ABSTRACT:

An elastic-plastic model is presented for describing the behavior of partially-saturated collapsible rocks. It extends the Mohr-Coulomb plasticity model to account for pore collapse behavior and considers the effect of partial saturation and capillary suction on the intergranular cohesion of the rock. Comparisons are made between theoretical and experimental results from hydrostatic compression and water injection tests on Pietra Leccese chalk showing the capability of the model in reproducing the behavior of such rocks.

RÉSUMÉ:

Un modèle elasto-plastic est presente pour les roches partiellement saturees collapsables. Le modèle est une extension de la plasticite de Mohr-Coulomb pour prendre en compte Ie comportment de collapse des pores. II considère de plus l'effet de la saturation partiale cr de la succion capilaire sur la cohesion intergranulaire de la roche. Une comparaison est faite entre les predictions theoriqucs et les resultats experimentaux obtenus durant des essais hydrostatiques et des essais d'injection d'eau sur la craie Pietra Leccese. Cette comparaison demontre la capacite du modèle à reproduire Ie comportment de telles roches.

ZUSAMMENFASSUNG:

Es wird ein elastisch-plastisches Modell fuer die Voraussage des mechanischen Verhaltens teilweise gesattigter, kollabierbarer Gesteine vorgestellt. Es erweitert das Mohr-Coulombsche Plastizitatsmodell durch die Beruecksichtigung des Porenkollaps-Verhaltens sowie der Effekte der teilweisen Sattigung und der kapillaren Saugwirkung auf die intergranulare Kohasion des Gesteins. Vergleiche zwischen theoretischen Vorhersagen und experimentellen Ergebnissen von hydrostatischen Kompressionsversuchen und Wasserinjektionstests an Pietra-Leccese-Kreide zeigen die Fahigkeit des Modells auf, das Verhalten dieser Gesteine zu simulieren.

1 INTRODUCTION

The paper addresses the behavior of a class of rocks which has been so far little studied within the rock mechanics discipline. In particular, it addresses the issues of partial rock saturation, and rock matrix collapse, which have been long recognized and studied in soil mechanics (for example Alonso et al. 1990, Toll 1990, Fredlund and Rahardjo 1993, and references therein), although limited modeling work has been performed with respect to collapsible soils (for example Lloret and Alonso 1980, Clemence and Finbarr 1981, Lagioia and Nova 1995). Partially saturated, collapsible rocks, such as chalks and weak sandstones have in the recent years become the focus of increasing attention because they are often encountered in oil and gas reservoirs. For improved oil recovery operations, these reservoirs are often water injected, thus altering their initial water saturation. Partial-saturated, collapsible rocks are characterized by relatively high porosity and low strength. The porosity may range from 25 percent to as high as 50 percent in some cases. The grains of such materials are forming a structure which is held together through weak cementation and/or in the presence of a wetting fluid through capillary tensions. An elastoplastic constitutive model which takes into account in a comprehensive manner the two effects of pore collapse and partial water saturation is presented. The model is used to back-analyze hydrostatic compression tests on Pietra Leccese chalk at various saturations, and simulate water injection tests on low saturation specimens (Papamichos et al. 1996). The Pietra Leccese chalk, is a high porosity, outcrop bio-calcarenite from the south of Italy. Its mineralogical composition is 93 % calcite, 6 % clays and micas, and 1 % quartz.

2 FORMULATION OF AN ELASTO-PLASTIC MODEL

In this section, the structure of an elastic-plastic model for partially-saturated, collapsible rocks is briefly outlined. A full description of the model is presented in Papamichos et al. (1996), together with its calibration procedure. The model captures the effect of capillary suction due to the partial saturation of the rock, and the collapse of the rock matrix as a result of the reduction of the capillary suction and ensuing reduction in intergranular cohesion.

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