Analysis on the causes of corrosion and scaling of downhole tubing in deep coalbed methane wells and the application of corrosion and scale inhibitor
In view of the serious problem of corrosion and scaling of downhole tubing in deep coalbed methane wells in the Daning-Jixian block,the cause analysis of corrosion and scaling were carried out based on the composition tests of produced gas,produced water,and corrosion and scale formation products.Morevoer,a composite corrosion and scale inhibitor was selected based on the corresponding mechanism astudy.The results show that the Ca2+ mass concentration in the produced water of deep coalbed methane wells is high(4 069.88-56 667.51 mg·L-1),which favors the formation of CaCO3 with HCO-3.In the process of gas reservoir devel-opment,the formation pressure gradually decreases,resulting in the continuous release of CO2 from the water during the generation of CaCO3,which promotes the generation of CaCO3.In addition,the reduced formation pressure leads to accelerated evaporation of the produced water in the wellbore,facilitating soluble salt extraction.The high partial pressure of CO2 in produced gas(0.16-0.99 MPa)makes it difficult to form effective FeCO3 corrosion passivation film on the surface of downhole tubing.In addition,the diffusion of metal ions under the scale layer is difficult,promoting Cl-in the produced water to migrate into the scale layer to maintain charge bal-ance and resulting in accelerated corrosion of downhole tubing.The selected composite corrosion and scale inhibitor exhibits high perfor-mance.Under the condition of 70℃,when the mass concentration is 300 mg·L-1,the scale inhibition rate is 91.39%,the static corro-sion rate is 0.058 mm·a-1,and the dynamic corrosion inhibition rate is greater than 90%.After injecting the composite corrosion and scale inhibitor,no obvious scaling phenomenon is observed in the produced water of the test well,and the Fe3+ mass concentration decreases from 281.93-378.60 mg·L-1 to 33.90 mg·L-1.The pump inspection cycle is extended from 139 d to 327 d.
deep coalbed methanethe eastern edge of Ordos Basincorrosion and scalingcorrosion and scale inhibitor
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