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Abstract

In this article we discuss mechanisms of stress-induced wellbore damage, concentrating on the drilling phase. Damage is usually considered to be negative; we phase. Damage is usually considered to be negative; we argue that yield and mechanical damage are often positive factors in stabilizing boreholes and in enhancing positive factors in stabilizing boreholes and in enhancing permeability. Various behavioral models for rock under permeability. Various behavioral models for rock under different situations are discussed, and conceptual models of damage and stresses around boreholes are presented. Finally, we discuss briefly issues of borehole stability in high stress fields during production, with respect to solids production management. production management

Introduction
Mechanical Damage

Mechanical damage is irreversible degradation of strength or stiffness, or alteration of flow properties, as a result of permanent changes in material fabric around the borehole. Permanent fabric changes are plasticity processes associated with yield which causes permanent processes associated with yield which causes permanent alteration of material properties. Recognising the importance of plasticity processes around wellbores, leading to alteration of material properties, will aid analysis and interpretation of geophysical log data, wellbore stability, and well productivity. This paper does not deal with fines migration and entrapment as a "skin" generating mechanism, nor with porous medium invasion by drilling mud particulate porous medium invasion by drilling mud particulate matter, nor with saturation alterations leading to wettability or relative permeability changes. We confine our discussion to mechanical damage linked to stress changes associated with borehole creation, and with processes implemented through that borehole. Throughout this paper, nonlinear, plastic behaviour is referred to as "yield".Wellbore damage may have positive or negative consequences; if massive shale sloughing leads to bore' hole loss, damage is a negative factor. Conversely, a positive aspect of wellbore damage is the continue sand positive aspect of wellbore damage is the continue sand production which occurs in some heavy oil reservoirs and production which occurs in some heavy oil reservoirs and greatly enhances productivity. Also, during drilling, plasticity processes reduce the stress concentration plasticity processes reduce the stress concentration magnitude in the wellbore wall, leading to stabilization of the wellbore region. Mechanical damage may occur during drilling, completion, or production. In shales, damage leading to negative consequences is most common during drilling. During completion and production, mechanical damage in sandstones, high porosity carbonates, coals, and tight gas sands affect resource development; these more permeable materials rarely give problems during drilling. permeable materials rarely give problems during drilling.

Extrinsic and Intrinsic Factors

Intrinsic factors are the rock properties themselves (Figure 1); these are functions of mineralogy, porosity, diagenetic history, cementitious material, granulometry, fluids, and so on. Mechanical properties include strength as a function of confining stress, deformability, absolute and relative permeability, the tendency of the rock to dilate when damaged, and so on. Extrinsic factors which may cause damage include mainly stress (o), temperature (T), and pore pressure (p). Changes in these may cause mechanical damage, and this causes irreversible alteration of the intrinsic properties of the rocks in the bore hole region. In other properties of the rocks in the bore hole region. In other words, changes in extrinsic factors may lead to plasticity processes that generate permanent rock fabric changes, processes that generate permanent rock fabric changes, causing the rock to behave differently if a similar stress path is later followed. Identifying yield processes and path is later followed. Identifying yield processes and quantifying them will aid understanding of process mechanics involved in bore hole stability, perforation damage and solids production.

Damage and Plastic Deformation

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