Abstract

With increasing difficulty of reservoir development and harsh geological conditions， higher drilling technology meets lots of challenges， especially in the large displacement wells and ultradeep wells. For Limit Extension Well， not only good drilling equipment is needed， but also drilling technology and basic theory are acquired. Bottom hole assembly （BHA） load transfer rule has important influence on Limit Extension Well. Therefore， on the basis of previous research， BHA load transfer rule is studied which considers the influence of drill string transverse vibration， and sensitivity analysis based on affecting factors of BHA load transfer rule. Research results show that transverse vibration of drill string has greater influence on the axial force and torque transfer. With the increase of friction coefficient and azimuthal variation， friction and torque of BHA increases linearly； with the increase of weight on bit （WOB）， friction and the rate of torque of BHA also goes up. The application of model considers drill string transverse vibration， which will not underestimate values of loads like previous dynamic models. It will support optimal BHA design and prevent early drill string failures. Therefore， the influence of transverse vibration cannot be ignored on load transfer in large displacement wells.

Key words： Drill string； Transverse vibration； Bottom hole assembly； Load transfer； Limit extension well

INTRODUCTION

With the increasing number of horizontal， large displacement horizontal and complex wells， it requires drilling equipment and drilling technology higher and higher for petroleum engineering. With the increasing drilling depth， not only advanced equipment is needed， but also the drilling technology and basic theory are acquired. BHA design optimization is very different compared with conventional design analysis， especially for large displacement horizontal well. It needs to consider more complex influence factors， such as optimize well track to reduce the friction torque， optimize drilling parameters to avoid serious drill string buckling， etc[1-4]. In addition， BHA vibration has great influence on BHA stress and drill string fatigue damage. Many scholars put forward a series of prediction methods to prevent drilling tool damage， but few scholars study the effects of the BHA load transfer rule under the condition of the BHA transverse vibration[5-9]. On the basis of predecessor’s studies， this paper study the movement of BHA load transfer rule considering BHA transverse vibration， which can be used to BHA design and optimizing， to provide the basic theory for reducing drill tool fatigue damage and drilling cost， to provide technical support for improving oil and gas well limit extension， to increase oil and gas production in signal well.

1. LOAD TRANSFER MODEL

Momentum theory is applied to reveal rule that inertial force and contact force of load transfer caused by transverse vibration along the borehole. BHA friction torque， buckling， vibration are considered in the model. Integrated axial force and torque transfer equation are corrected by considering additional contact coefficient of buckling degree and transverse vibration degree. Buckling analysis contains sinusoidal buckling and spiral buckling. Additional contact coefficient contains intermittent contact analysis between drill string and wellbore during transverse vibration. The comprehensive mechanics equations can be used for sensitivity analysis with different boundary loads and wellbore parameters. The model of load transfer is proposed to optimal BHA design in Limit Extension Well.

1.1 Fundamental Assumptions

（1） Sliding drilling or combined drilling mode；（2） BHA is homogeneous and isotropic；

（3） BHA has linear elastic deformation property；

（4）Transverse vibration is considered in BHA dynamic analysis on the influence of the axial load and torque；

（5） BHA is considered as many near-straight sections.

1.2 Modeling

1.2.3 Application of BHA Dynamic Transfer Rule Considering Transverse Vibration of String

For large displacement wells and horizontal wells， the force of drill string contain： friction force， collision vibration， transverse vibration generated by the “stick-slip”motion of drill string， and the vertical vibration produced by contact collision between the bit and bottom[13-15]. Drill string transverse vibration often occurs at the section of BHA in horizontal well. It has realistic guiding significance to calculate bottom-hole friction under the influence of transverse vibration. Torque dynamic transfer rule can be used to prevent drilling tool damage. Applications of theory research mainly contain：

（1） To design BHA and optimize drilling parameters with axial load and torque transfer rule in Limit Extension Well.

（2） To evaluate the influence of the downhole dynamic events on axial force and torque transfer along the BHA in Limit Extension Well.

（3） To support optimal BHA design and prevent early failures due to drill string buckling and fatigue.

（4） To compute the axial load and torque distribution along the BHA. Calculate the side force according to the stress on the bit and direction of the bit. These provide basic theory for horizontal well borehole trajectory controlling.

2.2 Calculation Result Analysis

The proposed model is applied to calculate drag and torque of M2 in Jilin oilfield， and BHA axial force and torque transfer along the drill string can be obtained. Considering the bottom drill transverse vibration effect， Figure 2， casing program in Table 1， well structure in Table 2. Drilling fluid density is 1078 kg/m3， weight on bit is 50 kN， rotary speed is 100 r/min， BHA length is 74.7 m， azimuth changes 10°， static friction coefficient is 0.35 that should be obtained from back-calculation using offset well data， dynamical friction coefficient is 0.28， and additional contact coefficient of transverse vibration is 1.0.

Sensible factors of BHA load transfer include： WOB， linear density of BHA， interspace between BHA and wellbore， dynamic friction coefficient of BHA， azimuth change， etc. Sensitivity analysis can determine axial force and torque transfer rule under dynamic load by changing dynamic parameters， and it provides basic theory for BHA design and mechanics analysis in Limit Extension Well.

The plots Fi versus Fi+1 of for varying friction coefficient and azimuth change along BHA section are displayed in Figures 5 and 7. The curves trend show： with the increase of friction coefficient and azimuth change， the friction force of BHA also increases. It is observed that the percentage of the values of torque increases with the friction coefficient increases.

The plots Ti versus Ti+1 of for varying friction coefficient and azimuth change along BHA section are displayed in Figure 6 and Figure 8. The curves trend show： with the increase of friction coefficient and azimuth change， the torque of BHA also increases. The reduction of the percentage Ti+1 per BHA section with Ti increases is a result of the less effect of drag force when higher axial force transferred to the drill bit.

Azimuth change and friction coefficient impact on torque are more apparent than the axial force. Azimuth change due to geosteering or wellbore placement issues increases the normal contact loads generated from torque and drag which consequently increasing the resultant frictional force and the axial compressive load. The total frictional force generated from torque and drag， buckling and vibration event will increase with the increase of dynamic friction factor of wellbore. The dynamic analysis of the downhole drilling events in Limit Extension Well and their effects on BHA components provides effective methods for predicting the limits of loads， torques and stresses before BHA service failure.

Therefore， to lower friction coefficient and to improve

borehole smooth degree are two important aspects for Limit Extension Well.

CONCLUSIONS

Compared with other analysis models， this paper proposes an axial force and torque transfer model that is a comprehensive dynamic prediction and analysis model considering drill string transverse vibration， drill string buckling and contact friction factors. The application of model considers drill string transverse vibration， which will not underestimate values of loads like previous dynamic models. It will be used to optimal BHA design and prevent early failures due to drill string buckling and fatigue.

The force of BHA transverse vibration and additional contact which caused by buckling has great influence on axial force and torque transfer. With increasing change of friction coefficient and azimuth， the friction force and the torque of BHA increase linearly. With increasing WOB， BHA friction force and the torque rate increase. Change of friction coefficient and azimuth has greater influence on torque transfer than friction force transfer.

Axial force and torque transfer rule is quite different among different well types during drilling process， such as complex structure well， extended reach well， etc.

Mathematical model of axial force and torque dynamic transfer is used to calculate drill string load distribution along the wellbore accurately， and it is also known as a comprehensive basic theory for BHA optimization in Limit Extension Well.

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