Simulation of Multi factor Coupling and Earthquake Prediction in Danjiangkou Reservoir Area

Transcription

1 Frontiers in Geotechnical Engineering (FGE) Volume 3, 04 Simulation of Multi factor Coupling and Earthquake Prediction in Danjiangkou Reservoir Area X. D. Xie *, S. M. Liu School of Mechanical and Automobile Engineering, Hubei University of Arts and Science, Xiangyang, Hubei, 44053, China School of Civil Engineering, Wuhan University, Wuhan, Hubei, 43007, China * xiangdongxie@63.com; cdcyk04@63.com Received 7 March 04; Accepted 8 June 04; Published 30 June Science and Engineering Publishing Company Abstract According to the geological structure characteristics of the Danjiangkou reservoir, a 3 D finite element model is established by software ANSYS, which includes the main fracture zones in reservoir area, the mechanical parameters of the rock changing with the depth and the coupling effect of the physical chemical interaction between the water and rock, seepage field and in situ stress field. Multi factor coupling stress field at each period corresponding to impoundment level is simulated by software FLAC3D through the model imported from ANSYS, and then the mechanism of reservoir induced earthquake in Danjingkou reservoir area after improving impoundment level is explored. Based on the research above, some earthquakes are predicted to happen in Songwan Guanfangtan Jiuxichuan, Shangsi Heitaoyuan, Linmaoshan Danjiangkou Qingshangang, Zishan ao Jiujunxian after the phase Ⅱ impoundment, with hypocenter depth about 9.5km and in yunxian, xijiadian, shiguguan, with hypocenter depth of km 4km and 9.5km around. Deep earthquakes are unlikely to be activated at the zones of intact rock around the Danjiangkou reservoir. Keywords Reservoir Induced Seismicity; Fracture Zone; Multi factor Coupling; Danjiangkou Reservoir Introduction Water resources are uneven distribution in China, rich in south area and shortage in north area. In this century, the South to North Water Transport Project is an important strategic step to promote social economic to a comprehensive, coordinated and sustainable development in China. Danjiangkou reservoir is the source of the midline of South to North Water Transport Project, which has provided adequate water for the northern region. However, seismic activity in the reservoir area has augmented after phase Ⅰ impoundment(967.), and the maximum reservoirinduced earthquake with magnitude of 4.7 was broken in Songwanin six years, since then there are just a few microseisms(xie, 00, Yi et al., 003). The impoundment in phase Ⅱ will increase the water depth to 70m; it is a common concern in the circle of civil and seismic engineering whether a new round of seismic activity would be caused with the increasing of water depth. Since the earthquake in Songwan in 973, many domestic experts and scholars have done a lot of research on the Reservoir induced earthquake in Danjiangkou, such research explored the seismic activity law mainly based on the reservoir geological conditions, the recent seismic activity, the changing of water depth and the simulation of tectonic stress field and hydrostatic stress field, but these research is not in depth on the mechanism of reservoir induced earthquake(li and Liu, 980, Gao, 980, Liu and Xu, 005,006). In fact, reservoir induced earthquake results from the multi factor coupling action, namely, with the increasing of impoundment, the rock in reservoir area will be deforming, the stress field will be adjusting, and then the rock structure and geometrical characteristics will be changing with the seepage of reservoir water and physical chemical interaction between the rock and reservoir water, therefore, the rock permeability is changed; on the other hand, the change of the rock permeability will cause the corresponding change of the rock mechanical behavior and stress state. In order to study the mechanism of 7

