Study On Productivity Calculation Methods For Fractured Gas Wells In Tight Gas Reservoirs

Tight gas in a broad sense includes tight sandstone gas and shale gas.. China has found a large number of tight gas fields, all of which need to be developed urgently. However, the occurrence mode and seepage mechanism of the tight gas are more complex compared with those of the conventional gas reservoir because tight gas belongs to low porosity low permeability gas reservoir. In addition, in most cases tight gas needs large-scale reservoir stimulation technology, such as multiply fractured horizontal well so as to achieve industrial gas flow. Therefore, studies on the seepage mechanism and productivity calculation methods of fractured horizontal well will provide a valuable reference for the development of unconventional gas resources in China.Although tight sandstone gas and shale gas both belong to unconventional low permeability gas reservoir with the same feature of low porosity and low permeability, they have great differences on the reservoir characteristics, occurrence mode as well as seepage mechanism. Therefore, this article will first conduct a comparative study between tight sandstone gas and shale gas about their reservoir characteristics and seepage mechanism and then summarizes the differences and similarities. Furthermore, we regard tight sandstone gas in Erdos basin and shale gas in Szechwan Basin as examples to illustrate the characteristics of these two kinds of tight gas, which will pave way for the research on seepage mechanism.Based on the concept of Knudsen number, the paper studies gas seepage mechanisms in nano-scale tight reservoir. It finds that the gas flow in tight gas reservoir is influenced by Kundsen number, and the throat radius is one of the main parameters influencing Knudsen number. Moreover, different scopes of Knudsen number correspond to relevant flow regimes, displaying specific characteristics of non-Darcy flow. The author deduces the mathematical expressions for apparent reservoir permeability and permeability correction factor and then anlyzes the effects of Knudsen number, temperature, and pressure on apparent reservoir permeability. Using the concepts of Flow Regime and Flow Scale, the author also builds three kind of ideal seepage models for tight gas reservoirs.After the seepage models are built, the article makes studies on the productivity of tight gas fractured vertical well. And the author here puts forward three concepts including apparent reservoir permeability considering slippage effects and Knudsen diffusion effects in the reservoir, apparent fracture permeability considering inertial non-Darcy effects, and apparent equivent radius. According to the former studies, four kinds of productivity equations for fractured vertical well in tight gas reservoir are deduced considering these effects respectively:(1) inertial Darcy effects in the fracture;(2) inertial non-Darcy effects in the fracture and threshold pressure gradient in the reservoir at the same time;(3) inertial non-Darcy effects in the fracture and slippage effects in the reservoir at the same time;(4) inertial non-Darcy effects in the fracture and Knudsen diffusion effects in the reservoir at the same time. Further, this paper will apply apparent equivalent radius into multiply fractured horizontal well and make use of the superposition principle of pressure drop to deduce the productivity equation of multiply fractured horizontal well in tight reservoirs by considering inertial non-darcy effects in the fracture, threshold pressure gradient, slippage and Knudsen effects in the reservoir.Based on the analyses above, it is found inertial non-Darcy effects and threshold pressure gradient will have adverse effects on the productivity of fractured wells, resulting in low well productivity. Slippage effects and Knudsen diffusion effects will improve apparent reservoir permeability and increase the productivity of gas wells, keeping the gas wells in tight reservoirs with the good production ability.