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A polycrystalline diamond compact (PDC) bit combined with a vertical steering tool known as Power-V has shown effective results in controlling deviation and increasing the rate of penetration in vertical wells. However, it has been found that the drill string is prone to stick–slip vibrations in this situation. At present, the mechanism of stick–slip vibrations in drill strings caused by Power-V has not been explored clearly. In this paper, an excitation model based on the working mechanism of Power-V is established, and the impact of lateral force on the PDC bit by Power-V is analysed. Based on a study of the structure and the force characteristics of PDC bits, a new dynamic torque model is proposed by using the Stribeck dynamic friction model. Then, the internal mechanism of stick–slip vibration induced by Power-V is revealed by combining the finite element analysis method of drill string dynamics with the new dynamic torque model. The stick–slip vibration characteristics caused by Power-V are clarified through a case study. This research indicates that Power-V can significantly increase the lateral force on the PDC bit during the drill process, which promotes deeper cutting between the gauge teeth of the PDC bit and the wellbore wall. Thus, the PDC bit experiences greater friction torque and rock breaking torque in the circumferential direction, which results in a greater susceptibility to stick–slip vibrations. [ABSTRACT FROM AUTHOR]

The vibration of a drill string can cause multiple hazards in the process of drilling operations. The conversion of excessive vibration energy of the drill string into the rock breaking energy can provide the dual effect of reducing the risks and improving the rock breaking efficiency of drilling. To address the task, a drill string absorption & hydraulic supercharging device has been developed. However, the design and exploitation of the drill string absorption & hydraulic supercharging device needs a comprehensive understanding of the characteristics of conversion of the drill string's vibration energy. Based on the drill string dynamics experimental device, a simulation test platform has been developed to evaluate the efficiency of conversion of the vibration energy. The analysis of the test results is used to evaluate the conversion characteristics of the vibration energy of the bottom hole drill string. The results show that the energy conversion efficiency depends on the elastic stiffness and installation position of the drill string absorption & hydraulic supercharging device, the overall rotational speed, and the WOB (weight on bit) of the conversion device. With increase in the elastic stiffness, the conversion efficiency of the vibration energy first increases and then decreases. The closer the installation position of the drill string absorption & hydraulic supercharging device is to the bit, the better the conversion efficiency. With increase in the rotational speed, the conversion efficiency of the vibration energy increases. With increase in the WOB, the conversion efficiency of the vibration energy first increases and then decreases. The research results provide the background for the development and application of speed raising tools based on conversion of the drill string vibration. [ABSTRACT FROM AUTHOR]

Friction between the drill string and the well wall is one of the key factors restricting the rate of penetration (ROP) in the process of sliding drilling in a horizontal well. The problem is effectively solved by applying the vibrating drag-reduction technology. In this paper, the authors propose a new type of mechanical rotary percussion tool driven by the positive displacement motor (PDM) and simulate the drag reduction effect of the tool in a horizontal well. The results show that the drag reduction effect depends on how the weight on the bit (WOB) is transmitted to the bit. When the excitation frequency is high, it causes severe fluctuation of the WOB. To maximize the overall WOB, the excitation frequency needs to be optimized. The WOB fluctuation contains not only the excitation frequency but also the double-excitation frequency and triple-excitation frequency. The research results can provide theoretical guidance for the application of the rotary percussion tool in horizontal wells. [ABSTRACT FROM AUTHOR]

It is generally recognized that the stick-slip motion of a polycrystalline diamond compact (PDC) bit is responsible for the low rate of penetration (ROP) and premature failure of the PDC bit when drilling in hard rock formations. To solve this problem, a torsional vibration damping tool was developed. A working principle model of the tool and a calculation model of the optimal spiral angle of the tool was established. The stability of the tool was analyzed, and the parameter range of spiral guideline thread stability was determined. The analysis revealed that the tool can automatically adjust the weight on the bit (WOB) and the rotary speed of the PDC bit through the spiral guide and the disc spring, which could suppress the torsional vibration of the PDC bit and convert part of the vibration load into cutting torque for rock breaking and thus to improve the rate of penetration (ROP). The optimal spiral angle of the tool is proportional to the axial stiffiless, bit diameter, and rock hardness of the drill string system and inversely proportional to the torsional stiffness of the drill string system and the reclining angle of the PDC bit. The stability of the tool is related to the spiral angle, length, and friction angle of the spiral guide. The tools were applied in test wells, and the results indicate that the torque fluctuations are significantly decreased and effectively extend the service life of the PDC bit. During the test process, the tool exhibits steady performance, the feasibility of the tool calculation model was verified. It is anticipated that the presented results will make such drilling a more feasible method for well services. [ABSTRACT FROM AUTHOR]

High-temperature and high-pressure wells are facing a lot of problems such as complicated stress on the production string, severe deformation, and annular pressure, which seriously restrict the pace of drilling and development of HPHT oil and gas resources in China. In this paper, theoretical analysis and software calculation are combined to study the stress and deformation of an HTHP well string. Based on the research results, the authors have established an algorithm and method of mechanical analysis for high-temperature and high-pressure wells. The method could be effectively applied to drill string analysis in high-temperature and high-pressure well conditions. Lastly, the developed method of analysis and calculation algorythm has been verified for the actual production conditions. Based on the proposed calculation method, we have evaluated the force, deformation, and trap pressure values of the drilling string and packer under different working conditions. The results can be used to assess the safety of field operations. In this paper we provide the algorithm and method for production string analysis in HTHP wells. [ABSTRACT FROM AUTHOR]

