Investigating the applicability of novel hydrate dissociation inhibitors in oilwell cement through molecular simulations.

In the field of hydrate formation cementing, the method of developing the low hydration exothermic cement systems cannot effectively solve the problem of hydrate dissociation caused by the hydration heat release of cement. Therefore, we proposed a new approach to address this issue by employing cement additives that can effectively delay the dissociation of hydrate. In our previous work, we designed a novel hydrate dissociation inhibitor, PVCap/dmapma, however, its applicability with cement slurry remains unverified. In this study, we established a more realistic model of oilwell cement gel based on experimental data. Additionally, we investigated the potential effects of PVCap/dmapma on the microstructure and mechanical properties of cement gel through molecular simulations. The results suggest that PVCap/dmapma has no negative effect on the performance of cement slurry compared to Lecithin. By adding PVCap/dmapma to cement slurry, the problem of cementing in hydrate formations is expected to be solved.

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.

An experimental study of the correlation between P-wave velocity and the physical properties of weakly cemented formations.

Deep water and shallow layers mostly feature weakly cemented formations, with complex geological structures, geological looseness, susceptibility to collapse. In order to obtain information on weakly cemented formation materials, weakly cemented argillaceous siltstone is simulated as the research object and the focus is on analysing the influence of ultrasonic frequency, density, particle size (porosity), and compressive strength on P-wave velocity and establishing the correlation relationship between longitudinal wave velocity and each parameter through indoor simulation experiments. The results showed that there is a linear relationship between P-wave velocity and ultrasonic frequency in terms of positive correlation as well as compressive strength. The nonlinear relationship between P-wave velocity and particle size (porosity) is a negative correlation, while the nonlinear relationship between sound velocity and density is a positive correlation. In addition, the influence of core height on P-wave velocity is analysed; it is found that as the core height increases, the velocity slightly decreases, and each ultrasonic frequency has an ultimate height for sound wave penetration. Through the response relationship between ultrasound and the physical properties of weakly cemented formations, indirect acquisition can be achieved, which is of great significance for the development of oil and gas in weakly cemented formations.

Preparation and Characteristics of a pH Intelligent Response Blocking Agent for the Microcrack in the Cement Sheath.

Microcracks in the annular cement sheath of oil wells frequently cause annular water channeling. Traditional cement squeeze technology has a low success rate in controlling this issue. Based on the conventional profile control water-blocking agent for underground in situ gelling and polyacrylic acid, a pH intelligent response microcrack-blocking agent was developed to block the microcrack in the cement sheath. The study investigated the influence of pH on the viscosity characteristics of the new blocking agent, the impact of polyacrylic acid on the compression recovery ability of the new blocking agent after gelling, the change in viscosity of the blocking fluid after flowing through the microcrack in the cement sheath, and the blocking effect after gelling. The results indicated that the new blocking agent has excellent viscosity-increasing ability with pH. After flowing through the microcrack in the cement sheath, the viscosity of the blocking agent increased significantly with the extension of the contact distance between the blocking agent and the microcrack in the cement sheath, which is very conducive to the retention of the blocking agent in the microcrack of the cement sheath. Polyacrylic acid had a negligible effect on the compression recovery ability of the blocking agent after gelling. At a fracture length of 5 cm, the pressure-bearing capacity of the blocking agent could reach 6 MPa.

Preparation and performance evaluation of non-foaming styrene-acrylic latex for cementing slurry.

The latex used conventionally for oil-well cementing can lead to serious foaming issues in the cement slurry, which not only affects the accurate measurement of the density of the latex-containing cement slurry, but also is detrimental to cementing construction. A large amount of a foam stabilizer used for latex preparation is mainly responsible for foaming of the latex-containing cement slurry. In this study, soap-free emulsion polymerization was conducted using 2-acrylamido-2-methylpropanesulfonic acid (AMPS), styrene (St), and butyl acrylate (BA) as the reaction monomers and the effects of the AMPS dosage, monomer ratio, reaction temperature and stirring speed on the performance of the latex were investigated. The optimum synthesis conditions included a 30% monomer concentration, an St : BA : AMPS monomer ratio of 5 : 4 : 6, a synthesis temperature of 85°C, a stirring speed of 400 r.p.m. and 1.5% of the initiator. As-prepared latex exhibited good filtration loss control, excellent freeze-thaw stability, and extremely low foaming of the cement slurry with the added latex, which was extremely beneficial for on-site cementing construction.

