Study On In-situ Stress Of Tight Reservoir Under Multi-field Coupling

With the rapid development of China's economy,the demand for oil and gas resources is growing.Exploration and development of unconventional reservoirs such as tight reservoirs are imminent.Ground stress plays an important role in the study of dense reservoirs as an important data indicator to understand,utilize and reconstruct reservoirs.The calculation results of high precision geo-stress have good field guidance significance for fracturing,well pattern layout,development plan optimization and shaft stability.Establishing accurate model and considering multi-field coupling is the basis and premise of obtaining high-precision ground stress.In this paper,on the basis of theoretical research,laboratory test and numerical simulation,the multi field coupling numerical simulation was carried out,and this paper simulated the change of the geostress state with the development of the mining time.Specific research contents are as follows:Based on the logging data,the conventional rock triaxial test and the empirical formula,the static Young's modulus,static Poisson's ratio and other rock mechanics parameters of the Junggar Basin Central 4 Block Dong 2 North 3D Zone were determined.At the same time,combined with geological data,seismic interpretation data and sedimentary facies data and with the help of the PETREL software,the 3D geological model of the Dong 2 North 3D Zone was established.And the model built by PETREL software were connected with finite element software ANSYS and COMSOL Multiphyics.By using the optimization module of finite element software ANSYS and writing the corresponding APDL language,the boundary stress condition of the Dong 2 North 3D Zone were inversed.Then the COMSOL Multiphysics software was used to calculate the multi-field coupling of the target reservoir at the place scale and to simulate the changing of geostress state during mining progress.The results show that:(1)Under the multi-field coupling,the initial maximum horizontal principal stress in the study area range was mainly concentrated in: 113MPa~134 MPa;and the minimum horizontal principal stress was mainly concentrated in: 106MPa~124 MPa.All the stress was expressed as compressive stress state.The geostress value was slightly increased compared to the case when the pore pressure and thermal stress was not taken into account.(2)After coupling,the direction of the maximum horizontal principal stress in the study area was the same as the maximum horizontal principal stress direction when it was not coupled: In the eastern region,the direction mainly presented as E-W;the direction of the main stress in the central fault development area gradually changed into NW-SE direction;and then in the western region,the main stress direction gradually turned into S-N direction.Of course all the results are consistent with direction of the well logging stress.The minimum principal stress direction was perpendicular to the direction of the maximum principal stress.(3)With the increase of mining time,the maximum horizontal principal stress direction changed slightly.The stress direction of the eastern region was changing from NWW-SEE to S-N.And the direction in the west tended to N-S direction.The minimum horizontal principal stress direction also changed slightly.The direction of stress in the eastern region was gradually changed from N-S to NEE-SWW,and the direction in the western region tended to E-W.(4)With the development of the mining time,the low stress region of the reservoir pore pressure was gradually expanded near the wellhead.And it preferentially extended along the direction of the fault.The direction of the fluid seepage was mainly from the outer strata to the wellhead,and the fluid seepage mainly extended along the fault.Around the wellhead area,the velocity of fluid flow increased,and the velocity of the fluid flow in the fractured zone was higher than that in the surrounding continuum fault and the fault core.The temperature was mainly transferred from the outside to the wellhead area of D8.In the D2,D7 and D701,the temperature was transmitted from the surrounding reservoir area to the wellhead area,and the direction of temperature was mainly transmitted along the fault extension direction.At the same time,the conduction velocity was faster than that of the surrounding porous media.