Abstract
Fluid withdrawal and pore pressure reduction change the effective stresses around a borehole and cause borehole instability associated with progressive localization of the damaged zone as well as potential fines production. Experimentally, chalk exhibits a complex geomechanics behaviour (pore collapse, shear failure, time/rate dependency) and modelling the behaviour of the borehole under in-situ and operational conditions requires the constitutive model to be capable of capturing the observations. This study presents a workflow that integrates rock mechanics testing on cylindrical specimens as well as specimen with a single lateral hole (SLH) and a finite element code, developed for chalk. The code incorporates post-peak softening as well as the rate dependency of the pore collapse stress in order to accurately predict the wellbore stability under in-situ stress conditions. The tested SLH specimen was CT imaged before and after testing for identifying the damaged zone and its extension. Backward numerical simulations of the SLH test data improved the accuracy of the estimated rock mechanics properties (post-peak failure and dilatancy) compared to the properties estimated by back analyses of standard triaxial tests with a single element simulator. The workflow is applied to predict the stability of a small lateral borehole (2 cm) created with Radial Jet Drilling technique with two different geometries: one with circular geometry created by a rotating nozzle; another with a circular hole with wing shaped cracks likely to develop when a static nozzle is used. Results of the wellbore stability analyses applying the chalk properties from the back analyses highlighted the importance of using experimentally verified post-peak failure and dilatancy parameters, together with a modelling tool capable of simulating shear strain localization incorporating the Cosserat approach. • This study integrates rock-mechanic tests and numerical simulations for open hole stability in chalk rocks. • Back analysis of standard triaxial tests using the single element simulator is carried out. • A Design and predictive model for the Single Lateral Hole test is validated. • Study shows the significant development of breakout zone during creep phase near the hole.
Highlights
A detailed wellbore stability analysis is becoming increasingly important in order to construct cost-effective open-hole completions, especially for drilling methods, such as deviated, extended reach and horizontal wells (Martins et al, 1999; Christensen et al, 2004; Tan et al, 2004), and conventional and high-pressure jet drilled multilaterals (Kamel, 2017; Reinsch et al, 2018a; Liao et al, 2020; Huang and Huang, 2019; Reinsch et al, 2018b; Medetbekova et al, 2020a)
Including a pore collapse yield surface is important for the compaction analysis, while the shear failure part of the yield surface is essential for the borehole stability analysis
A workflow developed for stability analyses and for obtaining the required rock properties is based on five main parts: 1. the basic rock mechanics testing for estimating the rock properties; 2. a rock mechanics testing method, called the single lateral hole (SLH), to study the wellbore stability under various stress paths, creep and flow conditions; 3. utilizing Computed Tomography (CT) imaging for identifying the damaged zone and its extension, and 4. backward numerical simulations of the SLH test data to improve the estimated rock mechanics properties and 5. forward numerical simulations utilizing the estimated properties to predict the stability of open-hole in chalk under reservoir in-situ stresses and operational conditions
Summary
A detailed wellbore stability analysis is becoming increasingly important in order to construct cost-effective open-hole completions, especially for drilling methods, such as deviated, extended reach and horizontal wells (Martins et al, 1999; Christensen et al, 2004; Tan et al, 2004), and conventional and high-pressure jet drilled multilaterals (Kamel, 2017; Reinsch et al, 2018a; Liao et al, 2020; Huang and Huang, 2019; Reinsch et al, 2018b; Medetbekova et al, 2020a). The yield mechanisms such as shear failure, pore collapse and tensile failure are the main deformation mechanisms describing the behaviour of many rock formations. To simulate the postfailure behaviour, a strain softening must be incorporated in order to capture the progressive damage associated with the reduction in the material strength Another complexity with chalk behaviour is that it undergoes timedependent deformation called creep, characterized by a continuous decrease in void space as time passes, while the effective stresses remain constant (Hickman, 2004). A rock mechanics testing method, called the single lateral hole (SLH), to study the wellbore stability under various stress paths, creep and flow conditions; 3. Forward numerical simulations utilizing the estimated properties to predict the stability of open-hole in chalk under reservoir in-situ stresses and operational conditions A workflow developed for stability analyses and for obtaining the required rock properties is based on five main parts: 1. the basic rock mechanics testing for estimating the rock properties; 2. a rock mechanics testing method, called the single lateral hole (SLH), to study the wellbore stability under various stress paths, creep and flow conditions; 3. utilizing CT imaging for identifying the damaged zone and its extension, and 4. backward numerical simulations of the SLH test data to improve the estimated rock mechanics properties and 5. forward numerical simulations utilizing the estimated properties to predict the stability of open-hole in chalk under reservoir in-situ stresses and operational conditions
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