Injection-induced thermo-poro-elasticity and boundary condition analysis for wellbore stability

Xinle Zhai, Kamelia Atefi-Monfared

Dans les comptes rendus d’articles de la conférence: GeoEdmonton 2018: 71st Canadian Geotechnical Conference; 13th joint with IAH-CNC

Session: Rock Mechanics and Engineering Geology II

ABSTRACT: Numerous studies have been conducted thus far to predict temperature and pore pressure variations within an injection layer. Available studies were developed assuming plane strain conditions, thus trivial deformations within sealing rocks. However, the response of a reservoir in the plane perpendicular to induced flow has substantial effects on in situ stress regime. Another common assumption is a content temperature at wellbore boundary from the very beginning of injection initiation. This assumption implicates a discontinuous jump in the temperature profile, from an initial state to fluid temperature. This paper presents new coupled closed-form thermoporoelastic analytical solutions for spatiotemporal pore pressure, temperature, and displacement evolutions induced in a reservoir confined with flexible sealing rocks, during early stages of injection when temperatures at wellbore change from an initial state to fluid temperature.

RÉSUMÉ: De nombreuses études ont été réalisées jusqu'à présent pour prévoir les variations de température et de pression interstitielle dans une couche d'injection. Les études disponibles ont été développées en supposant des conditions de déformation plane, donc des déformations triviales à l'intérieur des roches de scellement. Cependant, la réponse d'un réservoir dans le plan perpendiculaire au flux induit a des effets substantiels sur le régime de contrainte in situ. Une autre hypothèse courante est une température de contenu à la limite du puits de forage dès le début de l'injection. Cette hypothèse implique un saut discontinu dans le profil de température, d'un état initial à la température du fluide. Cet article présente de nouvelles solutions analytiques couplées thermoporoélastiques pour la pression porale spatio-temporelle, la température et les évolutions de déplacement induites dans un réservoir confiné avec des roches de scellement flexibles, au début de l'injection lorsque les températures passent d'un état initial à un fluide. 1 INTRODUCTION Injection of fluids with temperatures higher or lower compared to that of the in situ strata is a typical process in numerous energy and water production and/or storage operations. The resulting alterations in in situ pore pressure and temperature have substantial effects on stress state and deformations within a rock skeleton. Heat energy transfer into a reservoir rock occurs through two different mechanisms: conductive heat transfer, and convective heat transfer (Detournay and Cheng, 1988; Wang and Dusseault, 2003). In rocks with lower permeability, fluid flow from the wellbore into the rock takes place very slowly, thus thermal energy exchange between the injectant and the solid skeleton is mainly through conduction. In rocks with high permeability, fluid flow rate into the reservoir rock is rather high. The heat energy transferred to the rock via conduction is thus insignificant, and convection becomes the main heat transfer mechanism. Palciauskas and Domenico (1982) demonstrated the pore pressure and deformations in a rock due to the thermal loading using analytical methods. Results suggested the thermal effects to be irreversible and rather significant on pore pressure generation. Chenevert and Salisbury (1993) conducted a series of experiments on rock to obtain the variation of rock permeability with effective stress and porosity. Then the deformation of rock surrounding wellbore was studied in transient and steady-state flow. Rajapakse (1993) used Laplace and Fourier integration transforms to derive general solution for an axisymmetric stress analysis of a cylindrical borehole in an infinite poroelastic medium. The displacements, stresses and flow were presented based on modified Bessel functions of the second kind. The simplest theory describing the coupled response of a porous strata under induced pore pressures is the theory of isothermal poroelastic consolidation, originally proposed by Biot (1941). Rice and Cleary (1976) revised this theory considering the cdiscussed stress and pore pressure responses in plane problems. Considering only the conductive heat transfer, and assuming a constant temperature at the wellbore boundary, the solution to the general heat diffusion equation can be obtained using Laplace transform (Carslaw and Jaeger, 1959; Chen and Ewy, 2005). As the solution for temperature variations is yet complicated and difficult to implement in the field, the complementary error function has been typically adopted in the literature to obtain an approximate and simplified expression for early injection times and small radial distances from the injection source (McTigue, 1986; Wang and Papamichos, 1994). Two fundamental sources exist behind geomechanical variations occurring during a typical injection operation: pore pressure, and heat. Pore pressures at the wellbore are believed to continuously increase with injection initiation. As for the temperatures at the wellbore boundary, the reservoir response can be assessed in two stages. (1) The stage during which the rock temperature at the wellbore boundary changes from an initial state to the fluid temperature, referred to as the first stage in this paper. (2)

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