Research on Eccentric Displacement of the Trajectory Control Tool of High Build-up Rate Wellbore

Ding Feng1, a, Li Wu1, Jiawei Wang1, Xuhui Liu1, Hong Zhang1 and Hanxing Li2 1School of Mechanical Engineering, Yangtze University, Jingzhou, China

2CNOOC Research Center, Beijing, China afengd0861@yahoo.com

Keywords: High build-up rate; working principle; eccentric displacement; control.

Abstract. The trajectory control tool of high build-up rate wellbore is a kind of point-the-bit rotary steerable tool, which some characteristics like stepless speed changing, high build-up rate and smooth trajectory. This paper introduced working principle of the trajectory control tool of high build-up rate wellbore, focused on the method that control wellbore trajectory through synthesis eccentric displacement vector of guide mechanism, analyzed the synthesis eccentric displacement vector formation and decomposition principle of the tool. At the end the concrete structure are described.

1. Introduction

As a result of most domestic and foreign oilfields have entered the late period of development stage, a majority of new exploration blocks are in special environment, such as marine, offshore, desert area, which made the exploration development difficulty and cost are greatly increased. On the other hand, old oilfields in order to realize stable production has to face a great quantity of developing problem in marginal reservoirs, small reservoirs, complicated fault block oil and gas reservoir, thin reservoir. The evolution and development of well-types were promoted by exploration and development needs, consequently the proportion of complex structure wells such as extended-reach wells, horizontal wells in ultra-thin reservoir, multilateral wells were growing rapidly.

The trajectory control tool of high build-up rate wellbore is a kind of point-the-bit rotary steerable tool, which happened to be adapted to the current trends in reservoir development, solved current difficulties of drilling technology. The tool is suitable for exploration of complex structure wells such as vertical wells, directional wells, ultra-deep wells, extended-reach wells, horizontal wells in ultra-thin reservoir, long-distance horizontal wells, and multilateral wells. Compared to existing drilling tools, there were many advantages, such as higher control precision, better casing quality, smoother hole, further displacement extension, higher rotational speed and drilling efficiency.

2. The domestic and global development of point-the-bit rotary steerable tool

There were 2 kinds of point-the-bit rotary steerable tool on the global research; they were Halliburton's Geo-pilot and Schlumberger's latest product: Power-Drive Xceed.

Geo-Pilot system is mainly characterized by the lower part of drilling shaft can be rotated at an angle in the housing, which was equivalent to the formation of an adjustable corner. Geo-Pilot system achieved effectively guiding and controlling direction of the drilling bit through controlling bending of the drilling string. The advantage was that build-up rat would not be influenced by formation lithology, even in soft strata and heterogeneous formations. The disadvantage was that the drilling

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shaft would be prone to fatigue failure under high-intensity alternating stress. In addition, high-precision machining is the key to ensure rotary effect of this system.

Schlumberger's Power Drive Xceed had solved the problems that push-the-point rotary steerable tools encountered in soft formation and open hole sidetracking. Only near bit stabilizer of this system got in touch with the borehole, and it no longer needed to push pads or slowly rotating the tool to control the position. Electronic measuring device within the drilling tool can detected its location, and automatically adjusted the tool to high side of borehole. However, the working life needed to be improved [1-3].

Domestic research in this field is still in its infancy, and research on rotary steerable tools are mostly push-the-bit. Yangtze University has manufactured the principle prototype of this point-the-bit rotary steerable tool, and completed the various tests indoors. The largest build-up rate can reach 8.6°/30m.

3. Working principle of the trajectory control tool of high build-up rate wellbore

Control principle and implementation method: the trajectory control tool of high build-up rate wellbore didn’t rotate its housing, and made the shaft bend by a biasing device between non-rotating housing and the shaft. When the drilling direction needed to be changed during drilling process, the biasing device received control instruction, which made the shaft bending, and then the tool started to deflect.

