An important part of geothermal development is the re-injection of waste geothermal fluids into the reservoir, which can extend the life of the geothermal resource. However, the difficulty of recharge and the clogging problem in the recharge process are still a worldwide problem that restricts the sustainable development and utilization of sandstone geothermal reservoirs. The main factor affecting the recharge effect is physical clogging. In this paper, a series of simulation experiments were conducted to study the physical clogging in the recharge process of sandstone thermal storage in the Lubei area, which conclude that the degree of clogging is related to the particle size and concentration of suspended matter. At a certain particle size, the clogging is most serious when the experimental suspended matter concentration gradient is up to 140 mg/L. In addition, particle migration in clogging is related to the recharge flow rate, and the critical flow rate is 0.75–2 mL/min, which is experimentally derived, and it is recommended that the recharge rate should be controlled below 0.75 mL/min during the actual recharge. This paper provides a scientific basis for the efficient recharge of geothermal tailwater through the study of the clogging mechanism.

  • The mechanism of action of sandstone thermal storage recharge blockage is studied by theoretical analysis and simulation experiments.

  • The action process of suspension blockage and the mechanical characteristics of particle transport were studied.

  • A variation law of clogging is experimentally derived.

  • A solid foundation is laid for the efficient recharge of sandstone thermal storage.

Graphical Abstract

Graphical Abstract

In recent years, with the continuous exploitation of geothermal resources, there has been a loss in the balance of extraction and irrigation of geothermal water, resulting in the continuous decline of geothermal water levels and the formation of land subsidence and sinkholes. The unreasonable use of geothermal resources and the random discharge of thermal fluids have caused a large amount of wasted resources and caused more serious thermal and chemical pollution problems; geothermal recharge technology is an important technical means to guarantee the sustainable development of geothermal fields and protect the surface environment (Wang et al. 2022b). In order to solve these problems, a number of geothermal tailwater recharge experiments have been carried out in the Lubei area, with relatively significant results. ‘Taking heat without water’ by ensuring the geothermal tailwater is irrigated as much as possible is an important principle to realize for the full utilization of geothermal resources and resource conservation (Liu 2022).

Underground hot water resources are a kind of clean energy and a renewable resource, so they have high development and utilization value (Yang et al. 2020). Although the massive exploitation of geothermal water has reduced the consumption of fossil energy and lowered carbon emissions, the efficient utilization of geothermal water is still difficult in China, as the development of geothermal wells brings substances deep underground with complex compositions to the surface along with geothermal water (Yu & Ou 2020), and the direct discharge of geothermal water that has been used only once will cause thermal pollution and chemical pollution, and is a waste of water resources. At the same time, the groundwater table is not replenished, which can cause a series of environmental problems such as groundwater level decline, potential geological disasters, and resource depletion (Wang et al. 2022a). Fan (2009) proposed that human unreasonable exploitation and use of groundwater, ground subsidence, underground fractures, lava collapse, and other disasters will appear one after another, and geothermal tailwater recharge is particularly important for these problems.

Geothermal tailwater recharge is the re-injection of geothermal tailwater into the thermal storage with the main purposes of disposing of geothermal wastewater, reducing environmental pollution, restoring the heat production capacity of the thermal storage, maintaining fluid pressure in the thermal storage, and maintaining the exploitable conditions of the geothermal field (Wang et al. 2021). The recharge efficiency of geothermal tailwater is currently the primary problem that restricts the development and utilization of geothermal resources in China, and the problem of clogging is the main factor that affects efficient recharge (Raskovic et al. 2013); the proportion of each type of clogging is shown in Table 1, among which physical clogging is the most important factor that restricts the recharge efficiency in the recharge system. A schematic diagram of the clogging process during recharge is shown in Figure 1.
Table 1

Percentage of each clogging in recharge wells

Physical blockage
Chemical blockageMicrobial blockageGas blockageOther
Blockage type Clogging with suspended matter Particle transport 15 10 10 
Percentage % 50 10 
Physical blockage
Chemical blockageMicrobial blockageGas blockageOther
Blockage type Clogging with suspended matter Particle transport 15 10 10 
Percentage % 50 10 
Figure 1

Schematic diagram of clogging in the process of geothermal water recharge.

