Cased Hole Logs
Pulsed Neutron Logs
A pulsed neutron log provides a means of evaluating a formation after the well has been cased. It is of particular value for :
- evaluating old wells, where the original openhole logs are inadequate or nonexistent
monitoring reservoir performance over an extended period of time
monitoring the progress of the secondary and tertiary recovery projects
evaluating the formation, as a last resort, should the drillpipe become stuck
It is the most widely used and most direct logging method in cased holes at the present time. Other nuclear measurements are being developed that may eventually give superior results; these include the carbon/oxygen type logs and activation logs.
Though all commercially available tools are designed to measure the same formation parameters, their operating systems are all slightly different.
Principle of Measurement Regardless of the tool used, the principle of measurement remains the same. When a neutron generator is turned on for a very short period of time, a "burst" of neutrons leaves the tool. Since neutrons can easily pass through both the steel housing of the tool and the tubing/ casing, a "cloud" of neutrons gathers in the formation. Fast neutrons soon become "thermalized" by collisions with atoms in the formation. The most effective thermalizing agent is the hydrogen present in the pore space in the form of water or hydrocarbon. Once in the thermal state, a neutron is liable to be captured. The capture process depends on the capture cross section of the formation. In general, chlorine dominates the capture process. Since chlorine is present in formation water in the form of salt (NaCl), the ability of the formation to capture thermal neutrons reflects the salt content and, hence, the water saturation. The capturing of a thermal neutron by a chlorine atom gives rise to a capture gamma ray. Pulsed neutron tools therefore monitor these capture gamma rays. Thus, the common elements of all commercial pulsed neutron tools are a pulsed neutron generator and two gamma ray detectors at different distances from the neutron generator. Figure 1 illustrates a generalized neutron tool.
The cloud of neutrons produced by the initial neutron generator burst results in a cloud of thermal neutrons in the vicinity of the tool, which dies away as the neutrons are captured by chlorine atoms or other neutron absorbers in the formation. If there is plenty of chlorine present (i.e., high water saturation), the cloud of thermal neutrons disappears quite quickly. If, however, hydrocarbons are present (i.e., low water saturation), the cloud of thermal neutrons decays much more slowly.
The rate of decay is measured by monitoring how many capture gamma rays enter the gamma ray counter(s) as a function of time. Figure 2 plots the relative counting rate on the y-axis, and time, in microseconds, following the initial burst of fast neutrons, on the x-axis. Note that after a few hundred microseconds a straight-line portion of the decay curve develops. Note also how the water line has a steeper slope than the oil line. At later times note the background gamma ray count rate that remains substantially constant.
The y-axis in Figure 2 is logarithmic but the x-axis (time scale) is linear. Thus, the straight-line portions represent exponential decay. If N is the number of gamma rays observed at time t and No is the number observed at t = 0, then
N = No et/
where r is the time constant of the decay process. t is measured in units of time. It is convenient to quote values of t in microseconds (1 microsecond = l0-6 seconds). The capture cross section of the formation, the property of interest, is directly related to t by the equation:
S = 4550/t
where S is the capture cross section measured in capture units (CU).
Thus S is best measured by finding the straight-line portion of the capture gamma ray decay, and measuring its slope. This is accomplished in different ways by various commercially available tools.
On a typical pulsed neutron log as many as 9 curves may be displayed. Figure 3 illustrates a typical presentation:
Curve Name | Units | Logs Track | Remarks |
Sigma () | CU | 2 & 3 | Main curve |
Tau () | µ-sec | 2 & 3 |
|
Ratio | - | 2 | Pseudoporosity |
Near Counts | cps | 3 | Near detector, gate 1 |
Far Counts | cps | 3 | Far detector, gate 1 |
Monitor or Background | cps | 1 | Near detector, gate 3 |
Quality Control | - | 1 | Check of 7 loop |
Gamma Ray | API | 1 | Natural gamma |
Casing Collar Log | - | 1 | Both memorized and direct |
The Sigma Curve The curve, the principal pulsed neutron measurement, behaves rather like an openhole resistivity curve; i.e., it deflects to the left (high values of in wet zones and to the right (low values of in hydrocarbon-bearing zones or low-porosity formations.
Since values in shales are quite high, they tend to mask the effect of hydrocarbons, making shaly pay zones at first appear to be water-bearing. Figure 4 is a comparison of with resistivity.
The Tau Curve
is just another way of looking at . In fact, is the basic measurement of the tool (the decay time constant for the thermal neutron population). However, all interpretation equations for pulsed neutron logs are linear functions of . Thus, it is much easier to work with than with . It is recommended that be recorded on tape but left of f the log presentation, since its scaled reciprocal () gives exactly the same information in a form that is easier to work with.
Ratio Curve The ratio curve is a porosity indicator derived by taking the ratio of gamma ray counts seen during gate 1 at the near and far detectors. The ratio curve, behaving very much like a compensated neutron porosity curve, deflects to the right (low ratio) in low porosity or in the presence of gas. Figure 5 shows the ratio curve response to a pocket of gas trapped below a packer behind a tubing nipple. In the absence of any openhole porosity logs, the ratio can be used in combination with to find formation porosity.
