Workover operation involves the whole lifecycle of oil and gas field development, and is the core of the routine production, management, optimization and maintenance of oil and gas wells. Workover involves a tremendous business system, with overwhelming tasks, and huge team size. The total number of oil, gas and water wells for the PetroChina Co. Ltd. (hereafter referred to as PetroChina) is 36×10⁴, consuming a great amount of manpower and material resources every year on workover operation. Workover operation is an important aspect of maintaining normal operation and stabilizing and increasing production in oil and gas fields.

It is estimated that in the near future, with mature oilfields of China stepping into the ultra-high water cut stage, efficient utilization of low-abundance reservoirs would still be the main contributor for production capacity building, and meanwhile production per well constantly drops, which inevitably results in the increasing number of wells. Therefore, support and guarantee provided by workover, in terms of production stabilization and growth, cost saving and efficiency enhancement of oil and gas fields, gain increasing importance. This paper, by analyzing nature of workover technologies and summarizing key elements of workover technologies, systematically demonstrates the development status of the workover technology. Moreover, by comparing technologies of PetroChina with the equivalent ones in foreign companies, the future development direction of workover technologies has been proposed.

The workover technology is the technical practice in which technical professionals carry out repair and maintenance work of the wellbore, with specialized equipment and tools, in accordance with technical plans dependent on the reservoir rock property and status of the oil, gas or water well.

In order to ensure the normal production of oil, gas and water wells, workover is usually required to accomplish tasks such as “wellbore repair, dealing well downhole malfunction and adjustment of producing layers”. The scope of work includes^[5,6,7]: (1) completion operations such as wellbore cleaning up, pressure testing, fluid displacement, perforation and running production string; (2) running the tubular string with isolation tools to implement zonal production and injection; (3) pump-inspection operations such as running and retrieving of production tubing, pump rods and pumps; (4) supporting operations for acidizing and fracturing, including perforation and replacement of tool strings; (5) operations dealing with downhole malfunction such as sand plugging removal, removal and prevention of wax, sticking release and fishing; (6) operations to confirm channeling and identify leakage such as packer setting confirmation and casing damage inspection; (7) operations for pump replacement and parameter adjustment such as pump inspection, pump hanging depth adjustment and conversion from the producer to injector; (8) production testing operations to determine production capacity of oil and gas wells; (9) casing damage repair operations such as casing damage sealing, running smaller casing, casing reshaping, casing patching and casing replacement; (10) operations for potential tapping in old wells such as window-milling side tracking; (11) plugging and abandonment operations of oil, gas and water wells such as disposal treatments.

With demand for production stabilization and growth of oil and gas proposed by the national energy security, the production task is becoming increasing challenging. The number of wells grows, which is accompanied by the growingly heavy load of workover. Meanwhile, the nature of workover continuously evolves, and compared with the conventional operation, it is characterized by “high frequency of routine work, high labor intensity, extensive operation, and high environmental risks”. In the context of the current new times, new characteristics of workover have been expanded or emerged, mainly in eight aspects, namely “mission, production capacity, safety, wellbore, technology, quality, environment and development”.

(1) Mission: By the end of 2018, China's dependence on imported oil and gas has reached 69.8% and 45.3%, respectively^[8], and moreover it presents a tendency of continuous growth. Stabilizing and restraining such importing dependence has become the responsibility of the whole industry. Workover is an important aspect to ensure the normal operation and stable production as well as production growth of oil and gas fields. Safeguarding the national energy security is the new mission of workover operation development.

(2) Production capacity: Production per well continues to decline, and the number of oil, gas and water wells climbs up year by year. Under such circumstances, the volume of workover operation becomes increasingly large (Fig. 1), and workover operation regards maintaining production as its fundamental purpose. In 2018, PetroChina recovers production capacity more than 52×10⁴ tons through major repairs, which indicates the significance of workover operations in oil and gas production.

(3) Safety: Workover operations are typical high-risk operations, and safe practice is a basic requirement for workover operations. With increasing cases of complex well conditions such as high temperatures, high pressures and high sulfur contents, special operations such as snubbing and coil tubing operations grow, and unknown and concealed factors become prominent. Standardization of operations are difficult to achieve, and efforts of safe operation is constantly facing new challenges. Higher requirements are raised up upon workover equipment, tools, technologies and management.

