Understanding Calcium-Mediated Adhesion of Nanomaterials in Reservoir Fluids by Insights from Molecular Dynamics Simulations

Abstract

Interest in nanomaterials for subsurface applications has grown markedly due to their successful application in a variety of disciplines, such as biotechnology and medicine. Nevertheless, nanotechnology application in the petroleum industry presents greater challenges to implementation because of the harsh conditions (i.e. high temperature, high pressure, and high salinity) that exist in the subsurface that far exceed those present in biological applications. The most common subsurface nanomaterial failures include colloidal instability (aggregation) and sticking to mineral surfaces (irreversible retention). We previously reported an atomic force microscopy (AFM) study on the calcium-mediated adhesion of nanomaterials in reservoir fluids (S. L. Eichmann and N. A. Burnham, Sci. Rep. 7, 11613, 2017), where we discovered that the functionalized and bare AFM tips showed mitigated adhesion forces in calcium ion rich fluids. Herein, molecular dynamics reveal the molecular-level details in the AFM experiments. Special attention was given to the carboxylate-functionalized AFM tips because of their prominent ion-specific effects. The simulation results unambiguously demonstrated that in calcium ion rich fluids, the strong carboxylate-calcium ion complexes prevented direct carboxylate-calcite interactions, thus lowering the AFM adhesion forces. We performed the force measurement simulations on five representative calcite crystallographic surfaces and observed that the adhesion forces were about two to three fold higher in the calcium ion deficient fluids compared to the calcium ion rich fluids for all calcite surfaces. Moreover, in calcium ion deficient fluids, the adhesion forces were significantly stronger on the calcite surfaces with higher calcium ion exposures. This indicated that the interactions between the functionalized AFM tips and the calcite surfaces were mainly through carboxylate interactions with the calcium ions on calcite surfaces. Finally, when analyzing the order parameters of the tethered functional groups, we observed significantly different behavior of the alkanethiols depending on the absence or presence of calcium ions. These observations agreed well with AFM experiments and provided new insights for the competing carboxylate/calcite/calcium ion interactions.

Figure 1

Schematic of simulation setup. The atom colors are: Ca = green; C = cyan; O = red; H = white; S = yellow; Si = gray.

Figure 2

Snapshots of different calcite crystallographic surfaces. The atom colors are the same as in Fig. 1.

Figure 3

Snapshots of the simulation boxes after initial production runs with fixed silicon slabs in different reservoir fluids. In the fluids the ion colors are: Na⁺ = transparent blue; Cl⁻ = transparent orange; Ca²⁺ = green. Other atom colors are the same as in Fig. 1. Water molecules are removed for clarity. The alkanethiols in all three fluids start with similar order. The end carboxyl groups in seawater (SW) are surrounded by sodium ions, while those in brine (B) and calcium-doped seawater (CaSW) strongly chelate the calcium ions.

Figure 4

Radial distribution functions g(r) for the carboxylate groups with different ions or with water molecules in different reservoir fluids, confirming the complexation of the carboxylate groups by sodium ions in seawater (SW) and by calcium ions in brine (B) and calcium-doped seawater (CaSW).

Figure 5

Representative force measurements from the steered molecular dynamics (SMD) simulations in different reservoir fluids. The blue curves are with the silicon slabs approaching to the calcite surfaces (decreasing z), and the red curves are with the silicon slabs retracting from the calcite surfaces (increasing z). The stars represent the maximum retracting forces, F_max. The adhesion is much greater in seawater (SW) than in brine (B) or calcium-doped seawater (CaSW).

Figure 6

Snapshots of the simulation boxes at the maximum retracting forces (c.f. stars in Fig. 5) in different reservoir fluids. The atom and ion colors are the same as in Figs 1 and 3. Water molecules are removed for clarity. Note the difference in the order of the alkanethiols among the three fluids.

Figure 7

Representative traces of the order parameter, S = < (3 cos²θ − 1)/2>, where θ are the angles between the principal axes of the tethered alkanethiols and the pulling direction (z axis), from the SMD simulations in different reservoir fluids. Further examples are shown in the supplementary material. The blue and red curves represent the same sequences as in Fig. 5, and the stars represent the locations of the maximum retracting forces. While the change in order upon approach is similar for all three fluids, upon retraction in seawater (SW) the alkanethiols undergo the greatest variation in order.

Figure 8

Simulation snapshots of the retraction sequences in SW and B. Note in SW the alkanethiols are nearly all aligned perpendicularly to the surface (S ~ 0.7) at z = 3.0 nm when stretched and show “spring back” morphologies at z = 3.5 and 4.0 nm. Similar behavior was not observed in the B and CaSW fluids. The orange arrows in the snapshots for SW (z = 3.0 and 3.5 nm) are the average orientations of the alkane chains before and after pull-off, and the orange oval in B (z = 3.0 nm) highlights the salt layer on the surface of the calcite.

Figure 9

Summary of the maximum retracting forces, F_max, in different reservoir fluids on different calcite crystallographic surfaces. The error bars are from five independent simulations in each case. Adhesion in seawater (SW) is consistently two or three times higher than in brine (B) and calcium-doped seawater (CaSW). There are small differences in adhesion among the five different crystallographic directions, with the (10 $\overline{1}$0) and (10 $\overline{1}$1)Ca surfaces showing higher adhesion than the other three.

Figure 10

Density distributions of the calcite Ca (red) and CO₃ (black) ions along the z axis. The stars represent the surface Ca ions that can interact with the carboxylate groups on the AFM tips. Consistent with the adhesion results for seawater in Fig. 8, calcium lies closer to the surface for the (10 $\overline{1}$0) and (10 $\overline{1}$1)Ca crystallographic faces.