This paper presents a 3D numerical study on the mechanical behaviors of natural fracture under injection loading, and further proposes a practical workflow for designing a widened safe mud weight window (MWW) for drilling subsalt well sections with natural fractures. The original safe MWW value for subsalt well sections is usually narrow. More specific, for a subsalt well section with natural fractures, no safe MWW existed because of this unfavorable geological environment. Therefore, hydraulic plug technology was adopted to reinforce wellbore stability and widen the MWW. The value for opening a natural fracture under the given mud-weight pressure was numerically calculated, and then used for selecting the lost circulation material (LCM) treatment size. In addition, stress cage, which is quantified by enhancement of hoop stress at the wellbore surface, was numerically analyzed.
An illustrative example is performed with information from a well in a deepwater Gulf of Mexico (GOM) field. It is observed that a fracture with initial width of 0.5 mm opening to 0.664 mm will result in an increment of 0.55 lbm/gal mud weight. This solution is obtained with assumption of one pair of natural fractures. In the case of two existing pairs of fractures, a 1.1 lbm/gal increment of mud weight can be directly obtained for this set because of the linear property of the model,.
The case study described in this paper provides a workflow for widening the MWW for safe drilling through a naturally fractured subsalt well section. A best practice for using this type of MWW design is also presented.