| With the deepening exploration of oil and gas,the sandstone reservoirs with low permeability gradually became the important focus of the petroleum industry.The main exploration target of the KE gas field is the tight sand of the Lower Cretaceous Bashkichik Formation.After the long-term development,the formation pressure of the reservoir changed,which lead to the changes of in-situ stress,especially the two horizontal principal stresses.And the changes will cause more complex changes on the fracture pressure and caving pressure,which seriously affects the design of drilling and the construction of the gas field,thus affecting the long-time development of the gas field.Therefore,the accurately predicting of formation pressure and ground stress during mining is of great significance to oil and gas exploitation,especially the unconventional oil and gas.Taking KE field as the research object,based on the field data and the three-dimensional geological model of KE field,this paper analyzed the variation laws of formation pressure during the field development.This paper also used the finite element technology to carry out 4D stress analysis to analyze the influence of the changes of formation pressure on ground stress.Gained the following results:(1)The formation pressure in the initial state of KE field was analyzed.The result showed that there is only one pressure system in the KE gas field,the formation pressure in the initial state ranges from 110.4MPa to 123.5MPa,and the average value is 118.0 MPa,the coefficient ranges from 1.63 to 1.84,and the average value is 1.73.(2)The variation trend of formation pressure during the development was analyzed.The results showed that the formation pressure gradually reduced during the development of the filed.Reading from the plane map,it reveals that in the central part of the reservoir,where the well positions are densely distributed,the formation pressure changed faster.(3)The analysis of the initial tress of the field showed that the stress state of the work area is SH?Sv>Sh,and the difference between the maximum and the minimum horizontal stress was so obvious.The vertical principal stress is mainly related to the tectonic form,which is lower in the high part of the anticline and higher in the low part of the anticline.The horizontal principal stress is lower in the middle part and higher in the East and West part of the field.The horizontal principal stress near the faults are lower,while there is a small stress concentration in the North side of the large fault in the Northwest part.(4)The results of the four-dimensional geomechanical coupling analysis showed that during the development of the gas field,the horizontal minimum principal stress and the horizontal maximum principal stress changed with the decrease of the formation pressure in the whole area.The main variation area is the central part of the field,the ground stress in the central increases obviously,while the stress in the periphery decreases.(5)Based on the 3D geological model and the spatial distribution of formation pressure and stress in different mining periods,the three-dimensional spatial distribution of formation collapse pressure and fracture pressure in different periods of the work area are analyzed and obtained from the theory of mechanical stability.The comparison between the calculated mud density window and the drilling practice in different well positions in different periods of the field shows that the calculated mud density windows for safe drilling are consistent with the actual performance of the project,and they are reliable and suitable.The results show that the change of formation pressure have a significant impact on formation collapse pressure and fracture pressure in the process of gas field exploitation.It is of great significance for the scientific design and safe-efficient construction of new drilling in the field to consider the threedimensional spatial distribution of formation facture pressure and formation collapse pressure and its dynamic change. |
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