Corrosion and scale control technology used in the Permian CBM wells of southern Sichuan Basin

The coalbed methane in Junlian of southern Sichuan Basin is deep coalbed methane and there are over 10 coal seams. In recent years, corrosion and scaling of downhole string occurs extensively, which leads to pump inspection and off production and seriously impacts the development process. In this paper, 12 groups of scale samples were analyzed by means of XRD. It is shown that scale samples are the rare mixture of multiple inorganic minerals containing CaCO₃, FeCO₃ and SiO₂, so the conventional corrosion and scale inhibitor of Ca/Mg salt is not applicable and it is necessary to explore a new corrosion and scale control technology. Then, the dispersion, corrosion control, scale inhibition and bactericidal effects of 6 kinds of corrosion and scale inhibitors (EDTMPS, HEDP, ATMP, EDTMPA, PESA and PASP) and 2 kinds of bactericidal agents (1 227 and glutaraldehyde) were evaluated by means of laser grain size detection method, weight loss method, static scale inhibition method and vanishing dilution method. It is shown that glutaraldehyde has better bactericidal effect and it can kill the bacteria completely at 30 mg/L without influencing the grain size obviously. EDTMPS and PESA have better corrosion and scale inhibition effect. At the mixing ratio of 1∶1 and the concentration of 90 mg/L, scale inhibition rate and corrosion inhibition rate are up to 84.56% and 92.34% respectively and the grain size is decreased effectively. The eventually determined corrosion and scale control formula is 45 mg/L organic phosphorus scale inhibitor EDTMPS+45 mg/L polymer scale inhibitor PESA+30 mg/L glutaraldehyde bactericidal agent. It is injected through the annulus by using the uninterrupted pumping unit, and 3-6 d later, the mass concentration of Ca²⁺ in the produced water rises from 200 mg/L to 380 mg/L, that of Fe²⁺ drops from 14 mg/L to 8 mg/L and the number of bacteria is nearly 0. Long-term follow up shows that the pump inspection cycle caused by corrosion and scaling is extended from 665 d and 681 d to 749 d and 880 d, respectively. In conclusion, the corrosion and scale inhibitor dominated by EDTMPS, PESA and glutaraldehyde can effectively alleviate the corrosion and scaling of downhole string in this block.