Cement Slurry System Research Articles

Evaluation and Optimization of Cement Slurry Systems for Ultra-Deep Well Cementing at 220 °C

With the depletion of shallow oil and gas resources, wells are being drilled to deeper and deeper depths to find new hydrocarbon reserves. This study presents the selection and optimization process of the cement slurries to be used for the deepest well ever drilled in China, with a planned vertical depth of 11,100 m. The bottomhole circulating and static temperatures of the well were estimated to be 210 °C and 220 °C, respectively, while the bottomhole pressure was estimated to be 130 MPa. Laboratory tests simulating the bottomhole conditions were conducted to evaluate and compare the slurry formulations supplied by four different service providers. Test results indicated that the inappropriate use of a stirred fluid loss testing apparatus could lead to overdesign of the fluid loss properties of the cement slurry, which could, in turn, lead to abnormal gelation of the cement slurry during thickening time tests. The initial formulation given by different service providers could meet most of the design requirements, except for the long-term strength stability. The combined addition of crystalline silica and a reactive aluminum-bearing compound to oil well cement is critical for preventing microstructure coarsening and strength retrogression at 220 °C. Two of the finally optimized cement slurry formulations had thickening times more than 4 h, API fluid loss values less than 50 mL, sedimentation stability better than 0.02 g/cm3, and compressive strengths higher than 30 MPa during the curing period from 1 d to 30 d.

Experimental study on the factors affecting cement bond strength at the second interface of oil-gas well

The second interface of cementing refers to the interface formed by the formation and the cement sheath after cementing. The bonding strength of the second cementing interface is mainly generated through the interaction between the cement sheath and the formation. This study employs sandstone and limestone as experimental materials to investigate the influencing factors on the bonding strength of the second cementing interface by altering the water-cement ratio of the cement slurry system, adding ultrafine materials, and incorporating latex materials. In a progressive experimental approach, various types of cement slurries are selected, including pure cement slurry systems under different water-cement ratios (L1, L2), cement slurry systems with added nano-silica on the basis of pure cement systems (L3), and cement slurry systems with latex added (L4). The influence factors of the bonding strength of the second cementing interface of cement sheaths are explored through self-developed bonding strength evaluation molds and micro-characterization of the bonding interface. The results indicate that increasing the water-cement ratio or adding nano-silica can promote the generation of calcium silicate hydrate (C-S-H), resulting in higher early bonding strength but lower later strength under sandstone interfaces; the reaction of latex particles themselves to form a three-dimensional network structure can enhance the bonding strength. This research provides significant insights into the bonding laws of the second cementing interface and is of great importance to the cementing construction process.

Development and Evaluation of Insulation Materials for Deepwater Cementing.

Marine oil and gas resources are abundant in deepwater regions, where the shallow seabed harbors natural gas hydrate layers due to the cold temperature and high-pressure environment. Hydrates are prone to thermal decomposition, which can compromise the integrity of cement sealing and even lead to accidents like blowouts. While current low-hydrated heat cement systems mitigate hydrate decomposition from cement hydration heat during the waiting period for cementing, they do not address heat transfer from deep strata to shallow hydrate layers through fluid circulation in the tubing during deep oil and gas development. Rather than utilizing costly pipe insulation technology, adjusting the thermal conductivity of well cement emerges as a cost-effective approach to prevent heat loss in the wellbore to the hydrate layer and thus inhibit hydrate decomposition. The critical aspect lies in utilizing insulation functional materials. This study delves into the functional and structural design of insulation materials suitable for cement slurry systems in well cementing, outlining the preparation methods and processes for two insulation materials (SDBW and SDBW-II) tailored for deepwater well cementing. SDBW and SDBW-II are both nuclear shell structural materials. The core is made of high-strength hollow microspheres, produced by using a reverse suspension polymerization method to form spheres followed by high-temperature sintering. The shell consists of a wear-resistant BPA epoxy resin layer, with the surface of SDBW-II also containing highly reflective glass microspheres. Incorporating 20% of material SDBW into the cement reduces the thermal conductivity of the cement stone from 0.8 to 0.31 W·(m·K)-1 while achieving a compressive strength of 6 MPa after 24 h at 20 °C. Material SDBW-II offers both thermal resistance and reflection functions, increasing reflectivity (R) from 0.3 to 0.5. By adding 20% of this material to the cement, under the same conditions, although the compressive strength decreases to 4.2 MPa, the thermal conductivity can be reduced to 0.27 W·(m·K)-1. Furthermore, there is no significant change within 180 days, demonstrating long-term thermal insulation stability. These developed insulation materials can effectively improve the thermal insulation performance of the cement sheath, thereby maintaining the stability of the upper natural gas hydrate layer in the oil and gas production process of deepwater wells, providing an innovative solution for the long-term operation of deepwater oil and gas wells.

