Introduction to the theory and technology on downhole control engineering and its research progress

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 45, Issue 4, August 2018 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2018, 45(4): 754–763.

RESEARCH PAPER

Introduction to the theory and technology on downhole control engineering and its research progress SU Yinao* CNPC Engineering Technology R&D Company Limited, Beijing, 102206

Abstract: On the basis of reviewing the development history of drilling engineering technology over a century, this paper describes the technical and scientific background of downhole control engineering, discusses its basic issues, discipline frame and main study contents, introduces the research progress of downhole control engineering in China over the past 30 years, and envisions the development direction of downhole control engineering in the future. The author proposed the study subject of well trajectory control theory and technology in 1988, and further proposed the concept of downhole control engineering in 1993. Downhole control engineering is a discipline branch, which applies the perspectives and methods of engineering control theory to solve downhole engineering control issues in oil and gas wells; meanwhile, it is an application technology field with interdisciplinarity. Downhole control engineering consists of four main aspects; primarily, investigations about dynamics of downhole system and analysis methods of control signals; secondly, designs of downhole control mechanisms and systems, research of downhole parameters collections and transmission techniques; thirdly, development of downhole control engineering products; fourthly, development of experimental methods and the laboratories. Over the past 30 years, the author and his research group have achieved a number of progress and accomplishments in the four aspects mentioned above. As a research field and a disciplinary branch of oil and gas engineering, downhole control engineering is stepping into a broader and deeper horizon. Key words: oil and gas; drilling; downhole control engineering; research progress; development direction

Introduction As the major energy and strategic material of contemporary human society, petroleum affects the national economy and people's livelihood and national defense security. To extract crude oil and natural gas buried hundreds of meters or even several kilometers deep underground, it is necessary to drill and construct wells to form oil and gas channels that communicate underground reservoirs and the ground. Therefore, oil and gas well engineering is an important part of the upstream business of the petroleum industry, and it is a multi-disciplinary and multi-professional technical field which integrates multiple techniques, such as drilling, well completion (including cementing), well logging, testing, oil production, downhole operation, and reservoir stimulation, etc. While the downhole control engineering is one of its sub-disciplines, especially for downhole control issues and technologies. This paper describes the technical and academic background about the emerging of downhole control engineering, discusses its discipline framework, basic issues and major research contents, summarizes the research progress of downhole control engineering in China over the past 30 years, and envisions the future development of downhole control engineering.

1. Properties, purposes and tasks of downhole control engineering 1.1.

Universality of downhole control issues

In oil exploitation, many engineering problems, such as exploration, drilling, well completion, logging, oil recovery and well workover, are all related to the oil well itself. Control problems occur in various downhole production and operation processes. Taking the downhole control of drilling operations as an example, it involves a variety of control issues in safety control, quality control and cost control (Fig. 1). For each control problem, control tools or systems with one or more control modes can be developed. These are the research areas of downhole control engineering. 1.2.

Characteristics and difficulties of downhole control

The downhole control has its own inherent characteristics and difficulties, which are determined by the structures of the oil and gas well, the environment of downhole operation, and the properties of loads. (1) Small radial size. An oil and gas well is an elongated hole that can reach several kilometers in length starting from

Received date: 02 Feb. 2018; Revised date: 20 May 2018. * Corresponding author. E-mail: [email protected] Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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control equipment usually cannot be directly applied in the downhole control. Due to the particularity of downhole control, the conventional control systems, mechanisms, components and mature methods, which can be effectively applied in other industries and ground projects, are difficult to be simply copied to downhole control. Under such circumstances, downhole control engineering has to investigate these special problems to form a set of special design methods and techniques. 1.3. 1.3.1.

Basic issues of downhole control engineering Definition

Downhole control engineering is a discipline branch, which applies the perspectives and methods of engineering control theory to solve downhole engineering control issues in oil and gas wells. It is a combination of engineering control theory and downhole techniques in oil and gas well engineering. It is an application technology field with interdisciplinarity. From the engineering perspective, it involves drilling, well completion, well testing, well logging, oil production, well repair, and all other operations related to the downhole techniques of oil and gas wells. From the control perspective, it involves open-loop remote control and downhole closed-loop automatic control. 1.3.2.

