Total Porosity and Bound-Fluid Measurements From an NMR Tool

Abstract

New measurements recently available from a pulse-echonuclearmagnetic-resonance (NMR) tool enhance the usefulness of this logging technique. These include total NMR porosity and bound-fluid measurements. TotalNMR porosity provides a complete analysis of a reservoir's fluid volumes, including what is trapped in the very smallest pores associated with clays and very-fine-grained rock. This leads to improved permeability, irreducible water saturation, and effective porosity answers and, consequently, improved estimates of hydrocarbon producibility. Bound-fluid measurements, taken at logging speeds of up to 548.6 m/hr, provide irreducible water saturation and permeability when used in conjunction with another porosity device. This logging mode is favored when total NMR porosity is not required and when fast logging speeds are needed because of long log intervals, expensive rig rates, or poor wellbore conditions. Fig, 1 shows sample NMR logs taken from the Schlumberger Combinable Magnetic Resonance (CMR) tool. NMR signal amplitude is proportional to the number of hydrogen nuclei present in pore fluids and calibrated to give porosity (free from radioactive sources)and lithology effects. However, signal decay during each measurement cycle -called the transverse relaxation time, t2 - excites more petrophysicists. Signal amplitude is the sum of all decaying t2 signals generated by hydrogennuclei measured by the tool. Separating out the distribution of t2 values by a mathematical inversion process produces a t2 distribution. Thus, permeability and bound- and free-fluid porosity can be determined. Water in a test tube has a long t2 relaxation time of 3,500 milliseconds at25°C. However, water in rock pores has shorter relaxation times. For example, relaxation times for a sandstone typically range from 3 milliseconds for small pores to 500 milliseconds for large pores. Laboratory studies and field data show that t2 distributions in shales, shaly sands, some carbonate rocks, and heavy-oil reservoirs can have significant porosity amounts associated with t2of less than 3 milliseconds.