PetroChina started overseas oil and gas business in 1993, after over 20 years of hard work from basic development, to scale development and to optimized development, it has made brilliant achievements in this sector. PetroChina has built five major oil and gas cooperation zones in Central Asia-Russia, the Middle East, Africa, the Americas and the Asia Pacific, basically completing the strategic deployment of global oil and gas business^[1]. At present, PetroChina’s overseas oil and gas business includes management and operation of 88 oil and gas cooperation projects in 32 countries, including 58 oil and gas development projects, accounting for 2/3 of the total, and an annual oil and gas production capacity close to 2×10⁸ t. During the expansion of overseas business over 20 years, overseas oil and gas development technology has evolved from integrated application of domestic technologies to integrated innovation and to research and development innovation. In the course, a technology series for overseas oil and gas field development represented by high speed development of sandstone oilfield by natural depletion, overall optimization of development deployment of carbonate oil and gas fields, and cold recovery development of foam oil with horizontal well for ultra-heavy oil reservoirs has been established^[1,2]. This series of technologies have greatly improved the technical core competitiveness of PetroChina, and enabled PetroChina to avoid and reduce the risk of overseas investment to the greatest extent. With the sound support of these technologies, PetroChina's overseas oil and gas business has developed at high quality continuously and made huge economic benefits.

With the continuous changes in the internal and external situation of overseas business, overseas oil and gas development also faces a series of problems and challenges: (1) Most of the sandstone oilfields have entered the middle and late stage of development after high-speed development, facing the challenge of "double highs" (high water cut of more than 80%, and high recovery degree of more than 60%), so it is necessary to study development adjustment strategy and secondary development technology series for overseas high water cut sandstone oilfields^[1,2,3]. (2) The contradiction of waterflooding development in carbonate oilfields is prominent, making it difficult to keep stable production in these oilfields, so it is urgent to research high efficiency water and gas injection technology for large carbonate oil and gas reservoirs to enhance their oil recovery^{[1-2, 4]}. (3) Overseas projects under construction and to be constructed are mainly oil sand, shale gas, deepwater, polar and liquefied natural gas (LNG) projects, which belong to unconventional and new business fields, with high technical difficulty, high requirements and little mature domestic technology and experience to draw on, so it is necessary to develop economic and efficient development technologies for unconventional and deep-water oil and gas^[1,2]. It is an urgent task to develop a series of oil and gas development technologies suitable for characteristics and development stage of overseas oil and gas fields.

Over the past 20 years, accompanying the development of overseas oil and gas business, the development technology for overseas business has passed 3 stages from direct application of domestic mature development technology, to integrated innovation of overseas development technology, to research and development of overseas characteristic development technology. In each stage, the characteristics and key points of overseas development technology are different.

(1) Direct application of mature development technology in China (1993-1996). At this stage, the overseas business just started and we had no transnational production and management experience, the overseas oil and gas development was mainly concentrated in the field of tapping residual oil in conventional sandstone old oilfields. For example, mature development technology of domestic sandstone oilfields were applied in Talala, 6/7 block of Peru, Acheson of Canada, Caracoles of Venezuela and other old oilfields, opening the development and production of overseas oilfields from scratch.

(2) Integrated innovation of development technology suitable for overseas oilfields (1997-2008). In this stage, as a number of representative projects such as Sudan 1/2/4 area, Sudan 6 area, Sudan 3/7 area and Aktyubinsk of Kazakhstan were put into development consecutively, based on the integration and application of domestic mature technologies, overseas development technologies were innovated vigorously, giving rise to a series of oil and gas development technologies suitable for overseas oil and gas fields, for example, the high-speed development technology for overseas sandstone oilfield by natural depletion, the development technology for abnormal high pressure and ultra-low permeability carbonate reservoir, the waterflooding development technology for carbonate oilfield with condensate gas cap, the waterflooding development technology with horizontal well for thin carbonate oilfield, and the high-efficiency development technology for large high condensate oil reservoir^[1,2,3,4,5].

