Pertamina/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 premature pump failures and costly workovers to bring the wells back into 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 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/HC1 acid treatments performed on the wells to increase 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 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 wert effective at 1.5 ppm. Since the specialty chemical had precipitated in the container, ATMP was recommended.