Near and Far Count-Rate Display In track 3 the near and far count rates are displayed as an overlay ( Figure 6 ). When the correct scales are chosen for the near and far count rate displays, the result is a useful "quick-look" log with the following properties:
in gas Fl > Nl (dotted left of solid)
in shales Fl < Nl (dotted right of solid)
and in clean oil- or water-bearing zones, the two curves lie practically on top of one another.
Background and Quality Curves The background curve is a very insensitive natural gamma ray curve. Little movement shows on this curve except in "hot" zones, which are very radioactive. This curve is sometimes omitted without any great loss.
To summarize, the most important curves to work with are:
| for water saturation |
Ratio | for porosity |
GR | for shale content |
Near/far display | for gas indications |
Pulsed Neutron Interpretation
Capture Cross Sections
The capture cross section of a formation depends on the chemical elements present, and on their relative abundance. values vary over a wide range.
Common matrix materials (sand, lime, and dolomite) exhibit capture cross sections in the range of 8 to 12 CU. Pore-filling fluids such as water, oil, and gas also show a wide range, brines varying from 22 CU (fresh water) up to 120 CU (saturated brine). Oils, depending on the amount of dissolved gas they contain, range from 18 to 22 CU. Gases, depending on their gravity, temperature, and pressure, range from 4 to 12 CU.
Interpretation of Pulsed Neutron Logs Practical interpretation of pulsed neutron logs in clean formations is conceptually very simple. The total formation capture cross section () recorded on the log, is the sum of the products of the volume fractions found in the formation and their respective capture cross sections. Thus, in its simplest form:
log =
matrix • (1 -) +
fluid •
Figure 1 should clarify the mathematical relationship. If the "fluid" is a mixture of oil and water, the log response is described by
S log = S ma (1 -f) + S wf Sw + S hy f (1 - Sw)
By rearrangement of the equation, we have
Reservoir Monitoring-Time Lapse Technique Pulse neutron logs are useful for monitoring the depletion of a reservoir. The time lapse method is used. A base log is run in the well shortly after initial completion but before substantial depletion of the producing horizons. A few days, weeks, or even months of production are required to "clean up" near-wellbore effects of the drilling operation, such as mud filtrate invasion. Once a base log is obtained, the well may be relogged at time intervals over the life of the field, depending on production rate variations.
Successive logs may be overlaid so that changes in saturation can be easily spotted by changes in . A good example of this ( Figure 2 ) shows a base log and three additional logs at roughly six-month intervals. Note the rapid rise of the oil-water contact(s) with passage of time.
Log-Inject-Log The log-inject-log technique is used to find residual oil saturations. Once a base log is run, the formation is injected with waters of different salinities and logged again. In Figure 3 , the formation was injected with brine and logged, then injected with seawater and logged a third time. Provided the capture cross section of the seawater and brine flushes are known, all the unknown quantities may be normalized out and the residual oil saturation found, using
Note that it is not necessary to know either Sma or Soil. The technique has many variations, some using specially chlorinated oil that has a high capture cross section.
Inelastic Neutron-Gamma (Carbon-Oxygen) Logs
High-energy neutrons (14 Mev) produced by a pulsed-neutron source are directed into the formation, and the energy spectrum of gamma rays produced by the neutron bombardment is sampled at various times both during and after the neutron burst. Neutrons can interact with matter in two distinct ways to create gamma rays: by inelastic scattering with nuclei at high energies (>5 Mev) and, through capture or absorption, by nuclei at low energies (<.025 Mev). The gamma rays produced from each of these reactions have unique energies that depend on the type of nucleus with which the neutron reacts. By measuring the number and energy of gamma rays produced by neutron bombardment, the elemental composition of the formation can be inferred.
Applications These tools provide a measure of the oil saturation, C/O ratio; lithology, Si/(Ca + Si) ratio; porosity, H/(Ca + Si) ratio; shale, Fe/(Ca + Si) ratio; and salinity, Cl/H ratio, in open or cased holes. This logging method is used to determine the presence of hydrocarbons behind casing, regard-less of formation water salinity.
At present, reliable measurements can be made only with optimum borehole and formation conditions. The major interpretive uncertainty stems from the inability of the measurement to distinguish between carbon associated with carbonates (e.g., limestone, CaCO3) and carbon associated with hydrocarbons.
Depending on the tool used, the tool either (a) measures the number of gamma rays in two energy "windows," centered around the expected carbon and oxygen inelastic scattering energies during the burst and around the silicon and calcium thermal capture energies after the burst, or (b) employs a "spectral fitting analysis" to determine the yields of carbon, oxygen, calcium, silicon, and several other elements. This spectral fitting analysis uses three gates: the burst gate, the background gate, and the capture gate. The burst gate is at the source, the background gate cuts down on borehole interference, and the capture gate gives capture readings. The burst gate minus the background gate gives the inelastic spectrum and the capture gate gives the capture spectrum ( Figure 1 ).