Figure 1

Schematic diagram of clogging in the process of geothermal water recharge.

Close modal

At present, the research on the physical clogging effect and mechanism of large suspended particles (particle size >30 μm) is relatively mature, but the research on the clogging mechanism of intermediate particles (particle size of 1–30 μm) is relatively weak (Li et al. 2010). In the study of the mechanism of physical clogging during recharge, it was found in sand tank infiltration tests that the aqueous medium porosity was reduced due to the adsorption of suspended particles aqueous medium particles, thus causing physical clogging (Wu 2022). In addition to the difficulty of recharge, which is closely related to a thermal reservoir's fracture development and the smoothness of the underground hot water migration channel, the water quality characteristics of recharge also directly affect the recharge effect (Wan 2018; Liu et al. 2019). There are many examples of research looking to solve the difficulties of recharge technology: for example, Li & Wang (2021) analyzed the geological characteristics of thermal reservoirs and the difficulties of drilling and completion technology of recharge wells, and applied the drilling and injection completion process of oil and gas wells to geothermal recharge wells, and conducted research on drilling tool combinations, drilling fluids and completion technology, and developed drilling and completion technology for geothermal recharge wells, but there is little research on the mechanism of recharge clogging problems.

This research used an indoor one-dimensional sand column percolation test to design different sizes of concentration and particle size of suspended material to configure geothermal tailwater to simulate the phenomenon of suspended material clogging in the sand layer during recharge, and derive its mechanism of action, which shows that under the filtering and adsorption of suspended particles, to a large extent, the particles are deposited on the surface layer of the medium, causing the surface layer of the sand column to be clogged more seriously than the deeper part. The clogging simulation experiments were conducted at different concentrations to study the characteristics and laws of clogging. The microscopic mechanism of action of particle migration was studied through the mechanical properties of particle migration, and the critical flow rate of particle migration was derived from sensitivity experiments based on cores and geothermal tailwater obtained in the field. This paper differs from previous studies in the quantitative study of the physical clogging mechanism, and its innovation lies in the conclusion of a law of suspended clogging and the determination of the critical flow rate of recharge. The conclusions and understanding obtained from the study provide technical support for the study of sandstone geothermal tailwater recharge clogging, adding valuable experience for the management of recharge clogging in the future, and provide a scientific basis for the sustainable utilization of sandstone geothermal resources.

The process of suspension clogging

Current research on the mechanism of suspended matter clogging is mainly done through indoor simulation experiments, including the study of the clogging process and influencing factors. In terms of location, suspended matter clogging in the process of geothermal tailwater recharge is divided into surface clogging and deep-bed formation clogging. Surface clogging is formed under the action of filtration and sedimentation; when the diameter of suspended particles is larger than the diameter of the pores of the infiltration medium, the suspended matter is intercepted on the surface of the medium, and with the extension of recharge time, the thickness of the filter cake accumulates and the resistance of water passing through the filter cake increases until it is finally completely clogged, as shown in Figure 2. The main feature of internal clogging is that the final state of recharge is only the retention of suspended matter inside the medium, while the surface of the medium has no retained suspended matter, as shown in Figure 3.
Figure 2

Schematic diagram of surface clogging process.

Figure 2

Schematic diagram of surface clogging process.

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Figure 3

Schematic diagram of the deep-bed formation clogging process.

Figure 3

Schematic diagram of the deep-bed formation clogging process.

