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CMR-Plus Combinable Magnetic Resonance Tool

Oil identification and viscosity determination can be done in one pass by using the standard pulse sequences of the CMR tools for nuclear magnetic resonance (NMR) measurement. NMR oil identification is based on the three independent transverse relaxation (T2) mechanisms for pore fluids in the pore space of a rock:

  • surface relaxation mechanism
  • bulk fluid relaxation mechanism
  • molecular diffusion mechanism.

Their relation 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. It is the bulk fluid relaxation mechanism that is used to identify oil and determine its viscosity.

Figure 1. Oil in the pore space of a water-wet rock. 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.

Figure 2. Mean transverse relaxation time (<i>T</i><sub>2</sub>, log) versus viscosity for bulk oil samples (measured at room temperature). The bulk relaxation of oil is dependent on its viscosity, as shown by the mean transverse relaxation time (T2, log) plotted versus viscosity for bulk oil samples (measured at room temperature). 

The following examples illustrate the T2 response to heavy, medium, and light oils in water-wet sandstones drilled with water-base mud (WBM).

Example 1: CMR tool’s response to heavy, high-viscosity oil on top of water

Example 1: CMR  tool’s response to heavy, high-viscosity oil on top of water The bottom 15 ft of the 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 tool’s response was used to identify changes in hydrocarbon viscosity (heavy versus light), guiding the completion strategy.

Example 2: CMR tool’s response to medium-viscosity oil

Example 2: CMR tool’s 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. The log shows a zone that has no resistivity or mud log show but has T2 values longer than 210 ms, indicating oil. 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 tool’s response to light, low-viscosity oil on top of water

Example 3: CMR tool’s response to light, low-viscosity oil on top of water The bottom 20 ft of the 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 tool’s response was used to identify hydrocarbon viscosity, guiding the completion strategy.

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