Milled basalt ﬁ ber reinforced Portland slurries for oil well applications

The dispersion of short ﬁ bers to oil well Portland slurries may improve the compressive strength and fracture energy of the hardened cementing material. A study was carried out to investigate the e ﬀ ect of the addition of ball-milled basalt ﬁ bers (5% BWOC) to Portland slurries. Samples were prepared with and without silica ﬂ our (40% BWOC) in the composition and cured for 7 days under di ﬀ erent Bottom Hole Static Temperatures (BHST): 80°C (176°F) and 300°C (572°F). The mechanical properties and the microstructure of the hardened pastes were evaluated by compressive strength tests, X-ray di ﬀ raction and scanning electron microscopy. The results showed that milling basalt ﬁ bers was a cost e ﬃ cient method to adjust the length of the basalt wool ﬁ bers assuring slurry mixing and, therefore, adequate pumpability. The combined addition of silica ﬂ our and basalt ﬁ bers improved the fracture energy of samples cured at 80°C, therefore below the strength retrogression temperature. Curing at 300°C resulted in signi ﬁ cant ﬁ ber consumption by pozzolanic reactions that could not be prevented by the addition of silica ﬂ our. Therefore, ball-milled basalt ﬁ bers can be a cost-e ﬃ cient and en-vironmental-friendly solution to improve the mechanical properties of oil well cement slurries used below the retrogression temperature.


Introduction
Long-term production of oil and gas rely on cementing materials capable of withstanding crack growth and propagation, therefore ensuring the integrity of the cement sheath (Anjos et al., 2013;Costa et al., 2017;Han et al., 2011;Ichim and Teodoriu, 2017;Silva et al., 2018;Mangadlao et al., 2015;Soares et al., 2015;Souza et al., 2018).To that end, the addition of short fibers potentially increases the fracture energy of Portland-based cement materials (Bentur and Mindess, 2007).Different fibers have been used to reinforce Portland cement slurries, including polypropylene, polyvinyl (Qiu et al., 2017), polyacetal (Yang et al., 2017), steel (Beglarigale, 2017), carbon (Chen et al., 2018), cellulose (Cheng et al., 2018) and glass fibers (Khorami et al., 2017).Each type of fiber depicts advantages and limitations regarding density, geometrical aspects (length, diameter and aspect ratio), bonding to the cement matrix, and evidently cost and benefit.Studies have reported the potential use of basalt fibers (Anandamurthy et al., 2017;Fiore et al., 2015;Jalasutram et al., 2017;Jiang et al., 2014;Katkhuda and Shatarat, 2017;Ren et al., 2016;Vejmelková et al., 2018).On the downside, Berndt and Philippacopoulos (2002) have also studied the addition of different fibers, including basalt fibers, to class-G geothermal well cements containing 40% silica flour.They observed that the addition of 0.5% and 1.0% basalt fibers, 6 mm and 15 mm long, negatively affected the uniformity of the dispersion and the tensile strength of the hardened material.
Ball milled basalt fibers can meet most technical requirements at relatively low production costs.Care must be taken regarding the combination of high service temperatures and alkaline environments, since it can affect the integrity of basalt fibers due to its silica-rich composition.Cheng et al. (2018) studied oil well cement slurries reinforced with cellulose fibers that are also susceptible to alkaline environment.The addition of 15% silica fume improved both the mechanical strength and durability of the slurries.Khorami et al. (2017) tested Portland cement reinforced by silica-rich glass fibers with the addition of nanosilica.The pozzolanic reaction along with the filler action of nanosilica resulted in strong bonding at the fiber-cement interfacial zone.The durability of the matrix was also improved by the decrease in porosity and permeability.
Basalt fibers have also been used to reinforce concrete.Kabay (2014) observed that as the content of basalt fibers increased, the strength of concrete not only increased, but also larger deflections before failure and higher fracture energy values were achieved.Dias and Thaumaturgo (2005) reported that the fracture toughness of concrete reinforced with basalt fibers exceeded that of conventional unreinforced samples.
The objective of the present study was to assess the service parameter limits for the use of milled basalt fibers to strengthen oil well Portland cement slurries.Portland-basalt fiber mixes were hardened under two different Bottom Hole Static Temperature (BHST), i.e. 80 °C (176 °F) and 300 °C (572 °F).The former accounts for high temperature and geothermal wells, and the latter for wells subjected to steam injection for enhanced oil recovery.The effect of adding silica flour (40% BWOC) and ball-milled basalt fibers (5% BWOC) on the mechanical properties of Portland cement slurry was studied.The microstructure of the hardened materials was also characterized by X-ray diffraction and scanning electron microscopy.