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The microscopic mechanisms of deep-bed formation clogging are mainly filtering, precipitation, inertia, diffusion, and hydrodynamic force effects, of which, the filtering effect is when the diameter of suspended particles is smaller than the media pore width, so that the suspended particles collide with the pore wall and are thus filtered out. Sedimentation refers to the process of downward migration of suspended materials with water flow; some particles are precipitated by their gravity in the pore channels, causing a reduction in reservoir pore space, resulting in a decrease in the permeability of the reservoir. Inertia refers to when suspended matter in the water encounters an obstruction, and through inertia the particles maintain the original state of motion, offset the direction of the water flow, and collide with the pore wall, and are thus retained in the media pores. The diffusion effect is mainly for very small particles of particle size 0–2 μm in the reservoir interior; if the conditions change caused by the enhancement of fine particles, then Brownian Motion improves the chance of collision between the particles and the media so that it is easy to deposit in the pore space. The influence of the hydraulic effect on clogging is relatively weak. This means that when the infiltrating water flow is laminar, there is a certain velocity gradient between the water flow at the pore side wall and inside the pore, which causes the particles to move in a direction at a certain angle to the water flow under the action of lateral shear stress, making some particles become blocked in the pore channels, as shown in Figure 4. Generally, surface clogging and deep-bed formation clogging can occur simultaneously during recharge.
Figure 4

Schematic diagram of the microscopic action mechanism of deep-bed formation clogging. (a) Filtering effect. (b) Diffusion effect. (c) Inertia effect. (d) Sedimentation. (e) Hydrodynamic force effects.

Figure 4

Schematic diagram of the microscopic action mechanism of deep-bed formation clogging. (a) Filtering effect. (b) Diffusion effect. (c) Inertia effect. (d) Sedimentation. (e) Hydrodynamic force effects.

Close modal

Suspension clogging simulation experiment

Experimental setup and material

3.2.1.1. Experimental setup

The main part of the test device consisted of a plexiglass column, peristaltic pump, water supply tank, water control head and other devices, of which the plexiglass column is 100 cm high, with 11 cm internal diameter; the right side of the plexiglass column has four pressure measurement holes distributed from top to bottom. A stirrer in the water supply tank ensures the return water concentration is of uniform distribution, and a water control head device can freely adjust the difference between the upper and lower water level to facilitate the determination of the difference in water head on the clogging. The impact of the head difference on clogging can be easily measured. The experimental setup is shown in Figure 5.
Figure 5

One-dimensional sand column experimental setup. 1-Test bench; 2-Water supply tank; 3-Stirrer; 4-Peristaltic pump; 5,10-Fixed water head; 6-Overflow port; 7-Water inlet; 8-Pressure measuring hole; 9-Ruler; 11-Water outlet.

Figure 5

One-dimensional sand column experimental setup. 1-Test bench; 2-Water supply tank; 3-Stirrer; 4-Peristaltic pump; 5,10-Fixed water head; 6-Overflow port; 7-Water inlet; 8-Pressure measuring hole; 9-Ruler; 11-Water outlet.

Close modal

Other experimental equipment: used included: a content measuring instrument, flow meter, fixed head device, peristaltic pump, water pipe, straightedge bucket, stopwatch, measuring cylinder, beaker, rubber hose, several glass tubes, several water stop clamps, glass glue, wrench, standard sieve, scales, and so on.

3.2.1.2. Infiltration medium

Natural crystal was acid washed to make chemically stable high-purity quartz sand, which has 99% SiO2 content and strong corrosion resistance (Figure 6). Therefore, this high-purity quartz sand was used as the filling medium of the sand-filled pipe, and the particle size range was 0.155–0.195 mm.

3.2.1.3. Returning of hot storage tailwater

The composition of the recharge tailwater consists of two parts: water and suspended matter. The water source was Qingdao municipal tap water; the suspended matter was replaced by diamond sand which is stable in nature. Considering the different particle size classes of suspended particles in geothermal tailwater, two sizes of suspended particles, of 15 μm and 30 μm, were selected to configure the simulated recharge tailwater for the experimental study, and the recharge water with suspended concentrations of 20 mg/L, 60 mg/L, 100 mg/L, 140 mg/L and 200 mg/L was prepared for experiments with different suspended particle sizes and concentrations.
Figure 6

High-purity quartz sand.

Figure 6

High-purity quartz sand.

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Experimental procedure

  • (1)

    Reinstallation of geothermal tailwater: the recharge waters with suspended matter particle sizes of 0.040 mm and 0.060 mm were prepared for different suspended matter particle size experiments. Suspended matter concentrations of 20 mg/L, 60 mg/L, 100 mg/L, 140 mg/L, and 200 mg/L were used for different suspended matter particle size and concentration experiments.

