Nuclear magnetic resonance (NMR) technology in the past decade has emerged as a reliable, accurate reservoir description and evaluation tool, capable of helping identify the properties of reservoir fluids, quantify the volumes of fluids present, and determine a wide variety of reservoir characteristics in an expanding range of geologic and logistical settings.

The introduction of NMR wireline logging in the early 1990s and the MRIL®-Prime in the late 1990s was followed later in the decade by a logging-while-drilling (LWD) NMR tool for early identification and description of reservoir properties. In 2002, an NMR fluid-identification device was unveiled that enables producers to obtain laboratory-quality, in situ measurements of native fluids in real time and under reservoir conditions, when deployed as a component of an advanced reservoir description tool (RDT™).

These downhole NMR instruments have shown in case studies and hundreds of commercial applications the ability to achieve step-change improvements in formation evaluation and core analysis, by helping determine mineralogically independent porosity; porosity distribution, complete with a pore-size distribution in water-saturated formations; bulk volume irreducible fluid and free fluid; formation permeability; and hydrocarbon typing. Information from the NMR wireline logging tool, NMR LWD tool, and the NMR real-time fluid identification device can be integrated and interpreted to help distinguish low-resistivity and low-contrast pay zones, evaluate complex reservoir lithology, identify medium-viscosity and heavy oils, determine residual oil saturation, and enhance stimulation designs.

Within the past six months, Halliburton's logging and perforating business unit has conceived, initiated development of, and begun field testing another new NMR logging method. In about two dozen applications in the U.S. Mid-Continent region and Rocky Mountains, the new system has demonstrated the ability to identify natural gas reserves in tight formations. The new NMR activation procedure also helps reduce the complexity and increase the speed of acquiring NMR data. Though still in the early stages of commercialization, applications are expected to expand within the next couple of years to encompass the identification and quantification of gas in higher permeability reservoirs.

NMR logging basics
The major components of NMR logging instruments include:

• a permanent magnet, which generates a static magnetic field that magnetizes formation materials within a specified diameter around the well bore.
• an antenna surrounding the magnet, which transmits precisely timed bursts of radio-frequency energy into the formation in the form of an oscillating magnetic field.

The essential properties of reservoir pore fluids that affect the proton-decay echo trains are the hydrogen index (HI), longitudinal relaxation time (T₁), transverse relaxation time (T₂), and self diffusivity (D₀). HI is a measure of the density of hydrogen atoms in the fluid. T₁ is an indication of how fast the protons in the fluids relax longitudinally, relative to the axis of the static magnetic field, while T₂ indicates how fast the protons in the fluids relax transversely, again relative to the axis of the static magnetic field. D₀ is a measure of the extent to which molecules move at random in the fluid.

Each pore-fluid parameter generates a spectrum of magnetic resonance signals made up of the discrete decay, or relaxation, signatures of the hydrogen protons in reservoir fluids and gases. The activation sequences of NMR logging tools can be designed to maximize the echo spacing of water, oil and gas decay rates.

New T₁ logging capability
Halliburton's new NMR gas-logging method evolved out of ongoing efforts to continue expanding the performance envelope of existing NMR logging systems.

In the early 1990s, the primary NMR logging data acquired were T₂ measurements, which are degraded by fluid movement. Instrumentation existed to do T₁ measurements, but it was based upon a single-frequency tool and—like T₂ data—required stationary acquisition, at that time.

Eager to avoid the risk of becoming stuck in the well bore—a possibility that increases exponentially each time the tool string ceases movement—Halliburton continued developing NMR LWD technology and the NMR real-time fluid tester deployed as part of the RDT. The activation method for NMR LWD enables acquisition of magnetic resonance data as the drill pipe is rotating, while the MRILab analyzes moving fluid in the flow line of the RDT tool.

Since both methods deal with movement, both lended themselves to a new T₁-style measurement for the MRIL Prime wireline logging tool. A new activation method was created to enable the collection and analysis of T₁ spectrum signals while moving through the well bore. Because the new, mobile T₁ activation uses the same NMR method replied upon in NMR-while-drilling and in the fluid analyzer, any Halliburton engineer can provide the service, as long as they've received training on basic NMR logging.

