Estimating BHA directional tendency
BHA yield describes a build/drop or turn rate under specified conditions. An assembly can build, hold, or drop inclination and turn left or right. Its response depends on the assembly and the drilling conditions.
BHA Yield Surveying (BYS) is being developed to estimate these tendencies. Before drilling, modelling can compare assumed conditions. Surveys provide measured orientations as drilling progresses.
What the model uses
The mechanical model uses component dimensions and materials, stabilizer positions, bend settings, hole diameter, inclination, toolface, and mud density. It includes weight on bit when that input is supplied. Each estimate needs a record of the values and assumptions used.
The actual drilled response also depends on formation properties and the bit's cutting response. Bit–rock interaction requires further treatment when assessing a field prediction.
What the estimate means
Current development work searches for build and turn rates at which the modelled lateral bit reactions reach zero. These are mechanical equilibrium estimates for the assumed assembly and conditions.
Some cases have no resolved rate within the chosen search range. A solver can also fail to converge. An unresolved result must be reported as such. It does not establish zero yield.
Rates need stated units, such as degrees per 30 m. Under the current convention, positive build increases inclination and positive turn indicates a right turn. Negative rates indicate drop or a left turn. The units and sign convention must accompany each estimate.
Limits and validation
A converged model result does not establish field prediction accuracy. Field assessment requires suitable drilled cases, known conditions, and comparison with measured build and turn.
BYS estimates directional tendency. See BHA Misalignment for the separate problem of sensor misalignment and sag correction.
References
These sources explain directional response and modelling limits. They do not validate BYS performance.
- SLB, The Defining Series: Directional Drilling: assembly, operating, and formation factors that affect directional response.
- Xi et al., Study on factors affecting vertical drilling bottom hole assembly performance and a new bottom hole assembly design method considering formation uncertainties (2022): a vertical-drilling study that combines BHA mechanics with bit–rock interaction and formation properties.
- SLB, Enhancing BHA Tendency Modeling Accuracy Using Hybrid Physics and Machine Learning (2023): the public abstract discusses calibration and the need for suitable formation, bit, and steering data.