Thermogravimetry as a tool for measuring of fracturing fluid absorption in shales
Journal of Thermal Analysis and Calorimetry
Thermogravimetry as a tool for measuring of fracturing fluid absorption in shales
Krzysztof Labus 0 1
Ma?gorzata Labus 0 1
0 & Ma?gorzata Labus
1 Institute for Applied Geology, Silesian University of Technology , 2 Akademicka St., 44-100 Gliwice , Poland
Water-based fracturing fluids are used for gas shale stimulations. The fluids are pumped under pressure into the well to create conductive fractures in hydrocarbon-bearing zones. The chemical additives vary depending on the geological and technical conditions of the well. One of the requirements of good fluid coherence is its low absorbency relative to the formation rock. Standard water absorption tests for rocks are usually performed on cubic samples (50 9 50 9 50 mm) soaked in water. In case of rocks which are drilled from the borehole, obtaining such large samples is very difficult; therefore, an attempt was made to determine the rock absorption on small samples, using the TG analysis. Thermogravimetric (TG/DTG) experiments were conducted in temperature range 40-300 C in synthetic air environment. Shale rock samples were soaked in water and fracturing fluid. The absorption of the rocks is related to the shale's mineral composition, which was also determined by thermal analysis (TG/DSC).
Fracturing fluid; Shale
Hydraulic fracturing is one of the most popular methods of
stimulating hydrocarbons reservoirs in non-conventional
]. Fracturing is used for enhancing the
production of oil and gas from formations of very low
permeability, i.e., tight gas, coal and shale gas deposits. The
fluids are pumped under pressure into the well to create
conductive fractures in hydrocarbon-bearing zones. The
fracturing fluids used for gas shale stimulations consist
primarily of water but also include a range of additives.
The type and number of chemical additives are dependent
on the conditions of the specific well being fractured. In
case of water-based fracking fluids, the permeability
damage in shale formation can be caused by swelling of
clay minerals or by other physical and chemical
mechanisms. As a result of uncontrolled hydration, caused by
physicochemical interaction between fluid and shale or
clay rocks, the crystal structure of minerals is disturbed.
One of the requirements of good fracturing fluid is its low
absorption in the formation rock.
In the present study the TG/DSC method was used to
study the composition of shale rocks, and TG method for
absorption tests to examine the interaction of the specific
shale samples and fracturing fluid. Standard water
absorption tests for rocks are usually performed on cubic
samples (50 9 50 9 50 mm) soaked in water. In case of
rocks which are drilled from the borehole, obtaining such
large samples is very difficult; therefore, an attempt was
made to determine the rock absorption on small samples,
using the TG analysis. This analytical method was also
used by Corre?a and Nascimneto [
] for shale?fluid
interactions, regarding drilling fluids influence on wellbore
Fluid absorption in shale
Water absorption capacity is one of the basic physical
properties of rocks. Water absorption tests are usually
performed on dimensional stones which are used for
building purposes [
The conventional absorption tests include stone sample
cutting to usually cubic shape, as it was mentioned above.
The samples are oven-dried and then weighed. In the next
stage the dry sample is immersed in water for a specified
period of time and then weighed again. The difference in
mass (as a percentage) is the amount of water that
penetrated the stone sample. It is worth mentioning here that the
size and shape of the samples under examination are not
strictly specified [
Measured absorption typically ranges from less than a
per cent for granites and crystalline rocks up to 10?12 mass
per cent for the more porous sedimentary rocks (like
sandstones and limestones). Water absorption values of
siltstones and mudstones are relatively large?of about
]. Water-absorbing rocks are formed from minerals
that can hold water in their crystal structure or between
grain boundaries. In case of shales and other clay-rich
rocks, containing phyllosilicates, water absorption is often
accompanied by a change in the crystal sizes because of
their hydratation ease with which these minerals hydrate.
