Study on prevention and control measures of sandstone geothermal reinjection plugging

For chemical plugging, geothermal tailwater and formation water can be mixed under certain temperature and pressure conditions, and the compatibility and scaling type of geothermal tailwater and formation water can be determined by observing whether precipitation is generated (Zhang 2014). Microbial plugging is the formation plugging phenomenon caused by the poor water quality of the reinjection water source and the high content of organic substances such as carbon, nitrogen and phosphorus, which has changed the original water environment to a certain extent after reinjection. It mainly includes total nitrogen, total phosphorus, dissolved organic carbon concentration, temperature, dissolved oxygen, oxidation–reduction potential and organic matter concentration (Du et al. 2009). Gas plugging means that geothermal tailwater reinjection will carry gas back into the formation. When the gas content in the tailwater is relatively high, with the change of temperature and pressure, the dissolved gas in the tailwater will appear in the form of bubbles in the formation. The bubbles will gradually reduce with the hydrodynamic force, the pressure will become stable, and stay in the pore medium (Li et al. 2021). Therefore, given this series of plugging, it is urgent to study the prevention and control measures in the process of reinjection in the sandstone geothermal area of northern Shandong.

This paper puts forward corresponding prevention and treatment measures for various kinds of blocking. For physical blocking, the filtration performances of zeolite, ceramsite and quartz sand are compared, and the economical and efficient quartz sand filter material is selected as the filter material for primary filtration, and non-toxic and odorless PP cotton is selected as the filter material for secondary filtration. For chemical plugging, polyepoxysuccinate (PESA) was selected as the scale inhibitor through a simple evaluation experiment and comparison of the scale inhibition performance of PESA and hydrolyzed polymaleic anhydride (HPMA). Based on using the intermittent recharge method, chlorine dioxide is selected as the sterilizing agent, and an appropriate amount of organic phenol solution is added at the same time to achieve a better effect of eliminating microbial blockage. The gas blockage can effectively change the pressure difference at both ends of the bubble and reduce the gas blockage by using the method of regular lifting. The innovation of this paper is to design a set of reinjection and technical processes suitable for the sandstone geothermal area in northern Shandong, which is different from the traditional reinjection process by referring to a series of blockage prevention measures formulated, which can greatly improve the reinjection efficiency and thus improve the development and utilization efficiency of geothermal resources.

There are many kinds of filter materials on the market, and how to select the filter materials reasonably is the premise of economic and efficient reinjection. This section evaluates the performance of three kinds of filter materials, namely zeolite, ceramsite and quartz sand, studies the removal performance of suspended solids, and selects the most economical and efficient filter materials added to the filter during reinjection.

• (1)

  Weigh 50, 100, 150 and 200 g with electronic balance, and the median particle size is 10 μ mix m of carborundum with a proper amount of water, prepare four parts of raw water solution, put them under the stirrer and mix for 15 min, add the raw water solution into the raw water bucket with a volume of 1,000 L according to the concentration required by the experiment, and open the stirrer above the raw water bucket for mixing.

• (2)

  Three kinds of filter media, i.e. zeolite, ceramsite and quartz sand with a particle size of 0.8–1.2 mm, are selected, and the filter layer height is 100 cm.

• (3)

  Set the filtration rate of 10 m/h and the filtration period of 8 h, and measure the content of suspended solids in the filtered water with a suspended solids meter every 1 h.

Table 1 shows the concentration data of suspended solids in filtered water after filtration with three kinds of filter media when the concentration of suspended solids is 100 mg/L.

