Safety and Long-term Cementing Performance Design Method for Marine Natural Gas Hydrates

Abstract

The present invention relates to the technical field of oil and gas well cementing, and specifically discloses a method for designing the safe and long-term cementing performance of marine natural gas hydrates. According to the cementing construction process, the factors affecting the cementing quality of natural gas hydrates are systematically analyzed from three aspects: ① the stability of the hydrate formation during the action period of the preflush fluid and the requirement for increasing the interfacial bonding, ② the effect of the hydration heat of the cement slurry on the formation during cementing, and ③ the mechanical property requirements of the cement stone. Aiming at the problems mentioned above that affect the stable occurrence of natural gas hydrates and the cementing quality during cementing, a design method is proposed to prevent the decomposition of natural gas hydrates, improve the interfacial bonding of weak cementing formations and the cementing quality. The factors studied in the present invention are comprehensive, and a more targeted comprehensive cementing design method is obtained to ensure the safety of the cementing quality of natural gas hydrates and provide technical support for the efficient exploitation of natural gas hydrates.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas well cementing, and in particular to a method for designing safe and long-term cementing performance of marine natural gas hydrate. Background Art

[0002] Natural gas hydrate is an ice-like solid crystal composed of natural gas and water molecules. Natural gas is mainly composed of methane, so it is also called methane hydrate. It can be ignited in the air and is also commonly known as combustible ice. The energy density of natural gas hydrate is relatively high. Ideally, 1m 3 Natural gas hydrate can release 164m 3 Natural gas hydrate is considered to be one of the most promising new clean energy sources to replace coal, oil and natural gas in the 21st century.

[0003] At present, most of the trial production of natural gas hydrates in the sea area adopts traditional well construction methods. It can be seen that the exploitation of natural gas hydrates by well construction is the mainstream method in the world today. Cementing is an important part of the well construction process. The quality of cementing has a significant impact on the safety of the wellbore. Due to the influence of cement hydration heat, on the one hand, it will lead to the appearance of micro-annuli between the already well-bonded cement sheath and the well wall, and the gas will continue to erupt upward, resulting in serious consequences; on the other hand, the decomposition of hydrates will lead to instability of the formation in the area, and collapse may occur. In order to achieve the goal of safe well construction in hydrate formations and ensure the overall sealing performance of cement sheaths, it is necessary to conduct in-depth research on the development law of the two-interface cementation strength, the change law of the formation temperature field and stress field, and the integrity of the cement sheath and interface during the cementing process of natural gas hydrate formations, so as to provide technical support for cementing of hydrate formations.

[0004] A Chinese patent document with publication number CN113213785A discloses a high-strength, low-hydration thermal cementing low-hydration thermal cement and a preparation method thereof. In the cement system, low-heat silicate cement, 10%-20% of low-hydration active material, 4%-8% of reinforcing agent, 4%-6% of anti-shrinkage agent, and 0.5%-1.5% of early strength agent are used, which well solves the problem of cement slurry hydration heat and compressive strength. However, the performance of the cement system under low temperature conditions is poor.

[0005] The article "Simulation of the disturbance caused by the intrusion of drilling fluid containing thermodynamic inhibitors into natural gas hydrate formations" (Zhang Huaiwen et al., Science, Technology and Engineering, 2018 / 2) explored the disturbance law of hydrate thermodynamic inhibitors on hydrate layers. The process of two types of hydrate thermodynamic inhibitor drilling fluids, NaCl and ethylene glycol, invading hydrate rock samples was studied, but no solution was given to the contradictory problem of preventing the decomposition of hydrates in the formation and inhibiting the formation of hydrates in the wellbore.

[0006] "In the article 'Influence of Natural Gas Hydrate Decomposition on the Deformation of Soil Layers around Wellbores' (Wang Jing et al., Journal of Water Resources and Architectural Engineering, December 2017), a relevant model was established through the FLAC3D software to simulate the decomposition radius of natural gas hydrates at different horizons, and the soil deformation around the wellbore after decomposition was obtained. However, in this article, the relevant assumptions are not universal, and the thermodynamic effects during the cementing process are not considered. In summary, due to the complexity and particularity of the geological conditions, temperature, and pressure environment of natural gas hydrate reservoirs, the research on natural gas hydrate cementing technology focuses on the low-temperature and low-hydration heat cement slurry system, as well as the control of the stability of natural gas hydrate formations during drilling and production. However, reducing the hydration heat release of the oil well cement slurry system cannot meet the requirement that the cement stone has a relatively high compressive strength within a short time at low temperatures, and the waiting time for setting is too long."

[0007] Therefore, determining the relationship between the hydration heat release of oil well cement, the compressive strength of the cement stone, and the stability of natural gas hydrates is a key issue that urgently needs to be solved in natural gas hydrate cementing. In response to this key issue, a design method for the safe and long-term cementing performance of natural gas hydrates is proposed to ensure the cementing quality and lay a solid foundation for the safe and efficient exploitation of natural gas hydrates." Summary of the Invention

[0008] The concept of the present invention is to evaluate the working fluids used in the cementing construction process on the premise of ensuring the stable exploitation of natural gas hydrates. The factors affecting the stable occurrence of natural gas hydrates during the cementing process are studied from the following three aspects: ① the preflush fluid; ② the performance of the cement slurry system for cementing; ③ the mechanical properties of the cement stone."

[0009] Based on this, the object of the present invention is to propose a safe and long-term cementing performance design method for marine natural gas hydrates. Taking the cementing process as the premise, combined with the construction process in cementing, different analysis methods are adopted to ensure the stable exploitation of natural gas hydrates. For the preflush fluid circulated before cementing, in addition to the functions of separating the drilling fluid from the cement slurry, flushing the wellbore and casing wall, and improving the cement bond strength, it should also reduce the decomposition of hydrates in the reservoir section during circulation and have good rheological properties under low-temperature conditions. For the cement slurry used in cementing, the cement hydration kinetics theory is adopted to quantitatively analyze different hydration stages of different cement slurries, determine their key hydration kinetics parameters, and combine with the laws of thermodynamics to deduce the heat generation and heat transfer equation of the annulus-casing-formation. The hydration kinetics experimental parameters are converted into the temperature change under actual working conditions, and combined with the stable occurrence conditions of natural gas hydrates, the influence degree of the hydration heat release of the cement slurry on the natural gas hydrate formation in this system is obtained. At the same time, since the hydration heat release of the cement slurry system is related to the mechanical properties of the cement stone, it is necessary to improve the relevant properties of the cement stone, such as compressive strength, linear expansion rate, etc., on the basis of considering the stable occurrence of natural gas hydrates, so as to improve the bonding quality of the second interface and ensure the cementing quality. Through the research on the above-listed items, the key indicators of each factor are analyzed, and a comprehensive cementing design method is further obtained.

[0010] To further achieve the above object, the present invention adopts the following technical solutions: A safe and long-term cementing performance design method for marine natural gas hydrates. It includes the following steps:

[0011] ① The design of the preflush fluid is based on improving the displacement efficiency and the interface bonding strength. It is achieved by improving the compatibility between the preflush fluid and the drilling fluid and reducing the adhesion force of the preflush fluid to the casing and wellbore wall. The effects of various natural gas hydrate inhibitors are tested, and the inhibitor that prevents the decomposition of natural gas hydrates during contact with the formation and prevents the formation of natural gas hydrates in the wellbore and the appropriate dosage are preferably selected, and at the same time, it is ensured that the displacement efficiency can be improved and the interface bonding quality of the weak cemented formation can be improved;