(4) Wellbore: With deterioration of quality of oil and gas resources and complication of reservoir characteristics, the wellbore structure and condition is becoming increasingly complicated, and various targets have emerged for workover operations. From shallow wells to deep wells, from vertical wells, to directional wells and then horizontal wells, from low-pressure wells to meddle-high-pressure wells, from low-temperature wells to high-temperature wells, from conventional environments to highly-acidic environments, workover operations are doomed to more and more challenges.

(5) Technology: The workover technology constantly progresses and so does the technical capability, which is accompanied by continuously expanded scope of work. The technical development can be roughly divided into four stages. In 1955-1990, workover is dominated by maintenance work, and the auxiliary technical system such as routine pump inspection and sticking releasing is developed. In 1991-2000, workover operations are mainly repair-oriented, and technologies for directional well servicing, casing damage repair and treatments technologies for complex wellbore conditions are seen with rapid development. In 2001-2010, workover is represented by comprehensive workover, and innovation is seen in operation in a snubbing manner, complex casing-damage well treatment and “high temperature, high pressure and high sulfur content” well repair. Since 2011, workover is oriented at green high-efficiency operation, and development focuses on improving environmental friendliness, long horizontal wellbore operation and coil tubing operation.

(6) Quality: With growth in the number of wells each year, the annual workload has remained steady with slight declining, and quality has become a key indicator of workover operations. In 2018, the number of new oil, gas and water wells in PetroChina is 1.1×10⁴, while the maintenance workload is reduced by 606 times, compared with that in the previous year. The average annual maintenance operation per well was 0.401 times, decreasing by 0.016 times, and for pumped wells, the pump inspection cycle is 838 days, extended by 10 days.

(7) Environmental characteristics: Environmental protection requirements are becoming more and more demanding, and yet traditional extensive workover operations suffer from high emissions, high energy consumption, high risks, and high environmental pollution, which leads to failure of fulfilling the current requirements of the industry. “Green” jobs represented by clean operations and snubbing operations welcome their booms in development and results of large-scale application are remarkable.

(8) Development: The operation environment is harsh, the manpower work intensity is high, and the cost reduction and efficiency enhancement are seen with tremendous difficulties. The conventional operation mode dominated by manpower can no longer meet the requirements of high-quality development. It is the inevitable demand of the times to continuously promote the development of workover operation towards being automatic, intelligent, rapid, and efficient, so as to improve the operation environment, enhance the operation efficiency and lower down the work intensity.

After more than 60 years of development, responding to progressing of oil and gas field development and the diversified maintenance requirements of wellbores, a workover engineering technology system that embodies Chinese characteristics and adapts to the needs of sustainable development, a characteristics of the new era, has gradually taken shape.

Workover equipment, usually composed of surface lifting equipment (power, derrick, drawworks, rope, etc.), pumping equipment and wellhead equipment, is the core and also key of the workover operation technology. It can also be divided into the tractor hoist, workover rig. snubbing units, coil tubing operation rig and other downhole servicing rigs.

As oil and gas development shifts from conventional to unconventional resources, from shallow to deep zones, from vertical to horizontal wells, well types and operation conditions are becoming increasingly complex, which bring in new challenges for workover operation and consequently higher requirements for workover equipment. Given this, workover equipment eventually develops towards being high-power, automatic and environmental-friendly.

(1) Workover rig: The main manufacturers of conventional vehicle workover rigs are Sinopec Jianghan Petroleum Administration Fourth Machinery Factory, Nanyang Second Machine Petroleum Equipment Group Co., Ltd., CNPC Bohai Petroleum Equipment Manufacturing Co., Ltd., etc. In accordance with the "petroleum workover rig" industry standard, there are totally nines specifications, namely XJ350-XJ2250, with the maximum hook load up to 2250 kN, which can accomplish applications in oil, gas and water wells with depth smaller than 9000 m. The overall technical level has reached the international advanced level, and the manufactured rig fulfills domestic operation demand and meanwhile is exported overseas.

