International Journal of Scientific & Engineering Research, Volume 4, Issue 10, October-2013 598
ISSN 2229-5518
Comparison of Three New Portable Magnetics
and Miniperm Probes for Permeability Prediction
Timothy Yakubu Woma, David K. Potter, Ayoola W. Fagbenro
content indicates high permeability. The low sensitivity magnetic probe was found to be better than the high sensitivity magnetic probe in terms of corre- lation of magnetically derived illite content with probe permeability. Carrying out two magnetic susceptibility measurements using probes with high and low sensitivity could be a possible way of detecting natural cemented zones.
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Index Terms— Magnetics,Minipermeameter,Permeability,Portable,Probes,Reservoir,Susceptibility.
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He efficient exploitation of oil and gas and management of petroleum reservoir requires good knowledge of the geology of the particular reservoir. The knowledge re- quired is obtained from the analysis of vast subsurface data gotten from both down-hole measurements and laboratory measurements on core. Despite efforts by industry and aca- demia the acquisition and traditional analysis of core remains very expensive and time consuming. Besides the cost and time setback inherent in conventional core analysis, there are geological, petrophysical and geostatistical issues involved with routine (RCAL) core analysis and special (SCAL) core
analysis [1].
Recently, other sampling strategies and measure-
ments have been developed amongst which are the probe
permeability and probe magnetics. These techniques involve
cheap rapid core screening carried out at high resolution and in most cases measurements are non-destructive, thereby eliminating the need for cutting plugs. New probes that will enable better measurements and the further development of
these techniques are being designed and produced continual- ly.
Magnetic susceptibility is the ratio of the intensity of magnetisation to the applied magnetic field strength. Gener- ally materials are paramagnetic, diamagnetic or ferromagnet- ic (ferro - and ferrimagnetic).
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• Timothy Yakubu Woma is a lecturer in Pure and Applied Physics De- partment in Federal University Wukari, Taraba State, Nigeria, PH-
+2348133242540. E-mail: timothy.woma@gmail.com
• David K. Potter is a Professor of Petrophysics/Geophysics in Physics De- partment of University of Alberta, Canada, PH-7802481972. E-mail:
• Ayoola W. Fagbenro is a lecturer in Pure and Applied Physics Department
in Federal University Wukari, Taraba State, Nigeria, PH-
+2348054414889. E-mail: senator_ayoola@yahoo.com
Materials with positive susceptibility (X) such that (1+X) >1 are called paramagnetic materials. In the situation where sus- ceptibility (X) is negative such that (1+X) < 1 the material is said to be diamagnetic. Ferromagnetic materials differ from paramagnetic and diamagnetic materials in that they have very high positive susceptibility such that they are able to retain their magnetic field. The measurement of magnetic susceptibility is achieved by quantifying the change of force felt upon the application of a magnetic field to a substance. For liquid samples it is measured from the dependence of the natural magnetic resonance (NMR) frequency of the sample on its shape or orientation. Ivakhnenko and Potter in 2004 successfully used other methods to measure fluid susceptibil- ity, for example, Sherwood Scientific Magnetic Balance (MSB) Mark I and Magnetic Properties measuring System (MPMS2) SQUID magnetometer [2]. The susceptibility values of com- mon reservoir rock/ minerals and fluids as summarised by Hunt et al in 1995 and Potter et al in 2004 is given in table 1[3] and [4].
The main factors controlling permeability in clean sandstone include: grain size, shape, sorting, packing, degree of consolidation, cements (quartz overgrowth, barite etc) and fractures. Additionally in muddy sandstone clay content (es- pecially permeability controlling clays like illite or chloride) also control permeability while in shales the major factors controlling permeability are increased clay content (especially illite and chlorite), decreased quartz grain size and anisotropy [5]. Mikkelsen et al in 1991 and Vernik in 2000 also affirm that permeability depends on the amount of clay minerals like illite, chlorite and kaolinite present in a sample [6] and [7]. It has also been reported that the presence of illite can bridge pore space and create microporous rims that considerably reduces permeability with little effect on porosity [8] and [9].