Materials and methods
The samples used in the scope of this study were prepared using Class A Special Portland cement (Votorantim S.A., Brazil), silica flour (Halliburton, Ltd., Brazil) containing approximately 99% SiO 2 (Shahab et al., 2015;Li et al., 2015) and basalt fiber (Larocha, Ltd., Brazil) purchased in the form of stone wool flakes.Water from the city supplier was also employed.The wool flakes were ball milled using alumina balls as grinding media.Each milling batch consisted of 30 g of flakes and 168 g of alumina balls, milled for 10 min.The cement pastes were formulated to depict density of 1.87 g/cm 3 (15.6 ppg).A summary of all compositions tested is listed in Table 1.
A Chandler 80-60 mixer was used to prepare the samples.Portland cement, silica flour and basalt fibers were previously and manually mixed and then added to water, as the mixer rotated at 4000 ± 200 rpm.The addition took place during 15 s.The speed was then increased to 12000 ± 500 rpm during 35 s, and the mixer was then turned off.The mixing protocol was established by the American Petroleum Institute (API, 2013) to reproduce the mixing energy reached by field mixers.
After mixing, the slurries were poured into metallic molds (50.8 mm edge) and soaked in a thermostatic bath at 80 °C for 7 days, which is representative of high temperature wells (HT).The procedure for curing at 300 °C consisted of 4 days in a thermostatic bath at 80 °C, followed by a thermal cycle of 3 days in a model Chandler 1910 curing chamber at 300 °C under 2000 psi.After curing, uniaxial compression tests were performed using three samples of each composition and for each curing temperature using a Shimadzu AG-I 100 kN Universal Mechanical Testing Machine.The software used to control the machine was the Trapezium 2. The fracture energy values were then calculated from the area under the load-displacement curves after maximum stress.
Crushed samples were selected for SEM and XRD analyses.
Micrographs were obtained using a HITACHI TM3000 SEM using backscattered electrons.Sample preparation consisted in depositing a small sample fragment onto a carbon adhesive tape attached to the sample holder.The diffractograms were obtained by a Bruker D8 Advance Eco set-up using CuKα radiation source set to 25 kV and 40 mA.Data were collected in the 2θ range from 5°to 80°.Each sample was crushed and pulverized using mortar and pestle.The software programs used in the phase identification were the EVA and TOPAS from Bruker.

