Wednesday, December 7, 2011

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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Tuesday, December 6, 2011

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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Monday, December 5, 2011

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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