When evaluating the safety, reliability, and economic efficiency of casing strings in drilling operations, it is essential to define a reliable criterion of safety. According to the field statistics, when the casing tensile coefficient was in the range of 1.6-2.0, no casing failure happened in some deep and ultra-deep wells. On the other hand, the statistical data results need confirmation with an adequate scientific basis. In this paper, based on engineering experience and available research data, the authors have analyzed the types of axial load and the key factors influencing the casing tensile coefficient and proposed a new method to calculate the axial loads. Based on the reliability theory and processing quality reliability, the authors have established a method to predict and calculate the casing tensile coefficient. Based on the probability evaluation of the casing strength failure, the casing tensile coefficient is obtained. The method has been further verified by comparison with the field production data for deep wells of the Tarim Oilfield. The results show that the calculation method can reduce the acceptable casing safety value of the casing tensile coefficient to 1.5. The results have been applied and verified in the Tarim Oilfield, thus providing technical support for the development of deep oil and gas. [ABSTRACT FROM AUTHOR]

Casing drilling has been recognized as a practical method to reduce drilling costs and avoid particular drill string problems that often occur in conventional drilling processes. To gain a better understanding of the whirling motion of rotating casing, the energy method was adopted to calculate the critical whirling speeds of casing for both pinned-pinned and fixed-fixed types of support. The calculated critical whirling speed of rotating casing for fixed-fixed supports was found to be 2.35 tunes than that of pinned-pinned supports. To observe the motion patterns of rotating casing in different cases, experiments were conducted in a transparent wellbore. Rotary speed and inclination angle were found to be the two main factors that affect the motion pattern of rotating casing. For the case of 300 gallons per minute (gpm) flow rate with annular fluid of water whirling motion was observed to initiate with a rotation speed of 120 rpm. Steady whirling motion was observed at ~155 rpm and higher With increasing inclination angles, whirling motion was found less likely to occur The backward whirling was not observed in the experiments because the friction between casing and simulated wellbore was not large enough. The variations of torque as a function of rotary speed at different flow rates, inclination angles, and rates of penetration (ROP) were also investigated, and the results were found useful for predicting the quality of borehole cleaning. It was shown that flow rate, inclination angle, and ROP have notable influence on the torque required to rotate casing. With increasing inclination angle, torque was found to increase and reach the maximum value at the horizontal position. For a given ROP, torque was found to decrease with increase in flow rate. For a given flow rate, torque was found to increase with increased ROP. [ABSTRACT FROM AUTHOR]

Determination of the reasonable and safe speed for tripping or running casing to avoid downhole problems requires the development of a methodology for modeling the rheology of drilling fluid and pressure fluctuations. Based on the model of flow in a narrow channel, a new calculation model of pressure fluctuations for a Robertson-Stiff (RS)fluid in a vertical well is derived in this paper. Unlike the simplified pressure fluctuations prediction method based on flow in a narrow channel, this model considers the actual difference in stresses between the wall of the moving drill string and the borehole wall, yielding more reliable prediction values. Results indicate that pressure fluctuation values calculated by this model are in good agreement with the values measured under field cjnditions: the average deviation corresponds to 2%, the maximum deviation does not exceed 7%. The pressure gradient directly depends on the hipping speed, rheological parameters of the fluid, and the ratio of the inner and outer diameters of the annular space, i.e. the gradient increases with an increase in any of these parameters. The findings presented in this paper may be of interest for determining safe operation modes for wells with enhanced safety requirements, such as drilling with casing or in deep-water drilling, including selecting the tripping speed, the rheological parameters of drilling mud, and the optimal sequence of casing installation. [ABSTRACT FROM AUTHOR]

In horizontal well drilling, the success of the drilling operation mainly depends on optimal selection of the drilling trajectory. In this paper-, we propose a method for optimizing horizontal well trajectory design based on the numerical optimization methods PSO (particle swarm optimization) and AHP (analytic hierarchy process). We selected four functions as the parameters which affect the selection of the optimal solution: the shortest length of the well trajectory, the highest target hitting accuracy, the lowest cost of drilling, and the least drilling string friction. The key parameters for a horizontal well (the KOP (kick-off point), BUR (buildup rate), and hold angle) were calculated by ththe PSO method in Matlab. Applying the AHP mathematical tool for a systematic approach to decision-making, we ranked the solutions according to the engineering and design requirements. The new method proposed in this paper lets us overcome the limitations of the optimization algorithm and to recast empirical judgments of experienced analyzers in mathematical language, thus avoiding subjectivity in selecting the best solution. The method was applied to and tested on conditions of the Yanchang field. As shown by the experimental study, the proposed method is a simple and reliable method for selecting the optimal trajectory. [ABSTRACT FROM AUTHOR]