Juan Du1, Guixian Guo1, Pingli Liu1, Gang Xiong2, Pengfei Chen2, JinMing Liu1, Xiang Chen1. 1. State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China. 2. Research Institute of Natural Gas Technology, PetroChina Southwest Oil & Gasfield Company, Chengdu, Sichuan 610213, China.
Abstract
As an important part of the acid fracturing process of carbonate reservoir, the performance of acid fracturing working fluid directly affects the stimulation effect of oil wells. In this paper, formaldehyde (agent A) and ammonium chloride (agent B) were used as the matrix. Several aldehydes with different volume ratios were prepared. The acid ratio with the highest acid yield was selected by the sodium hydroxide titration experiment. The results show that when the volume ratio of agent A to agent B is 1:1.3, the acid production capacity is the strongest. The pH values at several time points in the process of acid reaction were measured by a pen pH meter. The relationship curve between acid production capacity and time was obtained. The acid production capacity increased with time. It tends to be stable after a certain time. The experiment of acid rock reaction kinetics shows that the reaction rate between acid and rock decreases with the extension of time. The reaction time can reach 6 h. The reaction rate of autogenic acid under different temperatures and concentrations ranges from 6.61 × 10-7 to 9.49 × 10-7 mol/(s·cm2) within 6 h. Therefore, it indicates that the reaction time between autogenic acid and carbonate rock is long and the reaction rate is low. It is beneficial to improve the acid treatment effect of the carbonate reservoir. The conductivity experiments show that at different temperatures, with the increase of sealing pressure, the etching ability of autogenic acid decreases. The etching effect is better at 338 K. After etching, the permeability and conductivity of the rock slab of 5 MPa closure pressure are 227 D and 72.62 D·cm, respectively. To sum up, this autogenic acid is an effective working fluid in the acid fracturing process of the carbonate reservoir. It can obviously reduce the reaction rate of acid rock and has certain conductivity.
As an important part of the acid fracturing process of carbonate reservoir, the performance of acid fracturing working fluid directly affects the stimulation effect of oil wells. In this paper, formaldehyde (agent A) and ammonium chloride (agent B) were used as the matrix. Several aldehydes with different volume ratios were prepared. The acid ratio with the highest acid yield was selected by the sodium hydroxide titration experiment. The results show that when the volume ratio of agent A to agent B is 1:1.3, the acid production capacity is the strongest. The pH values at several time points in the process of acid reaction were measured by a pen pH meter. The relationship curve between acid production capacity and time was obtained. The acid production capacity increased with time. It tends to be stable after a certain time. The experiment of acid rock reaction kinetics shows that the reaction rate between acid and rock decreases with the extension of time. The reaction time can reach 6 h. The reaction rate of autogenic acid under different temperatures and concentrations ranges from 6.61 × 10-7 to 9.49 × 10-7 mol/(s·cm2) within 6 h. Therefore, it indicates that the reaction time between autogenic acid and carbonate rock is long and the reaction rate is low. It is beneficial to improve the acid treatment effect of the carbonate reservoir. The conductivity experiments show that at different temperatures, with the increase of sealing pressure, the etching ability of autogenic acid decreases. The etching effect is better at 338 K. After etching, the permeability and conductivity of the rock slab of 5 MPa closure pressure are 227 D and 72.62 D·cm, respectively. To sum up, this autogenic acid is an effective working fluid in the acid fracturing process of the carbonate reservoir. It can obviously reduce the reaction rate of acid rock and has certain conductivity.
Carbonate reservoirs worldwide
are complex in structure, diverse
in form, and highly heterogeneous, which is one of the key and hot
areas in current oil and gas exploration and development.[1] Stimulation technology for the carbonate reservoir
is the key technology for exploration and development of the carbonate
reservoir. Acid fracturing is one of the main measures to increase
production of carbonate reservoirs with strong heterogeneity. As the
acid flows into natural fractures that intersect hydraulic fractures
(main fractures), it etches the walls of the natural fractures, which
then increases the natural fractures’ width and generates conductivity.[2] Among them, the performance of acid pressing
working fluid and acid additives directly affects the acid pressing
effect, especially in high-temperature deep Wells.[3,4] Due
to high reservoir temperature and fast acid rock reaction, the effective
length of acid-etched fracture after acid compression is short.[5] At the same time, the reservoir temperature is
high. It will cause serious acid corrosion of downhole string and
easy to cause equipment damage.[6]In order to solve these difficulties, technicians at home and abroad
have carried out in-depth research on the unconventional acid system.