Fig. 1. Schematic diagram of downhole control classification in drilling operation.

the ground surface; however, its diameter is often within half a meter. Moreover, the size of the well decreases gradually as the increasing well depth, and the smallest borehole diameter can be less than 0.1 m. Since engineers cannot reach the bottom of the well to participate in the operation process, this control is in the form of remote control or closed-loop automatic control. In particular, the limitation of radial size makes downhole tools or systems difficult to design and manufacture. (2) Coexistence of a variety of working media underground. Taking drilling as an example, solid (drill string) and liquid (drilling fluid) coexist downhole, and the drilling fluid in the actual working process is a multi-phase medium composed of non-Newtonian fluid, solid particles and even bubbles (under foam drilling conditions). The downhole system under multibody coupling has complex physical properties. (3) Extreme working conditions. Taking drilling as an example, the drill string works under conditions with high temperature (up to 200 °C or more), high pressure (up to 100 MPa or more), strong vibration (up to 500 g (acceleration of gravity)), strong loads (maximum axial tension load can reach nearly 1000 tons), erosion and corrosion. Therefore, the components and control technology that can be used in the ground

Research objects, purposes and properties

The research objects of downhole control engineering are all engineering control issues involved in various operating processes of oil and gas wells. The purposes of studying downhole control engineering are to theoretically identify the physical properties of the downhole control issues and the basic laws of the control procedures, and develop and provide effective control systems, techniques and methods in practice, and solve engineering problems optimally from both technical and economical aspects. Both the special structure of oil and gas wells and the extreme working conditions make the downhole control difficult and complex, and also determine the research properties of downhole control engineering, namely, it is a interdisciplinarity application technology field with high requirements on both theory and practicability. 1.3.3.

Discipline characteristic

Downhole control engineering is an application technology field which integrates theoretical research, product development and experimental research. It features multi-specialty and multi-discipline, which can be summarized as follows: it takes downhole as the object, control as the goal, mechanics as the basis, machine as the subject, fluid as medium, computer as the means, and experiment as the support. 1.4.

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Discipline framework of downhole control engineering

As a discipline branch, downhole control engineering con-

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sists of the following four basic parts. 1.4.1.

Study on theoretical basis

The theoretical basis of downhole control engineering is downhole system dynamics and control signal analysis theory. As the downhole system is a complex system with multi-body coupling effects, a series of theoretical models should be established to describe the dynamic characteristics of the downhole system, the purpose of which is to determine the distributions and variations of important physical parameters (such as velocity, acceleration, stress, displacement, pressure, amplitude, and frequency, etc.), thus establishing the basic theory of downhole system dynamics. There are some special requirements on downhole-system control signals. It is necessary to study its occurrence and transmission process, and determine its dynamic quality and stability indicators, which can be further used to optimize the control signal. 1.4.2.

Investigation on technological basis

The technological basis of downhole control engineering includes the downhole control mechanism with system design and the downhole parameter acquisition with transmission technology. For a variety of practical control signals applicable to downhole control, the corresponding signal generation, transmission, amplification and execution mechanisms must be designed. The typical structures of these mechanisms must be determined, and the structural libraries and characteristic simulation libraries for various types of signal control mechanisms must be established to reach the modular design level. Downhole parameters can be divided into state parameters and control parameters. State parameters are various geometric parameters and physical parameters that describe the working boundary and work process characteristics of the downhole system. Hence, many state parameters are used in the control process. It is necessary to study the measurement methods and means for different kinds of downhole state parameters and control parameters, as well as the methods and practical techniques of these parameter signals for the downhole short transmission and the two-way transmission between downhole and ground. 1.4.3.

Product development

The application objective of this discipline branch is to research and develop a number of control tools and control systems required for different downhole operations to solve actual production problems. Mechanical-electrical-hydraulic integration is often the basic feature of the downhole control system. It is a comprehensive application of downhole control mechanism design techniques. Due to the diversity and practicality of the developed products, downhole control engineering will have a highly targeted professional application range and can generate great economic and social benefits, and is expected to form a new high-tech industry. 1.4.4.

Laboratory construction and experimental research

The corresponding laboratory is a support for downhole

control engineering. The theoretical analysis results need to be verified by experiments. The key structural parameters in the design are generally determined by experiments. In particular, the coefficients related to real fluid systems can only be worked out through experiments. Therefore, in the downhole system dynamics and signal analysis theory research, downhole control mechanism design research, downhole parameter acquisition and transmission technology research, and product development, experimental research has a non-negligible role. The correlation among the above four parts is that, the product development is the main purpose of downhole control engineering research; the research of technological basis is the foundation of the product development; the research of theoretical basis is the basis of technical research; laboratory construction and experimental research are important means and support for the research of the theoretical basis, the research of the technological basis and the product development. The practical significance of the research of theoretical and technological basis is to form a modular design method, so that the product developers who did not engaged in the research of the theoretical and technological basis can complete the design and development of products by the approach of “building blocks” according to the above hardware structure library and characteristic emulation library. This will reduce the difficulty of product development, improve the product design level and expand the application scope of downhole control engineering.

