A Zeta Potentiometric Study on Effects of Ionic Composition and Rock-Saturation on Surface-Charge Interactions in Low-Salinity Water Flooding for Carbonate Formation

Abstract

As carbonate reservoirs are mostly oil-wet, the potential for the success of a waterflooding is lower. Therefore, a primary focus during waterflooding such reservoirs is on the ionic composition and salinity of injected brine which are able to impact the alteration of the rock wettability favorably by altering the surface charge towards a higher negative value or close to zero. The objective of this study is to employ zeta potentiometric studies comprising streaming potential and streaming current techniques to quantify the surface interactions and charges between the carbonate rock and fluid type as a function of the variations in its ionic state and rock saturation. Zeta potentiometric studies were conducted on carbonate rock samples to understand the behavior of different aqueous solutions by variation in the brine's salinity and ionic composition and the results were integrated with wettability studies. The concentrations of potential-determining ions (PDIs) such as SO42-, Mg2+ and Ca2+ in the injected brines are deemed responsible for altering the wettability state of the carbonate rocks. Several diluted brines (25%, 10% and 1% diluted seawater) and smart brines have been investigated. Smart brines were prepared by spiking the concentration of major PDIs. All zeta potential measurements were conducted using a specially designed zeta potentiometer sample-holding clamp capable of using the whole core plugs rather than pulverized rock samples. A major advantage of using the whole core sample is that the same core can be used in subsequent coreflooding tests, thus making zeta potentiometric results more relevant and representative for a particular rock-fluid system used in the study. The classical streaming potential and streaming current techniques were used for zeta potential measurement. The Fairbrother-Mastin approach was used where the streaming potential is measured against different pressure differentials. Measurements were also carried out for brines with rock samples of different states: oil-saturated, water-saturated and rock samples cleaned with organic solvents to determine any likely variations in surface charge interactions. The results of our experiments imply that the value of zeta potential either increases or becomes more negative with increasing percentage of dilution (25%, 10%, and 1%). This can be attributed to electrical double-layer expansion which is primarily caused by reduced ionic strength. Furthermore, with measurements done on smart brines, zeta potential value was also found to be increased when different diluted brines are spiked with ionic concentration of PDIs such as sulfate. This could have been caused by surface ion alteration mechanism where PDIs get adsorbed on rock surface causing possible detachment of oil droplets. Both the phenomena are known mechanisms for altering wettability towards more water wetness in carbonate rocks and are discussed in detail.

Role of Monovalent and Divalent Ions in Low-Salinity Water Flood in Carbonate Reservoirs: An Integrated Analysis through Zeta Potentiometric and Simulation Studies

“…By the addition of calcium and magnesium ions to the smart brine, a more positive value of zeta potential is observed whereas by addition of sulfate and phosphate ion to the smart brine, a more negative value of zeta potential is observed. A detailed analysis of results has been done by Gopani et al where it is reported that in order to separate adhered oil molecules from the rock surface, the zeta potential should be made either more negative or near zero [13]. A near zero value of zeta potential suggests a decrease in attractive forces keeping oil adhered to the rock surface and a more negative zeta potential suggests an increase in forces of repulsion between negatively charged oil and rock surface.…”

“…Al-Hashim et al found that favorable wettability state i.e., water-wetness can be achieved by addition of SO 4 2− and Mg 2+ ions in diluted seawater from carbonate rocks [12]. The increasing concentration of divalent anions such as sulfate and phosphate causes the zeta potential to become more negative while divalent cations results in polarity reversal at the rock surface [13].…”

Feature Ranking and Modeling of Mineral Effects on Reservoir Rock Surface Chemistry Using Smart Algorithms

“…These may be due to differences in the rock minerals used or sample preparations. Gopani et al 8 report the change in the surface charge of carbonate rock samples due to a change in the composition of ions (Ca 2+ , Mg 2+ , and SO 2– ) in brine and their ionic strength. These studies highlight the controlling role the rock mineral constituents play in molecule adsorption.…”

“…These may be due to differences in the rock minerals used or sample preparations. Gopani et al 8 Recently, we studied the effect of pH, salt type and concentration on clays, 7 iron minerals 25 and another reservoir constituent minerals 3 surface charge. In these studies, zeta potential measurements were used to infer conditions and well operations that provide surface conditions that would promote asphaltene adsorption.…”

“…Reservoir rocks are made of several minerals in varying proportions, with these minerals responsible for their surface chemistry and interactions. The effect of the mineral constituents has been a subject of research as many enhanced oil recovery processes are dependent on the change in the constituent mineral properties. − Thus, it has received enormous attention over the years. Of pertinent interest is the reservoir rock surface charge and how the constituent minerals control the net surface chemistry of the rock.…”