The thickener and crosslinking agent in crosslinking acid can generate
high strength stereoscopic network gel through crosslinking, which
can effectively reduce the reaction speed of acid rock, improve the
penetration depth and the conductivity of acid erosion fracture, and
prolong the validity period of acid compression.[7−10] Emulsified acid is a multiphase
system composed of an oil phase and acid phase. It has high viscosity,
good filtration reduction, slow speed effect, strong corrosion ability,
and small corrosion and does little damage to the strength of pipe
string. Emulsified acid is widely used in repeated acid pressure or
high sulfur gas well acid pressure, and the use of emulsified acid
in matrix acidification is also significantly increased.[11−15] Gelled acid can reduce the filtration loss of acid solution, form
a colloidal network structure, and reduce the diffusion of H+. Thickened acid can form wide cracks, small filtration loss, and
low friction and show good performance of suspended solid particles
in the process of acid pressing.[16−18] Autogenic acid is an
acid product produced in situ by chemical dissociation or reaction
of fluid injected into the formation. It has no acidity or weak acidity
on the surface, but acid is produced gradually at the bottom of the
hole temperature, and acid production is carried out gradually, which
can effectively reduce the acid rock reaction rate, prolong the acid
action distance, and reduce the corrosion effect on the underground
pipe string.[19−23]According to the mechanism of acid-rock reaction, acid-rock
reaction
speed is closely related to the ability of acid to release hydrogen
ions and reaction kinetic parameters. In recent years, many scholars
have conducted a lot of research and summarized the acid-rock reaction
behavior. Yoo used the apparatus repeatedly to investigate the reaction
kinetics of dolomite rock under the various concentrations of fresh
acid and spent acid. It was revealed that the dissolution rate and
diffusion coefficient for spent acid were higher than those of fresh
acid due to the higher kinematic viscosity and the lower pH of spent
acid.[24] Mahmoud used a new synthetic Gemini
surfactant as a fracturing fluid additive to stabilize the clay in
the fracturing of unconventional dense sandstone.[25] Kamal proposed to use the polyoxyethylene quaternary ammonium
gemini surfactants with different types of spacers as clay swelling
additives in completion fluids to mitigate the formation damage in
unconventional reservoirs.[26] Kalam reviewed
several waterless technologies for improving shale and ultratight
rocks conductivity and introduced some available simulation techniques
for monitoring the performance of these techniques.[27] Gomaa used an in situ-generated hydrofluoric acid (HF)
for sandstone acidizing, where an acid precursor (ammonium fluoride
NH4F) reacted with a suitable oxidizer (sodium bromates
NaBrO3) in an exothermic reaction. Upon application of
this new stimulation technology, the true production potential of
sandstone reservoirs can be achieved, well tubular corrosion will
be minimized, and handling hazardous chemicals such as HF will be
avoided.[28] Zhao synthesized a series of
cationic Gemini surfactants with different hydrophobic chain lengths
(C-4-C, n = 12, 14, 16). The surface tension, adhesion work, and
AFM are used to investigate the effect of hydrophobic chain length
and concentration of surfactant on surface activities adsorption morphology
and adsorption capacities.[29] Tariq investigated
the performance of the chelating agents, namely, ethylenediaminetetraacetic
acid (EDTA) and glutamic acid N,N-diacetic acid (GLDA), in creating sufficient fracture conductivity.
Several experiments/measurements were performed, the lab-scale outcomes
were reproduced numerically, and field-scale simulations were carried
out.[30]Therefore, the current research
on the acid rock reaction behavior
of carbonate reservoir. There are a large number of studies, both
on-site acid rock jet experiment and indoor experimental research,
such as acid rock reaction law, optimization of conventional acid
system, synthesis of surfactants.[31,32] However, there
are few studies on the true acid system of a carbonate reservoir.
