Oil identification and viscosity determination from T2 distributions
The three independent nuclear magnetic resonance (NMR) transverse relaxation (T2) mechanisms for pore fluids in the pore space of a rock are
- surface relaxation mechanism
- bulk fluid relaxation mechanism
- molecular diffusion mechanism.
This can be expressed as 1/T2= 1/T2s + 1/T2b + 1/T2d, where 1/T2 is the transverse relaxation time, 1/T2s is the surface relaxation, 1/T2b is the bulk fluid relaxation, and 1/T2d is the diffusion relaxation. This application update focuses on using the bulk fluid relaxation mechanism to identify oil and determine its viscosity.
Bulk relaxation refers to the relaxation time of a fluid in a large container—i.e., the fluid is not influenced by surface or diffusion relaxation mechanisms. Bulk relaxation in rocks can be the dominant relaxation mechanism in extremely large pores or when two or more fluids occupy the pore space of the rock. Because the hydrogen nuclei of the nonwetting fluid (oil) are prevented from contacting the grain surfaces of the rock, the dominant relaxation mechanism for the oil is bulk relaxation (Fig. 1). The bulk relaxation of oil is dependent on its viscosity (Fig. 2). Examples of the T2 response to heavy, medium, and light oils in water-wet sandstones drilled with water-base mud (WBM) are shown in the CMR log examples.
Oil identification and viscosity determination can be done in one pass by using CMR standard pulse sequences. CMR answers can provide valuable insight for finding oil in low-resistivity, low-contrast pay zones, determining viscosity for production estimates, identifying heavy oils, and distinguishing between gas and oil.
Example 1: CMR response to heavy, high-viscosity oil on top of water
The bottom 15 ft of this log shows long T2 distributions in the wet zone. Farther up the log, T2 shortens in the oil zone because the heavy oil has a short, or fast, bulk relaxation time. The T2 distributions correlate with the resistivity curves. In this case, the CMR response was used to identify changes in hydrocarbon viscosity (heavy versus light), guiding the completion strategy.
Example 2: CMR response to medium-viscosity oil
Laboratory NMR measurements on this 12% to 15% porosity formation show that when the rock is 100% water saturated, the T2 relaxation is shorter than 210 ms. When the rock is partially saturated with native oil, the T2 relaxation is longer than 210 ms. This log shows a zone that has no resistivity or mud log show but has T2 values longer than 210 ms. As a result, cores were shot, the zone tested, and, according to the geologist, this is is one of the best producing intervals in the field.
Example 3: CMR response to light, low-viscosity oil on top of water
The bottom 20 ft of this log shows that the water signal decays before 500 ms in this high-porosity formation. Farther up the log, T2 lengthens in the oil zone because this is a light oil, which has a long bulk relaxation time. The T2 distributions correlate with the resistivity curves. As in the first example, the CMR response was used to identify hydrocarbon viscosity, guiding the completion strategy.
Request More Information
|