Experimental Investigation to Enhance Oil Well Cement Compressive Strength by Using Nano Silica and Synthetic Fiber

Abstract


Introduction
Cementing is the procedure of forming hard cover around the casing for isolating it from various geological formations fluids and develop permanent zonal isolation (Dhorgham and Almahdawi, 2016).For efficient and successful cementing jobs, it is necessary to consider the drilling fluid characteristics, the use of spacers and flushes, the centralization of the casing, the maximization of the displacement rate, the slurry design for the downhole situation, the choice and testing of cement compositions, and the selection of the appropriate cementing operation (Bourgoyne et al.,1986).Predicting the stability of the wellbore typically requires the employing of these features (Majeed and Alhaleem, 2020).The cement slurry for the oil well is a combination of cement, water, and additives that was mixed at the ground and injected behind the casing.Components that are included in a cement slurry in order to modify or enhance a certain property.The slurry employed in the cementing operations must have the desired performance requirements, including low viscosity to simplify pumping, low permeability, suitable strength development, and high resistance to forceful fluid attack (Ravi et al., 2002).The compressive strength of cement has a significant impact on the longevity of the cement sheath as well as its structural integrity over the long term.A greater compressive strength results in a greater tensile strength, which increases the blend's tensile strain capacity.This leads to less cracking due to the tensile tension generated in the cement sheath owing to the borehole conditions.A further need for the cement slurry is that it be eco-friendly and does not cause any harm or contamination to any underground geological formations.Because it is exposed to a wide variety of depths, pressures, and temperatures, the cement mixture should be properly designed.In most cases, the cement design is established for a specific well by considering the circumstances already present downhole.This is then followed by testing in the laboratory to determine whether or not the design would be adequate (Assi and Almahdawi, 2020).Good compressive strength cement must be resistant to hard and aggressive formation, lost circulation region, carbon (IV) oxide and other toxic gases infiltration, and extreme temperatures (Awadh and Awad, 2020).The American Petroleum Institute (API) has been responsible for establishing and upholding international oil and natural gas industry standards since its founding in 1924.In most cases, the API standards are examined critically and revised every five years.The most recent version of the API standard (10 A) outlines six classes (Class A, B, C, D, and G) of oil well cement, each of which can be purchased in one of three grades: ordinary (O), moderate sulphate resistant (MSR), and high sulphate resistant (HSR) (API, 2010).Principally, cement mixture is set at conditions under API specifications (Assi, 2020).Claystone is the source of silica, alumina, and ferrite that are supplied to the cement mixture (Awad and Awadh, 2020).
Nanoparticles (NPs) are the simplest kind of structure with nanometer-scale dimensions (nm).Technically, a nanoparticle is any collection of bonded particles with a structural radius less than 100 nm.In recent years, nanomaterials have demonstrated a high potential for significantly improving the properties of cementitious material (El-Diasty and Ragab, 2013).Adding nanomaterials into oil-well cement enhances the fresh and hardened characteristics of cement.Some cement property improvements from nano-scale particles are early strength improvement, increased long-term tensile to compressive strength ratio, reduction in setting time, and microstructure change (Santra et al., 2012).Hadi and Abdul (2017) investigated the effect of Nano Alumina (NA) and Nano Silica (NS) on the strength and consistency of oil well cement when added to High Sulfate Resistance (HSR) class G cement.They evaluated compressive strength, density, free water, thickening time, and rheological parameters.According to the testing outcomes, the nanoparticle additives increase the cement's strength while enhancing its rheological qualities and reducing its free water content.It is believed that the pozzolanic reaction of NPs increases cement's strength.NA has a more pronounced effect on compressive strength than NS, but NS has a more significant effect on free water and rheology parameters.As the size of the Nanoparticles grows, the compressive strength of the cement reduces.The plastic viscosity, gel strength, and yield point all rise when the concentration of NPs is high (Hadi and Abdul, 2017).Ahmed et al. (2018) studied the use of polypropylene fibers (PPF) in class G oil well cement under conditions of high pressure and high temperature (HPHT).Cement was blended with different proportions of polypropylene fiber (0.25, 0.5, 0.75 % BWOC).Adding PPF (0.75 % BWOC) to cement shortened the thickening time from 317 to 78 minutes at 9300 psi and 220 °F.This is due to the fact that the addition of PPF improves the hydration of cement.The observations also indicated that the addition of PPF did not affect the quantity of free water.The inclusion of polypropylene fiber considerably improved the compressive strength (24 hours); this may be attributed to the fiber's high silica content, which triggered a pozzolanic reaction (Ahmed et al., 2018).The use of nanoparticles in Iraqi oil well cement class G, as demonstrated by Hadi and Al-Haleem (2020), leads to a reduction in thickening time and improved compressive strength at 38Co (53 and 47%) by adding Nano Silica (NS) and Nano Alumina (NA) of 2% BWOC respectively, and at 60C o (34 and 30%) by adding Nano Silica (NS) and Nano Alumina (NA) of (1 and 2%) in addition, the porosity of cement samples containing Nanoparticles was decreased by 37 and 50%, respectively, by adding NS and NA of 1% BWOC, which resulted in a compact microstructure of matrix (Hadi and Al-Haleem, 2020).
The purpose of this research is to increase the compressive strength of Iraqi class G cement by combining the addition of Nano Silica and synthetic fiber.This will bring this particular type of cement into conformance with the requirements established by the American Petroleum Institute and then can use in the petroleum industry.