(3) Research and development of characteristic overseas development technology (2009 to now). As the overseas oil and gas business expanded to large carbonate oilfields in the Middle East, complex carbonate gas fields in Central Asia, oil sands and shale gas in Canada, coalbed methane in Australia, Arctic LNG and deep-water oil and gas etc, through years of research on key bottleneck technologies restricting the development of overseas main business, a number of characteristic technologies, including overall optimization of development deployment technology for large carbonate reservoirs, high efficiency development technology for edge and bottom water carbonate gas field, collaborative development technology for gas cap and oil ring in carbonate reservoir with condensate gas cap, and cold recovery technology of foam oil with horizontal well for ultra-heavy oil reservoir have been established^{[1-2, 4, 6-7]} .

In more than 20 years of oil and gas overseas development practice, PetroChina has innovated a series of characteristic development technologies by combining overseas oilfield characteristics with domestic mature oil and gas field development technologies (Table 1). Among them, the most representative ones are high speed development technology by natural depletion for overseas sandstone oilfield, overall optimization of development deployment technology for large- scale carbonate reservoir, high efficiency development technology for edge and bottom water carbonate gas field group, cold recovery technology of foam oil with horizontal well for ultra-heavy oil reservoir, staged volume fracturing technology in horizontal wells for shale gas, and prediction of favorable coalbed methane reservoir and development technology with SIS horizontal wells (horizontal well connect another vertical well at toe end in coal seam from ground to coal seam). In the following section, the main characteristic innovative technologies and their application status will be elaborated.

The overseas sandstone oilfields represented by Sudan project are characterized by sufficient natural energy, good reservoir physical properties and oil properties, and extremely high investment environment risk^[1,2,3]. Limited by contract term and investment risk, many mature waterflooding development technologies for domestic sandstone reservoirs can’t be applied to overseas sandstone reservoirs^[8]. In view of this problem, based on in-depth study on the mechanism of high- speed development, five main control factors affecting the high-speed development by natural depletion, namely, the size of natural water body, crude oil flow capacity, difference between formation pressure and saturated formation pressure, effective thickness of reservoir, development technology policy, have been sorted out, quantitative analysis methods for them have been put forward^[3], overseas sandstone oilfield high-efficiency development mode, which can "make full use of the natural energy, delay water injection, recover investment rapidly, avoid investment risk", and technical policy of "sparse high-yield wells, large section commingled production, and large differential pressure production" have been innovatively established^[3]. At the same time, the major development indicators such as 500-1200 m spacing of initial well pattern, 140-200 t/d single well prorated production, 2.0%-2.5% peak production speed were made (Table 2). According to the permeability and oil properties of reservoirs in Sudan Oilfield, the fluidity matrix table has been established to classify the crude oil, then to work out well pattern density corresponding to the crude oil type (Table 3), and the well pattern and well spacing infilling technology enabling full use of natural energy has been created^{[3, 9-10]}. The wide application of high- speed natural energy development technology in overseas sandstone oilfields not only strongly supported the rapid production increase of two main projects in Sudan to 1500 × 10⁴ t/a, but also has supported the high-speed and efficient development of other overseas sandstone oilfields.

