Managing Radioactive Scale: Strategic Approach and Success Story

Abstract

Radioactive material contaminated, hard crystalline scale deposition on ESP & downhole equipment was resulting in frequent breaking of ESP shaft in a multilateral openhole producer. Extremely high workover cost in frequent replacement of downhole equipment and associated radioactive hazards, necessecitated a strategic MDT approach. Software usage and extensive laboratory backup helped finding the best possible option to mitigate this problem. Three possible well treatment options were investigated. Scale inhibitor squeeze option was studies through reservoir condition core flow and rejected because of probable formation impairment and placement difficulties in a complex well and reservoir geometry. Periodic soaking of ESP with scale dissolver was considered a viable option because of economics and operational ease. Laboratory studies were conducted to design the placement strategy. We developed a xanthan based high viscous self degrading polymer gel coushion which would support the scale inhibitor solution around the ESP for the necessary soaking period to prohibit flow into the pressure depleted formation. However this concept had to be abandoned because of flipping over of the fluids in laboratory observation and uncertainty of optimum soaking period downhole. The last and final option was to wait for the running ESP to fail and during workover the well could be completed with a macroni line for continuous injection of scale inhibitor. Judicial selection and optimization of inhibitor, continious monitoring of well treatment and well behavior has resulted in phenomenal success which is demonstrated by the fact that the ESP run life has been exceeded 700+ days against <100 days average runlife prior to treatment. The treatment also took care of radiation hazard and contaminated equipment disposal problem to a large extent. Introduction Inorganic scale build up inside the wellbore and near wellbore formation can smother a well in a very short productive period causing additional operating expenses in the range of millions of dollars. This statement is particularly true if the deposited scale is contaminated with NORM (naturally occurring radioactive materials). In addition to loss of production and workover cost, the full set of contaminated tubulars and downhole equipment needs to be either decontaminated at designated site or to be disposed following international regulations. Both the options are undesirable from the operational complication and expenditure point of view 1,2,3. Origin of NORM in oilfield scale - The NORM radionuclides are uranium, potassium, thorium, radium and radon. These elements are found in trace quantities throughout the earth's crust. These unstable radioactive elements undergo periodic decay and results in the formation of elements with different physical and chemical properties. Radium-226 which is a decay product of U-238 and Th-230 is water soluble and may therefore be found dissolved in aqueous phase associated with oilfield produced water. As far as radioactive scales are concerned, Ra-226 is the most common element responsible for radioactivity which is co-precipitated with Gr-II elements, particularly with barium as carbonates or sulphates. RaSO4 and BaSO4 have similar solubility and crystal structure hence Ra2+ substitutes for Ba2+ in the BaSO4 lattice. At lower temperatures RaSO4 is less soluble than BaSO4, increasing Ra substitution. There may be cases where Pb-210 is also a minor contributor 4. These types of radioactivity are also called TENR (technology enhanced natural radioactivity)4. This paper describes the laboratory and field experience, success achieved and lessons learned in dealing with radioactive strontium scale deposition and associated problems in BRH oil field.