Cementing is an important link in the process of well construction of oil and gas wells, and it is a relatively independent system engineering connecting drilling and oil production engineering. The cementing quality is related to the life of oil and gas wells and the production efficiency of oil and gas reservoirs. However, cementing operation is a hidden underground operation project involving a wide range of risks, so the safety construction and cementing quality are very important.

Therefore, domestic and foreign petroleum engineering and technical service companies have stepped up the research of cementing technology, especially after the blowout, fire and explosion accident of BP offshore drilling platform in the Gulf of Mexico, the particularity and importance of cementing operation have been paid more attention. At present, the cementing technology in foreign countries is generally developing in the direction of technical support, serialization, personalization and integration; the cementing quality is developing in the direction of ensuring long-term sealing quality and wellbore sealing; the cementing management and construction are developing in the direction of strengthening process control and management refinement.

With BJ service company of the United States as the representative, some companies put forward the concept of "long-term cementing", that is to achieve effective sealing in the whole well life cycle, so as to provide technical support for the long-term safety production of oil and gas wells. In order to improve the long-term sealing quality, BJ service company mainly starts from three aspects: one is to realize the seriation of cement slurry system and carry out the cement slurry formula design to meet the sealing requirements under different conditions; the other is to carry out the individualized cementing design according to the geological, drilling characteristics and down hole complex conditions; the third is to strengthen the site preparation and refine the site cementing construction; in recent years, BJ service company Lenbecher company promotes "integrated" cementing service and uses cementing optimization software to design cementing to improve the pertinence of the design; develops a series of supporting cement slurry systems to meet the requirements of cementing under different conditions; realizes the concept transformation of cementing management from extensive to refined process management, pays attention to details and strengthens process management.

According to the development trend of cementing operations at home and abroad, a series of matching cementing materials and slurry systems have been basically formed through in-depth and continuous research in recent years. Important progress has been made in ultra-high density and ultra-low density slurry, salt resistant latex cement, and ductile cement, which better meet the needs of cementing in complex situations and effectively support the stable production of oil and gas.

Development of cementing slurry syste

1.1 Ultra high density cement slurry

According to the theory of tight packing, ultra-high density cement slurry with a density range of 2.60-3.10g/cm³ was developed in the laboratory. Through reasonable deployment of weighting materials and particle size optimization, the pump ability and stability of cement slurry were coordinated and unified. The field application density was up to 2.72g/cm³, which solved the problem of ultra-high pressure cementing.

1.2.Ultra low density cement slurry

In view of the difficult problem of cementing in low-pressure and leaky formation and long seal solid section, the objective of minimizing the accumulation void ratio of spherical particles with different particle sizes and improving the solid phase in unit volume of cement slurry is realized by using reasonable particle grading, which effectively improves and improves the comprehensive performance of cement slurry and forms a denser cement stone. The developed ultra-low density cement slurry has the advantages of high compressive strength and rapid development of cement paste, easy to adjust the thickening time of cement slurry, anti channeling and anti leakage, which is beneficial to resist corrosive fluid and increase the sealing quality. The minimum density of field application is 1.10g/cm³.

1.3. Cement slurry system with large temperature difference

Through the design of molecular structure and the mechanism of water loss reduction and retarding, a high temperature resistant water loss reducing agent and a large temperature difference retarding agent were developed, which solved the problems such as the poor ability of water loss reducing agent to resist high temperature and salt, the narrow range of temperature difference for the retarding agent, and the poor stability of the slurry under high temperature. Based on a series of additives with high temperature and large temperature difference resistance, four sets of cement slurry systems with large temperature difference suitable for different temperature ranges (50-120℃, 80-180 ℃, 90-190℃, 70-180℃) are formed through the optimization design of close packing theory. The system has a wide range of applicable temperature, wide range of applicable temperature difference and rapid development of cement slurry column top strength, which is conducive to ensuring the cementing quality of deep well long sealing section with large temperature difference. In recent three years, PetroChina has promoted the application of more than 1000 wells.

1.4 Salt resistant latex cement slurry and supporting additives

In view of the technical problems of high-pressure channeling prevention, salt gypsum layer and salt water layer cementing in complex formation, through the design of latex particle structure and the use of special emulsifier system, the problem of latex stability in salt water and high price ion cement slurry was solved, and the salt resistance of cement slurry reached saturation. Latex can form film in cement, increase gas channeling resistance and improve the anti channeling performance of cement slurry. At present, the latex is resistant to 15% calcium, 30% salt and 180℃.

