Numerical Simulation And Productivity Prediction Of Multi-fractured Horizontal Well In Shale Gas Reservoir

Shale gas reservoir refers to an unconventional gas reservoir with continuous gas supply and continuous accumulation in shale by organic-rich shale serving as gas source rocks,reservoirs or cap rocks.As a special type of self-generated and self-storage gas reservoir,it differs largely from conventional reservoirs in terms of reservoir characteristics,gas occurrence state,migration mechanism,and development methods,etc.The conventional theory for developing oil and gas reservoir cannot be used directly to guide the development of shale gas reservoirs.All these restrict the economic and efficient exploitation of shale gas.In this paper,a Silurian Longmaxi shale gas reservoir in the W work area of the southern Sichuan Basin is studied.Through the understanding of shale reservoirs and the migration and production mechanisms of shale gas,based on the principles of seepage mechanics and oil and gas reservoir engineering,and adopting well testing and numerical simulation techniques,the geological characteristics of shale reservoirs,the seepage characteristics of fracturing horizontal wells,productivity models,and mining mechanisms were studied,with the following aspects been accomplished and some insights gained:(1)Based on the investigation of a great deal of domestic and foreign documents,a Silurian Longmaxi formation shale gas reservoir in the W work area of southern Sichuan Basin was studied.The shale reservoirs were characterized and evaluated in four aspects:rock minerals,reservoir properties,organic geochemistry,and gas-bearing properties of shale.The results showed that the marine black shale of the Longmaxi formation in the W work area is highly valuable for exploration and development.A three-dimensional geological model taking lithologic facies,organic carbon,and attributes and parameters into consideration was established for the shale gas reservoir in the W work area.The study has provided a reliable geological understanding for the numerical simulation of shale gas reservoirs.(2)Based on a large amount of domestic and foreign documents,a systematic and in-depth analysis of the occurrence state,migration,and gas production mechanisms of shale gas has been conducted.It mainly includes:? multi-scale flow and microscopic migration characteristics of shale gas in reservoirs;? Darcy seepage and non-linear seepage mechanisms of shale gas in micropores and fractures;? the adsorption-desorption mechanism of shale gas on the surface of organic matter;? shale gas production process.This study has laid a theoretical foundation for establishing a mathematical model that conforms to the seepage mechanism of shale reservoirs.(3)The characteristics and connotation of volume fracturing in the development of shale gas reservoirs were expounded.The sectionalized fracturing process for horizontal wells was introduced.The pressure dynamics of the multi-stage fracturing horizontal wells with infinite flow in the complex gas reservoir was analyzed.The influences of factors such as the half length of fracture,fracture conductivity,fracture gap,SRV radius,inner zone permeability,diffusion coefficient,Langmuir volume,and Langmuir pressure on gas well productivity were studied.It's understood that increasing the radius and permeability of the reconstruction area can increase the productivity of shale gas wells.Therefore,the scale of fracturing can be properly increased and volumetric fracture network system be enlarged in actual development to improve the effect of shale gas development.The study has laid a theoretical foundation for predicting the productivity of multi-fractured horizontal wells in shale gas reservoirs.(4)Taking the adsorption-desorption mechanism of shale gas into account and using the matrix as a gas source,the traditional dual-media model cannot accurately describe the natural cracks and multi-level secondary cracks that propagated and interconnected in the fracturing process.The flow of gas in the hydraulic main fracture was regarded as high-speed non-Darcy flow with the flow of gas in primary and secondary fractures regarded as Darcy seepage.A mathematical model for triple media seepage applicable to describing shale volume reconstructed reservoirs was established;A numerical model representing the development of multi-fractured horizontal wells in shale gas reservoirs has been established,including reservoir models and fluid models;Treating the hydraulic main crack as a discrete crack,the primary and secondary fractures were regarded as two equivalent porous continua.Based on self-similarity principle,a triple media ellipsoidal fracture network model was established.The exploitation mechanism of fracturing horizontal wells in shale gas reservoirs was studied,and the effects of different adsorption modes,high-speed non-Darcy flow,initial adsorbed gas content,and multi-level fracture permeability on the production dynamics of shale gas were analyzed.The results show that:? shale gas reservoir development must take the influence of shale gas adsorption effect into consideration,and delayed adsorption mode is an effective way to characterize the adsorption-desorption mechanism of shale gas.? the non-Darcy effect is significant in artificial fractures with high conductivity,which may lead to the decrease of seepage capacity and production capacity due to the existence of additional resistance.? in the middle and later stages of the development of shale gas reservoirs,the greater the initial adsorbed gas content,the greater the cumulative gas production of gas wells;? the fracture permeability at all levels is the most important factor affecting productivity of gas wells,of which hydraulic main cracks and secondary cracks are particularly significant.The research and understanding in this paper can provide a basic theoretical basis for guiding an efficient development of shale gas reservoirs.