King Fahd University Cased Hole Saturation Logs and Production Benefits HW my topic is about logging in cased hole, and I have three papers I want a combin

King Fahd University Cased Hole Saturation Logs and Production Benefits HW my topic is about logging in cased hole, and I have three papers I want a combined summary of them in this way:

Table of content

Introduction

Objective

Methodology

Results/Discussion

Conclusions

you can also use figures if it is possible

minimum words of 1500. SPE-195950-MS
A New Cased-Hole Porosity Measurement for a Four-Detector
Pulsed-Neutron Logging Tool
Gregory Schmid, Richard Pemper, Darrell Dolliver, Natasa Mekic, and Jon Musselman, Weatherford
Copyright 2019, Society of Petroleum Engineers
This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Calgary, Alberta, Canada, 30 Sep – 2 October 2019.
This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents
of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect
any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written
consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may
not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.
Abstract
A new cased-hole porosity measurement has been developed for a four-detector pulsed-neutron logging
tool. The measurement is based on a capture count rate ratio from two different detectors. To determine
an accurate porosity, the ratio is characterized in the laboratory in order to establish a ratio-to-porosity
transform. To account for varying measurement conditions in the field, environmental corrections, based on
laboratory studies and computer simulation, are applied. As an alternative to environmental corrections, the
capture ratio can also be actively compensated for the environment by using the results of a dual-exponential
fit to the capture time decay spectrum. In particular, we can compensate for the borehole fluid salinity by
using the borehole component of the dual-exponential fit, and we can compensate for the effective density
of the borehole environment by using an inelastic ratio derived from the capture subtracted burst yields. The
final porosity measurement has been shown to provide accurate results in the field through a comparison
with data from open-hole logs.
Background
Neutron Porosity Logging in Cased-Hole
The neutron porosity log is one of the most important well logs for petrophysical analysis (Ellis et al.
2007). Such logs can be acquired in both open-hole (OH) and cased-hole (CH) environments. The CH
log is particularly useful when there is either no OH log available, or when there is a suspected change
from OH conditions during the course of production, such as that due to compaction (Chan 2004) or acid
treatments (Shafiq et al. 2018). The measurement principle for neutron porosity is based upon the known
correlation between fluid filled porosity and the Neutron Migration Length, which is related to the slowing
down distance of fast neutrons. It has been shown (Ellis et al. 2007) that the Neutron Migration Length, and
hence the porosity, is related to the thermal neutron count rate ratio as measured in a tool containing two
thermal neutron detectors (e.g. He-3 tubes). Equivalently, it has been shown that the porosity is also related
to the neutron capture gamma-ray count rate ratio as measured in a tool containing two or more gamma-ray
detectors (Rose et al 2015). For both cases, tool characterization in appropriate laboratory formations can
establish a relationship between the measured ratio and the formation porosity.
2
SPE-195950-MS
For OH, the neutron porosity log is typically acquired with a Compensated Neutron Tool, which consists
of an AmBe neutron source and two thermal neutron detectors (Ellis et al 2007). For CH, on the other
hand, the porosity log is often acquired with a Pulsed-Neutron Tool, as it is already present in the tool string
for other purposes, such as oil or gas saturation measurements. A Pulsed-Neutron Tool consists of a 14
MeV pulsed-neutron generator and two or more gamma-ray detectors (Randall et al 1986, Roscoe et al
1991, Gilchrist et al 1999, Odom et al 2008). Instead of measuring the thermal neutrons themselves, the
Pulsed-Neutron-Tool measures the gamma-rays that result from the capture of the thermal neutrons in the
formation. This leads to a ratio-to-porosity transform that is similar in nature to the transform measured with
thermal neutron detectors. Although the Pulsed-Neutron-Tool can be run in both CH and OH completions,
different environmental corrections, or compensations, are required in order to account for the different
measurement conditions.
Environmental Corrections
The standard borehole environment is defined as an 8″ OH filled with fresh-water (FW). Any deviation
from this scenario requires a correction or active compensation. As will be shown later, the most
important environmental effects are the rock matrix (LITH), the borehole salinity (BSAL), and the casing
configuration (CSG), which bears directly upon the effective density of the borehole environment. Other
important environmental effects are the borehole size (BS) and the formation salinity (FSAL). Traditionally,
