Abstract

Unconventional oil and gas resources such as shale oil and gas, coal and rock gas, are becoming increasingly important in China's energy landscape and serve as a backup resource for ensuring domestic energy supply in the future. Due to the widespread density, low permeability, and lack of natural production capacity of unconventional oil and gas resources, it is necessary to create artificial oil and gas reservoirs through large-scale fracturing to achieve resource efficiency development. Due to the high stress of unconventional oil and gas resources in deep areas and the large scale of fracturing construction, it is very easy to encounter situations such as casing deformation and cement ring sealing failure, which seriously restricts the efficiency of resource development. Optimizing cementing materials and improving cementing quality are important measures and practical means to ensure the service of the wellbore. By reviewing the current status of research and development of unconventional oil and gas cementing materials, it is clear that there are inherent requirements for cementing materials in unconventional oil and gas development, such as compressive strength, anti channeling effect, crack resistance, and impact toughness. Composite cementing materials are continuously optimized to exert their synergistic effects at different scales. At the same time, the deep organic combination of cementing materials and intelligent technology remains the main direction for the development of cementing materials in deep unconventional oil and gas reservoirs in the future.

Ⅰ. Current Development Status of Cementing Materials

1. Fiber Cement Composite Material for Well Cementing

Fibers generally refer to soft and slender bodies with a length to diameter ratio of over 1000 times and a thickness ranging from a few micrometers to hundreds of millimeters. Fiber based toughening materials can be divided into high modulus fibers and low modulus fibers. Commonly used high modulus fibers include carbon fibers, brucite fibers, and basalt fibers; Low modulus fibers include polypropylene fibers, polyvinyl alcohol fibers, and polyester fibers. The main toughening mechanism of fiber materials is that when the fibers are pulled out, some external energy is consumed. At the same time, the bridging force of the fibers on the crack surface causes the crack to produce a closure effect, reducing the stress concentration at the crack tip, thereby limiting the generation and propagation of cracks.

Xiao Miao's research found that the compressive strength of cement with 9% microfiber substitution significantly decreased, but it could still maintain above 30MPa, while the tensile strength and tensile compression ratio both increased. The plastic deformation stage of cement stone increases, and the flexibility of cement is improved. Hao Huazhong et al. found through experiments that the optimal dosage of alkali resistant glass fiber is 2%. After 7 days of curing, the flexural strength, tensile strength, and impact energy of cement stone increased by 55.8%, 66.0%, and 45.1%, respectively. When Li Ming et al. used alkali treated bamboo fiber with a content of 0.5%, the flexural strength and tensile strength of cement stone increased by 28% and 11% respectively, the flexural compression ratio increased by 100%, and the elastic modulus decreased by 31%. The modified bamboo fiber has a significant effect on reducing brittleness and toughening of cement stone. Zhang Guofeng et al. modified the surface of inorganic non-metallic fiber materials by adding hydrophilic groups to enhance the bonding strength between fiber materials and cement slurry hydration products, resulting in good compressive and flexural strength as well as lower elastic modulus of cement paste. Shu Fuchang et al. used high-strength bundle shaped monofilament modified fiber XW-5 as a toughening agent. The small wellbore cement slurry containing toughening agent XW-5 had a small thickening effect on the cement slurry, good rheological properties, and good control of filtration loss. Egorova et al. found that adding 1% basalt fiber to BF cement slurry had the best effect, increasing its impact strength by 3.1 times. They also found that polypropylene fiber had no negative impact on the fluidity and pumpability of the cement slurry.

Chen Lichao et al. found that under impact load, the brittleness of conventional cement is mostly due to splitting failure, while the failure of cementing materials with polyvinyl alcohol fiber composite cementitious matrix is mainly due to crushing failure. Through SCB experiments, it was found that fiber cementing cement exhibits ductile fracture characteristics similar to metals, and it was analyzed that crack deflection, fiber pull-out anchoring, and particle size refinement effects are the key factors for enhancing the toughness, crack resistance, and energy absorption efficiency of fiber cementing cement. Cheng Rongchao et al. used high-strength organic polymer fiber groups with different scales and elastic moduli to form a fiber leak proof material SD66. The high modulus fibers were adsorbed on the surface of the low modulus fibers, improving their bonding strength with the cement matrix interface. Zou Shuang et al. developed a multi-scale fiber toughening agent BCE-230S by adding three different scales of inorganic fibers with high tensile modulus, high tensile strength, and good dispersibility to cement slurry. Compared with a single fiber, this toughening agent reduces the elastic modulus through synergistic effect, and improves the tensile strength, compressive strength, and impact strength of cement paste. Jia Jia et al. used a mixture of inorganic fibers and organic fibers that had undergone special surface treatment as toughening fibers. This fiber can effectively disperse and bridge in the medium, significantly improving the compressive and flexural strength of fiber cement stone and reducing its permeability. Gong Yingjie et al. used oil well cement fiber additives composed of organic and inorganic fibers of different sizes and shapes. Through surface treatment, the hydrophilicity was improved, which not only enhanced toughness but also overcame the problem of general fibers affecting the flowability of cement slurry. Hua et al. used fiber composite materials to utilize the high elastic modulus of microfibers, which can prevent crack formation in the early stage of cement cracking and improve the crack resistance of solidified cement. Chemical fibers have a lower elastic modulus and good ductility, which can bridge cracks and improve ductility during the crack propagation stage. The F27A leak proof toughening agent developed by Wu Yecheng utilizes the three-dimensional random distribution of mixed fibers of different sizes and the bridging effect at the edge of formation cracks to achieve leak proof effect. At the same time, the crack resistance effect of fibers on micro cracks of different sizes can effectively improve the toughness of cement rings.