2. Background for the emerging of downhole control engineering It is about 20 years since the concept of downhole control engineering has been proposed. Starting from promoting the success rate and accuracy of well trajectory control in oil and gas well drilling, downhole control engineering has gradually developed as a new field in oil and gas well drilling and completion. In the past decade, downhole control engineering has also been expanded in other downhole application sectors. Therefore, in order to deeply understand the technical and academic background for the emerging of downhole control engineering, it is necessary to review the development history of the oil and gas well drilling technology, especially the well trajectory control theory and technology in the past century. 2.1. Development of drilling technology in the past century Drilling engineering originated in China in the 3rd century BC, when ancient Chinese ancestors collected brines via drilling, which even achieved a certain scale of production in Zigong, Sichuan Province[12]. The modern oil and gas well drilling engineering started in the West. In the 19th century, foreign industrial countries used cable drilling to drill oil and gas wells, which is the foundation for the development of the modern petroleum industry in the 20th Century[3]. Between the late 19th and early 20th centuries, percussion drilling was

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replaced by rotary drilling as the main drilling method in the world. In the 1930s, downhole power drilling tools were invented, which accelerated the development of directional well technology in the 1950s to 1960s[45]. Since then, due to the introductions of engineering mechanics, computer technology and information technology, as well as the promotions of material science and downhole tools, the directional well technology for horizontal wells and extended-reach wells was further developed. The contemporary technology of oil well drilling engineering is a comprehensive system engineering which includes pre-drilling, drilling, completion, and oil and gas testing. It involves ground equipment, downhole tools, well trajectory control, drilling fluid and reservoir protection, measurement and testing, completion and cementing, and it is an application technology discipline involving several majors such as machinery, mechanics, chemistry and control. 2.1.1.

Development of well types

The development of well types can clearly reflect the development history of the oil and gas well drilling engineering technology in the past century, namely, from vertical wells to directional wells (deviated wells), to cluster wells, to horizontal wells, to extended-reach wells, to other special process wells (such as sidetracked horizontal wells. multi-lateral wells, inverted cluster wells, etc.). Before the 1950s, all the oil and gas wells worldwide were vertical. Due to the limitation of technical conditions, only vertical wells can be drilled to extract oil and gas in shallow layers. From the 1950s to the 1970s, this was a period which experienced the rapid development and popularization of directional wells and cluster wells worldwide. Directional wells are mainly drilled to solve problems such as ground obstacles, land lease rights, emergency fire fighting and make use of the natural deflection law of the formation. The development of cluster wells is to drill multiple directional wells (or vertical wells) on a wellsite in accordance with certain planning and design in order to save land occupation, reduce environmental pollution, increase drilling efficiency and reduce drilling costs. Horizontal wells are special directional wells with a maximum well deviation angle of about 90° and a horizontal extension in the reservoir. In the process of transition from directional wells to horizontal wells, a technological stage of highly-deviated wells was experienced. Highly-deviated wells are directional wells with a maximum well deviation angle of more than 60°. Since the extensions of horizontal wells in the reservoir are several times or even hundreds times longer than the conventional vertical wells and directional wells, the production of horizontal wells can be several times or even more than ten times higher than that of conventional vertical wells and directional wells. In addition, horizontal wells can solve the water and gas coning, hence it can greatly increase the single-well output, promote oil and gas recovery, and achieve

the results of fewer wells and higher production. Drilling directional wells means to solve the problems of the ground and the drilling engineering itself. The purpose of drilling horizontal wells is to solve the underground problems to increase production and recovery. The application of horizontal drilling began in the 1950s, however, it was widely applied since the 1980s and it was further generalized in the 1990s. Between the 1980s and the 1990s, there was a stagnation period for more than 20 years, which was exactly a kind of “scrutiny period”, just like the “scrutiny period” of directional wells from the initial trial to large-scale development, which also lasted for more than 20 years. This “scrutiny period” for a new technology is a reflection of calm analysis and review, a fine economic evaluation, a basic theory research, and the development and accumulation of a variety of supporting techniques. By a logical extension of this point, many “emerging” technologies, especially significant technological advances, have such a “scrutiny period”. This is a law. Extended-reach wells are the further development of highlydeviated wells. They are characterized by long highly-deviated steady inclined section and large horizontal displacement. Its development trend is to combine with horizontal well technology to form extended-reach horizontal wells. The technology of extended-reach wells began with the offshore drilling in the late 1970s. Its purpose was to maximize the control area of oil to reduce the number of platforms. Since the 1980s and especially the 1990s, the technology of extended-reach wells has developed rapidly. At present, extended-reach wells with a horizontal displacement of more than 10 km have been drilled[6], and the application field of this technology has been extended to onshore drilling, where extended-reach wells have been used to extract marine oil onshore and further reduce development costs. Special drilling techniques such as sidetracked wells and multi-lateral wells (also known as multi-bore wells) are showing increasingly vigorous development. The short radius and medium-short radius horizontal wells sidetracked from old wells by combining the old well sidetracking technology and the horizontal well technology can significantly increase the recovery rate and reduce the drilling cost. Multi-lateral wells enable multiple-zone production in a single well and are receiving increasing attention. In addition, the U.S. and Canada combined mining (metal ore and coal) technology with oil drilling technology to drill multiple “inverted cluster wells” using large-bore shafts and downhole artificial roadways, to produce extra-heavy oil by making use of gravity successfully. At present, the technology for drilling straight wells is still developing, with emphasis on deep wells and ultra-deep vertical wells under complex geological conditions. There are already a large number of wells whose well depths exceed 7 000 m worldwide. China drilled the Well Tashen 1, whose well depth reaches 8408 m. Germany set up the KTB project