Considering the difference between the susceptibil- ity of matrix minerals and permeability controlling clays, the sign of the raw magnetic susceptibility can be very useful for
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permeability and lithological zonations. Research in the past few years continuously show that excellent correlations exits between the net values of magnetic susceptibility and main permeability and lithological zones in a shallow marine shoreface Para-sequences as displayed in figure1 [10]; net susceptibility is generally negative in the high permeability clean sand units indicating the predominance of diamagnetic quartz and feldspar while in the low permeability muddy sand and shale units the net susceptibility has positive values indicating the higher percentage of paramagnetic illite clay and minor quantities of other paramagnetic and ferromagnet- ic minerals Processing the raw magnetic susceptibility into mineral content percentage provide even better correlation with key petrophysical properties. Potter et al in 2004 devel- oped a formula for calculating the mineral fraction assuming a two-component system [10]. The total susceptibility from a sample is expressed as:
Where XT = Total measured susceptibility, XQ = Known sus- ceptibility of quartz (from table1), XI = Known susceptibility of illite and FI is the fractional volume of illite which can also be expressed as:
The above equations are true for both volume susceptibility and mass susceptibility and illite content calculated using equation 2 from magnetic susceptibility measurements corre- lates with X-ray Diffraction derived illite content as shown in figure 2. The calculation of mineral (illite) content can be ex- tended to a whole range of other simple mineral mixtures for any given core material undergoing analysis, especially that magnetically derived illite content has been found to show very good correlations with horizontal plug permeability for a North Sea well [10]. Thus permeability in clean
ameter probes that provide high resolution, rapid, cheap and non destructive way of measuring permeability. The high resolution data from minipermeameter are at the lamina scale and can identify small scale heterogeneity such that key fea- tures are more likely to be identified.
Probe measurement data are less sensitive to missing core and improves depth matching to wire line log data. Minipermeameter estimate local absolute permeability by flowing gas through tubes sealed against the surface of core sample. Minipermeter are of two types: steady state mini- permeameters and unsteady state (or pressure decay) mini- permeameters. A new portable unsteady state air probe has been used in this study. Research in previous years have shown that in many cases core plug permeability and probe permeability measurements give very similar values [13]. However in some North Sea examples the core plug permea- bilities are higher than the probe permeabilities at compara- ble depths. The major reason for this variation is the fact that core plugs have been cleaned and dried whereas the slabbed core which is not cleaned has significant dried out hydrocar- bons, which are causing a slight reduction in the measured probe permeability values.
Comparison of three new portable probes has been carried out in this paper. The probes include one low sensitiv- ity magnetic susceptibility probe, one high sensitivity mag- netic probe and a minipermeameter probe (Tiny Perm II). The probes were used to carry out measurements on 45feet long slabbed core recovered from a shoreface reservoir. The results gotten from the measurements were processed after which a thorough analysis of the result comparing the probes fol- lowed.
TABLE 1
MAGNETIC SUSCEPTIBILITY OF COMMON RESERVOIR MINERALS AND
FLUIDS (AFTER [3] AND [4]).
sand(corresponding to lower magnetically derived illite con- tent) is expected to be higher than permeability in muddy sand (corresponding to higher magnetically derived illite con- tent), however, this is not true for low permeability naturally barite- cemented regions [11]. The naturally barite cemented
Type of
Mineral
Mineral Susceptibility Per unit mass (10-8m3/ Kg)
Susceptibility per unit Volume
(10-6SI)
regions are undetectable by the magnetic susceptibility tech- nique because barite that is a paramagnetic mineral has sus-
Diamagnetic minerals
Quartz -0.55 -13 to -17
Calcite -0.3 to -1.4 -7.5 to 39
ceptibility approximately the same as the susceptibility of
diamagnetic quartz.
Orthoclasse
Feldspar
- 0.49to -0.67
Core analysis using core plug results gives incom-
plete information about the reservoir as such sampling might
Paramagnetic
Kaolinite -2.0 -50
Illite 15.0 410
be biased. There is need to have sufficient samples that can
give information about the reservoir at the lamina scale espe-
cially for heterogeneous reservoirs that are very difficult to
manage. Corbett and Jensen in 1992 introduced the concept of
minerals
BVS Chlorite CFS Chlorite
13.6
52.5
sample sufficiency and developed rules-of thumb that help in
Pyrite 2.0 35 to 5000
estimating the optimum number of samples that will be
needed [12].
Probe Permeability allows one to obtain practically
sufficient number of samples that represent a particular core
interval. Probe permeability is measured using miniperme-
Ferromagnetic minerals
Magnetite 20,000 to
110,000
1,000,000 to
5,700,000
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The following instruments, equipment and other materials were used to carry out the analysis.
1. Bartington high sensitivity magnetic probe MS2E1.
2. Bartington low sensitivity magnetic probe MS2F.
3. MS2 display unit.
4. Standard calibration samples.
5. Connecting cables.
6. Vindum Engineering Inc. Tiny Perm II air miniper-
meameter probe.
7. 45 feet long slabbed core from a North Sea oil well.