Especially under the background of increasing exploration and development
of carbonate reservoirs in China, the study of the autogenous acid
system should be accelerated. This paper compares the acid production
capacity of agent A and agent B under different ratios. The acid system
with strong acid production capacity was selected. The reaction law
of acid was evaluated by laboratory experiments.
Experimental
Section
Material
Carbonate
Cores and Plates
In this
paper, the three rocks shown in Figure are used in the reaction kinetics experiment. The
three rock slabs shown in Figure are used for the conductivity test experiment. We
take some rock samples and grind them into powder. Then we make a
very flat sample. Next, we analyze the mineral composition of the
carbonate rock by the Rigaku Smartlab9K X-ray diffractometer (with
Cu Ko radiation at 40 kV and 40 mA); the test angle is 5–140°. Table shows the test results.
Figure 1
Experimental
carbonate cores.
Figure 2
Experimental carbonate
plates.
Table 1
X-ray Diffractometer
Test Results
composition
and content, %
sample
dolomite
calcite
chlorite
quartz
albite
dickite
olivine
Serpentine
core 1
97
1
1
1
core 2
98
1
1
core 3
98
1
1
plate 1
96
1
2
1
plate 2
91
3
3
2
1
plate 3
82
4
5
6
3
Experimental
carbonate cores.Experimental carbonate
plates.The results show that the mineral composition of the six rock samples
is mainly dolomite. The content of dolomite is the highest, up to
98%. The second is calcite. Therefore, the core (plate) used in the
experiment is a typical rock sample of dolomite and calcite in a carbonate
reservoir.
Potion
The potions
used in titration
experiments are NaOH solution (Chengdu Cologne Chemicals Co., Ltd.;
Analytical purity) and potassium hydrogen phthalate (Chengdu Kelon
Chemical Reagent Factory; Analytical purity).The autogenic
acid system studied in this paper mainly consists of two parts: formaldehyde
(Chengdu Cologne Chemical Co., Ltd.; Analytical purity) and ammonium
chloride (Chengdu Jinshan Chemical Reagent Co., Ltd.; Analytical purity).
Formaldehyde and ammonium salt can produce H+; this reaction
is a characteristic reaction, as shown in eq . As the H+ is consumed, the reaction
moves to the right and achieves a slow speed effect. This reaction
can be carried out at room temperature, with the increase of temperature
reaction speed.
Experiment Content
Formula
Ratio Optimized in Aldehydes and
Ammonium Salt
Through the NaOH titration experiment, the
ratio of the formula can be optimized in aldehyde and ammonium salt.
Before the experiment is performed, the concentration of the standard
configured fluid needs to be corrected. We use potassium hydrogen
phthalate for correction and record the volume of standard liquid
consumed by four groups of solutions. Equation calculates the concentrations of the four
groups of standard fluids and takes the average value to obtain 0.10–1
mol/L standard fluid after correction.In order to find the best ratio of
agent A and agent B in the autogenic acid system and optimize the
autogenic acid formula. We fix 1 wt % formaldehyde and change the
concentration of ammonium chloride from 0.8 to 1.4 wt %. After standing
for 2 h at room temperature, we obtain 0.061 in.3 autogenic
acid and correct NaOH standard solution for multiple titration experiments.
Then, we record the average volume of sodium hydroxide consumed to
obtain the total amount of hydrochloric acid released by autogenous
acid under different volumes.
Ability
of Autogenic Acid to Produce Acid
The ability of autogenic
acid to produce acid can be expressed
by the hydrogen ion concentration during an acid reaction. The pen
pH meter (Beekman Biotechnology Co., Ltd.; operating temperature 273–323
K) is used to measure pH in the process of acid reaction. Equation can calculate the
concentration of hydrogen ions in solution and make the curve of acid
concentration in 3 h.Then, on the one hand, we test the
effects of different temperatures (298, 313, and 323 K) on acid production
capacity with concentration unchanged; on the other hand, we test
the effects of different concentrations (C0.5, C3, C6, C7, C8, C9, C10, and C11) on acid production capacity with temperature
unchanged.