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to study the physical and chemical properties of the continental crust. It planned to drill an ultra-deep well with 10 000 m and actually drilled 9 001 m. The former Soviet Union drilled a well with the depth 12 262 m (Well Сг-3) in the Kola Peninsula for 17 years, which becomes a global open laboratory for the study of crust structure[3]. Compared with the beginning of the 20th century, in oil and gas well drilling, significant changes have taken place in the surface equipment, downhole tools, drilling fluids, etc. Moreover, the technical links such as well trajectory measurement, control, reservoir protection, and well completion have been established, so that drilling engineering becomes a modern engineering science discipline. From the development process of drilling engineering technology over 100 years, the following points can be drawn. Firstly, the demand of oil and gas exploration and development is always the strongest driving force for the advancement of drilling technology. Secondly, the evolution of well type shows that the drilling technology has shook off from the limitation of “simply completing a hydrocarbon channel” and is increasingly becoming an important way to increase the encounter ratio and the recovery ratio. Thirdly, the development trend of drilling technology in the future is to drill the exact wells which can reduce the cost of tonnage oil and improve overall efficiency. Meanwhile, drilling along reservoirs and drilling multiple underground oil and gas targets in a well will be the important technical directions[67]. Fourthly, the key to meet the needs of drilling engineering is to improve the well trajectory control technology. 2.1.2.

Development of trajectory control technology

Well trajectory control is one of the fundamental and key aspects of drilling engineering technology. The demand for oil and gas exploitation promotes the evolution and development of well type. The development of well type promotes the development of the theory and technology for well trajectory control, while the development of well trajectory control technology is the basis for the further development of well type. The modern well trajectory control theory and technology system is formed on the fusion of drilling engineering, engineering mechanics, machinery, control, computer, instrumentation, etc.. From the 1880s to the 1990s, the development of well trajectory control technology can be roughly divided into the following stages[3]. (1) The first stage (before the 1950s), the well drilling was performed by experiences. In the 1920s, the problem of well deviation of vertical wells and its seriousness were realized [8]. In 1928, stabilizers were firstly used in drilling. Researchers and drilling engineers tried to apply the weight of drill collar to reduce well deviation, and used the elastic theory of the beam to analyze the bending and stability of the drill string in the vertical well, to explore the techniques for controlling well deviation, however there was no breakthrough. (2) The second stage (from about 1950 to the early 1980s),

the template and software for well deviation control were provided through theoretical analysis. In 1950, a famous American scholar Lubinski A proposed a mechanical model of the bending drill string in vertical well[9], which was an important milestone in the history of well trajectory control research. Since then, a variety of theoretical algorithms have been developed, which can be divided into four kinds of representative methods, i.e., the differential equation method[9], the finite element method[10], the energy method[11], and the vertical and horizontal bending method[8, 12]. Their corresponding calculation charts and software have provided theoretical and technical supports for field application. At this stage, drilling engineering evolved from “technology” to “science“. The analyses of the force and deformation of the bottom hole assembly, the analysis of interaction between the formation and the drill bit, and the prediction of borehole trajectory constitute the three major aspects of this “science“; while the computer application is its external feature. Taking the analyses of the force and deformation of the bottom hole assembly as an example, in addition to the diversity of solving methods, the research evolved from one-dimension to three-dimension, from static to dynamic, and from small deformation to large deformation[89]. At this stage, the objects of the well trajectory control evolved from vertical wells to directional wells, cluster wells, and even horizontal wells. The control objectives evolved from the simple well deviation control to the controls of the rate of over-all angle change, the build-up rate and the rate of direction change. (3) The third stage (from about the early 1980s to the 1990s), the well trajectory was controlled in real time based on the measurement information provided by the steering tool. Well trajectory measurement instruments experienced the development from hydrofluoric acid bottle inclinometers, to single-shot inclinometers, to multiple-shot inclinometers, cabled inclinometer, to measure while drilling. As the steering tool allowed the well trajectory control to enter into the third stage, the accuracy of the trajectory control decision had been greatly improved. At present, the most advanced steering tool is measure while drilling, it overcomes the shortcoming of the cabled inclinometer, which can only be used in the “slide drilling” state; since measure while drilling can be used in the “rotary drilling” state. The measure while drilling with high configuration can measure more than 10 parameters, which include not only the directional parameters such as well deviation, azimuth and tool face, but also the working parameters such as weight on bit, torque on bit, downhole vibration and annular temperature, as well as geological parameters such as natural gamma, formation resistivity, density, neutron porosity, etc. This kind of instrumentation system is commonly known as a logging-while-drilling tool. 2.2. New issues and thoughts on well trajectory control technology The development from the first stage to the third stage is a major advance in well trajectory control technology, and it is