the environmental effects are applied by the use of correction charts (Ellis et al 2007). More recently, active
compensation has also been used (Zhou et al 2018).
Active Compensation of the Capture Ratio
Instead of applying environmental corrections based on expected downhole conditions, the capture ratio can
also be actively compensated using the specific downhole conditions sensed by the tool. In recent papers
(Rose et al 2015, Zhou et al 2018), the idea of using the burst ratio to compensate the capture ratio was
introduced. In the current approach, instead of using a burst ratio to compensate the capture ratio, we have
investigated the use an Inelastic Ratio. The Inelastic Ratio is based on inelastic yields, which are burst yields
with capture contributions removed. The specific technique used for extracting the inelastic yields will be
discussed later.
The Four-Detector Pulsed-Neutron Logging Tool
The four-detector Raptor 2.0 Cased-Hole Reservoir Evaluation (CRE) pulsed-neutron logging tool was
previously reviewed in (Schmid et al 2018) and is shown in Fig. 1 below. We will henceforth refer to this
as the Tool. The OD of the tool metal housing is 1-11/16 in. so as to allow passage through tubing.
Figure 1—Tool layout showing the neutron generator and the four LaBr3(Ce) gamma-ray detectors. The gammaray detectors are labeled Prox, Near, Far, and Long, in relation to their distance from the neutron generator.
The neutron generator is based on a sealed ceramic tube that produces 14 MeV neutron pulses via the D
+T fusion reaction. The firing sequence of the generator is the same as described in (Schmid et al 2018).
The generator is pulsed 600 times per second with a pulse width of 135 μs. The shape of the neutron burst
is square, with a rise time and fall time 12). Beyond this level (∼35
p.u.), a pronounced loss of sensitivity is precicted. Consequently, the porosity measurement described in
this paper is not strictly valid beyond ∼35 p.u.
The lithology effect of the measurement is shown in Fig. 7 below.
6
SPE-195950-MS
Figure 7—Overlaying the transforms of Fig.6 to show the lithology effect.
Environmental Corrections
Using MCNP calculations, along with measured laboratory data, a parameterized equation was derived
that offers corrections for different measurement conditions. In particular, the parameters included BSAL,
FSAL, BS, and CSG. The casing correction, CSG, is simply an “on” or “off” switch. If it is “on”, the
correction is derived as a function of BS. The final parameterized equation including all corrections is tacked
on as an additive term to the ratio-to-porosity transforms of Fig.6. Some examples of the correction terms are
shown in Fig.8 below. With the exception of BSAL, it can be noted that the corrections are mostly negative.
Figure 8—Example of limestone environmental corrections that are applied based on the input parameters.
Verification in Field Logs
The porosity algorithm discussed so far can be referred to as the standard algorithm. It is based on the
traditional approach of using environmental parameters entered by the user prior to logging (or after logging
during playback). In order to verify that this algorithm works properly, it was applied to field log data
SPE-195950-MS
7
acquired by the tool in three different wells: an OH well and two CH wells. In each case, the porosity
measurement was compared to an OH log taken with a compensated neutron tool. The results are shown in
Fig.9. Field Logs 1 and 2 were processed using a limestone matrix, and Field Log 3 was processed using a
sandstone matrix. The agreement between the tool and the OH logs is observed to be good. Over the range
of 0-40 p.u., there is a good match between data from the OH logs and that from the tool. The porosity
data of Field Log 3 shows noticeable fluctuation, but this is not noise. Rather, it is the rapidly oscillating
porosity of a complex lithology. The porosity fluctuations of the tool are seen to match well with the OH
porosity fluctuations.
Figure 9—Tool porosity logs compared to OH logs: Field Log 1 was OH and analyzed with a limestone matrix; Field
Log 2 was CH and analyzed with a limestone matrix; and Field Log 3 was CH and analyzed with a sandstone matrix.
Active Compensation of the Capture Ratio
The standard algorithm discussed thus far uses input parameters to implement the environmental corrections.
However, another approach is to perform active compensation of the capture ratio (in real time) so as to
reduce or eliminate the need for input parameters. In particular, we seek to compensate for both the borehole
salinity and also the effective borehole density (which is the borehole fluid density in conjunction with the
casing configuration). This can be done by using the borehole component of a dual-exponential fit to infer
8
SPE-195950-MS
the borehole salinity, and using a Prox/Far inelastic ratio to infer the effective borehole density. While the
approach to getting the borehole salinity from a dual-exponential fit has been discussed previously (Schmid
et al 2018), further explanation is needed regarding the use and extraction of inelastic yields.
The gamma-rays that we measure with the tool originate from two types of reactions: Thermal neutron