Nano-enhancing polymer as a fluid loss additive for ultra-high temperature cementing

The exploitation of ultra-deep oil and gas wells is accompanied by technical problems of ultra-high temperature, especially in the cementing process, which will directly deactivate many admixtures. By in-situ polymerization of the monomers and Nano-SiO2 to improve the temperature resistance, the resulting high temperature resistant composite polymer slurry system can show high efficiency in reducing fluid loss under the high temperature environment of 240℃. The synthesis of the high temperature resistant polymer was characterized by IR and 1H NMR, and its physicochemical properties were analyzed by DLS, GPC, TEM and Cryo-SEM. The synergistic mechanism of Nano-SiO2 on the high temperature resistance of the polymer was revealed by TGA. Using DLS, SEM and EDS, a series of cement slurry systems were used to analyze and compare the effect on fluid loss reduction of different adding methods of Nano-SiO2 in cement slurry at high temperature. The results showed that the grafted Nano-SiO2 polymer maintained effective adsorption capacity at high temperature without degradation, and its fluid loss reduction ability was significantly improved at high temperature. The high temperature resistant composite polymer is strongly adsorbed between cement particles to form a dense and robust spatial network structure. The CSH gel structure is optimized by the particle size gradation of the system, so as to obtain a dense cement cake, which is expected to be applied in high temperature cementing.

Analytical Solution for Predicting the Elastic Modulus of a Cement Slurry System with the Effect of Calcium Dissolution.

The dissolution of calcium ions in concrete in a low-alkalinity environment is an important factor causing a significant increase in the porosity of internal concrete, leading to a deterioration in its mechanical properties and affecting the durability of the concrete structure. In order to improve the reliability of concrete durability design and significantly increase the service life of concrete structures located in soft water environments, it is crucial to establish an analytical method to predict the elastic modulus (Edc) of cement slurry systems suffering from calcium dissolution. Firstly, the hydrated cement particles are regarded as a three-phase composite sphere composed of unhydrated cement particles (UC), a high-density hydrated layer (H-HL), and a low-density hydrated layer (L-HL). By introducing the equivalent inclusion phase (EQ) composed of UC and H-HL, the three-phase composite sphere model can be simplified into an equivalent hydrated cement particle model composed of EQ and L-HL. Finally, the Edc of the two-phase composite sphere composed of the equivalent hydrated cement particles and the porosity of the dissolved cement slurry system are solved by using elasticity theory. The effectiveness of the developed analytical method is verified by comparing it with third-party numerical results. Based on this method, the effects of hydration degree, volume ratio of calcium hydroxide (CH) to hydrated calcium silicate (C-S-H), and volume ratio of inner C-S-H to outer C-S-H on the Edc of the dissolved cement slurry system are analyzed. The parameter analysis indicates that among the three influencing parameters, the hydration degree has the greatest effect on the Edc of the dissolved cement slurry system. This study provides an analytical method for predicting Edc, which can provide some references for the durability design of concrete after calcium dissolution.

Laws of Hydration Heat of Cement and Hydrate Stabilization in Shallow Formations of Deep Seawater.

When cementing is required in marine deepwater hydrate formations, the heat released from the hydration process of oil well cement can easily lead to hydrate decomposition. It is necessary to clarify the initial phase transition temperature of the hydrate layer under the influence of cement waiting for setting so that it can meet the stability of the hydrate layer during cementing. In this paper, based on the actual conditions of offshore deepwater cementing, the coupled temperature field model of cement sheath hydration heat source-well wall hydrate decomposition is established by considering the hydration heat release during the cement waiting process and the phase change heat absorption of the well wall hydrate. Combined with the established model, the hydrate formation in the deepwater region of the ocean was selected and matched with suitable oil well cement for simulation. Through simulation, the critical temperature range (291-295 K) for the initial phase transition of the hydrate layer at 10-15 MPa was clarified and the relationship between the critical values of cement hydration heat release to maintain hydrate stability at different initial temperatures of the formation was established. The phase stability law of hydrates in the formation under cement sheath hydration heat release was revealed, providing a theoretical basis and guidance for the development of low-hydration heat release cement slurry systems.

