Combined Investigation of Effects of Contact Stresses, Pore Size and Rotary Dynamics on Mud Plastering in Prevention of Lost Circulation in Weak Zones during Casing Drilling

Abstract

While drilling with casing stands instead of the classic drill pipe (CwD), the reduced standoff between wellbore wall and the rotating, sliding and bending tubular plays a critical role by "crushing and hammering" the formation cuttings into the formation. This plastering effect has demonstrated to be not only beneficial to hinder the losses of drilling mud into the formation but also to improve wellbore stability and later productivity in terms of lower skin values. Field trials have established that monitoring of cutting sizes accumulated on the shakers and their correlation to the formation pore sizes (through offset match or well tests) can be an effective approach to improvise on the mud particle size for effective particle bridging and formation sealing. To enhance the mitigation of CwD-induced losses and formation damage, however, the horizon of investigation has to be broadened to include the size of radial clearance, rotating speed (RPM) and pump hydraulics. Assuming the cutting and mud smearing to be the result of point contact forces altering the near-wellbore stresses (confining stresses), the above parameters cause a repeated but unsynchronized invasion of particles in the initial micro fractures created due to bit interaction. Depending on the RPM, radial clearance and the resulting induced lateral drillstring movement, the magnitude of the contact force / hoop stresses increases within the original fracture-gradient limits. This improves formation sealability and results in an altered (enhanced) pore- and fracture gradient in the near-wellbore region. The paper discusses the effect of Geo-mechanical and mechanical aspects of Plastering during Casing Drilling in Weak or depleted wellbores. The experimental analyses incorporates the combined effect of the point contact-forces through base drilling parameters, alongside highlighting the approach for field mud particle-size improvisation. Altogether, a broader panorama concerning the contribution of Casing Drilling to Well Integrity has been presented, setting the path to further experimental work.