2.3 Reservoir Protection Drilling and Completion Fluid

The resources of conventional shallow oil and gas fields in China are gradually decreasing, while deep and ultra deep layers are accompanied by high temperature and pressure in the formation. The use of drilling and completion fluids with high solid content can easily block the oil and gas flow channels, cause reservoir damage, and affect the production of oil and gas wells or the discovery of new oil and gas fields.

2.3.1 Temporary Blocking Type Protection Oil and Gas Reservoir Drilling and Completion Fluid

In order to reduce or avoid damage to oil and gas reservoirs caused by drilling and completion fluids and improve single well production, domestic and foreign scholars have successively established "shielding temporary plugging, fine temporary plugging, and physical and chemical membrane temporary plugging" technology to protect oil and gas reservoir drilling and completion fluids. However, as the reservoir conditions for drilling become increasingly complex, previous protection techniques are difficult to meet the requirements. To address the issue of reservoir damage in tight shale oil and gas reservoirs, Sun Jinsheng et al. elaborated on the basic principles and research progress of reservoir protection technologies such as physical particle temporary plugging, chemical film-forming temporary plugging, underbalanced drilling completion, and interface modification, and provided future development directions for tight/shale oil and gas reservoir protection technologies. Jiang Guancheng and others took the lead in introducing biomimetics into the theory of protecting drilling and completion fluids in oil and gas reservoirs, and developed biomimetic temporary plugging protection technologies suitable for different oil and gas reservoirs, achieving the goal of "zero" damage in different permeability oil and gas reservoirs, greatly improving single well production, and being promoted and applied in major oil fields with significant results.

2.3.2 Liquid Casing Oil and Gas Reservoir Protection Drilling and Completion Fluid

In response to the problem of reservoir damage caused by special oil and gas formations such as "shielding temporary blockage, fine temporary blockage, physical and chemical film temporary blockage, and biomimetic temporary blockage", the drilling and completion fluid technology for protecting oil and gas reservoirs cannot effectively solve the problem of "well collapse, well leakage, and high friction". Jiang Guancheng and others continue to learn from nature and use the invented biomimetic super double sparse agent, biomimetic wall fixing agent, biomimetic bonding lubricant, and other treatment agents. We have achieved innovative results in a new technology for protecting reservoir drilling and completion fluids that can form a "liquid casing" while drilling. Main working principle: During the drilling process, the drilling and completion fluid composed of special treatment agents fills the wellbore of the oil and gas reservoir while drilling, seals the holes-fractures-caves, and strengthens the wellbore, forming a thin, dense, smooth, impact resistant, erosion resistant, and "casing" like high-strength sealing zone, completely blocking direct contact between the drilling and completion fluid and the oil and gas reservoir, to achieve the integrated goal of "protecting oil and gas reservoirs, increasing production" and "preventing and controlling drilling risks, ensuring safe drilling".

Indoor, Jiang Guancheng led team members to conduct a systematic study on the density, sealing strength, wettability, and lubricity of liquid casing, as well as the rheological properties, filtration and wall building properties, inhibition properties, enhanced wellbore capabilities, and oil and gas reservoir protection effects of liquid casing drilling and completion fluids. The results indicate that various performances are superior to previous drilling and completion fluid technologies for protecting oil and gas reservoirs, solving the shortcomings of previous technologies and leading the development of drilling and completion fluid technology for protecting oil and gas reservoirs. The liquid casing oil and gas reservoir protection drilling and completion technology has been successfully applied in over 1000 complex oil and gas wells in areas such as Zhaotong shale gas, Bohai Bay shale oil, Sulige and Songliao Basin tight gas, Jungar Basin tight oil, Shanxi coalbed methane, and other countries such as Chad. Compared with oil-based drilling and completion fluids or other high-performance water-based drilling and completion fluids used in the same block, this technology can reduce the average well collapse accident rate by 82.6%, reduce well leakage by 80.6%, reduce the complexity of sticking and sticking by 80.7%, increase mechanical drilling speed by 32.8%, and increase production by more than 1.5 times, with significant results; After being introduced by Schlumberger Company, it has been applied on a large scale in contracted areas such as Yan'an Baota, Zichang County, Ansai, etc., with an average drilling speed increase of over 30.1% and an output increase of over 1.6 times.

Overall, the drilling and completion of liquid casing oil and gas reservoir protection provides a supporting technology for the construction of China's largest coalbed gas field with an annual production capacity of over 4 billion cubic meters, the largest crude oil production base with an annual production capacity of over 30 million tons - Bohai Oilfield, the Bozhong 19-6 condensate gas field with a reserves of 100 billion cubic meters, the world's largest conglomerate oil field with an annual production capacity of over 3 million tons - Mahu Oilfield, and the Linxing Shenfu large tight gas field with an annual production capacity of 3 billion cubic meters.

