir transmissivity, skin factor, flow efficiency and system geometry. Buildup tests lead us to the average pressure of the reservoir, however, proper analysis of buildup test provides values of permeability, wellbore storage and apparent skin factor. Buildup and drawdown tests are currently analyzed using type-curve matching procedures, which involves trial and error and conventional techniques. Pseudopressure concept, which was used in this study, has shown to provide sufficient engineering accuracy in dealing with gas well test data. This study utilizes characteristic points, intersection and lines found on the pseudopressure and pseudopressure derivative plot to obtain fracture length, fracture conductivity, skin factor and reservoir permeability. It was found that changing the non-Darcy factor, D, the shape of the pseudo-pressure curve varies from the original gas curve shape at D=0, which implies that an additional skin effect is added at high rates. On the other hand, the pseudopressure derivative curve remains on its original shape, then, in spite of increasing non-Darcy effect factor, D, the pseudopressure derivative curve is not affected by this additional skin effect added to the system because of the non-Darcy flow effect. The interpretation technique was successfully tested with simulated and field examples.