Coiled tubing

Low-Frequency Water Hammer for Extended-Reach Applications

Horizontal wellbores typically encounter inefficiencies caused by friction during drilling and completion operations.

FMD test-loop facility.
Fig. 1—FMD test-loop facility.

Horizontal wellbores typically encounter inefficiencies caused by friction during drilling and completion operations. In severe cases, advancing the bit or downhole tools farther into the hole is almost impossible because of the inability of existing technology and methods to overcome downhole-friction forces. This problem is experienced in drilling, completion, and intervention activities and is particularly acute in coiled-tubing (CT) -deployed bottomhole assemblies. It is proposed that a downhole pressure-oscillation tool be developed that enables managing downhole pressure-oscillation amplitude, independent of flow rate. It also is proposed that such a tool, operating at low frequencies, could simultaneously encode downhole measurements within the pressure-pulse signal.

Introduction

Significant innovation has been applied to the challenge of improving weight transfer and delaying the onset of helical buckling when using CT in the horizontal section of wellbores. Numerous technical approaches have been tried with varying degrees of success and economic benefit. Examples include lubricants; tractors; friction-reduction tools; and larger-­diameter, tapered, or hybrid work strings.

Recently, the industry has shown interest in the performance of downhole friction-reduction tools in CT drillout operations. The general operating principle of this class of tools is to create a pressure oscillation at a particular frequency that induces motion in the CT. This motion reduces the friction force, improving weight transfer and extending the reach. The greater the pressure oscillation, the greater the induced motion and friction-force reduction.

The mechanical nature of commercially available downhole friction-­reduction tools requires that they be adjusted at the surface to generate a predetermined pressure-oscillation amplitude, in a defined frequency band, at a given flow rate. Flow rate and pressure amplitude correlate positively, which leads to an increase in backpressure as the flow rate increases and, conversely, a reduced backpressure and friction-force-reduction capability when flow rate decreases.

In plug-milling operations, managing downhole pressure-oscillation amplitude independent of flow rate would be helpful. During a short trip for hole-cleaning purposes, annular velocity could be increased by turning the friction-reduction tool off to minimize backpressure. While milling, the pressure-oscillation amplitude should be adjusted to the level required for ­friction-force reduction while maintaining adequate weight on bit at the current measured depth. Because the pressure-oscillation amplitude required for friction-force reduction varies between the heel and the toe of the well, the pressure-oscillation amplitude should be optimized for friction-force reduction and weight transfer at a particular measured depth in the well, rather than be preset to mitigate the onset of helical buckling at a modeled measured depth farther out in the lateral section. If the pressure-oscillation amplitude is too low to reduce the friction force sufficiently and extend the reach at greater measured depths, it would be useful to be able to increase the pressure-oscillation amplitude while maintaining the optimum flow rate for the downhole motor.

The proposed system design would be similar to that of a downhole measurement-while-drilling (MWD) tool such that a downhole pressure-­oscillation tool could be developed that simultaneously induces motion in the CT for friction-force reduction while telemetering data to the surface from the downhole sensors. Real-time downhole measurements, such as weight on bit, torque on bit, annular pressure, and differential pressure, provide information about downhole operating conditions. This information enables making improved decisions and taking action with respect to friction-force reduction, motor performance, and hole cleaning compared with decisions made on the basis of inference from surface-weight and -pressure measurements.

The first phase of this development was to investigate whether a flow-­modulation device (FMD), operating at frequencies less than 2 Hz and using design concepts similar to those in MWD applications, could produce sufficient pressure-oscillation amplitude to induce motion in the CT and provide sufficient friction-force reduction to improve weight transfer and delay the onset of helical buckling, thus enabling extended reach.

Method

A preliminary analysis was conducted to identify key elements that affect performance and to identify the resulting effects of the FMD in CT applications. Equations used in common CT calculations (detailed in Appendix A of the complete paper) were identified and include those for piston, ballooning, and buckling effects. From Equations A-1 through A-8 in Appendix A of the complete paper, the pressure-pulse amplitude, Δp, in Eq. 1 was selected as the governing variable for use in experimentation.

. . . . . . . . . . 1

 

Thereby, flow or fluid-injection rate, q, and time during which the valve is closed in each period, t, are the key variables in controlling Δp. As a result, pump rate (qp) and valve-closure time (tc) were selected as key variables and were incremented during testing. In Eq. 1, c=compressibility of the fluid upstream of the pulser and V=the inner volume of the CT string. Frequency variation also was proposed; however, given the low frequencies to be used and on the basis of previous FMD experimentation, it was concluded that the effect of frequency on reaction force was negligible. Testing was conducted with proprietary FMD technology, and a flow-loop test facility was designed and built for pulse-tool development, as shown in Fig. 1 above.
 

The primary pressure transducer was mounted at the tool inlet, allowing direct measurement of the pressure signal. Variable boundaries were defined on the basis of real-world CT-operation limits. Pressure and load measurements were collected for qp values from 80 to 160 gal/min and for tc values from 200 to 400 milliseconds. Values of Δp were expected to be between 100 and 1,600 psi. All tests were conducted with water.

Results

During experimentation, high-­magnitude-force oscillations (Fo) were achieved for all target flow-rate and pulse-width ranges. During each phase, observed Fo values greater than 1,500 lbf were achieved consistently, demonstrating the FMD capability to produce forces consistent with extended-reach application at all flow rates. To relate Fo to Δp, it was necessary to gather waveform-dimension data while sweeping through the various qp and tc values. These dimensions were derived from high-density recordings taken from real-time pressure-transducer and load-cell readings. Measurement points were chosen on the basis of characteristic pulse and force oscillations, allowing analogous comparison on a pulse-by-pulse basis.

This first data-analysis phase resulted in 219 individual data points, each with correlating Fo and Δp values. Initial hypotheses indicated a relationship between Δp and tc, as well as qp, which implied a further relationship between these parameters and Fo. Analysis was completed by plotting Fo vs. Δp on a per-tc and per-qp basis. However, no direct relationship with Fo was observed for ­either qp or tc at any given Δp. Further examination of recorded pressure data was conducted paying special attention to the rise and fall of the pressure wave. A correlation was observed between the falling edge of the pressure wave and the magnitude of Fo, as shown in Fig. 2. A decrease in this new parameter, slope mp, correlates directly to a decrease in measured Fo, as shown in Fig. 3.

jpt-2013-06-waterhammerf2.jpg
Fig. 2—Characteristic pulse-shape comparison of similar Δp pulses with varying Fo responses.

 

jpt-2013-06-waterhammerf3.jpg
Fig. 3—Pulses with similar Δp, demonstrating the effect of mp on the Fo amplitude.

 

Conclusions

It was confirmed that low-frequency FMDs are capable of producing high-magnitude force oscillations, consistent with applications in extended-reach operations. Observed pressure pulses were consistent with pulse characteristics of positive-pulse-MWD devices, verifying further application for inducing axial motion and simultaneous transmission of encoded data to the surface. Pulse and force-amplitude control was demonstrated, providing flexibility in any particular application. Analysis of pressure and force signals indicated FMD-pressure-response time is a key component in producing high-amplitude force oscillations at the device. Further study in a downhole environment is necessary to quantify the combined influence of pressure and force on extended-reach capabilities.

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 163883, “Investigation of Low-Frequency Water Hammer for Extended-Reach Applications,” by R. Macdonald, SPE, B. Jennings, and G. Vecseri, TeleDrill, prepared for the 2013 SPE/ICoTA Coiled Tubing & Well Intervention Conference & Exhibition, The Woodlands, Texas, 26–27 March. The paper has not been peer reviewed.