2.3.3 Soilless Water-based/Oil-based/Synthetic Drilling and Completion Fluids

Solid phase weighting materials such as barite and iron powder used in conventional deep and ultra deep high-density drilling and completion fluids can easily block oil and gas channels, causing irreversible damage to the reservoir. Solid phase free drilling and completion fluids using soluble salts as weighting materials can meet the weighting requirements of high-density drilling and completion fluids in deep wells, and have good reservoir protection effects. In response to the challenges of easy well leakage and difficult reservoir protection during the drilling process of Block 19-6 in Bozhong, Ma Yingwen et al. constructed a new type of high-temperature resistant and solid-free drilling and completion fluid using polyamine and potassium formate as inhibitors and weighting materials as the main agents. This system can withstand temperatures up to 200℃ and can stably exist for 7 hours in high-temperature environments. The high-temperature and high-density oil-based drilling and completion fluid developed by Sun Jinsheng et al. and Jiang Guancheng et al. has a density of up to 2.7 kg/L and a temperature resistance of up to 260℃. It can also resist contamination by distilled water with a mass fraction of 25% and CaCl₂ solution with a mass fraction of 15%. Compared to traditional oil-based drilling and completion fluids, soilless oil-based drilling and completion fluids have excellent rheological properties due to their absence of organic soil, which is beneficial for improving drilling speed and protecting reservoirs.

2.3.4 Deep Water Constant Rheology Drilling and Completion Fluid

During deep water drilling, drilling and completion fluids face many challenges, such as low mud line temperature and high bottom hole temperature, difficulty in regulating rheological properties, settling of barite, significant changes in equivalent cyclic density (ECD), and potential drilling risks such as wellbore instability and downhole leakage. Therefore, weakening the sensitivity of drilling and completion fluid viscosity and shear force to temperature, and maintaining its rheological parameters basically constant within a certain temperature range, is the key to solving this problem, thus developing a constant rheological drilling and completion fluid. The constant rheological synthetic drilling and completion fluid of MI-Swaco Company relies on rheological modifiers and organic soil to achieve "constant rheological". The constant rheological drilling and completion fluid developed by Halliburton Company uses an amide type rheological modifier instead of organic soil, which can ensure the adsorption of amide groups on the surface of the rubber particles at high temperatures and avoid low-temperature thickening of organic soil. The deep water constant rheological synthetic drilling and completion fluid HEM and FLAT-PRO systems developed by China Oilfield Services Co., Ltd. have exceeded the limit temperature of 180℃, surpassing similar international technologies at that time. Jiang Guancheng et al. established a biodiesel based constant rheological drilling and completion fluid system with a density of 1.2 g/cm3. The system has excellent environmental performance, can maintain constant rheological properties at 2-230℃, and can resist 5% seawater and 10% shale debris pollution, providing technical support for the development of deep water oil and gas resources.

2.3.5 Natural Gas Hydrate Protection Oil and Gas Reservoir Drilling and Completion Fluid

Wellbore stability is the core of safe and efficient drilling of natural gas hydrates, and high-performance wellbore working fluid is the key to maintaining wellbore stability. Considering the effectiveness of hydrate inhibition, environmental protection, and cost control, water-based drilling and completion fluids are still the preferred choice for natural gas hydrate exploration. Sun Jinsheng's team studied the effects of modified starch, sodium carboxymethyl cellulose (CMC), and xanthan gum (XC) under different temperature and pressure conditions on the formation of CH₄ hydrates. The study found that in conventional deepwater oil and gas and hydrate drilling processes, if the downhole temperature and pressure conditions have a strong effect on the formation of hydrates, XC is more suitable for preparing hydrate inhibitory drilling and completion fluids. In addition, to explore the types and mechanisms of damage to hydrate reservoirs, a simulation test method for wellbore working fluid intrusion into hydrate reservoirs was established based on nuclear magnetic resonance technology, exploring the decomposition characteristics of hydrates and the degree of reservoir damage under different liquid phase intrusion temperatures; It is proposed to introduce nano organic silicon materials into natural gas hydrate drilling and completion fluids to improve sealing performance and reduce reservoir damage caused by invasion. Wang et al. developed a composite reinforced wellbore stabilizer TA-PVA for natural gas hydrates. The experiment showed that the compressive strength of the core was increased from 0.22 MPa to 0.45 MPa, and the hydration expansion rate of bentonite was suppressed by 76.36%, effectively improving the stability of the reservoir wellbore. After more than 20 years of rapid development, China has achieved fruitful results in the exploration and development of natural gas hydrates. In 2017, China successfully conducted its first natural gas hydrate trial production in the Shenhu Sea area of the South China Sea, and in 2020, it successfully conducted its second trial production in this area, setting two world records of "total gas production of 86.14×10⁴m³, daily average gas production of 2.87×10⁴ m³" and conquering the drilling and production technology of horizontal wells in deep and shallow soft formations, We have achieved a significant leap from "exploratory trial production" to "experimental trial production".

2.3.6 Intelligent Drilling and Completion Fluid

As drilling targets shift from shallow to deep layer, deep water, from conventional to unconventional (such as shale oil and gas, tight oil and gas, coalbed methane, oil shale, etc.), and from traditional fossil energy to special underground resources (hydrates, dry hot rocks, etc.), drilling engineering faces increasingly complex surface and underground conditions, which brings great blindness to the selection of drilling and completion fluid types, treatment agents, formulas, and performance design. Therefore, designing and adopting drilling and completion fluid technology with intelligent features such as "self identification, self adjustment, and adaptation" helps to fundamentally solve the aforementioned technical challenges.