Performance experiment of ultra high temperature cementing slurry system

Introduction: The escalating pursuit of oil and gas reserves in China has led to increasingly deep and ultra-deep well drilling, encountering extreme temperatures that challenge the heat resistance capabilities of cement slurries. This study addresses the critical need for high-performance cement slurries capable of withstanding ultra-high temperatures during drilling and completion operations.Methods: A novel ultrahigh temperature conventional density (UHTE) cement slurry was formulated by incorporating high-temperature-resistant additives, namely DRF-1S and DRH-2L. An indoor testing framework was established to assess the slurry’s performance across various parameters: Evaluating the slurry’s ability to maintain uniform consistency over time. Measuring the amount of free water separated from the slurry under simulated downhole conditions. Assessing the slurry’s transition from fluid to solid state over time. Determining the compressive strength and other mechanical attributes after curing at elevated temperatures.Results: The experimental findings revealed the following: An optimal silica sand dosage of 50% was found to enhance the overall performance of the UHTE cement slurry. The developed UHTE cement system, fortified with DRF-1S and DRH-2L, demonstrated effective functionality within a temperature range of 180°C to 210°C, representing a significant advancement in high-temperature resistance. The slurry maintained API water loss below 100 mL, ensuring minimal fluid loss during placement. Adjusted thickening time allowed for adequate pumping and placement without premature setting. Settlement stability was maintained below 0.04 g/cm3, indicating excellent homogeneity and stability. Incorporation of the anti-decay material DRB-3S ensured a 28-day compressive strength exceeding 50 MPa at 210°C, signifying robust structural integrity under extreme thermal conditions.Discussion: This research provides a robust foundation for the practical application of UHTE cementing technology in ultra-high temperature environments. The successful development of the UHTE cement slurry system, characterized by its superior temperature resistance, controlled water loss, optimized thickening behavior, and exceptional mechanical properties, paves the way for field trials and further enhancements. Future work will involve testing the effectiveness of the new additive combination under actual downhole conditions and refining the formulation to develop an even more advanced UHTE cementing additive. These advancements will contribute to enhanced wellbore integrity, improved drilling efficiency, and increased safety in deep and ultra-deep oil and gas exploration endeavors in China.

Failure mechanism and control method of cement sheath sealing integrity under alternating thermal-stress coupling in geothermal wells

In geothermal wells, exposure to extreme conditions of alternating temperature-pressure coupling (T-P-C) may lead to repeated expansion and contraction of the casing and cement sheath. This can degrade the cement sheath sealing integrity, resulting in continuous annular pressure. Consequently, an elastic-plastic mechanical model of the casing-cement sheath-formation (C–S–F) system under T-P-C was established based on thermodynamic theory and elastic-plastic theory with consideration of additional heating strain and thermal load, and it is solved by global search in MATLAB. The elastic-plastic mechanical behavior and development of interfacial contact stress are investigated under four typical working conditions. The failure mechanisms include interfacial debonding and micro-annulus formation between the casing and cement sheath under strong alternating temperatures (SAT). Cycling of temperature and accumulated plastic strain caused by the cement sheath entering plasticity are the major causes of interfacial stress inversion, which leads to interfacial debonding during temperature unloading. The influences of the mechanical and thermodynamic parameters of the cement sheath on its sealing integrity are analyzed. Based on this, an integrity control method is proposed, which comprises a cement slurry system and considers cement mechanical and thermodynamic parameters and environmental conditions. The results provide an important reference for the optimal design of cement slurry systems and for preventing cement sheath integrity failures due to SAT.

