The oil and gas exploration industry continually searches for untapped hydrocarbon reserves. Over time, the reserves that are easiest to access have been developed and consumed, requiring operators to extend their search for fresh reserves into areas that are more technically challenging. Such challenges can include complex geological structures, thin or poor-quality pay zones, tight rock or marginal pressure envelopes in which to drill. Many of these challenges can be addressed by development of specialized downhole tools designed to evaluate the rock formations and help position wellbores optimally within the reservoir. However, all such tools depend on electronics, and a challenge is to produce tools that are able to function at high temperatures.
Demand for high-temperature tools is increasing, as is the temperature at which they are required to operate. As the trend toward higher temperatures and pressures continues, it is necessary to define further categories to distinguish “ordinary” high-pressure or high-temperature wells from those requiring more advanced tools.
Halliburton’s Quasar system is a new MWD and LWD (MWD/LWD) system capable of operating at temperatures up to 200 C (392 F) and 25,000 psi, placing it firmly in the ultra-HP/HT category (Figure 1).
The key challenges in high-temperature environments revolve around the electronic components of the system and the need to ensure they work reliably, are not damaged by the operating environment and provide measurements equivalent in accuracy to those provided by conventional systems. Techniques such as flasking and the use of eutectic materials for sacrificial melting are widely used in the wireline industry. However, these techniques are not appropriate for the drilling environment because of the much longer run lengths. Instead, the development of the Quasar system used a variety of other techniques to build reliable electronics for a 200 C environment. These included simplification of circuit designs to minimize the number of components required, screening of components to identify those most capable of operating at high temperature, careful modeling of heat flow patterns to eliminate hot spots in the electronics and removal of potentially corrosive elements from the atmosphere inside the tools. Rigorous testing at every level of the tool build process helps ensure the system performs as intended across its entire operating range.
The new system is built around the Quasar Pulse tool (Figure 2), which is an integrated base-services collar that provides all of the basic functions necessary for any MWD/LWD service. These include telemetry, directional survey, natural gamma ray, drillstring dynamics and pressure-while-drilling modules as well as power regulation and data storage functions.
The system is expanded further by the addition of the Quasar Trio suite of formation-evaluation sensors (Figure 3), providing multispacing resistivity, azimuthal density and neutron-porosity measurements. Accurate evaluation of gas zones, which are common in high-temperature environments, is only possible if all three measurements are available. A great deal of care was taken to help ensure the performance of these sensors in terms of their measurement range and accuracy is at least as good as their lower temperature counterparts. The system is the first in the industry to offer such a comprehensive suite of measurements at 200 C, allowing operators to evaluate high-temperature reservoirs while drilling without the need for subsequent wireline runs.
An operator in the Haynesville Shale wanted to drill a horizontal well to 6,887 m (22,595 ft) measured depth with a total vertical section of 3,070 m (10,072 ft). The expected circulating temperature at this depth was 182 C to 188 C (360 F to 370 F); therefore, the well could not be drilled with industrystandard high-temperature tools, which are typically rated to only 175 C (347 F). This restriction had limited the length of laterals in similar wells to about 1,524 m (5,000 ft). The operator deployed the Quasar Pulse service and was able to deliver the well successfully with a lateral section of more than 3,050 m (10,000 ft), which was a record for the Haynesville Shale. The operator would previously have had to drill two complete wells to achieve the same lateral footage. The Quasar Pulse service saved the operator the significant cost and time associated with drilling another well.
An operator in Southeast Asia wanted to reduce costs and achieve logging objectives in a batch of high-temperature offshore wells in which the anticipated bottomhole static temperature was 195 C (383 F). The past drilling practice for similar wells was to drill until reaching the maximum temperature rating of the MWD/LWD tools and then pick up a “dumb iron” assembly to drill to total depth (TD). This method resulted in uncertainty in the borehole position and loss of formation logging data. Another common practice was to use temperature mitigation techniques such as reducing rotary speed and circulating to cool the tools, but this significantly increased the amount of rig time required to drill the reservoir section.
The operator ran the Quasar Pulse service on five wells from the platform. Across the five wells the operator reported that the Quasar Pulse service saved about $1 million and more than 100 hours of rig time. This was accomplished by reducing the number of wireline runs, eliminating trips for tools that had reached their operating temperature limit and not having to use time-consuming temperature mitigation practices to reach TD.
Another operator was developing a gas field in which multiple wells had a bottomhole temperature that exceeded 200 C. With conventional equipment it is only possible to drill these wells using extensive temperature- mitigation procedures, including time spent circulating to cool the tools below their operating limit of 175 C. Even then it is sometimes impossible to drill to final TD without exceeding the tool specifications. Therefore, the operator was forced to complete the section without MWD/LWD tools in the drilling assembly. Additional time then had to be spent acquiring essential formation evaluation data with wireline tools.
To reduce the overall time spent on drilling and evaluating the well, the operator chose to run the Quasar Trio MWD/LWD triple-combo system to acquire all of the necessary formation evaluation data while simultaneously avoiding the need for temperature-mitigation procedures. The result was a comprehensive set of formation-evaluation data that included gamma ray, multispacing resistivity, azimuthal density and neutron-porosity measurements as well as measurements of downhole pressure for well control and directional surveys for well placement. The tools were exposed to a maximum temperature of 197 C (387 F), making this the first MWD/LWD triple-combo run to operate successfully at this temperature.
This article is based on the Society of Petroleum Engineers (SPE) paper 180592, “Taking the Heat: Logging While Drilling at Extreme Temperatures” by T. Parker and P. Cooper, presented on August 22-24, 2016, at the IADC/SPE Asia-Pacific Drilling Technology Conference and Exhibition in Singapore.
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