An Investigation on Reasons of Water Scaling in Oil Production and Water Injection Wells at Yanjia Oil Field

The reasons of serious water scale deposition in nearbore zones,bottom,and wellbore of oil production wells(OPWs) and water injection wells(WIWs) at Yanjia oil field of reservoir temperature 65—75℃ are investigated.The formation waters(FWs) from 7 OPWs are characterized by water type NaHCO_3,TSD in range 6.4—10.0 g/L,high HCO~-_3 concentration,1.6—3.7 g/L,and mild Ca~(2+)+Mg~(2+) concentration;the injection waters(IWs) from a water source well and an injection well — water type CaCl_2,TSD in range 32—33 g/L,high Ca~(2+)+Mg~(2+) concentration,3.1—3.8 g/L, and low HCO~-_3 concentration;all these waters above mentioned at 70℃ tend to CaCO_3 scaling as predicted by using CaCO_3 saturation index,SI,and,besides of FWs from 3 OPWs,to CaSO_4 scaling as predicted by using thermodynamical solubility of CaSO_4,S.For the mixture of FWs from 3 OPWs and the source well water at different ratio,the content of suspended substances is increased dramatically at 25℃ comparing with that for the FWs and the source well water alone and increased significantly again in 7 day of standing at 70℃,which indicates incompatibility of the FWs and the IWs;the suspended substances isolated and dried dissolve readily in dilute hydrochloric acid and hence are composed mainly of carbonates.The inorganic precipitates taken from a deep electric pumping well and a water injection well contain 90% of acid soluble substances(carbonates).The conditions for carbonate and sulfate scale formation such as temperature,pressure and pH value are discussed.