The experimental procedure included the quality control (Q.C) of the core, magnetic probes measurement and minper- meameter measurement. The details are as follows
The procedure started with Q.C. of the core with the aid of the pictures of the core before some sections were removed to ensure that the core sections were at the right depth and that the core had not been contaminated with particles of magnet- ic materials that will compromise the accuracy of the meas- urements. Then the core boxes were placed on a non- magnetic table and the cable was connected between the probes and the display unit positioned on a suitable plastic base.
The whole apparatus was then powered via an adap- tor connecting the display unit and the mains power supply. After this, the tool was set to the c.g.s. unit because the cali- bration sample is in c.g.s unit and the sensitivity knob was set to 0.1 c.g.s. Then the equipment was switched on and allowed to warm up for a moment after which measurement began.
The first measurement to be taken was that of the calibration sample to ensure that the calibration of the probe was still valid; the background reading was taken by holding the probe to the air and taking the measurement which was not- ed, then the probe tip was placed on the calibration sample and another reading was taken. The true value of the sample is the difference between the measured value and the back- ground (air) value, i.e.
The true value of measurement found from equation 3 above was compared to the standard known value of susceptibility of the calibration sample and the two values were the same confirming that the probe calibration was still valid and measurements could proceed. Then the probe tip was placed on the core section to be measured and measurement was taken at all measurable sections of the core. For every one foot of core measured the background reading was taken at the beginning and the end and averaged, and then the true value of susceptibility was found using (3). The instrument reading was automatically zeroed at the beginning of every measurement and the same procedure was used for both the high and low sensitive probes respectively.
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The core was first positioned properly on the measurement table, and then the probe was set to commence measurements after connecting the pressure transducer to the microproces- sor and control unit with the electric cable. The procedure out lined below was followed to take measurements:
1. Tiny Perm II was turned on, the plunger was pulled
out fully until the screen was reading “Push+Hold”;
2. The rubber nozzle was pressed firmly against the core surface to be measured;
3. Then the plunger was depressed completely. Imme- diately the current vacuum and measurement status bar was displayed on the screen, the plunger was
held in and the pressing of the nozzle to the core face continued until the vacuum reading was 0 and the status bar indicated that the measurement was com- pleted;
4. The result displayed on the LCD screen of the mi- croprocessor and control unit was recorded in the la-
boratory notebook;
5. The recorded Tiny Perm II result was cross-
referenced to the calibration curve to obtain the abso-
lute permeability at the core section measured;
The plunger was pulled out again and the procedure was repeated for each point of the core measured.
The raw magnetic susceptibility values obtained from meas- urements using the low sensitive magnetic probe and high sensitive magnetic probe were converted into illite content using (2). Figure 3 shows a plot of magnetically derived illite content against depth for both the low sensitive probe and high sensitive probe. The plot shows the distribution of illite content at different depths of the core. Comparison of the profiles obtained from the two magnetic probes shows a rea- sonable correlation between the two sets of measurements. Generally the illite content obtained from the low sensitive probe measurement are a little higher than the illite content obtained from the high sensitive probe except at depth be- tween 220ft-225ft. This exception could have arisen as a result of the presence of some natural cement or calcite whose sus- ceptibility is close to that of quartz so the low sensitive probe could not capture the difference while the high sensitive probe did. This suggests that repeating magnetic susceptibil- ity measurements using a lower sensitive probe can be help- ful in resolving the difficulty posed by natural cemented zones in magnetic susceptibility measurements of core.
It was stated earlier that illite content controls permeability, to confirm this, the magnetically derived illite content profile obtained from the magnetic probe measurements were com-
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pared with the permeability profile obtained from the minpermeameter probe measurements. Comparing the mag- netically derived illite content profile and the probe permea- bility profile shows that a good correlation exits between the two profiles. The high illite content at the top and bottom sections correlates with the low permeability at the top and bottom section, equally the low illite content at the middle section correlates with the high permeability in the middle section as shown in fig. 4. This correlation confirms what has been reported earlier by Potter in 2004 and Potter in 2007 for other silliclastic formations in the North Sea that very good correlation exist between magnetically derived illite content and horizontal plug permeability [10] and [11]. Thus magnet- ic susceptibility measurements can provide an alternative or complementary method for predicting permeability. This method is fast, less expensive, rapid and non destructive.
However, the regression coefficient between the magnetically derived illite content and probe permeability is not too good because the probe permeameter (Tiny Perm II) tool is not measuring the same volume as the magnetic probes tool. The magnetic probe measures a volume of
39.9mm3 while the probe permeameter measures a volume of about 100-900mm3. Also the probe measurements might not have been taken at exactly the same depth as a result of error due to parallax thus introducing small scale depth shifting issue. Additionally the effect of porosity on volume suscepti- bility would also have contributed to the poorer regression coefficient. It is also important to know that the physics of measurement for the magnetic probe and the miniperme- ameter are different and could also affect the regression coef- ficient.
