Evaluation of rock-bit interaction test under simulated ultra-deep well conditions based on similarity principle
Introduction
The depletion of shallow conventional oilfields coupled with ever-increasing energy demands urges oil and gas exploration and development targets to move toward deep and ultra-deep formations (Tulu, 2009; Norouzi et al., 2020). The American Petroleum Institute defines wells with drilling depth of more than 15,000 feet as deep wells and wells with a drilling depth of more than 25,000 feet as ultra-deep wells. In the process of ultra-deep drilling, the extreme environment found at bottom-hole could reach pressures up to 240 MPa and temperatures up to 250 °C (Ma et al., 2016). Under the extreme geological conditions mentioned above, the rock drillability gets lower with depth increase, and the abrasiveness of the drill bit gets more serious. The temperature field affects the physical and mechanical properties of the rock material and the variations in the temperature field causes thermal stress. In addition, the bottom hole confining pressure will increase the hardness and plasticity of the rock. From an economic perspective, the drilling rate of penetration (ROP) is the most important factor in determining the cost of drilling a well (Publications, 2007). Because of the concealment and complexity of oil drilling process, the comprehensive effect of various process parameters can only be measured in field tes. Therefore, the field test is not a completely ideal, economical and scientific test method. The high-temperature and high-pressure simulation test device can solve the problems of incomplete and inaccurate field test data, high test risk, lack of flexibility to change test plans and scientific and accurate acquisition of test data.
Since the 1950s, some institutes began to develop high-temperature and high-pressure simulation devices for the unnatural bottom-hole rock environment and the full-size drill bit drilling process. In 1956, Ekei used a small indoor drilling rig to conduct drilling tests on rock samples under different hydrostatic column pressure, confining pressure and pore pressure, for testing the effects of fluid properties and pressure on ROP and rock drillability, and studying the mechanism of rock fragmentation during drilling (Rollow, 1962). Rock drillability is measured by bit penetration rate method. To further understand the process and mechanism of rock fragmentation in rotary drilling, many researchers have adopted this method to study the interaction between a single bit tooth and rock under normal pressure and simulated downhole pressure, thereby find out the optimal mechanical parameters(Kelessidis, 2011a,b; Dougherty et al., 2014, 2015). The quantitative relationship between the geometric parameters of the wedge-shaped teeth and the rock crushing effect reveals the physical and mechanical properties of the rock during the drilling process, which contributes to improving the efficiency of mechanical fragmentation effectively (Schmidt, 1972; Howarth, 1986; Wijk, 1989, January; Thuro, 1997, April; Deng et al., 2007; Kelessidis, 2011a,b; Yarali and Kahraman, 2011). Mao (Mao et al., 2018) designed the most advanced micro-test device that can simulate the stress state of bottom hole pressure (including overlying rock, wellbore, formation and rock pore pressure). Through this equipment, it can simulate the rock breaking process of the drill bit in the field and evaluate the drillability of basalt, clay shale and sandstone samples under different wellbore pressures.
While the micro-test device can indicate some rules or trends, it is generally believed that the test results can only qualitatively explain certain aspects of the problem, rather than quantitatively evaluate and optimize the drilling parameters. As a result, several scientific research institutions carried out the research and design of full-scale drilling bottom hole pressure simulation test device, and used full-scale bit to carry out drilling test under the condition of simulating bottom hole pressure. Terra TEK, Inc. Drilling simulation test device is the first full-scale drilling simulation test device in the world. It consists of drilling rig, borehole simulator, drilling fluid circulation system, rock sample acquisition and processing, downhole tool test container and borehole stability test device. The full-scale drilling simulation test device of Schlumberger Cambridge Research Center was basically completed in 1992. The device can be used for drilling test of drill bit under φ 311.15 mm in rock samples with diameter of 600 mm and length of 1000 mm. After that, Japan National Institute for resources and environment, Japan National Oil Corporation Technology Research Center and Russia Drilling Technology Research Institute developed full-scale drilling simulation test devices to simulate different well depth conditions (Judzis et al., 2009; Kivade et al., 2015; Wu and Ye, 2019.).