Under standard experimental conditions, through the indoor filtration performance evaluation of zeolite, ceramsite and quartz sand in a filtration cycle, it is found that the average suspended solid concentrations at the filtration level of zeolite, ceramsite and quartz sand are 7.51, 6.63 and 5.42 mg/L, respectively, and the average removal rates of suspended solids are 92.49, 93.37 and 94.58%, respectively. Therefore, it can be concluded that the performance of the three filter materials for filtering suspended solids is: quartz sand > ceramsite > zeolite. At the same time, compared with other filter media, quartz sand filter media also have the following characteristics:

• (1)

  No impurities, high density, strong ability to retain dirt;

• (2)

  Stable chemical properties, high mechanical strength and strong compression resistance;

• (3)

  Good hydrophilicity, appropriate specific gravity, easy filtration and backwashing, and strong reproducibility;

• (4)

  High hardness, excellent corrosion resistance, no need to replace filter media frequently, long service life, reduced maintenance costs and maintenance time, low operating costs and high utilization efficiency.

Combined with the excellent filtration performance and its own characteristics of quartz sand filter material, quartz sand can be selected as the filter material during the tailwater reinjection, which can greatly improve the reinjection efficiency and reduce the reinjection cost. Since physical blockage accounts for a large proportion of the blockage in the whole reinjection process, a secondary filtration device can be set in the tailwater filtration process to greatly reduce the damage of physical blockage to the formation, set the primary filter tank and select the quartz sand filter material with strong filtering performance as the filter medium to filter most of the impurities. The secondary filter can use the non-toxic and odorless polypropylene particles as the raw material and PP cotton made through a series of processing as the filter material. The high-quality PP cotton filter element can be used in large quantities in tailwater purification. According to the experimental data, the curve of suspended matter concentration of filtered water with time was plotted in Figure 3, and the data were organized to plot the curve of average removal rate of suspended matter as shown in Figure 4. The main functions of PP cotton as the filter material are:

• (1)

  It can effectively remove all kinds of particulate impurities in the filtered liquid and has a large number of contaminants.

• (2)

  High filtration flow, low differential pressure, no chemical binders, more hygienic and safe.

• (3)

  It integrates a surface, and deep layer, has coarse and fine filtration, has good chemical stability, corrosion resistance, high-pressure resistance and low cost, and can effectively block rust, sediment, microorganism and other granular substances in water.

At the same time, the establishment of the corresponding automatic control system for geothermal tailwater treatment can achieve the goals of higher backwashing efficiency of the filter and more convenient blowdown of the filter tank, which can not only reduce labor intensity but also improve the treatment efficiency of geothermal tailwater, improve the water quality standard of the filtered tailwater and further meet the water quality requirements of the tailwater.

It can be seen from Figure 6 that the scale inhibition performance of PESA and HPMA increases with the increase of the dosage. When the dosage reaches 15 mg/L, the scale inhibition rates of PESA and HPMA remain at 91 and 84%, respectively, basically unchanged. At the same concentration, the scale inhibition performance of PESA is better than that of HPMA. This is because PESA introduces oxygen atoms into the molecules of HPMA, which improves its scale inhibition performance and reduces the dosage under the same scale inhibition effect. Therefore, according to the scaling characteristics of sandstone formation in the Lubei area, the PESA scale inhibitor can have a better scale inhibition effect.

The growth and reproduction of microorganisms in the underground environment are affected by many factors. Therefore, when formulating prevention and treatment strategies for microbial clogging, attention should not only be focused on microbial activity and growth inhibition but also fully consider the impact of reinjection water on clogging. In order to effectively solve the problem of microbial clogging in aquifers, it is necessary to start from two aspects of early prevention and late treatment, with prevention first and treatment second.

Reinjection water quality is an important factor that restricts the efficiency of artificial underground reinjection. The content of microorganisms in different water quality is different, and the quality of water quality directly determines the occurrence of reinjection well plugging and the degree of reinjection well plugging. It is very important to strictly control the reinjection water quality index. The level of nutrients in the reinjection water will affect the concentration of nutrients in the formation of raw water. The type and quantity of bacteria in the tailwater will increase the risk of microbial plugging. Before reinjection, the concentration of nutrients in the influent must be strictly controlled to prevent plugging. In the process of recharging, the endogenous respiration of microorganisms consumes cellular components and extracellular polymers. After stopping for a period of time, it is found that the extracellular polysaccharide secreted by microorganisms in the medium is completely degraded. Therefore, intermittent recharging can be used to alleviate the medium blockage. According to the growth cycle of microorganisms, the stopping time is generally 7–15 days.