2 Frontiers in Geotechnical Engineering (FGE) Volume 3, 04 reservoir induced earthquake thoroughly, a threedimensional finite element model of Danjiangkou reservoir area is established using ANSYS considering the characteristics of geological structure, the old fracture zones in reservoir area and the influence of stress field, temperature field, seepage field and physical chemical action between reservoir water and rock. Then the model is imported to FLAC3D to simulate the multi factor coupling stress field, and the mechanism of reservoir induced earthquake is analyzed deeply based on the simulation. FIG. 3D GEOLOGICAL STRUCTURE MODEL OF RESERVOIR AREA Establishment of Finite Element Model and Selection of Computation Parameters Establishment of Finite Element Model The finite element model of Danjiangkou reservoir area is a rectangular region with a length of 3.5km and a width of 08.4km, which is formed by cutting the area along the perpendicular direction of principal stress based on reference (Xie, 00). The model thickness and fracture s depths are all taken as 0km. In order to simulate the activity feature of the fault zone effectively, according to the geological data, the 500m width region along the middle line of Danjiang fault is taken as Danjiang fracture, the peripheral region with a width of 30km along Danjiang fracture is taken as Danjiang fault; the 00m width region along the middle line of Junyun fault is taken as Junyun fracture, the peripheral region with a width of 5km along Junyun fracture is taken as Junyun fault; the 00m width region along the middle line of Gonglu fault is taken as Gonglu fracture, the peripheral region with a width of 5km along Gonglu fracture is taken as Gonglu fault. Wafangchangzhoushan fracture is 00m width, and the peripheral fault is neglected for the fracture is too small. The reservoir depth is taken as 70m in order to rule the calculation model. The 3D geological tectonic new model is shown in the FIG. which is built by ANSYS. Selection of Computation Parameters Due to the different cover thickness and lithology in different place of Reservoir area, its calculation model is divided into layers along the depth in order to reflect the change of the lithology along the depth and the water level in reservoir, the layer thickness is 70m, 330m, 0.5km, km, km, km, 3km, 3km, 3km, 3km, 3km, respectively, from top to bottom. In the calculation geometry region, the cover to the east of Danjiang fracture mainly composed by carbonate and the thickness is taken as km approximately; the cover to the west of Danjiang fracture is divided into three parts by Junyun fracture and Gonglu fracture, the part to the north of Junyun fracture is mainly composed by sandstone and the thickness is taken as 5km about, the part to the south of Gonglu fracture is composed by diabase primarily and the thickness is taken as 8km about, the part between the Junyun fracture and Gonglu fracture is mainly composed by diabase weathered slightly and the thickness is taken as 3km about. Between the cover and the bottom of the model are comprised mostly by the volcanic rock. The paper used the differential method software(flac3d) for fluid solid coupling simulation by considering the rock in reservoir as equivalent continuous medium, at the same time, the composite solid element and Mohr Coulomb yield criterion are used. The surface rock mechanics parameters obtained from the groupʹs previous test data (shown in TABLE ), deep rock mechanics parameters were obtained from literature (Xie, 00)(shown in TABLE ), in which fault porosity is.5 times of that of corresponding intact rock, permeability coefficient is 0 6 times of that of corresponding intact rock; fracture porosity is 3 times of that of corresponding intact rock, permeability coefficient is 0 7 times of that of corresponding intact rock(xie, 00). Calculation Steps The calculation in the paper is implemented through two steps:() the bottom displacement in z direction is tied, so do the axial displacements on X positive side and Y negative side, then the axial displacements on X negative side and Y positive side are adjusting constantly to calculate the initial in situ stress field in Danjiangkou reservoir area; () on the basis of initial 8

3 Frontiers in Geotechnical Engineering (FGE) Volume 3, 04 TABLE PRE EXISTING TESTING RESULTS OF ROCKS IN DANJIANGKOU RESERVOIR AREA Rock type Sandstone Diabase weathered slightly Diabase Carbonate Volcanic rock Density(g/cm3) Young modulus(gpa) Poisson ratio Yield strength(mpa) Internal friction angle( ) Porosity(%) Permeability/(0 3 μm ) Depth /km TABLE MECHANICAL PARAMETERS OF ROCKS AT DIFFERENT DEPTHS OF DANJIANGKOU RESERVOIR AREA(XIE,00) Sandstone Diabase weathered slightly Diabase Carbonate Volcanic rock E/Gpa μ φ/ E/Gpa μ φ/ E/Gpa μ φ/ E/Gpa μ φ/ E/Gpa μ φ/ in situ stress field, the boundaries of sides and bottom of reservoir is set to be permeable, others are set to be impermeable. The initial saturation of the model is set to be zero. Applying the water load and