Time Dependent Borehole Stability: Laboratory Studies and Numerical Simulation of Different Mechanisms in Shale

Abstract

Experimental work and numerical modelling have been applied to study mechanisms related to time dependent borehole stability during drilling in shales. Fully coupled numerical modelling showed that consolidation effects may act on a time scale of days, due to the low permeabilities of shale. Creep testing of a North Sea Tertiary shale showed that above 80% of the failure stress, the observed strain rates indicated that delayed borehole failure could occur within a period of 15 days. When exposing outcrop shale with close to zero swelling clay content to non-native fluids, substantial changes in mechanical properties were observed. Elevated temperature (80 C) also seemed to cause a weakening both in static and dynamic properties. Introduction Stability problems during drilling in shales account for a significant amount of lost time and extra cost during drilling. Operational experience indicates that stability problems tend to increase with time after drilling, i.e. the longer a shale section stays open, the greater the risk of experiencing stability problems. Our qualitative and quantitative under- standing of the mechanisms responsible for this behaviour is still inadequate. This paper discusses some of the mechanisms involved in time dependent behaviour of shale, illustrated with examples from laboratory testing and numerical modelling. MECHANISMS RESPONSIBLE FOR TIE DEPENDENT BEHAVIOUR OF SHALE Four different mechanisms that may lead to delayed borehole failure will be discussed. There are two intrinsic mechanisms which control the stress-strain-time behaviour of a saturated rock which is exposed to a change in load:Hydrodynamic consolidation, characterized permeability, rock frame stiffness, fluid stiffness and porosity (re. Biot theory of consolidation).Creep, characterized by stress and time dependent strains at stress levels below the failure stress. In addition, we will discuss two extrinsic mechanisms:Shale-fluid interaction, characterized by a change in shale behaviour after extensive periods of contact with a non-native fluid (i.e. fluid that is not in chemical equilibrium with the formation).Temperature effects, i.e. mud temperature variations which may alter the stresses and which may also affect the mechanical properties of the formation. Other effects of a more indirect nature will not be discussed here, like time dependent boundary conditions in the borehole, such as changes related to mud pressure (e.g. surge, swab). EXPERIMENTAL SET-UP In order to characterize the mechanical behaviour of shale cores, specialized methods and procedures for triaxial testing have been applied. Fig. 1 shows the internal instrumentation in the triaxial cell, facilitating measurement of both static and dynamic response of the test sample. The shale samples (1.5" diameter and 3" length) by are installed in the triaxial cell, and the following general procedure is applied for triaxial testing:Loading to a predetermined level of consolidation (normally hydrostatic) with a set pore pressure.Consolidation of the sample under drained conditions, i.e. equilibration of pore pressure. P. 259

An Experimental Investigation of Shale Mechanical Properties Through Drained and Undrained Test Mechanisms

“…From a theoretical point of view, drained conditions mean that the PP in the sample is an independent variable, while for undrained tests, it is a dependent variable. In real-life borehole stability simulations, both drained and undrained (Horsrud et al 1994) rock mechanical properties are necessary because they depend on the time scale. In the case of a short borehole simulation, undrained rock properties can be used, whereas for long simulations, the drained rock properties are applied.…”

“…Generally, the acoustic properties of shale are significantly influenced by the heterogeneous nature of shale. Many authors have already implicitly analyzed this issue (Horsrud et al 1994Holt et al 1996;Lockner and Stanchits 2002;Fjaer et al 2008;Sarout et al 2007). …”

“…This is, for instance, the case for a normally consolidated material such as shale. Several authors also presented the PP response in sandstone-based rock strength in different ways (Skempton 1954;Horsrud et al 1994Horsrud et al , 1998Lockner and Stanchits 2002;Fjaer et al 2008). However, in the end, the same conclusion was noted.…”

“…18 implies that the UCS varied between 9.3 and 10.5 MPa, and the failure angle varied between 23.4°and 27.9°. In the case of North Sea shale, studied by Horsrud et al (1994), the UCS varied between 6 and 77.5 MPa, and the failure angle (b) ranged between 48°and 60°. The Young's modulus (E) correlated with a UCS of 6.55E (R 2 = 0.99).…”

“…In practice, fundamental models are calibrated against field cases and readjusted. Many investigators (Johnston 1987;Schmitt et al 1994;Horsrud et al 1994;Horsrud 1998;Spaar et al 1995;Holt et al 1996;Claesson and Bohloli 2002;Crook et al 2002;Søreide et al 2009;Fjaer et al 2008) have, instead, used outcrop shales to calibrate borehole stability models. The present work deals with data sets for Pierre-1 shale at room temperature and at the high confining pressure present at downhole conditions.…”