What Does NMR Really Contribute To Formation Evaluation?

NMR is not the end all to petrophysics. It is a new tool for understanding and describing the subsurface, both reservoir and non-reservoir rocks, which expands the petrophysical content of our logging data, providing direct estimates of irreducible water, clay bound water, and the permeability. NMR applications can be divided into standard and specialized deliverables. Standard deliverables are porosity and a subdivision of the pore space into Clay Bound and Irreducible Water and Moveable Fluids or perhaps into micro, meso, and macro pores. Specialized deliverables can be broadly grouped into those that address rock properties and fluid properties. So what is new? Porosity tools appeared in the 1950s; today, neutron and gamma-gamma density porosity data is collected on almost every well drilled. In addition, well-known crossplot techniques, e.g., neutron-density crossplots, are used to quantify shale content, differentiate light hydrocarbons and water. Porosity data and resistivity data are routinely used to compute saturations and locate oil-water and gas-water contacts. NMR provides a subdivision of the pore space that is not available from conventional log data. However, that subdivision does require interpretation parameters-the T2 cut-offs: the divisions between "Clay Bound" water, "Irreducible" water and "Moveable" fluid. Probably the most intriguing aspect of NMR measurements has been the promise to provide a non-core, continuous permeability measurement. Arguably, this has been achieved for clastic rocks, but carbonates, especially non-granular and diagenetically altered or fractured vugular rocks, still present a major challenge. NMR porosity measurements are especially useful because they only depend on the pore space, or, more precisely, on the fluids in the pore space. This strength for porosity measurements complicates the subdivision of the pore space. However, in the past 10 years, various researchers have used the different NMR responses of gas, conventional oil, and heavy oil for fluid identification and characterization. Such analyses are particularly attractive because the NMR measurements are independent of mineralogy and provide an independent source for this critical formation evaluation data