[0012] ② For the cement slurry used in cementing, the heat released during its hydration has a great impact on the natural gas hydrate formation. Under low-temperature conditions, it will affect the rate of the cement hydration reaction and greatly reduce the pozzolanic reaction of the active cementitious materials in the cement slurry, thereby resulting in slow strength development of the cement slurry and insufficient development of the microscopic pore structure of the cement stone, affecting the mechanical properties. Therefore, it is designed from aspects such as preventing the decomposition of natural gas hydrates, ensuring early strength at low temperature, improving the cementing quality, ensuring the safety of cementing, and the integrity of the wellbore during later exploitation. It should be ensured that the 50h hydration heat of the cement slurry for cementing is controlled within 105 - 118 J·g -1 , and under the ambient temperature of 14 °C, the maximum temperature of the cement slurry is controlled below 21 °C;

[0013] ③For the design of the mechanical properties of the cement stone, first, based on the software platform Abaqus, the bonding condition of the second interface of the natural gas hydrate cementing is determined according to the numerical simulation method. That is, first, the mechanical properties and constitutive relations of the natural gas hydrate formation are studied, and the widely used Cohesive Zone Model is used to simulate the interface bonding condition to obtain the distribution of the damage criterion of the second interface on the interface. Secondly, a simulated formation is established based on the basic formation mechanics information, and the bonding strength test of the second interface is carried out. Finally, according to the relevant results, the existing cement stone is evaluated, and the relevant mechanical property indexes for improving the cementing quality of the natural gas hydrate formation are proposed: the mechanical properties of the cement stone at 48 h should have the following characteristics: the elastic modulus should be 5 - 8 GPa, the bonding strength of the second interface should be greater than 1.2 MPa, and the compressive strength should be greater than 12 MPa;

[0014] ④According to the indoor cementing simulation experimental device, test whether the performance of the cementing fluid meets the requirements; if it meets the requirements, proceed to step ⑤, otherwise, repeat steps ① - ③;

[0015] ⑤Estimate the injection volume of the cementing fluid according to the relevant drilling parameters of the target interval, and inject the qualified cementing fluid into the formation according to the corresponding construction parameters until the cement slurry reaches the designed range.

[0016] Furthermore, in step ①, the natural gas hydrate inhibitors added to the preflush fluid include, but are not limited to, one or a combination of sodium chloride (7 - 10%) / ethylene glycol (8 - 10%), polyvinyl alcohol, N - vinyl pyrrolidone (0.5 - 1%), polyglyceramide ester (2 - 6%), N - methylacrylamide (2.5 - 7%), and polyethylene oxide (0.1 - 1.2%). Furthermore, in step ①, the materials for improving the interface bonding strength added to the preflush fluid include, but are not limited to, epoxy resin (6 - 12%), vinyl acetate latex (3 - 8%), and polyvinyl alcohol (1.0 - 3.2%).

[0017] Furthermore, in step ②, the low - heat - of - hydration materials added to the cement slurry include, but are not limited to, fly ash (35 - 55%), slag (35 - 60%), phase - change endothermic microspheres (7 - 15%), and solid - solid phase - change endothermic materials (8 - 18%).

[0018] Furthermore, in step ②, the early - strength materials at low temperature added to the cement slurry include, but are not limited to, liquid nano - silica (1 - 4%), nano - silica sol (2 - 6%), nano - early - strength agent of hydrated calcium silicate crystal nuclei (3.5 - 6.5%), and composite early - strength agent of lithium salt (1 - 3%) / potassium chloride (3 - 5%).

[0019] Further, in step ②, the expansion agent added to the cement slurry includes, but is not limited to, one or a combination of calcium oxide (2-5%), calcium sulfoaluminate (1-4%), and tricalcium aluminate (1-3.5%).

[0020] Further, in the safe and long-term cementing performance design method for marine natural gas hydrates, the density of the cement slurry is controlled at 1.4-1.6 g / cm 3 , and the density adjustment material added is a hollow glass microsphere lightening agent material and stabilizers such as microsilica.