  • (2)

    Loading sample: to prevent the loss of filtering material, the bottom of the Plexiglas column was filled with a 400 mm thickness bedding layer; the washed, dried and sterilized sand samples were loaded into the Plexiglas column in layers and compacted, during which pure water was slowly injected into the sand column to exclude residual gas in the column and gradually complete the process of filling the column with water. The total amount of pure water added during the loading process was recorded, and the overall height of quartz sand loaded into the sand column was 90 cm.

  • (3)

    After the water level in the pressure measuring tube was stabilized, the water level value was read and the initial permeability coefficient K0 of the sand column was calculated.

  • (4)

    Geothermal tailwater recharge experiment stage: the suspended liquid with a concentration of 20 mg/L and a suspended particle size of 0.040 mm was used to conduct a constant head recharge experiment, and the head values and discharge flow rates in each pressure measuring tube were recorded at different moments to obtain the permeability coefficients Kt of different layers in the Plexiglas column at different times. The final permeability coefficient K was obtained by also observing the formation of the infiltration clogging layer at the same moment.

  • (5)

    Separate recharge experiments were conducted with suspensions of different particle sizes and concentrations to investigate the clogging pattern. A total of 18 sets of experiments were conducted during an experimental period of 3 months. The specific experimental flow chart is shown in Figure 7.

Figure 7

Experimental flow chart.

Figure 7

Experimental flow chart.

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The permeability coefficient K is obtained by Darcy's law (Equation (1)), as follows:
(1)

where: Kt is the water medium permeability coefficient at moment t (m/d); Q is the water flow (m3/d); △x is the distance between any two pressure measurement tubes (m); △H is the difference between any two pressure measurement tube heads (m); d is the sand column inner diameter (m).

The ratio of the permeability coefficient of the water-bearing medium to the initial permeability coefficient of the sand layer at different periods, i.e. the relative permeability coefficient, is used to reflect the degree of clogging in the system:
(2)
where: Kt is the water permeability coefficient at moment t (m/d); K0 is the media initial permeability coefficient (m/d).

Analysis and discussion of experimental results

3.2.3.1. Different suspended matter particle size experiments

The particle size and concentration characteristics of suspended matter are the main reasons affecting the location of clogging generation. Following the variation of particle size and concentration of suspended matter in geothermal tailwater, experiments were conducted under the same hydrodynamic conditions (head difference △H = 105 cm), to quantitatively analyze the occurrence and development process of clogging according to the variation of relative permeability coefficient K’ in each section. The experimental control conditions are shown in Table 2.

Table 2

Experimental control conditions for recharge of each seepage section with different suspended particle sizes

GroupSand column height (cm)Seepage section (cm)△H(cm)Initial seepage velocity (m/d)The particle size of suspended matter (μm)Suspended matter concentration (mg/L)Refill time
90 AB: 0–20 105 16.7 15 200 90 
30 
BC: 20–40 15 
30 
CD: 40–90 15 
30 
GroupSand column height (cm)Seepage section (cm)△H(cm)Initial seepage velocity (m/d)The particle size of suspended matter (μm)Suspended matter concentration (mg/L)Refill time
90 AB: 0–20 105 16.7 15 200 90 
30 
BC: 20–40 15 
30 
CD: 40–90 15 
30 
Table 3

Control conditions for recharge tests in each seepage section with different suspended matter concentrations

GroupSand column height (cm)Seepage section (cm)△H(cm)Initial seepage velocity (m/d)The particle size of suspended matter (μm)Suspended matter concentration (mg/L)Refill time (h)
90 AB: 0–20 105 16.7 30 20 90 
60 
100 
140 
BC: 20–40 20 
60 
100 
140 
CD: 40–90 20 
10 60 
11 100 
12 140 
GroupSand column height (cm)Seepage section (cm)△H(cm)Initial seepage velocity (m/d)The particle size of suspended matter (μm)Suspended matter concentration (mg/L)Refill time (h)
90 AB: 0–20 105 16.7 30 20 90 
60 
100 
140 
BC: 20–40 20 
60 
100 
140 
CD: 40–90 20 
10 60 
11 100 
12 140 

