# A High Effective Acid System to Stimulate Sandstone Reservoirs

Authors: Bin Li
Journal: Geofluids
 (2022)

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## Abstract

Mud acid is regularly applied to recover or increase permeability of sandstone reservoir. However, due to its rapid reacting rate and heterogeneous displacement, the stimulation effect of conventional mud acid is relatively limited in sandstone formations. For mud acid, it is important to choose proper chemical additives to retard reaction rate and make it distributed homogenereously in formation. To overcome these shortcomings of mud acid, a new proprietary mud acid system has been developed for sandstone formations stimulation. The new system is composed of 2.0% multichemical solutions (MCS) +6.5%HCl+1.5% HF, and the combined solution contains nonionic surfactant, clay stabilizer, and dispersant which were specifically formulated. Core flooding experiments were conducted with the high effective mud acid; meanwhile, XRD and SEM analysis were performed before/after the core flooding tests. Retarded tests were also conducted for the new mud acid reacting with sandstone samples. Treatments with ten pores volume of the new high effective mud acid system were found to increase permeability of sandstone core approximately three times of its initial permeability. From the results of SEM and XRD analysis, most of clays were dissolved, and permeability was increased after core flooding by this new mud acid solution. On the basis of acidizing theory, the reaction retarded mechanism was explained about the new composite mud acid. Other properties of the new composite mud acid were also discussed. In comparison of conventional mud acid, the new composite mud acid can retard acidizing reaction rate and distribute evenly in formation; therefore, it can extend acidizing penetration and stimulate sandstone formation thoroughly. The new composite mud acid is more compatible and desirable than conventional mud acid, and it will be applied extensively for sandstone matrix stimulation.


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## Body

## 1. Introduction

In petroleum industry, matrix acidizing has been employed to remove formation damage caused by drilling, workover, or production process and to enhance the formation permeability [1, 2]. Mud acid has been extensively applied in matrix acidizing, which is composed of HCl and HF acid with different concentrations. HCl is used to treat calcareous minerals in sandstone, also to dissolve many types of iron containing minerals or scales, but HCl reacts with calcareous material rapidly to become spent acid without dissolving ability, resulting in limited acid penetration to sandstone formation [3–5].HF is a relatively strong acid, which has a unique ability to dissolve silicate materials. Laboratory studies confirmed that HF will preferentially attack calcareous materials, then clays and feldspars, and finally quartz [6, 7]. For this reason, HCl is usually mixed with HF to remove calcareous material and prevent calcium fluoride precipitation that would occur before HF contacts calcareous materials [8–10]. The rapid reaction of HF with clays is a definite disadvantage for applications involving deeper matrix stimulation.Two main limitations exist in the success of sandstone acidizing treatments. These are (1) limited penetration of reactive HF and (2) heterogeneous placement of mud acid in formations. Studies have shown that mud acid is spent within the first few inches from the wellbore due to rapid spending of acid within the matrix of sandstone reservoirs, thereby it causes excessive dissolution of mud acid near the wellbore and prevents deep stimulation in sandstone formations [11, 12]. In order to achieve deeper penetration, mud acid is required to be retarded to slow its reaction with rock, and various approaches have been employed in an attempt to increase effective mud acid penetration.In theory, retarding of HCl reaction with sandstone formations can be accomplished by reducing the contact of acid with rock [13, 14]. Based on this knowledge, couples of additives and acid systems have been developed to obtain retardation, such as surfactant retarders, gelled acids, emulsified acids, foamed acids, organic acids, encapsulated acid, and inorganic/organic retarder chemicals [15–17]. The formation will be damaged because gelled acid is not easy to be broken after acidizing; so, the acid system is not applied in sandstone reservoirs extensively. Also, emulsified acids are not used because it will bring many problems in oil treatment [18–20].Another disadvantage of mud acid applied in sandstone reservoirs is heterogeneous placement and permeability improvement in formations [21, 22]. Fluids pumped into the well will flow most easily into the high permeability, possibly undamaged portion of the completion. Thus, the largest portion of acid may be expended on only a small part of the total zone, and acidizing treatment may not be successful in removing damage from the entire production interval. Diverting agents are often added to the acid system to promote acid flowing into the entire perforated interval homogenereously. Diverting is a technique designed to alter the fluid injection profile in formation, and it can be classified as mechanical (straddle packer or ball sealers), solid (sand, oil soluble resins, graded rock salt, and benzonic acid fines), and chemical (viscous agents, natural gums, emulsions, acid swellable polymers, solid organic acids, foam, and other particulate diverters) [23, 24]. Chemical diverters are one of the most commonly utilized diverting techniques, and they are also the most difficult systems to use properly.Among chemical diverters, foam can block fluid flow by virtue of viscous properties. Mechanically, foams can flow like liquids and remain motionless like a solid. Major advantages of utilizing foam diverting include the following: (1) applicable over a range of pressure, temperature, and permeability; (2) ease of application; and (3) aids flowback of treatment [25–27]. Another advantage of foam is the ease of generating foam and varying its quality while pumping a job. Aqueous solutions as well as oils and alcohols can be foamed by nitrogen, carbon dioxide, or petroleum gases. Another significant benefit of foamed treating fluids is realized during flowback due to the ability of foam to transport released fines and insoluble out of the near wellbore area. This property can be of particular importance in the stimulation of underpressured reservoirs. Disadvantages of foam diverting include the increase in wellhead pressure due to foam’s lower hydrostatichead and potential surface and treating facility problems due to the foaming agent not being used in the correct application or concentration [28–30]. In general, the benefits of using foam slugs for diverting include the benefits listed for foamed fluids but at a considerably lower cost and less risk of system upsets during treatment flowback.For mud acidizing, it is important to choose proper additives to retard reaction rate and make mud acid distributing homogenereously in formation. Synergistic effect between chemical reagents can improve the overall performance of acid solution [31]. If one chemical surfactant can be developed to achieve both aims meanwhile, it will be helpful for the extensive application of mud acid. If the viscosity of mud acid is increased, the reaction rate with sandstone will be decreased. Also, if foam can be produced during mud acid flowing into sandstone reservoir, mud acid will be displaced uniformly in formation. For conventional foamed acidizing treatment, foams are created by pumping nitrogen or carbon dioxide gas. In fact, when mud acid reacts with sandstone, which mostly contains calcites, then carbon dioxide gas can be produced. Therefore, if the carbon dioxide gas can be used to form foam, it will be helpful to divert mud acid fluid into low permeable formations.The paper presents a series of experiments for mud acid added with a chemical composite, which has the functions of increasing viscosity and creating microfoams. This new kind of acid system can retard reacting rate with sandstone rock and can make fluid flow evenly into formations with different permeability. The experimental results show that the new mud acid system can increase formation permeability obviously. X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) were tested separately before/after core is flooded by the new proprietary mud acid. Using XRD and SEM, a mineralogical evaluation is obtained to show the acidizing ability of the new composite mud acid. The problems associated with conventional mud acid and its limitations can be overcome by the new composite mud acid, and the advantages of the new mud acid system are illustrated.