Taking Iraq and other large-scale bioclastic limestone reservoirs as research objects, by subdividing pore structure types, the relationship between micro-pore throat type and lithofacies has been established^[2] (Table 4) to solve the problems such as difficult prediction of high-quality bioclastic limestone reservoir and poorer development effect due to extremely strong heterogeneity^{[1, 4, 11-12]}, and 3D reservoir modeling with dual control of seismic and sedimentary facies based on the micro porosity-permeability relationship has been established, realizing integrated facies constraint modeling of bioclastic limestone reservoir using multiple kinds of data^{[1-2, 4]}. As Ahdeb and Halfaya carbonate oilfields are large in scale, have multiple oil-bearing layers and interlayers, and great reservoir difference, the development model of whole injection-production pattern of parallel horizontal wells for thin layer bioclastic limestone reservoir, the injection-production pattern of highly inclined horizontal wells as producers and vertical wells as water injectors for the hugely thick bioclastic limestone reservoir have been established^{[1, 4]} (Fig. 1), and the three-dimensional well pattern with the backbone well pattern in the main reservoir matching with the branch well pattern in the second main reservoir as has been worked out to solve the problems of complex spatial structure of well pattern of underground multi-layer systems and the limitation of ground facility and security. To balance between the rapid overall production construction and the best economic benefits of the oilfield, the multi-objective collaborative optimization technology of "production rate + investment scale + incremental benefit" has been innovated^{[1, 4]}. The deployment strategy of "laying horizontal well pattern in place in one step, and producing advantageous resources in priority, and resources on both wings steadily" has been worked out for Ahdeb oilfield. For Halfaya oilfield, the development strategy of "overall deployment, effective replacement by zones and layer series, producing high-yield layers in priority, making breakthrough in the center first, and expanding to both wings” has been made to save early investment and build the initial commercial production capacity in the shortest time with minimum investment, so as to realize the rolling development of the oilfield itself and maximize economic benefits. By using these technologies, Iraq's Ahdaeb project reached peak production capacity of 700×10⁴ t/a three years ahead of schedule, and the internal rate of return has increased by more than 3%; the Halfaya project rapidly built an annual crude oil production capacity of 2000×10⁴ t/a, and the investment in the construction of one million tons of production capacity was about 21×10⁸ yuan, setting a new low of the investment scale in constructing one million tons of production capacity in overseas projects of PetroChina. As a result, the regional crude oil production in the Middle East soared from 358×10⁴ t/a in 2010 to 8 000×10⁴ t/a in 2018.

The development object of the project on the right bank of Amu Darya River in Turkmenistan is complex marine carbonate gas reservoir, which is characterized by generally developed fractures, strong reservoir heterogeneity, complex gas water relationship, and active water bodies in some parts, making the deployment of well pattern and efficient development difficult^[1,2]. As the project has dozens of gas fields different in reserve scale and disperse in geographical location, and is limited by the product sharing contract term, the project faced great challenges in realizing orderly replacement and stable gas supply^[4]. To solve the above problems, from two aspects of single gas field and gas field group, a multivariable mathematical model considering factors such as gas reservoir structure, reservoir, fracture and water body was established with financial net present value as objective function to optimize key parameters such as well pattern and inclined well section length, distance of perforation to water layer and total well number simultaneously. Moreover, optimized overall fracture pore type edge bottom water gas reservoir development technology with highly deviated well has been created^{[1, 4]}. At the same time, on the basis of revealing the quantitative relationship between the gas production rate and the recovery degree of gas field at the end of stable production period, the overall optimization development model of gas field group considering the product sharing contract mode has been established. The improved genetic algorithm is used to solve the model, to get the optimal production sequence and production capacity scales of the gas fields in the group, that is, the overall collaborative optimization development technology of the gas field group based on the product sharing contract mode^{[2, 4]}. The high efficiency development technology for edge and bottom water carbonate gas field group has been fully applied to the production capacity construction on the right bank of Amu Darya River, realizing high efficiency development of the gas field and achieving good economic benefits. Compared with the conventional optimization technology of highly deviated well pattern, the productivity of the main Bereketli-Pirgui gas field increased by 20%, the total drilling footage reduced by 13%, and the financial net present value increased by 11%^[13]. The whole project on the right bank of Amu Darya River has built a natural gas production capacity of 170×10⁸ m³/a, kept stable production for 15 years, making outstanding contributions to the realization of diversified energy import, energy security and the construction of green China.