Materials
The materials that were used to prepare the cement slurry samples to be tested are:

Silicon dioxide nanoparticle
Silicon dioxide, also known as silica, is a chemical component that is denoted by the chemical formula SiO2.Seventy-five percent of the chemical components of cement is composed of silicates, which have a significant impact on the cement's strength and durability.Nano-silica is a highly effective pozzolanic substance composed of extremely thin silica that is applied to cement slurries for use in the oil industry, civil engineering, and building.It increases the cement's strength and durability and decreases its permeability.Nano-silica can enhance the compressive strength of cement, control fluid loss, reduce porosity and permeability inside the cement, and decrease the cement's setting time by speeding the hydration process, etc. Nano-silica particles fill the spaces within cement grains, creating a filler effect (Rai and Tiwari.,2018).The nano-silica used in this study was manufactured by Hongwu International Group Ltd. -China (Fig. 1a) which has the characteristics shown in Table 1.1b Commercially referred to as "Barolift," this synthetic cellulose fiber is 0.5 inches in length and produced by Halliburton Oil and Gas Company.It can be used with all fluid types without appreciably changing its flow characteristics.
This type of fiber was added for the first time to cement slurry by Hadi and Al-Haleem to improve the properties of oil well cement (Hadi and Al-Haleem, 2021).

Water
According to the API, the water used in this experiment is distilled water as shown in Fig. 1c.It is deionized water with 0% dissolved solids.The API standard uses a water-to-Class G cement (w/c) ratio of 0.44 in primary cementing operations, resulting in a cement density of roughly 15.66 lb/gal.

Cement
The cement employed in this study (oil well cement, Class G, HSR) is manufactured by the Babel cement factory (Fig. 1d).

Electronic balance
Every weight is measured with the use of an electronic balance.A very sensitive electronic balance was used to measure amount of the nanoparticles (Fig. 2a).

Constant speed mixer
OFITE model 20 constant speed mixer is used to prepare oil well cement slurry according to the API specifications 10A (Fig. 2b).

Water bath
Stuart model SWBD water bath Used to maintain the temperature of molds containing slurry at 100 °F and 140 °F (38 °C and 60 °C) for eight hours according API standards (Fig. 2c).

