A New Effective Method for Complex Fracture Evaluations in Fractured Sandstone Reservoirs

Abstract

Abstract Ultra-deep fractured low-porosity sandstone gas reservoir is the most realistic unconventional oil and gas resource to be developed in China. However, reservoir is naturally fractured, non-homogeneous and artificially fractured, which makes the traditional analytical well test analysis method not applicable. To address this problem, the fractures are classified and characterized into small, medium and long scales according to practical understanding. A multi-scale fracture characterization method is developed utilizing a dual-porosity model to depict uniformly developed small fractures, while a discrete fracture model is employed to characterize medium and long fractures. Further, a numerical well-testing model for ultra-deeply fractured low-porosity sandstone reservoirs is established, namely a multi-scale dual porosity dual-zone composite model. Results show that the flow regimes can be classified as: wellbore storage and skin effect, acid-fracture influence, inner zone influence, transition flow, outer zone influence, and boundary effect. Based on the established numerical well-testing model, sensitivity analysis of the parameters is performed and well-test curve plates are drawn. It is observed that an increase in the number and length of natural fractures resulted in a deepening of the "depression" in the pressure derivative curve and a decrease in pressure losses within the test well. The curve characteristics in the early stages are predominantly influenced by acid-fracture parameters, whereas the degree of "depression" and its manifestation is impacted by the storage ratio and interporosity-flow coefficient. Finally, field applications are carried out to verify the reliability of the model. Notably, the target blocks are characterized by well-developed yet heterogeneous natural fractures. The results show that our numerical well testing analysis method has demonstrated greater accuracy and effectiveness in analyzing and interpreting such reservoirs.