(2) Subbing units: The main manufacturers are Rongsheng Machinery Manufacturing Co., Ltd. of Huabei Oilfield, Hebei; Hebei New Tiehu Petroleum Machinery Co., Ltd.; CNPC Bohai Petroleum Equipment Manufacturing Co., Ltd. With respect to the industry standard of "Petroleum and Natural Gas Industries-Snubbing Units", three models of two types (standalone and assistant) of snubbing units have been specified, which is applicable to snubbing operation of oil, gas and water wells with a depth lower than 5,000 m and wellhead pressure below 35 MPa. Fabrication localization has been achieved^[9,10,11].

(3) Coiled tubing and operation units: PetroChina has developed coiled tubing, coiled tubing operation rigs and special supporting equipment (including injection heads, specialized online testing devices, small sand mixing pumps, etc.). In the Baoji Petroleum Steel Pipe Plant, China's first coiled tubing production line has been built, and even- and variable- wall thickness coiled tubing of specifications CT7-CT110 can be manufactured, with diameters ranging from 25.4 mm to 88.9 mm, which successfully replacing imported products, and greatly reduces the application cost. The CNPC Drilling Engineering R&D Institute has developed the first domestically-made coiled tubing operation unit, which evolves into three types of coiled tubing equipment, namely trunk-mounted, skid-mounted and towed. The pipe diameter is from 9.5 mm to 88.9 mm, and the maximum working capacity is applicable to well depth of 8000 m.

(4) Automatic workover equipment: Automated workover operation enjoys advantages of reduced labor intensity, labor cost saving and improved work efficiency. In recent years, the Daqing, Jilin, Xinjiang and other oil fields have carried out equipment R&D for automatic pipe running and retrieving in major repair, automated minor repair, and automated snubbing minor repair, which preliminarily achieves automatic operation of systems for surface pipe feeding, suspension, makeup, and aloft pipe racking. The number of operators reduces from 5-7 to 3, and small-scale applications of such systems have been seen.

With development of the workover technology, domestically-manufactured workover tools have been seen from nonexistence to coming-into-being, with continuously improve performance, which successfully solving workover problems such as casing damage inspection, falling object fishing, shock-assisted sticking releasing, and re-shaping, patching, milling and cutting of casing. The domestic tool has achieved specialization, serialization and standardization (Fig. 2), strongly supported development of the workover technology and ensured the normal production of oil, gas and water wells. In recent years, workover tools have made significant progresses in casing patching tools and zonal isolation tools.

(1) Casing patching tools: The casing patching tool is mainly used for casing reinforcement and zonal isolation. Research Institute of Petroleum Exploration & Development (RIPED), PetroChina, has independently developed an expandable pipe patching tool applicable to 139.7, 177.8, 244.5 mm and non-standard casing specifications. After repairing, for the 139.7 mm casing, the maximum drift diameter can reach 117 mm, the burst pressure is 60 MPa, the collapse pressure is 35 MPa, and the longest patching section is 150 m. This tool is able to achieve the rapid operation of integrated expanding and fishing, and fulfill operations requirements for the large-diameter layer isolation and adjustment and thermal-recovery wellbores with high temperatures up to 350 °C. It has been applied in over 1700 wells of 13 oil and gas field companies in PetroChina, with a success ratio of over 98%^[12,13].

(2) Isolation tools: Conventional isolation tools have formed four series of products, namely the compression, expansion, self-sealing and intrusion types, and are gradually developing towards special applications such as high temperatures/high pressures, self-expansion, cable-through and degradable tools. High-temperature and high-pressure packers with rated temperatures of 170 °C and rated pressures of 70 MPa, are able to meet the requirements of multi-stage fracturing and zonal injection. The expansion ratio of oil/water- induced self-expanding packer is increased from 1.5 to 3.0, and the pressure rating has exceeded 50 MPa. The bridge-plug isolation tool includes the drilling-easy bridge plug, soluble bridge plug, etc., with rated pressures up to 70 MPa^[14].

Along with the advancement of equipment and tools, the workover technology aims to meet the new development needs, always dedicates itself into developing workover technologies for various complex well condition and vigorously develop green workover technologies such as snubbing workover and clean operations, so as to provide technical support to maintaining the normal production of oil, gas and water wells.

