Experimental research on deterioration of mechanical properties of carbonate rocks under acidified conditions

Journal of Petroleum Science and Engineering xxx (xxxx) xxx

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Experimental research on deterioration of mechanical properties of carbonate rocks under acidified conditions Hao Zhang a, b, Ying Zhong a, b, *, Jiang Zhang a, b, Yongchun Zhang a, b, c, Jianchao Kuang a, b, Bin Yang a, b a b c

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology), Chengdu, Sichuan Province 610059, China College of Energy, Chengdu University of Technology, Chengdu, Sichuan Province 610059, China Petro-Engineering Research Institute, North China Oil and Gas Branch, Sinopec, Zhengzhou, Henan Province 450006, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Carbonate reservoirs Acid fracturing Reaction rate constant Activation energy Mechanical properties

Understanding the relationship among the individual factors (i.e., carbonate rock types, acid rock reaction ki­ netics, deterioration of rock mechanical properties) can provide practical guidelines that can be used for the design and optimization of acid fracturing operation. A comprehensively experimental study was conducted to investigate the acid rock reaction kinetics of carbonate rocks under acidified conditions, and how the acid rock reaction would influence the carbonate rock mechanical properties. In addition, the deterioration mechanism of the carbonate rock mechanical properties is determined by the qualitative analysis of casting thin section images. Results show that the limestone rock is more susceptible to acid dissolution than limy dolomite rock under the same acidified conditions. The limestone rock has relatively large reaction rate constant and low activation energy, however, it is reverse in limy dolomite rock. As a result, the mechanical properties of rocks with rela­ tively low cementation strength could be highly or lowly weakened. The reduction (in percentage, %) in me­ chanical properties of the limestone rocks with relatively high cementation strength is greater than that of the limy dolomite rocks with relatively high cementation strength. Casting thin section images show that the cementation and fillings of limestone and limy dolomite rocks are calcite and calcite-dolomite, respectively. The non-uniform (i.e., superficial and local deep) and uniform etched pattern (i.e., superficial and symmetrical) occur on the end surface of limestone and limy dolomite, respectively. Consequently, the mechanical properties of relatively high cemented limestone rocks are more significantly reduced than that of relatively high cemented limy dolomite rocks. This work is of great practical guidelines for the design and optimization of acid fracturing operations, as well as the efficient stimulation of carbonate reservoirs.

1. Introduction Acid fracturing is an effective production stimulation technology, which has been widely used in the development of carbonate reservoirs (Hung et al., 1989; Mumallah, 1991; Bazin, 2001; Abass et al., 2006; Zhu et al., 2015). Scholars have been focused on the researches of the acid rock reaction mechanism, acidizing fluid system, acid leakoff mecha­ nism, acid fracturing mechanistic model and so on (Nierode and Wil­ liams, 1971; Hoefner and Fogler, 1989; Buijse, 1997; Taylor and Nasr-El-Din, 2003; Xu et al., 2017; Li et al., 2018; Yin et al., 2018; Yoo et al., 2018). A variety of acidizing fluids (e.g., gelling acid,

self-diverting acid and emulsified acid) (Bazin and Abdulahad, 1999; Bazin et al., 1999; Lungwitz et al., 2007) and acid fracturing technolo­ gies (e.g., acid jetting, CO2 energized acid fracturing treatment) (Al-D­ hamen and Soriano, 2015; Beckham et al., 2015) have been developed to improve the production stimulation effectiveness of the acid fracturing. The effective fracture conductivity channels generated by acid frac­ turing is attributed to the uneven fracture surface caused by acid dissolution of partial minerals in the walls of fractures (Williams et al., 1979; Asadollahpour et al., 2018). The fracture surface roughness after the acidizing treatments is one of the primary factors that contribute to the production stimulation effectiveness of acid fracturing (Dong et al.,

* Corresponding author. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology), Chengdu, Sichuan Province 610059, China. E-mail address: [email protected] (Y. Zhong). https://doi.org/10.1016/j.petrol.2019.106612 Received 14 May 2019; Received in revised form 19 October 2019; Accepted 21 October 2019 Available online 24 October 2019 0920-4105/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Hao Zhang, Journal of Petroleum Science and Engineering, https://doi.org/10.1016/j.petrol.2019.106612