1.5. Ductile cement slurry

Based on the study of the toughening mechanism of cement stone, the stress model of cement ring is established. On the basis of improving the mechanical properties of cement stone, the developed cement series toughening materials can ensure the construction safety and meet the needs of annulus sealing, forming two sets of ductile expansion cement slurry systems, i.e. medium temperature (30-100℃) and high temperature (100-200℃). The application scope of ductile expansion cement has been extended from gas storage well to shale gas and tight oil and gas well cementing.

1.6. Anti CO²corrosion cement slurry

The underground acid gas will corrode the cement sheath in humid environment, which will reduce the compressive strength of cement stone and increase the permeability, so as to shorten the production life of natural gas wells. Aiming at the problem of cementing in high CO²gas field, the anti- CO²corrosion material was developed, and the anti-CO²corrosion cement slurry system with high temperature (70-180℃) and medium temperature (< 70℃) was formed. The comprehensive cost of the system is low, and the anti-corrosion effect is good, which can effectively guarantee the long-term sealing performance of cement stone under the condition of rich CO.

1.7. Phosphate brine mud

The phosphate cement slurry system was developed in combination with the matched water loss reducing agent and retarder, which provided a new technical way for cement ring anti-corrosion and cementing under high temperature in CO²environment. The cement slurry has good fluidity and channeling resistance, rapid development of compressive strength, good slurry stability and water loss control effect. The thickening time can be adjusted within the range of 50-150℃, with good comprehensive performance. At present, two wells have been tested on site.

1.8. Self repairing and self-healing cement

In order to solve the problem of gas channeling or annulus pressure after well cementation, the research of self repairing and self-healing cement has been carried out in recent years. The expandable core-shell polymer repair agent developed has a temperature resistance of 150℃. The repair agent has the structure of "core-shell" microcapsule, and the double functions of expansion and adhesion. According to the different underground environment, one or two of the effects of activating the expansion and adhesion of the repairing agent play a role at the same time. The repairing agent has the repairing ability to the broken cement stone and has no effect on the construction performance of the cement slurry.

1.9. Low hydration heat and low temperature cement

The main difficulties of cementing in deep water are low temperature on the surface, narrow safe density window, easy pressure leakage of formation, potential shallow water channeling or gas channeling problems, and high requirements for short-term and long-term sealing of annulus. Low density, short transition time, rapid development of compressive strength and low hydration heat in mud line are the requirements of cement slurry system. In recent years, great progress has been made in the laboratory research of deep-water low-temperature cementing slurry system. The hydration heat is more than 35% lower than that of ordinary cement, and other comprehensive properties meet the requirements.

2.  Development of cementing isolation fluid and materials

2.1. High temperature resistant isolation fluid

The key of high temperature resistant isolation fluid is to develop suspension with good performance. Through polymer modification, the temperature resistance and salt resistance of the isolation liquid were improved; the high temperature resistant suspension agent, together with weighting agent and auxiliary agent, was developed, and a 180℃isolation liquid with good compatibility and stability was developed. The system has strong temperature resistance, can maintain stable suspension performance under high temperature conditions, and effectively solve the contradiction between suspension and rheology.

2.2. High efficiency flushing isolation fluid

Aiming at the problem of low displacement efficiency in high temperature deep well and long sealing section under complex well bore conditions, the suspension stabilizer of isolation fluid, oil-based drilling fluid flushing fluid and special weighting materials are developed; the high-efficiency flushing isolation fluid system is developed, which overcomes the problems of poor flushing effect and low displacement efficiency in the cementing process of ultra long sealing section, and creates conditions for good cement slurry cementation. High efficiency flushing isolation fluid has been successfully applied in gas storage, deep well, ultra deep well and shale gas well.

2.3. Oil displacement type high density isolation fluid

In order to deal with the complex salt gypsum layer and the instability of shale gas well wall, high-density oil-based drilling fluid is generally used. By optimizing high-performance wetting reversal agent, a high-density oil displacement type isolation fluid was developed, which solved the cementing problem under the condition of oil-based drilling fluid. Laboratory experiments show that the developed oil displacement type high-density isolation fluid can completely dissolve the oil-based drilling fluid and achieve wetting reversal, which has a good effect of displacement and cleaning the oil-based drilling fluid, and creates conditions for improving the cementing quality under the condition of high-density oil-based drilling fluid.

3.  Difficulties and problems in cementing operation of the company

3.1. The operation object is complicated, and the material requirements are greatly improved

At present, PetroChina's oil and gas exploration and development are faced with "deep (deep), low (low pressure, low permeability, low abundance), sea (Ocean), unconventional (shale gas, tight oil, tight gas, etc.)" oil and gas resources, and a series of problems, such as poor quality of resources, complex objectives, strict safety and environmental protection, are unavoidable.