Kinetics of Acid and
Rock Reaction
Due to the limitation of pen pH meter’s
operating temperature,
it cannot accurately measure the pH in the process of autogenic acid
reaction above 323 K. Therefore, according to the acid rock reaction
kinetics experiment, the ability of autogenic acid to react with the
core at higher temperature is tested.We need to prepare three
groups (C5, C10, and C15) of 12.2
in.3 autogenic acid. Carbonate cores are wrapped in thermoplastic
tubes. Only the bottom of the core is left in contact with autogenic
acid. After sealing them, we should place them successively in an
oil bath at a speed of 20 rpm. Each concentration of autogenic acid
reacts with the core at different temperatures (313, 338, and 363
K). 1.22 in.3 acid samples are taken at certain intervals
(0.5, 1, 2, 4, and 6 h), diluted 1000 times, and filtered with an
8.668 × 10–6 needle filter. The spectrophotometer
shown in Figure (Beijing
East West Analytical Instrument Co., Ltd.) can measure the calcium
ion concentration in the solution, which tests the reaction ability
of autogenic acid to carbonate rocks. Then, the reaction rate between
acid and rock can be calculated according to eq .where J refers to the reaction
rate between acid and rock, mol/(s·cm2); C refers to the acid concentration at time T, mol/L;
and S/V refers to the area–volume
ratio, that is, the ratio of the reaction area of the rock to the
volume of the acid involved in the reaction, cm2/cm3.
Figure 3
Atomic spectrophotometer.
Atomic spectrophotometer.
Conductivity Test
We use the acid
erosion conductivity evaluation device (Chengdu Core Technology Co.,
Ltd.) to test conductivity. The experiment adopts the sequence of
etching first and conductivity later. The conductivity of C10 autogenic acid to acid etching cracks of rock plates was tested
at 313, 338, and 363 K.Etching ability test is to place two
iron wires between a pair of rock plates to form an acid etching channel.
Before etching, the rock plate should be fixed in the gripper and
placed in the water bath at the set temperature of preheat. Then,
we should configure 61 in.3 C10 autogenic acid
and open the advection pump. The carbonate rock plate is etched slowly
at a flow rate of 0.18 in.3/min. Five and a half hours
later, the etching is finished. The etched plates are taken out, and
the morphologic changes of the plates are observed with naked eyes
or a 3D scanner.Evaluation of conductivity is to test the permeability
and conductivity
of etched plates of water.[33,34] The etched plates are
loaded into the gripper and sealed. After adding a certain confining
pressure, we need to open the advection pump and empty the gas in
the pipeline at a flow rate of 0.61 in.3/min. After preparation,
we use the standard acid diversion software to set the closure pressure
from 5 to 60 MPa and the acquisition cycle to 30 s. Finally, we test
the permeability and conductivity of rock plates of water under different
pressures. Figure shows the flow of the specific experimental device.
Figure 4
Flow chart of the conductivity
test device.
Flow chart of the conductivity
test device.
Results
and Discussion
Formula Ratio Optimized
in Aldehydes and Ammonium
Salt
Figure shows the amount of NaOH consumed by autogenic acids with multiple
groups of volume ratios. The results show that ammonium chloride has
the highest acid production efficiency at 1.3. Therefore, autogenic
acid with a volume ratio of 1:1.3 will be used in subsequent experiments.
0.5 wt % CH2O + 0.65 wt % NH4Cl as C0.5, 3 wt % CH2O + 3.9 wt % NH4Cl as C3, and 10 wt % CH2O + 13 wt % NH4Cl at C10 are defined.
Figure 5
Titration results of ammonium salt at different volume
ratios.
Titration results of ammonium salt at different volume
ratios.
Acid
Production Capacities are Tested in Autogenic
Acid
According to the experimental scheme of acid production
capacity of autogenic acid, the dynamic process of hydrogen ion release
of autogenic acid in the reaction process can be described. We tested
the pH of autogenous acid at different acid concentrations and temperatures
within 3 h. The ability curves of autogenous acid to release hydrogen
ions in different environments were obtained. The results are as follows.
