Wellbore Strengthening – An Operators Evolution from Conceptual Well Problem Solving to Operational Success

Inadequate hole section formation strength, either known within detailed engineering planning or encountered unexpectedly during operational execution, can lead to downhole losses, high NPT, contingency casing string running and failure to achieve key well objectives. Several technologies and strategies can be utilized and executed, to work within narrow PPFG windows, avoiding formation breakdown and enable successful drilling, casing running and loss free cementing. This paper outlines the operators’ evolution, in relation to one of those technologies, Wellbore Strengthening, from initial concept well problem solving through to operational success. Wellbore strengthening has been around for many years, with success achieved, across many applications, to strengthen the formation above the original in-situ minimum horizontal stress. This is accomplished through the incorporation of a formulated blend of sized particles, previously typically calcium carbonate and graphitic material, but now also often including cellulose and fibrous material within the drilling fluid. By inducing pressure, the rock is fractured, the solids in the drilling fluid plug and seal the induced fracture, holding the fracture open, as the fluid is bled off behind the bridge, thereby creating additional strength via a hoop stressed zone. The formation strength, primarily of weak sands, has been increased by low 10’s of psi, to 100’s of psi, or ultimately pushed by specific drivers, up to many 1000’s of psi. The detailed design and testing of potential and selected wellbore strengthening formulations has been documented and considerable expertise is available within the oil and gas well drilling fluids companies to fully support the successful application of this technology. Wellbore strengthening application techniques can really be split into either of two types: Dynamic or Static, where the design of the wellbore strengthening formulation has common aims and technical objectives. However, the difference in complexity between the two techniques is significant, with static wellbore strengthening being little more than the combination of two common rig operations of open hole pill spotting and performing a LOT or FIT. However, dynamic wellbore strengthening is a much more complex, heavier on time and cost and more onerous to execute. This paper will present detailed examples of wellbore strengthening success, across several wells, what has been achieved, outlining practical lessons learnt, including some common wellbore strengthening misconceptions. Wellbore strengthening formulations are optimally developed where detailed geological input parameters, such as Dynamic Young’s Modulus, Poisson’s Ratio etc. are known, however a practical approach is outlined for where this data is unavailable, particularly applicable to exploration wells in new basins.