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the result of the introduction of engineering mechanics and computer technology into drilling engineering. However, the problems and difficulties in the well trajectory control technology cannot completely be solved only by mechanical models and calculation software. The reasons are as follows[13]. (1) The mechanical models of bottom hole assembly (BHA) were built on simplifications and assumptions, the calculated results have a deviation from the actual results. For example, the inside wall of the wellbore is often assumed as a regular smooth cylinder, but this is not the case in the field, which affects the contact state of the drill string with the wellbore wall and may affect the obtained lateral force. Although some analyses of the BHA dynamics take the effects of torque and the variations of weight on bit into account, there is still a gap between the simplified load spectrum and the actual one. In addition, some of the input parameters of the BHA model have only nominal properties, such as weight on bit and torque on bit, which are currently replaced by the measurements shown on the surface instruments. However, for directional wells and horizontal wells, due to the prominent frictional resistance, the actual weight on bit and torque on bit are far from their nominal values, which inevitably affects the calculation result of the lateral force of the drill bit. Strictly speaking, all the lateral forces determined by mechanical models and method are only nominal. (2) There are differences between the experimentally simulated anisotropy index of the formation and the actual downhole rock properties, such as confining pressure, the assumption of uniform and continuous rock anisotropy, etc. In addition, some of the experimental devices for measuring the transverse cutting index of the drill bit do not eliminate the effects of friction, which also causes systematic errors in the calculation results. (3) The existing control method basically relies on the dynamic analysis of the BHA to determine the drilling assembly. Once the drilling assembly is applied in the wellbore, the characteristics of the entire drilling system have been generally determined, and only the partial regulation can be performed, such as changing the weight on bit, varying the rotation speed or adjusting the tool face. If the mechanical properties of BHA are not accurately estimated or if the actual drilling trajectory deviates significantly from design due to the effects of strata and other random factors, drilling assembly has to be replaced. In particular, for the horizontal wells with oil sheet, the stringent requirements on the well trajectory often require frequent tripping and replacement of drilling assembly, leading to the increase of drilling costs. The mechanical model error, the measurement parameter error, the difficulty in acquiring some important calculation parameters in actual operation, the unchangeable BHA structure and characteristics, and the traditional control methods, all of them seriously restrict the effectiveness precision of well trajectory control technology, which makes it difficult to meet the accuracy and efficiency requirements of well trajec-

tory control for ultra-thin oil reservoirs and complex structural wells. How to overcome the error caused by the mechanical model of BHA in the actual drilling process? How to implement high-precision control when there are no accurate parameters available? How to timely change the inherent structure and inherent mechanical properties of BHA in drilling? How to drill into the target area when changes occur in the target location (caused by geological errors)? How to control the bit direction underground to make it accurately drill into the reservoir? These important issues caused deep thinking of the author and his colleagues in the mid-late 1980s. Considering that every major advance in drilling technology is the result of the introduction and combination of other disciplines and new technologies, meanwhile considering the commonality of drill bit and aircraft in attitude and trajectory control, and considering the successful application of engineering cybernetics in missile guidance[14], the author came up with the idea of introducing engineering control theory and aircraft guidance technology into oil and gas well drilling engineering to develop well trajectory control theory and technology. After a period of analogy, analysis and in-depth research, the author proposed a new research direction, namely, the well trajectory control theory and technology, in 1988, and hoped for “downhole closed-loop control“ and “use of means to solve problems”. The road in the beginning was difficult and cautious. From the judgment of the nature of the problem to the introduction or establishment of a new concept, from the thinking and definition of the connotation of the new field to the decomposition of a series of research topics, and from the deduction and determination of the system model, equations and boundary conditions to the conception and design of a patent program, the process was accompanied by repeated wandering, self-examination, self-questioning and self-verification and was basically conducted and completed in an “amateur” manner. At the end of 1991, the author discovered from technology news reported that some foreign counterparts were also working on or were beginning to work on this direction, and they also adopted the “closed-loop control” concept and formulation, which further strengthened the confidence of the author in continuing this research direction.

3.

Proposal of the downhole control engineering

In the late 1980s, the author proposed a new research field, well trajectory control theory and technology, and predicted that the well trajectory control technology would enter a new stage, “changing drilling” or even “automatic drilling” after the development process of the above mentioned three drilling stages. The background was as follows. From the perspective of demand, international drilling technology was focused on the vigorous development of horizontal wells and extendedreach wells, hence it was urgent to further improve the ability and degree of well trajectory control. From the perspective of technology, measurement while drilling (MWD) and logging

