The Key Technologies For Quantitative Evaluation For The Complex Deep Tight Gas Reservoirs

As the typical deep tight gas reservoir,TX₂ gas reservoir of the West Sichuan Depression,has the characteristics of buring deeply,complex geological conditions,multi-stage development of fractures and faults,complex accumulation mechanism and enrichment law,fracture controlling production and diversification of gas-water relationship.The unclear type of gas reservoir,the lack of theoretical guidance for productivity evaluation,the uncertain production decline mechanism for decline mode,and the ambiguity of dynamic reserve calculation methods of gas wells seriously restrict the increase of reserves and efficient development of the gas reservoir.Taking the Xu₂gas reservoir in Xinchang structural belt of West Sichuan depression as the research object,this paper focuses on the core problems for productivity evaluation and production decline model,and takes into account the optimization of dynamic reserve calculation methods.Under the guidance of multi-disciplinary theories such as mathematical geology,a productivity evaluation system based on the equivalent seepage resistance method of gas reservoir is established through theoretical derivation At the same time,the theoretical models of gas reservoir production decline for three types of conditions are systematically derived,the physical meaning of the model is given,and a unified description method of common production decline models based on Bernoulli differential equation is proposed.The research results have effectively guided the evaluation and development of Xu 2 gas reservoir in Xinchang structural belt of Western Sichuan depression.In terms of productivity evaluation,based on the principle of hydropower similarity,it is mathematically demonstrated that the stable seepage of compressible fluid can also use the equivalent seepage resistance method to solve the productivity equation,and the mapping relationship between the physical quantities of the principle of gas electricity similarity is established.Based on it,simplifing model according to the characteristics of fracture production control of Xu₂gas reservoir in Xinchang structural belt of West Sichuan depression,the gas well productivity equation describing fracture elements such as fracture length,fracture width,fracture density and the distance between fracture and wellbore comes ture.The results show that the gas well productivity increases rapidly with the increase of fracture length,fracture width and fracture density,and decreases rapidly with the increase of the distance between fracture development zone and wellbore.Besides,considering the turbulence effect,the productivity equation solved by the equivalent seepage resistance method combined with the pressure superposition principle is also a binomial productivity equation in form,in which the first-order coefficient of the productivity equation can indicate the degree of fracture development,which provides a theoretical basis for the productivity evaluation of Xu₂ gas reservoir.The actual gas reservoir development practice is consistent with the theoretical research conclusion.In the research of gas reservoir production decline model,based on the type and production characteristics of Xu₂ gas reservoir in Xinchang structural belt of Western Sichuan depression,the gas reservoir production decline model is mathematically deduced and analyzed from three aspects:two-phase stable seepage of water drive for gas reservoir,the actual production mode from constant production to constant pressure of gas drive for gas reservoir,and fracture control production.The results show that under the condition of two-phase stable seepage,the production decline model of gas well is consistent with the three decline models of Arps.The characteristics of decline curve depend on the characteristics of gas phase permeability curve.When the gas phase permeability curve is a power-law curve,the type of production decline of gas well is hyperbolic progressive decline.When the gas phase permeability curve is a linear curve,the production decline type of gas well is exponential decline.When the gas phase permeability curve is exponential curve,the production decline type of gas well is harmonic decline.As to decline model for actual working mode of gas well from constant production to constant pressure of gas drive for gas reservoir,similar to that of reservoir based on Hiles and Mott seepage model,it depends on seepage law following by fluid.That is when following the Darcy’s law,the production decline follows piecewise exponential decline;when the non Darcy high-speed or low-speed law is followed,the production decline follows hyperbolic decline,in which the value range of low-speed non Darcy hyperbolic decline index is between 0 and 1,and the value range of high-speed non Darcy hyperbolic decline index is less than 0.For fracture-dominated gas reservoirs,based on the analysis of Duong decline model,a new production decline model is proposed.The model is more general,including logistic decline model（LMG）,and can realize the optimal fitting of Arps decline model.Put it into practice,the effectiveness and practicability of the theoretical research is proved.And it provides a theoretical basis for the analysis of gas well production decline law in West Sichuan depression from the perspective of seepage mechanism,as well as a basis for index prediction and reserve evaluation model selection.In terms of optimization of gas well dynamic reserve calculation method,through investigation and combined with the characteristics of Xu₂ gas reservoir in Xinchang structural belt of West Sichuan depression,it is clear that the material balance method and the new production decline model are suitable for the calculation and evaluation of gas well dynamic reserve of this gas reservoir.The theoretical research results are popularized and applied to the evaluation and development of Xu₂ gas reservoir in West Sichuan depression,combined with grey relationship theory and neural network theory,and the productivity evaluation,quantitative analysis and data mining of main productivity control factors,post evaluation of the effectiveness of drainage and gas production process measures,extraction of gas phase permeability curve,fracture evaluation and evaluation of gas well dynamic reserves are realized,which provides technical support theoretical basis for the efficient development of the gas reservoir.