Geothermal resources development and land subsidence

Land subsidence is not only caused by regional tectonic subsidence, under-consolidated stratum densification subsidence, artificial foundation pit excavation and drainage, and ground load increase. Overexploitation of groundwater (temperature less than 25℃), geothermal fluid resources and oil and gas mineral resources in non-diagenetic reservoirs will cause reservoir pressure to drop and may induce land subsidence.

The geological structures causing land subsidence are mainly semi-cemented, interbedded with unequal thickness and loose sandstone and mudstone. Its consolidation characteristic is that the fluid pressure in the pore is equal to the formation pressure, and the formation stress is in equilibrium, which belongs to the normal consolidated formation. When excessive pumping occurs, the stress balance in the formation is destroyed and the fluid pressure in the pore of the formation is reduced. Because sandstone and mudstone are two completely different geological structures, their deformation characteristics are completely different when the fluid pressure in pores drops.

1) Normal consolidated mudstone has the ability to release water, which mainly depends on the strength (tensile strength) and elastic coefficient (Poisson's ratio) of rock and soil, but pore pressure has a great influence on the fracture pressure. Microscopically, we can take the isotropic and uniform elastic unit of the normal consolidated stratum (Figure 9- 1), where Δ x and Δ y are the horizontal geostress perpendicular to each other, Δ z is the overburden pressure, and Δ x = Δ y on the horizontal plane. Only when δz increases, the fluid wrapped in the pores of the unit will overcome the in-situ stress of δ x or δ y and release water. Due to the release of water, the relative displacement between the internal structures of the unit and the directional arrangement, rotation and slip of substances in the structure, the porosity becomes smaller, the unit is compacted and deformed, and finally the ground subsidence is caused. For mudstone, water release, compaction, consolidation and deformation are mostly irreversible plastic deformation, which will not rebound even after the water level is restored. Therefore, the land subsidence caused by mudstone is permanent and cannot be eliminated (lin li, 2006).

Figure 9- 1 stress diagram of normal consolidated stratum

2) The normally consolidated sandstone aquifer bears ground stress through the contact point of sand grains. The exploitation of geothermal fluid makes the water level drop, the increase of effective stress makes the sand grains arrange closely, the porosity decreases, the water-bearing sandstone compresses and the ground sinks accordingly. After stopping mining, when the water level recovers, the pore water pressure increases, the effective stress borne by sandstone decreases, sandstone rebounds, particle arrangement recovers, and land subsidence is eliminated. Therefore, the ground subsidence caused by pressure relief of water-bearing sandstone is temporary and recoverable. This point can be well proved in the relationship between the ground subsidence funnel and the water level decline funnel of the second water-bearing group in Tianjin rising at the same time and falling at the same time (but sometimes lagging). Although the arrangement of sandstone particles tends to be the most compact under the condition of repeated fluctuation of water level, some of the corresponding settlement is unrecoverable, but this part of compaction settlement accounts for a small proportion of recoverable amount. Generally speaking, most of the hydraulic pressure relief of sandstone caused by pumping belongs to elastic deformation and has the characteristics of recovery.

3) Overexploitation of shallow groundwater causes land subsidence.

Shanghai is a city with little geothermal exploitation but serious land subsidence. Due to the government's attention, the research on land subsidence in Shanghai was earlier and the research level was higher, and the control measures were taken, with good results. When analyzing the factors of land subsidence, they introduced analytic hierarchy process (AHP) to judge the variables that are difficult to analyze quantitatively, and finally obtained the proportion of each factor in the influence of land subsidence. Among them, the difference between groundwater exploitation and artificial recharge is obviously related to land subsidence, the proportion is about 60% ~ 70%, and the correlation coefficient is 0.9 153. Followed by the ground engineering load, the proportion is about 16.7% (Chen Zhengsong, 2009). The main measure taken is shallow groundwater recharge. Groundwater recharge started at 1965, and accumulated to about 600 million m3 in 2005, making the average land subsidence in Shanghai at present about10 mm/a. During this period, the accumulated land subsidence in Shanghai was only 0.2 18m, so it can be said that the recharge water "lifted" the earth.

Besides the coastal soft soil layer, the influencing factors of land subsidence in Tianjin area are mainly directly related to the over-exploitation of shallow groundwater. The results of settlement monitoring for many years show that the settlement amplitude increases in areas with large shallow groundwater exploitation and long duration, and slows down or even rebounds in areas with reduced exploitation. For the local sudden subsidence area, it is found through field investigation that it is related to human engineering activities such as artificial landfill, a large number of surrounding high-rise buildings, foundation pit dewatering, and high-density groundwater pumping in a short time.

4) Geothermal resources development and land subsidence

The research on the relationship between geothermal resources development and land subsidence rarely has targeted results. Some viewpoints simply correspond the amount of geothermal resources exploitation with the amount of land subsidence, and think that the large amount of geothermal resources exploitation contributes greatly to the settlement, while the small amount of geothermal resources exploitation contributes little to the settlement. Even within a year, the weather turned cold, the exploitation of underground hot water increased, and the land subsidence also increased. As the climate gets warmer, the exploitation of underground hot water decreases and the land subsidence decreases, depending on the performance at a glance. The fact is not so simple. First of all, there is a great difference between mining hot groundwater and mining shallow groundwater. Taking Tianjin as an example, the annual exploitation of shallow groundwater is more than 600 million, while the annual exploitation of porous geothermal resources is only150,000 cubic meters, a difference of more than 40 times. Secondly, in order to meet the heating demand of sedimentary basins, the exploitation layer of geothermal resources is often below 1000m, and the overlying sandy mudstone layer is thick and compact in structure, which has certain cementation and consolidation effect and is not easy to compress. However, the shallow groundwater exploitation layer is often 200-400m, or even shallower, and the sandstone and mudstone stratum structure is loose and underconsolidated. The geological structures of the two mining layers are obviously different, and their performances in land subsidence will not be exactly the same under the same water level fluctuation conditions. Even if the exploitation of deep geothermal resources will cause land subsidence, it is still a slow quantitative change stage and has certain concealment. As for the time length of quantitative change and the result of qualitative change, it should be put forward through experiments, research and monitoring mechanisms. Never make a simple analogy and blindly draw conclusions, which is hard to convince.

To study the relationship between geothermal resources development and land subsidence, we should do the following work: (1) collect land subsidence data; Master the geological background of the subsidence area; Find out the fault structure and formation lithology; Perform neutron density and bottom hole pressure logging; Take cores for indoor experiments to determine the relationship between formation stress and porosity, stress and permeability; Taking cores for indoor triaxial stress test to determine Poisson's ratio, Young's modulus and BIOT consolidation coefficient; Establish a prediction model to predict the coupling between reservoir pressure change and rock deformation; With the long-term monitoring of deep stratification standards, the settlement values caused by different settlement factors are stripped and then divided and ruled.