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Journal of Petroleum Science and Engineering xxx (xxxx) xxx

circular graph, erosion-destroyed areas). As a result, decrease the hy­ draulic fracturing initiation pressure and make the acid fracturing operation flexible. Therefore, acid etching may reduce the difficulty of acid fracturing operations due to the deterioration effect of acid etching on rock mechanical properties. In summary, the acid rock reaction during acid fracturing operation in carbonate reservoirs tends to alter the rock mechanical properties, in consequence, impacting the acid fracturing operations and effectiveness (i.e., disadvantages and advantages). Therefore, understanding the relationship among the individual factors (i.e., carbonate rock types, acid rock reaction kinetics, deterioration of rock mechanical properties) can provide practical guidelines that can be used for the design and optimization of acid fracturing operation. In this paper, the gelling acid and two kinds of carbonate rocks (i.e., limestone and limy dolomite rocks) from the Leikoupo Carbonate Formation in the Western Sichuan Basin in China were collected. A comprehensively experimental study was then conducted to determine the acid rock reaction kinetics of the two kinds of carbonate rocks under acidified conditions, and how the acid rock reaction would influence the carbonate rock mechanical properties.

2002). Effects of the rock uncontrollable inherent properties (e.g., mineral components, porosity, permeability and so on) and the variable conditions (e.g., temperature, pressure, acid concentration, flow rate and so on) on acid rock reaction rate were experimental researched and summarized by many scholars (Mumallah, 1991; Wang et al., 1993; Taylor et al., 2006). The acid etching process of minerals, also known as acid rock reac­ tion kinetics, have been researched for decades to describe the charac­ teristics of the reaction between acidizing fluids and rocks (Nierode and Williams, 1971; Mumallah, 1991; Wang et al., 2018; Yoo et al., 2018). The investigation of acid rock reaction kinetics is to determine the ef­ fects of various acid concentrations and temperatures on the acid rock reaction rate, yields the reaction rate constant, reaction order, activation energy, as well as acid rock reaction kinetics equation and Arrhenius equation. Those studies, therefore, accurately represents the process of acid rock reaction and must be essential for the design and optimization of acid fracturing operations (Navarrete et al., 1998). Meanwhile, the other potential influences caused by acid etching will significantly impact the acid fracturing operations and effectiveness, including disadvantaged and advantaged effects. For the disadvantaged effects: some publications have reported that the fracture conductivity is related to the rock mechanical properties, the alteration of rock mechanical properties will significantly influence the effectiveness of fracture conductivity (Cooke, 1975; Bartko et al., 2003; Garrouch and Jennings, 2017; Zhong et al., 2018; Jafarpour et al., 2019). The conductivity of acid-etched fractures in carbonate reservoirs, therefore, is strongly controlled by acid rock reaction and rock me­ chanical properties. Wormholing mechanisms have also been researched in previous work (Hung et al., 1989; Buijse, 1997; Dong, 2018; Xue et al., 2018), the minerals are dissolved to form the cylindrical pores in the wormholing process. There can be no doubt that the acid dissolution of rock minerals would destroy the rock structure, as well as the mineralogy of the artificial fracture surface (Bemer and Lombard, 2010; Kang et al., 2014; Liu and Mostaghimi, 2017; Lu et al., 2017), as shown in Fig. 1 (enlarged rectangular graph, fracture surface). Thus, the acid flow may lead to the alteration of the rock mechanical properties, consequently, resulting in adverse effects on the fracture conductivity. For the advantaged effects: fracture initiation and propagation are closely related to rock mechanical properties (Yang et al., 2004; Waters et al., 2011). Some publications demonstrated that when the injected acid fracturing fluids contact the formation, the acid fracturing fluids will leakoff into the areas around the wellbore (Hill et al., 1995; Zhou et al., 2007). Similar wise, acid dissolution of rock minerals would destroy the rock structure (Bemer and Lombard, 2010; Kang et al., 2014; Liu and Mostaghimi, 2017), and may weaken the rock mechanical properties, the erosion-destroyed areas are shown in Fig. 1 (enlarged

2. Experimental program 2.1. Materials used 2.1.1. Rock samples The cylindrical carbonate rocks used to conduct the experiments were collected from the Leikoupo Carbonate Formation, which is located in the Western Sichuan Basin in China. The carbonate rock samples were prepared in shapes of 25 mm diameter and 55 mm length to meet the requirements of the acid-rock reaction and mechanical properties measurements. The collected carbonate rock samples were classified into two types according to their mineral composition, they were limestone and limy dolomite rocks. The detailed information regarding the petrophysical properties, major mineral composition and burial depth of the rock samples were listed in Table 1. 2.1.2. Acidizing fluids It’s well known that the gelling acid is widely used in the acid frac­ turing of carbonate reservoirs in the field. The composition of the acid fracturing fluid was as follows: x% HCl þ 0.8% Gelling agent þ 4.0% Corrosion inhibitor þ1.0% Iron stabilizer þ1.0% Cleanup additive, of which x% equals to 10%, 15% and 20%. The gelling acid will be used to conduct the preconditioning of the carbonate rocks. 2.2. Methods and test procedures 2.2.1. Measurement of the acid-rock reaction kinetics The acid-rock reaction experimental device was used for the mea­ surement of the acid-rock reaction rate, as shown in Fig. 2. The exper­ imental procedure simulated the acid fracturing operation in the field (the rapidly moving acid fluid was injected into the carbonate rock) for 20 min. The rotating disk method (i.e., dynamic rotating test) was used to conduct this measurement. The rotational moving velocity of acid fluid was 4 m3/min, the system pressure in the acid container (i.e., displacement pressure) was 4 MPa, the confining pressure was 8 MPa. Besides, the weight of the carbonate rock was measured before and after the acid-rock reaction test to determine the acid-rock reaction rate. The acid-rock reaction kinetics equation was determined by measuring the acid rock reaction in cases of different acid concentra­ tions (i.e., 10%, 15% and 20%) under 90 � C temperature. The acid-rock reaction kinetics equation is expressed as (Lund et al., 1973; Li et al., 2016):

