Productivity evaluation of low porosity and low permeability reservoirs using differential pressure method: an example from the Cretaceous Bashijiqike Formation in Dabei area, the Tarim Basin

The Lower Cretaceous Bashijiqike Formation in Dabei area of Tarim Basin is a typical fractured tight sandstone reservoir with dual porosity structure consisting of low porosity and permeability matrix and fractures. This kind of dual porosity structure results in complex gas-water contact in the reservoir. It is always difficult to evaluate the productivity of this kind of reservoir by using the traditional geological and logging methods. Based on the theory of seepage mechanics, we believe that the formation pressure difference and capillary pressure are the parameters controlling productivity. The fluid can be produced from the porous media only when the formation differential pressure overtakes the resistance of capillary pressure generated by reservoir fluids of different phases. Based on analysis of a large amount of mercury injection data, we built a model through multiple regression analysis for calculating average pore throat radius with porosity and permeability. The capillary pressure of each phase and equivalent differential pressures can be estimated according to the distribution patterns of each phase in porous media. Finally, a model was built to predict productivity and fluid properties with the "Differential Pressure Method". Five tested layers in 3 wells were chosen to verify the accuracy of this model. The results show that the evaluation results are basically consistent with the test results.