] stated that water absorption in shale
decreases exponentially with time. In his experiment over
80% of total absorption took place after 5?6 days, and
equilibrium was reached in 10?20 days. If no discontinuity
or defect existed in shale structure, the depth of moisture
absorption would be very shallow. In the laboratory tests
the effective penetration depth is of about 6 mm [
] commented that rocks with bedding
planes perpendicular to the exposed face absorbed 50%
more water than ones with the bedding parallel to the free
surface. The result is anisotropic expansion and shrinkage
after capillary action and drying.
Several studies analysed the role of clay minerals on
water imbibition capacity of shales [
]. Imbibition is a
process of absorbing a wetting phase into a porous rock.
Spontaneous imbibition refers to the process of adsorption
with no pressure driving of the phases into the rock. Some
experimental investigations were also performed on shale
imbibition capacity towards hydraulic fracturing fluids
]. Currently, it is well known that the imbibition of
fracturing fluids is mainly controlled by the capillary
pressure, while the effects of clay absorption have not been
studied thoroughly [
]. Because different mechanisms
seem to be facilitating imbibition in the shale, there is not
so far a verified answer for the geomechanisms behind
those observations [
Phase composition of the selected samples was determined
with the use of XRD analysis. XRD measurements were
taken with the use of powder diffractometer Bruker-AXS
Advance D8 (Germany) of 2H/H geometry, equipped with
linear semiconductor detector Lynxeye and
energy-dispersive detector SOL-XE. Measurement conditions:
radiation CoKa/Fe filter, voltage 40 kV, current 40 mA, step
of 0.014 2H, step time 1.25 s (the sum of five
measurements with a step of 0.25 s) and digital processing of the
resulting data. For the measurement and calculation, the
Bruker Diffrac Suite software was used.
The rock samples were also observed with scanning
electron microscope FEI Quanta-650 FEG, equipped with
energy-dispersive analyzer (EDX)?EDAX Galaxy.
Standard water absorption test was performed for
comparison with thermogravimetric (TG/DTG) experiment
results. It was impossible, however, to obtain standard cubic
samples, as the rock pieces were comparatively small,
mostly taken from drill core. Moreover, examined rocks are
of slate structure, which is prone to splitting into plates. The
lump samples of mass from 20 to 60 g were dried in the oven
to 100 C for 45 min. After cooling in desiccators, the
samples were soaked in water for 24 h. After this time, the
samples were weighed and the mass differences before and
after water soaking were determined.
Thermal analysis was performed in Laboratory of
Geochemical Engineering (Institute for Applied Geology),
with NETZSCH STA 449 F3 Jupiter equipment, and
divided into two parts. Part I was intended to mineral
composition examination with the use of TG/DSC
methods. The analysis of the rock samples was carried out over
the temperature range of 40?1000 C, under synthetic air
atmosphere, with gas flow rate of 50 mL min-1.
Measurement heating rate was 10 K min-1. Rock samples were
powdered and put into the alumina (Al2O3) crucible in
amount of about 20 mg. The curves were interpreted with
the help of works of Wyrwicki [
] and Fo?ldva?ri [
II included TG analysis of lump samples (of about 50 mg),
shaped to fit the STA crucible. The analyses were
accomplished in four stages: 1?fresh sample, in air-dry
conditions, 2?pure (distilled) water saturated, 3?the
same sample saturated with fracturing fluid and 4?fresh
samples saturated with fracturing fluid. In each case the
saturation was carried out for 24 h. TG analysis was
executed in the temperature range of 40?300 C, under
synthetic air atmosphere, in open crucibles (without lids). The
gas flow rate was 50 mL min-1, and measurement heating
rate 5 K min-1.
The fracturing fluid used in the experiments was a fluid
prepared for hydraulic fracturing of shale formation and
consisted primarily of water (about 99%) and specific
? viscosity breaker (0.5 mL L-1),
? antibacterial agent (0.125 mL L-1),
? surfactant (2 mL L-1),
Clay minerals Calcite
aNames of the boreholes are restricted
foamer (3 mL L-1),
clay swelling inhibitor (2 mL L-1),
scale inhibitor (1 mL L-1).
The analysed samples, collected from deep boreholes from
Poland. Lithuania and Czechia, represent fine-grained
rocks, including siltstones, shales and marls.
