Cement Plugs: Stability and Failure by Buoyancy-Driven Mechanism

Abstract

Setting cement plugs off bottom is a well recognised problem in the industry. Different causes of failure have been identified over the years and many have been addressed by improving practices used in the field. A remaining problem is the instability of the lower interface between the cement and the fluid below, driven by the greater density of the cement. That is, once a cement plug has been injected into the wellbore it must resist the tendency to fall through the less dense drilling fluid below until the cement has set. The fluid properties which ensure stability have never been satisfactorily established and it is this gap which is addressed by the current work. Theoretical and experiment results are presented in this paper which allow the estimation of the yield stresses required of the fluids to stabilise a cement plug under any conditions of hole size, inclination and density difference between the cement and the fluid underneath. The experimental work shows that the model is somewhat conservative and this is discussed* in terms of the assumptions made in each case.