[0021] Compared with the prior art, the present invention has the following beneficial effects: ① For the preflush fluid and cement slurry involved in the cementing process, different influencing factors of the cementing working fluid on the cementing process of natural gas hydrates are analyzed according to the technological process, including: the flow performance of the cementing working fluid deteriorates under low-temperature conditions, the influence of the preflush fluid during circulation on natural gas hydrates, the heat exchange between the hydration heat release of the cement slurry and the formation heat of natural gas hydrates, and the bond strength at the second cementing interface and the mechanical property indexes of the cement stone; ② The factors studied are comprehensive, basically covering the technological process in the existing cementing construction. For the influencing factors of different working fluids, by extracting the key scientific problems of the influencing factors one by one and selecting appropriate evaluation methods, a safe and long-term cementing performance design method suitable for marine natural gas hydrates is finally formed; ③ For different well conditions, the geological conditions of natural gas hydrates, the saturation of natural gas hydrates, the water content, etc. are different, and the cement slurry systems used are also different. When analyzing different systems, according to the ideas and methods in this article, some performance parameters can be modified and further improved to obtain a construction plan for stable occurrence during the cementing process of natural gas hydrates under different conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] To more clearly illustrate the technical solution of the present invention, the content of the embodiments of the present invention that requires the use of drawings is introduced.

[0023] Figure 1 is a schematic diagram of the systematic analysis of the cementing design method of the present invention;

[0024] Figure 2 is a flow chart of the cementing performance design of the present invention;

[0025] Figure 3 is a schematic diagram of the evaluation of the bond strength of the cementing and the mechanical properties of the cement stone of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] To more clearly illustrate the technical solution of the present invention, the embodiments of the present invention will be described below with reference to the content of the drawings.

[0027] As Figure 1 , Figure 2As shown in the figure, a safety and long-term cementing performance design method for marine natural gas hydrates includes the following steps:

[0028] ① The displacement fluid plays roles such as diluting and dispersing the drilling fluid, flushing the wellbore and casing wall, and improving the cement bond strength during cementing operations. The spacer fluid needs to effectively separate the drilling fluid from the cement slurry to enhance the displacement effect and should have good fluidity and suspension force. During the cementing operation of natural gas hydrates, the working fluid is in contact with the natural gas hydrate formation for a long time, and the substances and energy it carries will interact with the formation, having a certain impact on the formation stability. In the actual process flow, although the preflush fluid has a short action time, it should still have a relatively low temperature.

[0029] According to the functions and performance requirements of the preflush fluid, conduct effectiveness tests on various natural gas hydrate inhibitors, select the inhibitor and appropriate dosage that can prevent the decomposition of natural gas hydrates during contact with the formation and prevent the formation of natural gas hydrates in the wellbore, and at the same time ensure that the displacement efficiency can be improved and the cementing quality of the weak cementing formation interface can be enhanced;

[0030] ② For the cement slurry used in cementing, the heat released during its hydration has a great impact on the natural gas hydrate formation, and it will also affect the relevant properties such as the curing of the cement slurry under low-temperature conditions. Therefore, design is carried out from aspects such as preventing the decomposition of natural gas hydrates, ensuring early strength at low temperature, improving the cementing quality, ensuring the safety of cementing, and the integrity of the wellbore during later exploitation.

[0031] First, measure the heat released by the cement slurry system using the isothermal calorimetry method, calculate the relevant parameters during the cement hydration process using the widely used K-D model, and establish a two-dimensional prediction model of the wellbore temperature field. At different stages, different calculation parameters need to be selected to obtain the temperature rise of the cement slurry. Secondly, introduce the change in cement temperature rise as the only heat source into the prediction model and perform numerical calculations in the solver to calculate the temperature change of the annulus-formation. Then, combined with the heat transfer calculation in Abaqus, more intuitively reflect the law of the heat release of the cement slurry hydration on the formation with time and wellbore distance, and combined with the conditions for the stable occurrence of natural gas hydrates, determine whether the heat of hydration of the cement slurry in this system needs to be regulated and the range of regulation.