Compressive strength measuring device
The OFITE model 250 Compressive strength measuring device which is located in the Petroleum Research and Development Institute of the Ministry of Oil was used to measure the compressive strength of oil well cement slurry (Fig. 2d).

Cement slurry preparation procedure
In accordance with the API 10A specification, the materials have been measured using an electronic balance.After the measurement process, the water, cement, and additives were mixed together inside a constant-speed mixer that was kept at 4000 revolutions per minute.During this time, the cement powder, with and without additives, was added uniformly.samples of cement slurry were prepared with different proportions of nano silica and synthetic fiber together additions.
According to previous studies, adding Nano silica in concentrations greater than 2% BWOC causes a reduction in compressive strength (Hadi and Al-Haleem.,2020).As a result, Nano silica was added in amounts between (0.5 and 1.5%) BWOC and (0.5 and 1.5 g) of synthetic fibers to improve the cement's compressive strength.The materials used to prepare cement slurry samples and their concentrations are explained in Table 2.

Procedure for testing compressive strength
The compressive strength qualities of cement determine its integrity and capability to withstand long-term loads.Unless otherwise specified, the maximum pressure utilized for curing is generally 3,000 psi (API).There are two ways to determine a material's compressive strength: crushing and nondestructive procedures.By API requirements, compressive strength tests are undertaken.The OFITE model 250 is an automated compressive strength tester utilized to assess the compressive strength of oil well cement by continuously applying force to the specimen (4 inch 2 ) until failure (Santra et al., 2007).
The specifications of the American Petroleum Institute impose temperatures of 38°C and 60°C in compressive strength testing, so the tests were carried out according to these conditions using the following procedure (API, 2002): 1. Clean and dry the plate's contact surface and molds.2. Cement slurry was poured into the prepared molds (2in×2in×2in) in a layer that was approximately one-half the depth and puddle of the molds.The mold is then covered with a clean, dry cover plate.Each sample must have at least three molds prepared.
3. The specimens were immersed in a water bath (as depicted in Fig. 2c) for eight hours at atmospheric pressure and curing temperatures of (38°C and 60°C).
4. Removing samples from the water bath before 45 minutes of using the strength device and placing them in a water bath maintained at 80 °F for 40 minutes.When the specimen is ready, it is inserted into the testing machine.
5. The specimens were positioned into a compressive strength device (Fig. 2d).Then, the specimen faces were subjected to a load.Then the compressive strength of each specimen is recorded.

Result and Discussion
The compressive strength of Iraqi class G cement samples prepared with different proportions of nano silica and Synthetic fiber (barolift) additives was examined under different temperature conditions (curing) for 8 hours according to the API specification 10A at 38°C and 60°C.

Compressive Strength of Cement at 38°C
The results of the compressive strength value at 38 °C for Iraqi cement class G with and without additives are shown in Table 3.
The results of the tests for the base sample of Iraqi cement class G (without any additives) at a temperature of 38 °C showed that the compressive strength value is acceptable and compatible with API specification 10A (exceeding 300 psi).As shown in Table 3, cement's compressive strength increases No additives 300 when the concentration of nano silica and synthetic fibers increases, and the values of compressive strength for all samples containing nano silica and synthetic fibers together in the same sample are acceptable and compatible with API specification 10A.The increase in compressive strength can be attributed to two main reasons.First, the addition of nano-silica led to an increase in the pozzolanic interaction with calcium hydroxide, which led to an increase in C -S -H and, as a result, an increase in the durability of cement and the small size of the added nano-silica particles, which have a large surface area led to fill small voids and pores and thus increase the packing efficiency.
The second reason is the addition of synthetic fibers, which can create strong cross-links with cement particles, in addition to the ability of fibers to control fluid loss and These results are similar to those obtained in the investigation of Ahmed et al. (2018).when using polypropylene fiber with oil well cement class G, it contributed to enhancement in the cement's compressive strength (Ahmed et al., 2018).

