Schlumberger

proVISION Plus Magnetic Resonance-While-Drilling Service

A two-dimensional measurement of pore and fluid properties

Magnetic resonance (MR) measurements, which reveal pore and fluid properties, consist of a series of manipulations of hydrogen nuclei in fluid molecules. The nuclei have magnetic moments, behaving like small bar magnets, and their orientations can be controlled by magnetic fields. The nuclei also spin, acting like gyroscopes.

T2 distribution

When the proVISION Plus tool approaches the formation, its permanent magnets align the nuclei, starting a measurement sequence. When the tool’s controlled antenna is applied, this alignment is followed by spin tipping, precession, and repeated dephasing and refocusing. Transverse relaxation and longitudinal relaxation limit how long a measurement will last. The relaxation time is a function of pore size and fluid properties.

Completing all these steps only takes a few seconds, and then the measurement is repeated, resulting in a collection of relaxation times. T2 distribution is this collection of all the relaxation times.


T2 distribution

Lithology-independent porosity

MR measurements—like neutron porosity logs—are sensitive to hydrogen. The MR signal amplitude is only proportional to the number of hydrogen nuclei present in the fluid. In contrast, the neutron log responds to all hydrogen in the formation, including those that are not part of the pore fluids. Thus, MR provides an accurate porosity measurement, free from chemical sources and independent from lithology effects since the signal includes only producible fluids and capillary-bound water.

 Lithology-independent porosity

Irreducible and producible fluid volumes

T2 distribution mimics pore-size distribution in water-saturated rock. The T2 decay can be further processed to quantify pore volumes associated within different ranges of T2. The volumes of interest typically are the bound fluid in small pores and free fluid that is readily producible from larger pores. An input T2 cutoff is usually selected to define both bound- and free-fluid volume. The same cutoff is used to define the capillary bound water that is used by reservoir engineers to assess the fluid flow in the reservoir.

Irreducible and producible fluid volumes 

Facies analysis and pore size distribution 

MR signal decay during each measurement cycle—the relaxation time—depends on pore sizes. Small pores shorten relaxation times, and the shortest times correspond to clay-bound and capillary-bound water. The relaxation times for large pores, which contain the most readily producible fluids, are longer. The distribution of relaxation times is a measure of the distribution of pore sizes. The relaxation times and their distributions can be interpreted to give other petrophysical parameters such as permeability, producible porosity, and irreducible water saturation. Other possible applications include capillary pressure curves, hydrocarbon identification, and facies analysis.

Facies analysis and pore size distribution 

Continuous permeability

One of the most important features of MR logging is the ability to provide a real-time permeability log. Log permeability measurements enable prediction of production rates to optimize completion and stimulation programs.

 Continuous permeability
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Magnetic Resonance LWD Evaluates Pore Sizes & Fluids in Real Time

proVISION Magnetic Resonance while Drilling
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