## 2. Experimental Studies

### 2.1. Materials

#### 2.1.1. Core Plugs

In these experiments, sandstone samples were from Baohe oilfield, located at Shandong province in China. Core plugs were cut from sandstone blocks provided by the oil service company, and the plug sample is1.0×2in. cylinder. The depth of the studied area ranges from 2735.1 m to 2740.2 m, reservoir temperature is 85°C, the porosity of the core samples ranges from 7.14% to 13.91%, and the permeability ranges from 1.21 mD to 13.91 mD. In laboratory, core plugs were flushed with alcohol for 24 hours to remove residual hydrocarbon and other contaminants according to treatment standard of reservoir cores. After being cleaned, the plugs were dried at 50°C for 12 hours, and then porosity and permeability of each core plug were measured using porosity-meter and dry nitrogen gas. These tests are at ambient temperature and at 2.5 MPa confining pressure.


#### 2.1.2. Acid

The regular mud acid is composed of HCl and HF, and the concentration of is based on mineral components of sandstone. The new proprietary mud acid is the blend of regular mud acid with multichemical solutions (MCS), which contain surfactant, clay stabilizer, and dispersant specifically formulated. The functions of the above reagents are to reduce the surface tension, prevent the expansion of clay minerals, and disperse the dissolved particles to avoid precipitation. In addition, Experiments of the compatibility of regular mud acid with multichemical solutions show that the liquid has no stratification and no precipitation. For different sandstones, the concentration of MCS will be altered proportionally to attain acidizing effect at the most.


### 2.2. Retarded Test

In order to testify the retarded property of the new mud acid system, sandstone rock was grounded into 100 mesh particles, and the sample of which diameter is lower than 100 mesh was weighted. An equal weight of one gram was reacted with excess acid at atmospheric pressure and temperature; meanwhile, the reaction time and the weight of rock particles were recorded.


### 2.3. XRD and SEM Test

In laboratory, mineralogical and pore structure evaluation of pre- and postflooding cores are obtained by use of X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The mineralogical evaluation is utilized to identify the dissolving ability of acid, and the SEM results are used to assess the effect of acid treatment on core’s porosity.Before core flooding experiments, the mineral contents of sandstone will be analyzed by XRD to provide clues for design of mud acid components. Meanwhile, the mineral analysis result before/after core flooding will be compared to determine which kind of minerals has been dissolved. The structure characteristics of pores and pore throats can be observed by SEM test, and mineral distribution also can be detected. The SEM results before/after core flooding can be compared to evaluate acidizing effect of the new mud acid system.


### 2.4. Core Flooding Experiment

To illustrate the effect of new mud acid system on sandstone, core-flooding tests were conducted, and the rock cores are taken from actual sandstone formation.


#### 2.4.1. Core Flooding Equipment

The scheme of core flooding apparatus was shown in Figure1. The flow was controlled by use of back pressure regulator, and a differential pressure transducer was used to measure the pressure difference between fluid inlet and outlet. Also, the pressure transducer was connected to computer acquisition to record the pressure drop during experiments; meanwhile, the results are tabulated and graphically displayed on computer in real time. A syringe pump was used to inject fluid into core plug at steady pumping rate, which can be set according to test requirement. When the flowing of fluid attains steady, the permeability can be calculated by pressure difference, flow rate, and core plug size.Figure 1
The scheme of the core flooding system.


#### 2.4.2. Core Flooding Procedure

In core flooding experiments, core plugs were first dried for 5 hours, and the pore volume of each core was calculated by the porosity tested previously. Plugs were seated in rubber sleeves at 5-10 MPa confining pressure to make sure that fluid flow only through the core plug and the clearance between core plug and rubber sleeve is airtight. Because the permeability of most cores is low, the injection flow rate was controlled at 1.0 cm3/min in all experiments, and the temperature is 85°C. Treatment fluids were injected as follows:
(1)
Injection of 5 wt% NaCl solution (3 pore volumes)(2)
Injection of the main acid treatment (10 pore volumes)Note that the injection direction is different for above procedure. The main acid was injected in a direction opposite to that direction in which 5 wt% NaCl solution was injected. The initial permeability was established in the first injection of 5 wt% NaCl solution, and the final permeability, which represents the result of acid stimulation, was calculated in the second injection of 5 wt% NaCl solution. Therefore, the data collected during the injection of main acid is not important for permeability evaluation. In comparison of the permeability values, the effect of acid can be evaluated, and the stimulation ratio also can be decided.