Results and discussion
The chemical composition and morphology of the milled fibers can be seen in Table 2 and Fig. 1, respectively.The basalt fibers used in this study were received as long entangled filaments with chemical composition consisting of more than 50 wt% SiO 2 , in addition to CaO, Fe 2 O 3 , and Na 2 O as major constituents.A preliminary study was carried out to establish the minimum milling time of basalt wool flakes to   (2018) reported that certain treatments performed on the surface of commercial basalt fibers may be responsible for greater adhesion of fiber-matrix interfaces and durability of concrete based composite materials.
The XRD patterns of silica flour and milled fibers can be seen in Fig. 2. Whereas silica flour shows crystalline structure, basalt fibers are essentially amorphous and therefore reactive.The interaction between the fibers and Portland cement hydration products, especially calcium hydroxide (CH), can be increased under high temperature.To overcome this problem, the addition of silica flour (40% BWOC) can reduce the calcium hydroxide contents by means of the pozzolanic reaction to form C-S-H gel.The filler effect attributed to the fine particles also reduces the permeability of the hardened slurry.
The compressive strength and fracture energy of all compositions tested herein are shown in Fig. 3.The strength of the samples cured at 80 °C (Fig. 3a) increased with the addition of 5% basalt fibers, from ∼23 MPa (BF0) to ∼33 MPa (BF5).An increase in fracture energy from ∼35 J (BF0S40) to 47 J (BF5S40) was also noticed from the combination of 5% basalt fibers with 40% silica flour.Such behavior can be related to the filler effect provided by silica flour at temperatures below 110 °C (Costa et al., 2017).The pozzolanic reaction between silica flour and calcium hydroxide in the pore solution of cement slurry is extremely slow at 80 °C (Ge et al., 2018).In addition, the increase in fracture energy in the fiber-reinforced cement slurry may be related to appropriate interfacial bonding, resulting from fiber milling.The effect of adding 40% silica flour to prevent the strength retrogression noticed in samples BF0 and BF5 is evident from samples cured at 300 °C (Fig. 3b) (Nelson and Guillot, 2006).The silica flour as well as the basalt fibers were involved in the pozzolanic reactions, as sources of silica.The addition of basalt fibers showed little contribution to strength or fracture energy at 300 °C, contrary to what was observed in the samples cured at 80 °C.The silica flour acted as a source of silica in the pozzolanic reactions to form C-S-H gel, and no longer as filler in the microstructure of the cement matrix.The behavior observed for samples BF0S40 and BF5S40 suggests the formation of stable xonotlite phases, responsible for the mechanical strength at high temperature, as later confirmed by XRD analysis.
SEM images of samples cured at 80 °C and 300 °C can be seen in Fig. 4. The chemical interactions between the basalt fiber and the cement slurry (Sim et al., 2005), specifically between SiO 2 and calcium hydroxide (CH), are illustrated in Fig. 4 (a).The reactions involving basalt fiber (BF) and the cement slurry during the hydration process can be expressed as: The reactions represented by equations ( 1) and ( 2) account for the hydration of C 3 S and C 2 S to form CH (Nelson and Guillot, 2006;Taylor, 1990).Reaction (3) shows the chemical interaction between BF and CH, which takes place as Si-O-Si bonds of the fiber which are broken by  OH − ions of the calcium hydroxide in solution (Bentur and Mindess, 2007).
Basalt fibers confined by amorphous C-S-H gel are illustrated in Fig. 4 (b) (Qiao et al., 2008).Milling may have contributed to better fiber-cement bonding.Moreover, the addition of silica flour could have acted towards reducing the porosity of the matrix by filling pores, which also increase the bonding between the fiber and the cement matrix.The combination of both effects improves the efficiency of fiber reinforcement, increasing the fracture energy of the cement.
The microstructure of BF5 sample cured at 300 °C is shown in Fig. 4  (c).The sample contained 5% BF and no silica flour.The fibers were likely consumed by the presence of CH (Hollis et al., 2006).In addition, well-developed broom-like pectolite formed, as can be seen in the microstructure (Nocun-Wczelik, 1999).Pectolite (NaCa 2 HSiO 3 O 9 ) is a sodium-containing calcium silicate phase that can be found in equilibrium with xonotlite, truscotite, foshagite and tobermorite.
The microstructure of BF5S40 sample, which contained 5% BF and 40% silica flour is shown in Fig. 4 (d).Due to the high curing temperature (300 °C), pozzolanic reactions involving both sources of silica (basalt fibers and silica flour) resulted in the formation of stable xonotlite [Ca 6 Si 6 O 17 (OH) 2 ] (Ge et al., 2018).
The crystallographic characterization of the samples cured at 80 °C and 300 °C can be seen in Fig. 5 and Fig. 6, respectively.The main phases present after curing at 80 °C (Fig. 5) are ettringite, Portlandite (CH) and C-S-H gel.C-S-H gel was identified as a phase with low crystallinity (Richardson, 2008;Yanagisawa et al., 2006).A small decrease in the intensity of Portlandite (2θ = ∼18°and ∼34°) and silica peaks (2θ = ∼28°) was noted for BF0S40 and BF5S40 samples.The addition of silica flour combined with basalt fibers improved the pozzolanic reactions for the formation of C-S-H gel.Therefore, the mechanical properties of the cement slurry might improve as a result of the proper bond between the hydrated products and the basalt fibers.
Fig. 6 (a) and (b) revealed the presence of Portlandite in samples BF0 and BF5.Differently from the behavior observed after curing at 80 °C, the addition of basalt fibers in the BF5 sample contributed to the pozzolanic reactions at 300 °C.Strength retrogression took place in the absence of silica flour above 110 °C.The presence of calcium chondrodite [Ca 5 (SiO 4 ) 2 (OH) 2 ] or reinhardbraunsite, responsible for most of the strength loss was also noticed (Anjos et al., 2013;Costa et al., 2017).The results corroborate the mechanical behavior observed in Fig. 3 (b).Fig. 6 (c) and (d) show more stable phases, such as xonotlite, which is formed above 150 °C by the conversion of tobermorite when the C/S ratio is close to 1.0 (Taylor, 1990).The basalt fibers along with silica flour contributed to the pozzolanic reactions.Contrary to what was observed from samples cured at 80 °C, xonotlite was observed instead of Portlandite.

Conclusions
The effect of adding ball-milled basalt fibers to oil well Portland cement slurries was investigated.The curing temperature as well as the presence of silica flour determined the microstructure, and therefore the mechanical properties of the hardened slurries.Curing at 80 °C revealed a material with high fracture energy by the combined addition of silica flour (40% BWOC) and milled basalt fibers (5% BWOC), which resulted in an increase in fracture energy from ∼35 J to 47 J. Curing at 300 °C resulted in strength retrogression of samples free from silica flour, regardless of the presence of basalt fibers.In addition, the basalt fibers were consumed by high temperature reactions due to their high contents of silica.Nevertheless, a slight increase in fracture energy was noticed due to the composition of the cement matrix.Therefore, the use of ball-milled basalt fibers is a cost-efficient and environmental solution to improve the fracture energy of oil well cement slurries used below the retrogression temperature.
Fig. 5 (a) and (b) show the presence of intense Portlandite peaks, similar to BF0 and BF5 samples, in addition to ettringite and C-S-H gel.Fig. 5 (c) and (d) show the presence of crystalline SiO 2 peaks corresponding to silica flour, in addition to ettringite, Portlandite and C-S-H gel.

Table 1
Composition of cement slurries (mass in g).

Table 2
Chemical composition of milled basalt fibers.