The key component of the biasing device is a set of eccentric mechanism with the combination of two eccentric circulars which can be controlled. When the underground automatic control is finished, the device will not rotate relative to the jacked fixed to the rotation centre so that it can always keep the rotation axis biasing the fixed direction and provides a fixed orientation angle. The single drive and combined movement in the inner and outer loop of the eccentric mechanism can realize the adjustable spindle control of the deflection angle, and it can also realize stepless deflection at any direction. High curve well trajectory control tool is mainly composed of these key parts, namely the measure and control nipple joint，electromagnetic clutch，high decelerate transmission and eccentric mechanism. It uses mechanical power to transmit. And the power comes from the drill pipe, drill pipe from the coupling to the rotation motion passed by the electromagnetic clutch high gear reduction ratio, after high reduction ratio gear reducer drive inside and outside, respectively, after the eccentric ring rotation, so that the spindle in different degrees of bending, provides for the drilling axis of a borehole are inconsistent with the inclination, to achieve stepless control drill inclination, to well trajectory control.

Fig.1 Structure chart of the trajectory control tool of high build-up rate wellbore

Cantilever

Drilling Shaft

Biasing

Focal BearingDriiling

Housing

Wellbor

Angular Displacement

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On the other side, the precise guide of the high curve well trajectory control tool is monitored by control system. It uses position sensor of the inner and outer loop and triaxial accelerometer to the measured signal to the controller for computing, and then the result of the operation through the communication terminal with the MWD measured hole deviation and azimuth Information transmitted to the ground monitor with the monitor. If you want to re-change well trajectory, issued from the ground to change conveyor shafts, bending angle and tool face instruction, sent by the controller via the communication terminal to perform to the biasing device, such surface and subsurface built the two-way communication channel, to achieve the well trajectory control purposes. In view of underground communications, detection and control instruments and equipment required power supply to work properly, so a high capacity and high temperature resistant battery short section as the underground power supply, battery temperature to overcome the past, the limited scope of the shortcomings of short life, and can in the high temperature, severe vibration, Impact, and bear a large acceleration under normal working conditions, for downhole tool provides persistent current, underground instrument and equipment to meet the electricity needs[4].

4. eccentric displacement vector of the trajectory control tool of high build-up rate wellbore

4.1 Composition of eccentric displacement vector

To set up coordinate, the wellbore of eccentric annular section can be regarded as the origin coordinates, then the high edge radical of wellbore(from wellhead to bottom)can be X axis direction. At the same time, the clockwise 90° of positive direction of X axis(right-hand rule) is Y axis direction. The wellbore centre 0 is set to beginning and the outer eccentric ring thick edge is ending, making the displacement vector as e1. Then the inner hole centre of outer eccentric ring is set to beginning and the inner eccentric ring thick edge is ending, making the displacement vector as e2, while the internal and external eccentric ring’s displacement vector is e. When e1 coincides with X axis and has the same direction with it while e2 coincides but has the opposite direction, the principle axes and the tool coax. Under this kind of circumstance, the tool is in free wheeling state and set the current internal and external state as the initial state. It is to set the angle between e1, e2 , e and the side of X axis direction as 1α , 2α , α (the clockwise is positive), and if the internal and external ring clockwise, the angle of the external ring is 1α , and the angle of internal ring is 180°+ 2α , then the co-vector of internal and external eccentric ring is :

21 eee += (1) If vector is deposed into X axis and Y axis, then there is :

2211 coscoscos ααα eeeex +== (2) 2211 sinsinsin ααα eeeey +== (3)

Next, the vector can be shown as:

eee

⎥⎦

⎤⎢⎣

⎡=⎥

⎦

⎤⎢⎣

⎡⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

αα

αα

αα

sincos

sinsin

coscos

2

1

21

21

(4) It will calculate that:

)cos(2 122122

21 αα −++= eeeee , 2211

2211

coscossinsinarctan

θθθθα

eeee

++

=

In the drilling process, the sensor can continuously test the angle of internal and external rotation of the eccentric rings by the compare between internal and external eccentric rings’ co-vector and set value. The adjustment is needed when there comes some deviation.

4.2 Decomposition of eccentric moving displacement

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When using the trajectory control tool of high build-up rate wellbore to drill, the eccentric displacement is an important affection to build-up rate. During the drilling process, to determine the eccentric displacement vector by according to the build-up rate, then deposing it into internal and external eccentric ring vector. At last, the angle of internal and external ring will be set as the displacement vector is not changed, the tool can drill forward through the determined build-up rate.