Acid Production Capacity is Tested at 298,
313, and 323 K
Autogenic acids can react to produce hydrogen
ions at room temperature and under heating conditions. We use a pen
pH meter to measure pH at different stages (0, 0.5, 1, 1.5, 2, 2.5,
and 3 h) during the acid reaction process. Then, we calculate the
concentration of releasing hydrogen ions by eq . Figure shows the change of hydrogen ion released from the
initial contact between formaldehyde and ammonium chloride to 3 h
after the reaction. The results show that the capacity of acid production
increased rapidly with time and tended to be stable after a certain
time. At 298 K, C3 reached a stable value of 3.8 ×
10–2 mol/L at 1.5 h. At 323 K, C6 reached
a stable value of 0.269 mol/L at 1 h. At 333 K, C9 reached
a stable value of 0.588 mol/L at 1 h.
Figure 6
Changes in the amount of hydrogen ions
released during 3 h of autogenic
acid reaction: (a) C3, (b) C6, and (c) C9.
Changes in the amount of hydrogen ions
released during 3 h of autogenic
acid reaction: (a) C3, (b) C6, and (c) C9.
Treatment
with Temperature at Constant Concentration
The content of
hydrogen ions in the solution of autogenic acid
is fixed after the acid reacts for 1.5 h. Acid formation ability under
this condition is affected by concentration and temperature. Figure shows the change
of treatment with temperature at constant concentration of autogenic
acid. The results show that the hydrogen ion content in solution increases
with the increase of temperature at the same concentration. After
multi-temperature treating, the increment of hydrogen ion content
of high concentration autogenic acid is obviously larger than that
of low concentration autogenic acid.
Figure 7
Treatment with temperature at constant
concentration.
Treatment with temperature at constant
concentration.
Treatment
with Concentration at Constant
Temperature
Figure shows the change of the concentration of hydrogen ions released
during the process of increasing the autogenic acid concentration
from C3 to C10 at fixed temperature. The results
show that the hydrogen ion content in solution increases with the
increase of concentration at constant temperature. The effect of temperature
on high concentration autogenic acid is greater than that on low concentration
autogenic acid. In the process of increasing the temperature from
298 to 323 K. The concentration of hydrogen ions in C9 solution
increased by 12 times as much as that in C3 solution.
Figure 8
Treatment
with concentration at constant temperature.
Treatment
with concentration at constant temperature.
Ability of Autogenic Acid to React with Calcium
Carbonate
According to the experimental scheme of acid rock
reaction kinetics, we can judge the reaction ability of acid and calcium
carbonate. We measure the concentration of Ca2+ in solution
by flame absorption with an atomic spectrophotometer and obtain the
hydrogen ion concentration of self-generated acid consumed by the
core within a certain period of time. The reaction rate of acid rock
is calculated by eq .
Calcium Ion Content in Solution
Figure shows the
Ca2+ concentration in the solution during the reaction
of autogenic acid with carbonate rock. Thus, Ca2+ concentration
increases with time and the solution did not stop reacting until 6
h later. It shows that autogenic acid has good retarding performance
and can prolong the reaction time of acid rock. The results show that
the temperature has a great influence on the calcium content of the
solution. The higher the temperature, the higher will be the concentration
of Ca2+ in the autogenic acid solution. Concentration also
has a certain effect on the content of calcium in the solution. The
higher the concentration, the higher will be the calcium concentration.
However, its effect is weaker than temperature.
Figure 9
Ca2+ concentration
in the reaction between autogenic
acid and carbonate rock.
Ca2+ concentration
in the reaction between autogenic
acid and carbonate rock.Figure shows
the increase in Ca2+ concentration in solution during the
reaction of autogenic acid with carbonate rock at 313 K. As can be
seen from the figure, the change trend of Ca2+ increment
over time in the acid reaction process is as follows: decrease first
and reach a stable value after 2 h.
