Understanding Chemical-Potential-Related Transient Pore-Pressure Response To Improve Real-Time Borehole (In)Stability Predictions
Authors: Tare U.A. , Mody F.K. , Mese A.I. 
Published: July 2010

Abstract
Chemical potential related borehole (in)stabilities in the field are predominantly time-dependent. With the intention of developing a real-time wellbore (in)stability modelling capability, experimental work was undertaken to investigate the role of the chemical potential of drilling fluids on transient pore pressure and time-dependent rock property alterations of shale formations. The current work presents the concept, and discusses the results of these undertakings.
The experiments using a Pore Pressure Transmission Test (PPT) apparatus expose formation (shale) cores under simulated downhole conditions to various salt solutions and drilling fluids. The uniqueness of this study comes from the fact that timedependent alteration in the pore pressure, acoustic and rock properties of formations subjected to compressive tri-axial stress are recorded during the PPT experiments. This eliminates the anisotropy-associated differences obtained when different samples are used to determine the relationship between these characteristic parameters of shale formations.
The main objective of this effort is to translate the results of the PPT tests to actual drilling conditions. At that point, the formation- drilling fluid chemical potential borehole instability model would be validated and updated in real-time to predict borehole (in)stabilities.

Introduction
Borehole instability in shales is a continuing problem that results in a substantial annual expenditure by the petroleum industry ($700 million according to conservative estimates). In the past, oil-based muds (OBM) have been the system of choice for difficult drilling. As environmental concerns restrict the use of oil-based muds, the petroleum service industry must provide innovative means to obtain OBM performance without negatively impacting the environment. Water-based muds (WBM) are attractive replacements from a direct cost viewpoint. However, conventional WBM systems have failed to meet key performance measures met with OBMs, especially while drilling high-angle, extended-reach well trajectories going through water-sensitive shale formations.
Historically, wellbore (in)stability problems have been approached on a trial-and-error basis, going through a costly multiwell learning curve before arriving at reasonable solutions for optimized operations and systems. Recent studies(1, 2) of fluidshale interactions have produced fresh insights into the underlying causes of borehole (in)stability, and these studies suggest new and innovative approaches to the design of water-based drilling fluids.
Since the identification of chemical-potential related instabilities in shales, a chemical-potential borehole stability analytical model has been proposed to describe the physio-chemical interaction(1, 3).
Advances in technology are providing new downhole sensors and tools that are increasingly able to collect and transmit information to the surface in real time, thus enabling field personnel to make faster and more informed decisions.
There are increasing economic demands on reducing rig downtimes associated with borehole instability problems. Thus, the anticipation and resolution of instabilities in the field using MWD/LWD data in conjunction with prior geophysical and shale/fluid interaction knowledge is becoming more critical.
In this study, transient pore pressure, compressional and shear wave velocities, and the deformation of shale cores under geostatic stress are measured.