Microbial growth and reproduction promote the formation of biofilm that blocks the surface and interior of the formation to form a microbial blockage. Sterilization and disinfection of the reinjection tailwater are important prerequisites to control the occurrence of microbial blockage. Sterilization treatment by adding disinfectant can reduce the occurrence of biological blockage. Common sterilization treatments include oxidant disinfection, ozone disinfection, chlorine dioxide disinfection, silver ion disinfection and ultrasonic disinfection.

• (1)

  Disinfection by adding oxidant: The traditional chlorination disinfection method is that chlorine dissolves in water and generates hypochloric acid, which damages the cell membrane by changing the permeability of microbial cells, interferes with and destroys the microbial enzyme system, and causes the coagulation and denaturation of microbial proteins and the release of proteins, nucleic acids and other substances, thereby causing bacterial death. The lower the pH, the better the disinfection effect.

• (2)

  Ozone disinfection: Ozone is a strong oxidant, and ozone disinfection is a biochemical oxidation reaction. It can destroy the structure of cell membrane, cause bacteria to distort and die, oxidize the enzymes needed to decompose the glucose in the bacteria, destroy the cell tissue, block the metabolism, and penetrate the cells to dissolve and die. Ozone has an inactivation effect on almost all bacteria.

• (3)

  Chlorine dioxide disinfection: Chlorine dioxide can prevent the synthesis and metabolism of bacteria, inhibit the synthesis of protein, change the capsid of viruses and cause the death of bacteria and viruses. The removal effect of iron and manganese is stronger than that of chlorine. It has the characteristics of rapid and efficient sterilization, a wide pH range, and is not affected by water hardness and salinity.

• (4)

  Silver ion disinfection: Silver ion exists in water soluble form, and its oxidation can inactivate bacterial protease and cause bacterial death.

• (5)

  Ultrasonic sterilization: The cavitation effect produced by ultrasonic produces the sterilization effect. When an ultrasonic wave propagates in a liquid medium, it causes a cavitation phenomenon, which can kill bacteria, inactivate virus, and even destroy the cell wall of small microorganisms.

For microbial clogging, based on intermittent reinjection, the above five sterilization treatment methods are compared. Considering the economy and convenience, chlorine dioxide can be selected as the sterilization agent in the process of tailwater reinjection, which has the advantages of a good sterilization effect and low price. At the same time, adding an appropriate amount of organic phenol solution can achieve a better effect of eliminating microbial clogging.

In order to prevent gas from blocking the formation, a bypass pipe can be added before the desander. When there is no sand in the water to be removed, the desander can be turned off and the water can pass through the bypass pipe to reduce the gasification caused by the reinjection water flowing through the desander. At the same time, an exhaust device is installed between the reinjection wellhead and the booster pump. The exhaust device is generally an exhaust tank with a simple principle to discharge the excess gas in the geothermal tailwater. At the same time, the bubbles carried in the tailwater are broken by changing the external conditions of the fluid. However, it must be noted that the automatic exhaust valve should be set on top of the tank body. When the gas accumulates to a certain extent, it can be released to the outside of the tank in time to ensure safety. If the capacity of the gas contained in the geothermal tailwater is high, the exhaust duct should be used to exhaust the released gas from the equipment room to prevent poisoning and fire caused by gas accumulation for a long time.

Under the condition of ensuring the efficiency of reinjection, reducing the reinjection flow rate is also one of the methods to slow down gas blockage. The smaller the flow rate of tailwater from the machine room to the reinjection well and from the wellhead to the aquifer, the longer the bubble floats, and the more conducive to the bubble dissipation. Therefore, the reinjection pipe should be as thick as possible, the diameter of the good pipe should be as large as possible, the reinjection pipe in the well should be shorter and the outlet of the pipe should be slightly higher than the static water level. For the coarse sand aquifer, it can be designed as one pumping and two loops, and there should be more reinjection wells for the aquifer below the coarse sand.