3.3.1. Maintenance work

The number of oil, gas and water wells has been increasing year by year, and meanwhile the quality of maintenance operations has continued to improve, which ensures the normal operation of oilfield production. With deepening into the earth of exploration and development practice and gradual utilization of unconventional resources, in order to adapt to different well types and well conditions, PetroChina has developed a series of maintenance operations (Fig. 3), covering simple servicing and maintenance work such as sand washover, pump inspection, simple fishing, replacement of downhole string or tools, and water plugging. In recent years, for maintenance work, outstanding progress has been made in pump inspection and wax removal and prevention operations.

(1) Pump inspection: For the complex well conditions such as high deviation, sand production, wax formation, scale precipitation and presence of gas, the “quintuple prevention” technology characterized by anti-biased wear, anti-wax, anti- corrosion, anti-scaling and anti-gas has been formed. For lifting approaches such as the screw pump and electric submersible pump and novel rodless lifting, the new pumped-well pump inspection technology such as cable-assisted operation and composite pipe operation has come into being. With the continuous improvement of the quality of work, the pump inspection cycle of the mechanical lifting system has been prolonged year by year, and the pump inspection operation and operating cost have been reducing every year^[15,16]. In 2017, the average pump inspection cycle of PetroChina is 836 days, which is 109 days longer than that in 2011 (Fig. 4).

(2) Wax removal and prevention: In view of problem of removing and preventing wax in wells equipped mechanical lifting, PetroChina has developed supporting technologies such as wax-free, conventional hot washing, high-pressure hot washing, solid-based wax prevention, wax inhibitors and electric heating. In 2018, these technologies are applied in more than 3.6×10⁴ wells in the Daqing oilfield, with excellent performance in wax removal and prevention. Over recent years, new technologies of wax prevention such as new alloy-based, ultrasonic, high-intensity magnetic and electromagnetic wax prevention have emerged one after another. These technologies have the technical advantages of one-time installation and being maintenance-free, and have been seen with small-scale applications.

3.3.2. Major repair technologies

Since the "Twelfth Five-Year", the major repair technology for oil, gas and water wells has developed steadily, and proposed 11 series of technologies such as casing damage inspection, small-drift-diameter channel opening, and sidetrack/side- deviating (Table 1), and the cumulative repaired casing-damage wells has reached 3.7×10⁴ wells, with cumulative oil production of about 500×10⁴ tons, and cumulative water injection of 9000×10⁴ m³. This is able to resume production and meanwhile efficiently improve the development well pattern. Among them, given production reality of oilfields, three special technologies, namely channel opening, sidetracking and top drive workover, have been developed, which result in good application performance.

(1) Channel opening technologies for no-channel and small- drift-diameter casing-damaged wells: For casing-damaged wells with drift diameters less than Φ50 mm and no channel, the channel opening technology with its essential reverse section-milling and reaming-milling has been proposed, and according to different casing damage types, six types of channel opening practices and one standard are developed. The technology has been applied in 130 wells, with a success ratio of 81.3%.

(2) Sidetracking: Given the challenging production re-initiation of long-term shut-in flooded wells, the Changqing Oilfield Company has solved the key problems of window milling of inclined wellbore section of directional wells, narrow-standoff cementing, and 88.9 mm small-diameter casing section multi-stage fracturing, and developed the directional well sidetracking technology to tap potential of remaining oil. 94 wells have been treated with this technology, the incremental daily oil production per well is more than 2 tons, and the annual restoring production capacity has exceeded 10×10⁴ tons. The Daqing Oilfield Company has put its side-deviating technology (similar to sidetracking) into extensive applications, which, compared with drilling a new wellbore, can save cost of relocation, land acquisition and ground supporting facilities, and in the meantime does not affect the original well pattern and development plan. More than 1300 wells have been treated with this technology. RIPED has developed the ultra-short radius sidetracking technology (with a radius of curvature of 2 m) and deep coring technology. While drilling a large-diameter horizontal wellbore through the reservoir, it can realize deep point- and azimuth- specific coring. With the reservoir property derived from collected core sample, the fine evaluation of and effective tapping potential of remaining oil can be achieved^[17]. The technology has been successfully tested in two wells of the Jilin oilfield. The wellbore size is Φ118 mm, the horizontal section is 30-50 m long. The length of the core sample in a single coring chamber is greater than 0.7 m, and the incremental daily oil production is over 1.3 tons, which demonstrates the broad application prospect of this technology.

