Nuclear logging while drilling

Gamma ray measurements have been made while drilling since the late 1970s. These measurements are relatively inexpensive, although they require a more sophisticated surface system than is needed for directional measurements. Log plotting requires a depth-tracking system and additional surface computer hardware.

Overview

Applications have been made in both reconnaissance mode, where qualitative readings are used to locate a casing or coring point, and evaluation mode. Verification of proper measurement while drilling (MWD) gamma ray detector function is normally performed in the field with a thorium blanket or an annular calibrator.

The main differences between MWD and wireline gamma ray curves are caused by spectral biasing of the formation gamma rays and logging speeds. When this method is used, the wellbore (which is generally inclined) is divided into multiple segments (often 4, 8, or 16). Incoming gamma counts are placed into one of the bins. From this, the segment densities and an average density are obtained. A coarse image of the borehole can be obtained when beds of varying density arrive in one segment before another. Azimuthal density can be run without stabilization, but it relies on the assumption that standoff is minimal in the bottom quadrant of the wellbore.

Rapid sampling

Another method is referred to as rapid sampling. In this method, statistical techniques are applied to rapid samples taken on incoming gamma counts. When the tool is rotating and there is a significant difference between mud weight and formation density, there will be an unexpectedly high standard deviation. This is used to create limits for a high- and low-count rate bin. The total counts arriving in the low-count rate bin are used to calculate a rapid sample density.

Ultra sonic calipers

Another method of obtaining density in enlarged boreholes relies on the constant measurement of standoff using a series of ultrasonic calipers. A standoff measurement is made at frequent intervals, and a weighted average is calculated. High weight is given to gamma rays arriving at the detector when the standoff is low, and low weight is given to those gamma rays that arrive when the standoff is high (Fig. 2). This method attempts to replicate the wireline technique of dragging a tool pad up the side of the borehole. The constant-standoff method can also be applied to neutron porosity tools.

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  Fig. 2—Use of ultrasonic measurements to compensate for mud effects. (From Economides, Watters, and Dunn-Norman, Petroleum Well Construction, © 1998; reproduced by permission of John Wiley & Sons Ltd.)

Neutron-porosity log tools

Although all Measurement while drilling (MWD) devices share the basic wireline configuration of a neutron source and two differently spaced detectors, the drillstring environment forces changes. A pad-mounted tool is not possible, increasing potential borehole effects. He-3 detectors with long central wires, the standards in wireline tools, are sensitive to the effects of vibration that can cause false counts. One service company uses a new neutron detector, an Li-6 scintillator. Because this detector can respond to gamma rays as well as neutrons, spectral processing is required to strip out the gamma ray counts that show up as low-height pulses. The detector absorbs essentially all incident thermal neutrons, resulting in a high counting efficiency, but the metal hatch over the detector acts as a filter, giving a substantial epithermal character to the response, which lies somewhere between the thermal and epithermal responses of a wireline tool. In another novel approach, multiple Geiger Marsden tubes arrayed around the circumference of the collar detect capture gamma rays. In principle, most detected gamma rays come not from neutron capture in the formation, but from capture in the iron of the collar. They thus reflect the neutron population near the detector as if they were neutron detectors. In practice, this tool can exhibit lithology and salinity effects. In formations containing siderite, some correlation between porosity and grain density has been observed from this design. This indicates that some of the gamma rays recorded by the detector do indeed come from the formation. A third design takes a wireline-like approach. The primary measurement uses banks of He-3 detectors at two different spacings from an americium-beryllium source. The source is centered in the drill collar rather than near the formation. Although this makes the source separately retrievable, it also filters and lowers the neutron flux at the formation. Source-to-detector spacings are similar to wireline tools. Porosity is calculated much as it is for wireline logs. A near-to-far count-rate ratio is taken. Shop calibration factors, borehole diameter, mud weight, salinity, temperature, pressure, and matrix corrections are applied to the ratio before finally calculating porosity. This procedure is superior to correcting porosities for these effects in that it reflects perturbations on what the tool measures: neutron slowing-down length, not porosity. Because of absorption of thermal neutrons by the drill collar, the measurement has an epithermal flavor. Each bank of detectors consists of three detectors distributed radially around the circumference of the drill collar. Examining the count rates from the three detectors allows for correction for tool position in the borehole.

Like the gamma ray API unit, some historical baggage accompanies the presentation and scaling of neutron-porosity logs. The curve is presented as porosity, frequently without reference to the matrix, even though the matrix does matter. Although the curve is most often scaled as apparent limestone porosity, as we have seen earlier, it is actually a measurement of the distance required for a neutron to slow down, referred to by physicists as the slowing-down length, L_s. Neutrons produced by an americium-beryllium (Am²⁴¹Be⁹) isotopic source have an energy of approximately 4.3 MeV, corresponding to a speed of more than 2000 cm/μs (44 million mph). Above approximately 0.1 eV (a mere 6,000 mph), neutrons slow down primarily through elastic collisions with the nuclei of atoms in the formation. Elastic collisions are like billiard-ball collisions: the nearer the nucleus struck is to the mass of the neutron, the more energy the neutron loses in the collision. This means that hydrogen, the nucleus of which has only a single proton (and which has altogether the same mass as the incoming neutron), is by far the most effective atom at slowing down a neutron (see Table 1). Neutron slowing down by elastic collisions (thermalization) may be visualized as a random walk (see Fig.2). The average straight-line distance that a neutron manages to get from the source before it comes to thermal equilibrium with the reservoir is the slowing-down length, L_s. Note how much longer the slowing-down length for limestone is than for water, as shown in the figure.

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  Fig.2 – Neutron scattering determines the size of a cloud of neutrons surrounding a source embedded in a formation.

