(Tianjin Geothermal Exploration and Development Design Institute)
Based on the analysis of the geological background and distribution characteristics of geothermal field in Tianjin, this paper makes a preliminary discussion on the characteristics of shallow geothermal energy in Tianjin, and puts forward some suggestions for the investigation and research of shallow geothermal energy in the future, so as to develop it more scientifically and reasonably.
Nowadays, mature heat pump technology makes the utilization of shallow geothermal energy a reality. Shallow geothermal energy has gradually entered the stage of energy utilization because of its advantages of large reserves, convenient utilization and little harm to the environment. Nowadays, more and more attention has turned to this energy treasure house with great potential. This new energy with great market prospect plays a positive role in alleviating the bottleneck of high-quality energy and improving environmental quality in China. However, at present, the promotion of heat pump system is very blind, and many projects are launched in a hurry without fully investigating and evaluating local energy reserves, which leads to the abnormal work of ground source heat pump system and affects the further promotion and application of ground source heat pump system. Therefore, the basic research of shallow geothermal energy is a very forward-looking topic.
Tianjin is surrounded by mountains and seas, and there are abundant shallow geothermal energy underground. Over the past 30 years, geothermal workers have carried out a lot of exploration and related research work in the field of geothermal research, and achieved fruitful scientific research results. However, due to the limitation of science and technology, the previous geothermal energy development only focused on geothermal water above 40℃, and the research on shallow geothermal energy was almost blank. Nowadays, with the development of high-tech such as heat pump, the utilization range of geothermal energy is greatly expanded, and the shallow geothermal energy with great potential is no longer sleeping. Related utilization has been carried out in several demonstration projects, and its basic investigation and research has also become the focus of users and management departments.
The formation of shallow geothermal energy is closely related to the geological background of this area. The stratum development and structural characteristics are different in different areas, and the geothermal flow is also different, so the energy contained is different. The geological background and characteristics of shallow geothermal energy in Tianjin are analyzed from the characteristics of geological structure and geothermal field.
Geological background of 1
Structural characteristics of 1. 1
In the geotectonic system of China, Tianjin belongs to the northern part of North China paraplatform, a first-class structural unit, which spans two secondary structural units, Yanshan platform fold belt and North China fault depression area. The Ninghe-Baodi fault is the boundary between them, and the bedrock is exposed or buried in the north of the fault, which belongs to the Jibaolong fold in the middle part of Yanshan platform fold belt. The faults developed in this area are mainly near east-west direction.
The south of Ninghe-Baodi fault is a plain area, which belongs to the northeast of North China fault depression and is a secondary structural unit. It is distributed in the east of Taihang Mountain uplift belt, south of Yanshan fold belt and extends eastward to Bohai Sea. It is a fault depression basin formed in Mesozoic and Cenozoic, which is further divided into three tertiary structural units from west to east: Jizhong Depression, Cangxian Uplift and Huanghua Depression, which are bounded by Paleogene Missing Line and Cangdong Fault respectively. Faults can be divided into three groups: NE-NE direction, NW direction and near EW direction. The NNE faults include Cangdong fault, Tianjin fault, Baitangkou west fault, Baitangkou fault and Baitangkou east fault, the NNE faults include Haihe fault, Hangu fault and Ninghe fault, and the EW faults include Wang Cao Zhuang fault and Niutihe fault. Among these faults, the NE-trending fault is the most active. Of course, as far as the overall relative characteristics are concerned, these groups of faults also have the characteristics of multi-stage activity and mutual shear. Faults are mostly inherited faults, which are generally characterized by large scale, steep occurrence, deep cutting and different tendencies. East-west faults generally tilt south; Most NE-NNE faults tend to the southeast and some to the northwest. NW-trending faults generally tend to the southwest. Most of these faults are normal faults, or mainly normal faults. Some of them pass through the crystal basement and the crust, while others pass through the Moho surface and reach the upper mantle. These faults control the development and evolution of the basin and its secondary and tertiary structural units. The alternating arrangement of uplift and depression and the cross distribution of multi-stage faults constitute the geological structure pattern of Tianjin.
1.2 general situation of formation development
The northern part of Tianjin is mainly mountainous and shallow bedrock areas, and the Cenozoic strata are relatively thin, generally around 200m; Due to the strong fault depression activity since Cenozoic, the southern plain has deposited huge Cenozoic strata, and the corresponding strata are well developed, which is a set of clastic deposits with interbedded sand and mudstone. From top to bottom, they are Quaternary, Neogene Minghuazhen Formation and Guantao Formation, Paleogene Dongying Formation and Shahejie Formation and Kongdian Formation respectively. The Cenozoic strata in Jizhong depression and Huanghua depression are more than 3000 meters thick, and the Cenozoic strata in Cangxian uplift area are 65430 meters thick. Mesozoic to Mesoproterozoic bedrock strata developed under Cenozoic.
