The benefits of casing while drilling have become apparent to the industry as complex wells are drilled through depleted reservoirs. Casing while drilling operations help to reduce lost circulation, provide wellbore strengthening, mitigate formation damage, and eliminate non-productive time (NPT). The plastering effect mechanism responsible for pulverizing and smearing the cuttings in the formation to increase the fracture gradient is under extensive research to effectively realize the above benefits. An analytical model used to predict the temperature of the drilling fluid downhole while drilling with a casing will provide an improved understanding of this plastering effect. An estimate of the downhole temperature increases attributable to the persistent contact between the casing and the borehole wall, a characteristic of casing while drilling, will add to the ongoing quantitative analytical studies.
This study proposes an analytical model to analyze the heat generated from the contact between the casing and the borehole wall during a casing while drilling operation and predicts the downhole temperatures of the drilling fluid at any depth of the well. The torque acting on the casing as a result of contact forces was used to model the heat generated, and a steady- state heat transfer solution is presented. The model also incorporates the heat dissipated downhole as a result of frictional pressure losses along the drillpipe, casing, and bottomhole assembly (BHA), as well as energy dissipated through pressure losses across the bit.
The paper presents four practical casing while drilling field cases to suggest potential applications for the proposed model. Downhole temperatures of the drilling fluid were calculated along the well profile, and the increase in mud temperature along the target zones was estimated. The effect of the increase in downhole mud temperatures while drilling with a casing is then analyzed in the context of improving the fracture gradient attributable to the plastering effect. In addition, the effect of drilling parameters on the increase in fracture gradient has also been presented. This simple analytical model can be applied to casing while drilling operations to enhance our understanding of the plastering effect and to use it to our advantage.