Compressive Strength of Cement At 60°C
The results of the compressive strength value at 60 °C for Iraqi cement class G with and without additives are shown in Table 4.No additives 1500 The results of the tests for the base sample of Iraqi cement class G (without any additives) were not acceptable according to API specification 10A (should be more than 1500 psi) this could be due to the effect of high heat treatment (curing) on early strength development.
The results indicate that at a temperature of 60 °C, there was an increase in compressive strength, and all samples with additives were acceptable and compatible with API specification 10A.
The nano silica fraction used was appropriate for all samples, and the synthetic fiber had a significant impact on the bonding process between cement particles, leading to an increase in the cement's strength.
This shows the possibility of employing nanoparticles to improve compressive strength for a variety of cement applications, including those outside of the petroleum industry, as established by Raouf et al. when using nanoparticles in concrete (Raouf et al., 2016) and similarly, Alobiedy et al. demonstrated that the use of nanomaterials improves the cement's mechanical properties (Alobiedy et al., 2019).

Compared the Results with a Study that Used Nano-Silica Alone
Hadi and Al-Haleem (2020), when utilizing nano silica alone as an additive to Iraqi oil well cement as shown in Fig. 3 Fig. 3. Compressive strength at 38°C and 60 °C vs. NS (Hadi and Al-Haleem.,2020)Compared to the results obtained by Hadi and Al-Haleem (Hadi and Al-Haleem.,2020),when utilizing nano silica alone as an additive with Iraqi cement and when using greater amounts than the amounts employed in this study, a drop in compressive strength values at 60°C was noted at a concentration of 2% BWOC of nano-silica, the reason for this is that adding nano-silica in large quantities leads to agglomeration of its particles.Therefore, a large amount of these NS particles cannot be uniformly displaced as these particles attract each other to form a strong bond, so they leave voids inside Cement matrix without fillers to produce a weak zone in the cement slurry, and only a small number of NS participates in the reaction, and this leads to a decrease in the compressive strength of the cement.
Comparing the results, we can conclude that when very low levels of nano silica are combined with synthetic fiber, it is possible to get compressive strength that is compatible with API specification 10A than when nano-silica is employed alone.Thus, getting an economic advantage by reducing the amounts of pricey nano-silica and substituting them with readily available synthetic fiber.

Conclusions
Various ratios of oil well cement, synthetic fiber, and nano-SiO2 were used to create the experimental samples for this investigation.All samples were immersed in the curing bath at temperatures of 38°C and 60°C for 8 hours.After curing, samples were removed, and their compressive strength was evaluated.Based on the obtained results, the conclusions were drawn: • In this nano silica and synthetic fiber coupling system, nano-SiO 2 behaved as a filler and nucleating agent, operating as C-S-H seeds in the pozzolanic reaction.Consequently, a rise in the cement's strength.• There must be accuracy in applying the concentrations of nanomaterials added to oil well cement to get a good result for cement properties.• The quantities of nano silica added to the cement can be reduced by replacing it with synthetic fiber, although it gives the same results, thus reducing the cost of nanomaterials.• Consequently, it is evident, based on this observation, that substituting oil well cement with synthetic fiber and nano-SiO2 can enhance compressive strength due to the synergistic effect.On the other hand, rheological investigations and an application study in the field are strongly advised to be carried out so that the applicability can be better understood.

Fig. 1 .
Fig. 1.The materials used which are: (a) Nano Silica (b) Synthetic Fiber (c) Distilled Water (d) Oil Well Cement Class G

Fig. 2 .
Fig. 2. The Apparatus used which are: (a) Electronic Balance (b) Constant speed mixer (c) Water bath (d) Compressive strength measuring device

Table 1 .
The characteristics of Nano-Silica

Table 2 .
compositions of cement slurry samples

Table 3 .
Compressive strength of Iraqi cement at 38 °C

Table 4 .
Compressive strength of Iraqi cement at 60 °C