Polysulphate: A New Eor Additive to Maximize the Oil Recovery from Carbonate Reservoirs at High Temperature
Authors: Khan Md Ashraful Islam , Piñerez Torrijos Iván Darío , Algazban Saja Hussam Aldeen , Strand Skule , Puntervold Tina 
Published: October 2022

Seawater injection is an EOR success in the North Sea carbonate reservoirs due to wettability alteration toward a more water-wet state, this process is triggered by the difference in composition between injection and formation water. "Smart Water" with optimized ionic composition can be easily made under laboratory conditions to improve oil recovery beyond that of seawater, however, in the field, its preparation may require specific water treatment processes, e.g., desalination, nano-filtration or addition of specific salts. In this work, a naturally occurring salt called polysulphate (PS) is investigated as an additive to produce Smart Water.

Outcrop chalk from Stevns Klint, consisting of 98% biogenic CaCO3, was used to investigate the potential and efficiency of the polysulphate brines to alter wettability in chalk. Solubility of polysulphate in seawater and de-ionized water and brine stability at high temperatures were measured. Energy Dispersive X-Ray and ion chromatography were used to determine the composition of the polysulphate salt and EOR-solutions, and to evaluate the sulphate adsorption on the chalk surface, a catalyst for the wettability alteration process. Spontaneous imbibition, for evaluating wettability alteration, of polysulphate brines into mixed-wet chalk was performed at 90 and 110°C and compared against the recovery performance of formation water and seawater.

The solubility tests showed that the salt was easily soluble in both de-ionized water and seawater with less than 5% solid residue. The de-ionized polysulphate brine contained sulphate and calcium ion concentration of 31.5 millimolar (mM) and 15.2 mM, respectively, and total salinity was 4.9 g/L. This brine composition is very promising for triggering wettability alteration in chalk. The seawater polysulphate brine contained 29.6 mM calcium ions and 55.9 mM sulphate ions, and a total salinity of 38.1 g/L. Compared to ordinary seawater this brine has the potential for improved wettability alteration in chalk due to increased sulphate content.

Ion chromatography revealed that the sulphate adsorbed when polysulphate brines were flooded through the core, which is an indication that wettability alteration can take place during brine injection, the reactivity was also enhanced by increasing the temperature from 25 to 90 °C. Finally, the oil recovery tests by spontaneous imbibition showed that polysulphate brines were capable of inducing wettability alteration, improving oil recovery beyond that obtained by formation water injection. The difference in oil recovery between ordinary seawater and seawater polysulphate injection was smaller due to the already favorable composition of seawater.

Polysulphate brines showed a significant potential for wettability alteration in carbonates and are validated as a potential EOR additives for easy and on-site preparation of Smart Water brines for carbonate oil reservoirs. Polysulphate salt, added to the EOR-solution, provides the essential ions for the wettability alteration process, but further optimization is needed to characterize the optimal mixing ratios, ion compositions, and temperature ranges at which EOR effects can be achieved.