1. Introduction
Cementing is one of the key tasks in well construction, the main objective of which is to provide zonal isolation along the well and mechanical support to the wellhead equipment throughout the life of the wellbore [
1,
2]. To achieve the required well integrity, various admixtures can be added to tailor the cement properties, both fresh (short term) and hardened (long term) [
3]. The properties of a fresh cement slurry (such as rheology, density, stability, etc.) play a major role in cement slurry placement in the annulus. The quality of cement slurry placement determines the mechanical and durability properties of a hardened cement paste [
4,
5].
Recently, nanotechnology research has attracted interest in the oil and gas industry due to the unique properties of nanomaterials. Nanomaterials are materials that have nanoparticles (NPs) embedded in their structures. NPs are ultrafine particles with sizes ranging from 1 to 100 nm in two or three dimensions, resulting in a very large specific surface area and, as a result, high chemical reactivity [
6,
7,
8]. Nanomaterials are produced using two approaches: top-down and bottom-up approaches. The top-down approach involves size reduction, where larger materials are broken into smaller sizes using methods such as mechanical attrition and etching techniques. Meanwhile, in the bottom-up approach, smaller materials (atoms or molecular components) are assembled to form larger materials through techniques such as synthesis and chemical formulation [
5,
9].
In the oil and gas industry, nanomaterials find applications in drilling, cementing, enhanced oil recovery, etc. In well cementing, nanomaterials have been reported to be able to influence the properties of fresh cement slurry, hydration kinetics, and the properties of hardened cement paste [
6]. Nanomaterials influence the properties of fresh cement slurry, such as pumpability, workability, consistency, compatibility, stability, etc. [
8]. In terms of hydration kinetics, they have been reported to accelerate the rate of cement hydration by providing additional nucleation sites for calcium silicate hydrate (C-S-H) gel precipitation and growth [
10]. The accelerated hydration kinetics reduce thickening time, allowing for early strength development [
11]. The addition of nanomaterials modifies the microstructure of the hydration products, consequently improving the mechanical and durability properties of the hardened cement paste. Because of their ultrafine sizes, they act as nanofilling agents that seal the micro- and nanopores in C-S-H gel, thus densifying and refining the micro- and nanopores (reducing the porosity and permeability), resulting in a more compact pore structure with enhanced strength and durability [
9,
12].
Since nanomaterials accelerate the rate of cement hydration, they can be used in cementing low-temperature wellbore columns encountered when drilling offshore and in shallow formations. Low temperatures can cause long wait-on-cement times due to the extended setting time and late strength build-up due to the delay in cement hydration kinetics, which can compromise well integrity. A long wait-on-cement time causes an extended drilling time and, as a result, increased operational cost [
13,
14]. Mostly, inorganic salts such as calcium chloride (CaCl
2) have been used as conventional accelerators; however, these can increase the permeability of the set cement, potentially affecting the robustness of the hardened cement [
15]. The chloride accelerators may also corrode the casing [
15]. These problems associated with conventional accelerators are among the factors driving the search for alternative additives for well cementing.
Nanomaterials are generally categorized as carbon-based (e.g., carbon nanotube, graphene, etc.), inorganic-based (e.g., metal oxides), or organic-based (e.g., polymeric NPs, dendrimers, etc.) nanomaterials [
5]. The most commonly researched nanomaterials in well cementing applications include nano-SiO
2, nano-Al
2O
3, nano-Fe
2O
3, nano-TiO
2, nano-CaCO
3, and nano-clays [
6,
16]. Nano-SiO
2 is the most researched among the oxides, and it has been reported to have a much greater influence on oil well cement properties than the others. When added to cement slurries, nano-SiO
2 speeds up early-stage cement hydration without appreciably slowing it down at later ages [
15]. The effectiveness of silica particles as accelerators is influenced by their size and surface area, as well as their pozzolanic reactivity [
14]. Nano-SiO
2 can accelerate cement hydration through a combination of three mechanisms: (1) its large surface area provides nucleation sites for C-S-H gel, thereby accelerating C-S-H precipitation and growth [
17]; (2) it reacts (pozzolanic reaction) with Portlandite (CH) crystals to form C-S-H gel (referred to as secondary C-S-H gel) [
16]; and (3) the particles act as early C-S-H seeds [
14,
18].