2 Geothermal field characteristics
According to the results of geothermal exploration for many years, the Cenozoic strata south of Ninghe-Baodi fault are well developed, with a thickness of more than 800m, which is an ideal thermal storage cover and has the conditions to store shallow geothermal energy. Within the exploration depth of 4km, the ground temperature distribution in the shallow crust is controlled by the geological structure pattern of alternating concave and convex bedrock, and it shows a certain strip distribution in the plane. The area with relatively high temperature is consistent with the uplift bedrock area, and the area with relatively low temperature corresponds to the depression bedrock area. With the contour line of geothermal gradient of 3.5℃/ 100 m as the boundary, the area is divided into 10 geothermal anomaly areas (Figure 1). Geothermal anomaly areas are mainly distributed in the northeast direction, some in the northwest direction, and most of them are distributed in the convex parts of Cangxian uplift and depression. Deep faults such as Haihe fault, Cangdong fault and Baitangkou west fault play an important role in controlling the distribution of Shanlizi geothermal anomaly area, Wanjia Wharf geothermal anomaly area and Wang Lan Zhuang geothermal anomaly area respectively. Therefore, the distribution of geothermal field on the plane is mainly influenced by geological structure.
3 Overview of shallow geothermal energy characteristics in Tianjin area
Most of the northern part of Ninghe-Baodi fault is mountainous, and only a few areas are shallow bedrock areas. The thickness of loose Cenozoic strata is mostly 2℃/ 100m ~ 300 m, which is not conducive to the maintenance of heat. In addition, due to the interference of strong groundwater runoff, the geothermal temperature is obviously reduced, which is obviously different from the normal geothermal warming law in plain areas, resulting in a generally low geothermal gradient of about 2℃/65438.
In the vast plain area south of the Ninghe-Baodi fault, the Cenozoic strata can be vertically divided into three geothermal zones from the surface to the underground: the surface layer is temperate, and its temperature is mainly affected by solar radiation heat flow, surface rocks and vegetation, buildings and other factors, with daily, annual and multi-year variation cycles. The temperature range decreases with the increase of depth, and it is a geothermal constant temperature zone at a depth of about 30m, which is basically unaffected by climate and basically stable. Below the constant temperature zone is the geothermal warming zone, and its temperature is mainly affected by the deep heat flow of the earth, thermal conductivity of rocks, geological structure, hydrogeology and other factors. The general rule is that the geothermal temperature also increases with the increase of depth, and the increase range varies with the different thermal heating rates.
Figure 1 Distribution Map of Geothermal Anomaly Area in Tianjin
From the distribution characteristics of geothermal field, it can be seen that the vast plain area south of Ninghe-Baodi fault contains a lot of shallow geothermal energy. Due to the different distribution of geothermal heat flow in different regions, geothermal energy reserves and renewable energy are different in the same depth range. Under the same energy demand, the collection depth of geothermal anomaly area is shallow, and vice versa. According to statistics, the geothermal gradient in the whole area is 2 ~ 6.31℃100 m, the buried depth at 25℃ 168 ~ 605 m, and the buried depth at 40℃ is 349 ~ 1355 m, with reference to the regional thermophysical parameters1. Equivalent to standard coal1.23x109t standard coal, equivalent to electric energy1.0x13kw h, and the ground temperature will also be periodic when the energy is periodically extracted from the shallow layers in isothermal layer and below (heat is extracted in winter and cold is extracted in summer).
Although shallow geothermal energy is rich and renewable, its development and utilization should be selective according to different hydrogeological conditions in different regions, such as collection scale and mining methods. When this extraction exceeds a certain amount, the ground temperature will "deform" irretrievably like a spring, which will not only affect the economy and safety, but also cause the destruction of underground hydrogeological conditions.
In a word, this area is rich in shallow geothermal energy. Geothermal energy, as a clean and low-grade renewable energy, is not only stable, natural and reusable all the year round, but also has great potential and great environmental protection performance, so it can not be ignored as a new energy source. As long as the energy collection design is reasonable and the technical means are appropriate, the energy balance of the heat pump system is relatively stable, and the development and utilization of shallow geothermal energy has broad prospects.