Fig. 1. Schematic diagram of acid fracturing. Both acid flow in artificial frac­ tures (enlarged rectangular graph, fracture surface) and acid etching in areas around wellbore (enlarged circular graph, erosion-destroyed areas) would destroy rock structure, consequently, weakening the rock strength.

(1)

J ¼ KCm 2

In which, J is the acid-rock reaction rate, mol/(cm ⋅s); K is the 2

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Table 1 Petrophysical properties, major mineral composition and burial depth of the carbonate rocks. Rock type

Porositya (%)

Air permeabilityb (mD)

Limestone Limy dolomite

1.41 4.98

0.041 0.443

a b c

Main mineral compositionc(%) Calcite

Dolomite

100 41

– 59

Burial depth (m) 6182–6252

Porosity is measured by using alcohol saturation method. Permeability was measured with nitrogen. Main mineral compositions of the rocks were determined by using DX-2700 X-ray Diffractometer technique.

Fig. 2. Schematic diagram of the acid-rock reaction experimental device, of which, the motor drive system is used for the dynamic rotating, the intermediate container full of high pressure gas is used to adjust the system pressure in the acid container, the monitor is used to control the system pressure and temperature.

reaction rate constant, mol/[cm2⋅s⋅(mol/L)m]; C is the molar concen­ tration of HCl, mol/L; m is the order of the reaction, dimensionless. The activation energy was determined by measuring the acid rock reaction in cases of different temperatures (i.e., 70 � C, 90 � C and 110 � C) under 15% HCl acidified conditions. The activation energy for acid-rock reaction could be determined according to the modified Arrhenius equation (Yoo et al., 2019):

Moreover, the components and types of the micro-fracture fillings and cement of the carbonate rocks were identified by using the rock casting thin section images, which helps to analyze the deterioration mechanism of the mechanical properties of carbonate rocks under acidified conditions.

Ea ln J ¼ þ lnðKo Cm Þ RT

3.1. Acid rock reaction kinetics of carbonate rocks under acidified conditions

3. Results and discussions

(2)

Where, Ea is the activation energy, J/mol; R is the universal gas constant, 8.314 J mol 1⋅K 1; T is the absolute temperature, K; Ko is the preexponential factor.

3.1.1. Reaction rate constant Reaction rate constant can be used to quantify the rate of a chemical reaction. Table 3 shows the results of the acid rock reaction rate in cases of different acid concentrations (10%, 15% and 20%) under 90 � C temperature. The acid rock reaction rate increases along with the increasing acid concentration. For instance, in cases of the limestone and limy dolomite rocks under acidified conditions, when the acid concen­ tration increases from 10% to 20%, the acid rock reaction rate increases from 1.283E-06 mol/(cm2⋅s) to 1.936E-06 mol/(cm2⋅s) (increased by 50.90%), and from 0.764E-06 mol/(cm2⋅s) to 1.258E-06 mol/(cm2⋅s) (increased by 64.66%), respectively. Moreover, the acid rock reaction rate of the limestone rock is larger than that of limy dolomite rock in the same acidified conditions (i.e., temperature and acid concentration). According to the acid rock reaction kinetics equation (i.e., Eq. (1)). The reaction rate constant K and reaction order m could be obtained from the experimental data according to the following equation, which is deduced by taking natural logarithms on both sides of Eq. (1):

