Dynamic Filtration of Bentonite Muds Under Different Flow Conditions

Abstract

The variation of key parameters describing t.he dynamic fluid loss behaviour of two wat.er-based drilling muds, barite-weighted and unweighted, has been systematically investigat.ed in an annular flow dynamic filtration cell designed to provide wellcontrolled laminar flows over a wall shear rate typical of those in the well bore during drilling.An axial section of the cell inner wall is formed from a 10 millidarcy ceramic filter. The permeability and low particle penetration decouples the cake filtration from formation invasion or formation damage effects. A variable annulus gap was designed to reveal possible Reynolds. number effects as well as the dominant wall shear rate effects.Numerous dynamic fluid loss histories at a filtration pressure of 280 p.s.i. (1.93 MPa) (typical duration 22 hours) show that the fluid loss can be described by three key parameters: one describing the early ('quasi-static') behaviour, a second describing the late ('dynamic') behaviour of near-constant fluid loss rate, and thirdly a timescale for the transition between these two regimes.The variation of these parameters with mud flow wall shear rates between 50 and 650 s-1 has been quant.ified empirically. The early behaviour is virtually independent of shear, but the timescale for transition to 'dynamic' behaviour decreases sharply with increasing shear rate from 5 hours to about ! hour. The final 'dynamic' fluid loss rate correlates approximately with the 0.6 power of the shear rate; other theoretically postulated variations are not supported. The irreversible deposition of mudcake under dynamic conditions reported by others has also been observed.The results have direct application to the modelling of fluid loss in drilling and in well completions. The quantitative laws obtained have also enabled some discrimination to be made between theoretical models for the fundamental mechanisms of mud particle deposition and erosion.