Figure 8a shows that, in the early stage of recharge, the relative infiltration coefficient of recharge water with suspended matter particle size of 15 μm in AB section decreased from 1 to 0.25, while the relative infiltration coefficient of recharge water with suspended matter particle size of 30 μm decreased to 0.15, both showing a rapid decreasing trend, and obviously the relative infiltration coefficient of the latter decreased more than that of the former. In the late stage of recharge, the AB section was recharged with different particle size suspensions, and the relative permeability coefficients of the media were reduced to about 0.06 after recharge to basically maintain a stable state; Figure 8b indicates that, in the early stage of recharge, the relative permeability coefficient of the recharge water recharge in the BC section with a suspended particle size of 15 μm decreased from 1 to 0.45, and the relative permeability coefficient of the recharge water recharge with a suspended particle size of 30 μm decreased to 0.4, although the decrease in the latter was greater than the former, but it was significantly greater than the relative permeability coefficient of the AB section, thus showing that the degree of clogging became smaller and smaller as the depth increased. It can be seen from Figure 8c that the relative permeability coefficients drop to about 0.77 for both particle sizes and basically stop changing, which further verifies the idea that the clogging degree becomes smaller with increasing depth.

Overall, the changes in the curve of the permeability coefficient from Figure 8 show that in the process of physical clogging caused by suspended matter, the size of suspended matter particle size can affect the time of clogging of thermal storage media: the larger the suspended matter particle size, the earlier the clogging occurs and the more serious the clogging is, while the degree of clogging becomes smaller as the depth increases.

3.2.3.2. Experiments with different suspended matter concentrations

In the recharge test, the relative permeability coefficients K’ of each percolation section AB, BC, and CD were measured with recharge time under a particle size of 30 μm and the concentration of suspended matter in the recharge solution of 20, 60, 100, and 140 mg/L, respectively (Table 3), and the experimental results are shown in Figures 9a, 9b, and 9c.
Figure 8

Relative permeability coefficient K’ time t(h) relationship curve for each seepage section under different suspended matter particle size conditions.

Figure 8

Relative permeability coefficient K’ time t(h) relationship curve for each seepage section under different suspended matter particle size conditions.

Close modal
Figure 9

Relative permeability coefficient K’ time t(h) relationship curve for each seepage section under different suspended matter concentration conditions.

Figure 9

Relative permeability coefficient K’ time t(h) relationship curve for each seepage section under different suspended matter concentration conditions.

Close modal

As can be seen from Figure 9a, for the sand layer in section AB, the K′ of the sand layer decreased to 0.71, 0.42, 0.32 and 0.26 after 90 h of recharge, indicating that the greater the concentration of suspension, the faster the rate of surface clogging and the higher the degree of clogging. As can be seen from Figure 9b, for the sand layer of the BC section, when the concentration of suspended solids in the recharge fluid is 20 mg/L, the permeability of the sand layer hardly changes during the recharge process, while under the conditions of 60, 100 and 140 mg/L concentration, the relative permeability coefficients of the sand layer decrease to 0.74, 0.58 and 0.5 at the end of the experiment, respectively, indicating that the suspended particles can migrate with the fluid in the water-bearing medium and affect the permeability of the sand layer in this section: the higher the suspension concentration, the more suspended particles are deposited, and the more obvious the reduction of permeability. From Figure 9c, it can be seen that for the sand layer of the CD section, the difference of aquifer permeability decrease is not obvious at 20 mg/L and 60 mg/L concentrations, indicating that the migration of suspended particles to the deeper part is somewhat restricted; when the suspended concentration is 100 and 140 mg/L, the sand column K′ become 0.76 and 0.75, respectively, at the end of the experiment, indicating that the clogging degree is no longer obvious with the increase of depth.

It can be concluded that the higher the concentration of suspended matter, the greater the degree of clogging, and the degree of clogging is different in different permeable sections. The suspended particles are first deposited in the surface layer of the sand column, which inhibits the downward migration of suspended particles to a certain extent, and the deeper the sand layer is located, the smaller the degree of clogging, i.e., clogging mainly occurs in the surface layer.