[0032] At the same time, since there is a strong correlation between the heat release of the cement slurry hydration and the mechanical properties of the cement stone, while reducing the heat release of hydration, it is necessary to take into account the mechanical properties of the cement. Low-temperature early-strength materials can be added to ensure the development of the early strength of the cement stone. Chinese patents with publication numbers CN113003962A and CN109266320A disclose several low-temperature early-strength materials. The low-temperature early-strength materials added to the cement slurry include but are not limited to calcium chloride, nano-silica, and calcium silicate hydrate-polycarboxylic ether nano-composite early-strength agent.

[0033] ③ Regarding the design of mechanical properties of cement paste, since most natural gas hydrate formations are not consolidated into rocks, the reservoirs are mainly fine-grained sediments such as silt and argillaceous silt, which are soft and weakly consolidated, and it is difficult to form effective bonding with cement paste; secondly, the mechanical properties of cement paste, such as shrinkage, elastic modulus, and compressive strength, will be affected by low temperature conditions. In order to achieve the goal of safe well construction in hydrate formations and ensure the overall sealing performance of cement sheath, it is necessary to study the development law of the two-interface bonding strength during the cementing process of natural gas hydrate formations.

[0034] Based on this, the research results of cementing strength at the cementing interface at home and abroad were investigated, and the cohesive zone model (CZM) theory was used to study the cracking and damage at the cementing interface. The bilinear cohesive force law was combined with the finite element method to simulate the entire process of material destruction. The research method refers to the numerical simulation research of Jiang Jiwei and others from China University of Petroleum (Beijing). In order to ensure the comprehensiveness of the analysis in this paper, it is necessary to conduct relevant research on the indoor physical research model of the cementing strength of natural gas hydrate formations to obtain a more accurate quantitative description.

[0035] ④ According to the indoor cementing simulation experimental device, test whether the performance of the cementing working fluid meets the requirements. The cementing simulation experimental device includes a pressure control module, a cement slurry pumping module, and a pressure and temperature sensor module pre-buried in the simulated bottom layer, which can be used to detect in real time the influence range of cement hydration heat release on the formation temperature and pressure changes.

[0036] ⑤ Estimate the injection volume of cementing fluid according to the relevant drilling parameters of the target layer, and inject the cementing fluid that meets the requirements into the formation according to the corresponding construction parameters until the cement slurry reaches the designed range. In summary, the design method for safe and long-term cementing performance of marine natural gas hydrates provided by the present invention can better solve the safety problem of natural gas hydrate cementing in the prior art. When analyzing different systems, some parameters can be modified and further improved according to the ideas and methods of this article, so as to obtain a construction plan for stable occurrence of natural gas hydrates in the cementing process under different conditions, which has good application prospects.

