Formation Micro-Imager Logs (FMI)

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HISTORY: 

In the late 1980’s Schlumberger introduced the concept of borehole electrical images by processing variations of the shallow microresistivity of wellbore walls recorded by modified versions of its Stratigraphic High Resolution Dipmeter Tool™.  Called the Formation Micro-Scanner™ (FMS), the tool measured closely spaced arrays of focused shallow resistivity readings that are related to changes  in rock composition and texture, structure, and fluid content [Serra, 1989].  Processing the data, in which a range of colors are assigned to the lateral (side-to-side) and vertical variations of the microresistivity along the wellbore, produces an image of the borehole wall.

   

WHAT DOES FMI MEASURES:  

Image logs are resistivity or acoustic devices that measure certain physical properties of the rock at or near the well that can be displayed as images of the wellbore, which can then be interpreted on a computer.  Typically rock properties are controlled by factors such as variations in composition, diagenesis, grain size, grain orientation, pore fluid variations, etc.  Image logs can provide detailed picture of the wellbore that represent the geological and petrophysical properties of the section being logged.

WORKING PRINCIPLE:

The current generation of tools, called the fullbore Formation Micro Imager™ (FMI), records an array of microresistivity measurements from 192 sensors on eight pads mounted on four orthogonally placed caliper arms.  The spacing and position of the pads provides 80% coverage of an eight-inch diameter hole and a resolution of 5 mm.  Other oil field wireline service companies have since developed similar high-resolution electrical borehole imaging tools. The FMI yields a continuous, high-resolution electrical image of a borehole (color-coded for resistivity values), and therefore complements whole cores cut in the  same well.

  

CORE AND FMI:  

If the FMI-derived image is of sufficient quality and calibrated against the core, it can provide a continuous survey of the formation in places where core is not cut, there was no core recovery, or when a core has beendamaged through handling, transportation, or plugging.  

In the figure FMI image of the 60 ft section representing  a Core . On the left, a
dynamic FMI image, on the right, a “Core View” simulating a core. The FMI
color scale presents a range of resistivities from conductive (black) to
resistive (white)

FMI ADVANTAGES:  

Determine net pay

The FMI fullbore formation micromager gives you microresistivity formation images in water-base mud. This is the preferred approach for determining net pay in laminated sediments of fluvial and turbidite depositional environments.

Visualize sedimentary features to understand structure

Sedimentary features features define important reservoir geometries and petrophysical reservoir parameters. The interpretation of image-derived sedimentary dip data helps you understand sedimentary structures.

Interpret seismic sections

Well-to-well correlation is difficult in deviated wells with sections of steep and varying structural dip. Greatly improve your structural interpretation of seismic sections with high-quality bedding dips to compute accurate logs of true stratigraphic thickness.

Get more data

Geological information from FMI borehole images helps with stochastic modeling of the sand-shale distribution. FMI images define channel heights superbly in amalgamated units. Other variables, such as the channel width and channel sinuosity, can be estimated using geological analogs, based on detailed sedimentological analysis of FMI image data.

Improve well construction plans

Borehole images improve your mechanical earth models, which in turn helps you optimize well plans. Better understanding of borehole stability can save you millions of dollars during field development.

Benefits

• Obtain accurate pay estimates
• Interpret formations accurately
• Improve reservoir descriptions
• Make decisions on site
• Get data in difficult environments, including deviated and horizontal wells
• Save time and money with complete interpretations in one image pass

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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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