Abstract

Wellbore instability in shales is a major problem in drilling and completion operations. Shale instability is primarily caused by mechanical failure of the rock around a wellbore, but physico-chemical interactions between drilling fluids and shales tend to exacerbate this process. Due to increasing environmental awareness water-based drilling fluids are being increasingly utilized. However improper application of water-based drilling fluids to drill sensitive clay-rich formations can lead to wellbore instability problems.

A major collaborative effort was undertaken to develop novel environmentally acceptable water-based drilling fluids with high membrane efficiencies to meet the requirements of the petroleum industry. This paper describes the development of these drilling fluids. Laboratory experiments on shale samples under realistic downhole conditions exposed to these drilling fluids show that shale stability can be achieved through an osmotic outflow of pore fluid and prevention/minimization of mud pressure penetration. This new generation of membrane efficient water-based drilling fluids generates exceptionally high membrane efficiencies (in excess of 80%).

A fundamental understanding of the key drilling fluidshale interaction mechanisms for shale stability and the application of experimental data to field conditions have resulted in an improved application of water-based drilling fluids to successfully achieve drilling objectives. Practical guidelines for maintaining shale stability with these new generation water-based drilling fluids presented here may be adopted for trouble-free drilling of shales and significant reduction in non-productive time.

Introduction

Shales make up over 75 percent of drilled formations and cause over 90 percent of wellbore instability problems. The drilling of shale can result in a variety of problems ranging from washout to complete collapse of hole. More typically, drilling problems in shales are experienced as bit balling, sloughing, or creep. Instability in shales is a continuing problem that results in substantial annual expenditure by the petroleum industry - $1.3 billion according to estimates. With the cost of drilling increasing, the need to drill extended-reach wells with long open hole intervals has also increased. In the past, oil-based muds (OBM) have been the system of choice for difficult drilling. Their application has been typically justified on the basis of borehole stability, thermal stability, fluid loss, lubricity, etc. 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. But, conventional WBM systems have failed to meet key performance measures that are met with OBM systems, especially while drilling high-angle, extended-reach well trajectories going through watersensitive shale formations.

Past efforts to develop improved WBM for shale drilling have been hampered by a limited understanding of the drilling fluid-shale interaction phenomenon. This limited understanding has resulted in drilling fluids designed with inadequately optimized properties that are required to prevent the onset of borehole instability problems. Historically, wellbore instability 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.

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