Application of Pressure Build-up Test to a Carbonate Gas Condensate Reservoir

Abstract

ABSTRACT The Yangshuiwu buried hill carbonate gas condensate reservoir has recently become the most promising exploration area in Huabei Oilfield (China). Fractures are commonly seen in this reservoir, which brings about challenges for reservoir engineers to accurately interpret the well test data obtained for this field. We first analyzed the effect of influencing factors such as phase behavior and the height of liquid column in the wellbore on the measured pressure curves. The phase behavior analysis shows that the retrograde condensation occurs in the late stage of the pressure drop. The maximum liquid volume of the retrograde condensate is only 1.62%, which has little influence on the well test curve. When the pressure gauge was placed at the middle depth of formation, the effect of “offset pressure” caused by condensate liquid could be eliminated. Moreover, dense connective fractures could be observed from the micro-resistivity image logging results, implying that a dual-medium reservoir model is more appropriate to characterize the carbonate reservoir. The dual-medium reservoir model was subsequently applied to interpret the pressure build-up data of well AT1X. The double logarithmic curves of pressure differences and pressure difference derivatives were obtained and their characteristics echoed well with the properties of typical gas condensate reservoirs. Finally, the reservoir parameters (including matrix permeability, skin factor, elastic energy storage ratio, and channeling coefficient) were successfully obtained based on the pressure build-up analysis. This study can shed light on improved characterization of gas-condensate carbonate reservoirs with abundant fractures.