Continuous Inclination Measurements and BHA Modeling Help Detect Microdoglegs

Fig. 1—3D view of pipe in borehole.

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Microdoglegs are a natural effect of any vertical or directional well that can explain a wide variety of downhole problems. A trajectory-prediction model able to calculate the inclination and azimuth approximately every 12 in. has been developed to estimate microdoglegs using standard surveys, bottomhole-assembly (BHA) data, and steering parameters. This new methodology combining downhole data measurements with drillstring-modeling analysis highlights the potential for drilling optimization and wellbore placement.

Wellbore Trajectory

Standard Surveys. Surveys are generally taken at an interval of every 95 ft, the length of one stand. While the general recommendation is to decrease the survey interval when building faster than 3°/100 ft, this is often neglected because there is no advantage seen in wellbore-positioning-uncertainty models.

The well path between each survey point typically is calculated using the minimum-curvature approach, which assumes a curve of equal angle along the surface of a sphere with only one radius in a 3D plane. Looking at the well as a whole, this approach appears logical and yields reasonable-looking trajectories; however, when examining more-frequent survey data, it becomes obvious how this method can mislead users to think that the well path is much smoother than it actually is. Continuous-survey measurements have enabled the industry to take a closer look at what is happening between survey points, in highlighting microdoglegs quite often undetected by standard surveys.

Continuous Surveys. Doglegs are generally discussed on a well level. Wells are analyzed for tortuosity looking at the change in trajectory from one survey point to the next. Little thought is given to what changes happen between those points unless a dysfunction occurs. Aggressive directional work can lead to the creation of microdoglegs, or doglegs on a scale of a few feet. Quick changes in direction create microdoglegs, which can contribute overall to higher torque and drag.

In examining continuous surveys, the actions of the directional driller can be seen clearly and doglegs can be examined more closely. While continuous-survey data have become more common in the industry, not all measurement-while-drilling (MWD) tools are equipped for the measurements and, generally, data must be processed at surface on the basis of the previous survey. In the absence of continuous-survey data, BHA modeling run on a step-by-step basis can aid in wellbore placement, failure analysis, and post-well evaluation.

Measurement Tool. A newly commercialized MWD tool designed for unconventional plays was used to gather the data used in this case study. This new MWD tool provides a full suite of measurements specially designed for directional drilling and geosteering to improve wellbore placement. It consists of a collar-based system to provide key measurements, including continuous inclination, shock and vibration, azimuthal gamma, downhole pressure, resistivity, weight on bit (WOB), torque on bit (TOB), and bending moment.

Rock/Bit/BHA Model

To reproduce the path taken by the bit/BHA system, a robust 3D rock/bit/BHA model is needed to predict the response of the BHA under a wide range of specific drilling scenarios and parameters. The model used for the reproduction of trajectories is the result of many years of research and field validation and couples a rock/bit model with a BHA model. The rock/bit model takes into account several key bit measurements such as the bit profile and the active gauge, the most important of which is the passive gauge of the bit. In order to describe the bit behavior fully, two parameters are used: steerability and walk angle.

Bit steerability is defined as the ability of the bit, under a lateral and axial force, to initiate a lateral deviation and is calculated as the ratio of lateral to axial drillability.

Walk angle is the angle measured in a plane perpendicular to the bit axis, between the direction of the side force applied to the bit and the direction of the lateral displacement of the bit.

Trajectory Prediction. The rock/bit/BHA model enables the calculation of the build/drop and turn rate of the directional system, coupling the bit behavior (steerability and walk) and the BHA. If one looks for the local response of the directional-drilling system over a distance of approximately 1 ft, a step-by-step approach is chosen to be able to reproduce local doglegs, such as the slide/rotary pattern of a steerable mud motor, or any borehole oscillations.

