Undoubtedly, the most significant advancement in formation evaluation methods of the 1990s has been the introduction and application of reliable, laboratory-quality downhole nuclear magnetic resonance (NMR) technology. The most advanced NMR logging system is based on the unique gradient field technology introduced by the Numar Corporation (Miller et al., 1990). The gradient field offers significant advantages over conventional homogeneous field tools and permits both rapid and flexible NMR data acquisition. When the Magnetic Resonance Imaging Log (MRIL®) service is run in combination with conventional porosity and induction tools, heretofore difficult formation evaluation problems are easily solved. The fundamentals of NMR logging and interpretation are illustrated with examples from around the world.

The MRIL service provides mineralogy-independent porosity data that do not require knowledge of the grain density. Conventional porosity tools (neutron and gamma-gamma density) are sensitive to a rock's mineralogy. The MRIL porosity is independent of mineralogy. However, the echo amplitude decay does depend on texture (e.g., surface-to-volume ratio of the pore system); thus, by combining conventional porosity tools and the MRIL device, it is possible to infer a rock's lithology. Furthermore, the MRIL porosity data permit one to directly determine the volume of clay-bound water and the effective porosity. By combining data from a short interecho acquisition (TE~0.6 ms) with the conventional 1.2-ms interecho data, it is possible to quantitatively differentiate between clay-bound water, irreducible fluids (e.g., capillary-bound fluid), and free or moveable fluids. This has proven extremely beneficial in the evaluation of low-resistivity formations that are easily overlooked when only conventional log data are evaluated, but which produce water-free hydrocarbons when completed (Taylor et al., 1995). The MRIL data allow one to identify and quantify these hydrocarbons that may be totally masked by high volumes of irreducible water.

At each depth, in addition to the NMR porosity data, a time series of echo amplitudes that decrease with time are recorded. The decay rate of the echo amplitudes is a measure of the pore space surface-to-volume ratio. Small pores have a large surface-to-volume ratio, while large pores have a small surface-to-volume ratio. The relative proportion of small and large pores can be determined by decomposing the recorded echo train into a number of underlying exponential decay terms. The NMR pore-size distributions are key to determining the irreducible water saturation, the moveable fluid saturation, and the permeability. Generally, rocks with only large pores (e.g., small surface-to-volume ratios) are very permeable while rocks with mixed pore sizes are less permeable, and rocks with extremely small pores and high irreducible water saturation have very low permeability.

The MRIL system's gradient field allows multiple, different MR measurements to be carried out simultaneously (Chandler et al., 1994). There are two NMR experimental parameters that can be varied — the recovery or wait time, TW, and the interecho spacing, TE. Because of the MRIL tool's unique gradient field, it is possible to simultaneously record MR echo data with two different recovery times. Hydrocarbons have longer T₁ buildups than water. Thus, by using two different wait times, it is possible to differentiate between oil, gas, and water (Akkurt et al., 1996). Data collected with two different wait times can be processed in either the time domain or in the T₂ domain. Generally, processing in the time domain minimizes the noise effects (line broadening) and provides more definitive hydrocarbon quantification.

Also, in a gradient field, the apparent T₂ decay time depends on the interecho spacing, TE. The longer the interecho time, the more rapid the apparent T₂ decay. The dependence of the apparent T₂ decay is inversely proportional to the pore fluid viscosity. Thus, by making simultaneous MR measurements with different TEs, it is possible to differentiate between heavy oil, light oil, gas, and water. The fluid with the lower viscosity (higher diffusivity) will be shifted to faster T₂ times in the differential TE (differential spectrum) measurement.

NMR logging, and in particular the MRIL service, has found a variety of applications worldwide. The tool has made it possible to determine fluid-filled porosity, independent of mineralogy. In shaly sands, it provides a mineralogy-independent effective porosity. When combined with resistivity data, saturation estimates are improved significantly. Even sands much thinner than the vertical aperture of the tool's antenna are properly counted; thus, reserve estimates based on MRIL measurements are more accurate than those available from other tools. The T₂ spectral data make it possible to differentiate between productive and nonproductive sands. When calibrated with sieve or laser particle-size data, the T₂ spectra can be interpreted in terms of grain size, allowing geologists to infer vertical changes n grain size (e.g., fine or coarsening). Petrophysicists can use the T₂ data to differentiate between clay-bound, capillary-bound, and moveable fluids. Furthermore, the data can be used to compute a pore size-based permeability. In certain locations, NMR permeabilities have replaced some or all core permeability data. In other locations, the NMR permeability is used to decide whether to complete and/or stimulate specific zones.

The ability to make simultaneous NMR measurements in adjacent sensitive volumes allows one to directly identify hydrocarbons and to estimate hydrocarbon properties. Because oil and gas have significantly different ratios of T₁:T₂, it is possible to differentiate gas and oil. This feature has been used to determine initial and residual oil saturations. Furthermore, the unique, well-defined gradient field allows one to estimate viscosity of the pore fluids by varying the interecho time. This permits the direct identification of heavy oil.

The ability to tailor acquisition for different applications is unique to NMR logging. Generally, it is necessary to plan the NMR logging and to tailor the acquisition parameters (i.e., T₂, DTW, or DTE logging, ...) to solve particular problems (i.e., porosity, permeability, hydrocarbon typing, ...). The MRIL tool, because of its well-defined gradient field, allows both faster logging and simultaneous acquisition of data for different purposes.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas