Use of a Downhole Mud Motor as a Pump for Drillstem Testing
- James S. Cobbett (Petroleum Development Oman)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- April 1982
- Document Type
- Journal Paper
- 859 - 861
- 1982. Society of Petroleum Engineers
- 1.5 Drill Bits, 1.6.6 Directional Drilling, 1.6.1 Drilling Operation Management, 3.1.1 Beam and related pumping techniques, 1.7 Pressure Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.10 Drilling Equipment, 2.4.5 Gravel pack design & evaluation, 2 Well Completion, 2.2.2 Perforating, 3.2.4 Acidising, 5.6.4 Drillstem/Well Testing, 3.1.3 Hydraulic and Jet Pumps, 1.6 Drilling Operations
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To drillstem test wells that do not flow to the surface, we have used the Christensen Inc. Navi-Drill, a downhole mud motor used for directional drilling, modified to act as a pump and run by the drilling fig. This method was used first on an exploration well in south Oman to produce oil from a zone that had shown only water in a conventional drillstem test.
One problem frequently encountered in testing exploration wells in Petroleum Development Oman's (PDO's) operations is that of formations at shallow depths [less than 2000 m (6,562 ft)] which, when drillstem tested, do not flow to the surface. The total influx is often water that previously has been lost into the formation during drilling and pretesting operations. This led to consideration of using a downhole pump with the fig on site for well testing. Conventional downhole pumps such as a sucker-rod pump, submersible electric pump, or hydraulic pump all require a permanent-type completion and considerable installation time. Also, well behavior and fluid data are required to design a workable pumping system. Then it was realized that a mud motor, of the type routinely used for directional drilling, could be modified simply and used as a downhole pump for this purpose. In this way, with the fig on site, a well could be purpose. In this way, with the fig on site, a well could be pumped to surface until a representative fluid sample and pumped to surface until a representative fluid sample and adequate data for reservoir analysis were obtained.
The type of mud motor suitable for this purpose is the positive-displacement, Moyno-pump type available from positive-displacement, Moyno-pump type available from both Smith Intl. Inc. (Dyna-Drill) and Christensen (Navi-Drill). The Moyno-pump is a progresssing cavity, positive-displacement pump, incorporating a helical rotor inside a helical stator. The continuous contact between rotor and stator creates a number of sealed cavities that progress along the pump as the rotor is turned. The stator is made from an elastomeric material to promote sealing. Since the pump is the positive-displacement type, it can be used to meter flow, giving a complete shutoff when the rotor is stationary.
The Moyno mud motor was developed from this pump for directional drilling. In this application the case (stator) is not rotated and the drill bit is attached to the rotor, which rotates when mud is pumped through it. The pressure rating of the device depends on both the number pressure rating of the device depends on both the number of stages (cavities) within the pump and on the pressure rating of each stage. As for the optimal sizes for well testing, both of these are greater for the Navi-Drill, chosen in preference to the similar Dyna-drill. The 12.07-cm (4.75-in.) Navi-Drill has five stages with a pressure rating of 4.0 MPa (600 psi) across the whole pressure rating of 4.0 MPa (600 psi) across the whole tool, and a specific output of approximately 1.3 dm3/rev (0.3 gal/rev).
Although the tool is rated for a maximum speed of 11.3 rev/s (680 rpm) as a mud motor, the maximum rotational frequency attainable in practice limits the output of the Navi-Drill in the pumping mode to, for example, around 1.5 dm3/s (35 bbl/hr) in the case of the 12.07-cm (4.75-in.) tool tested. However, remember that this flow rate is largely independent of pressure up to the maximum pressure rating. The rather low pressure rating means that only wells that will flow naturally to at least 400 m (1,300 ft) below drilling floor (BDF) can be pumped continuously, and this limits the tool's range of pumped continuously, and this limits the tool's range of application.
To use the Navi-Drill as a pump rather than a mud motor, three modifications were made, of which only two proved necessary. A shear pin to lock the rotor should have been installed to facilitate setting a packer below the tool. In the field trial this shear pin had been sheared already when the tool was received, and since no problems were encountered setting the packer, no shear problems were encountered setting the packer, no shear pin will be used in the future. Second, the bypass valve, pin will be used in the future. Second, the bypass valve, usually fitted above the Navi-Drill to permit the drillstring to fill and empty during trips in and out of the hole. was omitted. Third, the passageways through which around 1% of the fluid flows to cool and lubricate the main (lower) beating, bypassing the bit, were blocked off.
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