NMR FOR FLUID PROPERTIES AND POROSITY

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NMR  LOG'S FOR FORMATION FLUID PROPERTIES

Medical MRI relies on the ability to link specific medical conditions or organs in the body to different NMR behavior. A similar approach can be used with MRIL tools to study fluids in a thin zone a few inches from the borehole wall. MRIL tools can determine the presence and quantities of different fluids (water, oil, and gas), as well as some of the specific properties of the fluids (for example, viscosity). Both Medical-MRI devices and MRIL logging tools can be run with specific pulse-sequence settings, or “activations,” that enhance their ability to detect particular fluid conditions.

NMR AND MICRO POROSITY

Micro-porosity associated with clays and with some other minerals typically contains water that, from an NMR perspective, appears almost like a solid. Water in such micro-pores has a very rapid “relaxation time.” Because of this rapid relaxation, this water is more difficult to see than, for example, producible water associated with larger pores. Earlier generations of NMR logging tools were unable to see water in these micro-pores, and because this water was associated most often with clays, the porosity measured by these earlier tools was often characterized as being an “effective porosity.” Modern MRIL logging tools can see essentially all the fluids in the pore space, and the porosity measurement made by these tools is thus characterized as being a “total-porosity” measurement. Pore-size information supplied by the modern tools is used to calculate an effective porosity that mimics the porosity measured by the older NMR tools.

CALIBRATION OF NMR TO PETROPHYSICAL PROPERTIES IN LABs (AN ADAVANTAGE)

One of the key features of the MRIL design philosophy is that the NMR measurements of the formation made when the MRIL tool is in the wellbore can be duplicated in the laboratory by NMR measurements made on rock cores recovered from the formation. This ability to make repeatable measurements under very different conditions is what makes it possible for researchers to calibrate the NMR measurements to the petrophysical properties of interest (such as pore size) to the end user of MRIL data.

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NMR ( MRIL) Logging Basics

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Medical MRI and MRI Logging:

Magnetic resonance imaging (MRI) is one of the most valuable clinical diagnostic tools in health care today. With a patient placed in the whole-body compartment of an MRI system, magnetic resonance signals from hydrogen nuclei at specific locations in the body can be detected and used to construct an image of the interior structure of the body. These images may reveal physical abnormalities and thereby aid in the diagnosis of injury and disease. Magnetic Resonance Imaging Logging (MRIL®), introduced by NUMAR in 1991,1 takes the medical MRI or laboratory NMR equipment and turns it inside-out. So, rather than placing the subject at the center of the instrument, the instrument itself is placed, in a wellbore, at the center of the formation to be analyzed.

Comparison of the MRIL Tool to Other Logging Tools

Because only fluids are visible to MRI, the porosity measured by an MRIL tool contains no contribution from the matrix materials and does not need to be calibrated to formation lithology. This response characteristic makes an MRIL tool fundamentally different from conventional logging tools. The conventional neutron, bulk-density, and acoustic-travel-time porosity-logging tools are influenced by all components of a reservoir rock. Because reservoir rocks typically have more rock framework than fluid filled space, these conventional tools tend to be much more sensitive to the matrix materials than to the pore fluids. The conventional resistivity-logging tools, while being extremely sensitive to the fluid-filled space and traditionally being used to estimate the amount of water present in reservoir rocks, cannot be regarded as true fluid-logging devices. These tools are strongly influenced by the presence of conductive minerals and, for the responses of these tools to be properly interpreted, a detailed knowledge of the properties of both the formation and the water in the pore space is required. MRIL tools can provide three types of information, each of which make these tools unique among logging devices:

• information about the quantities of the fluids in the rock

• information about the properties of these fluids

• information about the sizes of the pores that contain these fluids

MRIL/NMR TOOL PRINCIPLE:

At the center of an MRIL tool, a permanent magnet produces a magnetic field that magnetizes formation materials. An antenna surrounding this magnet transmits into the formation precisely timed bursts of radio-frequency energy in the form of an oscillating magnetic field. Between these pulses, the antenna is used to listen for the decaying “echo” signal from those hydrogen protons that are in resonance with the field from the permanent magnet.