The heavy oil in the middle and deep layers of Orinoco belt in Venezuela is flowable and features "four highs, one low", high density (of 1.007-1.022 g/cm³), high asphaltene content (9%-24%), high sulfur content (hydrogen sulfide content of greater than 3.5%), high heavy metal content (greater than 500 mg/L), and relatively low viscosity (1000-10 000 mPa•s) underground. In the process of cold recovery, the crude oil must be flowable^[14]. Through physical simulation experiment of foam oil displacement, the mechanism of foam oil displacement has been revealed^{[1, 4, 15-16]}. Foam oil contains a large number of dispersed microbubbles, which can stay in the oil phase for a long time, significantly increasing the compressibility of fluid and the elastic driving energy. During the cold recovery process, foam oil flow can be formed under certain conditions, so cold recovery has quite high productivity, slow drop of reservoir pressure, and higher recovery factor (over 12%). A multi-component numerical simulation method was used to reconstruct the dynamic process of formation, rupture and coalescence of dispersed bubbles in foam oil (Fig. 2) and the three-segment development characteristic with bubble point pressure and pseudo bubble point pressure as dividing points^{[6, 17]} (Fig. 3). The initial foam oil productivity prediction formula and dimensionless IPR (inflow dynamic curve) model^[18] of horizontal well have been established to determine the policy boundary of cold recovery development technology for ultra-heavy oil reservoir, and to optimize horizontal well development design^{[4, 17]}. Finally, a whole set of cold recovery technology with horizontal well for ultra-heavy oil reservoir has been innovated. The application of this set of technology has enabled the economic and efficient development of ultra-heavy oil reservoirs in Venezuela, the Venezuela's MPE3 project built an annual heavy oil production capacity of 1000×10⁴ t in 2016, including 25 drilling platforms, 423 horizontal wells, with an average initial production of more than 100 t/d per well, and realized sustainable benefit development under the production scale of 1000×10⁴ t/d. From 2016 to 2018, PetroChina had a net profit of 47×10⁸ yuan, with an average unit operating cost of 130 yuan/t, achieving remarkable economic benefits.

PetroChina plans to keep the production of overseas oil and gas business at more than 2×10⁸ t annually in the future, of which production of carbonate reservoirs accounts for about 60%, that of sandstone reservoirs accounts for 30% and that of unconventional reservoirs 10%^[1]. However, to keep stable production, increase production and realize economic and effective development still faces a series of problems and challenges: it is difficult to stabilize oil production, control water cut, enhance oil recovery in old sandstone oilfields; it is difficult to maintain stable production in carbonate oil and gas fields; it is difficult to predict "sweet spots" in unconventional oil and gas reservoir development; and it is difficult to develop ultra-heavy oil and oil sand economically and effectively. Therefore, it is urgent to innovate and develop a series of suitable development technologies, including: technology to stabilize oil production, control water cut and enhance oil recovery (EOR) for old oilfields with high water cut; waterflooding/gas drive development and oil recovery enhancement technology for carbonate reservoir; key efficient development technology for complex carbonate gas reservoir; development technology for basement buried hill complex reservoir; sweet spot prediction and high economic efficiency development technology for coalbed methane, tight gas and shale gas; cold recovery, stable production and development effect improvement technology for ultra-heavy oil reservoir; and oil sand SAGD (steam assisted gravity drainage) high efficiency development technology etc^[1].

In the future, the overseas oil and gas development business will focus on four major areas: conventional sandstone reservoir on land, large-scale complex carbonate reservoir, unconventional reservoir and deep water reservoir. The key points are to speed up the production of large-scale carbonate reservoirs in the Middle East and maintain stable production, slow the production decline of carbonate reservoirs in Central Asia, improve the recovery rate of old sandstone oilfields with high water cut in Central Asia and Africa, promote the efficient development of different types of complex gas fields, and strengthen the research of economic and effective development technology for unconventional oil and gas, so as to keep the overseas oil and gas business production at more than 2×10⁸ t annually. Therefore, it is necessary to innovate a series of characteristic technologies to meet the needs of overseas oil and gas development, with the goal of^[1]: keeping the oil recovery enhancement technology for high water cut sandstone oilfield at the international leading position, making carbonate oil and gas reservoir development technology catch up the international advanced level; narrowing the international gap with and even overtaking the international level in unconventional and deep water oil and gas development technology.

4.2.1. Artificial intelligence reservoir prediction and 3D geological modeling technology

The traditional reservoir prediction and 3D geological modeling technology can’t meet the requirements of overseas geological research in pursuit of high-speed, high-efficiency and high-precision. The rapid development of artificial intelligence technology makes it possible to do high-speed, high- precision reservoir prediction and 3D geological modeling in the future. With the support of new information technology, we can efficiently and accurately collect all kinds of data needed using learning knowledge annotation and extraction methods such as big data, text analysis, image recognition, and knowledge mapping, etc^[19,20]; we can process data, curves and various geological information of specific area quickly by using machine learning method^[21]; we can correlate oil and gas reservoirs by using logging and seismic data and interpret and predict reservoirs intelligently through machine learning and deep learning and other technical means^[22]. We will do intelligent evaluation, diagnosis, prediction and optimization of reservoir prediction and 3D geological modeling through analogy with similar reservoirs to realize high-speed, high-efficiency and high-precision reservoir prediction and 3D geological modeling.

