Techniques Used To Monitor and Remove Strontium Sulfate Scale in UZ Producing Wells

Abstract

Abstract Scale deposition in completion strings is becoming a threatening problem to produce and safely operate wells completed in the Upper ZAKUM (UZ) oil field. Calcite or Calcium Carbonate (CaCO3) scale mostly found in the upper part of the production string, and Celestite or Strontium Sulphate (SrSO4) mostly found in the lower part of the production string, are the common type of scales encountered in Upper ZAKUM producing wells. Injection seawater (rich in Sulphate) and formation water (rich in Strontium ions) mix in the reservoir and/or wellbore under varying conditions resulting into Strontium Sulfate Scale formation in downhole equipment. While CaCO3 scales are possible to be removed by the use of common acids and wireline tools, Strontium Sulphate scale requires special techniques to remove chemically and/or mechanically, and present the most challenges to achieve complete removal. This work will describe ZADCO scale management strategy to monitor and remove Strontium Sulfate scale in Upper Zakum producing wells. A scale prediction simulator is used to identify wells with high scaling risk. Scale Risk Matrix (SRM) is being developed to classify the scale risk in each well. The Chlorides content, the percentage sea water in the produced water, the production rate, the percentage water cut and scaling index are the main parameters that are used to calculate the overall scaling risk for a certain string. The wells classified as high scaling risk wells, are included on a monitoring list for periodical scale checks by running gauge cutters on slickline. Scale samples are collected and sent to the lab for analysis and scale type identification. The category of strings with a scale thickness less than 0.25″ are treated with a chemical scale dissolver, the wells containing scale thicker than 0.25″ are treated with downhole cleaning tools run on Coiled Tubing. In 2005, ten wells severely scaled with Strontium Sulphate were mechanically treated using Coiled Tubing, Mills and motor, high pressure rotating tool with Sterling Beads* and Polymer as the cleaning fluid. Most of the job objectives were not completely accomplished due to severe hard scaling conditions. Introduction Upper Zakum field (4th largest known field in the world), operated by ZADCO, and located offshore Abu Dhabi. The field was discovered in early sixties. Oil production started in 1982. Seawater injection to support the reservoir pressure started in 1984. Water breakthrough started in 1990. Strontium Sulfate (SrSO4) scale was observed in 1991. Since 1991 there has been an increase in scale related problems such as interference with producing wells wireline operation and minor production losses. With more than 400 producers and more than 350 pressure support water injectors in UZ, a solid Well Integrity Management System (WIMS)1 was put in place by ZADCO that helps to identify, prevent and solve all the problems related to lack of integrity in each well. Scale management system is a major part of ZADCO WIMS. In 2001 ZADCO carried out a scale study to evaluate the scale risk in UZ field. It was found that the the maximum Scaling Index (SI) (need to explain what is scale index) for mixtures of UZ formation water and injected seawater at reservoir conditions is 0.57 for strontium sulfate which is only just above the positive threshold SI of +0.40 for the lowest zone of scale risk to the production problems. This would tend to indicate that UZ field would not have significant production declines due to scale.