Impact of Geo-Mechanical Effects in Reserves Evaluation of NaturallyFractured Carbonate Reservoir Using Rate Transient Analysis - A CaseStudy of Potwar Region

摘要

In Potwar Basin, Tight Naturally Fractured carbonate reservoirs usually have matrix porosity of the 2-3%and permeability less than 1 mD. It is very challenging to evaluate Gas initial in-place (GIIP) and reservesaccurately in these type of reservoirs. The objective of this study is to evaluate Gas in-place and tocharacterize the reservoir energy mechanism using rate transient analysis (RTA). A comparison has alsobeen made for Gas in-place from conventional material balance analysis and from rate transient analysis. In this study, rate transient analysis has been performed in a well, located in Potwar with thirteen years ofproduction history. The well is completed in Chorgali and Sakesar formations. Different scenarios of initialreservoir pressure, geo-mechanical effects during initial production of the well and aquifer volume weresensitized to evaluate the Gas initial in-place (GIIP) and reserves. Different type curves such as AgarwalGardner, Blasingame and flowing material balance and analytical aquifer modelling with / without geo-mechanical effects were applied and matched with the production history. Production logging data was alsoincorporated in the study. During history matching (gas-rate and bottom-hole flowing pressure), differentsensitivities were run to quantify the uncertain parameters and level of uncertainty in the simulator. While history matching and evaluating GIIP/reserves, it was observed that 8,060 psi reservoir pressuremeasured in the well was not stabilized and the reservoir was dominated initially by geo-mechanicaleffects. The best match was achieved with 10,935 psi initial reservoir pressure which was measured inthe first well of the field. Without considering this pressure and geo-mechanical effects, the best matchcould not be obtained. It was also confirmed by different type curves that reservoir was geo-mechanicallypressured during the early life of the well and also, it has a weak aquifer support during the late life ofthe well. The GIIP evaluated from these type curves are in the range of 75-90 BSCF whereas conventionalmaterial balance showed 60-70 BSCF indicative volume associated with the well. This study exhibiteda significant difference in GIIP/reserves evaluated from conventional material balance and rate transientanalysis, indicating remaining hydrocarbon potential in the region i.e. approximately 10-13 BSCF. If the static reservoir pressure is taking too long to stabilize, the conventional material balance approachalways depicts lower hydrocarbon volume associated with the well due to the tight and heterogeneousnature of the reservoirs. Therefore, conventional material balance cannot be applied in these reservoirs andunconventional approach such as rate transient analysis should be applied to evaluate GIIP / reserves andreservoir energy mechanism.