(197bi) Evaluation of Polymer-Calcite Interfacial Strength through a Uniaxial Tensile Simulation Study

Introduction. In energy applications, when the carbonate rock is weak and poorly consolidated, the hydrocarbon extraction process produces undesired particles which will impact on hydrocarbon productivity and increase environmental waste. Solids production control is essential to mitigate the problem. The solids production risk can be reduced by the injection of formation-strengthening chemicals into the formation. By doing so, the formation compressive strength will be improved through the bonding of formation grains, thereby enhancing the intergranular forces. However, screening the formation strengthening chemicals can be experimentally tedious and time-consuming. In this research, molecular dynamics (MD) simulation was employed to screen different polymer candidates. This can be evaluated from the stress-strain response and the adhesion properties from a surrogate polymer-calcite uniaxial tensile simulation study.

Simulation Methods. The main component of carbonate formation, calcite, was represented with calcite crystal planes (1 0 4). Classical atomistic MD simulations and pcff+ forcefield were employed to model the interaction of the polymer with the calcite. Polyacrylamide-based polymers were selected as potential candidates, including pure polyacrylamide (PAM), hydrolysed polyacrylamide with 33% charge density (HPAM 33%) and sulfonated polyacrylamide with 33% charge density (SPAM 33%). As a control case, polyethylene (PE), a non-suitable candidate was also modelled as it was used extensively in the polymer-graphene surface system in literature [1] and served as comparison.

For each separate case, the polymer matrix was filled between the upper and lower solids surface layer, each consisted of 3 calcite layers. A series of equilibration steps were performed under microcanonical, isobaric-isothermal, and isothermal-constant pressure in z-direction (NP_zzAT) ensemble following a temperature quenching process from 500K to 300K at 0 atm. Before the tensile simulation process in the production run, a vacuum region was added on the calcite upper layer, where the upper and lower calcite layers were fixed rigid while the polymer was allowed to move freely as deformable region. In the tensile simulation, the upper layer was pulled upward in z-direction with a velocity of 0.0001Å/fs and the polymer deformed slowly for 800ps. The stress-strain response was measured from the normal force acting on the calcite lower layer normalized by the calcite surface area, and the polymer strength properties were analysed. In a separate simulation, the polymer matrix was frozen, and the stress-strain behaviour was measured with both the polymer and calcite upper layer detached from the lower calcite surface completely, at which the tensile stress measured was surrogate for polymer adhesion strength to the surface.

Results and Discussion. The stress-strain response behaviour is depicted in Figure 1, while the polymer deformation behaviour can be observed in the simulation snapshot in Figure 2. For the deformable polymer cases, in general, when the calcite upper layer is pulled upward, the polymer attached to the layer will be pulled together and deform slowly. The reluctance of the polymer to be deformed causes an increase in tensile stress, and when the displacement increases past a critical limit, a void is created in the bulk polymer region. At this stage, the polymer experiences stress failure: the polymer reaches peak tensile stress and decreases drastically due to the damage in the polymer matrix. Preliminary studies shows that the peak stress value recorded for the polymer-calcite system is the same as pure polymer system, which suggests the value is attributed to pure polymer tensile strength. This indicates the polymers can adsorb strongly onto the calcite surface, and the damage failure only occurs in the middle polymer region, but not at the polymer-calcite interface. Hence, the frozen polymer cases are modelled to force the detachment of the polymer from the interface, where the tensile stress recorded is considered as interfacial strength from literature [1].

Polyacrylamide-based polymers have amide functional groups that interact strongly with ionic calcite structure. The maximum tensile stress in Figure 1 shows that polymer-calcite interfacial strength is significantly stronger than pure bulk polymer strength, validating the polymers can adsorb strongly to the calcite. In particular, the control case of PE has lower interfacial strength compared to other potential candidates. As shown in Figure 2, the PE has weaker adhesion and longer entangled chain away from the surface, while PAM system has stronger adhesion and complete adsorption to surface during the deformation process. Among the potential candidates, the HPAM33% shows better polymer strength due to the presence of carboxylate group in ionized form, which agrees with literature [2]. The results are further validated with the comparison to trend indication from the experimental results as shown in Figure 3, where the unconfined compressive strength (UCS) measured from the experimental study show similar trend to the polymers tensile strength in MD simulations.

Conclusion. Uniaxial tensile simulation studies allow the interpretation of the polymer deformation mechanisms and provide insights into the polymer-calcite interfacial strength analysis. It serves as a useful screening simulation tool to evaluate the suitable potential candidates for improved adsorption and formation strengthening performance.

References

1. Yuan, Z., et al., A criterion for the normal properties of graphene/polymer interface. Computational Materials Science, 2016. 120: p. 13-20.
2. Thomas, M.M., J.A. Clouse, and J.M. Longo, Adsorption of organic compounds on carbonate minerals. Model compounds and their influence on mineral wettability. Chemical Geology, 1993. 109: p. 201-213.