Mechanical properties of particle migration

In the process of geothermal tailwater recharge, the recharge flow rate is determined by the difference between the recharge pressure and the recharge thermal storage pressure, and the groundwater seepage rate increases with the increase of this pressure difference. At the moment of recharge, particles with poor cementation properties are easily dislodged and migrated and accumulate at pore throats causing clogging, referred to as particle migration clogging (Yu et al. 2021; Mao et al. 2022); in the petroleum industry, this is known as velocity sensitivity, which refers to the fact that micro-particles migrate with the fluid during core replacement, thus causing clogging of the core, and different injection rates affect the migration of micro-particles, thus causing a reduction in the rock formation permeability.

The solid phase particle clogging in the geothermal tailwater recharge process mainly comes from two aspects: first, pore surface deposition and pore throat interception caused by the invasion of foreign suspended particles in the recharge water; second, when the water quality of the recharge water is not compatible with the formation, sensitive clay minerals in the pore space will swell and disperse to produce new particles, while various removable particles previously deposited on the pore surface may also appear to disperse and migrate.

As shown in Figure 10, from the mechanical point of view, the above dispersion process of particles is mainly controlled by a complex mechanical system consisting of Van der Waals gravitational potential energy between particle surfaces, double electric layer repulsion potential energy, Born-Mayer short-range repulsion potential, hydrokinetic energy of injected fluid, the gravitational potential energy of particles, Brownian diffusion energy, and other factors. When the balance of these mechanical relationships is disrupted, it will cause expansion, dispersion, and particle migration of clay minerals in geothermal recharge wells, and form clogging at the pore throats, resulting in a serious decrease in reservoir permeability.
Figure 10

Particle migration characteristics.

Figure 10

Particle migration characteristics.

Close modal
Figure 11

Rock mineral and clay mineral content of a geothermal reservoir.

Figure 11

Rock mineral and clay mineral content of a geothermal reservoir.

Close modal

Particle migration simulation experiment

Rock type and physical characteristics

The rock type is mainly feldspathic sandstone with a high average quartz content, followed by feldspar, calcite, dolomite, and rhodochrosite, and high clay mineral content with volume fractions ranging from 8.00% to 25.00%, with an average value of 16.5%. The clay minerals contained include four minerals, montmorillonite, kaolinite, chlorite, and a small amount of illite, and the measured volume fractions were average (Figure 11).

Experimental setup and material

This experiment used the TC-III differential seepage replacement synthesis equipment shown in Figure 12; a sketch of the device is also shown in Figure 13. The experimental materials used were cores taken from sandstone thermal storage in the Lubei area (Nos. 2, 6 and 15) and geothermal tailwater taken from the sites.
Figure 12

The core repulsion test set.

Figure 12

The core repulsion test set.

Close modal
Figure 13

Flow chart of sensitivity experiment. 1-Repulsion power unit; 2-Sealed pressure-resistant vessel (containing fluid, acid-base solution, etc.); 3-Filter; 4-Valves; 5-Pressure gauge; 6-Peripheral pressure pump; 7-Clamp; 8-Return valve; 9-Flow meter.

Figure 13

Flow chart of sensitivity experiment. 1-Repulsion power unit; 2-Sealed pressure-resistant vessel (containing fluid, acid-base solution, etc.); 3-Filter; 4-Valves; 5-Pressure gauge; 6-Peripheral pressure pump; 7-Clamp; 8-Return valve; 9-Flow meter.

Close modal

Experimental procedure and evaluation criteria

  • (1)

    Clean the cores. Clean the cores and dry them for 3–5 days for gas permeability testing.

  • (2)

    Load the rock sample. Place the rock sample into the core holder and ensure that the fluid injection end of the rock sample is the same as the injection end of the gas-measured permeability to ensure constant permeability.

  • (3)

    Empty the rock sample repulsion device. Increase the surrounding pressure to 2 MPa; fill the intermediate vessel with simulated formation water, exhaust the intermediate vessel and core holder with a small flow rate by advection pump, close the exhaust valve when a continuous liquid droplet appears in the core holder exhaust valve, and the emptying of the repulsion system is completed.

