Abstract
The oil reservoir pressure declines due to oil production, and this decline will lead to reduction in the oil productivity. The reservoir pressure maintenance is a practice in the oil industry in which seawater is injected into the aquifer zone below the oil zone to support the reservoir pressure. Calcium sulfate scale is one of the most serious oilfield problems that could be formed in sandstone and carbonate reservoirs. Calcium sulfate may precipitate during the injection of seawater with high sulfate content into formation brine with high calcium content. Mixing seawater and formation water may cause precipitation of calcium sulfate, barium sulfate, and/or strontium sulfate. Seawater treatment does not remove the entire sulfate ions from the injected water. Low sulfate concentrations may cause damage. Enhanced oil recovery processes such as smart water injection, which originally is diluted seawater, may cause calcium sulfate precipitation as the reduction of water salinity will increase the sulfate precipitation and decrease its solubility. This study was conducted to investigate the damage caused by the deposition of calcium sulfate precipitation. A solution is proposed to prevent the damage due to calcium sulfate by using chelating agents. Several coreflooding experiments were conducted using Berea sandstone and Indiana limestone cores at reservoir conditions of pressure and temperature using seawater (high and low salinity) and formation water. Chelating agents used in this study are: EDTA (ethylenediaminetetraacetic acid), HEDTA (hydroxyethylenediaminetriacetic acid), and HEIDA (hydroxyethyliminodiacetic acid). HEDTA and HEIDA chelating gents are environmentally friendly and can be used in marine environment. High-salinity water injection caused severe formation damage, and the injectivity will decline faster compared to the low-salinity water injection. HEDTA and EDTA chelating agents at low concentrations performed better than HEIDA chelating agents in both Berea sandstone and Indiana limestone cores. HEDTA and EDTA chelating agents were able to prevent the damage due to calcium sulfate precipitation and enhanced the core permeability.
Highlights
The injection of modified seawater or low-salinity water increased the oil recovery from carbonate and sandstone reservoirs (McGuire et al 2005)
The objectives of this paper are to: (1) investigate experimentally the effect of the injected seawater on the reservoir permeability, (2) use EDTA, HEDTA, and HEIDA chelating agents to prevent the precipitation of calcium sulfate scale due to seawater injection, and (3) investigate the effect of using chelating agents on the reservoir permeability
The calcium sulfate precipitation highly damaged the core during seawater injection because of the high sulfate content
Summary
The injection of modified seawater or low-salinity water increased the oil recovery from carbonate and sandstone reservoirs (McGuire et al 2005). Lowering the salinity or total dissolved solids (TDS) of the injected water resulted in reducing the oil–rock capillary pressure, decreased the oil–water interfacial tension, and changed the wettability that caused relative permeability change (Tang and Morrow 1997). Low-salinity water flooding might cause formation damage in both sandstone and carbonate reservoirs. Carbonate formations contain high-salinity brines with high calcium concentration (up to 20,000 ppm) and this will cause calcium sulfate precipitation under the reservoir conditions of high pressure and temperature because the low-salinity water contains sulfate ions (Mahmoud 2014)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