Logging tools measure the size of the neutron cloud by looking at the falloff in neutron flux between two detectors. The falloff is inferred from a ratio of near-to-far neutron counting rates. As shown in the top half of Fig.3, this ratio can be uniquely related to slowing-down length. The data points in the plot represent a variety of lithologies and porosites, but all fall along the same trend line. Although it depends on details of lithology and fluid composition, conversion of slowing-down length to porosity is straightforward, as will be seen in the discussion of macroparameters later.

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  Fig.3 – In an alternate neutron-porosity log-interpretation process, the intermediate response parameter, slowing-down length, relates better to basic tool response. The data points in the top plot represent a variety of lithologies and porosities, but all fall on a single trend.

The standard neutron-porosity measurement counts all neutrons, not just the unthermalized or epithermal ones. They are designed this way because the epithermal count rate represents only a small fraction of the neutrons at a distance from the source. To get the count rate (and the precision) up, all available neutrons are counted. This is a textbook example of trading accuracy for precision. While elastic scattering and, thus, the hydrogen content of the formation control epithermal neutron flux, thermal neutrons can interact with the formation in other ways. In particular, they can be captured by other elements in the formation. Neutron capture is not dominated by hydrogen, as Table 2 shows. This means that the standard thermal neutron porosity is contaminated by the subsequent diffusion distance, L_d, that the thermalized neutrons travel before they are finally captured. This is illustrated by the vector sum in Fig.2. This total distance the neutron travels, slowing down plus diffusion, is called the migration length, L_m. Because of the enduring confusion around the matrix assumed for a particular display of neutron porosity and the tenuous relationship between what the tool measures and porosity, values would be better reported in migration length than apparent porosity. It is as if density logs were always displayed as density porosity without always reporting the assumed matrix and fluid densities.

These problems with the interpretation of the standard compensated thermal neutron log have started a search for a new neutron-porosity tool with a more epithermal character. Accelerator neutron sources have begun to appear in commercial porosity tools. These tools take a more sophisticated approach than the simple CNL-like count-rate ratios used by current pulsed-neutron lifetime tools to obtain porosity. For example, Schlumberger has fielded a neutron generator-based porosity sonde that measures several different "neutron porosities" with both thermal and epithermal characters. These different measurements of porosity can be compared and combined to improve the final porosity estimate. While the measurement may be a better hydrogen-index porosity measurement, it is not the same as a standard compensated thermal neutron-porosity log.

Measurement issues

All density measurements suffer if the drillstring is sliding in a high-angle or horizontal borehole with the gamma detectors pointing up (away from the bottom of the wellbore). To overcome this problem, orientation devices are often inserted in the toolstring. As the BHA is being made up, the offset between the density sleeve and the tool face is measured. Adjusting the location of the orientation device allows the density measurement to be set to the desired offset. While the drillstring is sliding to build angle, the density detectors can be oriented downward by setting the offset to 180°.

Neutron measurements

LWD porosity measurements use a source (typically americium beryllium) that emits neutrons into the formation. Neutrons arrive at the two detectors (near and far) in proportion to the amount they are moderated and captured by the media between the source and detectors. The best natural capture medium is hydrogen, generally found in the water, oil, and gas in the pore spaces of the formation. The ratio of neutron counts arriving at the detectors is calculated and stored in memory or transmitted to the surface. A high near/far ratio implies a high concentration of hydrogen in the formation and, hence, high porosity.

Neutron measurements are susceptible to a large number of environmental effects. Unlike wireline or LWD density measurements, the neutron measurement has minimal protection from mud effects. Neutron source/detector arrays are often built into a section of the tool that has a slightly larger OD than the rest of the string. The effect of centering the tool has been shown to have a dramatic influence on corrections required compared to wireline (Fig. 3). Standoff between the tool and the formation requires corrections of approximately 5 to 7 porosity units (p.u.) per inch. Borehole-diameter corrections can range from 1 to 7 p.u./in. depending on tool design. Neutron porosity measurements are also affected by:

• Mud salinity
• Hydrogen index
• Formation salinity
• Temperature
• Pressure

However, these effects are generally much smaller, requiring corrections of approximately 0.5 to 2.0 p.u.

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  Fig. 3—Effects of tool centering. (From Economides, Watters, and Dunn-Norman, Petroleum Well Construction, © 1998; reproduced by permission of John Wiley & Sons Ltd.)

Uncertainties in measurements

Statistical effects on nuclear measurements are quite significant. Uncertainties increase as ROP increases. LWD nuclear measurements can be performed either while drilling or while tripping. LWD rates vary because of ROP changes, but they typically range from 15 to 200 ft/hr, whereas instantaneous logging rates can be significantly higher. Tripping rates can range from 1,500 to 3,000 ft/hr. Typical wireline rates are approximately 1,800 ft/hr and constant. Statistical uncertainty in LWD nuclear logging also varies with formation type. In general, log quality begins to suffer increased statistical uncertainties at logging rates above 100 ft/hr. This limits the value of logging while tripping to repeating formation intervals of particular interest.

Noteworthy papers in OnePetro

P. Nelis, J. Dahl et al. 2006. An Assessment of The Benefits of & Quot;Secure Source & Quot; Systems For Nuclear Logging While Drilling Operations, SPE International Health, Safety & Environment Conference, 2-4 April. 98497-MS. http://dx.doi.org/10.2118/98497-MS

A. Badruzzaman, P.T. Nguyen et al. 1997. Nuclear Logging-While-Drilling Measurement: An Assessment, Middle East Oil Show and Conference, 15-18 March. 37745-MS. http://dx.doi.org/10.2118/37745-MS

See also

Nuclear logging

Acoustic logging while drilling

NMR logging tools

PEH:Drilling-Data_Acquisition