1. Introduction

Nanotechnology applications have pierced through oil industry in the four general petroleum disciplines: exploration, drilling, production and reservoir. The possibility to produce tailor-made nanomaterials with specific properties is expected to play a leading role in dealing with challenges encountered in different areas of the oil industry (Li et al., 2013). For example, nanoparticlesmay provide sophisticated measuring and imaging tools to improve field characterization techniques in oil exploration (Krishnamoorti, 2006, Pourafshary et al., 2009). Nanosensors compatible with hostile environments in deep wells are being developed for reservoir management (Zhang et al., 2011). Also, there has been a great focus on Enhanced Oil Recovery (EOR) using nanotechnology. Application of nanoparticles such as oxide nanoparticles, polymeric nanoparticles and nanoemulsions have demonstrated improved and cost-effective EOR processes (Del Gaudio et al., 2007, Morales et al., 2003, Zargartalebi et al., 2015). Nanotechnology has also led to the introduction of a new generation of smart fluids for drilling and completion applications. These smart fluids contain nanoparticles and provide advantages like wettabilityalteration, advanced drag reduction and sand consolidation, enhancing the drilling process (Matteo et al., 2012). Among others, nanopalygorskite has been found to deliver stable drilling fluid providing excellent penetration rates, enhanced lubricity, and improved wellbore stability at high temperature high pressure (HTHP) environments (Abdo and Haneef, 2013).

Nanotechnology is being increasingly introduced to improve the well cementing process. Conventional cement-based materials are naturally brittle, display low tensile strength and bulk shrinkage. These features result in poor performance in bottomhole condition and finally, costly repair operations, reduction in production capacity, environmental issues, and in worst case loss of the well (Jimenez et al., 2016). Application of nanotechnology in well cementing is a promising approach to convert conventional cement system into multifunctional, durable system to deal with imposed stressful conditions for entire life of the well. From cement sheath perspective, as hydrocarbon resources are becoming gradually depleted in easy-to-recover environments, hostile downhole conditions are more common (Jimenez et al., 2016). Additionally, thermal and stress cycles created by well events such as hydraulic fracturing, production, completion, steam injection, or remedial treatments during the well's life, poses significant challenges to the cement sheath integrity (Shadravan et al., 2014). Therefore, taking new approaches are required to design a cement system to withstand the potential severe conditions during well's life cycle. In order to enhance cementing process, nanospacer based on nanoemulsion technology has been introduced as a new method for enhancing mud displacement efficiency, resulting in better mud removal, wettability change and better casing/formation adhesion of the cement sheath (Maserati et al., 2010). In addition, nanomaterials have been applied in the oil well cementing industry to accelerate the cement hydration (Pang et al., 2014), improve the mechanical properties of the cement sheath (Mercado et al., 2012, Roddy et al., 2010, Roddy et al., 2013), control fluid loss, reduce the probability of casing collapse, and prevent gas migration (Roddy et al., 2013). In summary, nanotechnology has been applied by the oil well cementing industry to solve problems which cannot be solved by conventional approaches.

There are several reviews of nanotechnology application in construction materials, e.g. (Andersen and Geiker, 2009, Hanus and Harris, 2013, Pacheco-Torgal and Jalali, 2011), in concrete, e.g. (Antonovič et al., 2010, Mukhopadhyay, 2011, Pacheco-Torgal et al., 2013, Sanchez and Sobolev, 2010) and oil industry in general, e.g. (El-Diasty and Ragab, 2013, Esmaeili, 2011, Kong and Ohadi, 2010, Li et al., 2013, Matteo et al., 2012, Ponmani et al., 2013). The present review supplementing these reviews, focuses on the application of nanotechnology for preventing cement sheath failure thereby improving long-term zonal isolation of oil wells. The factors affecting zonal isolation are introduced in Section 2, and the factors compromising long-term zonal isolation are further dealt with in Section 2.1. The required properties of a cement system to provide the desired long-term performance are discussed in Section 3. The impact of nanoadditiveson the tensile properties and the bulk shrinkage of cement systems is reviewed in Section 4 and Section 5, respectively. This review is applicable for oil well cement sheaths in particular and cement-based materials in general.

2. Factors influencing zonal isolation

Primary cementing is the process of placing cement slurry in the annulus between the casing and the formation (Nelson and Guillot, 2006). The principal goal of primary cementing is to provide a complete zonal isolation in the well to ensure safety and prevent environmental problems while the well is operating economically (Hunter et al., 2007). The economic longevity of a producing oil and gas well is directly linked to the quality of the cementing job, and without complete isolation in the wellbore, the well may never reach its full production potential (Nelson and Guillot, 2006).