Novel material to enhance the integrity of CCUS cement sheath: Exploration and application of calcium aluminate cement

Carbon dioxide (CO2) emissions are being reduced using the technique known as CCUS (Carbon Capture, Utilization, and Storage). At present, cementing cement paste based on ordinary silicate cement (OPC) and compounded with CO2 corrosion-resistant materials are often used in CCUS wells. CAC cement is characterised by low alkaline hydration content and its study in CCUS wells has rarely been reported. Therefore, CAC cement will be a new cement material for CCUS wells. In this study, we have developed an experimental device to simulate the working conditions of CCUS based on the actual well conditions of CCUS and investigated the effects of stress-coupled CO2 corrosion environment on the performance of CAC cement. According to the experimental findings, CAC cement's compressive strength degraded at a rate of 30.6% after 60 days of CO2 corrosion, which was lower than that of OPC cement (38.3%). When compared to OPC cement, the peak strain and stress of CAC cement increased by 57.2% and 7.85%, respectively. Furthermore, compared to OPC cement sheath, the CAC cement sheath's bearing capacity increased by 8 MPa after stress corrosion, and the crack area of the CAC cement sheath after failure was considerably smaller at 275 mm3 than at 735 mm3. Additionally, after being subjected to stress-coupled CO2 corrosion, the integrity of the CAC cement sheath was much superior to that of the OPC cement sheath, offering a fresh viewpoint for building CCUS cement slurry systems.

Experimental Study on the Evolution Law of the Mechanical and Pore Characteristic Parameters of Set Cement under High- and Ultra-High-Temperature Treatments

Cement has been widely used as a structural material in many underground projects, and these projects often face high- or ultra-high-temperature environments, leading to the deterioration of the mechanical, porosity, and permeability properties of set cement, thereby increasing the risk of instability of underground structures. In response to this, two new temperature-resistant cement slurry systems were designed. Experiments were conducted on the changes in porosity and permeability of set cement after thermal treatment using low-field nuclear magnetic resonance technology (NMR), visual studies of pore and crack development were carried out using the argon-ion polishing field emission scanning electron microscopy (FE-SEM) and computed tomography (CT) methods. The research results show that as the thermal treatment temperature continued to rise, the compressive strength first increased (25 °C–200 °C) and then decreased (200 °C–600 °C). The porosity of the set cement first decreased (25 °C–115 °C) and then increased (115 °C–600 °C), and the penetration first slowly increased (25 °C–400 °C) and then rapidly increased (400 °C–600 °C). Visualization experiments were conducted on micro-cracks and the pore distribution of the set cement under high- and ultra-high-temperatures, which proved the evolution law of these characteristic parameters. The research results have vital reference significance for the protection of the structural stability of cement components when encountering high-temperature environments.

A viscosity-soluble type biomimetic self-healing cement slurry system

With the development of cementing slurry technology, cementing self-healing has become one of the most effective technologies to prevent and treat oil and gas channeling and improve long-term sealing capacity of cement sheath. To solve the technical problem of annulus oil and gas channeling caused by the long-term sealing failure of cement sheath, this paper researches and develops a kind of viscosity-soluble type biomimetic self-healing cement slurry system from the perspective of chemical biomimetics, based on the technical idea of “prevention before blocking”. Then, the engineering performance of this biomimetic self-healing cement slurry system and the mechanical property of self-healing set cement are evaluated respectively. In addition, the mechanical recovery capacity and permeability reducing rate of the self-healing set cement conserved with methane are evaluated by creating fractures through splitting. Finally, the healing mechanisms of the self-healing cement are analyzed by means of SEM microscopic test and infrared spectrography. And the following research results are obtained. First, the set cement has excellent mechanical property. When the compressive strength decreases by 15 %, the elastic modulus of set cement decreases by 41 %, showing good toughness to play the role of “first prevention”. Second, when meeting hydrocarbon, the damaged set cement is excited to generate a kind of viscosity-soluble type gelatinous ester substance, which re-bonds the skeleton of the set cement. When the self-healing set cement is conserved with methane, its compressive strength recovery rate is up to 94.5 %, bending strength recovery rate is 58.6 %, gas log permeability drops by 99 %, and liquid log permeability drops by 100 %, so as to realize “blocking later”. In conclusion, the viscosity-soluble type biomimetic self-healing cement slurry system can effectively achieve cementing integrity and long-term sealing integrity through self-healing after the damage of cement sheath, which provides a strong technical support for preventing the production safety hazard of sustained casing pressure in oil and gas wells and is beneficial to solve the technical problem of annulus oil and gas channeling caused by long-term sealing failure of cement sheath.