corresponding pore water pressure on the reservoir area, then the fluid solid coupling calculation is beginning after opening the mechanical process and fluid process together. And the calculation process is divided into three phases. The phase Ⅰ was from Oct. 969 to Oct. 973 with the water depth of 35m, the phase Ⅱ was from Nov. 973 to Dec. 03 with the water depth of 57m, and the phase Ⅲ is from Jan. 04 to Jan. 08 with the water depth of 70m. Simulation and Discussion FIG. DISTRIBUTION OF ANALYTICAL POINTS IN MULTI FACTOR COUPLING STRESS FIELD The influence of fluid solid coupling on the stress field is studied and the places of A, B, C and D are selected as representative points (shown in FIG. ) whose stresses are to be researched and analyzed, because the reservoir induced earthquakes usually occurred around the reservoir or on the fracture zone such as the places of A, B, C and D. The Calculation and Analysis of Stress Difference in Each Point The difference of stresses in each point in the reservoir area can be obtained by subtracting the decoupling stress field in Oct. 969 with the water depth of 35m from the coupling stress field in each period of impoundment. Taking the points of A, B, C and D as example, the relationship graphs between the stress difference and the depth at different water depth and different impoundment period are drawn in FIGs. 3, 4, 5, 6. FIG. 3 shows the relationship of the depth and the stress difference between coupling stress field from Oct. 969 to July 970 and decoupling stress field in Oct. 969 with the water depth of 35m. It can be seen from the curves in the FIG. 3 that shear stress and vertical principal stress are basically unchanged along the depth in this period, and the horizontal principal stresses have a large change along the depth. Here Sxx and Syy are the two horizontal principal stresses along axis of X and Y, respectively, and Szz is the vertical 9

4 Frontiers in Geotechnical Engineering (FGE) Volume 3, 04 principal stress, Sxy, Sxz, Syz are the shear stresses. The coupling horizontal principal stresses in the points of A, B and C decreased relative to the decoupling horizontal principal stresses, and the principal stress along Y axis decreased more significantly but no more than Mpa; the coupling principal stresses in the point of D increased relative to the decoupling principal stresses, and the principal stress along X axis increased more significantly but no more than Mpa. All these changes occur mainly in the depth from 5km to km. The reason of the changes is that the places in A, B and C have a good permeability, and the pore pressure is decreasing with the coupling effect, accordingly the horizontal principal stresses reduce, especially along the direction of Danjiang fracture because its permeability is better than other direction; the place in D has a low permeability, and the pore pressure is increasing with the coupling effect, accordingly the horizontal principal stress is increasing, especially along the direction of Junyun fracture. principal stresses along Y axis in the places of A, B and C decrease no more than 6Mpa, and the principal stress along X axis in the place of D increase no more than 3Mpa. This shows that the pore pressures in the places of A, B and C are becoming lower along the depth with the increasing of storage level and storage time for the good permeability; the pore pressure in the place of D is becoming bigger for the poor permeability, so that the horizontal principal stresses is bigger and bigger. FIG.3 INFLUENCE OF MULTI FACTOR ON STRESS FIELDS FOR 0 MONTHS AT STORAGE LEVEL OF 35M FIG. 4 is the relation curves of the depth and the stress difference between coupling stress field from Oct. 969 to Aug. 977 and decoupling stress field in Oct. 969 with water depth of 35m. This part considers the influences of coupling effect and the hydrostatic pressure effect causing by adding water depth of m. It can be seen from the curves in the FIG. 4 that the vertical principal stress in each point increases significantly, this trend is most notable in the places of A and B, then D and then C, which shows that hydrostatic pressure passes along the depth with a faster velocity in the fracture area than that in the intact rock, the horizontal principal stresses in each point has the similar changing trend with that in FIG. 3, the FIG.4 INFLUENCE OF MULTI FACTOR COUPLING ON STRESS FIELDS FOR 4 YEARS AT STORAGE LEVEL OF 57M FIG. 5 displays the relationship