Abstract

This paper presents a summary of the results of an experimental and theoretical study on filtration properties of water based drilling fluids under properties of water based drilling fluids under dynamic and static conditions. The experimental investigation of the filtration properties was conducted as a function of mud types, solids concentration, pressure and shear rate. A dynamic filtration equation was developed using a mass balance method based on the cake filtration theory prevailing in the chemical engineering industry. The results of experimental data showed substantial agreement with the filtration equation.

Introduction

It has long been recognized that the correct estimation of the filtration properties of drilling fluids especially under dynamic conditions, is particularly important during well drilling and completion operations as it can help the engineers diagnose and prevent a series of problems such as formation damage, prevent a series of problems such as formation damage, differential sticking etc. Throughout the history of drilling and completion operations, the quantitative modelling of drilling fluid filtration has been periodically addressed and most of these were based on periodically addressed and most of these were based on Darcy's law. The square root relationship between the cumulative filtrate volume and time has been successfully used to calculate the static filtration data. Filtration under dynamic conditions, however, has been investigated by a number of researchers and a number of equations has been published. Outmans is amongst those who first reported a theoretical study on dynamic filtration modelling and assumed that dynamic filtration finally reached equilibrium, which implies that the filtration rate and the cake thickness remain constant. He was able to predict the equilibrium dynamic filtration data.

Bezemer and Havenaar, from their experimental data, found that the equilibrium filtrate rate is directly proportional to the rate of shear at the cake surface.

Hassen proposed a series of equations to predict the static and dynamic filtration. The dynamic filtration process was distinguished by two stages, namely, non-equilibrium and equilibrium. He then used different equation to calculate the dynamic filtration at different stage.

Fordham et al suggested a dynamic filtration equation assuming that the fluid loss can be described by three key parameters: one describing the early "quasi-static" behavior, a second describing the late "dynamic" behavior of near-constant fluid loss rate, and thirdly a time-scale for the transition between these two regimes.

The above models can be used to estimate the dynamic filtration, especially at equilibrium, however, none has been accepted publicly due to the difficulties of their application. The equation proposed below can predict the filtration at any time and it is not necessary to subdivide the dynamic filtration process into several distinct phases. It is easy to process into several distinct phases. It is easy to use and provide accurate filtration data.

THEORETICAL DEVELOPMENT
External Mass Balance

Williams and Cannon first introduced the classic cake filtration equation, which was originally derived by Ruth in the chemical engineering industry by introducing Darcy's law and an overall external mass balance, to drilling fluid static filtration.

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