## 2.1. Materials

### 2.1.1. Core Plugs

In these experiments, sandstone samples were from Baohe oilfield, located at Shandong province in China. Core plugs were cut from sandstone blocks provided by the oil service company, and the plug sample is1.0×2in. cylinder. The depth of the studied area ranges from 2735.1 m to 2740.2 m, reservoir temperature is 85°C, the porosity of the core samples ranges from 7.14% to 13.91%, and the permeability ranges from 1.21 mD to 13.91 mD. In laboratory, core plugs were flushed with alcohol for 24 hours to remove residual hydrocarbon and other contaminants according to treatment standard of reservoir cores. After being cleaned, the plugs were dried at 50°C for 12 hours, and then porosity and permeability of each core plug were measured using porosity-meter and dry nitrogen gas. These tests are at ambient temperature and at 2.5 MPa confining pressure.


### 2.1.2. Acid

The regular mud acid is composed of HCl and HF, and the concentration of is based on mineral components of sandstone. The new proprietary mud acid is the blend of regular mud acid with multichemical solutions (MCS), which contain surfactant, clay stabilizer, and dispersant specifically formulated. The functions of the above reagents are to reduce the surface tension, prevent the expansion of clay minerals, and disperse the dissolved particles to avoid precipitation. In addition, Experiments of the compatibility of regular mud acid with multichemical solutions show that the liquid has no stratification and no precipitation. For different sandstones, the concentration of MCS will be altered proportionally to attain acidizing effect at the most.


## 2.1.1. Core Plugs

In these experiments, sandstone samples were from Baohe oilfield, located at Shandong province in China. Core plugs were cut from sandstone blocks provided by the oil service company, and the plug sample is1.0×2in. cylinder. The depth of the studied area ranges from 2735.1 m to 2740.2 m, reservoir temperature is 85°C, the porosity of the core samples ranges from 7.14% to 13.91%, and the permeability ranges from 1.21 mD to 13.91 mD. In laboratory, core plugs were flushed with alcohol for 24 hours to remove residual hydrocarbon and other contaminants according to treatment standard of reservoir cores. After being cleaned, the plugs were dried at 50°C for 12 hours, and then porosity and permeability of each core plug were measured using porosity-meter and dry nitrogen gas. These tests are at ambient temperature and at 2.5 MPa confining pressure.


## 2.1.2. Acid

The regular mud acid is composed of HCl and HF, and the concentration of is based on mineral components of sandstone. The new proprietary mud acid is the blend of regular mud acid with multichemical solutions (MCS), which contain surfactant, clay stabilizer, and dispersant specifically formulated. The functions of the above reagents are to reduce the surface tension, prevent the expansion of clay minerals, and disperse the dissolved particles to avoid precipitation. In addition, Experiments of the compatibility of regular mud acid with multichemical solutions show that the liquid has no stratification and no precipitation. For different sandstones, the concentration of MCS will be altered proportionally to attain acidizing effect at the most.


## 2.2. Retarded Test

In order to testify the retarded property of the new mud acid system, sandstone rock was grounded into 100 mesh particles, and the sample of which diameter is lower than 100 mesh was weighted. An equal weight of one gram was reacted with excess acid at atmospheric pressure and temperature; meanwhile, the reaction time and the weight of rock particles were recorded.


## 2.3. XRD and SEM Test

In laboratory, mineralogical and pore structure evaluation of pre- and postflooding cores are obtained by use of X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The mineralogical evaluation is utilized to identify the dissolving ability of acid, and the SEM results are used to assess the effect of acid treatment on core’s porosity.Before core flooding experiments, the mineral contents of sandstone will be analyzed by XRD to provide clues for design of mud acid components. Meanwhile, the mineral analysis result before/after core flooding will be compared to determine which kind of minerals has been dissolved. The structure characteristics of pores and pore throats can be observed by SEM test, and mineral distribution also can be detected. The SEM results before/after core flooding can be compared to evaluate acidizing effect of the new mud acid system.


## 2.4. Core Flooding Experiment

To illustrate the effect of new mud acid system on sandstone, core-flooding tests were conducted, and the rock cores are taken from actual sandstone formation.


### 2.4.1. Core Flooding Equipment

The scheme of core flooding apparatus was shown in Figure1. The flow was controlled by use of back pressure regulator, and a differential pressure transducer was used to measure the pressure difference between fluid inlet and outlet. Also, the pressure transducer was connected to computer acquisition to record the pressure drop during experiments; meanwhile, the results are tabulated and graphically displayed on computer in real time. A syringe pump was used to inject fluid into core plug at steady pumping rate, which can be set according to test requirement. When the flowing of fluid attains steady, the permeability can be calculated by pressure difference, flow rate, and core plug size.Figure 1
The scheme of the core flooding system.


### 2.4.2. Core Flooding Procedure

In core flooding experiments, core plugs were first dried for 5 hours, and the pore volume of each core was calculated by the porosity tested previously. Plugs were seated in rubber sleeves at 5-10 MPa confining pressure to make sure that fluid flow only through the core plug and the clearance between core plug and rubber sleeve is airtight. Because the permeability of most cores is low, the injection flow rate was controlled at 1.0 cm3/min in all experiments, and the temperature is 85°C. Treatment fluids were injected as follows:
(1)
Injection of 5 wt% NaCl solution (3 pore volumes)(2)
Injection of the main acid treatment (10 pore volumes)Note that the injection direction is different for above procedure. The main acid was injected in a direction opposite to that direction in which 5 wt% NaCl solution was injected. The initial permeability was established in the first injection of 5 wt% NaCl solution, and the final permeability, which represents the result of acid stimulation, was calculated in the second injection of 5 wt% NaCl solution. Therefore, the data collected during the injection of main acid is not important for permeability evaluation. In comparison of the permeability values, the effect of acid can be evaluated, and the stimulation ratio also can be decided.