2.2.2. Measurement of the deterioration degree of rock mechanical properties due to acid etching (1) Preconditioning of rocks To determine the deterioration degree of the mechanical properties of carbonate rocks under acidified conditions, the partial carbonate rocks were preconditioned by acidizing the end of rocks before used to conduct mechanical property measurements. The acid-rock reaction experimental device was used for the preconditioning of the carbonate rocks (Fig. 2). Detailed information of the carbonate rocks used for mechanical property measurements was listed in Table 2. The mechanical properties of the carbonate rocks were measured by using the RTR-1000 Rock Mechanics Servo Testing System manufac­ tured by GCTS company. Uniaxial compression tests were run at room temperature under 0 MPa confining stress. Triaxial compression tests were run at room temperature under 65 MPa confining stress. Thus, the elastic modulus and compression strength will be calculated to deter­ mine the deterioration degree of the mechanical properties of carbonate rocks under acidified conditions.

ln J ¼ lnK þ m ln C

(3)

The m and lnK are the slope and the intercept of the straight fitting line obtained by plotting lnJ versus lnC, respectively, as shown in Fig. 3 and Table 4. The results show the reaction rate constant K for limestone rock is much greater than that for limy dolomite rock under acidified conditions, they are 7.1742E-07 mol/[cm2⋅s⋅(mol/L)m] and 3.7206E3

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Table 2 Detailed information of the carbonate rocks used for mechanical property measurements. Rock type

Group

Sample #

Preconditioninga

Application

Limestone

L1

L1-A L1-B

(90 � C,

Uniaxial compression test

L2

L2-A L2-B

(90 � C,

Uniaxial compression test

L3

L3-A L3-B

(90 � C,

Uniaxial compression test

L4

L4-A L4-B

(90 � C,

Uniaxial compression test

L5

L5-A L5-B

(90 � C,

Uniaxial compression test

L6

L6-A L6-B

(90 � C,

Triaxial compression test

L7

L7-A L7-B

(90 � C,

Triaxial compression test

L8

L8-A L8-B

(90 � C,

Triaxial compression test

L9

L9-A L9-B

(90 � C,

Triaxial compression test

L10

L10-A L10-B

(90 � C,

Triaxial compression test

LD1

LD1-A LD1-B

(90 � C,

Uniaxial compression test

LD2

LD2-A LD2-B

(90 � C,

Uniaxial compression test

LD3

LD3-A LD3-B

(90 � C,

Uniaxial compression test

LD4

LD4-A LD4-B

(90 � C,

Uniaxial compression test

LD5

LD5-A LD5-B

(90 � C,

Uniaxial compression test

LD6

LD6-A LD6-B

(90 � C,

Triaxial compression test

LD7

LD7-A LD7-B

(90 � C,

Triaxial compression test

LD8

LD8-A LD8-B

(90 � C,

Triaxial compression test

LD9

LD9-A LD9-B

(90 � C,

Triaxial compression test

LD10

LD10-A LD10-B

– Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl) – Acid etching 15%HCl)

(90 � C,

Triaxial compression test

Limy dolomite

Table 3 Results of the acid rock reaction rate in cases of different acid concentrations (10%, 15% and 20%) under 90 � C temperature. Acid concentration (%)

Temperature (� C)

10 15 20

90

J (10

6

mol/(cm2⋅s))

Limestone

Limy dolomite

1.283 1.684 1.936

0.764 0.945 1.258

Fig. 3. Curves of lnJ versus lnC in cases of the limestone and limy dolomite rocks under acidified conditions.

rock under acidified conditions, they are 0.5602 and 0.6623, respec­ tively. This indicates that the effect of reactant concentration on acid rock reaction rate for limestone rock is slightly less than that for limy dolomite rock. Activation energy could also be used to quantitatively describe the acid rock reaction rate for rocks with different components under acidified conditions. Which will be discussed in the following section. 3.1.2. Activation energy Table 5 shows the results of the acid rock reaction rate in cases of different temperatures (i.e., 70 � C, 90 � C and 110 � C) under 15% HCl acidified conditions. The acid rock reaction rate increases along with the increasing experimental temperature. For example, in cases of the limestone and limy dolomite rock under 15% HCl acidified conditions, when the experimental temperature increases from 70 � C to 110 � C, the acid rock reaction rate increases from 1.537E-06 mol/(cm2⋅s) to 1.761E06 mol/(cm2⋅s) (increased by 14.57%) and from 0.836E-06 mol/(cm2⋅s) to 1.112E-06 mol/(cm2⋅s) (increased by 33.01%), respectively. More­ over, in the same case of acidified conditions (i.e., temperature and acid concentration), the acid rock reaction rate of the limestone rock is larger than that of limy dolomite rock. The acid rock reaction rate is related to the activation energy, which could be determined according to the modified Arrhenius equation (i.e., Eq. (2)) based on the data in Table 5. The –Ea/R and ln(KoCm) are the slope and the intercept of the straight fitting line obtained by plotting lnJ versus 1/T, respectively, as shown in Fig. 4. Table 6 shows the results of the activation energy and Arrhenius equation for acid rock reaction of limestone and limy dolomite rocks under acidified conditions. The activation energy for limestone rock is less than that for limy dolomite rock under 15%HCl acidified conditions, they are 3737.54 J/mol and 7768.24 J/mol, respectively. Thus, the higher the activation energy, the lower the acid rock reaction rate. The results indicate that the limestone rock is more susceptible to acid dissolution than limy dolomite rock under the same acidified conditions. Besides, the pre-exponential factor Ko is also calculated, then the