Preparation and application of self-emulsifying curing agent for CO2 high-pressure corrosion-resistant resin cement

In order to solve the problem that oily epoxy resin is not easy to disperse in a cement slurry system and its application is limited due to low stability, a new self-emulsifying curing agent ETB for oilfield cement slurry was prepared by using bisphenol F-type glycidyl ether (BPF), tetraethylenepentamine (TEP) and epoxypropyl trimethyl ammonium chloride (EPTAC) in three steps. The optimized synthesis conditions are n (TEP): n (BPF)= 2.6:1, 65 ℃ addition reaction for 4 h, n (EPTAC): n (TEP)= 1:1, and 75 ℃ for 2 h end-capping reaction. Finally, 20% glacial acetic acid is added for 30 min salt reaction to obtain the self-emulsifying curing agent ETB. The target product was obtained through Fourier transform infrared spectroscopy and nuclear magnetic resonance analysis. The stability shows that BPF resin lotion is uniform and stable compared to other lotions, with an average particle size of 580.9 nm. The application study of ETB emulsified BPF resin lotion shows that when the dosage is 6.0%, the fluidity index n of the resin cement slurry system is 0.887, the consistency coefficient K is 0.178 Pa.sn, the rheological property is good, the high-temperature and high-pressure water loss can be controlled within 50 mL, and the 48 h compressive strength increases by 36.05% compared with the blank cement paste. The high-pressure CO2 corrosion experiment shows that the penetration growth rate of resin cement stone is − 0.168%, which is much lower than the 455.6% of blank cement stone. Before and after corrosion, the compressive strength decline rate of blank cement stone is 25.07%, while the decline rate of resin cement stone is only 2.18%, indicating that resin cement stone has good corrosion resistance. TG-DTG and SEM were used to analyze the corrosion resistance mechanism: the BPF resin lotion was crosslinked and solidified in the cement slurry to form a film, and was tightly filled inside to form a three-dimensional network structure, thus improving the corrosion resistance.

Study on Bionic Self-healing Cement Slurry System

Aiming at the technical problem of gas channing in annulus caused by long-term sealing performance failure of cementing cement ring, based on the technical idea of “prevent before plugging” and from the perspective of chemical bionics, a kind of bionic self-healing cement slurry system of dissolving viscosity was developed. The indoor evaluation results show that the cement stone has excellent physical properties. When the compressive strength is reduced by 15%, the elastic modulus of the cement stone is reduced by 41%, showing good toughness and playing the role of “first prevention”. When the damaged cement stone is excited by hydrocarbon, a kind of soluble gel ester can be formed, and the cement stone skeleton can be re-bonded. Under the condition of methane curing, the recovery rate of compressive strength is 99%, the recovery rate of flexure strength is 58.2%, the recovery rate of gas permeability is 96.4%, and the recovery rate of liquid permeability is 100%, playing the role of “post-plugging”. Therefore, it is concluded that the adhesive dissolving bionic self-healing cement technology can effectively realize the late integrity and realize the long-term sealing integrity after the cement ring is broken through self-healing, which provides a powerful technical support for preventing the abnormal annulus pressure, a hidden danger of oil and gas well production safety.

Failure Analysis and Countermeasures for Cement Sheath Interface Sealing Integrity in Shale Gas Wells

Summary Shale gas development usually uses large displacement horizontal well and staged fracturing technology to increase operation production. The complex environmental and construction conditions often lead to wellbore sealing integrity problems in the shale gas production process. This study shows a new method for evaluating the sealing integrity of shale gas cement sheath interfaces, which aims to understand the failure mechanism during shale gas production and to propose countermeasures that can effectively improve the sealing integrity of cement sheath interfaces in shale gas cementing. The study results showed that the oil contamination of cement sheath interface will greatly weaken its sealing performance. After repeated cyclic loading, the sealing performance of the conventional and expanded cement sheath assemblies is damaged, and a gas channel is formed, which is caused by the combination of microcracks and microgaps. Furthermore, oil contamination of the cement sheath interface will accelerate its sealing failure. The addition of an expansion agent is helpful to solve the problem of microgap destruction, and the fibers or whiskers can alleviate the problem of tensile cracking. The field application in the three wells proved that the toughened expanded cement slurry significantly improved the sealing integrity of the cement sheath interface in shale gas wells. The research results can evaluate and predict the sealing performance of the cement sheath interface in shale gas wells under the conditions of staged fracturing and have some directional significance for the cement slurry system optimization in the field.

