Taking the Pulse of Older Reservoirs

As reservoirs age, operators need updated information about the oil, gas and water remaining in rock so reserves can be better extracted.

Many logging-while-drilling and wireline tools exist to assess initial reserves in place in an open-hole environment when there is a larger borehole. Once the well is cased and completed, getting that information typically requires a tool with a small outside diameter (OD) that can fit through a 2 3/8-inch completion. That often means miniaturizing components and sometimes making the smaller tools more powerful.

The latest answer is Halliburton’s IntelliSat pulsed-neutron logging service, expected to launch commercially before the end of the year. So far, it has been deployed in Alaska, the Lower 48 and the Gulf of Mexico, in temperatures ranging from 90 F to 240 F, in pressures of 1,800 psi to 14,300 psi and to depths of 1,500 ft to 6,450 ft in sandstones and carbonates.

In those deployments, it followed use of the legacy RMT-i tool, and the results of both tools matched “excellently,” said John Savage, Halliburton’s well intervention product champion for its wireline and perforating group.

When Halliburton set about creating a tool for the smaller cased-hole environment, there were a few musts on the design list: The OD couldn’t exceed 1 11/16 inch so that it could work in the 2 3/8-inch completion. It needed to be able to work in 350 F and 15,000 psi environments. It had to collect the same level of –or better– detail than the legacy RMT-i tool, which could operate in a 2 7/8-inch environment. And it needed to have the ability to generate 50% more neutrons than the legacy tool to reduce statistical uncertainty and enable faster logging speeds.

Halliburton contends IntelliSat checks those boxes.

“We're pretty well there on the hardware design, and now we’re just doing some tweaks on the analysis software,” Savage said. “We've reduced the tool package size, and we've got detectors now that aren't temperature-dependent anymore and that have much better spectral resolution.”

The tool can help operators understand where the oil-water or gas-water contact is occurring. Other applications include logging the flow of water outside the casing and evaluating placement of gravel packs. It also can calculate elemental yields, which is used to understand lithology and determine total organic carbons (TOCs).

“Total organic carbon is what they're looking for,” Savage said. Traditionally, that information can be obtained before the well is cased, and the IntelliSat service “allows us a way to do that after there's been casing put in the well.”

The tool uses a neutron generator to create neutrons, which in turn create gamma rays.

“What we’re measuring is those gamma rays” as they interact with the molecules in the reservoir, and for the best results, he said, “we want as many neutrons as possible.”

The energy level of the gamma rays indicates the different elements present in the reservoir, including carbon, oxygen, calcium and silicon.

The first challenge in creating the IntelliSat tool was developing a neutron generator that would create more neutrons than its predecessor. Improvements in the tube-manufacturing process and advancements in the power supplies have enabled a much higher output from the generator that is now more reliable, and with longer life.

“Our objective was a 50% increase in neutron generator output. We've actually exceeded that objective,” Savage said.

But generating the neutrons to create the gamma rays is only part of the process. The gamma rays have to be detected, and the detectors on Halliburton’s new tool are able to acquire the gamma ray information to generate high-resolution energy spectra and decay arrays. These same detectors are also sensitive enough to collect natural background radiation with the generator off, to calculate a potassium-uranium-thorium (KUTh) log for clay typing and to enhance the TOC calculation, he said.

Once the results are acquired, they must be analyzed.

“Having a very robust characterization is critical,” he said, adding that the tool’s modeling software is based on the Monte Carlo N-Particle code. The nuclear model is validated and optimized through thousands of experiments in the track lab under various borehole configurations and rock and fluid types, he said.

The IntelliSat tool can be run on a standard e-line unit with surface readout or run with a memory package on slickline or coiled tubing, or with quality-control surface readout data on digital slickline. With the addition of a sleeve, which increases the OD to 2 1/4 inches, the tool can operate in environments up to 20,000 psi and 425 F. For 30,000 psi environments, a 2.48-inch OD housing is available.

The expectation is the tool will launch later this year, as there is still some work to be done on the mineral side of the characterization software.

“We’re quite confident in our saturation models,” Savage said. “We're working hard on mineralogy [and] elemental yields” to be able to deliver lithology and TOC information to operators.