(3) Top-drive workover technology: The Daqing Oilfield Company carries out major repair operations using “guy rope-free servicing rig and top drive”. The operation adopts an electric-hydraulic drive mode, with load capacity of 35, 50, 70 and 110 tons, respectively. This technology is seen with advantages of small footprints, low one-time investment, high maneuverability, energy saving and environmental protection, and can meet the needs of oil and water well major repair operations in wellsite-limited and environmentally-sensitive areas.

3.3.3. Workover technologies for gas wells

Workover in gas wells is characterized by high difficulties and risks, and technical bottlenecks such as sticking-releasing, thief zone repair, gas pay zone protection and wellhead safety have been broken. In reference to the different characteristics of low pressure, medium pressure and high pressure gas reservoirs, a series of gas-well workover technologies is formed. Significant effect has been made in plugging removal and high temperature and high pressure gas well operations.

(1) Gas well plugging-removal technology: The Changqing Oilfield Company uses acid and alkaline scale-removing agents to improve the permeability of reservoirs in the near- wellbore zone, reduce the interfacial tension of the formation water, lower down the liquid-phase resistance, and improve the gas flow capacity. Such practices have been applied in more than 200 wells, with cumulative natural gas production growth of 2×10⁸ m³. The Southwest Oil and Gas Field Company has independently developed self-heating agent and 140-MPa injection device, and first proposed the hydrate- blockage removal technology for ultra-high-pressure H₂S-containing gas wells, which has been tested in two wells.

(2) High-temperature and high-pressure gas well operation technology: PetroChina has developed the wellbore integrity evaluation and management system, and through the well barrier monitoring and the annulus pressure calculation, risk level assessment, early warning and countermeasure response have been implemented. Full coverage of risk level management of ultra-deep (up to 7000 m), high-temperature (140 °C) and high-pressure (125 MPa) gas wells has been achieved. The workover technologies for cement injection under pressure and tubing cutting have been developed, and the snubbing workover technology of gas wells applicable to the case of pressures at 90 MPa and temperatures at 120°C has taken shape.

3.3.4. Workover technologies for horizontal wells

PetroChina has developed a series of horizontal-well workover techniques and supporting technologies such as sand washover and cleaning up, perforation, water detection and plugging, sticking-releasing and fishing, drilling-milling and casing-milling, and achieved the technical breakthrough of workover for horizontal wellbore sections with length lower than 1500 m. This ensures the normal operation of more than 9000 horizontal wells of PetroChina, and sustains effective development of low-permeability and unconventional oil and gas resources^[18]. The main innovative technologies include the booster-assisted fishing technology for horizontal wells, the water control technology for horizontal wells, and the perforating technology for horizontal wells

(1) The booster-assisted fishing technology for horizontal wells: The horizontal-well booster-assisted fishing tool string consists of the lifting-type releasable spear, downhole fishing booster, centralizer and supporting tools. The booster is anchored on the casing by hydraulic pressures, and can generate tensile forces of 500-650 kN at the top of the fish. The string section above the booster is unloaded, and the spear can be released. The maximum length of the captured sticking pipe string is 145 m.

(2) The water control technology for horizontal wells: The new technologies for water detection in horizontal wells such as “tractor + well logging composite instrument, neutron oxygen activation measurement + hydraulic transmission, mechanical-tool-based water detection and intelligent switch” have been proposed, and the preliminary realization of accurate water identification in the horizontal wellbore section has been achieved. Moreover, the new water plugging technologies, such as “zonal isolation, fixed-point plugging, selective nonselective plugging, and annulus chemical packer (ACP) segmentation plugging” have emerged, and PetroChina has preliminarily achieved the goals of fixed-point sealing, sealing of arbitrary water-producing sections, and production capacity restoration of the whole-section water-producing horizontal wellbore^[19].

(3) The perforating technology for horizontal wells: The perforating technologies for horizontal wells can be divided into the tubing conveyed, coiled tubing conveyed, cable hydraulic pumping conveyed, and the tractor conveyed. The operation capability for ultra-deep horizontal wells (5000 m), with horizontal section over 3000 m, temperatures up to 160 °C and pressures up to 140 MPa has been cultivated.

