Research On Influence Mechanism Of Coal Fines On Hydraulic Fracture Conductivity

The damage of coal fine to hydraulic fracture conductivity is one of the key factors restricting the continuous and stable production of CBM wells and the improvement of productivity.Aiming at the problem of damage to hydraulic fracture conductivity caused by coal in the process of coalbed methane well drainage,based on the solid-liquid two-phase flow experiment equipment in hydraulic fractures,the coal fine retention experiment and conductivity experiment in hydraulic fractures were carried out,and the comprehensive utilization theory was analyzed.The method combined with physical experiment has systematically studied the change of hydraulic fracture conductivity caused by coal fine.Finally,based on the above research,the coal fine control CBM well is proposed.The main research results are as follows:The coal fine retention coefficient is introduced,and the hydraulic fracture conductivity model considering the retention of coal fine is established.The influence of constant velocity and intermittent flow on the retention of coal fine in the drainage stage of single-phase flow and the retention coefficient of coal fine on hydraulic fracture conductivity are studied impact.With the increase of coal fine flow rate,the retention of coal fine decreases exponentially,which increases the porosity of propped fractures in a logarithmic manner,resulting in a logarithmic increase in the permeability and conductivity of propped fractures;the larger the particle size,the easier it is to stay,and the easier it is to reduce the porosity of propped fractures,resulting in a decrease in the permeability and conductivity of propped fractures.The retention coefficient of coal fine decreases exponentially with the increase of the flow velocity,and increases with the increase of the particle size of the coal fine;the retention coefficient ratio of the two flows increases linearly with the increase of the velocity,and the larger the particle size of the coal fine,the greater the flow rate when the retention coefficient ratio is 1.When the particle size of coal fine is the same,compared with the retention coefficient under discontinuous flow,the smaller the flow velocity,the greater the retention coefficient under constant velocity flow,and the greater the flow velocity,the smaller the retention coefficient under constant velocity flow;compared with the permeability and conductivity of propped fractures under discontinuous flow,the smaller the flow rate,the smaller the permeability of propped fractures under constant flow,and the greater the flow rate,the greater the permeability and conductivity of propped fractures under constant flow.During the injection experiment of different concentrations of coal fine suspension,as the concentration of coal fine suspension increases,the later the inflection point of the pressure difference curve appears,the greater the pressure difference at the inflection point,and the greater the stable pressure difference.In addition,the deposited coal fine inside hydraulic fractures increases linearly,and the permeability and conductivity of hydraulic fractures decrease linearly.However,the coal fine deposition amount and the coal fine deposition rate show different trends.The coal fine deposition rate first increases sharply to the peak with the increase in concentration,and then slowly decreases,and the coal fine deposition rate increases linearly;and as the residual rate of coal fine increases,the hydraulic fracture permeability damage is more serious,but the overall increase rate of the hydraulic fracture permeability loss rate slows down.It indicates that the damage of coal fine deposits to hydraulic fracture permeability mainly occurs at the initial stage of coal fine intrusion into hydraulic fractures.And the relationship between coal fine residual rate (R_cp) and hydraulic fracture permeability loss rate(?)is obtained:?=0.71578R_cp^0.4575.Based on the conductivity model established in Chapter 3,the changes in hydraulic fracture conductivity caused by the retention of coal fine are quantitatively analyzed,which provides an important basis for solving the problem of retention of coal fine in CBM wells.In the actual production process,CBM Wells after hydraulic fracturing should not only avoid repeated well shutdown,but also maintain a fast and stable drainage rate to reduce coal fine retention,in addition,the drainage speed should be accelerated beyond the pre-shutdown speed as soon as possible to discharge the residual coal fine when the well is shut down and started again.And based on Chapter 4,the technology of dust suppressant unblocking CBM wells is proposed.The field application results show that the concentration of coal fine produced by representative CBM wells rises significantly before and after the dust suppressant is injected.After the dust suppressant is processed,the representative CBM wells gas production of has increased,indicating that the use of dust suppressant plugging technology can help improve the gas production effect of CBM wells.