The addition of nano-Al
2O
3 to cement slurry has been reported to accelerate early cement hydration kinetics, resulting in improved mechanical properties in the hardened cement [
19]. The accelerating effect of nano-alumina on cement hydration can be viewed from two perspectives: the physical (filler) effect and the chemical effect. From a physical perspective, the presence of nano-alumina provides a greater surface area for the precipitation and growth of C-S-H gel, accelerating C
3S dissolution. From a chemical perspective, the presence of nano-alumina accelerates the aluminate (C
3A) reaction, as evidenced by an increased shoulder peak in the heat flow curve. It has been reported that nano-alumina (particularly the gamma phase) has a high dissolution rate, which contributes to the formation of additional ettringite at an early age, thus promoting the consumption of gypsum and thereby accelerating the C
3A reaction [
20]. It has also been reported that the presence of nano-alumina promotes the reaction between alumina and calcium hydroxide (CH), improving the strength of cement-based materials at later ages [
5,
21,
22]. The addition of nano-TiO
2 to cement slurry was reported to have similar effect in accelerating cement hydration and to improve compressive strength by the authors of [
23].
The use of nanomaterials is associated with two challenges, particularly at high dosages. The first challenge is attributed to the agglomeration effects caused by poor dispersion. Because of their strong inter-particle attraction, nanomaterials tend to agglomerate (form clusters) as the dosage increases, negatively affecting the workability of cement slurry, hydration, and compressive strength development [
8,
24]. The problem is more severe when dealing with cement slurries with low water/cement ratios [
25]. Another challenge is the reduction in slurry fluidity. Nanomaterials have large surface areas; therefore, they will adsorb large quantities of water, negatively affecting the workability of fresh cement slurry [
6]. These two challenges will negatively affect the mechanical properties and durability of hardened cement. Various studies have reported decreases in the mechanical properties of cement-based materials at high dosages of nanomaterials: Heikal et al. [
26] observed decreases in compressive strength when nano-Al
2O
3 content exceeded 2–6% (bwoc); Ma et al. [
27] found decreases in flexural and tensile strengths when the dosage of nano-TiO
2 was greater than 3% (bwoc); Maagi et al. [
24] reported decreases in compressive strength when nano-SiO
2 dosage was above 3% (bwoc). Significant efforts have been made to find the optimal dosages of nanomaterials. Despite extensive research, there are no explicit conclusions regarding the optimal dosages of nanomaterials, since they are a function of particle size as well as material composition. Therefore, the focus of this study was to evaluate the influence of material composition and dosage on the rheological and hydration properties of oil well cement using four types of nanomaterials (nano-SiO
2, gamma-nano-Al
2O
3, alpha-nano-Al
2O
3, and nano-TiO
2).
5. Conclusions
For the creation of this paper, the influence of various nanomaterials on the rheological and hydration properties of class G cement in the presence of polycarboxylate superplasticizer was experimentally studied. Based on our study, the following conclusions can be drawn:
The addition of nano-SiO2 (NS) and gamma-nano-Al2O3 (GNA) significantly increased the structure building ability of cement slurries, as evidenced by the increased values of apparent viscosity, gel strength, yield stress (Bingham plastic model), and consistency index (power law model) and a decrease in power law behavior index. This phenomenon can be attributed to the accelerated production of cement hydration products. The addition of NS and GNA altered the rheology and flow behavior of the cement slurries more significantly at higher dosages. In contrast, the addition of alpha-nano-Al2O3 (ANA) and nano-TiO2 (NT) tended to slightly reduce cement slurry rheology.
All nanomaterials can accelerate the early hydration of cement by reducing the induction time, with GNA having the strongest influence. This is attributed to its higher surface area and its ability to accelerate aluminate reactions, which is in great contrast to its chemically inert counterpart, ANA. The dosage of the nanomaterials influences the acceleration strength of ANA and NT more significantly than that of NS and GNA in the range from 3% to 7%.
All nanomaterials can increase the total hydration extent of cement during the late hydration stages due to their ability to mitigate sedimentation and provide better suspension of cement particles, which is similar to the function of a suspension aid (diutan gum). The addition of NS can provide additional increases in hydration extent due to the pozzolanic reaction and its stronger ability to serve as a nucleation aid for C-S-H, while the addition of GNA can accelerate a secondary hydration peak associated with aluminate reactions. The dosage effect on total hydration extent was relatively strong for ANA, NT, and NS in the range from 3% to 5% but weak for GNA from 3% to 7%.
Clearly, ANA and NT influence cement behavior primarily via physical effects, i.e., by increasing the surface area available for the precipitation and growth of C-S-H gel and acting as nano-filling agents. In contrast, NS and GNA can influence cement behavior via both physical and chemical effects, as NS can react with calcium hydroxide to form C-S-H gel (via the pozzolanic reaction) and further promote its nucleation and growth, while GNA can accelerate the aluminate (C3A) reaction. For oil well cement with a low C3A content, only NS can further increase the 7-day hydration extent compared to a slurry containing both a suspension aid (diutan gum) and polycarboxylate superplasticizer.