1. Introduction
The lithology of China’s karst stratum is predominantly limestone, and the geological conditions are complex. The soluble rock distribution area accounts for more than one-third of the land area, making it one of the countries with the world’s largest karst distribution areas and karst development types. Deep resource extraction, energy extraction, and urban underground space development are just a few of the underground initiatives that are becoming more prevalent as the social economy expands. Karst slopes, foundation stability, karst water gushing, mud outburst, and karst collapse negatively affect structure foundations and pose significant safety and investment risks to engineering construction. So, to carry out engineering construction in karst areas and to improve the stability of engineering structures and later safety maintenance, it is necessary to understand the characteristics of rock dissolution law in soluble rock areas, predict its development trend, and provide assurance for early identification and safety evaluation of karst geological disasters. Europe, the United States, and other countries and regions have carried out large-scale karst landforms [
1,
2], hydrogeology [
3,
4], karst environment [
5,
6], and other aspects to further develop karst research.
The research methods can be divided into dissolution mechanism theory research and karst detection research. Dissolution tests are one of the most significant contents and methods for researching the dissolution mechanism, which is a significant part of theoretical research to investigate further the controlling factors and impacts of carbonate rock dissolution. Since the 1970s, scholars have conducted experimental research on carbonate rock dissolution under thermal, hydraulic, and chemical factors. Eisenlohr et al. [
7], Sjoberg et al. [
8], Amrhein et al. [
9], Buhmann et al. [
10], and Ma et al. [
11] systematically studied carbonate rock dissolution and precipitation and summarized the kinetic factors. Plumer et al. [
12,
13] further found the calcite surface dissolution reaction law in saturated CO
2 water solution under different test conditions and its dynamic process. Alkattan et al. [
14], Pokrovsky et al. [
15], and Sjoberg et al. [
16] studied the dissolution rate of rock samples and their influencing factors. The study of a single mineral’s characteristics of dissolution under the control of a single factor in the research has advanced to the simulation of seepage and dissolution in the rock under the joint control of multiple means. Experimental conditions have evolved from early low-temperature, low-pressure conditions [
17,
18] to high-temperature, high-pressure conditions [
19,
20,
21] and from closed-system, static conditions [
22,
23] to open-system, flow conditions [
24,
25,
26]. Rock samples from limestone, dolomite, and other different sources [
27], different types [
28,
29,
30,
31], and different types of seawater, acetic acid, natural water body, and CO
2 water solutions [
30,
32,
33,
34,
35] are gradually increasing. This study investigates the effect of various external factors on carbonate rock dissolution by varying the experimental control factors. According to the study’s findings on the effect of temperature on carbonate rock dissolution, some researchers think that normal and medium temperature conditions are preferable to low and high temperatures for carbonate solution to facilitate carbonate rock dissolution [
36]. Concerning the impact of flow rate on the dissolution of carbonate rocks, Gao [
37] discovered that the dissolution of carbonate rocks by solution under flowing conditions is always greater than that under closed conditions, and Shao’s [
38] experiment confirmed that the change in flow rate has a significant impact on the dissolution rate. Robert et al. [
19], Yang et al. [
39], Shou et al. [
40], and Liu et al. [
25] investigate the impact of pressure on carbonate rock dissolution and have a consistent understanding of the pressure factor carbonate rock dissolution; that is, carbonate rock dissolution increases with increasing pressure. However, previous tests did not include the coupling of water flow speed and dynamic water pressure, and its dissolution effect and development law. The pH value of carbonic acid solution obviously influences the dissolution rate of rock samples [
41,
42,
43,
44], and the dissolution rate increases with decreasing pH value. Zhao et al. [
45] established that the dissolution rate in acid is approximately 20–400 times that in alkaline and neutral conditions.
