Determination of Fluid Properties and Reservoir Net Pay Cutoffs by Production Logging and Conventional Logs in Exploration Wells :A Case Study of the Granite Fractured Reservoir in JZ Oilfiled in Bohai Sea

Abstract

Abstract Since resistivity logging is much more sensitive to rocks than to hydrocarbons in granite reservoirs, conventional logging methods cannot always accurately evaluate fluid properties and identify pay zones. Drill Stem Test (DST) is often used to identify fluid properties and hydrocarbon production potentials. However, DST in granite reservoirs cannot accurately determine production from sub-layers which leads to inaccurate net pay zone identification and reserve estimation. The accurate determination of fluid properties and net pay zones in fractured granite reservoirs remains a challenging topic. This study proposes the use of Production Profile Logging (PLT)Logging in exploration wells in JZ oilfield characterized by fractured granite reservoirs, where DST could not characterize the sub-layers’ PLTs with in the thick layer of the combined test. With the use of open hole completion test, PLT logging can easily identify the production from sub-layers. The fluid density and water hold-up are consistent with the response of flowmeter logs. The method not only identifies production from sub-layers but also determines fluid properties. Meanwhile, it can quantify the production from each sub-layer according to PLT logging interpretation. It has satisfactory application in productivity evaluation in JZ oilfield. Results indicate that the use of PLT logging in four exploration wells in JZ oilfield is successful. Sub-layers are identified in DST based on PLT logging interpretations of flow rate, density, and water hold-up. The productivity index of each sub-layer is calculated by PLT logging interpretation. Combining lateral resistivity logs with sonic logging data, Draw porosity and ratio of the deep and shallow lateral resistivity plot based on each sub-layer’ production with PLT interpretation Logging results. Net pay cutoffs of JZ oilfield is determined to be φ≥3.0% and (RD/RS) *DT≥90. This method improves the accuracy of reserve evaluation and solves the problem that the estimates of net pay cutoffs are larger than actual values in DST. Compared with DST in fractured granite reservoirs, PLT logging not only leads to accurate determination of fluid properties and identification of net pay cutoffs but also largely reduces the costs. PLT logging is conventionally used to dynamically monitor cased hole wells. The novelty of this study is the successful application of PLT logging in reserve estimation. Compared to the conventional reserve estimation method based on DST, this new strategy accurately identifies net pay zones and determines net pay cutoffs in fractured granites to improve the accuracy of reserve estimation. Thus, the field can be developed more economically when oil prices are low.