SDN Gas Reservoir Fracturing Horizontal Well Productivity Analysis And Optimized Production Research

Situated at the southwest of SLG gas field, China's tight gas field with largest proven reserves, SDN reservoir is characterized by poor porosity and permeability, especially by heterogeneity in microscopic pore structure. Impaired by the adverse formation properties, fractured horizontal wells occupy a dominant position among production wells in SDN reservoir. However, present steady-state productivity equations for fractured horizontal wells in tight gas reservoirs often ignore the effect of heterogeneity in microscopic pore structure. Apart from that, initial production allocations for most wells calculated by conventional methods tend to be higher, resulting in shorter stable production periods reasonable for wells in this area. Therefore, productivity equation for fractured horizontal wells and proration method complying with SDN reservoir needs to be established.Based on geological microscopic formation features, method for calculating 2-D fractal dimension of capillary diameter was established by analyzing capillary pressure curves of rock samples, contributing to certify the fractal characteristics of the research formation. Then with the aid of fractal geometry and capillary seepage theories, the absolute permeability model and the apparent one considering slippage effect were deduced respectively on the basis of capillary bundle model. Moreover, the productivity model coupling gas flowing in formation, fractures and wellbores was finally built by means of dividing seepage areas into three parts and deriving each percolation equations. Meanwhile, instance data were utilized to verify the accuracy of this model by comparing results with one-point empirical formulas and modified isochronal well test. Additionally, typical parameters in this equation were also analyzed to show each impact on well's productivity. On the other hand, an optimized allocation model appropriate to fractured horizontal wells in the research area, involving productivity equation, wellbore pipe flow calculation equations and material balance equation, was established by restricting proration within practical constraint conditions. Based on this allocation model, stable production periods and recovery percent of reserves at the end of stable period were also analyzed and assessed.Based on the methods referred above, this research established a 2-D fractal dimension computation method showing that capillary diameter distribution of eight rock samples presented dimensional fractal features. Then the absolute permeability model and the apparent one were derived with parameters of clear physical meaning that can reflect physical characteristics of both reservoir and gas flowing in it. With the aid of apparent permeability model derived in this paper, a pseudo pressure expression considering both reservoir fractal features and slippage effect was derived, contributing to establish the productivity model. Sensitivity parameters analysis showed that open-flow capacity of actual wells increased with bigger fracture number, half length and flow conductivity and growing rates presented smaller. On the other hand, with the increase of capillary diameter fractal dimension and the decrease of capillary tortuosity fractal dimension, the open-flow capacity tended to be higher. Consequently, an optimized allocation equation based on the productivity model derived in this research was established, demonstrating better proration effect compared with conventional methods. This showed that later proration for fractured horizontal wells in the research area could be adjusted by this optimized production allocation model under present production conditions. The research results concluded in this paper can offer theoretical foundations and rational methods for fractured horizontal wells' productivity prediction and proration optimaization, contributing to provide useful experiences in productivity prediction and proration adjustment.