[0037] The above description is only a preferred embodiment of the present invention, so that those skilled in the art can understand or implement the present invention. The present invention cannot list all embodiments, and various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments described herein, but should conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for designing the safe and long-term cementing performance of marine natural gas hydrates, characterized in that, It is achieved by the following steps: ① The design of the preflush fluid is based on improving the displacement efficiency and the interfacial bonding strength. It is achieved by enhancing the compatibility and flushing effect between the preflush fluid and the drilling fluid, improving the interfacial bonding characteristics of the weakly bonded wellbore through the preflush fluid to enhance the interfacial bonding quality, and testing the effects of various natural gas hydrate inhibitors to determine the inhibitors and appropriate dosages that can prevent the decomposition of natural gas hydrates during contact with the formation and prevent the formation of natural gas hydrates in the wellbore, while ensuring that the displacement efficiency and the cementing quality of the weakly bonded formation can be improved; ②For the cement slurry used in well cementing, the heat released during its hydration has a great impact on the natural gas hydrate formation. At low temperatures, it will affect the hydration reaction rate of the cement and greatly reduce the pozzolanic reaction of the active cementitious materials in the cement slurry, resulting in slow strength development of the cement slurry and insufficient development of the microscopic pore structure of the cement stone, which affects the mechanical properties. Therefore, it should be designed from five aspects: preventing the decomposition of natural gas hydrates, ensuring early strength at low temperatures, improving the well cementing quality, ensuring the safety of well cementing, and the integrity of the wellbore during later exploitation. The 50-hour hydration heat of the well cementing slurry should be controlled within 105 - 118 J·g -1 , and at an ambient temperature of 14 °C, the maximum temperature of the cement slurry should be controlled below 21 °C; ③ For the design of the mechanical properties of the cement stone, first, based on the software platform Abaqus, the bonding situation of the second interface in natural gas hydrate cementing is determined by numerical simulation method. That is, first, the mechanical properties and constitutive relations of the natural gas hydrate formation are studied, and the Cohesive Zone Model, which is widely used, is adopted to simulate the interfacial bonding situation to obtain the distribution of the damage criterion of the second interface on the interface. Secondly, a simulated formation is established based on the basic information of the formation mechanics, and the bonding strength test of the second interface is carried out. Finally, based on the relevant results, the existing cement stone is evaluated, and the relevant mechanical property indexes for improving the cementing quality of the natural gas hydrate formation are proposed: the mechanical properties of the cement stone at 48 h should have the following characteristics: the elastic modulus should be 5 - 8 GPa, the bonding strength of the second interface should be greater than 1.2 MPa, and the compressive strength should be greater than 12 MPa; ④ According to the indoor cementing simulation experimental device, test whether the performance of the cementing working fluid meets the requirements; If the requirements are met, proceed to step ⑤; otherwise, repeat steps ① - ③; ⑤ Estimate the injection volume of the cementing working fluid according to the relevant drilling parameters of the target interval, and inject the qualified cementing working fluid into the formation according to the corresponding construction parameters until the cement slurry reaches the designed range.

2. The safety and long-term cementing performance design method for marine natural gas hydrates according to claim 1, characterized in that In step ①, the natural gas hydrate inhibitor added to the preflush fluid is one or a combination of the following: sodium chloride 7 - 10%, ethylene glycol 8 - 10%, polyvinyl alcohol, N - vinyl pyrrolidone 0.5 - 1%, polyglycerol amide ester 2 - 6%, N - methyl acrylamide 2.5 - 7%, poly(ethylene oxide) 0.1 - 1.2%.

3. The safety and long-term cementing performance design method for marine natural gas hydrates according to claim 1, characterized in that In step ①, the materials for enhancing the interfacial bonding strength added to the preflush fluid are epoxy resin 6 - 12%, vinyl acetate latex 3 - 8%, polyvinyl alcohol 1.0 - 3.2%.

4. The safety and long-term effective cementing performance design method for marine natural gas hydrates according to claim 1, characterized in that, In step ②, the low - heat - of - hydration materials added to the cement slurry are fly ash 35 - 55%, slag 35 - 60%, phase - change heat - absorbing microspheres 7 - 15%, solid - solid phase - change heat - absorbing materials 8 - 18%.

5. The safety and long-term cementing performance design method for marine natural gas hydrates according to claim 1, wherein In step ②, the low - temperature early - strength materials added to the cement slurry are liquid nano - silica 1 - 4%, nano - silica sol 2 - 6%, hydrated calcium silicate crystal nucleus nano - early - strength agent 3.5 - 6.5%, lithium salt 1 - 3%, potassium chloride composite early - strength agent 3 - 5%.

6. The safety and long-term effective cementing performance design method for marine natural gas hydrates according to claim 1, characterized in that, In step ②, the expansive agent added to the cement slurry is one or a combination of the following: calcium oxide 2 - 5%, calcium sulfoaluminate 1 - 4%, tricalcium aluminate 1 - 3.5%.

7. The safety and long-term cementing performance design method for marine natural gas hydrates according to claim 1, characterized in that The density of the cement slurry is controlled at 1.4 - 1.6 g / cm 3 , and the added density adjustment material agents are hollow glass microsphere-based weighting agents and microsilica stabilizers.

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