a

The method for preconditioning is the same as the method of the acid-rock reaction kinetics measurement, the experimental temperature and acid con­ centration are averaged at 90 � C and 15%, respectively. (2) Measurement of the rock mechanical properties.

07 mol/[cm2⋅s⋅(mol/L)m], respectively. Thus, the greater the reaction rate constant, the larger the acid rock reaction rate. This indicates that the limestone rock is more susceptible to acid dissolution than limy dolomite rock under the same acidified conditions. Besides, the reaction order m for limestone rock is slightly smaller than that for limy dolomite 4

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Table 4 Parameters and equations of the acid rock reaction kinetics in cases of the limestone and limy dolomite rocks under acidified conditions. Rock type

Temperature (� C)

K (mol/[cm2⋅s⋅(mol/L)m])

m (Dimensionless)

Acid-rock reaction equation J ¼ KCm

Limestone Limy dolomite

90

7.1742E-07 3.7206E-07

0.5602 0.6623

J ¼ 7.1742E-07C0.5602 J ¼ 3.7206E-07C0.6623

measurements show that the cementation strength of the carbonate rocks is various (Fig. 5 and Fig. 6). Thus, we use 60 MPa and 100 MPa (i. e., compression strength) to classify the rock cementation strength in the uniaxial and triaxial compression measurements, respectively. Then the rock cementation strength can be divided into two categories (i.e., (a) and (b) in Figs. 5 and 6). Rocks with relatively low cementation strength include the rocks of group L1, L2, L6, LD1, LD2, LD6, LD7, the cemen­ tation strength of the rest rocks are relatively high. In addition, the rocks with relatively high cementation strength are divided into two types according to their mineral composition (i.e., limestone and limy dolo­ mite, (b-1) and (b-2) in Figs. 5 and 6). Therefore, those rocks can be divided into three categories according to their cementation strength and mineral composition. They are (a) rocks with relatively low cementation strength, (b-1) limestone rocks with relatively high cementation strength, and (b-2) limy dolomite rocks with relatively high cementation strength. A comparison of the data in Figs. 5 and 6 indicates that the compression strength of both limestone and limy dolomite rocks reduced after being exposed to acidified conditions. For example, in the uniaxial compression measurements (Fig. 5), the compression strength of carbonate rocks in group L1 (limestone rocks) before and after being exposed to acidified conditions are 44.3 MPa and 30.9 MPa (reduced by 30.25%), respectively. The compression strength of carbonate rocks in group LD1 (limy dolomite rocks) before and after being exposed to acidified conditions are 56.0 MPa and 50.7 MPa (reduced by 9.46%), respectively. Similar results are also observed in the results of triaxial compression measurements (Fig. 6). For instance, the compression strength of carbonate rocks in group L6 (limestone rocks) before and after being exposed to acidified conditions are 89.7 MPa and 52.6 MPa (reduced by 41.36%), respectively. The compression strength of car­ bonate rocks in group LD6 (limy dolomite rocks) before and after being

Table 5 Results of the acid rock reaction rate in cases of different temperatures (i.e., 70 � C, 90 � C and 110 � C) under 15% HCl acidified conditions. Temperature (� C)

Acid concentration (%)

70 90 110

15

J (10

6

mol/(cm2⋅s))

Limestone

Limy dolomite

1.537 1.684 1.761

0.836 0.945 1.112

Fig. 4. Curves of lnJ versus 1/T in cases of the limestone and limy dolomite rocks under acidified conditions.

Arrhenius equation can be determined according to Eq. (4). � � Ea J ¼ Ko Cm exp RT

(4)

We can, therefore, conclude that the limestone rock is more sus­ ceptible to acid dissolution than limy dolomite rock under the same acidified conditions (i.e., same acid concentration and experimental temperature). Following the acid rock reaction tests (i.e., determine the reaction rate constant and activation energy), we have also conducted rock uniaxial and triaxial compression measurements to determine the influence of acid rock reaction on rock mechanical properties under acidified conditions. 3.2. Deterioration of mechanical properties of carbonate rocks under acidified conditions 3.2.1. Classification of carbonate rocks according to their cementation strength and mineral composition Results of the initial compression strength (i.e., differential stress for rock failure) obtained from both uniaxial and triaxial compression

Fig. 5. Initial and acid-etched compression strength values of carbonate rock samples obtained from uniaxial compression measurements.