  • (4)

    Conduct experiments. Different injection rates (0.1, 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 6.0 mL/min) are used to inject the simulated geothermal tailwater into the cores, and the permeability of the cores at different injection rates are measured and recorded.

  • (5)

    End the experiment. During the experiment, as the flow rate increases, once the rate of change of rock permeability is greater than 20% (corresponding to the flow rate increases of the previous point) that is the critical flow rate: if the critical flow rate occurs, the flow interval can be increased; if the injection flow rate has not reached 6.0 mL/min or the injection pressure gradient is greater than 2.0 MPa/cm, end the experiment. The experimental flow chart is shown in Figure 14.

Figure 14

Experimental flow chart.

Figure 14

Experimental flow chart.

Close modal

In this experiment, three sets of experiments were conducted for three cores, which took one month. The velocity-sensitive damage index (clogging rate) is proportional to the permeability damage and inversely proportional to the critical flow rate, i.e., the higher the permeability damage and the smaller the critical flow rate, the stronger the velocity sensitivity. The formula for calculating the velocity-sensitive damage index is as follows, and the velocity-sensitive evaluation criteria are shown in Table 4.

Table 4

Velocity sensitivity evaluation criteria

Velocity Sensitivity Index (Clogging Rate)Degree of damage
<0.05 No damage 
0.05–0.3 Weak damage 
0.3–0.5 Moderate to weak damage 
0.5–0.7 Moderate to strong damage 
>0.7 Strong damage 
Velocity Sensitivity Index (Clogging Rate)Degree of damage
<0.05 No damage 
0.05–0.3 Weak damage 
0.3–0.5 Moderate to weak damage 
0.5–0.7 Moderate to strong damage 
>0.7 Strong damage 

The velocity-sensitive damage index (clogging rate) is evaluated with the following formula:
(3)
where: Rv-velocity-sensitive damage index; KD-clogging rate; Kmax-maximum core permeability, ×10−3μm2; Kmin-minimum core permeability.
Table 5

Degree of core damage

Core numberKmaxKminCritical Flow RateClogging rateDegree of damage
0.9 0.48 0.47 Moderate to weak damage 
81.52 63.76 0.5 0.22 Weak damage 
15 39.45 17.33 0.75 0.44 Moderate to weak damage 
Core numberKmaxKminCritical Flow RateClogging rateDegree of damage
0.9 0.48 0.47 Moderate to weak damage 
81.52 63.76 0.5 0.22 Weak damage 
15 39.45 17.33 0.75 0.44 Moderate to weak damage 

Analysis and discussion of experimental results

The experimental data of three cores were analyzed, the clogging rates were calculated, and the clogging of the formation caused by particle migration was judged according to the evaluation criteria, as indicated by the test analyses (Figures 15, 16, and 17; Table 5). The purpose of velocity-sensitive evaluation experiments is: to determine the critical flow rate of particle migration damage due to flow rate, and the degree of geothermal reservoir damage; to provide a basis for determining a reasonable experimental flow rate for the next various damage evaluations; to provide a basis for determining a reasonable recharge rate.

While previous authors have only analyzed the sensitivity of oil and gas field reservoirs, this experiment analyzed the sensitivity of geothermal reservoirs. It can be seen that, at the beginning of the particle migration experiment, as the injection flow gradually increased, the permeability of the three rock samples showed sawtooth-shaped fluctuations, which was due to the geothermal tailwater entering the rock samples, which washed away some of the particles in the original pores of the samples, making the permeability have a small upward lift, and as the particle migration experiment continued, the permeability of all the rock samples showed a decreasing and relatively gentle trend, only declining relatively significantly after the injection flow reached the critical flow. However, the decline was not great and the reservoir showed moderately weak damage in general, which, combined with the analysis of the clay mineral composition test results, is due to the presence of the clay minerals kaolinite and illite in the reservoir that caused the particle migration clogging. It is indicated that this sandstone thermal reservoir will be affected by particle migration to a certain extent, and the critical flow rate is in the range of 0.75–2 ml/min. Care should be taken to control the recharge rate below the critical flow rate of 0.75 ml/min during actual recharge.