4.2.2. Secondary development and EOR technology for sandstone oilfield after high-speed development

In order to quickly recover investment after acquisition by PetroChina, high-speed development has been generally adopted in overseas sandstone oilfields. At present, these oil fields have entered the late stage of "double-high" development with high water cut and high recovery degree in general, facing problems such as rapid rise of water cut, high comprehensive water cut, high recovery degree, low reservoir pressure maintenance level and high decline rate^{[1, 3]}. Moreover, in these oilfields, the injection production well pattern on the plane is imperfect, and general water waterflooding mode is adopted vertically, so the control degree and producing degree of waterflooding reserves are low^[3]. In order to effectively stabilize oil production and control water cut, further improve oil recovery and meet the demand of continuous and efficient development of sandstone oilfield, the development technology for overseas sandstone oilfield will go towards the following two directions in the future.

(1) Artificial waterflooding and natural water drive collaborative development technology. After the high-speed development of natural energy, the distribution characteristics of remaining oil in the overseas sandstone oilfields are quite different from those in domestic early waterflooding oilfields. Therefore, it is necessary to find out the distribution law of remaining oil in these oilfields after high-speed development and work out a quantitative description method, explore evaluation method of natural energy intensity, and make the adjustment strategy and technical policy of the collaborative development of natural water drive and artificial waterflooding. On this basis, we will develop quantitative description and local infilling adjustment technology to tap remaining oil after high-speed development, and innovate a set of collaborative development technology of natural energy drive and artificial waterflooding.

(2) Secondary development technology for overseas oilfield. Referring to the concept of secondary development and adjustment of domestic old oilfields, the concept and method of secondary development of overseas old sandstone oilfields are established with the aim of deepening understanding on the underground reservoir, transforming the development mode, optimizing the process technology and strengthening the technical and economic evaluation^[3]. Since the overseas old oilfields are mainly developed by natural depletion or general waterflooding currently, the working thought of "overall control, mode conversion, well pattern reorganization, layer system subdivision, water shut-off and displacement control, overall optimization" is to be substantiated to form the development and adjustment strategy and secondary development technology series for overseas old sandstone oilfields.

4.2.3. Oil recovery enhancement technology by water or gas injection for overseas carbonate reservoir

As the main oil and gas reservoirs developed overseas turn from sandstone reservoirs to carbonate reservoirs, the output of overseas carbonate reservoir will make up for over 60% of the total^[1], and carbonate reservoir will become the main expansion field and core of overseas development business in the future. The overseas carbonate reservoirs represented by those in the Middle East have large differences in geology and reservoir characteristics from carbonate reservoirs in China, which are characterized by large scale, huge thickness and strong heterogeneity^[1,4]. With early construction of production capacity completed, these reservoirs are now facing with the problem of increasing and stabilizing production. Therefore, the research and development of water and gas injection technology to improve oil recovery for overseas carbonate reservoir will become the future research direction. The research on these technologies will focus on three aspects:

(1) Quantitative evaluation of reservoir heterogeneity and integrated 3D modeling technology for carbonate reservoir. As it is difficult to quantitatively describe and delineate spatially the heterogeneity characteristics of Iraq's large-scale bioclastic limestone reservoir, it is urgent to find out the causes of the heterogeneity of the bioclastic limestone reservoir and evaluate the microscopic pore structure of the reservoir, establish the logging quantitative identification standards and methods of different types of rocks^{[1, 4]}, find out the origins of and work out quantitative identification methods for the high-permeability layer, look for methods to identify quantitatively and predict distribution of the interlayer at multi-scale, and make clear the spatial distribution law of high permeability layer, so the heterogeneity of large-scale bioclastic limestone reservoir in Iraq can be quantitatively evaluated and modeled in 3D.