Numerous factors may contribute to loss of zonal isolation. The physical state of the cement system develops initially from a liquid slurry during placement, through a permeable and weak solid, to an (almost) impermeable solid after hardening (Nelson and Guillot, 2006).

The physical process of gas migration, the most common and most dangerous form of annular flow, is convenient to categorize according to when it happens during the cementing operation. Immediate gas migration (during placement), short-term gas migration, and long-term gas migration are the three major categories of gas migration during the life of the well (Nelson and Guillot, 2006). Immediate gas migration occurs from the start of the cementing job to the end of cement placement. It is essential to control the thickening time and viscosity during placement to preserve the flowability of the slurry and ensure a complete placement of slurry in the annulus. Short-term gas migration can occur anytime between a few minutes to a few days after the end of cement placement. Properties impacting zonal isolation in this period include fluid loss (filtration), free fluid development (bleeding), gel strength development, chemical shrinkage, and permeability (Nelson and Guillot, 2006). The required short-term properties are summarized below (Nelson and Guillot, 2006, Ravi et al., 2002, Shadravan et al., 2014, Williams et al., 2011):

• •

  Slurry stability

  • -

    Non-settling under static and dynamic condition

  • -

    Limited free-fluid (bleeding) (<0.25 vol %)

   

• •

  Limited fluid-loss (filtration) (<50 ml/30 min)

• •

  Static gel strength development (time to transition from 48 Pa (100 lbf/100 ft2) or critical static gel strength to 240 Pa (500 lbf/100 ft2) less than 45 min)

• •

  Low permeability

• •

  Limited chemical shrinkage during the transition phase from slurry to final set

Usually, short-term properties have been in focus by industry rather than the long-term properties. However, even when the cement slurry is placed properly and its short-term properties provide adequate zonal isolation, the long-term zonal isolation can be compromised. In general, the required long-term properties of a cement system are:

• •

  Volume stability

• •

  Chemical stability

• •

  Mechanical stability under static/cyclic loads

• •

  Thermal stability under static and cyclic thermal condition

In addition to optimal cement system performance, long-term zonal isolation requires that the drilling and cement placement procedures are done ideally, i.e. proper casing centralization, adequate mud removal, complete cement placement, adequate cement-formation/cement-casing bond, and no cement slurry contamination.

2.1. Factors compromising long-term zonal isolation

In order to identify the origin of factors leading to loss of long-term zonal isolation in cemented oil wells, the causes of problems are analyzed from the time when the cement slurry is placed in the annulus. Initially cement hydration is the main cause of distress inside the cement sheath; later the different well operations affect the stresses in the cement sheath.

Typical operations during a well's lifetime and the main influencing parameters for each operation are listed in Table 1. When designing the cement system, it is important to consider depth dependent temperature and confining pressure during and after placement (ISO10426-1, 2009) and the temperature and pressure conditions during possible later operations. All loads have a direct influence on the radial, axial and tangential stresses to the cement sheath (Ramos et al., 2009), see Fig. 1. Most of the operations listed in Table 1 can be expressed as cyclic loading with different frequency and amplitude; thus, the cement sheath performance under cyclic loading is critically crucial compared to the case of static loading. In HPHT cyclic loading situations, tensile failure of cement sheath occurs according to the theoretical and preliminary tests (Teodoriu et al., 2013).

Table 1. Possible main operations/events during the life of an oil well after cement placement, after (Hunter et al., 2007).

Operation/events	Typical time of occurrence after cement placement	Typical duration	Main affected property/load	Range of variation/example value
Cement hydration	Start earlier	Months	Autogenous (bulk shrinkage)	Volume reduction: 1–5% (Reddy et al., 2009)
Pressure testing	Hours	Hours	Pressure	50 MPa in a 244 mm, 69.9 kg/m casing results in a strain in the casing-cement interface of 1.25 millistrains (James and Boukhelifa, 2008)
Subsequent drilling	Days	Week	Mechanical load (shock wave), pressure, temperature	Drill string rotating at 100 rpm for 50 h Pressure variation if the mud weight has been changed to drill the next section (Thiercelin et al., 1998) Temperature increase of the cased section when the mud, which has been heated by the formation being drilled, returns to the surface via the annulus (Thiercelin et al., 1998)
Well completion	Weeks	Weeks	Pressure, temperature
Perforation	Weeks	Day	Pressure, mechanical load (shock wave)	Pressure increase to values in excess of 40 MPa (Thiercelin et al., 1998)
Hydraulic fracturing	Years	Day (x times)	Pressure	Differential pressure: (7–55) MPa (Shadravan et al., 2014, Williams et al., 2011) Pressure increase is more damaging because the fluid injection lasts from minutes to hours (Thiercelin et al., 1998)
Hydrocarbon production	Year	Years	Pressure, temperature	Depends on production zone depth Pressure and temperature fluctuation mainly concerns the near-surface casing sections (Thiercelin et al., 1998)
Fluid injection	Years	Months (x times)	Pressure, temperature	Steam injection: a temperature increase of 250 °C results in a strain in the cement sheath at the casing interface of 3.25 millistrains (James and Boukhelifa, 2008) Injector well: Max. $\mathrm{}\mathrm{}\mathrm{}$results in 420 pressure cycles from (0–8) MPa (Pedersen et al., 2006)
Formation movement	Decades	Hours	Mechanical load	Dependent on formation creep-characteristics