2. Whisker Composite Materials for Well Cementing

Whisker toughening materials can be mainly divided into two categories: organic whiskers and inorganic whiskers. Inorganic whiskers such as calcium carbonate whiskers and magnesium oxide whiskers are widely used in well cementing. The toughening mechanism of whisker materials is that the SiO₂ on the surface of the whisker participates in the cement hydration reaction, generating more C-S-H, which tightly wraps the whisker with the hydration products, enhancing the bonding effect between the whisker and the cement-based material interface; On the other hand, whiskers refine the pore size structure of cement paste and improve the density of the internal structure of cement paste. Yu Jiamin's research found that potassium whiskers can reduce the elastic modulus of cementing cement, increase the ultimate stress and strain, and achieve the effect of reducing brittleness and increasing toughness. Aluminum whiskers can effectively improve the compressive strength of cement paste while significantly enhancing its tensile strength, making the cement sheath more flexible. He Yuxin and others found that the main hydration product of the modified desulfurized gypsum whisker cement paste is C-S-H gel. The pore structure of the cement paste is more dense than that of the neat cement paste at the same age. Unhydrated whiskers are tightly inserted into the cement paste, improving the toughness of the cement paste through crack bridging, and alleviating the damage of external forces on the overall structure. Partially hydrated desulfurization gypsum whiskers participate in the hydration reaction to generate an appropriate amount of ettringite, improving the strength of the system.

3. Polymer Composite Materials for Well Cementing

Polymer materials mainly include resins, latex, and latex powder, which rely on their adhesive properties to achieve toughening effects. The latex particles can be evenly dispersed in the cement slurry. As the cement hydration reaction proceeds, the latex particles accumulated on the surface of the hydration products form a continuous film and are connected with the hydration products, thus forming a network structure composed of polymer and hydration products that penetrate and composite with each other. Finally, a polymer film covering C-S-H gel is formed. The polymer film filled in the pores reduces the rigidity of the cement paste, enhanced the impact resistance of cement stone. Song Weibin et al. introduced polymer flexible polymer (BCG-300S) to form a flexible polymer film that inhibits penetration, preventing fluid intrusion into cement slurry and achieving anti channeling effect. At the same time, when the cement stone is subjected to external impacts, the flexible polymer can disperse stress and increase the deformation ability of the cement stone. Song Benling and others used BCG-300S toughening and anti channeling agent as toughening material, and silica as high-temperature stabilizer to produce high-temperature resistant, toughening and anti channeling cement slurry. This cement slurry system solves cementing problems such as formation porosity, associated gas, and heavy oil thermal recovery, and the formed cement stone does not deteriorate in strength after being cured for 5 days at 315℃. Yuan Jinping and others used butadiene styrene latex DRT-100L to prepare cement slurry, while optimizing the particle size of quartz sand and iron ore powder to improve the compactness of cement stone. This cement slurry system can reduce the elastic modulus, effectively prevent the tensile, shear failure, and microcracks of cement stone, improve displacement efficiency, and ensure good interfacial bonding strength. Luo Changbin et al. developed a tough cement slurry system based on the toughening material DRE-100S and latex powder DRT-100S using the theory of tight packing, combined with matching admixtures. The elastic modulus of the system was reduced by 30% compared to conventional cement paste, and the 24-hour compressive strength was higher than 20MPa, exhibiting low elastic modulus and high strength characteristics. Guo Jinzhong et al. pointed out that the dosage required when using only latex is too large, and the compressive strength loss of cement stone is too large, so it needs to be used in conjunction with fiber toughening materials.

The toughening agent ZR-1 developed by Ma Jun and others is mainly composed of polymer elastic materials, particle reinforced materials, and some active materials. Polymer elastic materials play a role in improving the toughness of cement paste, particle reinforced materials fill the grain boundaries and cavity defects of cement paste itself, and active materials increase the spatial network structural strength of the cement matrix. Gao Jichao and others have developed an elastic cement slurry system with the polymer flexible polymer toughening material BCE-310S as the core. It has the characteristics of good fluidity, stable slurry, low water loss, adjustable thickening time, rapid strength development, and short static cementation transition time. Moreover, its cement paste exhibits stress-strain behavior close to that of an ideal elastic-plastic material under confining pressure. The cementitious composition invented by Bingamon includes hydraulic cement, free lime and alkali ion sources, calcium carbonate sources, calcium sulfate sources, and organic components. The compressive strength can reach 1.4 MPa after 8 hours. Hua Sudong and others used phosphogypsum PG as the main raw material, compounded with an appropriate amount of reinforcing material to prepare phosphogypsum based cementitious materials (PGS). At 50℃ and 80℃, the compressive strength of the cured PGS after 1 day of curing was 8.9 MPa and 13.9 MPa, respectively, and the compressive strength after 120 days of curing was 29.6 MPa and 30.4 MPa, respectively.