Compared with other sensor measurement tools such as wireline logging, logging-while-drilling (LWD) tools must stay downhole for extended periods of time and are subject to high vibration. While ratings of 25,000 psi and 350ºF may not be considered HPHT for some drilling environments, they are generally categorized as high pressure, high temperature for LWD tools, said Alan Rennie, operations specialist for PathFinder Energy Services.
It’s “fairly straightforward mechanically to get systems that will withstand 25,000 psi – there’s just a design and manufacturing cost associated with it,” he said. But mechanical failures are not the issue with improving HPHT ratings for LWD tools. The real challenges, he said, are with electronics surviving downhole in temperatures up to 350ºF. “There are very few electronic components — be it a resistor, an inductor, a capacitor or a memory chip — rated by a manufacturer above even 300ºF. We pre-screen electronic components to meet our high-temperature requirements and system-test them to make sure they function.”
With that said, tools in PathFinder’s Survivor series, one of the its newer tool suites, are rated to 25,000 psi and 350ºF, and consist of MWD directional survey, annular pressure, resistivity and nuclear porosity services.
The array wave resistivity (AWR) is a dual-frequency (2 MHz and 500 kHz) borehole compensated wave propagation tool with three transmitter-to-receiver coil spacings. Twelve resistivity curves with different diameters of investigation are provided, and advanced resistivity analysis compute six dielectric assumption independent resistivities. Mr Rennie noted that the 2 MHz frequency is an industry standard and the 500 kHz frequency was added to increase the diameter of investigation of the measurements and to provide other phase and attenuation resistivity values for improved geological interpretation.
The AWR also provides a gamma ray and tri-axial inclination measurement at the bottom.
The slim density neutron standoff caliper (SDNSC) provides the porosity service (neutron porosity, density), where two in-line ultrasonic transducers provide standoff and borehole caliper measurements. The standoff measurements are used to weight the density data. The caliper data are used to define the borehole size while rotating.
One significant feature of the SDNSC, Mr Rennie pointed out, is that it uses a Californium 252 neutron source in its system to provide neutron porosity, compared with other systems that use the traditional Americium-Beryllium (AmBe) sources. The Californium 252 is smaller, more readily available and has a shorter half-life than AmBe. “It’s more environmentally friendly because of its shorter half life, and it can be recycled,” he explained.
An operator in the Gulf of Mexico required LWD triple combo measurements in 7 ½-in. borehole. The overall objectives were to provide 4 ¾-in. nominal collar size gamma ray, resistivity, density, neutron porosity and caliper measurements to characterize a hydrocarbon target. This was accomplished by acquiring, processing and presenting high-resolution real-time and recorded LWD triple combo formation evaluation data to define the lithology, formation fluid volume and types.
The LWD triple combo formation evaluation log plot (above) shows a shale to sand gamma ray response change at xx415 ft MD. This is the beginning of the hydrocarbon zone. In track 2, the resistivity at xx415 ft MD increases significantly with a corresponding density/neutron cross-over in track 3.
Note that the density and neutron curves are computed on a sandstone matrix. In the reservoir, the sandstone formation contains shaly intervals. The calculated density/neutron cross-plot porosity in the hydrocarbon zone is greater than 30%. The lower section of the hydrocarbon zone ends at xx504 ft MD. The operator did not run wireline triple combo services due to the quality of the LWD formation evaluation data.
This article is partly based on SPE 109940, “An LWD Tool Suite for Formation Evaluation in HPHT Environments,” by A. Rennie, P. Boonen, PathFinder Energy Services, presented at the 2007 SPE Annual Technical Conference and Exhibition, 11-14 November 2007 in Anaheim, Calif.