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while drilling (LWD) technologies provided the necessary foundations. Moreover, a few foreign drilling technology service companies successively developed products such as remote-controlled variable diameter stabilizers and remote-controlled variable-bend joints, which proved that it was feasible to give downhole execution tools an adjustable structure and characteristics, i.e., to achieve “changing drilling”. However, due to the limitation of technology, “automatic drilling” cannot be fulfilled at that time. In addition, the downhole execution tools at the time were generally open-loop remote control; moreover, the instruments and tools were separated from each other and a complete closed-loop control system was not yet formed. From the late 1980s to the early 1990s, well trajectory control technology developed toward closed-loop and systematic. Eight companies from the United States, France, the United Kingdom, Norway and Germany started to develop the practical systems for automated control of well trajectory. In China, the author completed the feasibility study, conceptual design and project decomposition of the “well trajectory control theory and technology”, and divided the investigations in this field into basic research, product development and experimental methods, and completed and declared the invention patent of “automatic hole drift angle controller” between 1988 and 1990[1516]. Based on this, out of the consideration of the following points, from 1992 to 1993, the author further proposed the new concept about “the theory and technology on downhole control engineering”, and expanded the scope of the study from the original well trajectory control to the engineering control issues in all aspects of oil and gas wells: (1) Foreign studies in this field mainly focus on the development of several typical system products, and the related companies set up barriers to each other and the results of relevant theoretical and method study were rarely reported. (2) The deep development of products needs guidance of the common basic theory and design methods on the downhole characteristics of oil and gas wells, which could bring about greater innovation, but there was no report on the relevant research results at that time. (3) The foreign products are limited to the automatic control of wellbore trajectories during drilling, and their principles and methods could be extended to other downhole operations and used for the development of related new tools and new systems. (4) Further research on the control mechanism and system design method based on the new theoretical research results and the formation of a modular design method enable the innovation and creation from designers without engaging in basic research, hence the success rate would be even greater. The proposal of downhole control engineering has promoted the development of drilling and other downhole technical research. Taking drilling as an example, after undergoing the prior three stages, drilling technology began to enter into the stage of “changing drilling” and developed toward the “automatic drilling” stage.

4. Main research contents of downhole control engineering The main research contents of downhole control engineering include the following four aspects. 4.1. Investigations about dynamics of downhole system and analysis methods of control signals Specifically, it includes the dynamics model of the downhole system (drillstring-inner fluid-annular fluid); the load properties of the downhole system and the initial and boundary (state of bottom hole and borehole wall) conditions; quantitative description of several physical fields (pressure field, velocity field, flow rate field, and dynamics field) of the downhole system; dynamic analysis and stability evaluation method of downhole control signals; analysis of downhole control signal transmission process, analyses of transfer function and frequency spectrum characteristics; and screening of downhole practical control signal. 4.2. Designs of downhole control mechanisms and systems, research of downhole parameters collections and transmission techniques Specifically, it includes the design methods and principles of signal generation, transmission and actuating elements; the design and analysis of generation, transmission and actuating elements of various practical control signals; the computer simulation of the generation, transmission and actuating elements of various practical control signals; the overall design method and principle of the downhole control system; the system synthesis and modular design methods for different control signals; the intelligent basic algorithm and system simulation of the downhole control system; the dynamic debugging method and technical evaluation of downhole control system; structural module library and simulation software library for various control signal subsystems (organizations); a variety of parameter acquisition methods and device (sensor) design; and the methods to improve the signal transmission characteristics (frequency and quality). 4.3. Development of downhole control engineering products With respect to product development, the control tools and systems should be developed according to the control requirements of various downhole operations (such as drilling, well completion, well testing, logging, oil recovery, and workover). For example, for the drilling process, a variety of open-loop remote-controlled downhole tools and closed-loop automatic control tools and instrument systems are mainly being developed based on the geometric guidance and geosteering of the well trajectory. 4.4. Development of experimental methods and the laboratories

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The laboratory should be capable of testing the results of

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the above-mentioned various theoretical studies, determining critical structural dimensions, assembling and tuning experimental prototypes, etc. Because of the particularity of the structure and working conditions of oil and gas wells, it is necessary to study the corresponding effective experimental methods and form relevant technical standards and operating procedures, and it is also necessary to design and develop special experimental equipment according to experimental requirements.