3.3.5. Snubbing operation technologies

With continuous efforts to tackling problems in snubbing operation, breakthroughs have been made in terms of technical bottlenecks such as production tubing blockage and wellhead equipment in snubbing applications, and the technology stepping-over from water wells to oil, gas and water wells, from minor repair to major repair, completion, and fracturing, from low-pressure to medium- and high- pressure cases, have been achieved. Six sets of two series of snubbing operation technologies with pressure-bearing capacity of 21 MPa and 35 MPa respectively, have been proposed. With widening of the application field, a characteristic technology represented by high-temperature well operation and snubbing-assisted fracturing has been formed. Since the large-scale promotion of this technology across PetroChina in 2010, the workload has been increasing every year, with implemented operation in 3.6×10⁴ wells, reduced discharge of injection water over 1800×10⁴ m³, and more than 1600×10⁴ m³ of water injection attributed to reduced workover operation time. The performance in terms of production stabilization, energy saving, and emission reduction in oilfields is remarkable. The primary innovative technologies include the snubbing operation technology in fireflooding and SAGD applications, the snubbing- assisted fracturing technology, and long-life elastomer cores for annular blowout preventers

(1) The snubbing operation technology in fireflooding and SAGD applications: The Liaohe Oilfield Company has developed the snubbing operation technology for thermal recovery, with temperature tolerance of 220 °C and treating pressure of 14 MPa. The sealing element for the blowout preventer is made of high-temperature resistant perfluoro-rubber, and temperature resistance and reduction are realized by the cooling circulation system. Snubbing operation in thermal-recovery wells can greatly reduce the well-occupying period, by over 58 days for SAGD wells and by more than 8 days for fireflooding wells. The average avoided production loss due to workover operation per well reaches 600 tons.

(2) The snubbing-assisted fracturing technology: The Daqing Oilfield Company has innovatively developed the high-pressure straight-pipe integrated fracturing practice (for pump rates no more than 5 m³/min), which overcomes the technical challenge of stripping/replacing production tubing and fracturing string, and achieves timely fracturing, rapid completion and environmental-friendly post-fracture operation. In 2018, 60 wells are completed using the snubbing- assisted fracturing technology. The average operation time per well is shortened by half, and the average reduced liquid discharge per well is 540×10⁴ m³. The average wellhead pressure after fracturing reaches 11 MPa, and the average incremental daily oil production per well is 5.7 tons.

(3) Long-life elastomer cores for annular blowout preventers: The rubber core of the annular blowout preventer is the essential sealing component for snubbing operation, of which the dynamic sealing performance and effective lifecycle are the tow key factors affecting the efficiency of the snubbing operation system. For a long time, a huge gap existed between performances of domestic rubber cores and imported products, which was a key issue that restricts improving of speeds and efficiencies of snubbing operation. RIPED has developed a multi-hardness-range, long-life composite rubber core suitable for applications in oil and water wells. It has been through tests in more than 130 wells and presents a service lifecycle exceeding 3.3×10⁴ m, which is 6.5 times of that of the domestic rubber core and 1.3 times of that of the imported product.

3.3.6. Coiled tubing operation technologies

Since the “Twelfth Five-Year”, coiled tubing operation has been seen with leap-forward development. It has been promoted across 11 oil and gas field companies such as Daqing, Changqing and Xinjiang, and the annual operation workload is experienced with an annual average increase by 35%. The operation periods of wellbore drifting and flushing, sand washing and plugging removal are reduced by 40%, and the comprehensive cost is lowered down by 20%, which demonstrates outstanding outcome in cost reduction and efficiency improvement.

(1) The auxiliary tools and technologies for coiled tubing operation have been fully localized, and six groups of workover technologies, namely well flushing, plugging removal, string cutting, squeeze cementing, drilling-milling and gas lifting, have been developed, with rapidly improving overall technical levels.

(2) The Changqing Oilfield Company, based on feasibility and safety evaluation of process integration using velocity strings through the whole operation, together with the factory- like completion operation pattern, proposes the well-cluster velocity-string factory-like operation workflow involving “four single-run steps” of wireline operation, well-cluster preparation, velocity string running using coiled tubing and production re-initiation by nitrogen injection. The overall operation period is reduced and the operation efficiency is raised up by 40%.