Evaluation method for reservoir damage of cementing slurry

The damage of cement slurry to the reservoir is directly related to the production of oil and gas wells, and is a problem that must be solved for the efficient and economic development of oil and gas reservoirs. According to the characteristics and field conditions of reservoir damage caused by cement slurry, this paper analyzes the main factors affecting the degree of reservoir damage, puts forward the technical index requirements of the evaluation device, and establishes the evaluation method of reservoir damage caused by cement slurry in case of loss of injection. The results show that the influence of temperature on the damage of the cement slurry layer is small, and the influence of pressure difference on the damage to the cement slurry layer is large; When the pressure difference is 22 Mpa, the penetration damage rate caused by cement slurry to the pore center without mud cake attachment is about 43%. After a day of core maintenance, the permeability damage rate will reach 61.2%; The damage of cement slurry to the pore micro-fractured core is very serious. After 1 day of maintenance, the damage to the core permeability after cement slurry solidification is as high as 83%, with an average of 76.5%; When the risk of leakage is low, the cement slurry system with low density and low water loss can be used for cementing operation. The filtrate of cement slurry should be better matched with the formation clay minerals, formation water, and drilling fluid filtrate. When the risk of leakage is high, high-density cement slurry, balanced pressure cementing, pressure control cementing, and other technologies to avoid leakage can be used for cementing operations. When the cementing conditions are more difficult, the cementing slurry system can be used to dissolve the cementing slurry system at a later stage. The research results provide theoretical data support for the study of reducing the damage of cement slurry to the reservoir.

Microencapsulated phase change material-cement composites for cementing the natural gas hydrate layer

To cement natural gas hydrate (NGH) layers in the deep and ultra-deep water environment, cement slurries must display both low heat of hydration (HOH) and high early strength. Herein, chemical precipitation was used to prepare a microencapsulated phase change material (MPCM) comprising a core of phase change materials and a shell of silica. The MPCM could effectively control the temperature increase during cement hydration and improve the early strength of cement pastes by accelerating hardening. Comprehensive physical and chemical analyses were carried out to identify a suitable ratio of MPCM and its underlying mechanisms. A laboratory device was further used to test the effect of MPCM-cement composite slurries on the thermal stability of NGHs. The properties of this new cement slurry system meet the requirements for cementing NGH layers in deep and ultra-deep water areas. This cement composite has broad application potential in the oil and natural gas industry.

Study of Early-Age Hydration, Mechanical Properties Development, and Microstructure Evolution of Manufactured Sand Concrete Mixed with Granite Stone Powder.

This study explored the potential of granite stone powder (GSP) as a supplementary cementitious material (SCM). The 72 h early hydration process stages of GSP-mixed slurry were analyzed in depth, and the mechanical properties of manufactured sand concrete (MSC) mixed with GSP were investigated. Physical phase types, morphological characteristics, and pore structure evolution were investigated using an X-ray diffractometer, scanning electron microscope, and mercury intrusion approach (MIP). Atomic force microscopy was used to show the interface transition zone between aggregate and slurry in phase images, height images, and 3D images, allowing quantification of ITZ and slurry by calculating the roughness. Gray entropy analysis was used to evaluate the significance of the effect of pore size distribution parameters on mechanical strength, and the GSP-content-mechanical-strength gray model GM (1, 1) was established to predict mechanical strength. The results indicate that, compared with the reference group, the GSP cement slurry system exhibited a delayed hydration process acceleration rate, with a 1.04% increase in cumulative heat of hydration observed in the 5% test group and an 11.05% decrease in the 15% test group. Incorporating GSP in MSC led to decreased mechanical properties at all ages, with significant decay observed when incorporation ranged from 10% to 15%. Although the type of hydration products remained unchanged, there was a decrease in the number of C-S-H gels and gel pores, while large pores increased, resulting in increased porosity and roughness of the interface transition zone and slurry. Large pores (>1000 nm) were found to have the greatest influence on mechanical strength, with gray correlation above 0.86. The GM (1, 1) model yielded accurate predictions, showing good agreement with measured data and thus it can be identified as belonging to a high-precision prediction model category. These findings provide theoretical support and a reference for applying GSP as an SCM, laying the groundwork for data-based specification development.