Coal bed gas well cementing is low temperature, easy to leak out and easy to hurt; large volume fracturing of tight oil gas and shale gas well is very demanding for cementing quality, cement ring sealing and long-term sealing, which puts forward higher requirements for cement slurry system and cementing material performance.

3.2. The problem of annulus pressure is prominent, and the integrity of wellbore seal is increasingly valued

With the increase of the number of natural gas wells drilled, the extension of production time, the acceleration of the construction of gas storage, and the continuous increase of shale gas and tight gas development, the economic and efficient development mode of "horizontal well + volume fracturing" has been promoted, but some wells have problems of annular air channeling or annular pressure, which affects the safety of production. At present, the integrity of wellbore seal and the quality of long-term cementing are paid more and more attention.

3.3. It is difficult to control the activity cost

PetroChina adheres to the development policy of "quality, efficiency and sustainability", strictly controls the investment return standard, and reduces the total investment. Therefore, the cementing industry is naturally affected. The oil companies should strictly control the operation cost, the cementing materials should be of good quality and low cost, and the cementing quality should be strictly guaranteed without increasing the cost, so the difficulty it faces can be imagined.

4.  Future research direction

In recent years, PetroChina has made great progress in the research, development and application of cementing materials and cementing slurry system, but there is still a certain distance for materials suitable for cementing in complex deep wells, ultra deep wells, complex natural gas wells, tight oil and gas wells under large volume fracturing conditions, complex gas storage wells, deep complex salt gypsum wells, low-temperature deep-water cementing in the ocean, etc., with special cement The research of slurry, admixture and high performance multifunctional cement slurry system is lagging behind.

Compared with foreign countries, the current admixture products are single in variety and insufficient in refinement; lack of high-performance high-temperature retarder and loss reducing agent; lack of in-depth research on external admixture, lack of high-performance weighting agent and reducing material; lack of serialization, mainly concentrated in the stage of medium and low temperature, medium and high temperature and general-purpose admixture, lack of ultra-low temperature and ultra-high temperature water Compared with foreign countries, the slurry system has a certain gap in ultra-high density cement slurry, ultra-low density cement slurry and toughness cement slurry.

In order to meet the needs of cementing in the increasingly complex wellbore environment, it is necessary to deepen the research on the mechanism of cementing materials, accelerate the research on new cementing materials, strengthen technical reserves, strengthen independent innovation, focus on solving the key technical problems in cementing production, and improve the guarantee and market competitiveness of cementing engineering technology for exploration and development. The following research work is recommended:

1) Mechanism study

It mainly includes the research on the mechanism of the relationship between cement slurry and cement stone, the research on the mechanical properties and long-term sealing mechanism of cement stone, the research on the mechanism of enhancing the sealing ability of cement sheath, casing and formation, the research on the sealing failure criterion and sealing failure mechanism of cement sheath, the research on the tight packing mode and the interaction mechanism between materials (mainly to achieve the minimum water demand and good tight packing); elastic-plastic Study on the mechanism and modification technology of cement stone (to achieve high strength, low modulus of elasticity, high impact toughness and long-term sealing of the annulus).

2) Perfect cement slurry system and performance

It mainly includes ultra-high density and ultra-low density cement slurry, large temperature difference cement slurry system and series of admixtures, high strength and toughness cement and series of admixtures, anti-corrosion materials and anti-CO2 corrosion cement slurry system, low temperature and high temperature salt resistant cement slurry system and supporting admixtures, amps type fluid loss reducer and corresponding cement slurry system, latex salt resistant cement, etc. A lot of work has been done on the research of the above cement slurry system, and good results have been achieved in the field application. However, it is necessary to further improve the admixture and cement slurry formula, improve the reliability and field applicability.

3) Research and development of new cementing materials and additives

It mainly includes research on high-performance high-temperature retarder, high-performance high-temperature loss reducing agent and stability of cement slurry at high temperature, research on broad-spectrum admixture (mainly refers to the admixture that is not sensitive to temperature, pressure and other conditions), research on anti shrinkage material and micro crack material of cement stone, research on ultra-high temperature material of cement stone, research on self repairing material, research on non cement base material Materials, supramolecular materials, nanomaterials, etc. The above materials have just carried out preliminary research or less research. In order to provide technical reserves for cementing and long-term sealing of complex wells, more research efforts are needed.