capture; and fast neutron inelastic. Since the thermal neutron population in the vicinity of a detector will
depend upon the fast neutron slowing down distance, the capture yields are going to be sensitive to fluidfilled porosity. However, the inelastic yields are much more sensitive to the number density of nuclei rather
than the fast neutron slowing down distance. As such, the inelastic yields should not be very sensitive to
porosity. This fact can be used to compensate the capture ratio for the effective density of the borehole
environment (given that changes in the borehole environment tend to dominate the inelastic response). The
technique to extract inelastic yields from the time specta will now be discussed.
As previously shown in Fig.2, the gamma ray time spectrum has a burst region followed by a decay
region. The burst gamma rays include both inelastic and capture, while the decay gamma rays are pure
capture. In order to properly extract the inelastic yield, the capture component must be removed from
the burst. This can be done using the dual-exponential fit. Fig. 10 shows the three steps that are needed.
The predicted inelastic of Step 3 is the inelastic yield as extracted from the MCNP simulated tool data
using capture subtraction of the burst. The actual inelastic in Step 3 is the true inelastic yield as calculated
internally by MCNP. It can be observed that the predicted and computed inelastic yields are in good
agreement.
Figure 10—MCNP simulated time spectra showing the three steps involved in extracting the inelastic yield from the burst.
For the MCNP time spectra shown in Fig.10, the burst starts at time 0 and ends at time tBEND=150 μs,
while the dual-exponential fit to the decay data starts at time tDEF=180μs and runs to the end of the decay.
Note that there is a 30 ss time gap between the end of the burst, and the beginning of the dual-exponential
fit. Regarding the dual-exponential fit, it is obtained using the following equation:
(1)
where TCDEF(t) is the total thermal capture in the dual-exponential fit window, and ABH, AF, τBH, τF are the
amplitudes and decays times for the borehole and formation components, which are determined by a least
squares minimization of Eq.1 to the decay data beginning at tDEF.
In Step 2 of Fig.10, the total thermal capture curve is first extended so as to now start at the end of the
burst window. This is done by simply using Eq.1, and allowing the time variable, t, to start at tBEND, the end
of the burst. Once this is done, the total thermal capture during the burst, TCBURST(t), is calculated. This is an
SPE-195950-MS
9
inverse mathematical problem, but can be solved exactly by assuming a square shape for the neutron burst
output (a good approximation in this case). Using the result of the dual-exponential fit from Eq.1, we get:
(2)
(3)
(4)
where TCBURST,B(t) is the total thermal capture in the borehole during the burst, and TCBURST,F (t) is the total
thermal capture in the formation during the burst.
In Step 3 of Fig.10, the inelastic yield, I(t), is then calculated as:
(5)
where Burst(t) is the measured burst data versus time.
For calculating an inelastic ratio, the inelastic yield from Eq.5 is summed over the limits of the burst
window (0-150 s) for each detector. It should be noted that this yield is not corrected for epithermal effects.
Although epithermal effects are usually negligible, they can become noticeable in some cases, such as
small borehole size coupled with low porosity. In this case, we can make use of the 30 s gap between the
burst window and the decay window. If no epipthermal neutrons are present, the inelastic curve of Eq.5
will go to zero immediately following the burst, as shown by the black dashed line in Fig.10. However, if
epithermals are present, the inelastic yield will contain a tail in the gap region. This strength of this tail will
be proportional to the epithermal strength in the burst window, and a correction can be applied.
In Fig.11, we show how the Prox/Far inelastic ratio can be used to compensate the Prox/Far capture ratio.
The points on this plot are MCNP calculations for Limestone at three different porosities (2p.u., 18 p.u. and
25 p.u.) and many different borehole fluid and casing configurations. The lines are to guide the eye, and
show the effect of increasing the effective borehole density at a given porosity. The Prox/Far capture ratio
is seen to depend upon both the effective borehole density and the porosity. The Prox/Far inelastic ratio, on
the other hand, is seen to depend only upon the effective borehole density. As such, we can use the inelastic
ratio to correct the capture ratio for the effective borehole density. Since the relationship between capture
ratio and inelastic ratio has a shape that is (to a good approximation) independent of porosity, any given
measurement can be propagated along the known curve shape until it intersects with the conditions that
correspond to FW OH. This procedure constitutes the capture ratio compensation.
10
SPE-195950-MS
Figure 11—MCNP simulated Prox/Far (P/F) inelastic and capture ratios for three
different porosities and various different borehole fluid/casing configurations.
To s…
Purchase answer to see full
attachment