| With the rapid development of the world economy, the demand of energy is growing. The fields related to oil and gas exploration and development are also continuous expanding, extending from the old to the new area, from land to ocean, from shallow to deep, which lead to the increase of drilling risks and difficulty. As the technical equipment can not fully meet the requirements of these special operations areas, the efficiency of the drilling operations are low, especially drilling in the areas for safety window of narrow density which prone to leakage and lead to serious loss of back blowout accident caused huge property damage and personal injury. For pressure control of narrow density window drilling, conventional well control technology is helpless, so the study of effective well control technology is very essential to meet the needs of well controls. Compared to other conventional and unconventional well control technologies, micro-flow control system is an effective well control technology to deal with drilling pipe sticking, lost circulation and blowout incident for its advantages of simple to operate, reducing non-working hours and decreasing force on the casing shoe. Although researches on the micro-flow MPD have been made in foreign countries, shortcomings are still existed and efforts are still needed to further realization of tasks such as raising drilling speed, finding reservoir and drilling in narrow density window and so on. The objective of this paper is to present a set of micro-flow MPD ground automatic control system based on the flow rate monitoring at bottomhole, drilling mud flow rate and pressure monitoring at ground and study the key technologies of automation hardware design and ground control software program design, including the design of overall scheme of automatic control, electro-hydraulic linkage adjustment control, annulus hydraulic system, throttle opening control, special working conditions.On the basis of extensive investigation on the present status of MPD technologies development at home and abroad, the micro-flow MPD ground automatic control system is established, including the design of ground automated control, flow rate control strategy and micro-flow ground monitoring software. The computing system of upper computer receives the real-time data from the lower computer such as inlet flow rate, outlet flow rate, inlet pressure, outlet pressure, well depth and formation pressure to calculate the opening degree of throttle. At the same time, continue to read the data of flow rate. This process is repeated until meets the requirements of different drilling conditions.To meet the requirement of micro-flow automatic control of ground testing and the debugging of throttle opening, a set of SYZ18model ground micro-control simulating derrick is designed based on the hydraulics similar principles. On the basis of the CFG23model long auger pile frame of drill rigs, the modification on derrick system, rotary system, wellbore system, drill string system and excitation system are conducted. The simulating derrick can not only satisfy the need of ground automatic micro flow control experiments but also realize the simulation of gas. drilling mud and gas-drilling phase two-phase flow in the wellbore. Meanwhile, the flow conditions of drilling fluid can also be observed by the supervise hole in the simulation wellbore.Also, the hydrodynamic mathematical model and computing method for drilling operations are improved:(1) With consideration of factors, such as virtual mass force, interphase resistance, angular frequency, gas slip and so on, a model for pressure velocity mathematical model in gas-drilling mud and gas-oil-water-drilling mud flow in micro-flow control drilling operations is established. According to the uniqueness principle of pressure wave propagation velocity, a novel method for predicting the position of gas influx has been proposed based on pressure response time plate;(2) The model for compensation balance control of back pressure is proposed to control throttle valve in real-time by compensation of gas slippage pressure drop, which can maintain the balance between bottomhole pressure and formation pressure.(3) In gas-drilling mud two-phase flow, the model for surge pressure in gas-liquid two-phase flow is proposed with consideration of real time wave velocity.(4)To consider the variations of back pressure, void fraction, compressibility factor, and so on, the spatial sliding grid and multiple substitutions approaches are used to solve the mass and momentum conservation equations of multiphase flow to decrease the time complexity of computing and trace the gas migration velocity in real time.(5) Since phenomenon of saw tooth shaped caused by the transform of flow pattern is not in accord with the actual condition of multiphase flow in the wellbore, the cubic spline function interpolation method is adopted to eliminate the saw tooth shaped pressure, flow rate and void fraction curves.VB.NET is adopted to program the micro-flow automatic control software. The influence factors of ground micro flow control are analyzed based on case study. The analysis focus on the gas migration velocity, annulus void fraction, two-phase flow in gas solubility, the pressure wave velocity, pressure response time, throttle core suffered transient pressure induced by two steps linear valve shut and throttle closing scheme. Research results of the study can provide theoretical and practical guidance for the design of micro-flow controlled pressure drilling and field operations. |
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