Cementing is an important operation in the drilling process. During the cementing process, mud cake, fracturing, perforation, and so on will cause holes in the cement sheath. Thereby, the size effect will reduce the cement strength, which will seriously affect the cementing quality. Several hole types were drilled to study the mechanical properties and damage mechanism of oil well cement. The stress distribution and failure process of cement containing a hole were studied using RFPA2D software. The experimental and simulation results demonstrate that an internal hole has an obvious effect on the cement performance. The hole will change the cement bearing capacity and affect the fracture direction. Three crack types exist: tensile, shear, and far-field. Both 2 and 5 mm vertical eccentric holes can reduce the cement tensile stress. Furthermore, the specimen tensile strength decreases with an increase in the diameter of holes. Horizontal eccentric holes with diameters of 2 and 5 mm will increase the cement tensile strength. Among these, the sample L2-2 exhibits a long crack path, high energy consumption, and a remarkable enhancement effect.

Hydraulic fracturing technology is an important means to efficiently exploit unconventional oil and gas reservoirs. As the development of oil and gas fields continues at a high rate, the life cycle of oil and gas wells has been significantly shortened. Fracture sealing is often used to transform oil and gas reservoirs, maintaining long-term economic development benefits. Multiple high-conductivity channels were created between the borehole and the reservoir through temporary sealing of fractures near the contaminated zone. This extended the recovery range and further improved the recovery of oil and gas. A mathematical model was developed to predict the distribution of stress around the artificial fracture prior to the rupture of the seal. Finite element software was used to model the stress distribution around a reservoir containing natural and artificial fractures. We discuss the mechanical conditions for the initiation of a new fracture and the optimal timing for fracture sealing. The prediction of the propagation and propagation trajectories of the new fracture is revealed, and the behavior rules for the initiation and steering propagation of the new fracture are clarified. These results can facilitate theoretical studies and on-site technical optimization of fracture sealing.

The performance of the temporary plug is a key factor in determining the success of loss-circulation control and temporary plug diversion fracturing. Due to the complexity of geomechanics and working conditions, current commonly used temporary plug agents face problems such as low plug strength and efficiency, large filtration losses due to failure to form filter cakes, and slow degradation affecting the recovery of fracture conductivity. A novel idea for the development of a novel water-soluble polymer plug for fracking is proposed, namely, low-to middle-molecule weight + reinforced chain rigidity + supramolecular aggregation. Using sodium bisulfite and potassium sulfate as initiators, AA, AM, and AMPS as grafting monomers, and SM as hydrophobic functional monomers, the AM-AA-AMPS-SM copolymer was prepared by polymerization. The developed new temporary plugging agent was completely degraded at 70°C for 5–8 h by carrying out experimental evaluation tests, such as water absorption expansion rate, swelling kinetics, density, post-dissolution viscosity, strength of the temporary plugging agent and post-degradation conductivity. After degradation, the viscosity of the solution is 2.5–3.6 mPa s with good fluidity and no gel remnants. The density of the temporary plug material is about 1.14 g/cm3. The absorption expansion rate was 25.8 g/g. The pressure is 60.1 MPa when the thickness of the granular temporary plug is 0.4 cm. Under experimental conditions, the fracture conductivity was found to be 69–123 D*cm at a closing pressure of 30 MPa after degradation of the temporary plug. The test results demonstrate that the new temporary plug agent, with its high plug strength, temperature-controlled degradation, reflux stability and effective self-support after degradation, can meet the requirements of drilling plug and temporary plug fracturing technologies.

Low-porosity, extra-low-permeability tight sandstone oil and gas reservoirs have become an important research area for hydraulic fracturing. Sand-mud interlayer is often developed in this type of reservoir in the longitudinal direction. The heterogeneous distribution of oil layers and mud shale barrier layers in the longitudinal direction will lead to restrict propagation of hydraulic fractures in the fracture height direction, which affects the effective stimulated reservoir volume significantly. In this paper, a finite element model of hydraulic fracture propagation in sand-mud interlayer was established based on the real characteristics of the target block formation to study the hydraulic fracture penetrating propagation feasibility under different combinations of fracturing parameters in the tight sandstone reservoir in the target block, and the model verification was completed with the field data. A total of three types of variable orthogonal combinations of 7 kinds of viscosities of fracturing fluid, 5 kinds of injection rate and 8 kinds of thicknesses of barrier layer were carried out to explore the penetration feasibility of hydraulic fracture under the commonly used combination of low injection flow rate and high viscosity in offshore and high injection flow rate and low viscosity in onshore shale gas reservoirs. And the feasibility evaluation plate of the hydraulic fracture penetration propagation in sand-mud interlayer is established. The method for evaluating the feasibility of hydraulic fracture penetrating propagation in sand-mud Interlayer under different combinations of fracturing parameters can be well applied to the tight sandstone reservoir in this paper. And it will provide theoretical guidance for hydraulic fracture design and fracturing parameter optimization. In addition, the method can also be applied to the fracturing development of shale reservoirs with sand-mud interlayers.

During the thermal recovery of heavy oil thermal recovery wells, improving the mechanical properties and integrity of the cement ring is of great significance for the safe and efficient exploitation of heavy oil resources. This paper studies the relative properties of calcium aluminate cement and three kinds of slags under the conditions of 50°C × 1.01 MPa and 315°C × 20.7 MPa. CAC-slag composite material performance was evaluated using the cement paste compressive strength and permeability tests to study the physical properties of CAC with blast furnace slag. X-ray diffraction analysis, scanning electron microscopy (SEM), and thermal analysis (DSC/TG) were carried out to investigate the mineralogical composition of CAC with blast furnace slag. Results show that adding blast furnace slag did not affect the performance of cement slurry. Moreover, C2ASH8 curing occurred at low temperature, the microstructure of CAC paste was compact, and the permeability resistance was improved, thus improving the low-temperature properties of neat CAC. When cured at a high temperature, the CAC paste was mainly hydrated with C3ASH4 and AlO(OH), which had a well-developed crystal structure. Adding blast furnace slag can improve the CAC resistance to high temperature.