The carbonate areas in Sichuan Province are mostly old, few, marginal and poor areas, and mountainous areas account for the vast majority of the total area, and most of them are located in areas with complex terrain, high mountains and deep valleys and blocked traffic. Man-made packhorses are still the main means of transportation in these areas, and the ecological environment in these areas is very fragile. In most areas, the bedrock is exposed, the cultivated land is few, the soil layer is thin, the soil is barren, the soil and water conservation is poor, the precipitation is unevenly distributed during the year, and the surface regulation ability is poor. In the dry season, there is a serious water shortage. As long as it doesn't rain for more than ten days, it is difficult for people and animals to drink water. People and animals have to rely on people to carry horses and cows to get water several kilometers away, not to mention normal industrial and agricultural production water. The problem of water shortage seriously restricts the local economic development.
Gongxian County is located in the south of Sichuan Province, with superior natural conditions and rich light, heat, water, soil and biological resources. It is a region with great potential for resource development. The area is rich in mineral resources, mainly coal, limestone and pyrite. The contradiction between supply and demand of water resources in this area has become increasingly prominent, which has become an important constraint factor for social and economic development.
The county is dominated by low karst and Zhongshan, and the mountainous area accounts for more than 80%, mostly sloping land with a slope of more than 25, which is not conducive to reclamation and planting. Besides the river, there are many karst canyons and karst basins in mountainous areas, and the surface water resources are scarce. According to the demand of local social and economic development planning for water resources, the existing urban population in Chang Xun is 80,000, and in 2065, the urban population will reach10.6 million, and the demand for urban domestic water and production water will reach10×10.04m3/d. At present, urban water mainly depends on decentralized water supply from several units, and the daily water supply capacity will be zero, which is the same as that of Chang Xun. Changxun River, where Chang Xun is located, is seriously polluted by mining, and the existing water supply facilities are difficult to meet the needs of life, production and township development. The pressure of urban water supply is also increasing, especially in the peak period of water use, and the phenomenon of water shortage is very obvious.
According to the former Ministry of Geology and Mineral Resources' Plan for Poverty Alleviation and Water Exploration in Southwest Karst Poverty-stricken Mountainous Areas and the Layout of Poverty Alleviation and Water Exploration in Sichuan Karst Poverty-stricken Mountainous Areas, the Sichuan Provincial Department of Geology and Mineral Resources organized and implemented the demonstration project of groundwater exploration in Chang Xun, Gongxian County, Yibin City, 65438-0999. Its purpose is to carry out groundwater exploration and establish water supply demonstration sites in southern Sichuan, which is seriously short of water in karst rocky mountainous areas, so as to provide scientific basis for solving the problems of human and animal water use, farmland water supply and ecological environment protection in poor karst rocky mountainous areas. Through one year's water exploration work, some achievements and understandings have been obtained.
2 Karst hydrogeological conditions
The formation and distribution of karst water in Sichuan are obviously controlled by regional structure and natural conditions. Carbonate rocks in different regional structures and natural areas are quite different in lithology, thickness, distribution area, exposure conditions and karst degree. According to this feature, Chang Xun, which is seriously short of water, was selected for water supply demonstration in Gongxian from 65438 to/kloc-0 in 1999. Combined with the distribution, burial and water-rich characteristics of carbonate groundwater, karst water is abundant, but its distribution is extremely uneven and controlled by structure. Seven water wells were drilled for the local area free of charge, of which two were abandoned because there was no water, and five were actually completed. Solved the drinking water problem of 30 thousand people. Practice has proved that it is feasible to solve the problem of water shortage in cities and towns and drinking water for people and animals in rural areas from the technical conditions and well completion technology when drilling wells in the vast carbonate areas.
2. 1 Karst and karst water are obviously affected by lithology and structure.
Carbonate rocks of Permian Maokou Formation and Qixia Formation in this area are pure, thick and soluble, which are good dissolution media for surface water and groundwater, with strong karst development. Carbonate rocks of Triassic Jialingjiang Formation and Leikoupo Formation contain about 10% of clastic rocks, so surface depressions, funnels and sinkholes are mostly developed along the contact parts between carbonate rocks and clastic rocks.
This area is the turning point of Changning anticline in structure, and carbonate rocks are distributed in bands. Under this specific structural condition, lateral dissolution is strictly limited, and karst develops longitudinally along the turning point and its two wings, and the surface dissolution boundary is longer.
