Tahe Oilfield Wellbore Stability Study In Hard Brittle Shale

The oil reservoir of Tahe oilfield is under 5000 m, and has complicated geological conditions, which will cause abnormal situations during the process of drilling, such as lost circulation, hole collapse, well kick and sticking. Those accidents directly result to the lower mechanical speed and the longer drilling cycle, and make the drilling and well completion engineering have to face serious challenges. Therefore it is very necessary to know and analysis the reasons of the wellbore stability.Based on the collection and analysis of field data, the main reasons of wellbore instability have been found. Combining with experiment and theory, the paper has raised integrated measures to ensure the wellbore stability, the study is divided into two directions: Studying the typical wells data of Tahe oilfield to find the reasons of wellbore instability, using log data from the field and rock mechanics parameters from rock mechanics experiments, and combining with analysis of the wellbore rock stress state and the Mohr-Coulomb failure criterion, three pressure profiles of wellbore stability will be calculated, then the safe drilling fluid density window will be determined; According to analysis of Tahe oilfield formation characteristics of hard brittle shale at 5400~6500m which has micro-cracks and easily broken, and ensuring the effective drilling fluid density window, we can pick up the good drilling fluid system which is polymeric alcohol sulfonated anti-sloughing drilling fluid that has strong abilities of anti-sloughing, inhibitory and plugging, then combining with the integrated measures which can ensure the wellbore instability during the drilling, so we can apply it to the drilling field of TH12344 and get the desired effect.Those integrated measures not only solve the wellbore instability of TH12344 during the drilling, but also have positive significance for the successful completion of future drilling and well completion in Tahe oilfield.