Enhancing the efficiency in drilling operations can lead to significant reduction in the overall costs of wells construction. Efficiency is generally achieved by maximizing drilling rate of penetration (ROP) while minimizing non-productive time (NPT) such as loss of circulation, well control and stuck pipe incidents. Reducing the uncertainties associated with subsurface formations, especially formation stresses around the wellbore, is one key aspect to decreasing the frequency of NPT incidents.
Understanding wellbore stresses and the mechanical properties of subsurface formations is essential to optimize drilling surface parameters and drilling fluids properties to ultimately maximize drilling ROP and minimize NPT. Determination of the geomechanics drilling mud window in real time allows drilling operations to proactively prevent fractures and/or formation instabilities. The availability of the mud window in real time will enhance the rig reaction time to any abnormalities experienced while drilling to maintain bottomhole pressure (BHP) consistently within the window.
The drilling mud window is constrained by a maximum and a minimum mud weight (MW) boundaries. The lower limit represents the stability gradient and the upper limit represents the fracture gradient. Drilling with a MW below the lower limit may cause formation instabilities such as caving and swelling that could lead to more severe consequences such as stuck pipes. Exceeding the upper limit MW may induce formation fractures that lead to loss of circulation that increases the risks of well control incidents.
The developed model automatically and continuously calculates formation mechanical properties such as Young's modulus and Poisson's ratio using sonic logging while drilling (LWD) data. Based on formation specific correlations, the model then determines the in-situ stresses, induced stresses, and principle stresses. The fracture and stability gradients can be determined and converted to a MW for easier communication with the drilling crew. The maximum and minimum MWs are displayed as curves in real time that allows immediate adjustments to drilling parameters and/or drilling fluid properties.
Geomechanics studies that contain the mud window are usually conducted pre-drilling using offset wells data, and these studies are often updated post-drilling only, which does not reduce the uncertainties associated with them. A real time model maintains the window relevant and up to date with the new data generated from the well.