Faults and fractures obviously control the formation and extension direction of karst pipelines, especially in the folded fault zone where fractures are developed and rocks are broken. Groundwater runoff is gentle and activities alternate, and dissolution is progressing smoothly. Large karst caves and karst pipelines are well developed, and karst water is very rich. For example, Yudong underground tunnel develops along the Kutang depression fault, and the long axis of subsidence is generally near the north-south direction, which is mainly controlled by the north 15 ~ 25 west and north 12 ~ 20 east faults.
At the end of the Wu Dong Movement, there was regression in this area, and Permian carbonate rocks emerged from the water surface, which suffered long-term dissolution and erosion, and ancient caves of different scales developed. These Ancient Karst Cave have been proved in coal exploration. When drilling, the drilling tools have been hollowed out and water gushed out. In coal mining, when the floor is exposed, a large amount of karst water gushes out, such as the spring water exposed in the pit of Changxun Coal Mine. The development depth of ancient karst is generally in the range of 50 m, and it is mostly distributed on the top ancient denudation surface along the bedding plane, which has no hydraulic connection with modern karst.
2.2 Characteristics of recharge, runoff and discharge of karst water
The recharge of karst water is mainly atmospheric precipitation in outcrop area, followed by surface water, fissure water in overlying non-karst layer and mine water.
Karst landform is developed in limestone exposed area, and atmospheric precipitation is the main source of karst water, which directly supplies groundwater through gullies, troughs, funnels and tiankeng. Surface water flowing through limestone areas also leaks a lot. Fissure water and mine water are also the main recharge sources of karst water in this area, especially coal mine groundwater permeates karst strata in the form of dispersed flow. Therefore, the karst water supply source in this area is sufficient.
The movement and circulation of karst water are controlled by tectonic conditions and hydrological network, with longitudinal runoff as the main flow path, which is generally discharged to the downstream low-lying areas for local recharge.
The discharge of karst water in the area is mainly controlled by hydrological network, which is generally concentrated in rivers, gullies or low-lying areas. In Jinshawan sag and Chang Xun sag, groundwater is discharged radially around the sag; Groundwater in Tangba area is discharged through the runoff of Yudong underground river in Gongxian County.
2.3 Karst water is abundant, but its distribution is extremely uneven.
Karst water in this area mainly occurs in underground river systems and fissures, and there are three forms: confluence, dispersion and pipeline flow.
The limestone of Maokou Formation and Qixia Formation are mainly distributed in the two wings and the dip end of Changning anticline. Karst water is very rich in karst depression, mainly in large-scale pipeline flow, and there are also karst springs discharged along the fissure. The flow rate of Daquan underground river is 30 ~ 184 L/s, and according to the drilling data, the development depth of underground karst cave is more than 50 m. When the underground karst cave is not exposed by drilling, the water inflow is very small, and the unit water inflow is 0 ~ 2.455 l/(s m), which shows that the groundwater distribution is extremely uneven.
Leikoupo Formation and Jialingjiang Formation are mainly distributed in the wing of long anticline. Because there are about 10% clastic rocks, the karst development degree is moderate, and the developed karst landforms are mainly depressions, funnels and sinkholes. Karst water is rich and shallow, which is mainly convergent and divergent. The spring flow is mostly 0. 1 ~ 10L/s, and the unit water inflow is 0.107 ~ 0.58l/(s m).
Different karst landforms in the area clearly reflect the buried depth of karst groundwater. From watershed to valley, the intensity of karst development is from weak to strong, and karst water is from deep to shallow. Generally, the buried depth of karst water level in valley area is less than < 25 m, the buried depth in karst valley area is less than < 50 m, and the buried depth in peak valley and low mountain canyon is greater than 100 m.
2.4 The water quality is good, and it is suitable for people and animals to drink.
The water quality types of Triassic karst water are HCO3-Ca and HCO 3-Ca Mg, with salinity ranging from 0. 1 ~ 0.3g/L and pH ranging from 7.4 to 8.1. Because of the strong alternation of water cycle and excellent water quality, it is suitable for domestic, agricultural irrigation and industrial water use.
The water quality type of Permian karst water is SO4. Calcium bicarbonate sulfate. Sodium and sodium bicarbonate. Calcium sulfate. The salinity is 0.5 ~ 1.5 g/L, the total hardness is 300 ~ 600 mg/L, and the pH value is 7. 1 ~ 7.7. Except for sulfate, other chemical and toxicity indexes generally do not exceed the standard, and the water quality is good, which is basically suitable for drinking, irrigation and industrial water. Due to the recharge of mine water, its water quality is obviously worse than that in 1960s. Sulfate increased from 4.8 ~ 182.5 mg/L to 80.0 ~ 588.0 mg/L, and salinity increased from 13 1.0 ~ 548.0 mg/L to 332.8 ~ 638.0 mg/L.