Effects of microstructure on compressive strength of silica sand-enhanced oil well cement at a wide temperature range.

The influence of microstructure of silica-enhanced cement on the mechanical performance of cement is difficult to describe. In this study, we used the scanning electron microscope and image processing method to investigate the relationship between the complicity of cement microstructure and compressive strength under various temperatures and curing times. Fractal dimension was applied to describe the complicity of silica-enhanced cement. The relationships among compressive strength, fractal dimension, temperature, curing time and pore structure of cement were identified. The results show that curing time directly controls the complicity of microstructure of silica-enhanced cement and compressive strength by altering the pore orientation and macropore ratio in silica-enhanced cement. The curing temperature affects the complicity of cement microstructure and compressive strength indirectly by changing the ratio of micropore and small pore. The fractal dimension of silica-enhanced cement shows good correlation with compressive strength. Pore size distribution is the most important factor that influences the complicity of cement matrix and compressive strength of silica-enhanced cement. When building up the macroscopic mechanical performance model of silica-enhanced cement, we should consider the influence of pore size distribution in cement under different curing temperatures and times on the complicity of cement microstructure.

A novel hydrophobically associating water-soluble polymer used as constant rheology agent for cement slurry.

During the process of well cementing in deep water, the cement slurry experiences a wide range of temperature variation from low temperature at seabed to high temperature in downhole. The elevated temperature affects the rheology of cement slurry. The change of rheology of cement slurry could influence the safety of cementing operation. The aim of this paper is to develop a new kind of hydrophobically associating water-soluble polymer (NHAWP) as an additive to prepare a constant rheology oil well cement slurry, which can be used at temperature range from 4°C to 90°C. The acrylamide, 2-acrylamide-2-methylpropionic acid and stearyl methylacrylate were applied to synthesize the NHAWP by the inverse microemulsion polymerization. Test results indicate that the critical association temperature of NHAWP is 45°C. The critical association temperature is independent of NHAWP concentration, salt concentration and alkalinity of solution. When the temperature is below 45°C, NHAWP shows little influence on the viscosity of solution. When the temperature is above 45°C, the NHAWP forms spatial network structure by intermolecular hydrophobic association and thus increases the viscosity of solution significantly. The NHAWP also displays good thermal stability and excellent salt and alkali resistance properties. In addition, the NHAWP shows nearly no negative influence on the basic properties of cement slurry, which indicates that the NHAWP can be used as a constant rheology agent to prepare a cement slurry with constant rheology in the temperature range of 4°C to 90°C.

Adsorption mechanism of Pb2+ in montmorillonite nanopore under various temperatures and concentrations.

Adsorption of lead (Pb2+) onto the montmorillonite (Mt) surface is one of the key approaches to remove Pb2+ in geological and environmental engineering. Temperature and initial Pb2+ concentration are two essential factors that influence the adsorption capacity of Mt on absorbing Pb2+. However, the nanoscale governing mechanism of temperature and initial concentration on Pb2+ adsorbing of Mt is still unclear. This research performed comprehensively molecular dynamics (MD) simulations to investigate how temperature and initial concentration affect the dynamic Pb2+ adsorption of Mt nanopore. The Pb2+ removal ratio shows a two-stage variation with the increase of initial Pb2+ concentration. Temperature controls the maximum initial Pb2+ concentration for complete Pb2+ removal by changing the maximum adsorption energy of Mt. Temperature also influences the maximum adsorption capacity and Pb2+ removal ratio of Mt nanopore indirectly by changing diffusion and hydration state of Pb2+. The initial Pb2+ concentration corresponding to the maximum adsorption energy coincides with the maximum initial Pb2+ concentration determined by the Pb2+ removal ratio. Lower adsorption energy and higher level of hydration and diffusion make Pb2+ absorbing on Mt surface become more difficult, reducing the Pb2+ adsorbing capacity of Mt. The initial Pb2+ concentration influences adsorption capacity and Pb2+ removal ratio not only via altering the quantity of Pb2+ but also through controlling the adsorption energy of Mt, as well as the diffusion and hydration state of Pb2+. With the increase of initial Pb2+ concentration, the hydration of Pb2+ is weakened while the adsorption energy of Mt and diffusion of Pb2+ are enhanced.

