Pulsed neutron lifetime logs

This page provides an overview of Pulsed-Neutron-Lifetime (PNL) devices and their applications.

How PNL logs work

PNL logs measure the die-away time of a short-lived neutron pulse. They probe the formation with neutrons but detect gamma rays. Chlorine has a particularly large capture cross section for thermal neutrons. If the chlorine in the formation brine dominates the total neutron capture losses, a neutron-lifetime log will track chlorine concentration and, thus, the bulk volume of water in the formation. For constant porosity, the log will track water saturation, S_w. The neutrons are little affected by steel casing, so this is the standard cased-hole saturation tool. Like other nuclear tools, modern PNL tools incorporate two detectors for borehole compensation. These detectors also permit the calculation of a ratio porosity. This ratio porosity is similar, but not identical, to that of a compensated neutron-porosity tool. They differ because the energy of the neutrons from the pulsed accelerator source is higher than the energy from the isotopic source used in compensated neutron logging. Also, the neutron-lifetime tools detect capture gamma rays rather than direct neutrons.

The basis of operation is similar to the other nuclear radiation transport tools in that the tool infers a cross section. In this case, the tool measures the time required for a pulse of neutrons to be absorbed by a formation. The mechanism by which the neutrons disappear is primarily thermal neutron capture. The time evolution of a pulse of neutrons follows the usual exponential decay law:

....................(1)

where Σ_abs is the total neutron capture cross section of the formation expressed in capture units (c.u. = 1000 × cm²/cm³, which has units of cross-sectional area per unit volume). The total capture cross section for a formation follows the standard linear volumetric mixing law discussed above:

....................(2)

where V_i is the volume of a particular constituent (mineral or fluid) of a formation and Σ_i is the capture cross section of that constituent. Because the corrected tool reads the total capture cross of the formation, this equation forms the basis of interpretation. For example, in the case of a clean sand with a porosity that is filled with oil and water, the tool reading will be

....................(3)

If porosity is known either from openhole logging or the ratio porosity measured by the pulsed neutron tool itself, the various cross sections can be looked up in a table, and it is a simple matter to solve for S_w. Table 1 lists the capture cross sections for several materials that commonly make up reservoirs. Several things can be observed on the basis of this table and the response equation above. First and foremost, for this measurement to be very sensitive to replacing oil by water in the pore space, the water salinity needs to be higher than 50,000 ppm NaCl. Otherwise, the oil and water capture cross sections are so similar that the measured formation cross section will not change perceptibly when one is substituted for the other. These sensitivities can be evaluated easily by setting up a simple spreadsheet and varying the values in the equation above. Eq.3 also suggests the value of running this log in a baseline monitor mode or time-lapse mode. Differences between successive logging runs will depend only on differences in fluid volumes because the terms involving the unchanging rock matrix will subtract out. In this way, no explicit knowledge of the capture cross sections of the minerals and clays is required to interpret saturation changes.

....................(4)

While it is rarely done, this method is particularly valuable if a baseline run is made early in the production history of the well before S_w has had a chance to change significantly.

• 

  Table 1

Applications

Log-inject-log measurements for residual oil saturation

In a variation on the baseline monitor mode of operation suggested above, residual oil saturation can be determined by a log-inject-log procedure. In this procedure, a pulsed neutron log is run over a zone of interest to get a baseline reading. Then, a brine of contrasting salinity is injected into the formation while logging pass after pass with the pulsed neutron tool. Over time, all of the movable, original formation fluids are displaced by the new brine until S_orw (residual oil saturation to waterflood) is achieved. Differences between the preinjection-pass and the final-pass formation capture cross sections give direct access to S_orw. As a bonus, changes in the capture cross-section profile over time during injection highlight permeability variations in the formation.

Mechanical integrity

Oxygen-activation flow logging may be used as a test of well integrity and zonal isolation. This is a stopwatch measurement. The neutron generator activates the oxygen in a slug of water. The time it takes the slug to move from its birthplace at the generator until it is opposite one of several remote detectors is measured. The flow velocity is just the distance from source to detector divided by the transit time. Because of the short half-life of oxygen, a particular source-to-detector spacing will be optimal only for a narrow range of flow rates. This procedure works equally well for flow inside and behind pipe. In principle, similar measurements can discern distance to the flow.

