Research On Borehole Stability In Junggar Basin

Junggar Basin has adverse geological conditions, complicated structure and deep-buriedreservoir. Due to the complicated geological structure, the steep & broken landform as wellas unstable borehole have become teriible probelms. Accoring to the data on the mines thathave been competed in the central district of Junggar Basin, stability of borehole wall is animportant factor to restrict the construction cycle locally. The situation of many mines hasbecome more complicated due to the collapse of borehole walls, thus causing huge economiclosses. With the deepening of exploration and development, an effective technology isrequired so as to reduce costs and investment risks. In Junggar Basin, Sinopec hasconstructed many deep mines with short development cycles, but until now there haven'tbeen enough basic research on this type of mines. Due to the low level of exploration &development, lack of awareness of environmental factors for well drilling, great difficulties indesigning reasonable mine structure, and unreasonable density of drilling fluid, thedifficulties in mine construction are greatly enhanced. For instance, the studies on groundstress are lacked. In the exploration & development of oil and gas, ground stress is an veryimportant index that is related to the stability of borehole wall, deformation and damage ofcase pipes, etc. Moreover, the stress can help to perfect the design of drilling engineering anddesign reasonable methods to drill mines and stabilize boreholes, thus avoiding the risk indrilling projects.To sovle the serious problems in drilling projects in Junggar Basin, a series of effectivemeans and methods were put forward. By analyzing some advanced and mature technologiesat home and in accordance with the practical geological environment for drilling, a largeamount of information on rock core in the main districts of Junggar Basin was analyzed, andan indoor rock mechanics test was carried out. Moreover, in accordacne with the test results,the rock mechanics sections of many districts in the basin were established by use of loginformation. The indoor test of ground stress was conducted by means of Kaiser Effect Test,DSA Test, and Paleomagnetic Test and so on, thus obtaining a lot of test data. In addition,accoring to the dipmeter log, conventional log and spot drilling log, an comprehensiveanalysis on ground stress of Junggar Basin was conducted, and the corresponding section of ground stress was established. Based on an analysis on stress around borehole, someadvanced domestic and foreign methods for forcasting strata collapse pressure andbreakdown pressure were summed up and analyzed, the forecasting model was established,and the sections of strata collapse pressure and breakdown pressure in the main districts werealso established, thereby determining the safe densitywindow of drilling fluid.An statisfactory effect in outcome application was achieved, while the precision of ourground stress section and rock mechanics section was both more than 85%. Compared withthe testing parameters of the reference well, those of the well-drilled wells increasedsignificantly, and almost no borehole wall sloughing based on mechanical factors occurredin these wells.