Utilization of metakaolin-based geopolymer as a mud-cake solidification agent to enhance the bonding strength of oil well cement-formation interface.

This research work designed a novel mud-cake solidification method to improve the zonal isolation of oil and gas wells. The calculation methodology of mud-cake compressive strength was proposed. The optimal formula of activator and solid precursors, the proper activating time and the best activator concentration were determined by the compressive strength test. The effects of solid precursors on the properties of drilling fluid were evaluated. Test results show that the respective percentage of bentonite, metakaolin, slag and activator is 1 : 1 : 0.3 : 0.8, as well as the optimum ratio of Na2SiO3/NaOH is 40 : 1. The optimum concentration of activator is 0.21 and the activating time should be more than 10 min. The solid precursors did not show any bad influence on the rheological property of drilling fluids. Even though the compressive strength decreased when the solid precursors blended with barite, the strength values can still achieve 8 MPa. The reaction of metakaolin and activator formed cross-link structure in the mud-cake matrix, which enhanced the connection of the loose bentonite particles, lead to the significant enhancement of shear bonding strength and hydraulic bonding strength. This mud-cake solidification method provides a new approach to improve the quality of zonal isolation.

Bentonite-Acrylamide Hydrogels Prepared by the Nonmixing Method: Characterization and Properties.

A significant amount of research has been conducted on bentonite-acrylamide hydrogels. These gels are usually prepared by uniformly mixing bentonite with reactive monomers. Herein, a new preparation method of bentonite-acrylamide hydrogels has been proposed to cater to one novel application of bentonite-acrylamide hydrogels. In this method, bentonite-acrylamide hydrogel was obtained by pressing bentonite into a thin mud cake and extruding a mixed liquor of acrylamide, a cross-linking agent, an initiator, and water into the thin mud cake and then subjecting the system to water-bath curing. The effects of extrusion pressure, extrusion time, and acrylamide concentration on the tensile strength and elemental composition of bentonite-acrylamide hydrogel were investigated. The results show that the tensile strength of the bentonite-acrylamide hydrogel first increased and then tended to be stable with the further increase in extrusion pressure and extrusion time. As the concentration of acrylamide increased, the tensile strength of the bentonite-acrylamide hydrogel increased first and then decreased slightly. With the increase in extrusion pressure, extrusion time, and acrylamide concentration, the contents of C and N elements in the thin mud cake gradually increased and then tended to be stable, which reflects the state of the monomer entering the thin mud cake. In addition, the elemental composition of the bentonite-acrylamide hydrogel was analyzed via the scanning electron microscopy-energy dispersive X-ray spectrometry method, and it was found that the composition of the hydrogel was relatively uniform in the direction of mixed liquor extrusion.

Effect of carboxylic group on the compatibility with retarder and the retarding side effect of the fluid loss control additive used in oil well cement.

The retarding side effect and the compatibility with other additives are the main problems that limit the field application of the synthesized fluid loss control additive (FLCA). The effect of the type and content of carboxylic acid groups on the retarding side effect of FLCA and the compatibility between FLCA and the retarder AMPS-IA synthesized using 2-acrylamido-2-methyl propane sulfonic acid (AMPS) and itaconic acid (IA) was studied in this paper. The type and content of carboxylic acid group have a great influence on the fluid loss control ability, the compatibility with retarder and the retarding side effect of FLCA. FLCA containing IA or maleic acid (MA) shows better compatibility with retarder than FLCA containing acrylic acid, but the retarding side effect of FLCA containing MA is weaker than that of FLCA containing IA. Thus, MA is the most suitable monomer for synthesizing FLCA having good compatibility with retarder AMPS-IA.