of the depth and the stress difference between coupling stress field from Oct. 969 to Jan. 08 and decoupling stress field in Oct. 969 with water depth of 35m. This part considers the influences from coupling effect and the hydrostatic pressure causing by adding water depth of 35m. It can be seen from the curves in FIG. 5 that the vertical principal stress difference in the place of a increases along the depth, this means the hydrostatic pressure has a fast transmission to the bottom of the model for the good permeability near the place of A. The principal stress differences in the directions of X and Y increase at first, then decrease and then increase gradually along the depth after phase Ⅱ impoundment in the fracture zone of point A. This change means the hydrostatic pressure increases quickly in the depth of km for the good permeability of carbonate rock, then the pore pressure decreases in the depth of km to 0km for the good permeability of volcano strata caused by physical chemical interaction between the seepage water and volcano rock, and then the pore pressure increases swiftly in the depth of 0km to 0km because the bottom surface of the model is impervious. The vertical principal stress differences in the places of 0

5 Frontiers in Geotechnical Engineering (FGE) Volume 3, 04 B and C increase weakly and then decrease gradually along the depth. even if the water depth keeps constant. The stress differences along Y axis in the places of B and C in FIG. 6 reduce by 5Mpa about relative to the corresponding stress in FIG. 5, and reduce by 0Mpa in the place of D. This shows the permeability along the direction of Y axis with the lower normal stress is becoming better in the places of B, C and D with the increase of coupling time. FIG.5 INFLUENCE OF MULTI FACTOR COUPLING ON STRESS FIELDS FOR 4 YEARS AT STORAGE LEVEL OF 70M The horizontal principal stress differences in the places of B and C reduce along the depth, especially in the depth of 5km to km, but no more than 8MPa. This phenomenon indicates the pore pressure becomes smaller and smaller for the permeability is better and better with the increase of storage level and storage time. The vertical principal stress difference in the place of D increased first and then decreased along the depth, but the changing range is under 5Mpa. This means the hydrostatic pressure transports down to the bottom poorly in the rock near the place of D for the bad permeability. The horizontal principal stress difference in the direction of X axis increases along the depth, but in the direction of Y axis has the adverse trend, and the changes of the two stress difference are very obvious in the depth of 7km to km. This indicates the pore pressure in the direction of X axis increase for the poor permeability and the pore pressure in the direction of Y axis decrease for the good permeability. FIG. 6 gives the relationship of the depth and the principal stress differences between coupling stress field from Oct. 969 to Jan. 08 and decoupling stress field in Oct. 969 with the water depth of 35m. This part considers the effects of multi factor coupling and the hydrostatic pressure causing by adding water level of 35m. The coupling time increases by 0 years in this diagram relative to that in FIG. 5. It can be seen from the curves in the FIG. 6 that the three principal stress differences in the place of A increase by one time relative to those in FIG. 5. This phenomenon indicates that the hydrostatic pressure can pass down to the bottom continuously and the permeability of the rock can enhance further with the increase of coupling time FIG.6 INFLUENCE OF MULTI FACTOR COUPLING ON STRESS FIELDS FOR 4 YEARS AT STORAGE LEVEL OF 70M The Calculation and Analysis of Stress Circle in Each Point Taking points of A, B, C and D as an example, the horizontal principal stresses in each point corresponding to different storage level and storage time, which can be used to draw the stress circle and failure envelope, are calculated. Because of the biggest horizontal principal stress differences always appear near the depth of 9.5km at different storage level and storage period, so the stress circles and failure envelopes in the depth of 9.5km are listed in the paper (shown in FIG. 7). The sign in FIG.7 signifies the stress circle of the decoupling horizontal principal stress at the storage level of 35m; the sign signifies the stress circle of the horizontal principal stress calculated with coupling effect for 4 years at the storage level of 57m; the sign 3 signifies the stress circle of the horizontal principal stress calculated with coupling effect for 3 years at the storage level of 70m; the sign 4 signifies the stress circle of the horizontal principal stress calculated with coupling effect for 3 years at the storage level of 70m. According to the calculation results: the horizontal principal stress circles in the places of A, B and C are all no more than the failure envelope at different depth