Figure 10
Increase in Ca2+ concentration
during acid rock reaction
at 313 K.
Increase in Ca2+ concentration
during acid rock reaction
at 313 K.Among them, the stable increment
of calcium ions in C5 solution is 3.77 × 10–2 mol/L, that in C10 is 4.49 × 10–2 mol/L, and that in
C15 is 4.58 × 10–2 mol/L. This indicates
that autogenic acid has been releasing hydrogen ions within 6 h of
reaction. It has strong release ability in the first 2 h. Hydrogen
ions can also be released stably in the process of 2 to 6 h.
Rate of Acid Reaction with Rock
The Ca2+ concentration increases in a certain time during
the reaction of autogenic acid with carbonate rock. It can reflect
the hydrogen ion concentration consumed by acid generation in this
time period so as to calculate the acid rock reaction rate. Because
1.22 in.3 samples are taken at each time point of the experiment,
the area volume ratio in eq varies with time. Table shows the acid-rock reaction rates of autogenic acids
at different temperatures and concentrations within 6 h. The reaction
rate of acid rock is 6.61 × 10–7 mol/(s·cm2) to 9.49 × 10–7 mol/(s·cm2). The results show that the reaction rate between autogenic
acid and carbonate rock is very low. It is beneficial to improve the
acid treatment effect of the carbonate reservoir.
Table 2
Reaction Rates of Autogenic Acid Rocks
at Different Temperatures
autogenic
acid type
temp, K
reaction time, h
area volume
ratio, cm2/cm3
Ca2+ concentration, mol/L
reaction
rate, mol/(s·cm2)
C5
313
6
4.25 × 10–2
0.304
6.61 × 10–7
C10
313
6
4.25 × 10–2
0.325
7.07 × 10–7
C15
313
6
4.25 × 10–2
0.344
7.54 × 10–7
C5
338
6
4.25 × 10–2
0.394
8.57 × 10–7
C10
338
6
4.25 × 10–2
0.409
8.89 × 10–7
C15
338
6
4.25 × 10–2
0.421
9.17 × 10–7
C5
363
6
4.25 × 10–2
0.416
9.05 × 10–7
C10
363
6
4.25 × 10–2
0.426
9.28 × 10–7
C15
363
6
4.25 × 10–2
0.439
9.49 × 10–7
Table shows the
reaction kinetics experiments of autogenic acids used in this paper
and those used previously. It can be seen that the reaction rates
of used autogenic acid A, B, and C are all on the dot product of 10–6. However, the quantitative product of the reaction
rate of autogenic acid prepared from formaldehyde and ammonium chloride
is 10–7, which is significantly lower than other
types of autogenic acids.
Table 3
Comparison of Kinetic
Parameters of
Different Autogenic Acid Reactions
The conductivity test of acid etched fracture is one
of the important
indexes for calculating the effective action distance of acid and
evaluating the effect of acid compression. Through this test, we can
understand the fracture diversion morphology of artificial fracture
and natural fracture after acid rock reaction under high temperature
and high pressure. According to the experimental scheme of diversion,
the three plates were first etched with C10 autogenic acid
at different temperatures to establish artificial fracture channels. Figure shows the change
of face morphology before and after etching of each pair of plates. Figure shows the scanning
results of autogenic acid etched rock slabs before and after scanning
with 3D scanner at 363 K.
Figure 11
Comparison of the front and rear faces of the
lithographic plate
etching at different temperatures: (a) 313 K etched plates 1, (b)
338 K etched plates 2, and (c) 363 K etched plates 3.
Figure 12
3D scanning of the rock plate face.
Comparison of the front and rear faces of the
lithographic plate
etching at different temperatures: (a) 313 K etched plates 1, (b)
338 K etched plates 2, and (c) 363 K etched plates 3.3D scanning of the rock plate face.The results show that C10 autogenic acid has a certain
etching effect on the three plates at different temperatures. However,
the etching morphology is not ideal. It can be observed from the plate
change of the face before and after the plate etching that the acid
solution at 313 K has poor etching effect on the rock plate and there
are only a few etching marks on the end face of the rock plate. The
acid solution at 338 K had a better etching effect on the plate, and
several short crack channels were produced on the end face. The etching
effect of acid solution at 363 K on the injection end of rock plate
is better.3D scans show that the slabs are intact before the
acid etching.
