Proposed Steering Mechanism Reduces Tortuosity in Horizontal Wells

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This paper describes a new approach to evaluating the effectiveness of the rotary-steerable-system (RSS) steering mechanism on wellbore tortuosity in horizontal wells. Discussing the relative merits of a push-the-bit steering mechanism vs. a point-the-bit steering mechanism is an oversimplification; neither mechanism can deliver the wellbore quality the industry demands from RSS. The present study introduces the continuous proportional steering method (CPSM) and demonstrates how this mechanism can provide superior wellbore quality by reducing wellbore tortuosity.


For this study, 18 wells were chosen on the basis of geology, trajectory, and bottomhole assembly (BHA). The study focused on the horizontal 6⅛- and 5⅞-in. production sections, where the wellbore quality is more critical. These sections typically require minimum changes in inclination for reservoir navigation, and any tortuosity resulting from the steering mechanism can be identified easily.

The goal of this study is to evaluate the accuracy of RSS steering mechanisms available in the industry. The study was performed on wells drilled by different steering mechanisms yet logged by the same wireline service provider. Analysis of the wireline data shows that drilling with CPSM significantly reduces average tortuosity by 4 to 5 times, average dogleg severity (DLS) by 3 to 4 times, and average angle change (AAC) by 5 to 6 times. Results show that intervals drilled with CPSM are more continuous and smooth compared with push-the-bit and point-the-bit mechanisms.

RSS Mechanisms

Currently, three different steering mechanisms are considered, push-the-bit, point-the-bit, and CPSM systems.

In a typical push-the-bit steering mechanism, a force is formed against the wellbore side wall, causing the bit to push on the opposite side wall and leading to a direction change. The multiple pads mounted on the body of the RSS are activated one at a time by diverting some of the mud flow through a controller valve. The controller will orient the valve in the desired direction. Therefore, each pad that crosses against the valve will be activated, whereas the other pads will be deactivated. Consequently, the pads will go into a continuous cycle of opening and closing to meet the directional plan.

Alternatively, a typical point-the-bit steering mechanism causes the bit direction to change relative to the rest of the tool by tilting the bit with an internal deflection running through the RSS.

While drilling tangent or horizontal intervals, the diverter valve in a push-the-bit system will be closed because no directional change is required. No mud flow will activate the pads, thereby deactivating the steering system. In a point-the-bit system, continuous bit tilt is not applied in tangent or horizontal sections because no directional change is required.

System deactivation commonly will lead to the BHA dropping inclination because of gravity. At a certain threshold, the system will be reactivated to the build angle and will correct the inclination to remain within the desired value. Overshooting also occurs commonly when passing the correct inclination angle. Therefore, the wellbore will oscillate around the planned target well path until it reaches the well or section total depth.

In contrast, the CPSM forms the directional control by use of pads mounted on a slow-rotation sleeve to ensure precise steering. Each pad is activated by a separate hydraulic motor controlled by a downhole processor connected to a hydraulic-actuator piston. The electronic processor ensures the application of the precise amount of force to the pad relative to the desired bit direction vector. The pressure to the three pads is applied continuously with no switched-off cycle, so the effect of gravity is eliminated in tangents and horizontal sections.

Wellbore Tortuosity

The term “wellbore tortuosity” has different definitions, and, until now, no commonly agreed definition of this phenomenon existed. Here, wellbore tortuosity can be generally defined as any unwanted deviation from a straight line or the well plan.

Wellbore tortuosity is not a measure of the complexity of a 3D well plan, although the term has sometimes been used incorrectly in this context, but rather a measure of the inevitable, unwanted undulations around the well plan. Theoretically, wellbore tortuosity can be evaluated by comparing the planned well path with the actual surveys that will determine the actual well path. No industry standard exists yet to evaluate wellbore tortuosity numerically.

Wellbore-Tortuosity Estimation

Average Excess DLS Over Plan. Average excess DLS over plan can be calculated from the directional planned well path and the actual well path derived from the survey. The average excess DLS over plan is a measure of how the total actual drilled dogleg compares with the total planned dogleg. The tortuosity will be accurate because of the length interval reduction that was used to analyze the tortuosity from the high-density wireline data with a 0.25-ft interval. Therefore, the method with the wireline data is currently the most accurate way of measuring wellbore tortuosity.

AAC Criterion for Horizontal Wells. The AAC can be considered the average of the absolute deviations from 90° of each individual inclination measurement. The absolute deviation was obtained because the inclination can be above or below 90° because of poor directional control or deliberate geosteering. The AAC provides a weighted average in case surveys, which are not taken at equal distances. All the survey data used in this study were taken in equal length intervals (i.e., 0.25 ft) to optimize calculation accuracy. Normally, the larger the AAC value is, the more severe the wellbore undulations are.

Wellbore-Tortuosity Evaluation

Wellbore tortuosity was examined for a number of directional planned well paths and associated with actual drilled well paths. The data were used to compare the performance of the sections drilled with the CPSM mechanism with those drilled with the push-the-bit and point-the-bit steering mechanisms.