Polish and Lithuanian samples B-1, B-2, N-1, L-1, L-2,
P-1 and W-1 are lower Silurian shales and siltstones, dark
grey in colour. They were collected from boreholes (from
depth of about 3000 m), in the area of Baltic Basin. Baltic
Basin extends from Northern Poland to Lithuania, and it is
one of the most interesting areas in Europe in terms of
shale gas exploration. The significant unconventional gas
and oil resources are accumulated in laterally extensive,
organic rich source rocks. The analysed rocks of Baltic
Basin are mostly composed of quartz, muscovite and clay
minerals (Table 1, Fig. 1). Sample from Nieste?powo (N-1)
exhibits different mineral composition, as it is a
chloriterich shale?the chlorites present are Fe-clinochlore and
chamosite. The sample from Buso?wno (B-2) contains some
amount of calcite, apart from quartz and muscovite.
Sample from Prabuty (P-1) represents limestone.
Rocks from Czech Republic are represented by samples
K-1, NM, N-2 and M-1. They are classified as marls,
however, rich in organic matter and, in consequence, dark
in colour. These samples were collected from boreholes
(also from the depth of over 3000 m), from the formation
Fig. 2 XRD pattern of sample
of Upper Jurassic Mikulov Marls [
]. This formation is
built of 1400-m-thick organic rich rocks, which sourced
oils in the Vienna Basin in Czech Republic and Austria
]. The main minerals forming sampled rocks are calcite
and clay minerals, with minor amount of quartz, muscovite
and feldspars (Fig. 2). In case of sample from Morkuvky
borehole (M-1), some chlorites (chamosite) were stated.
The results of rocks examination analysis are shown in
Table 1. The mineral composition determined with the use
of a range of methods (provided in the above paragraph)
differs, depending on the measurement method.
Thermoanalytical methods are believed in general of a little use for
determining the mineral composition of argillaceous rocks.
Difficulties in interpretation arise from the chemical
composition and crystalline structure of clay minerals, the
relatively small mass and enthalpy changes involved, the
presence of other components such as sulphides and
organic material which oxidise during heating [
Nevertheless, thermal methods are used traditionally as
complementary ones in case of sedimentary rocks
examination , as they provide data on clay minerals and
organic matter content, which is difficult to obtain by X-ray
Figure 3 presents the examples of TG/DSC curves for
samples B-2 and N-2, as a result of Part I of thermal
In the diagram for sample B-2 (Buso?wno) (Fig. 3a), the
dehydration of clay minerals is visible in the range from 40
to 270 C (mass loss?0.69%). In the range of
A ACa P MKu Q
Mu A Q SQ Ca D CaCa
Position 2? /?
270?600 C, exothermic reactions overlap, resulting in a
total loss of sample mass of 3.97%. Combustion of organic
matter gives exotherm with a maximum of approx. 392 C.
Successive exotherm, with maximum at 484 C, is related
to the decomposition of pyrite, present in the sample.
Between 600 and 800 C, a significant mass loss (6.72%) is
observed, which can be interpreted as calcite degradation.
In case of sample N-2 (Nemcic?ky) (Fig. 3b) in the
temperature range from 40 to 280 C, the process of clay
minerals dehydration takes place, connected to loss in mass
of 3.53%. The dome exotherm, with a maximum at 505 C,
is associated with the combustion of organic matter,
resulting in a loss in the sample mass of 3.98%. The
endothermic reaction, clearly visible on the DSC curve, in
the temperature range of 600?840 C, is the effect of
calcite degradation. This is accompanied by a significant mass
loss of 20.49%. The last exothermic peak (907.5 C) is
derived from spinel phase crystallization.
Water and fracturing fluid absorption measurements
As it was described above, during Part II of thermal
analysis the rock?s samples were analysed in four stages and
0?blank test?fresh sample, in air-dry conditions,W?
distilled water saturated,F?the same sample saturated with
fracturing fluid,F2?fresh samples saturated with fracturing
The results of the measurements are provided in Table 2
and compared to the results of standard water absorption
method (A) (Fig. 4).
