Study On The Evolution Law Of Fracture Seepage Of Anthracite Under Superheated Steam Injection

Coalbed methane(CBM)reservoirs in China generally have the characteristics of"low permeability and poor desorption".In-situ heat injection can not only promote CBM desorption,but also improve the permeability of the reservoir,which can realize the efficient development of CBM.The seepage characteristics of the fractures are the key factors determining the desorption,migration and heat injection-heat transfer of the coal seam,and the key to the success or failure of the in-situ heat injection enhanced CBM mining engineering.Aiming at this key problem,this paper takes Jincheng anthracite in Shanxi Province as the research object to systematically study the evolution of fracture seepage characteristics during cyclic intermittent superheated steam injection,as well as the evolution of seepage characteristics of proppant-containing fractures during long-time superheated steam injection.Finally,the law of heating deformation and the evolution law of fracture seepage of anthracite under different heat injection methods have been obtained,and the corresponding influence mechanism has been discussed in depth,and the following conclusions have been drawn:(1)Under the electric heating method,within 300°C,with the increase of temperature,the axial deformation of anthracite expands linearly as a whole,and the hydrostatic pressure has a significant inhibitory effect on the thermal deformation.Affected by the interaction of factors such as triaxial stress,and uneven thermal expansion,the fracture permeability is positively correlated with temperature when the volume stress is 15 MPa(σ_a=σ_c=5 MPa),and it is negatively correlated with temperature when the volume stress is 30 MPa(σ_a=σ_c=10 MPa).(2)Under the superheated steam heating method,the surface temperature of anthracite can be divided into four stages with injection time:rapid heating stage,phase change heating stage,slow heating stage and constant temperature stage.The thermal deformation expands continuously with the increase of temperature,and the higher the temperature is,the larger the deformation is;as the temperature tends to the heat transfer limit,the deformation also tends to be stable;during the cooling process,the anthracite is compressed and deformed with the decrease of temperature.With the increase of hydrostatic pressure,the thermal deformation decreases.Comparing the changes of anthracite permeability before and after heat injection,it is found that:when the initial fracture permeability is large,after superheated steam injection at 400°C(inlet temperature of 300°C)for 12 hours,the fracture permeability is increased to some extent,and after cooling,the fracture permeability is slightly reduced by triaxial stress compression,but it is still higher than the value before heat injection;when the initial fracture permeability is small,the fracture is affected by thermal expansion and the fracture permeability is extremely low and cannot be injected with superheated steam.(3)Based on the fracture permeability of anthracite is enhanced after cooling,a cyclic intermittent injection scheme is proposed for the problem that the initial fracture permeability is too low to be injected,i.e.,injecting superheated steam,cooling and injecting again,and so on for several cycles.The law of fracture permeability of anthracite with proppant during long-time superheated steam injection is studied.It is found that the fracture permeability of anthracite continued to decrease with increasing cycle time under the conditions of injecting superheated steam at 200°C(inlet temperature of 200°C),300°C(inlet temperature of 250°C),and 400°C(inlet temperature of 300°C),and cannot be restored to the initial level after cooling.The main reasons for this may be:the limiting effect of triaxial stress,thermal stress and the water-lock damage of residual water saturation caused the fracture to close and prevented the permeability from increasing.Therefore,intermittent heat injection is not effective in increasing the fracture permeability.(4)In response to the problem that the fracture permeability cannot be effectively increased due to low initial permeability,the technical idea of adding proppant to increase the initial permeability is proposed.The law of permeability of anthracite fracture with proppant during long-time superheated steam injection is studied.It is found that under the conditions of superheated steam injection at 200℃(inlet temperature of 195℃),250℃(inlet temperature of 215℃)and 300℃(inlet temperature of 235℃),the fracture permeability with proppant is gradually decreased with injection time and cannot be restored to the initial level after cooling;while under the conditions of superheated steam injection at 350℃(inlet temperature of 260℃)and 400℃(inlet temperature of 300℃),the fracture permeability gradually increased with injection time,and the increase reached 18.93%～23.81%.Comparing the fracture surface characteristics before and after the 72 hours and 12 hours with superheated steam injection at400°C,it is found that the number of new microfractures increase substantially,the existing microfractures expand further,and the degree of aperture increases.The number of microfractures under the effect of long-time superheated steam injection is more,the length is longer and the degree of connectivity is better.Therefore,the selection of inlet superheated steam temperature above 260°C and the addition of proppant can significantly improve the heat injection effect of coalbed methane reservoirs.