The behaviour of silica nanoparticles in multi-phase ionic solutions: enhanced oil recovery implications

Sofla, Saeed Jafari Daghlian (2019) The behaviour of silica nanoparticles in multi-phase ionic solutions: enhanced oil recovery implications. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Conventional oil production methods produce approximately one-third of the initial oil in place from a reservoir, on average. The remaining oil is a large attractive target for Enhanced Oil Recovery (EOR) techniques. Recently, the potential of using nanoparticles in EOR methods has been explored with some promising results from preliminary evaluations. However, the application of nanoparticles in real oil reservoirs is limited by knowledge gaps. The stability of nanoparticles in the injection or formation water containing diverse types and concentrations of ions is a challenge. It is still unknown whether and under which conditions nanoparticles can self-assemble at the oil-water interface and alter the oil-water interfacial properties. The wettability alteration capacity of nanoparticles usually investigated by contact angle measurements is affected by subtle experimental artefacts; hence, the result of conventional contact angle measurements may not be reliable to evaluate the effect of nanoparticles on the wettability of substrates. Moreover, the mechanism of wettability alteration by nanoparticles is not clear yet. The stability of nanoparticles in the aqueous phase is the primary challenge to using nanoparticles in reservoir conditions. Nanoparticles are extremely unstable in high salinity seawater or formation water. Typically, seawater or formation brine is used for water-flooding and EOR purposes. Therefore, if we want to modify the fluid-fluid or fluid-rock properties by injecting nanoparticle enhanced water, then the stability of the nanoparticles in high salinity seawater or formation brine is extremely important. A novel method to stabilize silica nanoparticles in seawater is proposed. First, the stability of silica nanoparticles in the presence of different ions is investigated. The results show that the presence of multivalent counter-ions in the electrical double layer of nanoparticles can destabilize silica nanoparticles. To reduce the concentration of positive multivalent ions around silica nanoparticles, a method called “H+ protected” is proposed and its effectiveness is tested by particle size, turbidity, zeta-potential, and pH measurements. Experimental results show that the H+ protected method obtained by adding hydrochloric acid (HCl) to the solution, can effectively stabilize silica nanoparticles in seawater. By investigating the controlling parameters of nanoparticle attachment at the interface (bulk suspension properties including the concentration of nanoparticles, concentration of HCl, salinity, size and charge of nanoparticles and operating conditions i.e., temperature and pressure) and coupling them with nanoparticles’ stability in the solution, the conditions under which silica nanoparticles can reduce oil-water interfacial tension are experimentally investigated. The maximum IFT reduction occurs when there is a packed monolayer of nanoparticles at the oil-water interface. For instance, increasing nanoparticles’ concentration and salinity to their optimum value would lead to achieving smaller IFT values. Further increasing the concentration of nanoparticles and salinity beyond the optimum value can destabilize the nanoparticles and increase their average size in the solution, which can reduce the number of nanoparticles at the interface and thus increase the IFT value. In general, the minimum IFT occurs when the surface energy reduction due to the adsorption of nanoparticles is minimum, i.e., the chance of nanoparticles desorbing from the interface due to thermal fluctuations (especially in the elevated temperatures) is high and aggregation of nanoparticles in the bulk solution is initiated. We believe that IFT reduction is partially but not fully responsible for incremental oil recovery greater than water-flooding alone. We test our hypothesis by conducting silica nanoparticles in seawater flooding experimentally and comparing the results with simulations that examine the effect of a) IFT reduction only and b) the effect of altering the relative permeability, wettability, and IFT reduction. The mechanism of wettability alteration by silica nanoparticles is investigated. The impact of experimental methods in conventional contact angle measurements on the wettability alteration data is evaluated. In conventional contact angle measurements, the rock samples are either aged with (immersed in) nanoparticles-fluid before conducting the experiments or contacted with the nanoparticles-fluid before the oil droplet is attached to the rock substrate. In both cases, nanoparticles exist in the oil-rock interface before initiating the contact angle measurements (pre-existing nanoparticles). A real reservoir scenario would be to inject the nanoparticle-fluids into an already established equilibrium condition of oil-water-rock. Hence, the contact angle measurements are modified using a new displacement contact angle method to represent the injection of nanoparticle-fluids into a reservoir. The impact of pre-existing nanoparticles on the contact angle measurements is examined for simple (n-decane, NaCl brine, pure substrates) and complex (crude oil, seawater, and reservoir rock) systems at various wetting conditions of the substrates (water-wet and oil-wet). The effect of the surface and nanoparticle charge on the contact angle is evaluated by adjusting the aqueous phase salinity. We also differentiate between the disjoining pressure mechanism and diffusion of silica nanoparticles through the oil phase by testing the attachment of nanoparticles on the rock surface. The results illustrate that a substantial portion of the wettability alteration ability of nanoparticles reported in the literature may be attributed to the method of measuring the contact angles where nanoparticles can adsorb at the rock sample before contact angle measurements. Silica nanoparticles are shown to further reduce the contact angle (make the substrate more water-wet) only when we have water-wet condition initially. Under oil-wet conditions, nanoparticles cause no notable change on the contact angle. The synergic effect of structural disjoining pressure and capillary pressure reduction might be a possible mechanism of wettability alteration in the water-wet conditions. In oil-wet conditions, the only possible mechanism is capillary pressure reduction. This chapter is presented as a paper in the International Symposium of the Society of Core Analysts.

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