Decision-Making Factors for Re-Running a Power Section in Directional Drilling
Directional drilling companies and operators face significant pressure to reduce operational costs, which can lead to the consideration of re-running power sections, such as mud motors, to save on equipment expenses. While this might seem like a viable option, several important factors must be carefully weighed before making the decision to re-use a motor, as the risk of performance degradation and eventual failure is heightened.
1. Aging and Degradation of Elastomers
Elastomers in the motor’s stator are a critical component, and their performance declines with age. Exposure to heat, light, ozone, oxygen, and radiation causes the elastomer to degrade, leading to hardening, cracking, and a loss of mechanical properties. Before re-running a motor, an internal inspection of the stator is essential. Techniques like using strong backlighting or a borescope can help evaluate the internal condition of the stator for signs of degradation.
Motor storage conditions also affect the stator's shelf life. For new or newly relined stators, the recommended shelf life varies depending on storage conditions:
Proper storage practices, such as keeping stators covered, out of direct sunlight, and painting them light colors, can help extend their life. However, once a stator has been used, its remaining shelf life decreases substantially, especially if exposed to harsh environments such as oil-based mud (OBM) or high temperatures exceeding 240°F. For such cases, stators should be relined after each run to avoid premature failure.
2. Heat and Hysteresis Effects
When operating downhole, the power section is subjected to extreme heat, both from the ambient bottom hole temperature and from dynamic heating effects. This is primarily due to hysteresis, a phenomenon in which elastomers—due to their viscoelastic properties—generate heat when deformed. As the rotor and stator nutate, friction concentrates heat in the middle of the stator lobes, causing the elastomer to degrade much faster. Temperatures can exceed 350°F (177°C), accelerating the aging process from years to mere days or hours.
Even if a stator appears intact after a run, with no visible chunking or significant wear, internal aging has still occurred due to the combination of hysteresis and ambient heat exposure. In less demanding applications, such as low-temperature, water-based environments, re-running the stator might be feasible, but the risk of failure is significantly higher compared to using a freshly relined stator.
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3. Loss of Rate of Penetration (ROP) Due to Stator Wear and Slip
Re-running a power section also poses the risk of diminished drilling performance. As the stator wears out, its fit with the rotor loosens, leading to slip within the power section. Wear can result from tight motor fits, suboptimal rotor condition, or the pumping of solids through the motor at high flow rates. These factors can cause the stator profile to deteriorate, reducing its efficiency and causing a noticeable drop in rate of penetration (ROP). A worn-out stator could lead to power section slip, making the motor less effective, and increasing operational costs through lower productivity.
4. Fluid Compatibility and Chemical Absorption
Another important consideration when re-running a stator is its exposure to drilling fluids and wellbore gases, especially in the case of oil-based muds (OBM). Elastomers can absorb chemicals from the drilling fluids, which may weaken the compound, degrade the rubber-metal bond, and compromise the overall integrity of the stator. This is particularly problematic in oil-based fluids, where chemical interaction can cause the elastomer to lose critical components of its composition, further decreasing its mechanical strength.
5. Motor Performance and Stall Events
Studies have shown that frequent stall events, especially micro-stalls, can significantly damage motor elastomers, even though they may be difficult to detect with surface-level data. Research from Texas A&M University used downhole sensors to monitor motor performance and observed that stalls and near-stalls are a direct cause of motor elastomer damage. When a stall occurs, the energy buildup is violently dissipated by the elastomer, causing cumulative damage over time. This leads to a gradual decline in motor performance, eventually resulting in motor failure and necessitating a trip to replace the motor.
Micro-stalls are particularly concerning because they often go unnoticed, but the damage they cause accumulates and can severely impact motor lifespan and efficiency.
Conclusion
While re-running a power section may offer short-term cost savings, the risks associated with elastomer aging, stator wear, fluid incompatibility, and performance degradation must be carefully considered. Factors such as the operating environment, the type of drilling fluid used, and the motor’s wear profile all play a crucial role in determining whether it is worth re-running a used motor. In many cases, the potential for lower ROP, increased failure risk, and added downtime may outweigh the cost benefits, especially in high-demand or high-temperature drilling conditions. Thus, the decision to re-run a motor should be made with a thorough understanding of the associated risks and the motor’s current condition.
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