(3) Coiled tubing-based horizontal well sand washing: The coiled tubing equipped with auxiliary sand washing tools solves the problems such as failure of circulating in the case of downhole string sticking and sand washing in long horizontal sections, shortens the operation cycle, and reduces the operation cost. The maximum depth for application of horizontal well sand washing exceeds 4500 m and the length of the horizontal section surpasses 1500 m^[20].

3.3.7. Clean operation technologies

PetroChina adheres to the principle of “put prevention first, and combine prevention and correction”, grasps four key aspects of operation, namely “wellbore, wellhead, surface and waste and tailings”, and develops clean operation technologies such as wellbore blowout-prevention control, rod, pipe and wellbore cleaning, wellhead liquid collecting platform, and negative-pressure recycling unit. In 2018, these technologies are seen with cumulative application over 13×10⁴ wells, with oil sludge reduction exceeding 7.5×10⁴ tons. The coverage of cleaning operations in environmentally-sensitive areas has reached 100%, and the overall coverage is above 60%.

By formulating comprehensive and normative standards of workover equipment, tools and practices, PetroChina has established the standardized system for workover technologies and improved the overall technical level. Through the implementation of position management standardization and operation site standardization, and practice standardization, PetroChina has successfully constructed the standardized system for workover operation management, which realizes no blind spots for safety management, no hidden dangers for on-site equipment and no violations in personnel operation, and thus ensures safe and efficient production.

Over recent years, workover operation has been evolving towards standardization, specialization, mechanization and informatization. Comparison of equipment, tools, technologies and informatization with oversea companies indicates that major workover technology in PetroChina has reached the international advanced level, and yet in some areas, gaps remain. (1) Localization of conventional workover rigs has been achieved, and the resultant product is seen with mature technologies and price advantage, and can fulfill demand of domestic operation. However, novel types of workover rigs such as grid-power, top-drive and automatic rigs still need further R&D efforts. (2) Domestically-made workover tools have abundant product lines and complete specifications, and is able to basically meet operation requirements. Nevertheless, some gaps still remain in aspects such as automatic downhole tools, inspection tools and tools for high temperature and high pressure applications, and dedicated R&D work is in urgent need. (3) The overall technical performance of the major repair technology is relatively high. Still, weakness is seen in casing inspection/detection technologies of PetroChina, compared with those in oversea companies, and technologies for complex casing damage repair and ultra-short-radius sidetracking for potential tapping also demand for further development. Workover technologies applicable to gas wells and horizontal wells have preliminarily taken shape, and can basically meet the requirements of conventional workover operation. As for “high temperature, high pressure and high H₂S content” gas wells and horizontal wells with lateral sections over 1500 m, the workover technology apparently still falls behind that of leading foreign companies. (4) The snubbing operation technology for oil and water wells is basically mature and seen with large-scale application. But performance of critical components such as hydraulic systems and rubber cores of blowout preventers for gas wells still lags behind that of products of foreign companies. Snubbing operation equipment for high-pressure gas wells is dependent on importing, and corresponding operation is dominated by external service providers. (5) On an overall basis, coiled tubing and corresponding operation units stay at equivalent levels with products of foreign companies, and technologies for operations inside production tubing and casing are mature. The large-tubing-diameter large-volume trailer coiled tubing units and coiled tubing units for high temperature, high pressure, and ultra-deep wells are still seen with gaps behind products of foreign companies. (6) The A5 production operation system and workover database has been developed and put in promoted applications across each oil and gas field companies. Still, standardization of the management system and application of big data technologies need to be improved. The on-site real-time monitoring system is limited to small-scale application, and the coverage needs to expand. The expert decision-making system needs to be improved.

With the increasing diversity and complexity of workover targets, future development of the workover technology mainly has the following eight orientations:

(1) Enhance understanding of characters of reservoirs and improve target-pertinence of workover plans: The complexity of reservoirs is represented by high water cut, low permeability, ultra-deep depth, ultra-high/low pressure, high temperature and presence of acid matters, which decides that the workover technology has to be reservoir-specific. Understanding of effects of sensitive factors, optimization of operation plans, selection of favorable tools, and identification of applicable practices are all in demand. Auxiliary technologies such as understanding of characters of reservoirs, regional casing-damaged well treatments and working fluid system optimization based on reservoir characteristics also need to be developed.