(2) High efficiency waterflooding development technology for bioclastic limestone reservoir. In the future, overseas carbonate reservoirs need to maintain stable production for a long time by means of waterflooding. However, the contradiction between waterflooding pressure recovery and rapid water cut increase of horizontal well in thin-layer bioclastic limestone reservoir is increasingly prominent, while high-efficiency waterflooding of huge thick bioclastic limestone reservoir is a world-class problem. Therefore, it is urgent to study the overall waterflooding, oil stabilizing and water control, and comprehensive adjustment technology with horizontal well for thin-layer bioclastic limestone reservoir, study high-efficiency waterflooding development technology for huge thick bioclastic limestone reservoir in the future^{[1, 4, 23-24]}, and work out the injection-production mode, zoning and layered waterflooding development technology policy for the huge thick bioclastic limestone reservoir and high-permeability layer^{[1, 4, 25-27]} to ensure high efficiency utilization of the limited water resource and high efficiency waterflooding development and enhance sweep efficiency and waterflood development effect of huge thick bioclastic limestone reservoir.

(3) Water and gas injection adjustment and EOR technology for complex carbonate reservoir with gas cap and oil ring. After years of oil-ring waterflooding and gas cap production, the complex carbonate reservoirs with gas cap and oil-ring in Central Asia are facing a series of problems like low formation pressure level, low reservoir production degree and difficult collaborative development of gas cap and oil ring, it is necessary to further develop three-dimensional modeling and remaining oil quantitative evaluation technology for dual- medium carbonate reservoirs^[28]; figure out main factors affecting the development effect of waterflooding and gas flooding, formulate development technology policy of waterflooding and gas flooding for dual-medium reservoir^[29], and find new adjustment technology and development mode of waterflooding and gas flooding for complex carbonate oil and gas reservoir with gas cap and oil ring in the middle and late development stage^[30]. At the same time, the adaptability of EOR technologies, such as alternate gas and water flooding and surfactant flooding, need to be studied, to create a set of EOR technologies for complex oil and gas carbonate reservoir with gas cap and oil ring in the middle and late development stage.

4.2.4. Economic and effective development technology for overseas unconventional oil and gas

Unconventional oil and gas resources are rich in world, accounting for 1/3 of the total resource amount. With newly added reserves accounting for about 45% of total in the world^[31,32], unconventional oil and gas resources have become an important supplement to conventional oil and gas resources and an important field of resource allocation of major oil companies. At present, PetroChina has many large unconventional oil and gas projects overseas, involving heavy oil, oil sand, tight gas, shale gas, etc. with remaining recoverable reserves of more than 20×10⁸ t. But due to lack of low-cost and efficient development technology, the development of some overseas unconventional oil and gas is in a state of loss or stagnation. Therefore, it is urgent to innovate economic and effective development technology for overseas unconventional oil and gas. The technologies need to be developed are in three aspects:

(1) The technology to stabilize and optimize production and enhance recovery in cold recovery for ultra-heavy oil reservoir. With the continuous horizontal well cold recovery, the ultra-heavy oil reservoir in Venezuela is witnessing formation pressure drop, production decline, and rise of GOR, so the cold recovery gradually gets worse in development effect, decreases in potential, faces big challenges in keeping stable production. It is necessary to further study distribution law and quantitative description technology of remaining oil in horizontal well cold recovery, and formulate development technology policy stimulating foam oil flooding, and develop secondary foam oil non thermal recovery, pressure keeping and single well production enhancement technology for ultra-heavy oil reservoir^[1], to create a set of technologies to stabilize and optimize production in cold recovery for ultra-heavy oil reservoirs, which is mainly based on infill drilling and pressure maintaining technology^[6]. Meanwhile, we should actively study the EOR technology of thermal recovery after cold recovery, especially the new generation of steam flooding and in situ combustion technology^[7], pay attention to the research of EOR technology of injection of non- thermal medium, especially the injection of miscible gas, non-condensate gas, polymer, and chemical agent etc^[7], to build up a set of EOR technology for ultra-heavy oil reservoir in the late stage of cold recovery.

