Scale deposition mechanism and low-damage underbalanced drilling fluids for high CO2 content gas reservoirs

Factors affecting the solubility of CO2 and CaCO3, including temperature, pressure, salinity of solutions, and pH value have been studied and then the mechanism of scale deposition in gas reservoirs with high CO2 content has been analyzed. The solubility of CO2 reduces with the increase in temperature and salinity, whereas it increases with the increase in pressure; the solubility of CaCO3 reduces with the increase in temperature, it increases with the increase in pressure and increase in pH value can promote the forming of CaCO3. Taking the typical formations of high CO2 content in Jilin Oilfield as an example, the prediction of CaCO3 scaling was conducted, indicating that there might be scale deposition after the drilling fluid filtrate invaded into the formations. Since the gas reservoirs was characterized by deep burial, high temperature, low pressure coefficient, fracture developing, etc., two kinds of drilling fluids, oil-in-water drilling fluid and solid-free formate drilling fluid, were selected for the underbalanced drilling. The anti-pollution test and dynamic damage tests showed that they all exhibited excellent anti-pollution performance to CO2 and remarkable formation protection. The field applications in Longshen No.2 and No.3 wells proved that the two drilling fluid systems worked well and the returned permeability of the field drilling fluid to cores was more than 85%, which met the demands of underbalanced drilling.