Diagnostic Study Of Tracer Production Profiles In Fractured Horizontal Coalbed Methane Wells

China’s coal bed methane（CBM）reservoirs are characterized by low saturation,low permeability,low reservoir pressure and high degree of metamorphism,making it difficult to achieve efficient exploitation using conventional technology.Currently,horizontal well fracturing technology is one of the effective means to increase the production capacity of CBM.This technology can effectively increase the permeability of the reservoir through the modification of the seepage channel to achieve efficient development.The reservoir fracturing effect can be accurately evaluated by the output dynamics and fluid properties of the rejection fluid.Tracer monitoring,as an effective means of accurate flow characterization,can directly track,trace and mark the flow trajectory of fluids,visually indicating and feeding back the spatial characteristics of fluids,and is increasingly used in monitoring the effect of segmental fracturing in horizontal wells.This thesis takes CBM reservoir as the research object,and conducts diagnostic research on the tracer output profile of CBM fractured horizontal wells through indoor experiments,theoretical research combined with field construction effect evaluation.Firstly,through the experiments of compatibility,interference,isothermal adsorption equilibrium,adsorption kinetics,microscopic characterization and system optimization,an anti-sorption tracer system suitable for fracturing monitoring of CBM reservoirs was constructed.The experimental results showed that the rare earth elements were well-matched with the fracturing fluid system,with no precipitation and no reaction after the addition of rare earth elements;the mutual interference between rare earth elements was weak,and the average error was within 5%.The extraction and complexation of rare earth elements was then used to construct an anti-adsorption tracer system,which consists of rare earth elements,extractant（D₂EHPA）and complexing agent（EDTA）,and can reduce the adsorption rate of lanthanide tracer from 90.33%to 13.43%.Secondly,a physical model of circular pipe fracture is constructed based on the basic characteristics of CBM reservoir,tracer adsorption properties combined with seepage theory.Further,by combining the one-dimensional convective diffusion and isothermal adsorption equations,a convective diffusion model of tracer considering adsorption is established,and the equations of tracer concentration in injection,topping and return phases are constructed respectively.The sensitivity analysis shows that with the increase of diffusion and retention coefficients,the peak of tracer returner concentration curve decreases,the slope of the curve becomes slower,the time of seeing the agent is earlier,and the returner process is longer.In addition,the peak number of tracer rejection concentration curve is the same as the number of cracks,and the peak value of the rejection concentration curve decreases and the slope of the curve becomes slower with the increase of the number of cracks.Finally,the on-site fracturing effect of horizontal wells in Shaanxi North CBM Well B area was monitored and diagnosed.The results show that the half-seam length of each fracture section in the direction of fracture Well B-1 to observation Well B-2 is above 130 m,with the longest half-seam length（212 m）in the fourth fracture section and the shortest（133 m）in the third section.The half-seam length of each fractured section in the direction of Well B-1 to Well B-3 is less than 120 m,except for sections 5 and 6.Among them,section 6 has the longest half-seam length（209 m）and section 4 has the shortest（73 m）,and the diagnostic results are more consistent with the use of 4-dimensional imaging for monitoring.In this thesis,through the construction of an anti-adsorption tracer system and the establishment of a tracer rejection model,the horizontal well fracturing tracer output profile in well area B was diagnosed,and the fracturing effect of each section was finally analyzed.The research results are of great significance to the optimization of the later development plan of the well area,the design of fracturing construction in the same type of reservoir,and the long-term efficient and stable development of CBM well production.