4. Suggestions on investigation and evaluation of shallow geothermal energy
4. 1 Investigate the characteristics of shallow geothermal energy and the parameters needed to evaluate the shallow geothermal energy reserves.
Because the horizon containing shallow geothermal energy in Tianjin area is not the focus of geothermal work in the past, there is a lack of data of Quaternary and even the upper member of Minghuazhen Formation, including accurate data of thermophysical parameters such as rock and soil density, rock and soil specific heat, fluid density, average specific heat and rock and soil porosity. At the same time, in the geophysical exploration test of geothermal wells, there is basically no geophysical logging in the shallow layer of 500 meters, and there is no systematic data to check its lithology. Therefore, the layout of investigation work should focus on discovery. The main data and parameters needed are: formation lithology and distribution range, buried depth, vertical geothermal gradient change, rock and soil density and specific heat capacity, rock and soil porosity, etc.
4.2 Comprehensive collection of basic geological data in the whole region
In the ground temperature investigation and related research work, collecting data is a basic work that can not be ignored, and it plays a vital role in understanding the research degree and work arrangement in this field. It mainly collects the data of basic geology, tectonic geology, drilling (including oil wells, geothermal wells and some Quaternary wells), geophysical prospecting (including digital earthquake, magnetotelluric sounding, high-precision gravity and aeromagnetism), geochemical exploration, hydrology and geothermal, etc., and makes comprehensive analysis and arrangement to understand the work degree and existing problems in the work area, and arrange the workload for the problems to be solved to avoid duplication of work. In the past, the research focused on deep geothermal wells, but little attention was paid to the data of Quaternary wells and the top of the upper member of Minghuazhen Formation. Therefore, the collection of this data is a key point to analyze the geological structure characteristics and strata in this area (especially the lithology, distribution, occurrence, buried depth and thickness of strata between 13.5 ~ 40℃).
4.3 Carry out field shallow geothermal geological survey.
Field investigation is the main means of geological investigation and the basis of geothermal field investigation, so this project is a necessary part of the investigation. By investigating the distribution of existing wells, the temperature of water intake section and aquifer, the utilization status of shallow geothermal energy and the exploitation amount of geothermal energy in the whole region, we can better understand the current situation of available work in the region and provide a basis for the layout of all work.
4.4 Geophysical exploration
(1) Secondary interpretation of existing geophysical results. Based on the data of artificial earthquake, gravity and magnetic logging and borehole geophysical logging, this paper analyzes the stratigraphic development, structural position and fault development characteristics in this area, finds out the activity and age of faults, and determines its influence on shallow geothermal. In the investigation and study of shallow geothermal energy, geophysical exploration is different from previous work. First of all, the existing geophysical data, especially gravity and magnetic data, should be redeveloped in combination with the data in recent years and other work, and then appropriate geophysical work should be arranged in necessary areas.
(2) Investigation of geothermal flow and geothermal field. In the investigation and study of shallow geothermal energy, one of the most critical parameters is geothermal flow value. The spatial variation law of ground temperature in this area is found out mainly by borehole temperature measurement, and the ground temperature gradient of different strata is determined. At the same time, the rock samples of the corresponding interval are collected to determine the thermal conductivity of the rock.
5 drilling work
In the investigation and study of shallow geothermal energy, how to obtain reasonable parameters is the key to solve the problem. The existing drilling holes can be used to obtain parameters, but some existing conditions can not be met, and some fields are even blank, such as vertical geothermal gradient and formation thermophysical parameters. Therefore, it is generally necessary to arrange the necessary drilling work, and the drilling should be combined with the local utilization planning and demand as far as possible, and the drilling should be arranged in a representative position. According to different sedimentary environments, the sediments are arranged from north to south and from west to east, in order to find out lithology, structure, structure, geothermal change, thermal reservoir permeability, geothermal fluid pressure, its physical properties and chemical composition, and obtain representative calculation parameters.
6. Establish a demonstration base for comprehensive utilization of shallow geothermal energy.
In the middle and late stage of shallow geothermal energy survey, in order to promote the socialization of the results, we should build a demonstration base for comprehensive utilization of shallow geothermal energy, design heating and cooling areas, select sites, purchase equipment, install and debug, and track and evaluate the operation. Finally, a comprehensive investigation and evaluation report should be submitted to provide an example for the applicability of heat pump technology in this field.
Nowadays, energy has become the "compass" of economic development, and Tianjin is rich in shallow geothermal energy. It is urgent to study and develop it, which will certainly make positive contributions to Tianjin's sustainable development and environmental protection.