(2) Carry out research campaign for major repair technologies and overcome challenges in puzzling well treatments: To-be-repaired problematic wells such as ill horizontal wells, corrosion wells and wells with severe casing damage are seen with increasing quantity, and conventional workover technologies and tools fail to meet operation requirements. Continuous efforts have to be made to develop technologies for downhole accurate identification, sand production treatments, corroded casing repair and ultra-short-radius sidetracking.

(3) Enhance input in R&D for horizontal well operation, and develop auxiliary technologies for long horizontal wellbore section scenarios: In 2017, the total quantity of wells in the US reaches 99×10⁴, and the quantity of horizontal wells is 12.6×10⁴, accounting for 12.8% of the total. Compared with the US, PetroChina has only about 9000 horizontal wells, which accounts for 2.5% of the total well quantity. Given this, with exploitation of unconventional oil and gas resources, increasing quantity of horizontal wells and extended length of horizontal wellbore sections, technologies for workover, water detection and plugging and casing reshaping in long horizontal wellbore sections are in urgent needs for development.

(4) Improve the snubbing operation technology, and expand application range for snubbing: Focus should be placed upon research of snubbing equipment and technologies for gas wells, so as to improve performance of essential components of domestically-made products and breaks the technical monopoly of foreign companies. Meanwhile, the application range and scale should be improved, in order to provide technical support to safe and environmental-friendly operations in oil, gas and water wells.

(5) Raise up operation capacity of coiled tubing and improve operation quality of special operation: Key problems in coiled tubing for high-pressure/ultra-deep/ultra-long-lateral- section wells and nonmetallic intelligent coiled tubing equipment and operation technologies should be tackled. The range of special operations should be expanded, and extensive applications of conventional operations should be promoted. Operation approaches should be changed, for the purpose of safe and clean operation.

(6) Development automatic workover technologies, and progress towards artificial intelligence: Efforts should be made to greatly improve control precision, reliability and operation efficiency of the core system, with the help of industrial manipulators, visual identification, automated control and artificial intelligence, in order to overcome technical bottlenecks of major and minor repair as well as automatic snubbing operation. In the meantime, the testing scale should be enhanced, so as to save cost and enhance efficiency.

(7) Promote clean operation in an all-round manner and build environmental-friendly oil and gas fields: Sustained efforts should be made to tackle key problems in clean operation technologies and auxiliary processes, and technical integration, improvement, updating and replacement are encouraged. Clean operation should be promoted in an all-round manner, so as to achieve green production of hydrocarbons and build environmental-friendly oil and gas fields.

(8) Perfect informatization and achieve workover operation resource sharing: Building and application of the workover informatization platform, site monitoring system and remote-control expert decision-making platform should be accelerated, to sufficiently exploit cluster effects of “Internet + Technology” and achieve standard, unified, safe and efficient management of workover operation.

After years of development, the workover technology of PetroChina basically fulfills operation demands in different historical times, and efficiently supports the robust development of oil and gas exploration and exploitation. With the accelerated tasks for reservoir growth and production capacity building in PetroChina, exploration and development progress towards the “low-permeability, deep-burial, offshore and unconventional” resources, which is accompanied by increasing diversity in oil and gas reservoir types and complexity of wellbore configurations. Cost-effective recovery of hydrocarbon resources from wells with complicated conditions such as high-temperature high-pressure wells, deep wells, highly-deviated wells, horizontal wells and highly-sour gas wells is inseparable from the technical level and high-quality development of workover operation. Integrated application of massive water injection and fracturing technologies and efficient exploitation of hard-to-recover and unconventional resources are tightly related to the continuous improvement of support provided by workover operation. The workover technology will play more and more important roles in supporting significance, with respect to production stabilization and stimulation, cost saving and efficiency enhancement.

During the “Thirteenth Five-Year” and a certain subsequent period, sustained efforts should be made by PetroChina to tackle key problems in automatic/intelligent operation equipment, snubbing operation in “high temperature, high pressure and high H₂S content” gas wells, water detection and plugging in long horizontal wellbore section, coiled tubing operation in ultra-deep wells, clean operation and workover informatization, and help the workover technology evolve into a “sharp sword” for maintaining normal operation and stabilizing production rates in oil and gas fields, and serve as the booster for production stabilization and stimulation, cost reduction and efficiency enhancement in PetroChina.