Boron has a very high neutron-absorption cross section that greatly reduces the neutron lifetime measured by a pulsed-neutron tool. This makes it a useful tracer when used in conjunction with pulsed-neutron logging. It has been exploited in mechanical-integrity testing by injecting borated water into a well. Any place to which the boron-tagged water finds its way will stand out on the pulsed-neutron log.

Gravel-pack logs

In another example, silicon activation is used to evaluate gravel-pack quality. Gravel packs are placed in oil and gas wells to prevent the accumulation of formation material that otherwise would clog wellbores and production facilities. In the conventional logging method for gravel-pack evaluation, a nonfocused density tool detects the density contrast between packing material and completion fluid. When a pulsed-neutron log is used, it detects activation gamma rays from silicon and aluminum in the packing material that have a half-life of approximately 2.24 minutes. Of the other common downhole elements, oxygen has a much shorter half-life (7.2 seconds), and chlorine, sodium, and iron have half-lives of 30 minutes or longer. Thus, a judicious choice of logging speed can maximize sensitivity to silicon and aluminum. Because the threshold for silicon activation is high (4 to 5 MeV), the measurement is very shallow, maximizing sensitivity to the gravel-pack region.

Other applications

Pulsed-neutron logging tools have been applied in nonconventional ways to solve several production-logging problems. Not all of these applications make use of neutron die-away time. Instead, they monitor gamma rays from neutron activation of specific elements that can be thought of as tracers. In other applications, the pulsed-neutron ratio porosity can be used for excavation-effect-style gas interpretations.

Induced gamma ray spectroscopy tools

A final class of neutron logs, neutron-induced gamma ray logging, records the energy spectra of the induced gamma rays. Because elements excited by neutrons emit characteristic gamma rays, such spectra can be analyzed for elemental concentrations. Most commonly, carbon and oxygen concentrations are used to determine oil saturation, although more detailed geochemical information lies buried in the spectra.

Nomenclature

e	=	natural logarithm base
Sw	=	water saturation
Vi	=	volume of a particular constituent (mineral or fluid) of a formation
Σabs	=	total neutron capture cross section
Σi	=	capture cross section of ith formation component
Σw	=	capture cross section of water
ϕ	=	porosity

Subscripts

i	=	item count or index
t	=	time

Noteworthy papers in OnePetro

Ierubino, J.V. and Ginest, N.H. 1989. Use of Pulsed Neutron Logging Techniques To Prove Protection of Underground Sources of Drinking Water. Presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 8-11 October 1989. SPE-19616-MS. http://dx.doi.org/10.2118/19616-MS

Barnette, J.C., Copoulos, A.E., and Biswas, P.B. 1992. Acquiring Production Logging Data With Pulsed Neutron Logs from Highly Deviated or Non-Conventional Production Wells With Multiphase Flow in Prudhoe Bay, Alaska. Presented at the SPE Western Regional Meeting, Bakersfield, California, 30 March-1 April 1992. SPE-24089-MS. http://dx.doi.org/10.2118/24089-MS

Maher, T. and Trcka, D. 1999. Inflow Fluid Typing in Screened Horizontal Completions Using aPulsed Neutron Holdup Imager. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1999. SPE-56646-MS. http://dx.doi.org/10.2118/56646-MS

Roscoe, B.A. 1996. Three-Phase Holdup Determination in Horizontal Wells Using a Pulsed-Neutron Source. Presented at the International Conference on Horizontal Well Technology, Calgary, Alberta, Canada, 18-20 November 1996. SPE-37147-MS. http://dx.doi.org/10.2118/37147-MS

External links

Use this section to provide links to relevant materiHingle, A.T. Jr. 1959. The Use of Logs in Exploration Problems. Technical Program, 1959 Annual Meeting, SEG, 38.al on websites other than PetroWiki and OnePetro

See also

Nuclear logging

Neutron porosity logs

PEH:Nuclear_Logging