2.5 Enrichment of Karst Water in Demonstration Area
The different lithology of carbonate rocks has great influence on the intensity of karst development. Pure limestone is easy to form underground pipelines and corrosion cracks. In the contact area between clastic rocks and carbonate rocks, due to the limitation of clastic rocks, corrosion cracks and karst pipelines are easy to form in this contact area. Where karst is strongly developed, it is easy to form large underground pipelines, which is a good place for karst water to gather. This is the general law of karst water enrichment in this area. However, where karst development is weak, underground corrosion cracks often form.
In low-lying areas, it is beneficial to the enrichment of karst water, which is due to superior catchment conditions, such as Jinshawan sag and Chang Xun sag, which are centralized drainage areas of groundwater. The transverse valley zone is usually the local erosion datum. When crossing the karst aquifer, karst water collects in these places and discharges into the valley, such as the Jade Empty Underground River and the Changning River.
Structurally, this area is located at the west turning end of Changning anticline (figure 1), with secondary folds and vertical tensile fractures, accompanied by multiple groups of compressive-torsional and tensile fractures, and groundwater is easy to be enriched. Due to good structural and topographic conditions, karst in Jinsha Bay and Chang Xun karst depression is very developed, and karst water has the characteristics of shallow burial and large amount of water.
The patrol area is mainly divided into three karst groundwater systems:
(1) Shawan karst groundwater system: Triassic carbonate rocks are exposed, which are structurally located in the northern wing of Changning anticline. The measured total spring flow is 3.34 L/s. This area is the main water supply source of Furong Mining Bureau, and the daily groundwater exploitation amount reaches 1.6× 104 m3. Most of the water-bearing channels exposed by drilling are less than 1 m in diameter, and they are still mainly dissolved holes and cracks, and no large pipeline system has been formed.
(2) Groundwater system of patrol station: Permian carbonate rocks are exposed, structurally located at the turning end of Changning anticline, separated from Jinsha Bay by sandstone and mudstone in the north and Silurian mudstone in the south, and the surrounding groundwater is discharged around the patrol station depression. Affected by coal mining, the water volume of karst springs has been greatly reduced compared with that before the 1970s, and some springs have even dried up, with a total spring flow of 30 L/s. The karst water in the depression has the characteristics of shallow burial and large water volume, karst fractures and pipelines are developed, and the aquifer is buried more than 60 m.
Figure 1 Schematic diagram of arc turning zone of Changning anticline
(3) Yukong underground river system: developed in limestone of Qixia Formation and Maokou Formation, located in the south wing of Changning Anticline, and the outlet of underground river is located at Changning River in Gongquan Town. The length of underground river runoff is 7.5 km, the hydraulic gradient is 5.7‰, the maximum flow is 244 1l/s, and the minimum flow is 184 L/s, with an annual average. This underground river runs through the surface watershed between Changning River and Changxun River, and Tangba area is the upstream runoff area of the underground river. Because the surface is covered by Quaternary, there is no underground river here. According to the construction of No.4 drilling hole, the karst development depth is more than 60 m, and the hole depths are 30.76~34.0 m, 36.54~37.29 m, 43. 10 ~ 44.20 m and 54.30~55.45 m, respectively, all of which are water-bearing caves filled with a lot of sandy pebbles and clay. For other boreholes constructed in Tangba area, the buried depth of groundwater level is generally 50 m, which shows that its water-bearing capacity is extremely uneven.
3 Determination of completion technology of exploration-production combined well in demonstration area
3. 1 Determination of exploration and production joint hole
Through the hydrogeological survey of1∶ 50,000, the occurrence, distribution and burial characteristics of karst groundwater in the demonstration area are preliminarily understood, and the water-rich blocks are delineated. As the demonstration area is a town, considering the requirements of urban planning and development and the possible karst collapse caused by groundwater pumping in karst areas, the initial well location is determined to avoid affecting the town.
Geophysical methods such as electrical sounding, induced polarization sounding and audio-frequency magnetotelluric field are used to analyze and compare the preliminarily determined drilling positions, and combined with the geological structure of the demonstration area, the exploration and mining combined holes are determined. There is a water supply well of Furong Mining Bureau in Jinshawan sag, and the selected well is located in the wing of the secondary syncline. The hole position determined by the patrol depression is in the syncline between patrol and Baixiangyan-Shizitan anticline. Tangba sag is located at the intersection of Kutan high fault and Renjiagou normal fault.
