Study and Treatment on Carbonate/Bicarbonate Pollution in Shale Gas Wells in Sichuan

Shale gas drilling in Sichuan basin is moving from the block Wei-202 to block Wei-204, where the water based drilling fluids used in the second interval were generally contaminated with CO₃^2- and HCO₃^-. The source of the contamination is the Changxing, Longtan and Maokou formations. Contamination of the drilling fluids resulted in fast increase in viscosity and gel strengths, and it was difficult to bring under control the increase in viscosity and gel strengths. To deal with this problem, the source of CO₃^2- and HCO₃^- was first analyzed. It was believed that CO₂ invasion took place in drilling the Changxing and Longtan formations, and some mud materials contain high concentration of HCO₃^- (>10,000 mg/L). Measures have been taken to prevent the contamination from mud materials. Using pH meter, the concentrations of CO₃^2- and HCO₃^- can be accurately measured. Preventing measures and two treatment methods (CaO and CaCl₂) were discussed, and it was concluded that CaCl₂ is the best additive to deal with the contamination. Previously CaO had been used to treat out the CO₂ contamination on well Wei-204H23-5 and well Huang-206, but the results were not satisfactory. After using CaCl₂ to treat out the CO₂ contamination, the concentration of HCO₃^- was reduced from 15,420 mg/L to 1120 mg/L, and the viscosity of the mud was reduced from above 300 sec to 54 sec, which was the viscosity of the mud before HCO₃^- contamination. The viscosity of the mud in well Huang-206 was 180 sec after HCO₃^- contamination, and was reduced to 57 sec, as that before the contamination. This method of eliminating CO₂ contamination has the advantages of is high efficiency, cost-effective and strong pertinence. Keep in mind that the technical measures should be well prepared to avoid contamination of the mud by both Ca²⁺ and HCO₃^-.n recent years, with the transfer of the construction block from Wei202 to Wei204, the pollution problem of carbonate/bicarbonate in the water base drilling fluid was widely encountered. The pollution occurred mainly in Changxing, Longtan and Maokou, and the viscosity/shear increased rapidly and was difficult to control. In view of this problem, the sources of pollution were firstly discussed, and the analysis showed that CO₂ intrusion and coal interlayer in Changxing and Longtan were important pollution sources of carbonate/bicarbonate. Some materials contained a large amount of bicarbonate (＞10 000 mg/L), and measures had been taken at present. The accurate test of carbonate/bicarbonate can be achieved by using a pH meter. Secondly, preventive measures and two treatment schemes are discussed, and it is suggested that CaCl₂ treatment is more suitable for this block.Finally, the CO₂ pollution treatment technology of Wei 204H23-5 and Huang 206 was explored. Two wells were treated with the CaCl₂ solution when the CaO treatment was not ideal. HCO₃⁻ in Wei 204H23-5 decreased from 15,420 mg/L to 1120 mg/L, FV ＞300 s decreased to 54 s as pre-pollution state. After the pollution, the FV of Huang 206 well was up to 180 s, and after the treatment, the FV was reduced to 57 s as pre-pollution state. This method has the advantages of high efficiency, economy and strong pertinence, but technical measures should be implemented to avoid the double contamination of drilling fluid by Ca²⁺ and HCO₃⁻.