Many scholars performed an experimental study by self-designing experimental equipment, constantly updating and enhancing various control factors of the experiment from the standpoints of solution preparation, pressure system, and hydrodynamic system, and further exploring the dissolution effect of multifactor coupling of carbonate rocks. There are two solution preparation systems: organic acid preparation [
46,
47] and carbonic acid solution preparation [
22]. The most common organic acid in buried diagenetic oilfield water is acetic acid, which is produced from deionized water and pure acetic acid reagent. She et al. [
48] and Yang [
49] use acetic acid solution. Although it is simple to prepare and produce accurate results, its applicability is only limited. Carbonate solution preparation, the most crucial dissolution medium for carbonate under near-surface supergene conditions, is the other type of solution preparation. This preparation technique is employed by Wang et al. [
50], Shao [
26], Gao [
37], and Fan et al. [
51] for their experiments. Gas and liquid must be introduced into a larger container that can be used continuously. It has many flaws although it has a broad range of applications. The instrument is large and difficult to override, the water supply tank is positioned at a high point, and the reaction vessel has a large volume. In earlier testing, the larger vessel’s volume reached 1500 L. However, the long-term dissolution test could not be performed even with such a massive tank. Second, the configuration time is long, and the dissolution reaction time of most experiments is 7 days, with some going as long as 15 days. Even with the magnetic drive stirring device, the reaction will be slow. Third, the accuracy is low, and there are numerous impacting factors in the reaction process, including the absolute pressure of the gas–liquid system, liquid temperature, contact area and time between gas and liquid, and so on. The flow rates of water and CO
2 must be constantly adjusted, and adequacy must be judged by the numerical value of the liquid surface pressure gauge to achieve the required pH value. Because the entire process is based on experience and repeated attempts, judging the gas–liquid reaction’s adequacy and the test results’ accuracy is impossible. The pressure system in the equipment is primarily pressurized by nitrogen and is actuated by a high-pressure pump and a back-pressure valve: nitrogen pressurization means that a high-pressure N
2 gas tank provides a specific pressure value. For example, Liu et al. [
25] formed a pressurizing and stabilizing system by sealing a high-pressure water tank, an airbag, and a pressure reducer with an automatic compensation function, which can meet the constant pressure test to exclude the influence of pressure change factors, such as constant pressure and variable flow rate. Another method of pressurization is the use of a high-pressure pump and a back-pressure valve, with the pressure supplied by the high-pressure pump driving device and stabilized by the back-pressure valve controller. Although Fan et al. [
51], Jiang et al. [
52], and She et al. [
53] achieved pressure value regulation using this method, However, the test did not perform the simultaneous regulation of the hydrodynamic system, that is, the pressure is variable while the flow rate is constant. The test equipment’s hydrodynamic system can simulate two types of water flow state: static erosion and osmotic erosion. Contact erosion occurs when there is no water head or hydraulic gradient. For example, Zhao et al. [
45] and Huang and Song [
54] used the direct immersion method to perform static erosion, and while the test procedure was simple, the actual water flow state could not be simulated. The other flow state is seepage erosion with water head or hydraulic gradient, which is classified into two types based on whether or not the flow rate changes. Fan et al. [
36,
51] and Jiang et al. achieve the fixed flow rate. Jiang et al. [
52] use the methods of fixed time, fixed flow rate, and fixed total amount. She et al. [
53] and Gao et al. [
37] employed a liquid pump to drive the flow rate, leading to a constant flow rate. Su [
55] and Shao [
26] applied a liquid pump to control the flow rate, which enabled them to observe the impact of temperature on the dissolution rate of carbonate rocks at different flow rates. However, they did not include the pressure factor, such as the flow rate.
Currently, most research on carbonate rock erosion is based on a single environmental equilibrium state with less emphasis on nonequilibrium state systems in different occurrence environments, focusing on the relationship between control factors, such as temperature, velocity, pressure, and the erosion effect. Because the test solution preparation process is complicated, the reaction time is long, and the accuracy is low, the hydrodynamic and pressure systems cannot be controlled simultaneously. In this paper, the experimental equipment for carbonate rock dissolution under dynamic water pressure is developed, and dynamic dissolution tests of carbonate rock under different flow rates and pressures are performed to address the problems of carbonated water preparation and parameter setting of the test system. The development trend of karst under thermal–hydraulic–chemical coupling conditions is predicted, and previous experimental equipment limitations are overcome. More water flow rates and pressures can be simulated in the future, allowing for a more comprehensive study of carbonate rock’s hydrochemical kinetic parameters and dissolution characteristics.