Geothermal tailwater reinjection can be divided into three types: gravity reinjection, pressurized reinjection and vacuum reinjection. In the case of no booster pump pressurization, the geothermal tailwater after utilization is directly injected into the reinjection well for reinjection, namely gravity reinjection, also known as non-pressure self-flow reinjection. This method applies to the aquifer with low water levels and good permeability. When the booster pump is used for pressurization, the used geothermal tailwater is injected into the reinjection well for reinjection, that is, pressurized reinjection. If natural reinjection is used, vacuum reinjection can be used when the reinjection water carries a lot of bubbles. The ultimate purpose of the three is to recharge the geothermal tailwater to the same formation as the production well to maintain the formation pressure and supplement the formation energy.

According to the various prevention and control measures proposed for plugging, a set of tailwater reinjection process is designed. The geothermal water is extracted from the geothermal well and then transferred to the reinjection well after heat exchange and utilization to ensure that the tailwater can be efficiently reinjected to the corresponding formation. The specific process is as follows:

• (1)

  Sterilization and disinfection: the geothermal tailwater generated after heat exchange and utilization should be sterilized and disinfected first to prevent the impact of microbial blockage.

• (2)

  Desanding: the cyclone desander is used to remove the sand and impurities in the geothermal tailwater and reduce the workload of the rear filter.

• (3)

  Control flow rate: in order to prevent the damage caused by particle migration to the formation, a flow rate controller is installed to control the flow rate that is lower than the critical flow rate when the formation is affected by velocity sensitivity;

• (4)

  Scale removal: a scale inhibitor is added to remove the scale caused by chemical action.

• (5)

  Secondary filtration: according to the formation characteristics of the geothermal field, the suspended particles with different particle sizes in the geothermal tailwater are filtered, and the secondary filtration device is designed, with the primary filtration accuracy of 50 μm, the secondary filtration accuracy is 5 μm, this process can not only filter the suspended solids in the reinjection fluid but also filter out some microorganisms, effectively preventing physical and biological plugging during reinjection in the well. During installation, pressure monitoring instruments should be installed at both ends of the filter inlet and the outlet, and the backwashing time should be determined according to the pressure change to ensure the filtering effect.

• (6)

  Exhaust: an exhaust device at the reinjection wellhead is installed to remove the gas in the geothermal tailwater. The working pressure should be higher than the maximum working pressure of the reinjection system and set as the last process to ensure that the gas in the geothermal tailwater is fully released before reinjection.

This paper mainly aims at the plugging problem of geothermal tailwater in the process of reinjection, formulates corresponding prevention measures, and establishes a complete set of reinjection and technical processes. The main conclusions are as follows:

• (1)

  By comparing the performance of three commonly used filter materials, it is concluded that the ability of the three filter materials to remove suspended solids is quartz sand > ceramsite > zeolite. According to the filtration ability of the three filter materials, the economical and efficient quartz sand filter material is finally selected as the filter material for primary filtration.

• (2)

  By comparing the scale inhibition performance of PESA and HPMA, the scale inhibitor PESA with green environmental protection and high-scale inhibition efficiency was selected.

• (3)

  For microbial clogging, the method of intermittent recharge is proposed, and five common sterilization treatment methods are introduced, including oxidant disinfection, ozone disinfection, chlorine dioxide disinfection, silver ion disinfection and ultrasonic disinfection. Finally, cheap chlorine dioxide is selected as the sterilization agent in the process of tailwater recharge.

• (4)

  For gas plugging, the working principle of the exhaust device is introduced, and the method of back lifting is proposed to exhaust the blocked gas. It is necessary to control the number of back lifting to protect the original formation structure.

• (5)

  Finally, a complete set of reinjection process flow is given for various types of plugging. Compared with the traditional plugging treatment process, the new process will greatly improve the reinjection efficiency and can be applied in engineering practice. Through efficient reinjection, the goal of improving the economic and efficient development and utilization of geothermal resources will be finally achieved.

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

The authors declare there is no conflict.