Analysis of and Solutions to the CaCO3 and CaSO4 Scaling Problems Encountered in Wells Offshore Indonesia

Abstract

Abstract Pertmina/MAXUS Southeast Sumatra Petroleum, Inc. is the largest offshore oil producer in Indonesia. CaCO3 and CaSO4 scaling in and around the submersible pumps of Pertamina/MAXUS' Farida/Zelda reservoir wells led to pre-mature Pertamina/MAXUS' Farida/Zelda reservoir wells led to pre-mature pump failures and costly workovers to bring the wells back into pump failures and costly workovers to bring the wells back into production. Twenty-four well brines were analyzed on-site to production. Twenty-four well brines were analyzed on-site to accurately determine brine chemistries and scale samples were analyzed to determine exact composition. Well histories were studied to find correlations of procedures which led to scaling problems. Saturation Indices, developed at Rice University and problems. Saturation Indices, developed at Rice University and presented in the paper, were applied to the problems to give presented in the paper, were applied to the problems to give insight into the causes of the intermittent, but costly scale formation. Discussions were held with a submersible pump consultant and pumps were examined to provide additional data for the analysis. Thirteen scale inhibitors were evaluated at 225 F (107 C) and 300 psia (2.07 MPa) in a 1.1% CO2 atmosphere using a flow simulator developed at Rice University to find the most effective scale inhibitor to use in the wells to mitigate scale formation. The CaSO4 scaling problem was determined to be a result of HF/HCl acid treatments performed on the wells to increase productions rates. These acid treatments were followed by KCl productions rates. These acid treatments were followed by KCl seawater overflush solutions. The problem can be eliminated through the limited use of strong acid stimulation treatments and by putting a fresh water plug between the formation and the seawater or using scale inhibitor in the seawater overflush and kill fluids. Calcium from the dissolved CaCO3 in the formation due to the acid environment precipitated with sulfate ions from seawater to form the CaSO4 scale. CaCO3 scale formed due to the increased temperature, the use of rotary gas separators and the decreased pressure in and around the submersible pumps. Since production rates need to be maximized and increased temperatures production rates need to be maximized and increased temperatures and decreased pressures are necessitated by the production scheme, it has been decided to use chemical scale inhibitors in the wells. The wells are being squeezed to protect them from CaCO3 scale formation. Wells that had phosphonate residuals from previous inhibitor squeezes had longer pump runs than wells without inhibitor protection. One specialty proprietary chemical and generic aminotrimethylene phosphonic acid (ATMP) were found to be the most effective scale inhibitors in flow through testing and were effective at 1.5 ppm. Since the specialty chemical had precipitated in the container, ATMP was recommended. precipitated in the container, ATMP was recommended Introduction The wells of Pertamina/MAXUS, particularly the Farida and Zelda fields, in Indonesia have a tendency to scale in and around the submersible pumps. The type of scale is primarily calcium carbonate, but some calcium sulfate scale has been identified. The latter occurs to a large extent after acid stimulation treatments. Scale formation in the pumps can cause significant and serious damage to the pump components and results in short pump runs which heavily impacts field economics. Although scale has been reported primarily from the Farida and Zelda fields, scale was observed on or in the pumps from the Cinta, Yvonne, Rama and Intan fields (see Table 1).A review of carbonate chemistry reveals the basic reasons for the formation of scale in and around the submersible pumps. calcium carbonate (CaCO3) solubility has an inverse relationship with temperature or stated more simply, CaCO3 scale becomes more insoluble with increasing temperature and a solution at equilibrium with CaCO3 will precipitate the solid as the temperature is increased (Figure 1). The tendency to form CaCO3 also increases with increasing pH (as the solution becomes less acid). The decrease in total pressure around the pumps allows dissolved carbon dioxide (CO2) to escape from solution as a gas causing an increase in pH with a subsequent increase in the tendency to form solid CaCO3 scale. Finally, the decrease in total pressure decreases the solubility of solid CaCO3 and therefore increases the probability that CaCO3 scale will form. In summary, the increase in temperature (up to 100 F) around the submersible pumps and the pump motors and the decrease in pressure, both contribute to the formation of CaCO3 scale in and around the pumps and in the reservoir and perforations. (For a review of the basic chemistry of CaCO3, see 1, 2, 3, 4).Calcium sulfate (CaSO4) scale formation is somewhat dependent on temperature, but is typically precipitated due to a decrease in pressure or an increase in the relative concentrations of calcium and/or sulfate. CaSO4 solubility is fairly independent of pH and therefore, can readily precipitate in an acid environment pH and therefore, can readily precipitate in an acid environment (e.g. after an acid stimulation treatment). P. 233