Due to the extremely low porosity and permeability of shale, fracturing is often used to develop shale gas reservoirs. During shale fracturing, extremely high fracturing pressure may invalidate the integrity of the cement sheath and bring hidden dangers to the safe development of shale gas. This paper compares the stress state of the three-dimensional unperforated and perforated finite element models of casing-cement sheath-formation to obtain the influence of perforation on the stress concentration and failure area of the cement sheath. The stress state comparison incorporates three sets of perforation models with different hole densities and diameter verifies the stress concentration law of perforation on the cement sheath. By studying the effects of the elastic modulus and Poisson’s ratio of the cement sheath on the maximum tensile and compressive stresses of the cement sheath under fracturing pressure, the integrity failure form of the cement sheath and the measures for integrity failure are obtained. The results show that the peak stress concentration of the perforated cement sheath is about twice the normal value of the stress. The failure area is the two ends of the perforation, the shape is similar to the bottom surface of the elliptical cone, and the thickness is up to half of the wall thickness of the cement sheath. The line length of the largest failure area is 4.5 times the diameter of the hole. Under fracturing conditions, the cement sheath generally undergoes tensile failure. The lower the elastic modulus and Poisson’s ratio of the cement sheath, the smaller the tensile stress of the cement sheath, and the less likely it is to cause tensile failure.

Conventional fluid loss additives have difficultly controlling the water loss of cement–metakaolin slurry with semi-saturated brine cement slurry and limiting it to less than 50 mL (30 min)⁻¹. This paper describes the development of an anti-salt fluid loss additive for metakaolin–cement systems. This study adopted the aqueous solution polymerization method; selected four kinds of monomers, namely 2-Acrylamido-2-methylpropane sulfonic acid (AMPS), N,N-Dimethylacrylamide (DMAA), acrylamide (AM), and methyl acrylate (MA); and performed a single-factor experiment on the proportion of monomer, reaction temperature, initiator dosage, and developed fluid loss additive, which has a high salt tolerance and temperature tolerance. This fluid loss additive can resist salt until saturation, and it can control fluid loss in 24 mL·(30 min)⁻¹ when its dosage is 2%. The fluid loss additive can achieve the effect of fluid loss reduction by increasing the filtrate viscosity, forming a flexible elastic adsorption layer via adsorption, and blocking mud cake pores.

A finite element model, which includes reservoir rock, cement stone, casing, and perforation channels, was developed. The purpose of the study is to create a geomechanical model of the zone around the well, which includes support elements and perforation channels. This model will help predict changes in the productivity coefficient of a terrigenous reservoir and determine the most efficient mode of operation of a producing well. In order to exclude the stress concentration within the casing–cement stone and cement stone–rock, the numerical model applies contact elements. As a result, structural elements slip, while the stresses are redistributed accurately. The numerical simulation of a stress state in the near-well zone was carried out by using the developed model with differential pressure drawdown on the terrigenous reservoir, one of the oil fields in the Perm region. It is shown that the safety factor of the casing reaches roughly 3–4 units. The only exceptions are the upper and lower parts of the perforations, where this parameter is close to one unit. The safety factor of cement stone accounts for 2–3 units. However, parts with its lowest value (1.35) are also concentrated near the perforation channels. In order to analyze the change in permeability, the dependence of the safety factor on effective stresses was taken into account. Therefore, it was found that, in the upper and lower parts of perforations, the stresses decreased, while permeability rose by up to 20% of the initial value. An increase in differential pressure drawdown, on the contrary, can lead to a permeability reduction of 25%, especially in the lateral parts of the perforations. Areas of rock destruction under tensile and compressive forces were identified by using the Mohr–Coulomb criterion. It is estimated that with an increase in pressure drawdown, the areas of rock destruction under tensile force disappear, while the areas of rock destruction under compression increase. After further analysis, it was found that, with the maximum pressure drawdown of 12 MPa, the well productivity index can decrease by 15% due to the reservoir rock compaction.