Table 6 Results of the activation energy and Arrhenius equation for acid rock reaction of limestone and limy dolomite rocks under acidified conditions. Rock type

Acid concentration (%)

Modified Arrhenius equation

Ea (J/mol)

Ko

Arrhenius equation

Limestone Limy dolomite

15

lnJ ¼ 449.5472/T-12.0695 lnJ ¼ 934.3561/T-11.2806

3737.54 7768.24

2.50E-06 4.72E-06

J ¼ 5.74E-06exp(-449.55/T) J ¼ 1.26E-05exp(-934.36/T)

5

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Fig. 6. Initial and acid-etched compression strength values of carbonate rock samples obtained from triaxial compression measurements.

Fig. 8. Alteration of rock compression strength obtained from triaxial compression measurements.

exposed to acidified conditions are 78.2 MPa and 69.6 MPa (reduced by 11.00%), respectively. Thus, the mechanical properties of carbonate rocks exposed to acidified conditions would change, a detailed discussion of the alter­ ation of the carbonate rock mechanical properties upon exposure to acidified conditions will be given in the following section. 3.2.2. Alteration of the carbonate rock mechanical properties upon exposure to acidified conditions Results of the alteration of rock mechanical properties (i.e., compression strength and elastic modulus) are shown in Figs. 7–10. Obviously, the rock mechanical properties are weakened due to acid etching, moreover, the characteristic of the reduction of rock mechani­ cal properties varies with the rock categories as discussed earlier. Thus, in each figure, the results are divided into three categories according to the rock categories as discussed earlier. For the alteration of compression strength of rocks exposed to acid­ ified conditions (Figs. 7 and 8). Three performance can be observed: i-) The compression strength of rocks with relatively low cementation strength are highly or lowly weakened. For example, in the uniaxial compression measurements (Fig. 7(a)), the group L1 rocks has the highest compression strength reduction degree, which is up to 30.25%, however, the group LD1 rocks has the lowest compression strength reduction degree, which is as low as 9.46%. Similar, in the triaxial compression measurements (Fig. 8(a)), the group L6 rocks has the highest compression strength reduction degree, which is up to 41.43%,

Fig. 9. Alteration of rock elastic modulus obtained from uniaxial compression measurements.

Fig. 10. Alteration of rock elastic modulus obtained from triaxial compression measurements.

however, the group LD7 rocks has the lowest compression strength reduction degree, which is as low as 10.21%. ii-) Limestone rocks with high cementation strength are highly weakened. For instance, the minimum reduction degree of compression strength in uniaxial and triaxial compression measurements are 46.84% (Fig. 7(b-1), L5) and

Fig. 7. Alteration of rock compression strength obtained from uniaxial compression measurements. 6

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50.08% (Fig. 8(b-1), L7), respectively. iii-) Limy dolomite rocks with relatively high cementation strength are slightly weakened. For example, the maximum reduction degree of compression strength in uniaxial and triaxial compression measurements are 10.07% (Fig. 7(b2), LD4) and 13.40% (Fig. 8(b-2), LD10) respectively. For the alteration of elastic modulus of rocks exposed to acidified conditions (Figs. 9 and 10). Results show that the decrease (in per­ centage, %) of elastic modulus of rocks with relatively low cementation strength ranges from 18.94% to 66.27% (Fig. 9(a)) and from 16.13% to 50.60% (Fig. 10(a)) in the uniaxial and triaxial compression measure­ ments, respectively. The decrease (in percentage, %) of elastic modulus of limestone rocks with relatively high cementation strength ranges from 17.84% to 62.26% (Fig. 9(b-1)) and from 13.69% to 54.56% (Fig. 10(b-1)) in the uniaxial and triaxial compression measurements, respectively. However, the decrease (in percentage, %) of elastic modulus of limy dolomite rocks with relatively high cementation strength only ranges from 11.93% to 15.21% (Fig. 9(b-2)) and from 3.29% to 11.92% (Fig. 10(b-2)) in the uniaxial and triaxial compression measurements, respectively. Therefore, results indicate that the reduc­ tion (in percentage, %) in elastic modulus of rocks with relatively low cementation or limestone rocks with relatively high cementation strength are greater than that of limy dolomite rocks with relatively high cementation strength. According to the above results and discussions. We can, therefore, make conclusions that the mechanical properties (i.e., compression strength and elastic modulus) of carbonate rocks would weaken under acidified conditions. The mechanical properties of rocks with relatively low cementation strength could be highly or lowly weakened. However, the reduction (in percentage, %) in mechanical properties of the lime­ stone rocks with relatively high cementation strength is greater than that of the limy dolomite rocks with relatively high cementation strength under the same acidified conditions (i.e., same acid concen­ tration and experimental temperature). Thus, in the following section, the detailed discussion of the effect of acid rock reaction on the deteri­ oration of mechanical properties of the relatively high cemented car­ bonate rocks is given.

