Total Porosity and Bound-Fluid Measurements From an NMR Tool
- Greg Gubelin (Schlumberger Wireline & Testing) | Austin Boyd (Schlumberger Wireline & Testing)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1997
- Document Type
- Journal Paper
- 718 - 718
- 1997. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating, 1.2.3 Rock properties
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New measurements recently available from a pulse-echonuclearmagnetic-resonance (NMR) tool enhance the usefulness of this loggingtechnique. 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 andvery-fine-grained rock. This leads to improved permeability, irreducible watersaturation, and effective porosity answers and, consequently, improvedestimates of hydrocarbon producibility. Bound-fluid measurements, taken atlogging speeds of up to 548.6 m/hr, provide irreducible water saturation andpermeability when used in conjunction with another porosity device. Thislogging mode is favored when total NMR porosity is not required and when fastlogging speeds are needed because of long log intervals, expensive rig rates,or poor wellbore conditions. Fig, 1 shows sample NMR logs taken from theSchlumberger Combinable Magnetic Resonance (CMR) tool.
NMR signal amplitude is proportional to the number of hydrogen nuclei presentin 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 amathematical inversion process produces a t2 distribution. Thus, permeabilityand 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 smallpores to 500 milliseconds for large pores. Laboratory studies and field datashow that t2 distributions in shales, shaly sands, some carbonate rocks, andheavy-oil reservoirs can have significant porosity amounts associated with t2of less than 3 milliseconds.
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