The MRIL-Prime tool can be operated at nine separate frequen-
cies. The use of multiplefrequencies allows independent information to
be obtained from multipleconcentric cylinders,thereby improving the
signal-to-noise ratio,enabling faster loggingspeeds, and permitting
different pulse-timingsequences for complexdata acquisition.

Because a linear relationship exists between the proton resonance frequency and the strength of the permanent magnetic field, the frequency of the transmitted and received energy can be tuned to investigate cylindrical regions at different diameters around an MRIL tool. This tuning of an MRI probe to be sensitive to a specific frequency allows MRI instruments to image narrow slices of either a hospital patient or a rock formation. Fig. 1.2 illustrates the “cylinders of investigation” for the MRIL-Prime tool, which was introduced in 1998. The diameter and thickness of each thin cylindrical region are selected by simply specifying the central frequency and bandwidth to which the MRIL transmitter and receiver are tuned. The diameter of the cylinder is temperature-dependent, but typically is approximately 14 to 16 in.

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PULSED NEUTRON-NEUTRON

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PULSED NEUTRON-NEUTRON BASIC PRINCIPLE: 

The formation  is subjected  to a pulse of high-energy neutrons  (14MeV)  from  a  neutron  generator.  These pulses  are  repeated  at  a  certain  repetition  rate of 20 pulses per second. The thermal neutron population is sampled between pulses and  its rate of decay can be computed. 

NEUTRON INTERACTIONS: 

Neutrons  may  interact  with  matter  in  a  variety  of modes. Because  it  is rare  that neutrons are absorbed until they lose most of their energy, each neutron often has  many  interactions  in  the  process  of  losing  its energy. This process is also called slowing down. The characteristics  of  some  of  these  interactions  can  be used  to  predict  the  formation  properties.  In  well logging,  there  are  four  major  types  of  interactions between a neutron and a  target nucleus  that can be used for  formation  evaluation:  inelastic  scattering, elastic scattering, absorption or capture of  fast and  slow or thermal neutrons. The type of interaction that is most probable  to happen  is  function of neutron energy.

LIFE OF A NEUTRON:

The path a neutron takes as it scatter, is the change in direction at a certain distance, sometimes even toward the source  is erratic, but on average,  it gradually moves away  from  the  source.  With  each  interaction,  the neutron loses  some  of  its  kinetic  energy.  This

continues, until  it has a value  just above  thermal energy (0.025 eV). Up to this point the velocity of the neutron was so much higher than the target formation nuclei, that  the  target  nuclei  could  be  treated  as  being stationary. This  is  no  longer  the  case,  now  that  the neutron's  energy  is  just  above  thermal  energy.  The energy  of  the  thermal  neutron  is  about  equal  to  the

thermal  vibration  energy  of  the  formation  nuclei. Subsequent collisions will, on  the average, maintain the neutron's energy  in equilibrium with  the  thermal energy of  the  formation nuclei.

HYDROGEN  INDEX

The neutron  energy  loss  for  any particular  collision depends upon the mass of the neutron and the mass of the  element  or  particle  being  struck,  which  is demonstrated  in picture below.

 

In  the  first  instance,  the  particle  being  struck  has  a larger mass  than  the neutron. A small amount of energy is transferred to the particle, but the neutron bounces back, retaining the majority of its kinetic energy. In the second  case,  the  neutron  basically  runs  over  the particle, transferring some energy to it and continues with most of  its energy. The  greatest  energy  loss  results,  when  a  neutron

collides with a particle or atom of an equal mass. This  is shown  in  the  third  instance. Here, all or nearly all of  the neutron's kinetic energy  is passed  to  the equally massed particle, which  is similar  to what happens when  two equally massed billiard balls collide head-on.

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