limy dolomite (LD) rocks range from 4.38% to 13.40%, and from 3.29% to 15.21%, respectively. However, the reaction rate constant of lime­ stone (L) rocks is 7.1742E-07 mol/[cm2⋅s⋅(mol/L)m], the reduction (in percentage, %) of compression strength and elastic modulus of relatively high cemented limestone (L) rocks range from 46.84% to 77.36%, and from 13.69% to 62.26%, respectively. Opposite results are observed in Fig. 12. The deterioration degree of mechanical properties (i.e., compression strength and elastic modulus) of relatively high cemented rocks, obtained from uniaxial and triaxial compression measurements, is negatively correlated to the activation energy. Results indicate that the lower the activation energy is, the more serious the deterioration of rock mechanical properties is. For example, the activation energy for limestone (L) rocks is 3737.54 J/mol, the reduction (in percentage, %) of compression strength and elastic modulus of relatively high cemented limestone (L) rocks range from 46.84% to 77.36%, and from 13.69% to 62.26%, respectively. However, the activation energy for limy dolomite (LD) rocks is 7768.24 J/mol, the reduction (in percentage, %) of compression strength and elastic modulus of relatively high cemented limy dolomite (LD) rocks range from 4.38% to 13.40%, and from 3.29% to 15.21%, respectively. It was clear the greater reaction rate constant or the lower activation energy is expected to contribute to more serious mechanical properties degradation of carbonate rocks upon exposure to acidified conditions. We can, therefore, conclude that the relatively high cemented limestone rocks are more susceptible to mechanical properties deterioration than the relatively high cemented limy dolomite rocks under the same acid­ ified conditions (i.e., same acid concentration and experimental temperature). 3.2.4. Effect of acid etching on the residual stress in carbonate rocks with relatively high cementation strength In addition to the effect of acid etching on rock compression strength and elastic modulus, it’s also observed that the acid etching would in­ fluence the residual stress in carbonate rocks with relatively high cementation strength. The stress-strain curves are shown in Fig. 13 (obtained from uniaxial compression measurements) and Fig. 14 (triaxial compression measurements). Obviously, the post peak behavior of limestone rock in uniaxial and triaxial compression measurements changes from brittle to ductile, which means the residual stress in limestone rock is changed. This can be attributed to the alteration of the internal stress in limestone rock caused by acid etching (Friedman, 1972; Engelder and Geiser, 1983). However, the post peak behavior of limy dolomite rock in uniaxial and triaxial compression measurements is nearly unchanged and brittle, which means the residual stress in limy dolomite rock is nearly unchanged. Therefore, the acid etching significantly influences the residual stress in limestone rocks by destroying the skeleton structure and intergran­ ular cementation, however, little impacts that in limy dolomite rocks.

3.2.3. Effect of acid etching on the deterioration of mechanical properties of relatively high cemented carbonate rocks Fig. 11 shows that the deterioration degree of mechanical properties (i.e., compression strength and elastic modulus) of relatively high cemented rocks, obtained from uniaxial and triaxial compression mea­ surements, is positively correlated to the acid rock reaction rate con­ stant. Results indicate that the larger the reaction rate constant is, the more serious the deterioration of rock mechanical properties is. For example, the reaction rate constant of limy dolomite (LD) rocks is 3.7206E-07 mol/[cm2⋅s⋅(mol/L)m], the reduction (in percentage, %) of compression strength and elastic modulus of relatively high cemented

Fig. 11. Curves of deterioration degree of rock mechanical properties versus acid rock reaction rate constant (the left figure shows the reduction of rock compression strength in percentage, the right figure shows the reduction of rock elastic modulus in percentage). 7

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Journal of Petroleum Science and Engineering xxx (xxxx) xxx

Fig. 12. Curves of deterioration degree of rock mechanical properties versus activation energy (the left figure shows the reduction of rock compression strength in percentage, the right figure shows the reduction of rock elastic modulus in percentage).

Fig. 13. Stress-strain curves of carbonate rocks before and after acid etching (uniaxial compression measurements).

Fig. 14. Stress-strain curves of carbonate rocks before and after acid etching (triaxial compression measurements).