## 2.4.1. Core Flooding Equipment

The scheme of core flooding apparatus was shown in Figure1. The flow was controlled by use of back pressure regulator, and a differential pressure transducer was used to measure the pressure difference between fluid inlet and outlet. Also, the pressure transducer was connected to computer acquisition to record the pressure drop during experiments; meanwhile, the results are tabulated and graphically displayed on computer in real time. A syringe pump was used to inject fluid into core plug at steady pumping rate, which can be set according to test requirement. When the flowing of fluid attains steady, the permeability can be calculated by pressure difference, flow rate, and core plug size.Figure 1
The scheme of the core flooding system.


## 2.4.2. Core Flooding Procedure

In core flooding experiments, core plugs were first dried for 5 hours, and the pore volume of each core was calculated by the porosity tested previously. Plugs were seated in rubber sleeves at 5-10 MPa confining pressure to make sure that fluid flow only through the core plug and the clearance between core plug and rubber sleeve is airtight. Because the permeability of most cores is low, the injection flow rate was controlled at 1.0 cm3/min in all experiments, and the temperature is 85°C. Treatment fluids were injected as follows:
(1)
Injection of 5 wt% NaCl solution (3 pore volumes)(2)
Injection of the main acid treatment (10 pore volumes)Note that the injection direction is different for above procedure. The main acid was injected in a direction opposite to that direction in which 5 wt% NaCl solution was injected. The initial permeability was established in the first injection of 5 wt% NaCl solution, and the final permeability, which represents the result of acid stimulation, was calculated in the second injection of 5 wt% NaCl solution. Therefore, the data collected during the injection of main acid is not important for permeability evaluation. In comparison of the permeability values, the effect of acid can be evaluated, and the stimulation ratio also can be decided.


## 3. Results

### 3.1. XRD Results before Flooding

Since the design of acid concentration depends on mineral contents of sandstone, it is required to detect the mineral contents of each core plug by XDR analysis technique before core flooding.The XRD result of A1 core is shown in Figure2, which indicates the sandstone core consists of 54.2% clay, 17.6% limestone, 13.4% ankerite, and 5.4% quartz. This core plug will be flooded by the proprietary composite mud acid.Figure 2
XRD result of A1 core plug.


### 3.2. SEM Results before Flooding

The scanning electron microscope (SEM) technique was utilized to obtain microstructure of samples as small as few microns in diameter. The SEM result of A1 core is illustrated in Figure3. It shows that illite and smectite appear to bridge pore throats, and most pores are not interconnected through pore throats. Thereby, the permeability can be improved by dissolving the minerals plugged at pore throats, and the porosity can be enlarged by acid etch of quartz grains.Figure 3
SEM result of A1core plug.


### 3.3. Core Flooding

#### 3.3.1. New Multichemical Mud Acid

The new proprietary mud acid, consisting of 2% multichemical solutions (MCS), 6% HCl and 1.5% HF, is applied in the flooding experiments for a group of core plugs. The A1 core plug was injected sequentially with 3 PV of 5% NaCl solution, 2% MCS+ 6% HCl+1.5% HF, and 3 PV of 5% NaCl solution. The injection rate is controlled at 1.0 cm3/min, and the stable pressure drop data was shown in Figure 4. At the stage of injecting the first 1.5 PV of the conventional mud acid, the pressure drop increased from 1.42 to 1.53 MPa and then decreased to 1.05 MPa after injecting the remaining 8.5 PV of the composition. After the final 3 PV of 5% NaCl solution was injected, the pressure drop kept at 1.04 MPa steadily. In comparison of the pressure drop at the two injecting stages of NaCl, it illustrated that the new composite mud acid enhanced the flowing ability of core plug dramatically.Figure 4
Pressure drop across A1 core injected with composite mud acid.Figure5 shows the result of permeability improvement by injecting the new mud acid system through the A1 core plug. The new mud acid system increased the permeability about 300% by injecting approximately 10 pore volumes of the core plug; thus, the acidizing effect is significant. The multichemical solution is capable of increasing viscosity, creating microfoams, and decreasing surface tension. Therefore, after the conventional mud acid is added with this solution, the complex fluid will decrease acid reacting rate with sandstone and will flow evenly into pores of the core plug. The result may be attributed to the superior ability of the new mud acid system to slowly but thoroughly dissolve clay and feldspar components.Figure 5
Permeability of A1 core flooding by composite mud acid.Another series of five experiments were also performed where the new mud acid system which contains 2%MCS, 6%HCl, and 1.5%HF was flooded to react with a group of sandstone plugs. The results of permeability enhancement were given in Table1. For the cores, the permeability ratio of after and before flooding is 2.94, and it indicated that the core was stimulated thoroughly. Although the other five cores were also enhanced, the results were different with each other, and the average permeability enhancement ratio is about 307%. From the experimental results, it is considered that the application of the new mud acid system is desirable and prospective for sandstone stimulation.Table 1
The flooding results of group core plugs.
No.Length, cmDiameter, cmK1, mdϕ1, %K2, mdϕ2, %K2/K1ϕ2/ϕ115.412.5412.5212.8936.7615.592.941.2125.382.5410.539.7623.6512.882.251.3235.332.549.559.1432.3211.523.381.2645.452.5413.4113.2623.5814.981.761.1355.372.548.037.9334.6810.784.321.3665.282.547.487.6428.469.093.811.19