As the eccentric ring displacement co-vector is set to e, and its modulus value and angle will change as it changes. However, the e needs to be determined by the build-up rate and tool face. The wellbore of axis end section is set to coordinates origin, then the high edge radical of wellbore(from wellhead to bottom)can be X axis direction. At the same time, the clockwise 90° of positive direction of X axis (right-hand rule) is Y axis direction. There is a coordinates. The modulus value of e can be calculated by the angle of drill axis and tool axis and the intersection angle γ of drill axis and tool axis can be calculated by the set build-up rate. Then the relation between |e| and γ is:

( ))( 1

22

21

2

LLLLLe

+−−

=γ (5)

Here, L1 shows the distance between upper axis and eccentric ring, mm; L2is from end axis to eccentric rings, mm; L is the distance between two bearings, so L = L1 + L2, mm; γ is the angle of drill bit axis and tool axis, °; |e| is the modulus value of e, mm.

Therefore, to define build-up rate and the angle of tool face is helpful to make determined to the modulus value of e and its direction. From formulas (2) and (3), it will be obvious that, when the value of e, e1, e2 and α are known, using two uncertain numbers make two formulas clear and calculate the 1α and 2α with two pairs of answers, as following:

⎪⎪⎩

⎪⎪⎨

⎧

=±−+

=

=±−+

±=

1,0)2

arccos(

1,0)2

arccos(

222

21

222

2

111

22

221

1

kkee

eee

kkee

eee

παα

παα

m

When the co-vector of eccentric ring displacement was set, there existed two kinds of answers of angle between internal and external eccentric ring [5-8].

y=kx

Track of shaft centre

Track of outer ring centre

Maximum eccentric displacement

Fig.4 Motion track of eccentric rings

5. Reality of structure

The trajectory control tool of high build-up rate wellbore is made up of measurement and control sub, electromagnetic clutch, deceleration device, eccentric mechanism, dynamic sealing device and bearing stuffs, whose proto is just like the figure 5. Measurement and control sub mainly includes battery sub that serves for underground measurement and control, controller, position transducer that

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can measure the angle of eccentric rings, three-axis acceleration transducer, information transfer sub, hardware circuit and measurement platform controlling software. It is that ten short section of glass pipe, the control circuit board and three-axis digital acceleration transducer placed inside a steel pipe in which to measure the position of the eccentric ring angle differential sensor arrangement for detecting the rotation angle of the eccentric ring make the battery sub effecting.

Fig.3 Principle prototype of the trajectory control tool of high build-up rate wellbore

6. Conclusions

(1) The trajectory control tool of high build-up rate wellbore makes trajectory control reality by a equivalent bent joint structure which comes from the oriented organization’s synthetic eccentric displacement vector.

(2)Displacement synthesis is combined of two eccentric displacement synthesis derived from the ring, which controls the rotation of two eccentric rings, can control well trajectory.

(3) The trajectory control tool of high build-up rate wellbore adjusts the deviation angles through monitoring the angles of two eccentric rings. While the tool is in the inclined state, two eccentric rings start to rotate to the specified angle. And when the tool is in the deviation state, the two rings stay their angle and achieve continuing control of wellbore trajectory.

References

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[4] Li Hanxing, Jiang Wei, Jiang Shiquan. Analysis on control of eccentric displacement vector of the controllable eccentralizer rotary steerable drill ing tool. China Offshore Oil and Gas, 2008(3):184-187.

[5] Keng H.Kok, Hughes-Jones HUW et al. Case history：A robust point-the-bit rotary steerable system with at-bit imaging and 3D geosteering service integral to optimal wellbore placement in a complex thin sand reservoir. SPE 121186, 2009

[6] D Stroud,M Russell and Peach. Development of the Industry's First Slimhole Point-the-Bit Rotary Steerable System. SPE 84449, 2003

[7] Tetsuo Yonezawa, Edward J.Cargill et al. Robotic Controlled Drilling：A New Rotary Steerable Drilling System for the Oil and Gas Industry. SPE 74458, 2002

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[8] Stuart Schaaf, Demos Pafitis. Application of a Point the Bit Rotary Steerable System in Directional Drilling Prototype Well-bore Profiles. 62519, 2000

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