The main aim of this paper is to compare the three new port- able probes used for measurement in this project. Having confirmed that magnetically derived illite content can be used to predict permeability it is necessary to compare the two new magnetic probes to know which is most suitable for permeability prediction.
Fig. 5 shows the correlation between low sensitivity magnetic probe magnetically derived Illite content and miniperme- ameter permeability, while figure 6 is the correlation between the high sensitivity magnetic probes magnetically derived Illite content and minipermeameter permeability. The low sensitivity probe Illite content gives a better correlation with permeability than the correlation between the High sensitivi- ty probe Illite content and permeability. This means that the low sensitivity magnetic probe is better than the high sensi- tivity magnetic probe for permeability prediction.
The high sensitivity probe gave poorer correlation with permeability because its use is not completely compati- ble with the simple two component system of illite and quartz used for determining magnetically derived illite con- tent outlined in (2). It is able to record slight differences in susceptibility as a result of the presence of other permeability controlling clay minerals other than illite. Previous X-ray
diffraction performed on the core shows that besides illite and quartz that are the main minerals in the core, there are other clay minerals like pyrite, mica and kaolinite in small quantities [14]. Unlike the high sensitivity probe that gives a complex profile, the low sensitivity probe can not differenti- ate between small differences in susceptibility but rather up- scales the susceptibility thus subscribing to the simple two component system of illite and quartz given in (2).
Fig. 1. Correlation between Net Susceptibility Values and Main Permeabil- ity and Lithological Zones in a N. Sea Oil Well (From [10]).
Fig. 2. Showing Good Correlation between Magnetically Derived Illite Con- tent and XRD Derived Illite Content (from [4]).
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190
195
200
205
210
215
220
225
230
235
illite conte nt-de pth plot
High sensitive probe
low sensitive probe
240
1.00
0.10
0.01
0.00
illite conte nt(fr ac.)
Fig. 3. Magnetically Derived Illite Content against Depth Plot from High
Sensitive and Low Sensitive Magnetic Probes Measurements.
Fig. 5. Correlation between Low sensitivity magnetic Probe Illite Content and Minipermeameter Permeability.
Low Senstive Illite Content - Depth plot
Illite content(fraction)
Permeability - Depth plot
permeability (mD)
High Sensitive Illite Content - Depth plot
1.00
0.10
0.01
0.00
0.001 0.1 10 1000 100000 10000000
190
190
illite content (fraction)
193
193
1.000
0.100
0.010
0.001
196
199
202
205
208
211
214
217
220
223
226
229
232
235
196
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223
226
229
232
235
190
193
196
199
202
205
208
211
214
217
220
223
226
229
232
235
Fig. 4. Correlation between Magnetically Derived Illite Content Profile and
Permeability Profile.
Fig. 6. Correlation between High sensitivity magnetic Probe Illite Content and Minipermeameter Permeability.
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The oil well used in this study has low permeability zones at the top and bottom regions while the middle section of the core has high permeability.The amount of illite in the oil well is concentrated at the top and bottom sections of the cored interval while the middle section of the cored interval has a lower amount of illite content. Both low sensitivity magnetic probe and high sensitivity magnetic probe gave magnetically derived illite content that correlated with probe permeability. Thus magnetic probes can be used in a rapid, cheap and non- destructive way to predict permeability.
The low sensitivity magnetic susceptibility probe is better than the high sensitivity magnetic susceptibility probe in terms of correlation with permeability. In the absence of routine core analysis data, the low sensitivity probe can be used to predict permeability for quick decision making. It may be unnecessary to spend energy and resources to design and construct higher sensitivity magnetic probes for the pur- pose of permeability prediction. The simple two component (illite and quartz) system of determining magnetically de- rived illite content may not be completely compatible with high sensitivity probe measurement in cases where there are other clay minerals in the core.
Finally, repeating magnetic susceptibility measure-
ments using a different sensitivity probe may be helpful in
detecting calcite and/or natural cemented zones in a core.
Plotting the magnetically derived illite content against depth
for both high sensitive and low sensitive magnetic probes on
the same graph will produce a profile that could be used for
detecting the natural cemented or calcite zones. These zones
will have the two profiles crossing each other with a noticea-
ble separation between the low sensitive probe magnetically
derived illite content-depth profile and the high sensitive
probe magnetically derived illite content-depth profile.
The authors are grateful to Petroleum Technology Develop- ment Fund (PTDF) for their funding and Institute of Petrole- um Engineering, Heriot-Watt University Edinburgh for the use of their facilities.
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