Numerical approach for wellbore stability and minimum required mud weight
Wellbore stability for drilling oil and gas wells is a vital art in drilling engineering that is related to geomechanical properties of the rock as well as the design of well profile. The major factors in the wellbore stability study are the minimum required mud weight and the well profile. A Microsoft Excel model is built to determine required minimum mud weight to drill a stable well, based on the mathematical approaches of most common rock failure criteria regarding drilling operations; Mohr-Column, Mogi-Column, and Modified-Lade. It provides the feasibility of selecting one of the three rock failure criteria, based on user requirement to match the geomechanical model proposed for the area. Moreover, it allows the feasibility to calibrate the calculated UCS values to match actual values and get reliable results. The model structured on analysing wellbore stresses and calculating the minimum mud weight that compensates the resultant of these stresses to control wellbore stability. Creating the model aims not only to determine minimum mud weight, but also to optimize well trajectory to reduce potential drilling hazards that reflects on safety, time, and cost.
The model works according to certain assumptions, include that rock is elastic and homogeneous. Result of the model is a graph that indicates the locations, where rock may undergo failure, and minimum required mud weight for wellbore stability with zero-degree breakouts failure. It also allows to investigate the effect of depth, inclination, and azimuth on minimum required mud weight considering the three faulting regimes. The model is verified for offshore deviated wells, and it can be used for onshore and vertical wells. It has been applied on an actual case study at Gulf of Suez – Egypt – and its results were compared with the results of a commercial model to check their reliability.
Results of the models highlights many conclusions; wells parallel to minimum horizontal stress are more stable than wells parallel to maximum horizontal stress in case of normal faulting regime. Well inclination has significant effect on the expected minimum mud weight, in case of reverse faulting regime.