Fractured tight sandstone formation is an important unconventional oil and gas accumulation distributed in a wide region of Tarim oilfield in China at great depth, and has several distinct features: high-pressure/high- temperature drilling environment, low rock matrix permeability, well-developed natural fracture system, and highly anisotropic in-situ stress. Even using highly inhibitive oil-based fluid during drilling operation, mud loss and wellbore collapse still occurred frequently. Moreover, in many wells when the mud weight is added, borehole conditions become not better but worse. Contrary to the traditional field experiences, collapse pressure itself has a mud weight window.

According to the field observations and literature reviews, we built a model based on the poroelastic theory, coordinate transformation technique and temperature field simulation. We found that classical Mud Weight Window is no longer being appropriate for wellbore analysis in fractured tight sandstone foremation. Instead, a Crtical Mud Weight Window is proposed, which properly explained unusual drilling problems.

A mud weight optimization strategy is also discussed to minimize the shear failure region around a borehole. In addition, a flow chart including the experimental equipments and calculating method is presented.This approach is applied to explain the drilling problems in 8 vertical wells drilled through Bashijiqike formation. It is observed that the new method may provide more accurate instructions for mud weight selection.

Finally, we presented a field case study in Keshen block, Tarim field. In this case, the engineers used our method to choose the mud weight. As a result, this model offers reasonable mud weight that is useful to mitigate borehole instability in fractured tight sandstone formation.

You can access this article if you purchase or spend a download.