Modeling of Water Flooding Effects During Enhanced Oil Recovery
Merzouk Zatout
ABSTRACT
Secondary oil recovery using the water injection method was adapted to enhance oil production from the Algerian X-field by maintaining reservoir pressure. The Albian aquifer was used for this purpose. This operation caused many problems with the production facilities. Indeed, several solids, including halite, gypsum (rarely anhydrite), calcite, dolomite, barite, and iron oxides, were deposited in the reservoir, on the downhole completion equipment, along the tubing, and in the surface equipment. On the other hand, the diagenetic sequence of the reservoir sandstones is marked by late cements such as anhydrite, dolomite, barite, and pyrite. To understand the formation of late diagenetic cements and solids observed after water flooding, and the factors involved, a geochemical simulation of the individual and the combined effects of Cambrian oil brine and Albian injection water was carried out. The PHREEQC software was used with two different thermodynamic databases, Pitzer.dat and llnl.dat. The Pitzer database is suitable for highly concentrated waters (brines) and tested for high pressure and temperature brines. The llnl.dat database is suitable for high pressure and high temperature. It gave good results, especially for speciation determination. Thus, a combination and comparison of these two databases was used.
The results obtained by the PHREEQC simulation perfectly reproduced the observations made. Indeed, the late diagenetic iron cement crystallized because of the supersaturation of the oil brine in hematite and goethite during its installation in the Cambrian HMD trap after the oil secondary migration. This favored the crystallization of the late anhydrite and dolomitic cement. In addition, interactions between sulfate-rich Albian water and Cambrian oil brine containing barium, iron, calcium and strontium explain the precipitation of anhydrite, calcite, barite, and iron oxides. The high concentrations of sodium and chlorine in the oil brine, and the progressive decrease of temperature along the hydrocarbon production pathway explain the precipitation of halite throughout.
This study shows that it is useful to simulate the water flooding operation with the starting water flooding and reservoir brine ionic compositions to ovoid salts precipitation by pretreating the injection water or including specific chemical inhibitors of insoluble salts.
