Analysis and optimization of control algorithms for RSSTSP for horizontal well drilling
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Steering control algorithm plays an important role in a rotary steerable system for horizontal well drilling, including the determination of the well trajectory, vibrations, stability, durability among other variables. This work develops a control algorithm for three static push-the-bit rotary steerable systems (RSSTSP) (TSP is the abbreviation of “three static push-the-bit”). Based on the structure, mechanism, and working process of the RSSTSP, mechanical and mathematical models are proposed to determine the required steering force (amplitude and direction) to move the drill bit from a point to another. Additional equations are constructed to overcome the non-uniqueness and then compute the optimal forces for the three pads to achieve the required steering force. Moreover, a new control algorithm of RSSTSP is developed, considering the steerability, stability, durability, favorable area, unfavorable area, maximum usable magnitude of steering force. The proposed control algorithm is also applied to a new RSSTSP and tested on a GU-693-P102 well for validation. It is found that each pad force changes smoothly, the drilling tool is stable, and the well trajectory is consistent with the design, demonstrating that our proposed control algorithm is robust and effective for RSSTSP for horizontal well drilling.
KeywordsHorizontal well drilling Rotary steerable system (RSSTSP) Control algorithm Steering force Pad force Dogleg
With the technological development for unconventional oil and natural gas productions, the numbers of horizontal and three-dimensional multi-target directional wells have increased significantly (Ozkan et al. 2011; Jia et al. 2014; Orem et al. 2014). Drilling equipment not only needs to meet requirements for desired drilling trajectory, but also needs to work reliably in more complex stratum and harsher operating conditions, which presents significant challenges on the drilling technology (John et al. 2000; Kaiser and Yu 2015; Ikonnikova et al. 2015). In recent decades, rotary steerable systems (RSS) have developed very rapidly, in their capability to provide continuous rotation, constant steering, and smoother boreholes (Weijermans et al. 2001; Drummond et al. 2007; Hakam et al. 2014). RSS ensure steering the borehole when drill string is rotating, and are usually used together with a logging while drilling (LWD) system. Geological parameters are analyzed in real time, and then precise control of the directional trajectory is achieved based on geological conditions (Haugen 1998; Tribe et al. 2001; Torsvoll et al. 2010). So far, Schlumberger’s PowerDrive, Baker Hughes’ AutoTrak, and Halliburton’s Geo-Pilot have been the main representative technologies (Stuart et al. 2000; Tribe et al. 2001; Bian et al. 2011; Wang et al. 2014). RSS can be divided into static bias and dynamic bias according to different bias units and can also be divided into push-the-bit and point-the-bit approaches according to different directing principles. PowerDrive and AutoTrak belong to “push-the-bit,” while Geo-Pilot belongs to “point-the-bit” type. PowerDrive is of dynamic bias, while AutoTrak is of static bias (Schaaf 2000; Fontenot et al. 2005; Wang et al. 2014). This article focuses on three static push-the-bit (RSSTSP) like AutoTrak systems. RSSTSP have three stretching pads, which press against the well bore thereby causing the bit to press on the opposite side resulting in a direction change (Niu et al. 2013; Wang et al. 2014, Marck and Detournay 2016). Steering control algorithm plays an important role in any RSSTSP, and a robust control algorithm is the key factor to achieve the desired control effects (Seifabad and Ehteshami 2013; Hakam et al. 2014; Kremers et al. 2015).Because many of these techniques are not available to the public, the research papers for control algorithm for RSSTSP are very few in the published literature. Zhang and Yu analyzed the configuration and deviation principle of RSSSTP (Zhang 2000). Cheng and Jiang studied control method, using biasing displacement vector (Cheng et al. 2010). Du and Liu studied multi-solution and uncertainty for controlling three pad forces. Models were established with two pads and adapted to adjust and control magnitude and orientation of steering force vector, and another pad extended to wellbore without force (Du et al. 2008). Due to non-unique solutions, Li et al. (2015) proposed to use three pads working at the same time, where one pad force should have a maximum or minimum value determined by relative position of pad and steering forces. The aforementioned papers mainly deal with how to calculate steering force. However, vibrations, stability, and durability were not considered. Therefore, based on the structure and work process of RSSTSP, this work develops mechanical and mathematical models for the steering force and pad force. A new control algorithm is developed considering factors of steerability, stability, durability, “favorable area,” and “unfavorable area.”