Ratios of element yields (C/O, Si/Ca, Cl/H, etc.) are normally presented. Given a constant porosity and lithology, an increase in the carbon-oxygen ratio indicates an increase in oil saturation. It should be noted that by taking elemental ratios, any variations in neutron output from the source are normalized.
Note the following considerations:
· The log is generally run in cased holes when conditions are not favorable for pulsed neutron logs because of low formation water
salinities.
· Optimum formation conditions are high porosity (>20%), low water salinity (<50,000 ppm NaCl), and consistent or known lithology. The log can be useful where salinities are unknown or variable.
· Depth of investigation is very shallow for measurements on the inelastic scattering spectrum. This limits the tool's openhole use and forces consideration of the effects from the casing annulus.
· Optimum borehole conditions are a small-diameter hole and constant fluid composition in the casing. If an oil-water contact or varying salinities are expected in the casing, a fluid displacer should be considered.
· At present, the statistical uncertainty in analyzing the spectrum is the tool's limiting feature. Advances in detector design and spectrum analysis should
solve these problems.
Figure 2 shows a continuous carbon/oxygen log. The curves it presents are:
Track 1 | Monitor |
| Silicon correlation |
Tracks 2 and 3 | Silicon-calcium ratio (capturespectrum)Carbon-oxygen ratio |
Figure 3 shows an inelastic neutron gamma log of the sort that employs spectral filtering. The data it records are:
Track 1 | Ion-Indicating ratio Fe/(Si+Ca) |
|
|
Tracks 2 and 3 | Lithology indicator (Si/(Si+Ca) |
Cement Bond Logging
This variant of acoustic logging makes use
of the observation that on acoustic logs run inside casing with good cement bonding, the amplitude of the signal detected at the receiver is much reduced, while in unsupported casing the signal remains strong. The log format may include a gamma ray and casing collar log for depth control, a transit-time curve, and an amplitude measurement for evaluation of bonding. There may also be a "signature" or a "variable density" display of the actual waveforms. These displays aid both quality control and log evaluation. In Figure 1 , a typical cement bond log presentation, GR and casing collar logs are omitted.
Measurement Principle A cement sheath bonded to the casing can be intuitively predicted to attenuate sound propagation in the pipe. CBL tools are able to differentiate between "no cement" and "solid cement." In the in-between range, however, these tools are not yet able to provide unambiguous answers to the question, Will the cement job prevent high-pressure fluid flow in the annulus? Even so, the tool is a valuable and much-used adjunct to completion work.
Cement bond logs began as auxiliaries to the acoustic log, run with tools designed for D-type logging. The information supplied was important enough to motivate development of special CBL tools, which now do the majority of the bond-logging measurements.
The chief problem with acoustic-type CBL tools is that the casing-signal attenuation is not directly related to the degree of hydraulic sealing provided by the annular cement. Hence, no matter how accurately the attenuation is measured, answers are still in terms of probabilities, except in the extreme conditions of perfect or no bonding.
Figure 2 illustrates the interplay of cement presence, bonding, signature, variable density display, and amplitude.
A CBL log should always include a section above the presumed cement top, where the pipe is completely unbonded. This gives one endpoint for the log; the amplitude curve should never read higher than this. The other endpoint is given by the zero point on the log scale. The curve never reads zero, but comes close (2-3 mv) in well-bonded pipe.
The paradox of acoustic-amplitude-type CBL logging is that the signal of most interest is zero or near it, but the equipment triggers on a finite signal in normal operating mode. As the signal approaches zero, it gets harder and harder to fine-tune the system to pick up the right signal. To correct this, the more sophisticated tools allow a detection window set at a selected time interval after the first pulse. This time is normally close to the casing transit time.
As with normal interval transit time logging, good quality control with the CBL requires the use of an oscilloscope picture. With most equipment, this is the only way to be sure that the amplitude measurement is made on the first-arriving half-cycle of acoustic energy, essential for meaningful interpretation. Figure 3 illustrates this concept.
In normal logging mode, the system triggers on the first arriving (El) half-cycle, measuring both its single-receiver travel time (time from transmitter to receiver) and its amplitude. Two things can prevent this: (1) weak signals in well-bonded pipe can go below the detection threshold and (2) in hard-rock country, it is possible for formation signals to arrive ahead of casing signals. In the first case, cycle skips appear on the log ( Figure 4 ), and the amplitudes recorded in the "skip" intervals are not interpretable. In the second case, the transit-time curve departs from the fairly straight-line value of casing transit time, and begins to follow formation variations. The scale is not directly correlatable, since the CBL transit time is a 3-ft single-receiver measurement and is not borehole-compensated. Normal casing transit time is 3 ft X 57 µsec/ft plus the travel time from tool to casing and back again, usually around 250-260 µ sec.
Most CBL tools assume in-phase arrivals through all sides of the casing, meaning that the tool must be centered. The degree of centering can be judged from the transit-time curve. A poorly centered tool produces shorter transit times. Centering may be virtually impossible in deviated holes or large casings.
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