Patent Application
20080050259
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
As described above, conventional Moineau drilling motors have used an elastomeric helical cavity component bonded to a steel housing. However, due to the behavior of the selected elastomer material in various competing conditions, there have been inevitable tradeoffs in the choice of a desired elastomer material. Such tradeoffs typically result in the selected elastomer having at least one less-than-optimal material property (e.g., lower-than-desired resilience, suboptimal processability, and/or inadequate dynamic properties) and as described above, these tradeoffs tend to compromise various stator fabrication and/or performance metrics.
Lower than desired elastomer resilience results in inadequate torque transmission. As described above, elastomeric materials with insufficient resilience undergo excessive deformation at high torque loads (due to the low rigidity of the elastomer), which allows drilling fluid to pass from one cavity to the next without producing any work. The result is a loss in rotor RPM (and therefore drill bit RPM). In severe conditions the rotor can stall in the stator. Several material properties may be measured to determine the resilience of an elastomeric material. Such properties include, elastic modulus (e.g., at tensile strains of 25 and 100%), compression modulus (e.g., at compressive strains 5, 10, and 15%), and hardness (Shore A).
While increased elastomer resilience is known to reduce RPM drop-off (thereby improving torque transmission), it is also known to degrade elastomer processability. As described above in the Background section, conventional wisdom in the downhole drilling industry suggests that resilient elastomer materials are not suitable for downhole stators due to inherently high viscosity (poor flowability of the pre-cured elastomer) at conventional injection molding temperatures. The processability of the elastomer is particularly important in longer and/or smaller diameter stators. Longer stators (e.g., greater than 20 feet) are often used in an attempt to minimize RPM drop off. Smaller diameter stators (e.g., less than four inch diameter) are commonly used in side tracking or other coiled tubing applications. It is known to those of skill in the art that increasing stator length and decreasing lobe diameter significantly increase the required pressure and time (and therefore expense) required to fabricate a stator via injection molding.
One measure of processability commonly used in the art is a property referred to as Mooney viscosity (e.g., measured according to ASTM D1646). Mooney viscosities in the range from about 20 to about 60 are sometimes considered to provide suitable processability. However, such measurements can be difficult and time consuming. Rheological properties can also be used to determine both the processability and the resilience (rigidity) of an elastomer. For example, the minimum torque, ML, as determined via ASTM D2040, tends to be a good indicator of elastomer processability, while the maximum torque, MH, tends to be a good indicator of elastomer resilience. An elastomer typically has good processability (suitable flowability at conventional injection molding temperatures) when ML is in the range from about 1.0 to about 4.0 lb·in when measured at 380 degrees F. with no preheat. High elastomer resilience (for reducing RPM drop-off) is typically indicated when MH is in the range from about 75 to about 110 lb·in as also measured at 380 degrees F. with no preheat. Conventional stators typically have an MH of about 55 lb·in or less.
Often increasing the resilience of an elastomer also degrades the dynamic properties of the elastomer. Such degradation of the dynamic properties is known to cause localized heating of the elastomer lobes due to the viscoelastic behavior of the elastomer (and its poor thermal conductivity). This in turn can result in thermal degradation of the elastomer and ultimately in failure of the stator (due to a phenomenon referred to in the art as “chunking” in which the stator lobes become embrittled and subsequently crack and tear apart). The dynamic properties are typically determined in the art by measuring a quantity referred to as tan δ, which is the ratio of the loss (or viscoelastic) modulus to the storage (or elastic) modulus. Increasing tan δ typically indicates increasing viscoelastic behavior and therefore degraded dynamic properties. While there is no universally agreed upon industry standard measurement technique for determining tan δ, the Applicant has found that a 250 degree F. tan δ value as determined in an RPA, after cure temperature sweep at a frequency of 10 Hz and a strain of 7% provides a suitable indication of the dynamic properties of a stator elastomer for use in a downhole stator. Tan δ values of less than about 0.25 typically indicate suitable dynamic properties; however, the Applicant has also found that stators employing highly resilient elastomers can accommodate somewhat compromised dynamic properties via reducing the strain in the interference fit between rotor and stator.
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In one exemplary embodiment, elastomer formulations including Nysyn 33-3 nitrile butadiene rubber (having 33 percent acrylonitrile and a Mooney viscosity of 30), at least 15 parts of a phenolic resin plasticizer per 100 parts nitrile rubber, and at least 60 parts carbon black per 100 parts nitrile rubber have been found to have both desirable resilience and processability (e.g., ML in the range from about 1.0 to about 4.0 and MH in the range from about 75 to about 110). Such formulations have also been found to have desirable dynamic properties (e.g., a 250 degree F. tan δ value of less than about 0.25).
Table II lists exemplary formulations A, B, C, and D in accordance with the present invention as well as a prior art formulation STD. It will be appreciated that this invention is not limited by the precise formulations listed in Table II. The artisan of ordinary skill will readily recognize that the various components in those formulations may be substituted with suitable equivalents. In the exemplary embodiments shown, Akrochem P55 phenolic resin is utilized. It will be appreciated that the invention is not limited to any particular phenolic resin. It will also be understood that Akrochem P55 also includes from about 6.5 to about 8.5 percent of a hexa cross-linking agent.
Table III lists characteristic properties measured for the formulations listed in Table II. These properties were determined in accordance with the test methodologies listed in Table IV.
With reference now to
Exemplary embodiments of this invention advantageously obviate the need for the above described tradeoff in elastomer rigidity and processability. Moreover, exemplary embodiments of this invention may even obviate the need for stators having rigid, metallic helical cavity components (except perhaps in the most demanding applications).
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