### 3.4. XRD Analysis after Core Flooding

After 16 pore volumes injected, XRD on coreflood was performed to detect the mineral contents. The result can be compared with the mineral contents before flooding experiments, thereby what have been dissolved by acid will be determined. Since HCl react rapidly with the iron containing minerals of pyrite and siderite, it may lead to iron precipitation. Because the new mud acid system contains surfactant, which will play the role of sequestering, therefore, the iron precipitation can be avoided or prevented in the process of acidizing. Figure6 is the XRD analysis result of A1 core after flooding by the new mud acid system, and the comparison of mineral contents was listed in Table 2. It is confirmed that feldspars and micas have been dissolved completely by HF; meanwhile, clays and quartz were also dissolved partly by HF. Limestone and other iron containing minerals were removed by HCl. In addition, due to the decrease of mineral composition in core plug A, the proportion of clay increases, but the absolute content of clay is relatively reduced. The dissolving ability of the new mud acid system is more superior than conventional mud acid.Figure 6
XRD result of A1 core after flooding.Table 2
The mineral content comparison of A1 core before/after acid flooding.
Core plug AMineral types and contentsClay, %Quartz, %Potassic feldspar, %Feldspar, %Limestone, %Ankerite, %Pyrite, %Siderite, %Micas, %Before acid flooding5.23.35.217.15.42.71.65.354.2After acid flooding2.3006.220.46.30064.9


### 3.5. SEM Analysis after Core Flooding

After the core plug was flooded, a thin section or slice was cut from the core plug at the position of acid outlet. SEM analysis was conducted on the rock section.A thin slice was also cut from A1 core plug flooded by the creative composite mud acid, and the SEM result was shown in Figure7. It shows that cores were enlarged and interconnected after acidizing, and most of the clays have been removed. The quartz crystals were also etched or dissolved by acid, and most parts of clays filled at pore throats were dissolved. Because cores were interconnected with each other, thereby the permeability of the core plug was enhanced significantly by acidizing. These results confirm that new mud acid give a higher stimulation than conventional mud acid in sandstones. Meanwhile, the results indicate that the new mud acid system reacts extensively with both the clay and quartz fraction, and the reaction with clays is retarded hence allowing deeper penetration of live acid.Figure 7
SEM result of A1 core.


### 3.6. Retarded Test

For retarded test, an equal weight of one gram was reacted with excess acid at atmospheric pressure and temperature; meanwhile, the reaction time and the weight of rock particles were recorded. The reaction time was plotted against dissolved ratio of sandstone, which is the weight ratio of dissolved rock against the particle sample (Figure8).Figure 8
Retarded test of composite and conventional mud acid.At the time of 200 minutes, the dissolved weight ratio is 25% for conventional mud acid, and its dissolving capacity attained the maximum value, which no longer increased. For the new mud acid system, its dissolved weight ratio is 10% at the time of 200 minutes, but the ratio was rising along with time, and it would take more than 300 minutes to attain the max dissolving limit. It is evident that the retardation of the new mud acid is superior to the commonly used mud acid. Reaction rate of the new mud acid system with sandstone was much lower than conventional mud acid; therefore, the penetration of the new mud acid system will be much deeper. The difference in response between these two acid systems was attributed to the superior ability of the new mud acid system to slowly but thoroughly dissolve and sequester the siderite and chlorite (iron containing) components.


## 3.1. XRD Results before Flooding

Since the design of acid concentration depends on mineral contents of sandstone, it is required to detect the mineral contents of each core plug by XDR analysis technique before core flooding.The XRD result of A1 core is shown in Figure2, which indicates the sandstone core consists of 54.2% clay, 17.6% limestone, 13.4% ankerite, and 5.4% quartz. This core plug will be flooded by the proprietary composite mud acid.Figure 2
XRD result of A1 core plug.


## 3.2. SEM Results before Flooding

The scanning electron microscope (SEM) technique was utilized to obtain microstructure of samples as small as few microns in diameter. The SEM result of A1 core is illustrated in Figure3. It shows that illite and smectite appear to bridge pore throats, and most pores are not interconnected through pore throats. Thereby, the permeability can be improved by dissolving the minerals plugged at pore throats, and the porosity can be enlarged by acid etch of quartz grains.Figure 3
SEM result of A1core plug.


