Organic/Inorganic Deposition in Oil Producing Wells from Carbonate Reservoirs: Mechanisms, Removal and Mitigation

Abstract

Abstract Several wells in a carbonate reservoir have experienced significant reduction in their oil productivity. Bailer samples collected from these wells showed that they were damaged with organic/inorganic deposition. Analysis of the collected material showed that the organic deposits consisted of asphaltene and associated paraffins while the inorganic deposits were mainly calcite with ankerite, anhydrite and small amounts of quartz and halite. The formation of calcium carbonate scale is anticipated to occur due to CO2 degassing during production. In addition, these wells produce commingled oil from two carbonate reservoirs which allow their formation waters to mix together; therefore, scaling tendency prediction showed a positive saturation index for calcium carbonate. The sulfate scale in the form of anhydrite is anticipated to be formed due to mixing of high sulfate source (seawater), used to maintain reservoir pressure, with a calcium rich source (formation water). The colloidal instability index from SARA analysis indicated that the reservoir oil in this field has a moderate instability to potentially precipitate asphaltenes. Additionally, the interaction of low pH scale inhibitor with the formation oil and presence of charged particles such as calcite will destabilize asphaltenes in oil. Two techniques were examined toward dissolving the organic/inorganic deposition including a two-stage and a single-stage method. The two-stage method involves the use of solvent preflush to remove the organic coating material from the whole deposits followed with the use of HCl acid to remove the remaining inorganic deposits. An emulsified HCl acid/solvent is used in the single stage method to remove the whole deposits. The paper discusses different analytical techniques used to identify the nature of the damaging material, mechanism of organic/inorganic deposition, and the optimization of dissolver/solvent systems to remove formation damage and restore productivity of damaged wells. It also assesses and optimizes the current used scale inhibitor squeeze (SIS) treatment program.