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PCP systems for directional and horizontal wells
Because of the inherent curvature (angle build sections) and angled bottomhole segment of directional and horizontal wells, optimization of a progressive cavity pump (PCP) system design for such applications begins with the drilling program. The proposed well geometry, or directional plan, should take into consideration the design and operation attributes of a PCP system, including equipment selection, to contend with potential rod/tubing-wear and rod-string fatigue problems, the preferred pump seating location for achieving optimal production rates throughout the well life, and possible issues related to gas and solids production.
The first line of defense against rod/tubing-wear and sucker-rod fatigue problems in deviated and horizontal wells is a good wellbore profile (see previous sections on rod-string/tubing wear and rod loading). Ideally, the planned angle build rates should be kept as low as practical, and additional monitoring is typically required during drilling to ensure that the well closely follows the prescribed path. Note that slant wells (wells spud at an angle on surface), which typically have no planned curvature, often provide a good alternative to deviated wells for shallow reservoir developments as a means to avoid rod/tubing-wear problems. With slant wells, it is important to ensure that the well profile remains straight and does not “drop down” into the target bottomhole location. If it is not possible to avoid high wellbore curvature (> 5°/30 m [5°/100 ft]) in directional or horizontal wells, it becomes even more important to obtain the smoothest wellbore profile possible. Fluctuations in wellbore curvature and curvature reversals usually lead to severe wear. Therefore, drilling programs should include clauses that specify both maximum curvatures (i.e., dogleg severity) and allowable rates of change in curvature.
Experience has clearly demonstrated that closely spaced surveys (< 20 m [65 ft]) help to prevent large local curvature fluctuations and can typically be justified from an overall capital- and operating-cost perspective. Closely stationed directional surveys are also helpful in determining rod centralization requirements at a later stage. Caution should be exercised when specifying rod strings for directional and horizontal wells (i.e., where the pump is seated in the build section) based on directional surveys with long survey intervals (> 30 m [100 ft]) because the survey data may not reveal high-curvature segments that exist in the actual wellbore. This is illustrated in Fig. 1, which compares the dogleg severity established along a directional well based on the widely stationed openhole survey data recorded during drilling and a subsequent closely stationed gyro survey run in the cased wellbore. The gyro survey depicts significant variations and much higher curvatures. Therefore, if unexpected wear problems occur repeatedly at one or more locations along a directional or slant well, the survey may have provided a poor representation of the actual well curvature, and appropriate wear mitigation strategies must be taken to prevent additional failures.
Fortunately, PC pumps can operate effectively at high well angles, even beyond horizontal. However, attention is required when the pump seating interval is selected to avoid potential wear, pump inflow, and gas interference problems. This is illustrated by Fig. 2, which presents a vertical section plot of a horizontal well that was inadvertently drilled with a “trap” at the base of the build section. Because the severe sand plugging and gas slugging problems that occurred with the pump initially seated within the angle build section led to several workovers, the operator was forced to try seating the pump in the horizontal section beyond the trap at the location shown. Although the equipment options were quite limited and wear problems were still a concern for this well, it was successfully pumped in this configuration through the use of a continuous-rod string and a larger pump that could be run at low speeds.
Ideally, the pump should be seated as low as possible in directional and horizontal wells to maximize intake pressures. As mentioned, use of long, small-diameter tail joints should be avoided as a means to lower the intake position because of the inherent pressure losses. Depending on casing size, reducing the wellbore curvature over the planned pump seating interval may be important to prevent the pump from having to operate in a bent configuration. This condition would negatively affect pump life and increase the potential for wear and rod-string fatigue failures directly above the pump. Operating a PC pump while bent may also introduce the possibility for fatigue failures of the rotor within the stator. Close attention to the wellbore inclination and curvature is also important in the selection of an optimal pump intake location to ensure that the intake will not be positioned against the high side of the casing, thus increasing the potential for gas interference problems. This is especially crucial in horizontal wells, which are more prone to gas-slugging conditions as a result of elevation variations along the horizontal section. Sand production should also be taken into consideration in establishing the pump intake position. Given that sand transport capabilities are reduced in the casing relative to the smaller-diameter production tubing, seating the pump at nearly horizontal will reduce the potential for problems resulting from sand buildup.
Achieving proper rotor space out (i.e., positioning of the rotor within the stator) is also more difficult in directional and horizontal wells because rod weight is normally a key parameter used during rod-string installation to determine when the rotor enters the stator downhole. Because of friction and the non-vertical-well profile, the weight of the rod string is partially supported by the tubing in such wells. Experience and close attention to other details, such as recording accurate measurements of tubing/rod-string component lengths and monitoring for rotation, become more important in these applications.
- Matthews, C.M. and Dunn, L.J. 1993. Drilling and Production Practices To Mitigate Sucker-Rod/Tubing-Wear-Related Failures in Directional Wells. SPE Prod & Oper 8 (4): 251-259. SPE-22852-PA. http://dx.doi.org/10.2118/22852-PA.
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