6 Frontiers in Geotechnical Engineering (FGE) Volume 3, 04 and different storage period, just the horizontal principal stress circles in the place of D reach to failure envelope in the depth from surface to 0km at the storage level of 70m FIG.7 STRESS CIRCLE OF HORIZONTAL PRINCIPAL PRESSURES IN EACH POINT AT THE DEPTH OF 9.5KM The place of A, which is in the intersection zone of Wafangchang zhoushan fracture and Danjiang fracture and covered mainly by carbonate rock that can be corroded easily and become leakage, has a big initial horizontal principal stress difference although it is not up to destruction state after impoundment. In view of the above, it can be speculated that the Songwan earthquake occurred in the end of the year of 973 is mainly due to the reduction of resisting shear capacity caused by water infiltration, lubrication, physicalchemical effects between water and rock. Some deep earthquakes with a certain magnitude will be induced again on the line of Songwan guanfangtan jiuxichuan because the horizontal principal stress circleenlarges toward the failure envelope in the depth of 6km tokm after phase Ⅱ impoundment. The place of B, which is near the intersection zone of Junyun fracture and Danjiang fracture and has the same geological condition as that in the place of A, cannot cause earthquake for the two horizontal principal stresses are equal by and large before phase Ⅱimpoundment. But some deep earthquakes with a certain magnitude will be induced around the place of B such as Linmaoshan danjiangkouqingshangang and Zishanao jiujunxian for the horizontal principal stress difference increase rapidly in the depth of km to km after phase impoundment. The place of C, far away from the fracture zone, in the edge of the Dan reservoir covered by intact rock which is not conducive to water infiltration and has a large cohesion and internal friction angle, will not be induced earthquakes for the horizontal principal stresses is nearly equal before phase impoundment, and after phase impoundment the horizontal principal stress difference increases rapidly but it is still far from the failure envelope. The place of D is located in Junyun fracture covered mainly by diabase weathered slightly which has little water infiltration and physical chemical effects; accordingly has small water lubrication and corrosion. Thus the deep earthquake cannot be induced before phase impoundment, although the horizontal principal stress difference is large. After phase impoundment, the horizontal principal stresses difference increases continuously and reached the failure envelope, especially in the depth of km to 4km and 9.5km around, so some deep earthquakes with a certain magnitude will be induced along the line of Junyun fault such as Junxian, Xijiadian, and Shiguguan. Conclusions A 3 D finite element model is established by software ANSYS according to the geological structure characteristics of the Danjiangkou reservoir, then the model is imported to software FLAC3D to simulate the multi factor coupling stress field at different period and impoundment level. The mechanism of reservoir induced earthquake in Danjingkou reservoir area after improving impoundment level is explored and some valuable results are acquired. First, the influence of multi factor coupling is notable on principal stresses in reservoir area, especially in the fracture or fault area, but is negligent on the shear stresses. The permeability in the direction of minimum horizontal principal pressure is better than that in the direction of maximum principal pressure in the condition of same initial permeability (see FIG. 3); second, the hydrostatic stress transports along the depth in the fracture zone of reservoir area with a faster velocity than that in the intact rock for the fracture zone has a good permeability. The hydrostatic pressure on the bottom of the reservoir will gradually transport to the depth of 0km, namely, the bottom of the model (see FIGs. 4, 5, 6); third, the reduction of resisting shear capacity of the rock caused by water infiltration, lubrication, physical chemical effects between water and rock is the main reason of Songwan reservoir induced earthquake occurred in the end of the year of 973; last, after phase impoundment, with the gradually increasing of coupling effect and physical chemical effect between water and rock, some earthquakes of a certain magnitude with hypocenter of 9.5km will be induced in the places such as Songwan guanfangtan jiuxichuan, Shangsi heitaoyuan,