2. Test Equipment and Apparatus
A set of YYDR-2 rock hydrodynamic pressure dissolution test equipment (
Figure 1) based on microreaction technology was created using the most recent continuous flow microreaction technology. The test equipment consists of three systems: carbonated water preparation, dissolution reaction, and dissolution reception (
Figure 2). All of them are controlled and set by computer system control software. The instrument is composed of 316-L stainless steel. The following are the main technical parameters: the high-pressure constant-current infusion pump has a flow rate control range of 0.1–100 mL/min, a pressure of 10 MPa, and an accuracy of 1%. The flow range of the gas flow controller is 5–1000 sccm, the pressure is 10 MPa, and the accuracy is 0.1%. The back-pressure valve has a back-pressure range of 0.2–10 MPa; the gas separator has a pressure range of 10 MPa. The pH measuring range is 2–14, and the precision is 0.02. The temperature control range of the constant temperature water bath box is room temperature +5–200 °C, with a 0.1 °C accuracy.
2.1. Carbonated Water Preparation System
It is developed a carbonated water preparation system for real-time online configuration of solutions with different concentrations, which consists of four parts: liquid supply, gas supply, gas–liquid mixing, and storage: The liquid is extracted using a peristaltic pump, which extracts and precisely controls the liquid supply. The carbon dioxide gas is supplied by a carbon dioxide cylinder (with a two-stage pressure-reducing valve), and the flow value is precisely controlled in real time by a gas mass flow controller. The liquid is mixed with CO2 gas using the most recent continuous flow microreaction technology. The microchannel is used to carry out a gas–liquid intensified reaction to ensure rapid and sufficient mixing of gas and liquid, resulting in a carbonic acid aqueous solution with a specific pH value and stable solubility. After the gas–liquid reaction, the final 1-L solvent bottle is used as a carbonic acid aqueous solution container. The built-in pH probe and liquid level sensor can accurately monitor the pH value of the solution at any time. The preparation system can achieve a sufficient and rapid gas–liquid reaction while ensuring a steady carbonic acid aqueous solution supply.
2.2. Dissolution Reaction System
The dissolution reaction system comprises a pressure and hydrodynamic device, a temperature control device, and a sample reaction device. For example, the pressure and hydrodynamic control device raises and stabilizes the liquid pressure via the back-pressure valve and adjusts the flow rate via the high-pressure constant-current infusion pump to ensure that the solution can complete the flow rate adjustment at different pressures. Afterward, the solution flows into the sample reaction device, allowing four sample silos to be simultaneously placed within 13 mm and 25 mm. Next, the liquid enters through the upper cover’s liquid inlet, is evenly separated into four parts by the umbrella-shaped design at the top of the inlet filter seat cover and then flows into the four main reaction chambers by the similar design at the bottom of the inlet filter seat cover. The liquid is dispersed as it flows in, fully contacting the rock sample. As it flows out, 316-L stainless steel is used in the outlet filter seat cover. Then, the sample bin and preheater are placed in the oil bath thermostat to form a temperature control device, where a 316-L stainless steel coil is used as the preheating device to quickly and accurately preheat the solution delivered by the high-pressure infusion pump to the required temperature of the test, thereby realizing the room temperature −200 °C temperature control, with a larger more extensive temperature control range than the previous water bath.
2.3. Solution Receiving System
Solution condensation, gas–liquid separation, decompression and exhaust, and liquid receiving are the four components of the solution receiving system. The condensing unit also uses a 316-L stainless steel coil to achieve condensation by increasing the liquid flow distance, preventing the temperature of the reacted solution from becoming too high and damaging the test device. On the other hand, the condensing unit increases the flow distance between the sample bin and the gas–liquid separator and cools the reacted solution via air cooling. The reacted solution enters the gas–liquid separator, through the condensing device, and the gas and liquid are separated into two paths. The back-pressure valve connects the separated gas to the outlet to adjust and stabilize the fluid’s circulating pressure. The steady flow valve ensures that the separated liquid flows out of the system steadily and smoothly. The receiving container collects the reacted solution and detects additional parameters.