The principle of the step-by-step approach is to calculate the BHA deflection at different bit depths in small step increments. A recommended step length is approximately 1 ft and no greater than 2 ft. Fluctuations in WOB and toolface are accounted for, and formation changes can be accounted for when unconfined-compressive-strength and dip/strike data are available. The result is a calculated trajectory that more closely resembles the path the BHA actually creates during drilling.

Case Study

The well analyzed in this case study is a typical US shale well drilled in the Eagle Ford play. The well is characteristic of shale wells, with a 9⅝-in.-casing shoe set high in the vertical section at approximately 4,000 ft. A curve with planned doglegs of approximately 10°/100 ft is landed in the shale with a 6,000-ft lateral following it.

The MWD tool was modeled with two sets of sensors, one for inclination at 49.8 ft from the bit and another for bending moment and downhole WOB and TOB at 52.4 ft. During the run, the MWD tool captured and sent to surface several different measurements, including downhole WOB, downhole TOB, MWD bending moment, and continuous inclination.

Because of the deflection of the MWD tool inside the wellbore, the inclination of the tool is different than the wellbore direction (BHA misalignment, also known as sag), as illustrated in Fig. 1 above. One first calculates the wellbore trajectory using the step-by-step approach and then estimates the trajectory of the MWD tool inside the first calculated trajectory to represent the trajectory of the measurement tool better, using a 3D stiff-string model.

The calculated trajectories using the step-by-step approach are in good agreement with actual continuous inclination measurements and also provide new and novel information that is not available by traditional surveying methods.

Conventional 90-ft surveys are spaced too far apart to see every change in the well path along the way. A large part of what adds tortuosity to a well is directional work, but, with a snapshot taken only every 90 ft, a large portion of the steering is unseen. The BHA model coupled to the rock/bit model enables reproduction of the tortuosity of the well path with fair accuracy, which is key for wellbore placement and torque and drag.

Using a stiff-string torque-and-drag model with calculated contact points, it is possible to see the contribution to torque and drag that these microdoglegs can make. Small changes in the trajectory can increase significantly the number of contact points, and, over the length of a 16,000-ft well, these contact points add additional drag.


The consequence of any directional well is tortuosity and doglegs on a global and local scale. Such doglegs can be of no significance or can cause a multitude of problems. Standard surveys taken on a scale of every 95 ft at best show only a part of the story and at worst can be misleading. Slide/rotary patterns can be seen clearly in continuous surveys and are accurately reproduced using an advanced drillstring model. Knowledge and understanding of these patterns can be extremely valuable to the well-construction process.

Microdoglegs can help explain unexpectedly high levels of torque and drag seen in some directional wells. Explaining trends in torque and drag can be quite helpful in preventing failure during completions operations. Wellbore placement is also becoming more and more critical, with wells being drilled closer together to optimize recovery. The subject well revealed a true-vertical-depth difference of almost 20 ft between the standard surveys and the continuous surveys. Bending stresses also provide an interesting additional method for drillstring analysis. The ability to model bending stresses accurately has interesting implications for drillstring-fatigue and -failure analysis.

Continuous inclination surveys are a valuable tool, and they are generally underused. While common in many tools, there are still numerous wells being drilled without the guidance of continuous surveys. In these cases, the ability to calculate and reconstruct the trajectory can be invaluable. Having a full view of the trajectory, including unexpected changes in direction over a short distance, can have implications for post-well analysis and for evaluation of completions programs and design, and can aid in failure evaluation. When continuous surveys are not an option, computer modeling has been proved to be a robust and useful tool in furthering well analysis.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 184074, “Microdogleg Detection With Continuous Inclination Measurements and Advanced BHA Modeling,” by K.A. Mills, SPE, and S. Menand, SPE, DrillScan, and R. Suarez, SPE, Nabors, prepared for the 2016 SPE Eastern Regional Meeting, Canton, Ohio, USA, 13–15 September. The paper has not been peer reviewed.

Continuous Inclination Measurements and BHA Modeling Help Detect Microdoglegs

01 December 2017

Volume: 69 | Issue: 12


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