7 Frontiers in Geotechnical Engineering (FGE) Volume 3, 04 Linmaoshan danjiangkou qingshangang and Zishanaojiujunxian, other earthquakes of a certain magnitude with hypocenters of km to 4km and 9.5km around will induced along the Junyun fault such as Junxian, Xijiadian, Shiguguan, but the deep earthquakes unlikely happen in the intact rock around the reservoir bank (see FIG. 7). ACKNOWLEDGEMENTS This work was supported by the Natural Science Foundation of Hubei Province (Project Number: 0FFC050 and 0CDB67). REFERENCES B. Chen, N. Li Coupling FEM analysis of deformation fields seepage field temperature field of porous media. Chinese journal of rock mechanics and engineering, 00, 0(4): 467~47. C.H. Liu, C.X. Chen, X.T. Feng Study on mechanism of slope instability due to reservoir water level rise. Rock and soil mechanics. 005, 6(5): 769~773. L.D. Yang, Z.X. Yang Seepage coupling analysis and simulation of anisotropic saturated soil. Chinese journal of rock mechanics and engineering, 00, (0): 447~45. L.X. Yi, Y.T. Che, G.T. Wang Retrospection and prospect of the research on reservoir induced seismicity. South china journal of seismology. 003, 3(): 8~379. P. Li Seismic and geologic research in the district of Sanxia and Danjiangkou. Beijing: Earthquake press, 994. P. Li, X.S. Liu Research of Junyun fracture zone and Danjing fracture zone and discussion of seismic and geologic questions in the area of Danjiangkou reservoir. Anthology of Danjingkou reservoir induced seismicity. Beijing: Earthquake press. 980, 4. S.M. Liu, L.H. Xu Numerical simulation of tectonic stress field at Danjiangkou reservoir area. Chinese journal of rock mechanics and engineering. 004, 3(3): 407~40. S.M. Liu, L.H. Xu Finite element simulation of hydraulic pressure stress field in Danjiangkou reservoir area. Journal of hydraulic engineering, 005, 36(7): 863~869. X.D. Xie Study on mechanism of reservoir induced seismicity based on theory of fluid solid coupling in the Danjiangkou Reservoir area. Wuhan: Ph.D. Thesis of Wuhan University, 00. X.D. Xie, L.H. Xu Simulation of the seepage field in the danjiangkou reservoir area. Advances in Civil and Engineering, Trans Tech Publications, 0. X.M. Gao Seismic activity of Hanjinag and Danjiangkou Reservoir. Anthology of Danjingkou reservoir induced earthquake. Beijing: Earthquake press. 980, X.Y. Kong High seepage mechanics. Hefei: University of science and technology of China press, 00. Z.Z. Shen Coupling analysis of viscoelastic stress field and seepage field on the foundation of Sanxia dam. Engineering mechanics, 000, 7():05~3. XiangdongXie worked in Hubei University of Arts and Science. He went to Wuhan University for pursuing PhD in 007 and achieved degree in 00, then has been in the University of Manitoba for the studying on the design of piezoelectric harvester from 0.09 to and 04.0 to as a visiting scholar and a post doctoral research fellow, respectively. He mainly engaged in researches on reservoir induced earthquake and design of piezoelectric harvester. And now, research on the Hubei ProvinceNatural Science Foundation (No.0FFC050). Sumei Liu works in the School of Civil Engineering, Wuhan University, Hubei, China, as an associate professor. Her main research interests are as below: i) reservoir induced seismicity (RIS), focusing on the mechanism of RIS and its evaluation; ii) vibration performance of structures; iii) Durability of concrete structure. In recent years, Professor Liu took part in three projects related to RIS as the main member. The projects were Study on RIS Adopting Geodynamical Method, Reservoir Induced Seismicity in Danjiangkou Reservoir and Study on RIS Adopting Multi Fields Coupling Method, and the third project was supported by the National Natural Science Foundation of china. And now, Professor Liu is studying on the threshold level of pore pressures in inducing reservoir seismicity (Hubei ProvinceNatural Science Foundation, No. 0CDB67). 3