ABSTRACT

A new experimentat technique for the investigation of drilling-fluid induced borehole instability in shales is introduced. Pressure transmission transients are used to study pressure penetration in shales, the latter being a main shale-destabilising mechanism. The pressure transmission method provides a powerful tool for the fundamental study of shale-fluid interactions and for development of improved water-based mud formulations to promote borehole stabilisation in shales. The capabilities of the method are illustrated by permeability measurements made in low-permeability shales. In addition, a wide range of conventional and new mud systems is screened and the shale-stabilising performance of these systems is assessed. It is argued that the strategy for promoting borehole stability in shales through improved mud formulation should be based on maintaining pressure isolation between wellbore and formation.

1. INTRODUCTION

Borehole instability in shales is a prime technical problem area in oil- and gas-well drilling. Costs for the industry spent on shale stability problems have been estimated at $500 million/year. Shale drilling is notoriously difficult when using water-based drilling fluids, which are now replacing technically superior, but environmentally unacceptable, oil-based muds. Past efforts to improve water-based fluid formulations for shale drilling have been hampered by an imperfect understanding of the fluid properties required to prevent the onset of borehole failure. Recent studies of shale-fluid interactions [1–4], however, have revealed many of the underlying causes of borehole instability and have suggested a new approach to water-based mud design. This development has been aided by a growing awareness that the design of improved water-based muds greatly benefits from laboratory techniques that can realistically simulate downhole shale-fluid interactions. As a result, new downhole simulation techniques are replacing inaccurate and outdated techniques in drilling fluid rerearch. This paper introduces a new technique based on pressure transmission measurements, for monitoring fluid transport and associated pore-pressure effects in shales.

2. DRILLING FLUID INDUCED BOREHOLE INSTABILITY IN SHALES

Shales are heterogeneous, low-permeability media of which the matrix consists to a large extent of clays. The forces that act on a shale system can be divided into mechanical and physico-chemical forces. The former include the in-situ stresses, the pore-pressure and mechanical forces in the cementation that may develop in response to tensile or corressive loading. The physico-chemical forces in the clay parts of the shale include the well-known van der Waals force and the double-layer repulsion. Also, at small platelet distances, a variety of short-range forces become important, e.g. oscillatory- and structural forces (for a recent review, see ref. [5]) All physico-chemical forces combined give rise to the 'hydration stress', which usually is repulsive and has a well-defined direction between two clay platelets. In the inhomogeneously distributed ensemble of clay particles and other shale materials, the preferential direction of the hydration stress is usually averaged out. The hydration effects are then described in terms of a scalar quantity: the 'hydration pressure'.

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