In 2016, Iranian scholar Ghojogh explicitly proposed for the first time that the application of intelligent materials/fluids in drilling can produce "intelligent drilling and completion fluids", which can help successfully drill in difficult drilling areas where conventional drilling and completion fluids are difficult to handle. In recent years, exploratory research has been conducted on various intelligent drilling and completion fluids with pressure sensitivity, salt response, pH response, temperature sensitivity, shape memory, intelligent temporary plugging, and magnetic response. Jiang Guancheng's team formulated a saturated salt water drilling and completion fluid with two salt responsive zwitterionic polymers, namely high molecular weight low ionic strength (HvL) and low molecular weight high ionic strength (LvH), as the core. Research has shown that salt responsive polymer drilling and completion fluids have excellent salt resistance, thermal stability, and resistance to bentonite and shale cuttings pollution; At the same time, AM-AMPS-TAC polymer with salt response characteristics has been developed, and the drilling and completion fluid prepared has the best salt and temperature resistance, and has low filtration loss. This team also formulated a high-temperature resistant reversible emulsified drilling and completion fluid with a synthesized pH responsive reversible emulsifier RE-HT as the core, which has good performance and a filter cake removal rate of 98.98%. It has good application prospects in complex deep well drilling. Sun Jinsheng and others summarized the technical research and development direction, methods and application prospects of intelligent materials (shape memory alloy, self-healing gel, membrane materials and bionic materials) in the field of drilling and completion fluids. However, at present, intelligent drilling and completion fluids are only in their early stages and still face many problems and challenges, which require joint research by a large number of scientific researchers in the future.

3.Suggestions for Future Development

The National Development and Reform Commission and the National Energy Administration have released the "Action Plan for Energy Technology Revolution and Innovation (2016-2030)", which has included the development of 10000 meter high-temperature and high-pressure oil and gas extraction technology and equipment in the national oil and gas technology major special strategic plan for 2020-2035. Therefore, oil and gas drilling is developing towards deep and ultra deep layers. The development of high-temperature geothermal wells and the need for deep drilling require increasingly high performance requirements such as temperature resistance, environmental protection, and oil and gas reservoir protection of drilling fluids. It is recommended to focus on tackling key issues in the following areas for future deep layer and deep water oil and gas drilling and completion fluids.

(1) . To conquer key materials and technologies for high-temperature, high-pressure, and high salt drilling and completion fluids with temperature resistance of 260~300℃, compressive strength of 2.6~3.0 MPa, and density of over 2.6 g/cm³.

(2) . To conquer polar temperature drilling and completion fluids with temperature resistance less than -30℃ to meet the needs of future ultra-low temperature oil and gas drilling in Antarctica and other areas.

(3) . To do research on deep-water constant rheological environmentally friendly drilling and completion fluids, environmentally friendly natural gas hydrate drilling and completion fluids suitable for horizontal wells, and hydrate horizontal well wall stabilization technology.

(4) . To develop drilling and completion fluids that form high-quality liquid casings while drilling. Using the chemical materials in the drilling and completion fluid to form a high-strength sealing zone resembling a "casing" on the wellbore while drilling, sealing all holes-fractures-caves to meet the needs of safe, efficient, and environmentally friendly drilling.

(5) . Integrated leakage prevention and plugging in fractured and malignant formations. By increasing the density of drilling and completion fluids, strengthening the sealing and anti collapse performance, developing adaptive anti leakage and plugging materials, expanding the safe density window, and achieving efficient plugging.

(6) . To develop intelligent drilling and completion fluids and intelligent reservoir protection technologies based on intelligent materials, big data, cloud computing, 4G/5G networks, artificial intelligence, and other technologies, enabling them to have the functions of "self identification, self adjustment, adaptation, self transmission, and self maintenance", achieving the goals of "safety, efficiency, economy, environmental protection, and intelligence".

(7) . To develop ecological and environmentally friendly drilling and completion fluids. Developing eco-friendly drilling and completion fluid treatment agents using natural biodegradable materials, constructing an eco-friendly drilling and completion fluid system, and truly achieving coordinated development between the petroleum industry and environmental protection.

4.Conclusion

Although deep layer and deep water oil and gas resources are very abundant, they face unprecedented technological challenges. Drilling and completion fluids are the core of ensuring safe, efficient, and economical drilling. In the future deep layer and deep water drilling process, drilling and completion fluid technology will inevitably face many unknown challenges. It is necessary to start from key scientific issues, utilize the latest basic theories developed by other disciplines, conduct interdisciplinary research, and develop original drilling and completion fluid technology. Through a deep analysis of the development history and principles of drilling and completion fluid technology at home and abroad, it can be concluded that vigorously developing liquid casing drilling and completion fluids, intelligent and ecological drilling and completion fluids, is one of the important development directions in the future. It can provide strong technical support for breaking through the "ice and fire dual sky" of deep water drilling and completion fluids and for deep water oil and gas exploration and development.