A Comprehensive, Practical Approach For Wellbore Instability Management

Abstract

In the northwest shelf, a region of growing importance for oil and gas production in Australia, drilling delays and well suspension before reaching targets have occurred frequently because of wellbore instability. As a result, the economic benefits from new drilling technologies are significantly reduced because of high additional drilling costs. To improve drilling efficiency and minimize drilling costs, it is crucial to develop and apply a practical approach for optimizing wellbore instability management.This paper initially presents guidelines, in the form of design charts, for efficient wellbore stability analysis and wellbore profile design. A comprehensive, practical approach that uses the design charts for wellbore profile design is described subsequently. The application of the design charts is demonstrated through field case studies for a range of in-situ stress regimes.Effects of In-Situ Stress Regime. The variation of wellbore stability with in-situ stress regime, which was developed from safe mud-weight windows determined for each of the in-situ stress regimes and material strength categories (lower bound, mean, and upper bound strength), is shown in Fig. 5. It can be seen that drillability from wellbore stability consideration in relation to the in-situ stress regime can be divided into three broad areas.• Area 1 represents those stress regimes in which drilling operations of any trajectory can be conducted without any instability problems if appropriate mud weights are adopted. The stress regimes in this area are close to hydrostatic; that is, the differences between v , H , and h are small.

Analysis of borehole collapse and fracture initiation positions and drilling trajectory optimization

“…The anisotropic poroelastic models mainly embody the effect of rock poroelastic anisotropies on borehole stability; however, Aadnøy (1988) concluded rock anisotropic elasticity parameters had a minor influence on fracture pressure. Chen et al (2002) found the safe mud-weight window was not significantly affected by rock poroelastic anisotropies for most of the drilling formations encountered.…”

Determinación de presiones críticas de la ventana de lodo, aplicando modelos analíticos y caracterizaciones geomecánicas

“…Para que exista estabilidad entre el hueco perforado y las rocas circundantes, debe predominar el equilibrio de todos los esfuerzos, es decir, que estos sean lo más parecidos posible. Para ello, deben controlarse la dirección del pozo y el peso del lodo [12]. La deficiencia en el control del peso del lodo puede generar problemas como colapsos (o comúnmente breakout) o fracturas [13].…”

Shear failure analysis of a shallow depth unsupported borehole drilled through poorly cemented granular rock

“…Although some researchers state that non-linear approaches can deliver more accurate results, they have acknowledged that some input parameters in such models are hardly available (Fleming et al, 1990;Woodland, 1990). On the other hand, Tan et al (1999) and Chen et al (2002) showed that in ordinary oil-filled wellbores, the minimum (critical) support pressure required to keep the wellbore stable is not significantly influenced by the elastic anisotropy. In this paper for a vertical borehole the stresses which were generated on an element at the borehole wall due to far field stresses were calculated based on the elastic theory.…”

“…The in situ stress state can be defined in terms of the principal stresses, σ v , σ H , and σ h . The ratio of minimum horizontal stress to the vertical stress (k o = σ h /σ v ) ranges from 0.3 to 1.5 and the ratio of maximum horizontal stress to minimum horizontal stress (σ H /σ h ) varies from 1 to 2 in most of oil fields and exploration boreholes (Herget, 1988;Tan et al, 1993;Chen et al, 2002). Based on the tectonic and faulting condition, Anderson (1951) suggested three different stress regimes for the underground stress conditions.…”