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Fig. 1. Cross-section through formation, cement sheath and casing. Radial stress ${\mathit{}}_{}$, and tangential stress ${\mathit{}}_{\mathrm{}}$, are indicated, after (Thiercelin et al., 1998).

The potential failure of cemented oil wells compromising zonal isolation can be divided into three groups: formation-cement debonding (outer microannulus), casing-cement debonding (inner microannulus), and failure of cement sheath itself (Liu and Vipulanandan, 2012). Formation-cement debonding, can be caused by bulk shrinkage, incomplete mud removal, stiffness difference between formation and cement sheath and/or temperature and pressure change inside the casing. Casing-cement debonding can form due to stiffness differences and/or pressure and temperature fluctuation inside the casing. Failure of the cement sheath itself can be due to pressure and temperature fluctuations and bulk shrinkage causing tensile failure; or cyclic loads, formation pressure and formation subsidence and movement, which can lead to shear failure (Liu and Vipulanandan, 2012, Nelson and Guillot, 2006, Teodoriu et al., 2013). In general, cement sheaths fail to perform as designed mainly because of too low tensile properties and volumetric instability (McCulloch et al., 2003, Nelson and Guillot, 2006).

3. Requirements to cement systems to provide long-term zonal isolation

The ability of a cement system to provide long-term zonal isolation depends on the mechanical properties of the cement sheath, the formation, geometry of the cased wellbore and the well condition (Thiercelin et al., 1998). As described in the previous section, cement sheaths typically fail because of volumetricinstability and imposed tensile stresses beyond the tensile strength of the cement sheath.

The tensile strength requirement strongly depends on the loading during the life of the well and the mechanical properties of the formation. The Young's modulus of the cement sheath should be lower than that of the formation to mitigate the tensile strength requirement when the wellbore pressure or temperature increase. For flexible cement sheaths, the tangential stress is less tensile and more compressive near the casing-cement interface. This reduces the required tensile strength to prevent cracking (Thiercelin et al., 1998, Williams et al., 2011). In contrast, in case of wellbore pressure decrease, the tensile strength requirement increases with increasing Young's modulus of the formation, and debonding is more likely to occur when the cement system is placed in a stiff formation because it constrains cement deformation (Thiercelin et al., 1998). As all types of loading might be encountered in a well's life, it's important to consider an optimum Young's modulus for the cement sheath according to the formation's mechanical properties.

Each well has its special condition and a custom made design is needed. Therefore, it is necessary to determine the stresses that can occur in a well during the entire service life and identify the risks of cement sheath failure (McCulloch et al., 2003). In Table 2, it can be observed that during different loading modes a cement system with low Young's modulus is needed to survive under the imposed stresses. Moreover, the Young's modulus affects the requirements to tensile and compressive strengths, and a reduction of Young's modulus will decrease the required tensile strength to maintain zonal isolation. Consequently, two common mechanical characteristics of a cement system which can survive in oil and gas wells stress conditions are as below:

• 1.

  The ratio of cement sheath-to-formation Young's modulus: A suitable value would be 0.5 (Williams et al., 2011).

• 2.

  The ratio of tensile strength to Young's modulus σt/E, of cement sheath: A higher ratio is more favorable for elevated stress environments. The requirement is well-specific. (Nelson and Guillot, 2006, Williams et al., 2011)

Fig. 2 illustrates schematic load-extension curves for a target system and cement system with and without fibrous or flexible material. The target system presents a high strength and low Young's modulus illustrating the potential combined impact of fibrous and flexible additives. The target system has the potential to prevent cement sheath failure.

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Fig. 2. Schematic load extension curve of a target system (see text) compared to the response of a reference system and systems containing fibrous or flexible particles (Own data).

4. Improvement of tensile properties

To create a long-term zonal isolation, a flexible cement system exhibiting high σt/E with high resistance to fatigue and low shrinkage is required. The impact of nanofibers, nanoparticles, flexible particles and hybrid addition of these additives on tensile properties are discussed.

