Key drilling technologies for HTHP wells with narrow safety density window in the Yingqiong Basin

The safety density window of the high temperature and high pressure (HTHP) wells in the Yingqiong Basin of the South China Sea are quite narrow, and even some of them have no windows, so complex downhole accidents happen frequently in the process of drilling, e.g. overflow, mud loss, and coexistence of blowout and mud loss in the same interval, and consequently many wells are forced to be completed ahead of schedule and even abandoned. In order to solve the drilling problems caused by narrow safety density window, a set of drilling technologies and support process for HTHP wells with narrow safety density window have been formed based on years’ practice and exploration, such as optimizing the casing depth to widen the safety density window, injecting the cement into the weak formation to increase its pressure bearing capacity, adopting small-size drilling tools to reduce the circulating pressure loss significantly, selecting high-temperature elastic lost circulation material (LCM) to plug the induced fractures, and applying nano-meter loss circulation spacer and high-density slurry of manganese tetraoxide to deal with the cementing fluid loss and pressure stabilization of narrow safety density window. In this paper, the principles and field application effects of all these technologies were discussed in detail. These technologies are applied in over 10 HTHP exploration wells in the Yingqiong Basin, western South China Sea. It is shown that these technologies can effectively cope with the harsh well conditions, e.g. bottom hole pressure as high as 212 ℃, formation pressure coefficient over 2.30 and safety density window as narrow as 0.04. And as a result, the occurrence rate of drilling complexities is reduced greatly. The research results provide the reference for the similar wells with narrow safety density window.