Thus, the consideration of the effect of acid etching on residual stress in limestone rocks is critical for the design of acid fracturing in carbonate formations.

that in limy dolomite (Fig. 15(b)). In other words, the micro-fractures in limestone rock are more severely eroded by acid than that in limy dolomite, which may be attributed to the different degree of acid rock reaction between the two kinds of rocks. Therefore, the skeleton struc­ ture and intergranular cementation of limestone rock may be seriously destroyed due to the strong acid dissolution of micro-fracture fillings, consequently, alter the internal stress in limestone rock and weaken the rock strength a lot. However, the slight acid dissolution of microfracture fillings may hardly lead to the alteration of the internal stress in limy dolomite rock, as a result, weaken the rock strength slightly. To prove this deduction, the rock casting thin section images are used for the qualitative analysis of the components and types of the microfracture fillings and cements. As shown in Fig. 16: i-) The microfracture in limestone rock is cemented and filled with calcite. ii-) The

3.3. Deterioration mechanism of rock mechanical properties under acidified conditions The images of the end surface of carbonate rocks (i.e., limestone and limy dolomite) before and after acid etching (Fig. 15), show that the carbonate rocks are severely etched. The end surface of the rocks become uneven and many visible micro-pores appear along the microfracture. However, the magnitude of the dissolved micro-pores be­ tween limestone rock and limy dolomite rock is different. The dissolved pores in limestone rock (Fig. 15(a)) are more obvious and larger than 8

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Journal of Petroleum Science and Engineering xxx (xxxx) xxx

Fig. 15. Images of the end surface of carbonate rocks before and after acid etching.

micro-fracture in limy dolomite rock is cemented and filled with dolo­ mite and calcite. According to the discussion earlier, the limestone rock is more susceptible to acid than limy dolomite rock under the same acidified conditions. Therefore, the calcite cements in the micro-fracture of limestone rocks are more easily eroded by acid than dolomite-calcite cements in the micro-fracture of limy dolomite rocks, as a result, the etched pattern of the end surface of the two kinds of rocks is different. The schematic diagram of the etched patterns are shown in Fig. 17, they are: (a) Non-uniform etched pattern (i.e., superficial and local deep), which occurs on the end surface of the limestone rock, as a result, altering the internal stress in limestone rock and weaken the rock strength a lot. (b) Uniform etched pattern (i.e., superficial and sym­ metrical), which occurs on the end surface of the limy dolomite rock, the internal stress in limy dolomite rock changed little, and the rock strength is weakened slightly.

determine the acid rock reaction kinetics of the two kinds of carbonate rocks under acidified conditions, and how the acid rock reaction would influence the carbonate rock mechanical properties. Understanding the relationship among the individual factors (i.e., carbonate rock types, acid rock reaction kinetics, deterioration of rock mechanical properties) can provide practical guidelines that can be used for the design and optimization of acid fracturing operation. Results obtained from acid rock reaction experiments have shown that the limestone rock is more susceptible to acid dissolution than limy dolomite rock under the same acidified conditions (i.e., same acid concentration and experimental temperature). The limestone rock has relatively large reaction rate constant and low activation energy, how­ ever, the limy dolomite rock has relatively small reaction rate constant and high activation energy. The degree of alteration of the rock mechanical properties upon exposure to acidified conditions has been determined by conducting the rock mechanical properties tests. Results show that the mechanical properties (i.e., compression strength and elastic modulus) of carbonate rocks would weaken under acidified conditions. The mechanical

4. Conclusion A comprehensively experimental study has been conducted to

Fig. 16. Casting thin section images of the limestone and limy dolomite rocks. 9

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Fig. 17. Schematic diagram of rock structure before and after acid etching (a) Non-uniform etched pattern (i.e., superficial and local deep) occurs on the end surface of limestone rock (b) Uniform etched pattern (i.e., superficial and symmetrical) occurs on the end surface of limy dolomite rock.

properties of rocks with relatively low cementation strength could be highly or lowly weakened. However, the reduction (in percentage, %) in mechanical properties of the limestone rocks with relatively high cementation strength is greater than that of the limy dolomite rocks with relatively high cementation strength. In addition, the acid etching significantly influences the residual stress in limestone rocks, however, little impacts that in limy dolomite rocks. The deterioration mechanism of the carbonate rock mechanical properties is determined by the qualitative analysis of casting thin sec­ tion images. The cementation and fillings of limestone and limy dolo­ mite rocks are calcite and calcite-dolomite, respectively. Therefore, the non-uniform (i.e., superficial and local deep) and uniform etched pattern (i.e., superficial and symmetrical) occur on the end surface of limestone and limy dolomite, respectively. Consequently, the mechan­ ical properties of high cemented limestone rocks are more significantly reduced than that of high cemented limy dolomite rocks.

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