## 3.3. Core Flooding

### 3.3.1. New Multichemical Mud Acid

The new proprietary mud acid, consisting of 2% multichemical solutions (MCS), 6% HCl and 1.5% HF, is applied in the flooding experiments for a group of core plugs. The A1 core plug was injected sequentially with 3 PV of 5% NaCl solution, 2% MCS+ 6% HCl+1.5% HF, and 3 PV of 5% NaCl solution. The injection rate is controlled at 1.0 cm3/min, and the stable pressure drop data was shown in Figure 4. At the stage of injecting the first 1.5 PV of the conventional mud acid, the pressure drop increased from 1.42 to 1.53 MPa and then decreased to 1.05 MPa after injecting the remaining 8.5 PV of the composition. After the final 3 PV of 5% NaCl solution was injected, the pressure drop kept at 1.04 MPa steadily. In comparison of the pressure drop at the two injecting stages of NaCl, it illustrated that the new composite mud acid enhanced the flowing ability of core plug dramatically.Figure 4
Pressure drop across A1 core injected with composite mud acid.Figure5 shows the result of permeability improvement by injecting the new mud acid system through the A1 core plug. The new mud acid system increased the permeability about 300% by injecting approximately 10 pore volumes of the core plug; thus, the acidizing effect is significant. The multichemical solution is capable of increasing viscosity, creating microfoams, and decreasing surface tension. Therefore, after the conventional mud acid is added with this solution, the complex fluid will decrease acid reacting rate with sandstone and will flow evenly into pores of the core plug. The result may be attributed to the superior ability of the new mud acid system to slowly but thoroughly dissolve clay and feldspar components.Figure 5
Permeability of A1 core flooding by composite mud acid.Another series of five experiments were also performed where the new mud acid system which contains 2%MCS, 6%HCl, and 1.5%HF was flooded to react with a group of sandstone plugs. The results of permeability enhancement were given in Table1. For the cores, the permeability ratio of after and before flooding is 2.94, and it indicated that the core was stimulated thoroughly. Although the other five cores were also enhanced, the results were different with each other, and the average permeability enhancement ratio is about 307%. From the experimental results, it is considered that the application of the new mud acid system is desirable and prospective for sandstone stimulation.Table 1
The flooding results of group core plugs.
No.Length, cmDiameter, cmK1, mdϕ1, %K2, mdϕ2, %K2/K1ϕ2/ϕ115.412.5412.5212.8936.7615.592.941.2125.382.5410.539.7623.6512.882.251.3235.332.549.559.1432.3211.523.381.2645.452.5413.4113.2623.5814.981.761.1355.372.548.037.9334.6810.784.321.3665.282.547.487.6428.469.093.811.19


## 3.3.1. New Multichemical Mud Acid

The new proprietary mud acid, consisting of 2% multichemical solutions (MCS), 6% HCl and 1.5% HF, is applied in the flooding experiments for a group of core plugs. The A1 core plug was injected sequentially with 3 PV of 5% NaCl solution, 2% MCS+ 6% HCl+1.5% HF, and 3 PV of 5% NaCl solution. The injection rate is controlled at 1.0 cm3/min, and the stable pressure drop data was shown in Figure 4. At the stage of injecting the first 1.5 PV of the conventional mud acid, the pressure drop increased from 1.42 to 1.53 MPa and then decreased to 1.05 MPa after injecting the remaining 8.5 PV of the composition. After the final 3 PV of 5% NaCl solution was injected, the pressure drop kept at 1.04 MPa steadily. In comparison of the pressure drop at the two injecting stages of NaCl, it illustrated that the new composite mud acid enhanced the flowing ability of core plug dramatically.Figure 4
Pressure drop across A1 core injected with composite mud acid.Figure5 shows the result of permeability improvement by injecting the new mud acid system through the A1 core plug. The new mud acid system increased the permeability about 300% by injecting approximately 10 pore volumes of the core plug; thus, the acidizing effect is significant. The multichemical solution is capable of increasing viscosity, creating microfoams, and decreasing surface tension. Therefore, after the conventional mud acid is added with this solution, the complex fluid will decrease acid reacting rate with sandstone and will flow evenly into pores of the core plug. The result may be attributed to the superior ability of the new mud acid system to slowly but thoroughly dissolve clay and feldspar components.Figure 5
Permeability of A1 core flooding by composite mud acid.Another series of five experiments were also performed where the new mud acid system which contains 2%MCS, 6%HCl, and 1.5%HF was flooded to react with a group of sandstone plugs. The results of permeability enhancement were given in Table1. For the cores, the permeability ratio of after and before flooding is 2.94, and it indicated that the core was stimulated thoroughly. Although the other five cores were also enhanced, the results were different with each other, and the average permeability enhancement ratio is about 307%. From the experimental results, it is considered that the application of the new mud acid system is desirable and prospective for sandstone stimulation.Table 1
The flooding results of group core plugs.
No.Length, cmDiameter, cmK1, mdϕ1, %K2, mdϕ2, %K2/K1ϕ2/ϕ115.412.5412.5212.8936.7615.592.941.2125.382.5410.539.7623.6512.882.251.3235.332.549.559.1432.3211.523.381.2645.452.5413.4113.2623.5814.981.761.1355.372.548.037.9334.6810.784.321.3665.282.547.487.6428.469.093.811.19


## 3.4. XRD Analysis after Core Flooding

After 16 pore volumes injected, XRD on coreflood was performed to detect the mineral contents. The result can be compared with the mineral contents before flooding experiments, thereby what have been dissolved by acid will be determined. Since HCl react rapidly with the iron containing minerals of pyrite and siderite, it may lead to iron precipitation. Because the new mud acid system contains surfactant, which will play the role of sequestering, therefore, the iron precipitation can be avoided or prevented in the process of acidizing. Figure6 is the XRD analysis result of A1 core after flooding by the new mud acid system, and the comparison of mineral contents was listed in Table 2. It is confirmed that feldspars and micas have been dissolved completely by HF; meanwhile, clays and quartz were also dissolved partly by HF. Limestone and other iron containing minerals were removed by HCl. In addition, due to the decrease of mineral composition in core plug A, the proportion of clay increases, but the absolute content of clay is relatively reduced. The dissolving ability of the new mud acid system is more superior than conventional mud acid.Figure 6
XRD result of A1 core after flooding.Table 2
The mineral content comparison of A1 core before/after acid flooding.
Core plug AMineral types and contentsClay, %Quartz, %Potassic feldspar, %Feldspar, %Limestone, %Ankerite, %Pyrite, %Siderite, %Micas, %Before acid flooding5.23.35.217.15.42.71.65.354.2After acid flooding2.3006.220.46.30064.9


## 3.5. SEM Analysis after Core Flooding

After the core plug was flooded, a thin section or slice was cut from the core plug at the position of acid outlet. SEM analysis was conducted on the rock section.A thin slice was also cut from A1 core plug flooded by the creative composite mud acid, and the SEM result was shown in Figure7. It shows that cores were enlarged and interconnected after acidizing, and most of the clays have been removed. The quartz crystals were also etched or dissolved by acid, and most parts of clays filled at pore throats were dissolved. Because cores were interconnected with each other, thereby the permeability of the core plug was enhanced significantly by acidizing. These results confirm that new mud acid give a higher stimulation than conventional mud acid in sandstones. Meanwhile, the results indicate that the new mud acid system reacts extensively with both the clay and quartz fraction, and the reaction with clays is retarded hence allowing deeper penetration of live acid.Figure 7
SEM result of A1 core.


