| In order to solve the problem of complex and accident prone drilling condition in Weixinan oil fields of Zanjiang Division, CNOOC, the author modeled current tectonic stress field in Weixinan area by elastic-plastic incremental limited unit modeling method. The fracturing or fissure belts, controlled by recent tectonic stress field, and their influences to stability of borehole wall were studied, after 3D distribution of current tectonic stress was determined. Then, the borehole wall stability belts were forecasted and some drilling advices were provided.Most of the fractures formed in Paleogene in Weixinan depression. The tectonic evolution since Cretaceous was divided into four tectonic epochs, according to regional tectonic evolution research, study in development of dominating depression faults with displacement-length method, and in mechanism and evolution stages for fractures. They are as follows: middle epoch of Early Cretaceous - Paleocene (135-52 Ma, maximum principal compression stress orientation was NE-SW trending), Eocene - Oligocene (52-23.5 Ma, maximum principal compression stress orientation was WNW-ESE trending), Neogene - Early Pleistocene (23.5-0.78 Ma, maximum principal compression stress orientation was nearly S-N trending), and Neotectonic period (Middle Pleistocene - Holocene, 0.78 Ma-, maximum principal compression stress orientation was NW trending). By study in growth of dominating depression faults and moving of subsidence center of depression, it was found that the boundary fault of Weixinan depression was oriented in NE trending and as normal faulting extended in NW-SE orientation before Paleocene, and then which moved mainly as strike-slip fault in Eocene and Oligocene. No. 1, No. 2, No.3 and No.4 faults, oriented in ENE or nearly E-W directions with en echelon type, were derived from Weixinan boundary fault, and moved as normal faulting in Eocene and Oligocene, thus a dextral strike-slip fault system formed and controlled the development of fault and fissure belt in Weixinan area. In the influence of tectonic stress fields since Neogene, that fault system is generally trans-compression and relatively close.A lot of faults and fissures developed in the strata of Weixinan area, with stronghydration and anisotropic, the mudstone layers in Second member of Weizhou formation and Second member of Liushagang formation are researched especially. The inhomogeneity of strata was studied in detail, particularly to research sedimentary lithofacies in each borehole, two plan views and 11 profiles based on seismic data. It was divided into 13 mediums with different strength in that area, and which is favorable to improve the precision of numerical modeling.Depending on comprehensive research, making the 3D estimated modeling for tectonic stress and considering the condition of inclined borehole used logging data, the stress value was estimated in 17 boreholes and 18418.05 meters in length. Some rock mechanics parameters were also estimated, i.e. Young's modulus (E) and Poisson ratio (v). The author combined the data of fault plane solution and diameter change of borehole wall and got the recent attitude of maximum principal compression stress was 320° Z5° in Weixinan region.Used the modeling parameters as similar as to real geological status, the modeling in two plan layers for project areas of 11-1N,11-4N and 12-1 and in 11 profiles were performed, and in which the estimated and modeling parameters of current tectonic stress field were well coincided, and also supported by the borehole history data. Then, the borehole wall stability was forecasted rather detail and some corresponding advices in drilling engineering were proposed, according to the modeling data of recent stress field and division of borehole wall stability belts.
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