Use of high-accuracy and high-density wireline data, normally recorded over very short lengths (i.e., typically 0.25 ft), is the most accurate method to assess wellbore tortuosity.

Tortuosity analysis was performed on an interval of the section with a certain objective; that is, to hold the inclination at the same true vertical depth in the reservoir rather than on a complete section. This approach enables a direct comparison of the intervals drilled with CPSM with those drilled with the push-the-bit and point-the-bit steering mechanisms.

In Field A, six wells were selected and investigated; one well was drilled with the CPSM steering mechanism, four wells used a point-the-bit system, and one used a push-the-bit system. The study discovered that a push-the-bit steering mechanism was not able to keep a good constant inclination while drilling a lateral section, compared with the other two systems studied. The push-the-bit steering mechanism delivered an average tortuosity of 19.01°/ft, an AAC of 0.94°/ft, and doglegs of 10.45°/100 ft. The point-the bit system was second with an average tortuosity of 18.29°/ft, an AAC of 0.214°/ft, and doglegs of 10.30°/100 ft. The CPSM steering system demonstrated the best performance with an average tortuosity of 5.65°/ft, an AAC of 0.01°/ft, and doglegs of 3.8°/100 ft.

Only two wells were identified as having similar characteristics in Field B. The first well was drilled using the CPSM steering mechanism and carried an average tortuosity of 0.05°/ft, an AAC of 0.01°/ft, and doglegs of 1.02°/100 ft. The second well was drilled using a point-the-bit system as a steering mechanism and had an average tortuosity of 23.15°/ft, an AAC of 0.30°/ft, and doglegs of 12.52°/100 ft. In this field, it was not possible to evaluate the push-the-bit system because no well had the same previously selected characteristics.

Three wells were chosen in Field C. Each well was drilled with a different steering mechanism. The CPSM had an average tortuosity of 4.531°/ft, an AAC of 0.176°/ft, and doglegs of 3.25°/100 ft, showing better inclination-hold performance when compared with other systems. Alternatively, the point-the-bit mechanism presented the worst performance of the three wells, with an average tortuosity of 22.85°/ft, an AAC of 0.47°/ft, and doglegs of 12.37°/100 ft. It was followed by the push-the-bit system, with an average tortuosity of 16.54°/ft, an AAC of 0.257°/ft, and doglegs of 7.84°/100 ft.

As in Field B, only two wells were selected for Field D. CPSM and push-the-bit systems were evaluated. Again, the CPSM demonstrated better accuracy compared with the push-the-bit system. The CPSM was able to deliver a well with an average tortuosity of 3.62°/ft, an AAC of 0.02°/ft, and doglegs of 2.8°/100 ft, compared with push-the-bit system that had an average tortuosity of 14.51°/ft, an AAC of 0.117°/ft, and doglegs of 8.2°/100 ft.

Finally, five wells were chosen in Field E. The push-the-bit system was not able to maintain good performance or hold the inclination while drilling the horizontal section, revealing an average tortuosity of 12.08°/ft, an AAC of 0.168°/ft, and doglegs of 6.97°/100 ft. The point-the-bit system followed with an average tortuosity of 6.01°/ft, an AAC of 0.267°/ft, and doglegs of 2.75°/100 ft. Nevertheless, the wireline survey showed that the CPSM steering mechanism had an average tortuosity of 3.53°/ft, an AAC of 0.056°/ft, and doglegs of 2.75°/100 ft.

Because of differing results between push-the-bit and point-the-bit mechanisms in all five fields, an alternative approach was chosen to compare the hold-inclination performance factors such as tortuosity, AAC, and dogleg averages across all five fields (18 wells) investigated. The behavior in each steering mechanism while drilling a tangent section can be clarified by plotting them on an enlarged scale plot. Fig. 1 illustrates the behavior of the CPSM, point-the-bit, and push-the-bit mechanisms.

Fig. 1—Average tortuosity vs. steering principle.


Fig. 1 shows that the CPSM system revealed an average tortuosity of 3.49°/ft across all 18 wells analyzed. The push-the-bit system saw the second-best performance with an average tortuosity of 14.84°/ft, followed by the point-the-bit system with 17.88°/ft. On the basis of these results, the CPSM mechanism is able to deliver better hole quality and avoid microdoglegs. Regarding tortuosity, the CPSM mechanism had an impressive 4.25-times improvement when compared with the push-the-bit mechanism.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 28497, “Effect of the Rotary-Steerable-System Steering Mechanism on Wellbore Tortuosity in Horizontal Wells,” by Albaraa Alrushud, SPE, and Muntasar Mohammad, SPE, Baker Hughes, a GE Company, and Victor Oliveira, SPE, and Bader Zahrani, Saudi Aramco, prepared for the 2018 Offshore Technology Conference Asia, Kuala Lumpur, 20–23 March. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission.

Proposed Steering Mechanism Reduces Tortuosity in Horizontal Wells

01 November 2018

Volume: 70 | Issue: 11


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