Reservoir effectiveness evaluation based on comprehensive reservoir quality evaluation indexes of well logging: A case study on the Permian Qixia Fm in the western Sichuan Basin

In the western Sichuan Basin, the Permian Qixia Fm grained dolomite reservoirs are currently the new focus of natural gas exploration and development. The Qixia Fm is characterized by developed dissolved vugs and fractures, low matrix porosity and strong heterogeneity, so evaluation faults tend to occur if reservoir effectiveness is evaluated by means of the traditional porosity evaluation, and the consequently the predicted gas well productivity is more deviated from the actual measurement. In this paper, the characteristics of Qixia Fm reservoirs in this area were firstly analyzed. Then, the method and criterion of the effectiveness evaluation of Qixia Fm reservoirs in the western Sichuan Basin were established by means of numerical simulation and special logging data processing. And the following research results were obtained. First, surface porosity of matrix pores and secondary dissolved vugs calculated based on the special processing of conventional logging and electric imaging logging can be used to evaluate the reservoir properties of reservoirs. Second, deep and shallow dual lateral logging in combination with electric imaging logging and Stoneley wave energy data can be used to evaluate the filtration capacity of reservoirs. Third, the criterion of effective Qixia Fm reservoirs is established, including storage coefficient ≥0.6, deep lateral resistivity <5000 Ω·m, surface porosity >0.6%, Stoneley wave attenuation ≥10% and comprehensive reservoir evaluation index ≥0.25, and the evaluation criterion can well characterize reservoir quality and gas well productivity. The field application results of this evaluation method and evaluation criterion indicate that the coincidence rate of reservoir effectiveness has increased from 70% to over 90%. Thus, the technical difficulties related to the heterogeneous carbonate reservoir effectiveness evaluation are basically solved. Besides, they provide a basis for the determination of production test layer selection, completion engineering and development scheme so as to cut down the natural gas exploration and development cost.