Nuclear magnetic resonance (NMR) logging technology takes another step forward as the tool is modified for logging-while-drilling (LWD) applications.

Operators have long dreamed of a precision measurement tool capable of withstanding the rigors of a drilling environment. At the top of the wish list was replacement of the nuclear magnetic resonance wireline tool with one that would run while drilling and still receive the same information, with the same quality, to feed into existing processing and interpretation programs.
The industry's first NMR LWD tool was introduced recently. Instead of gathering data with a wireline rigged after drilling, the operator can gather data about the reservoir and its contents while a hole is being drilled. Some of this information will be available in real time, transmitted over a slow communication channel through pressure pulses in the mud column.
Fluid typing and diagnosis
Use of magnetic resonance imaging (MRI) began in the medical field. The MRI scan displays only fluid-rich material when the human body is scanned. This technology was transferred to downhole measurements to analyze gas, oil and water held in pore spaces in reservoir rock.
In downhole NMR measurements (wireline or LWD), a magnet in the tool energizes hydrogen atoms around the borehole. Next, a powerful high-frequency signal is sent out, and faint echoes are received from the hydrogen. The received signal carries information about the amount of hydrogen present: its state - solid, liquid or gaseous - and its immediate environment (the pore space). The NMR tool provides the following information, all of it important to a reservoir engineer:
• quantity of fluid in the reservoir. For example, if the received echo is one-tenth of its maximum strength, the rock must contain 10% porosity.
• types of reservoir fluids. Oil, gas and water each have a distinct NMR signature and can be differentiated from each other.
• sizes of the rock pores. Capillary-bound water has a unique NMR signature. Quantifying the amount of bound water present improves the estimate of rock permeability.
The next step
As wireline NMR tools gained wider acceptance in the marketplace, the next logical step was to investigate whether NMR measurements could be made during the drilling process and not after it. Clearly, the earlier NMR data is available for a reservoir, the more valuable this information is.
As late as 1997, there was a belief in the E&P industry that true NMR-while-drilling measurement would not be possible due to the random and sometimes violent motion of the tool string. This motion is detrimental to the accuracy of an NMR reading, obliterating fluid typing and pore size information.
The challenge has been to construct a robust tool capable of withstanding the extreme forces present in a drillstring while simultaneously carrying out a delicate measurement. Numar tackled that challenge, winning a Hart's E&P Meritorious Engineering Award in 2001.
Four major obstacles were overcome in the development of the LWD tool:
• investigation of the effects of drillstring motion;
• mechanical strength to support an entire drillstring;
• power delivery to support hundreds of hours of uninterrupted operation; and
• access to data.
Overcoming drilling motion
Early on, the first NMR wireline tools still relied on the Earth's magnetic field using T1 (time constant 1), a measurement made when hydrogen atoms become magnetically polarized. The current generation of wireline tools uses T2 mode (time constant 2), a measure of how fast hydrogen becomes depolarized. While the T2 data acquisition mode is efficient, it also is sensitive to motion. The constant, random lateral motion of an LWD tool tends to destroy T2 information, creating a dilemma: How could the wireline measurements be replicated?
To get an LWD measurement, it made sense to reapply some of the earlier T1-based nuclear magnetic logging concepts from the 1950s to get to the core of the issue. If there is drilling, the instrument selects T1 measurement mode; if drilling is stopped, the T2 mode is selected, essentially turning the system into a wireline tool without a wireline.
Figure 1 shows the MWD principle. Figure 1a shows the annulus of sensitive volume. In Figure 1b, naive use of T2 logging (transversal relaxation time) while the tool is in motion would result in signal loss due to the random motion of the drillstring and the attached tool. Figures 1c and 1d indicate that when switched to T1 mode (longitudinal relaxation time), motion tolerance is achieved.
First, a saturation pulse initializes a large annulus around the tool. After a predefined time interval, a readout sequence determines the amount of magnetization from a small annulus within the initialized volume. As long as the readout annulus falls within the initialized area, the result is not affected by random tool displacements. This process is repeated for several time intervals, resulting in a T1 polarization profile for the formation.
Different than wireline
Although the NMR LWD data looks in many respects like wireline data, the tool does not share any physical components with its wireline counterpart. Rather, the tool was built from the ground up to have exceptional strength and longevity in a very adverse environment.
Wireline tools are powered through a cable from the surface. Since there is no such umbilical in LWD, only two options for power generation were available: a turbine or batteries. Here again, the T1 mode made the choice easy. This measurement mode turned out to be a real power miser, and a single battery charge is sufficient to power the tool continuously for 200 hours.
Wireline cables also function as transport medium for data up and down the line. In LWD, most of the data acquired is recorded. In this case, a permanent memory of 200 MB can hold data acquired in 200 hours or more and can be downloaded at the surface within minutes. A real-time data link for an important subset of the data will be supported in the future.
Early tests successful
Early prototypes did much of their work in the Gulf of Mexico, overcoming the traumatic impact of a drill becoming a measuring tool inside the wellbore. Violent or not, the earth yielded its fluid-typing secrets to the geologists. There were two jobs, each including a conventional LWD triple-combo and an MRIL-WD tool and each availing the geologists a rigsite-processed "quick look" after drilling, said Sperry-Sun log analyst Joe Beck. "When back onshore, a detailed analysis was provided, and the geologists were satisfied with the results," Beck said.
The tool has since been deployed in a Gulf of Mexico deepwater exploration well as part of the bottomhole assembly (BHA). The LWD section contained sonic, neutron, density and resistivity tools. The plan was to drill the bottomhole section in a single run with a 105/8-in. bit and synthetic mud, from casing to total depth in 3 days. Programmed to wait for the trip-in time, the tool was initialized according to the job plan installed in the BHA and then tripped in. After the job was run, the tool was tripped out, the memory read and the tool laid down.
The data recorded consisted of motion-tolerant reconnaissance log data over the entire bit run and over repeat sections as well as additional hydrocarbon-typing data in wiping mode over the target zones. The data matched the wireline data collected from other instruments and verified the porosity and free-fluid data. The reconnaissance log provided total porosity, free-fluid index and bound-fluid volume.
In addition, the tool provided the standard NMR wireline answers: total porosity, free-fluid volume, capillary-bound water, hydrocarbon typing and permeability estimates in the wiping, evaluation logging mode. After the drilling phase, the well also was logged with the wireline counterpart for comparison (Figure 2).
The production version of the tool, which will be built in quantities, has two sections: electronic and sensor/antenna. The magnet in the antenna section is dimensioned to produce a circumferentially uniform field of 120 gauss at a diameter of 14 in., corresponding to a mean operating frequency of 500 kHz. The electronics section with its battery packs can operate for 200 hours and supports the sending of data in real time, recording up to 200 MB of data downhole.
Looking into the future
The NMR LWD tool is well-suited for high-cost offshore exploration and development wells, where holes are drilled with 8½- to 105/8-in. drill bits. In these wells, breakthroughs in reservoir characterization have been achieved by computing hydrocarbon volumes directly from NMR wireline data. The new tool carries forward these advances in reservoir evaluation technology by making it occur sooner in the well construction process.