| With the continuous increase in global oil and gas consumption,conventional methods of oil and gas development are inadequate to meet the demands of social development.This has led to a growing focus on exploiting tight reservoirs,which are characterized by low porosity and permeability.Hydraulic fracturing has become an indispensable measure for the efficient development of the tight reservoirs.Fracturing fluids flowback plays a crucial role in well production,as it carries valuable reservoir information.However,the potential reservoir information carried by flowback data has been largely overlooked and underutilized.The regularities governing gas and water production after fracturing remain unclear,and a lack of clear understanding persists regarding the distribution and migration mechanisms of fracturing fluids within the reservoir.Additionally,determining the optimal shut-in time after hydraulic fracturing poses a challenging problem.This article delves into the study of tight volcanic gas reservoirs in the Songliao Basin,shale gas reservoirs in the Sichuan Basin,and sandstone gas reservoirs in the Ordos Basin.The primary focus is on investigating the core scientific problems associated with fracturing fluid absorption and flowback.By conducting core experiments,the study evaluates the ability of the tight reservoir to absorb fracturing fluid and create fractures.To gain a comprehensive understanding of reservoir characteristics,the study employs theoretical derivation and core experiments to examine the mechanisms driving fracturing fluid absorption and flowback.This analysis aims to uncover the microscopic distribution of fracturing fluid within the reservoir and the energy behind flowback driving forces.Through the utilization of field data and numerical simulations,the research explores the flowback characteristics and patterns observed in tight reservoirs.It investigates the influence of reservoir characteristics and engineering measures on flowback.Furthermore,an artificial fracture diagnostic model is developed using flowback data.This model enables the calculation of volume and width distribution of artificial fractures,providing a quantitative analysis of the distribution of fracturing fluid within fractures and reservoirs at a macroscopic scale.Ultimately,the study presents a comprehensive evaluation method for fracturing,well shut-in,and flowback processes specific to tight gas reservoirs.This methodology is applied in the field,serving as a practical tool for evaluating and assessing these reservoirs.The research findings reveal that the characteristics of tight reservoirs have a significant impact on flowback,specifically in terms of fracturing fluid absorption and fracture creation ability.In the process of liquid absorption within tight reservoirs,it primarily occurs in small pores.As fluids spread throughout the reservoir,the flow direction varies between early-stage and mid-to late-stage fluids.Early-stage fluids predominantly flow from small to large pores,while mid-to late-stage fluids reverse course and flow from large to small pores.The driving energy for early-stage flowback primarily stems from rock elasticity,fracturing fluid pressure,and gas expansion.On the whole,the overall driving energy for flowback depends on factors such as fracture compressibility,fracture network structure,and formation pressure.In different types of reservoirs,variations are observed in the gas-water ratio and cumulative gas production.Shale gas wells demonstrate V-shaped trend in the gas-water ratio,whereas volcanic gas reservoirs exhibit a J-shaped trend.To evaluate artificial fractures effectively,fracture diagnostic models based on flowback data prove to be valuable tools.By utilizing these models,it is possible to assess the efficacy of artificial fractures accurately.Furthermore,a fracturing-shut-in-flowback comprehensive evaluation method holds promise for assessing the effectiveness of onsite fracturing and optimizing flowback systems specifically for tight reservoirs. |