A Rocky Mountain NMR T₁ Example.
Tracks 1, Depth & Track 2 contain GR, depth, and resistivity, respectively. In Track 3 is the T₁ Spectra where the T₁ signature of gas can be easily seen to the far right of the track at the anticipated long T₁ time ~~4000ms. Track 4 contains NMR volumetric data from the NMR T₂ acquisition. In Track 5 is the T₂ Spectra and the red line delineates where a T₂ gas signal would be expected at this particular reservoir temperature and pressure.

The idea behind the mobile T₁ logging method is the same as for other downhole NMR tools: i.e., fluids in the reservoir will distinguish themselves from one another according to their T₁ relaxation times. For example, the relaxation times for clay-bound water and capillary-bound water will be only a few milliseconds (ms) and for free water anywhere from 50 to 100 ms. The relaxation time for gas, meanwhile, typically ranges from 3,000 to 5,000 ms.

Because NMR logging tools image only the hydrogen protons in reservoir fluids and gases, the instruments can provide information that is either difficult or impossible obtain with conventional logging technology. For example, an operator can struggle to locate and evaluate gas reservoirs, if faulty water resistivity data skew the results of saturation calculations. Similarly, excessive clay in a targeted formation can impair neutron density data, making it difficult to detect gas based upon the traditional neutron-density cross over. The presence of gas also can be masked if clay-bound water or reservoir mineralogy degrade resistivity calculation.

Tight gas logging in the Rockies
Halliburton proposed using the mobile NMR T₁ logging method on two tight gas wells in the Rockies after porosity proved inadequate to ensure the accuracy of two existing NMR logging fluid identification methods—time-domain analysis and enhanced diffusion method.

Because gas has a relatively long T₁ relaxation time when compared to pore water and little or no light oil was anticipated to complicate the T₁ signal distribution, it was predicted that gas and water in the two wells would be separated by their different positions within the T₁ spectrum.

Plans called for the NMR T₁ activation to be acquired in the down-log mode in both wells and an up-log T₂ activation, based upon 1.2 ms single inter-echo spacing with a 9.5-second polarization wait time. Down-log objectives were to obtain a gas indication from the T₁ spectrum; up-log objectives were to obtain clay-bound water porosity, capillary-bound water volume, free or movable fluid volume, and total NMR porosity, then calculate a continuous permeability. An NMR saturation analysis was to be run on the T₂ data using true resistivity for a gas-water saturation analysis.

In both wells, the NMR T₁ activation was logged down at 12 feet per minute (ft/min), with a running average of 16 levels. Although no field log or field-presentable data were available from the down-log, the data generated were transmitted via satellite to Halliburton's Denver computing center for processing. The NMR T₂ activation was up-logged at 9 ft/min, with a running average of 16 levels over the same log intervals as the down-log; a field log of the up-log was created and transmitted, while the binary/array data was transmitted to Denver for processing.

As anticipated, the proton signatures generated by the NMR T₁ log were shown to offer much greater differentials than the NMR T₂ log for distinguishing gas from reservoir fluids. Gas identification by the NMR analysis and the T₁ signals were in exceptionally good agreement, with a couple of exceptions. As these wells are tested, it will be determined how well this NMR T₁ method correlates with production.

In addition to pinpointing and accurately quantifying gas zones in the two wells, NMR T₁ logging results helped manage logging costs on the two wells by minimizing rig time, and provided invaluable insights into the design and placement of stimulation treatments and completions.

T₁ logging in the future
Although initial applications of the new NMR T₁ logging system have focused on gas in tight reservoirs, the technology within the next few years is expected to become capable of pinpointing and assessing the producibility of gas zones in many different types of reservoirs. That's because Halliburton has successfully applied NMR logging technology around the world to detect oil and gas in both clastic and carbonate reservoirs, exhibiting porosities ranging from as much as 30 percent porosity to as little as 3 percent, and resistivities as low as 1 ohm. Presently, one near-term goal is to improve the commerciality of the T₁ logging activation by providing real-time results and real-time log-quality control on the monitor screen for the field engineer.