Structure and work process of RSSTSP
The RSSTSP works as follows: The value and direction of steering force are first determined based on the current actual point and the expected point on the ground. The steering force is then transmitted downhole. Based on steering force, each pad force is next determined in accordance with predetermined control algorithms downhole. Lastly, the pads are pushed out by applying hydraulic pressure, and the expected steering force and well trajectory are realized. In the whole process, a robust algorithm for steering force and pad force is the key factor for achieving the desired control effects.
Maximum usable steering forces (Amax)
Amplitude and direction of the required steering force
Consider a RSSTSP used in a drilling operation. The amplitude and its direction of the required steering force should be determined based on the current point and orientation of the drill bit and the target point and orientation one would want the drill bit to be.
Amplitude of the required steering force (Ak)
If the value of A k is greater than 100%, it would not be drilled to the target point. In this situation, it is necessary to redesign well trajectory and redetermine target point until the value of A k is less than 100%.
Therefore, once A k is obtained, the amplitude of the steering force F can be calculated using Eq. (4).
Direction of steering force (αk)
The “+” indicates increase in “build force” or “walk force,” and the “−” indicates decrease in “build force” or “walk force.” Build rate and walk rate are also determined by steering force and the direction angle of steering force α k .
Full working to advance the inclination, while the azimuth is not changed.
Both the inclination angle and azimuth angle are all advanced.
Full working to increase azimuth, while the inclination is not changed.
The inclination angle decreased, while the azimuth angle increased.
Full working to decrease inclination angle, while the azimuth angle is not changed.
The inclination angle and the azimuth angle all decreased.
Full working to decrease azimuth, while the inclination is not changed.
The inclination angle and the azimuth angle all decreased.
No working. The inclination and the azimuth were controlled by BHA.
A new algorithm for pad forces (F1, F2, F3)
Non-uniqueness for pad forces
The angle (α1) between pad 1 and high side is measured by the RSSTSP system. Once F and α k are determined, there are three unknown parameters F1, F2, and F3. However, there are only two equations in Eq. (8). The solutions are thus not unique. To determine F1, F2, and F3, an additional equation must be established.
An additional equation for optimal pad force
For example, if the required steering force falls in area 1 (in which there is no pad), then area 4 (in which there must be a pad) is an unfavorable area. We thus set the pad force in area 4 with an unfavorable force: F1 = F uf, which gives an additional equation to Eq. (8), so that a set of solutions for all the pad forces can be uniquely found. If the steering force falls in area 2 (in which there is a pad), then area 2 is the favorable area. In this case, a favorable force should be assigned to pad 2: F2 = F f, which gives an additional equation. If the steering force is in area 3, then area 6 is the unfavorable area and F3 = Fuf becomes an additional equation. If the steering force is in area 4, then area 4 is the favorable area and F1 = Ff gives as an additional equation. If the steering force is in area 5, then area 2 is the unfavorable area and F3 = Fuf becomes an additional equation. If the steering force is in area 6, then area 6 is the favorable area and F2 = Ff becomes an additional equation.
A new algorithm for pad forces
According to the relative status of the steering force, one of the pads can be determined in either the favorable and unfavorable areas, following the procedure detailed in “Direction of steering force (α k )” section. Hence, there are six possible situations.
Integrating the steer capability, stability, durability, favorable area, unfavorable area, maximum usable magnitude of steering force, a new control algorithm of RSSTSP can easily be written using Eqs. (12)–(18).
The results demonstrate the well is consistent with design track as designed. The maximum dogleg rate is 5.92°/30 m, and well trajectory is smooth. It validates that our proposed control algorithm is robust and effective for RSSTSP systems.
According to the current and targeted build and walk rates, this work establishes a method to calculate the work efficiency (A k ) and direction of the required steering force (α K ). The calculated results offer key instructions to transmit from ground to downhole for a RSSTSP drilling process.
Considering maximum usable magnitude of steering force, steerability, stability, durability, favorable area and unfavorable area, a new algorithm for assigning each pad an optimal force is developed to achieve the required steering force to move the drilling bit from point A to point B.
The present new control algorithm is applied to a new RSSTSP, and a field test is carried out in a GU-693-P102 well for validation. It demonstrates that the new control algorithm is robust and effective for RSSTSP systems.
The research is mainly supported by Natural Science Foundation of Hei Long Jiang Province (No. QC2017042).
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