## 3.6. Retarded Test

For retarded test, an equal weight of one gram was reacted with excess acid at atmospheric pressure and temperature; meanwhile, the reaction time and the weight of rock particles were recorded. The reaction time was plotted against dissolved ratio of sandstone, which is the weight ratio of dissolved rock against the particle sample (Figure8).Figure 8
Retarded test of composite and conventional mud acid.At the time of 200 minutes, the dissolved weight ratio is 25% for conventional mud acid, and its dissolving capacity attained the maximum value, which no longer increased. For the new mud acid system, its dissolved weight ratio is 10% at the time of 200 minutes, but the ratio was rising along with time, and it would take more than 300 minutes to attain the max dissolving limit. It is evident that the retardation of the new mud acid is superior to the commonly used mud acid. Reaction rate of the new mud acid system with sandstone was much lower than conventional mud acid; therefore, the penetration of the new mud acid system will be much deeper. The difference in response between these two acid systems was attributed to the superior ability of the new mud acid system to slowly but thoroughly dissolve and sequester the siderite and chlorite (iron containing) components.


## 4. Discussions

The new mud acid system formulated by combining the multichemical solutions with HCl and HF has shown some highly beneficial and unique properties. The most unique features of this new mud acid system are as follows: (1) the reaction rate with clays is retarded compared to conventional mud acid systems. (2) It displays homogeneous profile for acid displacement by foams’ diverting. (3) It exists other beneficial properties, such as suspending clay fines and avoiding precipitations.


### 4.1. Retarded Effect of Acid

The reaction between acid and sandstone is heterogeneous reaction, which characterizes their reaction only on the contact surface. This reacting process can be composed of three steps: the first step is that hydrogen ion (H+) transmits onto the rock surface; the second step is that hydrogen ion reacts with carbonate, clays, and feldspars; and the final step is that the reaction products, such as Ca2+, Mg2+, and CO2, leave off the rock surface. The acid reaction rate with sandstone depends on the above three steps and especially on the slowest one of them.In fact, the reaction between hydrogen ion (H+) and sandstone is so fast that it will instantly finish when hydrogen ion (H+) contacts with sandstone. After H+ reacts with sandstone, some reaction products, such as Ca2+, Mg2+, and CO2, will accumulate in liquid layer near sandstone surface. This liquid layer, containing reaction products, is very thin and is called as diffusion layer. Its character is different from the inner part of acid, since there is no ion concentration difference in the inner part of acid; however, there exists ion concentration difference in the diffusion layer at the direction perpendicular to sandstone surface (Figure 9). In this diffusion layer, reactants and reaction products will transmit in opposite directions under the action of ion concentration gradient. Therefore, this ion transmitting is named as ion diffusion. Besides this ion diffusion action, there also exists convection action in the diffusion layer, which is caused naturally by density difference. In words, hydrogen ion (H+) is dependent mainly on convection and diffusion action that it could transmit from diffusion layer to rock surface. The rate, which H+ attaints to the rock surface from diffusion layer, is called as mass transfer rate.Figure 9
Concentration distribution in diffusion layer for acid-rock reaction.Based on the above three reacting steps, the acid reaction rate with sandstone will depend on the mass transfer rate of H+, the H+ reacting rate on rock surface, and leaving rate of reaction products from rock surface. Among of the three rates, the mass transfer rate of H+ determines the whole reacting rate between acid and sandstone, because the mass transfer rate of H+ is much slower than the other two rates. Therefore, if the mass transfer rate of H+ can be lowered down, then the whole reacting rate between acid and rock will become slower. When the surfactant is added into the conventional mud acid, the composite liquid will become sticky, and there will form a viscous, thin layer, or film on the sandstone surface. This viscous layer or film will control the mass transfer rate of H+ and make it decrease dramatically. Meanwhile, kinetic studies have shown that the reaction rate of mud acid with clays and feldspars are directly related to both the hydrogen ion concentration (principally from the HCl) as well as the effective concentration of HF [32–34]. In the case of HF action on pure silica (equivalent to quartz sand grains), these same kinetic studies show that the reaction rate is related only to the HF concentration, and that this reaction is primarily controlled by thermodynamic factors. Therefore, in comparison with the conventional mud acid, the reaction rate of the composite mud acid will be retarded significantly.Besides, the effect of viscosity on acid penetration, foams, produced by surfactant, also will be helpful to deepen acid penetration in sandstone formation. Because foams can lower the liquid saturation and accumulate at the surface of sandstone rock, thus, plenty of foams can lower the liquid permeability and decrease the amount of live acid that leaks off from rock pores. Therefore, foams will prevent acid reaction with sandstone at the rock surface, and more live acid will penetrate deeply into sandstone formation.


