Drilling a well safely is the cornerstone of our industry. Fields with complex geology add another set of challenges due to the higher uncertainty on formation dips, fractured or weak formation to name a few. The most unstable zones in the well construction process are thinly bedded shales. Their laminated nature creates variations in rock strength between the different layers making them very susceptible to rock failure. Unfortunately, these formations are very common and can occur at any stage of the well construction.
Traditionally, a one dimensional (1D) geomechanical pre-drill model is prepared using offset data (conventional triple combo with sonic data) and is used for mud weight calibration and to prepare the appropriate action plan to be implemented during the drilling of the well. While drilling, real-time triple combo, and acoustic data are acquired, refining the 1D geomechanical model, reducing the uncertainty with each newly acquired data point, enabling safer drilling.
The new workflow adds another crucial component: Rock strength characterization. Rock strength comprises the Unconfined Compressive Strength, the Tensile Strength, and the Friction Angle. These factors are critical to estimating the shear failure during wellbore stability analysis. Rock strength is obtained from rock laboratory tests, but when unavailable, it can be modeled and further refined while drilling from sonic measurements and porosity if the angle between the wellbore and the formation is known while drilling.
The pre-drill model is built using dip information from surface seismic data. During the drilling process, dip information needs to be gathered throughout the overburden regardless of lithology. Using a deep directional resistivity tool commonly used in well placement, the dip of the formation is computed continuously in the shales and the sands. The 1D geomechanical model is now refined in real-time with the triple combo, multipole acoustic, and dip data. The additional data acquired in real-time drastically reduces the uncertainty, not only, of the mud weight calibration, but also the newly added component of rock strength.