When the injection flow rate of the rock samples in the study area increased to a certain value, microparticles such as kaolinite and illite were dislodged, and with the increasing flow rate of the injected water, the previously dislodged particles were migrated into narrow pores and throats. However, the core in this study area is relatively dense, the pore and throat radii are small and the microfractures are narrow, so the particles do not undergo a large degree of migration, which is not easy to observe, so the next section analyzes the magnitude of migration before and after particle migration by laser particle size measurement (Figure 18).
Figure 15

Velocity sensitivity curve of rock sample No. 2.

Figure 15

Velocity sensitivity curve of rock sample No. 2.

Close modal
Figure 16

Velocity sensitivity curve of rock sample No. 6.

Figure 16

Velocity sensitivity curve of rock sample No. 6.

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Figure 17

Velocity sensitivity curve of rock sample No. 15.

Figure 17

Velocity sensitivity curve of rock sample No. 15.

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Figure 18

Schematic diagram of particle migration.

Figure 18

Schematic diagram of particle migration.

Close modal

Comparative analysis of grain size of core sand before and after particle migration

The length of the core used in this experiment is relatively short, and the particle migration itself is difficult to observe with the naked eye. To more accurately describe the effect of particle migration out of the sand within the core, particle size analysis of the core before and after the experiment was carried out using a laser particle size meter, which can be selected for automatic test measurement. After the instrument itself has been operated to detect the optical path and eliminate bubbles, the sample to be measured is slowly added manually in the sample window, and the particle size curve and other parameters can be analyzed. The experimental setup is shown in Figure 19.
Figure 19

Laser particle size analyzer.

Figure 19

Laser particle size analyzer.

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Figure 20

Particle size analysis before and after migration of core No. 2 particles.

Figure 20

Particle size analysis before and after migration of core No. 2 particles.

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Figure 21

Particle size analysis before and after migration of core No. 6 particles.

Figure 21

Particle size analysis before and after migration of core No. 6 particles.

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Figure 22

Particle size analysis before and after migration of core No. 15 particles.

Figure 22

Particle size analysis before and after migration of core No. 15 particles.

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As can be seen from the results curves before and after the particle migration experiments by the laser particle size meter (Figures 20, 21 and 22), the overall core particle size cumulative mass percentage after the experiments is smaller than that of the original cores, and the median particle size is slightly lower, indicating a certain degree of a velocity-sensitive particle migration phenomenon; however, the magnitude of particle size change is not very obvious, verifying the moderately weak velocity-sensitive damage in this sandstone geothermal reservoir.

  • (1)

    It is concluded from the study that both surface clogging and deep-bed formation clogging can occur, and in most cases both types of clogging will occur at the same time, mainly surface clogging. It is concluded that the permeability coefficient of the water-bearing medium shows the law of ‘rapid decline – stabilization’ in the process of recharge, i.e. serious physical clogging occurs in the initial stage of recharge.

  • (2)

    The particle size of the suspended matter can affect the time of clogging of the thermal storage medium. When the concentration of suspended matter is known, the larger the particle size is, the earlier the clogging occurs and the more serious the clogging is. In this case, the experimental particle size is 30 μm.

  • (3)

    The degree of clogging of the aqueous medium increases with increased concentration of suspended particles in the recharge fluid, at a certain suspended particle size, set to 30 μm; the experimental concentration gradient was set to 20, 60, 100, and 140 mg/L, and the degree of clogging was largest when it was 140 mg/L; at the same time, with the increase of depth, the degree of clogging becomes smaller and smaller, that is, the clogging is more serious in the surface layer.

  • (4)

    By analyzing the mineral content of sandstone thermal reservoir rocks, the mechanical properties of particle migration were studied, and particle migration simulation experiments were conducted. The results showed that velocity-sensitive damage should be reduced by controlling the recharge flow rate below 0.75 mL/min, while the moderately weak velocity-sensitive damage of this target sandstone thermal reservoir was verified by a laser particle size analyzer.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict of interest.

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