A finite-element scheme has been developed, including a section of reservoir rock, cement stone, casing and perforation channels. It is noted that in order to exclude the occurrence of stress concentrators at the casing – cement stone and cement stone – rock contacts, contact elements are specified in the numerical model, due to this, structural elements «slip», but at the same time, a reliable redistribution of stresses is carried out. Numerical simulation of the stress state of the near-wellbore zone using the developed model with varying depression on terrigenous reservoir for the conditions of one of the Perm Region oil field is carried out. It is shown that the safety factor of the casing is about 3–4 units, with the exception of small areas near the upper and lower areas of the perforations, where this indicator is close to one. For cement stone, the safety factor was 2-3 units and however, areas with its lowest value (1.35), also concentrated near the perforation channels, were note. To analyze the change in permeability, the dependence of this characteristic on effective stresses was used and it was found that zones of reduced stresses and an increase in permeability up to 20% of the initial value are detected in the upper and lower parts of the perforations. With an increase in depression on the reservoir, permeability decreases, especially in the lateral sections of the holes, where this parameter can decrease by 25% of the initial value. Using the Coulomb-Mohr criterion, areas of rock destruction from stretching and compression were identified. It is noted that with increasing depression, the areas of destruction under tensile stresses disappear and the areas of destruction under compression increase. An analysis of the change in the productivity coefficient depending on the depression showed that when creating a maximum depression on the 12 MPa formation, the productivity coefficient of the well can decrease by 15% due to compaction of the reservoir rock caused by an increase in effective stresses.

Lost circulation is a major technical bottleneck restricting safe and efficient while-drilling plugging operations. Polyacrylamide gel has become a widely used plugging material in drilling engineering. Unlike rigid, cement-like plugging materials, the gel system formed in this study does not develop a hardened, consolidated structure capable of anchoring or binding the drill bit during subsequent drilling operations, thereby eliminating the risk of bit-sticking. Nevertheless, the gel possesses sufficient elastic (viscoelastic) structural strength—reflected in its storage modulus (G′)—to effectively resist deformation and displacement under differential pressure, thereby providing reliable fracture-sealing performance, which effectively prevents pipe-sticking risks. However, high-concentration PAM molecular chains easily stretch and entangle in aqueous solution, triggering an abnormal increase in initial viscosity and poor pumpability. Although Ca²⁺ can inhibit the premature water absorption and thickening of PAM to maintain system fluidity, an excessively high Ca²⁺ concentration will suppress the hydrolysis of Al³⁺ and hinder the formation of hydroxyaluminum—the key crosslinking component of the gel system. To solve the above contradiction, a CaCl₂-tolerant delayed-release crosslinker was synthesized. ZnO was selected as a carrier to adsorb and immobilize polynuclear hydroxyaluminum complexes hydrolyzed from an inorganic aluminum crosslinker at 70 °C, realizing the controlled delayed release of the crosslinker. The microstructures and chemical bonding were characterized by SEM elemental mapping, FT-IR and ²⁷Al MAS NMR. The results confirm that abundant aluminum species are uniformly loaded on the ZnO surface to form stable Zn–O–Al covalent bonds, and the loaded aluminum exists mainly in the form of hydroxyaluminum. With increasing temperature, the Zn–O–Al bonds gradually break and slowly release hydroxyaluminum species. A novel delayed crosslinking gel system was ultimately optimized, composed of 6% CaCl₂, 10.4% PAM and 3% ZnO loaded with polynuclear hydroxyaluminum. The system exhibits excellent delayed gelation behavior, with a fluidity loss time longer than 120 min and a gelation time over 200 min. It maintains favorable fluidity within 30–90 °C, and the formed gel shows a stable elastic modulus (G′) and viscous modulus (G″). Moreover, the system achieves a plugging rate of more than 90% and a breakthrough pressure above 5 MPa, demonstrating superior comprehensive plugging performance for while-drilling plugging applications.