A Novel Method of Screening Drilling Muds Against Mud Pressure Penetration for Effective Borehole Wall Support
Authors: Tan Chee P. , Zeynaly-Andabily Mohammad E. , Rahman Sheik S. 
Published: December 2007

Chee P. Tan, SPE, Australian Petroleum Cooperative Research Centre, CSIRO Petroleum, Victoria, Australia; Mohammad E. Zeynaly-Andabily and Sheik S. Rahman, SPE, Australian Petroleum Cooperative Research Centre, Centre for Petroleum Engineering, U. of New South Wales, Kensington, Australia

Abstract
This paper describes the fundamental concept of the mud pressure penetration mechanism and a novel approach which can be used to screen drilling muds against pressure penetration through the quantification of mud support reduction with time as the drilling mud invades the rock material. The reduction of the effective mud support will lead to a less stable condition at the borehole wall.
The variation of breakthrough pressure and apparent permeability with cumulative pore size distribution of a synthetic shale have been quantified for a polyacrylamide an ester-based and an oil-based mud system. Drilling fluid and shale properties required for the model, which are determined using analytical and laboratory techniques, are presented herein. The mud pressure penetration model is verified with laboratory simulation of the mechanism for the polyacrylamide mud invading the synthetic shale.
The model provides a fundamental basis for analysing the time-dependent support reduction by different mud systems. The mud pressure penetration apparatus provides an effective method of screening different drilling muds (types and compositions) in providing the effective mud support on the borehole wall for a given shale. The study conducted showed that the effectiveness of these mud systems in providing the support is largely dependent on the adhesion and viscosity of the filtrates, and permeability of the rock material.

Introduction
Wellbore instability, experienced mainly in shale sections, is one of the principal causes of drilling delays and in some cases, even suspension of wells prior to reaching the target. These instabilities may be induced by either in-situ stresses that are high relative to the strength of the formations or physico-chemical interactions of the drilling fluid with the shales. The instability can be managed more efficiently through a better understanding and development of a capacity to conduct laboratory simulation and model the penetration of the mud pressure into the formation and subsequent wellbore failure due to the interaction.
This paper describes the fundamental concept of the mud pressure penetration mechanism and a novel approach which can be used to screen drilling muds against pressure penetration through the quantification of mud support reduction with time as the drilling mud invades the rock material.
Drilling fluid and shale properties required for the model, which are determined using analytical and laboratory techniques, are presented herein. The variation of breakthrough pressure and apparent permeability with cumulative pore size distribution of a synthetic shale have been quantified for a polyacrylamide, an ester-based and an oil-based mud system. The mud pressure penetration model is verified with laboratory simulation of the mechanism for the polyacrylamide mud invading the synthetic shale.

Fundamental Concepts of Mud Pressure Penetration Mechanism
When drilling under an overbalance condition without an effective barrier present at the wellbore wall, mud pressure would penetrate progressively into the formation. Due to the low permeability of shales, the mud pressure penetration would result in an increase in pore pressure near the wellbore wall. The increase in pore pressure reduces the effective mud support which leads to a less stable wellbore condition. This time-dependent stability issue depends strongly on the mud filtrate and pore fluid properties and the rock material composition.
Breakthrough Pressure. Penetration of mud filtrate, which involves displacing a wetting fluid from a porous medium by a non-wetting fluid, requires a threshold differential pressure.
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