5. Main research progress and technology development trend of downhole control engineering Since 1988, the author and his research team have conducted basic research and technical research on downhole control engineering, and achieved a number of achievements and progress in the research of downhole system dynamics, controllable signal analysis, downhole control system and mechanism design, frontier technology research and development, experimental method research, laboratory construction, etc. The properties of the well trajectory control system were determined. The system is a multi-objective and multi-interference complex system. Based on the current understanding, it is still a grey system. Therefore, it is difficult to completely describe it with a deterministic model, and it is necessary to add a control section (closed-loop automatic control or openloop remote control). The controlled plant, controlled variable, reference input, and disturbance of the system were determined. For example, the formation variations and other random interfere during the drilling are regarded as the disturbance quantity, which provides the possibility of establishing a systematic analysis and control model. A series of signals that could be used for downhole control were studied, and a number of new concepts and methods were proposed. A total of more than 22 kinds of control signals in 4 categories (mechanical, hydraulic, geometric, and other categories) were listed and studied, from which the control signals were screened. Considering the particularity of the downhole control system, concepts and terms such as controllable signal, closed-loop control, control chain, and master control signal were further proposed, which was helpful for the analysis and design of the system and mechanism. The quality, characteristics and generation methods of a variety of controllable signals were mastered. The concept and research methods of downhole system dynamics were proposed to be the theoretical basis for analysis and modeling of the downhole control system. It combined the tubular mechanics based on solid mechanics and the downhole annulus hydraulics based on fluid mechanics, and took the drill string (or other string, such as casing string and work string), fluid in the tube and annulus fluid as a system for analysis and modeling, to achieve “fluid-solid coupling”. It was different from the traditional analysis method in which tubular mechanics and annulus hydraulics were isolated in

analysis. Therefore, it could reflect the physical characteristics of the downhole system more accurately. A variety of methods were used to establish the basic equations for downhole system dynamics, and the practical boundary conditions were abstracted from the characteristics of the operation process and equipment, both of which make the solution more universal. On this basis, the basic equation could be simplified as the corresponding mechanical equations for a specific technique. If only the drill string is studied and the annulus is ignored, the dynamic equations of the drill string can be obtained. If only the annulus is studied and the drill string is ignored, the basic equations can be evolved into the annulus hydrodynamic equations. Therefore, the traditional string mechanics and annulus hydrodynamics become the special cases of the downhole system dynamics. The basic issues that the downhole control system of oil and gas wells, in particular, the dynamics analysis of the drilling process, are to determine the true frequency spectrum and characteristics of the drill bit when working at the bottom of the well. Due to the scarcity of the relevant research works at home and abroad, theoretical research and experimental research on the variations of the drill bit loads in drilling were conducted; meanwhile, the special nipple for data acquisition was developed, and massive relevant data from experiments and actual drilling cases were collected and analyzed. The relevant rules were summarized, so that the mechanical analysis of the drilling system was established on solid experimental basis. Using the analysis method of downhole system dynamics, a study was conducted on the longitudinal vibration of the drill string for the drillstring-liquid coupling system in two cases (trip in and trip out), and the longitudinal vibration equations and boundary conditions of this solid-liquid coupling system were established and solved by the difference method. The influences on the fluctuation pressure and back pressure difference during trip in and trip out were analyzed quantitatively, and the correlation between the load of the drill string and the variation of the hook load was demonstrated, which provided a theoretical basis for the prediction of downhole working conditions, the analysis of downhole accident causes, and the determination of control signals for well trajectory control system. The theoretical and experimental research on characteristics of transmission channels was carried out. According to the requirements of the information transmission from the downhole to the ground, from the ground to the downhole, and from the downhole to the downhole, the physical modeling and theoretical analysis of various transmission methods, such as liquid pulses, electromagnetic waves and acoustic waves, etc., were carried out, which were supported by the corresponding experimental verifications. The research results provided the theoretical basis for channel design, pulse generator development, and information coding, etc. The objectives, basic structure and design principles of the

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remote control system and automatic control system for well trajectory were studied. The objectives and mathematical descriptions of the remote control system and automatic control system for well trajectory were given. Moreover, based on the function allocation, the basic structure and control method of the system were proposed and the basic design principles of the control system were elucidated. On this basis, the application characteristics of various methods such as switch control, fuzzy control and adaptive control in downhole control were further studied, providing a basis for system design and development. For the downhole controller, which is a key component of the downhole closed-loop intelligent drilling system, its function structure, hardware module composition and software structure model and its design principles and methods were analyzed, which laid the foundation for the physical design of the downhole controller. The concepts and modular design methods for the downhole control system and mechanism design were proposed. Based on the special downhole operating environment and characteristics of oil and gas wells, the design principles of the downhole control mechanism and system were researched and set up. The main control signals and structural forms of various downhole control mechanisms were investigated, such as hydraulic-control diversion mechanism, hydraulic-drive follower mechanism, displacement control mechanism, gravity signal mechanism, gravity edge finding mechanism, stroke-controlled cylinder diameter adjustment mechanism, downhole changeable angle mechanism, rotary cylinder diameter adjustment mechanism, pin groove mechanism, locking mechanism, umbrella-shaped cylinder diameter adjustment mechanism and back pressure differential mechanism, etc. The characteristics and transfer functions of signal generation, amplification, transmission and execution in these practical mechanisms were determined, and the hardware structure library and emulation software library of these mechanisms were established. Using the control chain and modular design method proposed by the author, the relevant mechanisms can be combined in an orderly manner to get a variety of technical solutions that can achieve the same control purpose, and these technical solutions can be compared, evaluated and optimized to form a design proposal, which provides a theoretical basis and guarantee for technological development. The library integrates hardware and software, and it is opened. It is a powerful tool for designers to engage in technical innovation, which can reduce the application threshold of manufacturing plants and field technicians, and can expand the scope of application. The measurements of numerous downhole engineering parameters and geologic parameters were studied, such as well deviation, azimuth, tool surface, weight on bit, torque, temperature and vibration for engineering parameters, as well as resistivity (near-bit resistivity and azimuthal resistivity), natural gamma and neutron porosity for geological parameters.