4.2. Nanoparticles

Nanoparticles having a very high surface area-to-volume ratio can act as nuclei for cement hydrates promoting cement hydration (Lindgreen et al., 2008, Pang et al., 2014). As reinforcement and as filler, they are densifying the microstructure resulting in a reduced porosity (Sanchez and Sobolev, 2010). Distribution of nanoparticles in cement slurry also increases the matrix viscosity (Choolaei et al., 2012), which might impair the homogeneous dispersion of nanoparticles in the slurry. For all nanoparticles, the most significant problem is to achieve a homogeneous dispersion, which depends on a few critical factors such as concentration, matrix viscosity and attractive forces (Parveen et al., 2013). High nanoparticle concentration leads to relatively high agglomeration tendency (Sanchez and Sobolev, 2010) and increased viscosity of the cement slurry. Potential properties of nanocomposite can only be achieved by an effective dispersion of nanoparticles in the matrix (Parveen et al., 2013). It has been observed that addition of 5% (Stefanidou and Papayianni, 2012) and 3% of nano-SiO2 and 5% of TiO2 (Li et al., 2006) decreased the flexural strength of the concrete. This was attributed to the weak zones caused by agglomeration of nanoparticles.

The impact of addition of nanosized particles on tensile strength of cement mortar and paste are summarized in Table 5. Addition of 5% nanosized bentonite to oil well cement paste resulted in 54% increase in tensile strength compared to a reference mixture with microsized bentonite (Roddy et al., 2013). Fig. 3 illustrates the impact of various metal oxide nanoparticles with the same size, on tensile strength of concrete with water-to-binder mass ratio (w/b) of 0.4 after 28 days of curing. System containing 4% nano-SiO2 showed significant enhancement in tensile strength (Nazari and Riahi, 2011c). SiO2 (Nazari and Riahi, 2011c), TiO2 (Nazari and Riahi, 2011d) and ZnO2 (Nazari and Riahi, 2011e) nanoparticles performed similarly in concrete. Up to 4% increase in nanoparticle content resulted in tensile strength enhancement, but beyond 4% led to tensile strength reduction. The optimum concentration for system containing Al2O3(Nazari and Riahi, 2011a) and Cr2O3 (Nazari and Riahi, 2011b) was 1%. In almost all of the studies, addition of nanoparticles in relatively low concentrations resulted in noticeable tensile strength improvement. However, still there is lack of information about their impact on Young's modulus (Nazari and Riahi, 2011a, Nazari and Riahi, 2011b, Nazari and Riahi, 2011c, Nazari and Riahi, 2011d, Nazari and Riahi, 2011e).

Table 5. The impact of nanosized particles on tensile strength σt, and Young's modulus E, of cementitious systems.

Particle	Content (wt. %)	Size	w/b	System type	Curing time (d)	Flexural strength (MPa)	σt(MPa)	σt/σt.Ref	Ref.
Bentonite	5	32-38 μm	–	Paste	3	–	1.1	–	(Roddy et al., 2013)
Nanobentonite	5	–	–				1.7	1.54b
Nanoclay platelets	0	–	0.48	Paste	56	5.4	1.5	–	(Hakamy et al., 2014)
	1					7.2	2.1	1.40
	2					7.1	2.0	1.33
	3					6.8	1.9	1.27
Nano- SiO2	0	15 nm	0.5	Mortar	28	4.9	2.2	–	(Li et al., 2004)
	3					5.8	2.6	1.18
	5.2					6.2	2.7	1.23
Nano-MgO	0	30 nm	0.5a	Mortar	28	3.0	1.1	–	(Moradpour et al., 2013)
	1					5.1	1.9	1.73
	3					3.5	1.3	1.18
	5					3.2	1.2	1.09
Nano-Fe2O3	0	30 nm	0.5	Mortar	28	4.9	2.2	–	(Li et al., 2004)
	3					5.8	2.6	1.18
	5.2					6	2.7	1.23

a

w/c.

b

Reference system contained the same content of bentonite in microscale (32–38 μm).

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Fig. 3. The impact of nanoparticles with average size of 15 nm on tensile strength of concrete with w/b of 0.4, at the age of 28 days of curing, data from (Nazari and Riahi, 2011a, Nazari and Riahi, 2011b, Nazari and Riahi, 2011c, Nazari and Riahi, 2011d, Nazari and Riahi, 2011e).

In addition to long-term properties, presence of nanoparticles in cement slurry has a major impact on its short-term properties which should be considered in designing cement system. Thickening time, slurry stability (free-fluid, sedimentation and fluid-loss), rheological properties, gel strength development, porosity and permeability are strongly influenced by concentration and type of nanoparticles. Incorporation of nano-SiO2 to oil well cement slurry resulted in zero free-fluid, significant reduction in dormant period, and reduced porosity and permeability (Choolaei et al., 2012).