### 4.2. Other Advantages

In the multichemical solution, it also contains clay stabilizer and dispersant. The clay stabilizer will chemically stabilize clay fines and leave clays in a water-wet state. The dispersant will make fines suspending in the spent acid liquid, to prevent solids plugging at pores and throats. Besides, it can increase the viscosity of acid and sustain stable foams, and the nonionic surfactant also can remain good water-wetting characteristics and low surface tension after acidizing. These functions ensure that the new composite mud acid is generally nonemulsifying with most formation fluids and prevent the secondary precipitation in spent acid liquid. In summary, the combined constituents in the multichemical solutions are fully compatible and provide a synergistic mud acid system that is safe to handle and use. It also exhibits most of the desirable properties required for sandstone matrix acidizing.


## 4.1. Retarded Effect of Acid

The reaction between acid and sandstone is heterogeneous reaction, which characterizes their reaction only on the contact surface. This reacting process can be composed of three steps: the first step is that hydrogen ion (H+) transmits onto the rock surface; the second step is that hydrogen ion reacts with carbonate, clays, and feldspars; and the final step is that the reaction products, such as Ca2+, Mg2+, and CO2, leave off the rock surface. The acid reaction rate with sandstone depends on the above three steps and especially on the slowest one of them.In fact, the reaction between hydrogen ion (H+) and sandstone is so fast that it will instantly finish when hydrogen ion (H+) contacts with sandstone. After H+ reacts with sandstone, some reaction products, such as Ca2+, Mg2+, and CO2, will accumulate in liquid layer near sandstone surface. This liquid layer, containing reaction products, is very thin and is called as diffusion layer. Its character is different from the inner part of acid, since there is no ion concentration difference in the inner part of acid; however, there exists ion concentration difference in the diffusion layer at the direction perpendicular to sandstone surface (Figure 9). In this diffusion layer, reactants and reaction products will transmit in opposite directions under the action of ion concentration gradient. Therefore, this ion transmitting is named as ion diffusion. Besides this ion diffusion action, there also exists convection action in the diffusion layer, which is caused naturally by density difference. In words, hydrogen ion (H+) is dependent mainly on convection and diffusion action that it could transmit from diffusion layer to rock surface. The rate, which H+ attaints to the rock surface from diffusion layer, is called as mass transfer rate.Figure 9
Concentration distribution in diffusion layer for acid-rock reaction.Based on the above three reacting steps, the acid reaction rate with sandstone will depend on the mass transfer rate of H+, the H+ reacting rate on rock surface, and leaving rate of reaction products from rock surface. Among of the three rates, the mass transfer rate of H+ determines the whole reacting rate between acid and sandstone, because the mass transfer rate of H+ is much slower than the other two rates. Therefore, if the mass transfer rate of H+ can be lowered down, then the whole reacting rate between acid and rock will become slower. When the surfactant is added into the conventional mud acid, the composite liquid will become sticky, and there will form a viscous, thin layer, or film on the sandstone surface. This viscous layer or film will control the mass transfer rate of H+ and make it decrease dramatically. Meanwhile, kinetic studies have shown that the reaction rate of mud acid with clays and feldspars are directly related to both the hydrogen ion concentration (principally from the HCl) as well as the effective concentration of HF [32–34]. In the case of HF action on pure silica (equivalent to quartz sand grains), these same kinetic studies show that the reaction rate is related only to the HF concentration, and that this reaction is primarily controlled by thermodynamic factors. Therefore, in comparison with the conventional mud acid, the reaction rate of the composite mud acid will be retarded significantly.Besides, the effect of viscosity on acid penetration, foams, produced by surfactant, also will be helpful to deepen acid penetration in sandstone formation. Because foams can lower the liquid saturation and accumulate at the surface of sandstone rock, thus, plenty of foams can lower the liquid permeability and decrease the amount of live acid that leaks off from rock pores. Therefore, foams will prevent acid reaction with sandstone at the rock surface, and more live acid will penetrate deeply into sandstone formation.


## 4.2. Other Advantages

In the multichemical solution, it also contains clay stabilizer and dispersant. The clay stabilizer will chemically stabilize clay fines and leave clays in a water-wet state. The dispersant will make fines suspending in the spent acid liquid, to prevent solids plugging at pores and throats. Besides, it can increase the viscosity of acid and sustain stable foams, and the nonionic surfactant also can remain good water-wetting characteristics and low surface tension after acidizing. These functions ensure that the new composite mud acid is generally nonemulsifying with most formation fluids and prevent the secondary precipitation in spent acid liquid. In summary, the combined constituents in the multichemical solutions are fully compatible and provide a synergistic mud acid system that is safe to handle and use. It also exhibits most of the desirable properties required for sandstone matrix acidizing.


## 5. Conclusions

A new mud acid system has been developed, and it has proven to be effective in overcoming the drawbacks associated with sandstone stimulation by conventional mud acid. It can be concluded as follows:(1)
The new proprietary mud acid is the blend of regular mud acid with multichemical solutions, which contain surfactant, clay stabilizer, and dispersant specifically formulated(2)
Retarded tests testify that the retardation of the new mud acid is superior to the commonly used mud acid. Reaction rate of the new mud acid with sandstone was much lower than conventional mud acid; hence, the new composite mud acid can dissolve components of siderite and chlorite slowly and thoroughly(3)
Based on XRD and SEM analysis before core flooding, the composition of HCl and HF is designed properly. The results of XRD and SEM analysis were compared before/after core flooding, it indicated that most of clays and some parts of quartz were dissolved by the new composite mud acid, and the permeability was increased distinctly(4)
The new mud acid system retains an important property that can slower acidizing reaction rate significantly, and the penetration of live acid can be deeper than conventional mud acid in sandstone matrix stimulation. The limitation of rapid spending rate for conventional mud acid can be overcome, and deep penetration can be achieved by the new composite mud acid



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*Source: 2874912-2022-07-19.xml*