The measurement methods, models and experimental methods for these downhole parameters were studied, and practical techniques (techniques, devices and software) were formed. Research on downhole wireless short-transmission technology was carried out from the establishment of models to the formation of practical technology and to the application in tool systems. The methods for improving the transmission rate of the downhole information, including pulse generator design method, technology development, and coding method, were researched, and a combined code encoding method was proposed and used in the actual instrument system. The characteristics of the continuous pressure wave signals in the drilling fluid were analyzed and the treatment methods were studied, including the code modulation rules, signal mathematical model, frequency spectrum characteristics, transmission characteristics, detection and processing methods, and pressure phase shift keying signal demodulation and decoding, etc., which provided the theoretical basis for the study of continuous wave transmission technology. The design of the downhole generator was carried out. The developed downhole generators were used in actual tooling systems to provide the power supply guarantee for downhole measurement and control. A study on downhole control experiment method was conducted, and a downhole control engineering laboratory was built. In order to cooperate with the research and development of continuous wave transmission technology, experimental facilities, such as long wind tunnel and short wind tunnel, were designed and constructed. Based on basic theory, design method and laboratory construction, the author and his research team further committed to the research and development of downhole control tools and systems, such as remote-controlled stabilizer with variable diameter, positive-pulse wireless MWD system, near-bit geo-steering drilling system, pressure measurement while drilling system, wireless EMWD system, continuous wave wireless MWD system, automatic vertical drilling system, rotary steering system, seismic-while-drilling system, neutron porosity measurement tools, and remote-controlled layered mining system, etc. These control tools and systems with independent intellectual property rights have obtained a number of national invention patent authorizations, some of which have been industrialized and applied on a large scale, and achieved significant technical and economic benefits; while some others are being transformed from prototypes to products. These scientific research achievements have enriched and expanded the technical connotation and application scope of downhole control engineering. Currently, as a new research field and an emerging discipline branch in oil and gas engineering, downhole control engineering is developing to a greater extent and depth, which is due to the development trend of contemporary science and technology, and the universality of downhole control issues in

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oil and gas wells, and the requirements of the related process engineering for control technology. The development of cybernetics, information theory and system theory have promoted the rapid development of control technology and its large-scale application in many fields, which has greatly enriched the connotation and degree of the related engineering technologies. Generally, cybernetics improves the function of engineering technology, system theory improves the integrity of engineering technology, and information theory improves the precision of engineering technology. Their cross and integration with computer technology, engineering mechanics and mechanical science provide guarantee for the development of well control technology for oil and gas wells. The development directions and trends of downhole control engineering are as follows. (1) The downhole control technology is expanding its applications from drilling to other downhole processes, such as well completion, oil production, logging, testing, downhole operation and reservoir stimulation. (2) The control technology is developing from monomer tools or instruments to those with highly integrated, systematic and suitable for special operations. (3) The control system is developing towards real-time, automatic and intelligent. (4) The working characteristics of the downhole control system are developing toward high performance indicators (such as high temperature, high pressure, large capacity, high transmission rate, etc.), high reliability, long working life and strong adaptability. (5) The downhole control technology will combine more tightly with the hydrocarbon exploration and development, which will gradually become a direct means of increasing the discovery rate, the recovery rate and single-well production. Its basic goal is still to reduce the cost of oil per ton.

6.

leum industry, so as to generate the great economic and social benefits, and contribute to the promotion of the core competitiveness of China's petroleum engineering.

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Conclusions

25(6): 663–672.

This paper reviewed the background for the emerging of downhole control engineering and the development process of the related technologies. The importance, the role and the value of the theoretical innovation and technological innovation have been demonstrated again. Driven by demands and innovation, the downhole control engineering is bound to become increasingly perfect under the constant efforts of the relevant researchers, and the downhole control technology will become a high-tech with broad application prospects. It will provide significant technical support for underground engineering and even exploration and development for petro-

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[12] BAI Jiazhi. Applying longitudinal and transverse bending beam theory to solve stress and deformation of BHA. SPE 10561, 1982. [13] SU Yinao. New thinking on borehole trajectory control. Acta Petrolei Sinica, 1993, 14(4): 117–123. [14] QIAN Xuesen, SONG Jian. Engineering cybernetics. Beijing: Science Press, 1980. [15] SU Yinao. Automatic well slope controller: 90109809.4. 1991-08-21. [16] SU Yinao. Progress in research on downhole control engineering. Beijing: Petroleum Industry Press, 2001.