5. Summary and Conclusions
The self-developed carbonate dissolution test equipment was used in this study to conduct dynamic dissolution tests of carbonate rocks at different temperatures, flow rates, and hydrodynamic pressures. The dissolution effect and development law of carbonate rocks under complex conditions were discussed. The following are the main conclusions:
Self-designed and manufactured a set of carbonate rock hydrodynamic pressure dissolution test equipment simulating multifactor coupling conditions, which outperforms the previously closed dissolution test equipment. First, the smaller gas–liquid mixing microreactor was used for the first time in dissolution equipment. The micro-mixing technology ensured rapid and complete mixing of CO
2 and aqueous solution. Second, the exact configuration of carbonic acid solutions with different concentrations was completed online in real time, ensuring safe, continuous, stable, and efficient gas–liquid mixing, breaking the limitation of pH parameter selection and the stability of previous test instruments, and significantly reducing the volume of carbonated water preparation device [
26,
37,
50,
51]. Next, the test system employs computer control software to control and set related devices and parameters, which reduces the difficulty of using the instrument, ensures numerical value accuracy, and enables intuitive observation. Third, the pressure and hydrodynamic device realize the combined regulation of solution dynamic water pressure and speed, raises and stabilizes the liquid pressure via the back pressure valve and adjusts the flow rate via the high-pressure constant-current infusion pump to ensure that the solution can complete the flow rate adjustment at different pressures. It breaks through the previous test hydrodynamic system that cannot be regulated simultaneously with the pressure system [
26,
51,
52,
53,
55];
The test simulates carbonate rock’s dissolution effect in different environmental factors (such as water chemical conditions, temperature conditions, dynamic water pressure conditions, and water flow speed conditions). In the analysis of the carbonate rock dissolution rate, the dissolution amount of the carbonate rock is directly proportional to the water flow speed [
37], the dynamic water pressure [
19,
25,
39,
40], and the influence of the flow rate on the dissolution rate is more significant than the dynamic water pressure [
38]. When the water flow rate is high (75 mL/min), the change in hydrodynamic pressure slightly impacts the dissolution rate (the average dissolution rate is 5.68 mg/g). However, when the flow rate is low (15 mL/min), the hydrodynamic pressure change significantly (the average dissolution rate is 16.76 mg/g). In addition, when the hydrodynamic pressure is low (2 MPa), the change in water velocity significantly impacts the dissolution rate (the average dissolution rate is 32.23 mg/g). On the other hand, when the hydrodynamic pressure is high (6 MPa), the change in water velocity has little impact (the average dissolution rate is 21.15 mg/g). To realize the coupling of water flow speed and dynamic water pressure, it breaks through the coupling of two factors that cannot be achieved by previous test, and also lack of research of the dissolution effect and the development law under the coupling action;
CO
2 aqueous solution significantly influences limestone dissolution and is the most important carbonate dissolution medium in the near-surface supergene condition. Following dissolution, visible dissolution grooves and grooves appear on the surface of rock samples, along with new calcium carbonate deposits. The changes in the solution’s pH value, conductivity value, and Ca
2+ ion concentration show the dissolution rate and degree. The values after dissolution are greater than those before the test. The pH values were changed at +0.40, +0.80, +0.60, and +0.88, which are directly proportional to the change in the dissolution rate [
57,
58]. The conductivity values were +66 µs/cm, +28 µs/cm, +52 µs/cm and +22 µs/cm, and the Ca
2+ ion content was +13.05, +6.48 and +10.25, respectively, which were inversely proportional to the change in dissolution rate [
55]. The pH value generally follows the law of “first increasing and then decreasing, tending to be stable with a little fluctuation” during the test, which can effectively reflect the dissolution process of limestone under actual conditions.
The experimental study shows carbonate dissolution is proportional to the water flow velocity and dynamic water pressure close to the ground surface. The runoff change of groundwater near the surface obviously influences the dissolution rate. In addition, the karst develops more in the groundwater runoff than in the groundwater’s recharge and discharge areas. In the case of a slow groundwater flow rate, the dynamic water pressure greatly influences the dissolution rate, and the karst develops more. Therefore, its study has important theoretical significance and engineering application value as it provides technical means and guarantees for the early identification, karst development, and safety evaluation of karst geological disasters.
In subsequent studies, the author will present the influence and change of the law of the coupling effect of water flow velocity change, hydrodynamic pressure change, thermal–hydraulic–chemical, and other factors on the dissolution effect in a subsequent paper.