Study on the Oil Well Cement-Based Composites to Prevent Corrosion by Carbon Dioxide and Hydrogen Sulfide at High Temperature

Complex wells with high temperature and the presence of carbon dioxide and hydrogen sulfide acid gas require the use of high-temperature and high-density anti-corrosion cement slurry for cementing operations, and conventional cement slurry does not have the advantages of high density, high-temperature resistance, or corrosion resistance. In order to avoid the severe corrosion of cement slurry by carbon dioxide and hydrogen sulfide at high temperatures, solid phase particles with different particle sizes are combined with polymer materials to form a dense, high-density, high-temperature- and corrosion-resistant cement slurry. In this paper, we consider the use of manganese ore powder weighting agent, composite high-temperature stabilizer, inorganic preservative slag and organic preservative resin to improve the corrosion resistance of cement slurry, design a high-density cement slurry that is resistant to high temperature and carbon dioxide and hydrogen sulfide corrosion, and evaluate the performances of the cement slurry at 180 °C. The results show that the manganese ore powder weighting agent effectively improves the density of the cement slurry. Using composite silica fume with different particle sizes as a high-temperature stabilizer can ensure the rheology of the cement slurry and improve the ability of the cement sample to resist high-temperature damage. The use of slag and resin as preservatives can effectively reduce the corrosion degree in cement slurry. The high-temperature corrosion-resistant cement slurry systems with different densities designed using these materials exhibit good rheological properties, with water loss of less than 50 mL and a thickening time of more than four hours. The compressive strength decreased by less than 5.8% after 28 days at high temperatures. After being corroded by hydrogen sulfide and carbon dioxide (total pressure 30 MPa, 16.7% hydrogen sulfide and 6.7% carbon dioxide) under high temperature (180 °C) for 30 days, the corrosion depth of the cement sample was less than 2 mm, the reduction of compressive strength was low, and the corrosion resistance was strong. These research results can be used for cementing operations of high-temperature oil and gas wells containing hydrogen sulfide and dioxide.

Rheological and mechanical properties of oil-well cement reinforced by hybrid inorganic fibers

The mechanical integrity of cement sheath during reservoir stimulation is critical for oil and gas wells. It is essential to optimize and design a cement slurry system that not only meets the primary cementing requirements but also has a good mechanical performance. In this study, effects of hybrid inorganic short-cut carbon fiber and basalt fiber on the rheological and mechanical performances of shale gas well cement were investigated. The rheological performance of prepared cement reinforced with hybrid inorganic fibers was researched by the viscometer, filtration loss meter, and consistometer. And the cement stone samples were tested by pressure testing machines. According to rheological behavior experimental results, the Power-law model was most suitable for describing the rheological behavior of slurries among the three models used in this study, and the rheological parameters, including consistency coefficient, yield point, and plastic viscosity of the studies cement slurries with fibers slightly increased in a suitable engineered addition. Combination of carbon fiber and basalt fiber can compromise the effect of the addition of a single fiber on the rheological properties of slurry and does not affect the workability and pumpability of slurry in terms of the fibers' additive amount of this study. Moreover, the fibers can also improve the sedimentation stability of cement slurry without significantly affecting its filtration loss and thickening time. In terms of mechanical properties of cement stone, no markedly difference in compressive strength was found between reference slurry and slurries with fibers, while the addition of carbon fiber and basalt fiber significantly improved the tensile strength and flexural strength, and also mitigated the brittleness of cement stone. The findings of this study can offer a key reference for the cement slurry design of oil and gas wells requiring reservoir stimulation.

Effects of ethylene diamine tetraacetic acid and calcium nitrate on high-temperature cementing slurry in a large temperature difference environment

In wellhole environments with large temperature differences, high-temperature retarders can slow the strength development of the top of a cement slurry column, thus affecting construction progress and safety of cementing projects. Herein, to address the problem of slow strength development of the top of a cement slurry caused by the addition of the high-temperature retarder GH-8L, the synergistic effect of the co-addition of ethylene diamine tetraacetic acid (EDTA) and calcium nitrate (CN) is considered. The experimental results show that the thickening time of the cement slurry system mixed with EDTA and CN exceeds 300 min at 150 °C and 75 MPa and its strength at 48 h is >14 MPa. As demonstrated by the hydration heat tests, EDTA and CN promote the hydration heat release rate of the GH-8L cement slurry at low temperature. The synergistic mechanism of EDTA and CN in the GH-8L doped cement slurry promotes the initial dissolution of cement particles, significantly increases the content of C-S-H and CH in the hydration products, and promotes the formation of needle-like NO3–AFt ettringite. The cementing slurry, with the co-addition of EDTA and CN, can not only meet the requirements of thickening time and cementing with a large temperature difference but also compressive strength.