Development and Testing of Insulated Drillpipe
- J.T. Finger (Sandia Natl. Laboratories) | R.D. Jacobson (Sandia Natl. Laboratories) | A.T. Champness (Drill Cool Systems Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2002
- Document Type
- Journal Paper
- 132 - 136
- 2002. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.1.5 Processing Equipment, 1.6.1 Drilling Operation Management, 1.12.6 Drilling Data Management and Standards, 5.3.2 Multiphase Flow, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.2 Pipelines, Flowlines and Risers, 1.14 Casing and Cementing, 4.1.2 Separation and Treating, 1.10 Drilling Equipment, 1.7.5 Well Control, 1.12.1 Measurement While Drilling, 1.11 Drilling Fluids and Materials, 1.6 Drilling Operations, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.9.2 Geothermal Resources, 3 Production and Well Operations, 2.4.3 Sand/Solids Control
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The Geothermal Research Dept. at Sandia Natl. Laboratories, in collaboration with Drill Cool Systems Inc., has worked to develop and test insulated drillpipe (IDP). IDP will allow much cooler drilling fluid to reach the bottom of the hole, making possible the use of downhole motors, electronics, and steering tools that are now useless in high-temperature formations. Other advantages of cooler fluid include reduced degradation of drilling fluid, longer bit life, and reduced corrosion rates.
This article describes the theoretical background, laboratory testing, and field testing of IDP, including structural and thermal laboratory testing procedures and results. We also give results for a field test in a geothermal well in which circulating temperatures in IDP are compared with those in conventional drillpipe (CDP) at different flow rates. A brief description of the software used to model wellbore temperature and to calculate sensitivity in IDP design differences is included, along with a comparison of calculated and measured wellbore temperatures in the field test. There is also analysis of mixed (IDP and CDP) drillstrings and discussion of where IDP should be placed in a mixed string.
Sandia first considered IDP during an investigation of energy extraction from magma, which would have involved drilling into extremely high temperature (>1600°F) molten rock. This presented a fundamental problem - the damaging effect of high formation temperature on drilling tools, materials, and processes.1 When drilling fluid gets too hot, it can irreversibly deteriorate in its ability to carry the drilled cuttings, increase drillpipe corrosion by orders of magnitude, shorten the life of bits and other downhole tools, prevent the use of downhole motors and electronic instrumentation/ tools, and even affect borehole stability and well control. Each of these problems can be attacked by individual technology developments, but all can be solved or greatly mitigated by simply cooling the downhole environment with a lowertemperature drilling fluid. IDP delivers drilling fluid to the bottom of the hole at a much lower temperature than CDP.
More detailed calculations and measurements are presented later, but an example of IDP's effect in a geothermal well is shown in Fig. 1. In this and other fluid-temperature plots, the right side of the curve shows the temperature in the annulus, and the left side shows the temperature in the drillpipe. Several important points are illustrated by the following calculation.
Bottomhole fluid temperature is reduced from 401°F with CDP to 166°F with IDP. This is the temperature that steering tools, mud motors, and other electronics must survive.
Maximum fluid temperature (not at the bottom of the hole) is reduced from 406 to 214°F, the maximum temperature seen by drilling-fluid additives.
The mud return temperature is higher with IDP, 196°F vs. 163°F. This may mean that mud coolers are necessary, but it also means that more heat is being removed from the hole, which will be beneficial for logging and cementing after drilling.
These curves describe temperatures in holes that have been drilled from surface with either CDP or IDP - they do not reflect conditions that would exist while running measurement-whiledrilling (MWD) tools into a hot hole already drilled and shut in, for example. This kind of operation has been successfully performed in exploratory geothermal drilling at temperatures greater than 660°F by staging into the hole with a top drive and circulating cooler fluid after adding each stand of pipe,2 but IDP would make the process quicker and allow operation at even higher temperatures. (In the hole described in the Ref. 3, it required 14.5 hours to run in the hole to 8,500 ft.) In this kind of operation, it is essential to know the downhole temperature, so having an MWD tool in the drilling assembly is an important advantage.
An alternative approach to this scenario is described in the Wellbore Cooling section.
The IDP described in this paper was constructed by welding a 3.5-in. outer diameter (OD) by 3.068-in. inside diameter (ID) liner tube inside conventional 5-in. drillpipe (Fig. 2) and filling the annulus between the tubulars with an insulating material. With the compressive strength of the insulation to support the liner tube against internal pressure, it can be made of a thinner material, saving cost and weight and preserving the ID as large as possible.
Two criteria, compressive strength and conductivity, drive the choice of an insulating material. Required compressive strength can be estimated from the diameters of the drillpipe and liner, along with the expected internal pressure, but typical values are less than 8,000 psi.
Insulation conductivity does not need to be exceptionally low because there is an alternate heat-flow path through the uninsulated tool joints. That is, at less than a nominal value of conductivity, further improvement in the insulation has little effect on the total heat flow because of the heat flow through the tool joints. Drilling-fluid temperatures are calculated for three values (k=0.05, 0.3, and 1.0 B/hr-ft-F) of insulation conductivity (Fig. 3), and it can be seen that the variation in insulation properties has little effect relative to the difference between even the poorest insulation and conventional pipe. A number of materials have conductivity in the range of 0.2 to 0.8 B/hr-ft-F, and a selected fracture proppant was chosen as the insulation for the IDP.
IDP Structural Considerations
Because IDP is heavier (approximately 33 lb/ft) than CDP (approximately 19.5 lb/ft), drillstring weight could become a problem in deeper wells. Tensile stress induced by string weight is highest at the top of the drillstring. The other controlling stress is the hoop stress in the liner tube caused by mud weight and pump pressure; the liner stress is partially supported by the insulation and is highest at the bottom of the drillstring.
If principal stresses in the liner tube are combined and tested by the von Mises criterion, we can identify failure conditions. A plot of combined stress (Fig. 4) for various conditions shows that in a 20,000-ft well, failure (i.e., stress greater than the assumed 8,000- psi yield limit) may occur at both the top and bottom of a full string of IDP (Curve A) if the liner tube does not get full support from the insulation. On the other hand, both a 15,000-ft string of IDP (Curve B) and a mixed 20,000-ft string with 10,000 ft of IDP at the top and 10,000 ft of CDP below (Curve C) have liner stresses well within the assumed yield limit.
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