The full-scale drilling simulation test device is more similar to the actual drilling site situation, but it also has some defects, such as huge equipment volume, high cost and so on. In addition, both the micro-bit drilling test and the full-scale drilling test were mostly carried out under normal temperature without in-situ stress or triaxial iso-stress conditions, which cannot simulate the influence of differential in-situ stress and temperature on drilling efficiency and optimization of drilling parameters during ultra-deep drilling. Currently, there is no research or development report on rock-bit interaction equipment that simulates the ultra-deep drilling environment with triaxial stress up to 240 MPa and temperature up to 200 °C, which hinders the simulation of real downhole rock drilling coupling, wellbore stability and bit evaluation.
In order to evaluate the drilling efficiency and bit wear in ultra-deep formation, the parameters of ultra-deep drilling simulator were calculated based on the similarity principle, and then the design and manufacture of ultra-deep rock-bit interaction simulator were completed. Considering the influence of formation differential stress change on bit footage, the effects of weight on bit (WOB) and rotary speed on torque, bit wear and rate of penetration (ROP) were studied for the first time under the conditions of in-situ stress of 240 MPa and temperature of 200 °C.
Section snippets
Parameter calculation of ultra-deep drilling simulator based on similarity principle
To develop an ultra-deep drilling simulation device based on the similarity theory, the similarity criterion must be derived first. There are many methods to evaluate the similarity criterion, such as the similarity conversion method, the dimensional analysis method and the matrix method. This simulation device parameter calculation selects the similarity conversion method in the equation analysis method to derive the similarity criterion. This method has a rigorous structure and a clear
Manufacturing of ultra-deep drilling simulator based on similarity principle
After obtaining the parameters of the simulation device based on the similarity theory, the design and manufacture of the drilling test simulation platform with the maximum stress of 240 MPa in the triaxial direction were completed. The physical simulation test equipment for ultra-deep drilling is mainly composed of servo motor, drill bit, axial loading cylinder, horizontal loading cylinder, bit cooling system, control system and signal acquisition system. Fig. 2 shows the design of loading
Physical simulation test study of rock-bit interaction in ultra-deep formations based on similarity theory
The American Petroleum Association defines deep wells as wells with a depth of more than 15,000 ft (4572 m) and ultra-deep wells as wells with a depth of more than 25,000 ft (7620 m) (Spaar et al., 1995, January). In order to study the influence of different confining pressure, WOB and rotary speed on bit interaction under ultra-deep geostress condition. The drilling footage efficiency and bit wear model of diamond bit under the conditions of in-situ stress of 240 MPa and temperature of 200 °C
Results and discussion
An ultra-deep drilling simulation device based on similar theories is an evaluation platform that can help optimize suitable rock breaking tools and drilling parameters. In order to evaluate the drilling efficiency, bit wear under ultra-deep formation conditions, and to study the influence of the change of differential stress on the drill bit footage, the following test data were obtained through a self-designed and manufactured ultra-deep rock drill interaction physical simulation test device
Conclusions
In this work, we calculated the parameters of the ultra-deep drilling simulator based on the similarity principle, and then completed the design and manufacture of the ultra-deep rock-bit interaction simulator. In order to evaluate the drilling efficiency, bit wear under ultra-deep formation conditions, and investigate the influence of the change of differential stress on the drill bit footage, the test data were obtained through a self-designed and manufactured ultra-deep rock drill
Contributor roles taxonomy (credit)
Zhongming Zhou: Investigation, Validation, Formal analysis, Data curation, Writing – original draft. Shouding Li: Methodology, Formal analysis, Data curation, Resources, Writing – original draft, Writing – review & editing, Supervision, Project administration, Funding acquisition. Xiao Li: Resources, Investigation, Funding acquisition. Dong Chen: Formal analysis, Bo Zheng: Data curation
Declaration of competing interest
All authors declare that they have no conflict of interest.
Acknowledgments
We thank the editors and anonymous reviewers for their helpful and constructive suggestions and comments. The work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA14040401), the National Natural Science Foundation of China (42090023), the Key Deployment Program of Chinese Academy of Sciences (No. ZDRW-ZS-2021-3-1, YJKYYQ20190043, ZDBS-LY-DQC003, KFZD-SW-422), the National Key Research and Development Program of China (No. 2018YFC1504803
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