At 363 K, the color of slab inlet end turns yellow. It shows that
the autogenic acid has a certain etching ability at the slab injection
end. The blackening of the etched part is disturbed by the iron wire
between the two plates. The color of the rest remains unchanged. It
shows that the etching effect of autogenic acid on the rest of slate
is poor.Table and Figure show
the conductivity
test results of etching slabs. When the closure pressure is 5 MPa,
the temperature is 338 and 363 K. The permeability of the etched plate
is 210.16–226.99 D. The conductivity is 67.23–72.62
D·cm. The permeability and conductivity of rock slab etched at
313 K at the same condition are only 36.77 D and 11.76 D·cm,
respectively. The conductivity of the plate decreases with the increase
of the closure pressure. When etched at 338 and 363 K, the conductivity
decreases significantly between 20 and 40 MPa. The decrease of etching
at 313 K is the largest at 10 MPa, and then, it becomes smaller.
Table 4
Permeability and Conductivity of C10 Autogenic
Acid at Different Temperatures under Different
Closure Pressure
group number
etching temp, K
closure pressure,
MPa
differential
pressure, kPa
actual flow, in.3/min
permeability, D
diversion
capacity, D·cm
1
313
5
6.38
0.58
36.77
11.76
313
10
18.91
0.55
11.91
3.81
313
20
21.01
0.5
9.66
3.09
313
30
21.01
0.48
9.18
2.94
313
40
21.01
0.47
9.11
2.91
313
50
21.01
0.46
8.83
2.83
313
60
21.01
0.44
8.62
2.75
2
338
5
1.01
0.57
226.99
72.62
338
10
1.34
0.56
167.63
53.63
338
20
1.57
0.56
143.89
46.03
338
30
2.32
0.56
97.43
31.17
338
40
5.05
0.54
42.79
13.68
338
50
13.4
0.54
16.68
5.33
338
60
15.56
0.5
12.84
4.11
3
363
5
1.14
0.59
210.16
67.23
363
10
1.56
0.56
157.48
50.38
363
20
1.77
0.54
136.36
43.62
363
30
2.96
0.53
92.39
29.55
363
40
7.65
0.53
34.71
11.11
363
50
15.6
0.53
13.72
4.38
363
60
18.36
0.48
9.74
3.12
Figure 13
Conductivity
of C10 autogenic acid at different temperatures
under different closed pressure.
Conductivity
of C10 autogenic acid at different temperatures
under different closed pressure.
Conclusions
(1)
When the volume ratio of formaldehyde to ammonium chloride
is 1:1.3, the acid production efficiency is the highest. Autogenic
acid can release hydrogen ions at room temperature and under heating
conditions. The amount of hydrogen ions released increases with time
and tends to be stable after a certain time. Several groups of experiments
confirmed that the higher the acid concentration and temperature,
the lower the pH of the autogenic acid, and the stronger the acid
generating capacity.(2) Acid rock reaction kinetics experiments
show that the reaction
rate of acid and rock decreases with time. The reaction time can reach
6 h. Acid-rock reaction rates of autogenic acids at different temperatures
and concentrations within the 6 h range from 6.61 × 10–7 mol/(s·cm2) to 9.49 × 10–7 mol/(s·cm2). It shows that the reaction time of
autogenic acid and carbonate rock is long and the reaction speed is
low, which is beneficial to improve the acid treatment effect of the
carbonate reservoir.(3) The conductivity experiment shows that
the etching ability
of autogenic acid decreases with the increase of closure pressure
at different temperatures, among which 338 and 363 K have a better
etching effect. The permeability and conductivity of etched slabs
are 210.16–226.99 D and 67.23–72.62 D·cm at 5 MPa
closure pressure. At 313 K, the etching effect is poor, and the permeability
and conductivity are only 36.77 D and 11.76 D·cm.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13