3.2 Completion Technology
In order to improve drilling efficiency, air DTH hammer drilling method is adopted, which greatly improves the well completion speed. Single hole completion time increased from1~ February to 1 ~ 2 weeks. A total of 7 exploration and production combined holes were constructed, of which 5 boreholes yielded 6,825m3/d of water, which achieved the expected purpose of this exploration. The water stop depth of construction drilling is below the shallow karst development zone to avoid karst collapse that may be caused by pumping water. See table 1 for water intake horizons and pumping test data of each borehole.
Table 1 Summary of Drilling and Pumping Results
4 Significance and understanding of groundwater exploration and demonstration in karst rocky mountain
It is of great significance and remarkable social benefits to carry out demonstration projects of groundwater exploration in karst mountain areas with serious water shortage. Through the above work, the buried distribution, recharge runoff and enrichment factors of groundwater in different landforms, different structural parts and different types of carbonate rocks are further understood. In the demonstration project, water is taken by drilling wells in various ways, which opens up a feasible road for the development and utilization of groundwater in karst rocky mountainous areas with serious water shortage, and solves the problems of drinking water for people and livestock and some agricultural irrigation.
Understanding of 4. 1
There are various structural systems and complex geological structures in southern Sichuan, and the water-rich areas mainly formed by geological structural factors have different shapes. There are mainly folded and fractured water storage structures in the area, and the karst hydrogeological units divided by geological structural factors are as follows:
Nose anticline type: the axis extension is short, the dip angle of the two wings is slow, the water-bearing rock groups are alternately distributed in a semi-ring belt shape, and they are connected with the two wings of the high-order anticline in a "T" shape or a "V" shape, with developed faults, mainly Junlian nose anticline and Xunsichang nose anticline.
Turning airfoil: At the arc turning position and the dipping end of the anticline wing, tension cracks and plane cracks in the vertical plane are developed, which are easy to enrich groundwater, mainly including Jinshawan and Chang Xun strip-shaped corrosion depressions in Changning Shuanghe anticline.
Open syncline type: the occurrence of the core is gentle, the inclination of the wing is slightly steep, and different water-bearing rock groups are alternately distributed in annular strips, which is beneficial to the migration and enrichment of groundwater to the core in the form of circumferential runoff, and often forms closed or semi-closed strong water-bearing gravity basins, such as Longgongpo syncline, Muai syncline, Tianyusi syncline and Datianba syncline in Shuanghechang County.
Short-axis anticline type: the axis is open, the occurrence of the two wings is relatively gentle, the water-bearing rock groups are distributed annularly along the two wings, or exposed in the core in a sheet shape, and there are relatively cemented and isolated traps around it, which is typical of Wujiaobao anticline.
Types of cracks with water plugging and water filling at the same time: the compressive and torsional fracture zone in the area is generally dense, and the scale of the fracture zone is small. When the lithologic and geomorphological conditions are favorable, especially when the soluble rock is in contact with the insoluble rock, one side of the soluble rock is relatively rich, and the other side is blocked by water. Such faults include Yinjiawan Jikou fault and Zhucaojing fault.
Water-conducting fault type: some faults in the horizontal inference layer and the inclined strike area often lead to hydraulic connection between adjacent aquifers, resulting in cross-layer recharge and water conduction, such as Yufengting fault in Junlian and Dengtanba.
Types of fault intensive zones: complex parts of different tectonic systems and high anticline cores often form a wide range of faults and fracture zones, with dense springs and large flow, such as Shuanghe, Meihe-Xingwen zones in the axis of Shuanghe anticline in Changning.
4.2 socio-economic benefits and political impact
In the demonstration work of 1999, we received strong support and help from the superior departments, local governments at all levels, relevant departments and local people from beginning to end. From the beginning of planning and site selection, the local government assigned a special person to help solve the difficulties encountered in the work. During the construction process, leaders of the government and relevant departments visited the site for many times, showing concern and sympathy for the production and life of on-site engineers and technicians. After the completion of the well, raise funds in time to improve the supporting facilities of the well, and immediately solve the people's living and production water. Local newspapers and TV stations made a special interview report on the project of finding water and drilling wells, which was affirmed and evaluated by the local government and the masses. There are 5 wells in the patrol area, with a water output of 6,825m3/d, which can solve the drinking water problem of more than 30,000 people and greatly alleviate the water shortage problem in Chang Xun. It can be regarded as a successful pilot of groundwater exploration and demonstration project in carbonate water-deficient areas of Sichuan Province, which has produced great social impact and sensational effect.