Evaluation of a new treatment to remove calcium sulfate scale: Lab studies and field application

Scale deposition has been a significant problem in the oil and gas industry. Scale deposition has an adverse impact on production and injection operations. The precipitation of scale in the near-wellbore area will diminish the productivity of the production wells and loss of the injectivity of water injection wells. This paper presents a new non-corrosive agent to remove calcium carbonate, and calcium sulfate scales. Two sets of coreflood experiments were conducted to evaluate the chemical to remove anhydrite depositions. Coreflood experiments were conducted by using the new scale remover to stimulate dolomite cores with anhydrite content of 30 wt% at 250°F. Other experiments were conducted with the Bandera sandstone cores. Calcium sulfate was precipitated inside the pores and then the chemical was used to treat the cores to remove this damage. Effluent samples were collected and analyzed with ICP for sulfur and calcium concentrations. CT scan was used to detect the dissolution and the deposition of scale inside the cores. The chemical was applied on several oil wells from the Nukhul sandstone formation. Coreflood results show increasing the permeability of the dolomite core to 4 times of its initial value and ICP results show 1:1 calcium to sulfur concentrations. The second coreflood experiments showed the ability of the dissolving agent to remove the precipitated scale completely with (final permeability =initial permeability) with no effect on clays. In the field case, the treatment was conducted and placed using a coiled tubing without using additional additives such as corrosion inhibitors, intensifiers, and iron control agents. The success of this treatment was realized by increasing the well production 4 times its initial oil production per day. Field data and the lab results highlight that the de-scaling solution is an effective fluid for calcium sulfate scale.