The example of the results obtained for each sample
from TG measurement is given in Fig. 5. In case of the
sample K-1 (Kobyli), 1.27% loss of mass (stage 0) is a
result of yielding hydration water of clay minerals, which
are present in the rock (essential water). The same sample,
after soaking in distilled water, during the test losses 7.80%
of mass; and this value could be assumed to represent water
absorption capacity of the rock. This sample is very rich in
clay minerals (illite, kaolinite); hence, the absorption
includes imbibitions into the clay minerals structure. The
real water absorption into the rock could be calculated as
the difference between the result of stage ??W?? of the
experiment and hydration water (result of stage 0), which
in this example equals 6.53%. Consequently, the ??real??
fracturing fluid absorption could be calculated as a result of
subtraction of measurement result in stage ??0?? from
??F2???given in the last column of Table 2.
When the same sample is soaked in fracturing fluid
(stage F), the measured absorption is lower (in case of
sample K-1 it is 6.62%). It should be noted, however, that,
especially in case of shale rocks, which are fissile in fabric,
the former water saturation (W) is able to disturb the
original rock structure. On the other hand, the fresh
samples, which were soaked in fracturing fluid, have lower
fluid absorbance capacity (in case of sample K-1?6.02%).
The comparison of the obtained results from the
absorption experiment is provided in Fig. 6. As it is visible
from the graph, the most distinctive is the sample P-1, due
to very low values of absorbance capacity. The mass loss in
case of dry sample (stage 0) is close to 0% (Table 2),
which is the evidence of the absence of clay minerals in the
rock, as confirmed by SEM analysis (Fig. 7). Sample P-1 is
a limestone, composed of calcite, in which grains are
tightly packed. According to TG/DSC analysis, the amount
of calcite was calculated to be about 92%. The lack of clay
minerals is a primary cause for very low values of water
absorbance capacity (0.22%) and real fracturing fluid
absorbance capacity (0.06%).
The other rocks absorb fracturing fluid in amounts from
0.90 to 4.75%. The highest values of fracturing fluid
absorbance (over 3.40%) revealed marl samples K-1 and
N-2. As it can be noticed from Fig. 6, the initial content of
clay minerals, which is demonstrated by mass loss of fresh
sample (stage 0), is not the crucial parameter in
determination of the absorbance of the rocks.
On the basis of observation of the morphology of the
rocks by SEM, it can be stated that the structure of the rock
and the presence of intergranular spaces are important for
the fluid absorption in the studied rocks.
It should be noted here that the results obtained from
thermogravimetric analysis are generally consistent with
standard water absorption tests. When we compare water
absorption (A) from standard measurement and ??real??
absorption calculated from TG measurement as ??W-0??
value, the significant correlation is visible (Fig. 4).
Obviously, the geometry and size of the sample are very
Fig. 7 SEM image of sample P-1 (Ca calcite, Py pyrite)
important, and this is the reason of average higher values of
water absorption for small samples used in TG experiment
in comparison with much more bigger lump samples used
in standard test.
TG method was proved to be an appropriate tool for
measuring water and fracturing fluid absorbance in the
rocks. This method enables the measurement on very small
(of about 50 mg) samples, in case the rock sample size is
insufficient (which is common in case of samples from
deep wells) to carry out the standard water (or other fluid)
Water absorption capacity in examined rocks ranges
from 0.07 to 6.53%. In case of most of the rocks, this value
falls below 5%, which is regarded as low water absorption.
Only one sample of marl (K-1) from Mikulov formation
reveals high water absorption, reflecting high clay minerals
Fracturing fluid absorption is in the range of 0.06 to
4.75%. The highest values were recorded for marls
samples; in some cases fracturing fluid absorption capacity was
even higher than water absorption. This phenomenon
indicates that the used fluid is not suitable for fracturing
clay?carbonate rocks (marls).
High fluid absorption is related to the presence of
minerals of micaceous habit (e.g., muscovite, chlorite) and clay
minerals. These phyllosilicates contribute layered structure
of shale rocks and are partially responsible for the fate of
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