(2) The economic and effective development technology to enhance the SAGD effect and oil recovery of oil sand. At present, the SAGD development technology is applied by PetroChina in Canada's oil sand project, which has high cost and poor efficiency. How to improve the development effect and enhance recovery effectively and economically of SAGD are problems commonly recognized in the oil industry. The main research directions include: research on multiple thermal fluids (N₂, CO₂, steam) assisted SAGD and superheated steam SAGD^{[7, 33]} to improve development effect, and research on the new generation of in situ combustion technology^[7,34], so as to greatly improve the recovery of oil sand.

(3) Sweet spot prediction and high-precision intelligent staged fracturing technology through horizontal well for tight oil and gas reservoir. Unconventional tight oil and gas reservoirs, including shale oil and gas and coalbed methane, are characterized by large-scale continuous distribution, integration of source and reservoir, formation control of oil and gas, huge resource scale and no obvious boundary of oil, gas and water. These reservoirs are mainly developed by selecting "sweet spot section", horizontal well, and fracturing, so they are typical "artificial oil and gas reservoirs"^[35,36]. Therefore, it is necessary to develop multi-parameter comprehensive prediction technology for sweep spot section and low-cost high- precision intelligent staged fracturing technology through horizontal well^[37,38]. The fracturing technology enables reconstruction of the stress field, temperature field, chemical field and seepage field and formation of "artificial high permeability area through integrated fracturing, injection and production^[35,36].

4.2.5. Supporting technologies for efficient development of deep-water oil and gas reservoirs in the sea area

The marine oil and gas resources are rich globally, and low in overall exploration and development level. At present, the recoverable reserves of remaining oil and gas in the global waters are 1840×10⁸ t, and the resources to be discovered are 1293×10⁸ t, accounting for 43.7% and 42.2% respectively^[31,32]. Deep and ultra-deep waters are hot areas, and the ocean is the main battlefield for resource allocation of major international oil companies. However, PetroChina has little technology for and experience in deep-water oil and gas reservoir development. In the face of more complex geological conditions, less data, larger well spacing and more difficult marine engineering, it is urgent to develop a series of low-cost development supporting technologies for deep-water and ultra-deep water reservoirs, mainly including deep-water ultra-deep water gravity flow reservoir characterization and prediction technology, quantitative prediction technology of remaining oil distribution at large well spacing, optimization of integrated well arrangement in deepwater oil and gas reservoir and offshore engineering, high-efficiency development technology policy, development adjustment and EOR technology, development strategy and optimization technology, offshore automatic drilling and completion technology, and deepwater oil recovery technology for deepwater oil and gas field^[39,40], so as to make overseas deepwater oil and gas development technology of PetroChina catch up with the world's advanced level as soon as possible.

Looking back, PetroChina's overseas oil and gas business has achieved a leap forward development from scratch, from small to large, from weak to strong in the development course of more than 20 years. Meanwhile, overseas oil and gas development technology has also gone through the development process from integration application of domestic technologies, integrated innovation, research and innovation, with a series of characteristic technologies for overseas oil and gas field development building up, represented by the high speed development technology for sandstone oilfield, the optimization of overall development deployment technology for carbonate oil and gas fields, and the foam oil cold recovery technology with horizontal well for ultra-heavy oil reservoirs, which have greatly enhanced the core competitiveness of PetroChina and provided sound technical support for the rapid expansion of PetroChina’s overseas oil and gas business.

Looking to the future, the overseas oil and gas business is facing more complex and changeable cooperation environment, and thus higher challenge to achieve high-quality and efficient development. So, it is necessary to give full play to the important supporting role of scientific and technological progress in the development of overseas business. In the future, the overseas oil and gas development business needs to carry out continuous scientific research on the short board and bottleneck technologies, to maintain the international leading position in the technology of stabilizing oil production and controlling water cut and enhancing oil recovery in high water cut sandstone oilfield, reach the international advanced level in the technology of waterflooding and gas flooding to enhance oil recovery in the carbonate oil and gas reservoir, and quickly catch up the international level in the technology of unconventional and deep-water oil and gas development in the sea area, so as to provide strong technical support and guarantee for high-quality development of overseas oil and gas business of PetroChina.