SYNOPSIS:

Extensive laboratory and field investigations have led to validation of the Differential Strain Curve Analysis (DSCA) as a technique to determine the in situ stress field from measurements performed on available core. The method is based on the assumption that a rock specimen retrieved from its downhole environment, will expand due to the generation of randomly oriented microcracks. Their density is proportional to the stress magnitude differential. Careful monitoring of a rock specimen's behaviour upon reloading reflects the past stress history. Statistical analyses compensate for the localized inherent inhomogeneities. DSCA results are compared with data generated by other techniques for several geological formations.

RESUME:

Un grand nombre d'investigations en laboratoire et sur le terrain ont mene à la validation de l'Analyse de la Courbe des Deformations Differentielles en tant que technique pour determiner le champ de contraintes in situ à partir de mesures effectuees sur carottes. La methode est basee sur l'hypothèse qu'un echantillon de roche, detache de l'environnement souterrain, se dilatera par microfissurations dont la densite est proportionelle à la difference de contrainte. Des mesures precises du comportement de cette roche lors de son rechargement permettront done d'obtenir une idee de son chargement initial. L'introduction de methodes statistiques permettront, en outre, d'ecarter les problèmes des non-homogeneites locales. Des resultats obtenus par cette methode ont ete compares avec les donnees obtenues par d'autres techniques. Plusieurs formations geologiques ont ete considerees et l'Analyse de la Courbe des Deformations Differentielles semble donner de bons resultats.

ZUSAMMENFASSUNG:

Ausgedehnte Labor- und Felduntersuchungen haben bestatigt, daß die Differentialanalyse von Dehnungskurven eine geeignete Methode ist, das Spannungsfeld vor Ort durch Messungen an vorhandenen Kernen zu bestimmen. Die Methode beruht auf der Annahme, daß eine Gesteinsprobe, wenn sie von den in der Bohrlochsohle herrschenden Bedingungen entfernt wird, sich durch zufallig orientierte Mikrorisse ausdehnt, deren Dichte proportional zu der Gröse der differentialen Spannung ist. Ein sorgfaltiges Verfolgen des Verhaltens einer Gesteinsprobe, wenn die Spannung wieder erhöht wird, zeigt daher den Verlauf der Spannung in der Vergangenheit. Eine statistische Analyse ermittelt den Einfluß der unvermeidlichen lokalen Inhomogeneitaten..Ergebnisse der Differentialanalyse von Dehnungskurven werden fuer mehrere geologische Formationen verglichen mit Ergebnissen, die durch andere Methoden gewonnen wurden. Die Methode erscheint vielversprechend.

1. INTRODUCTION

Rock formations are frequently fractured to increase production in the oil and gas fields. The attitude of these induced hydraulic fractures is governed by the magnitude and orientation of the pre-existing stress field. As sophisticated stimulation techniques become available, more "difficult" reservoir developments are contemplated. However, the constantly increasing well-completion costs make it increasingly vital to anticipate the geometry of these man-made features as their pattern and spacing influence the recovery efficiency. This is especially true for low-permeability formations in which the drainage area is of limited extent; hence, massive hydraulic fracture treatments are usually required. Similarly, an appreciation of the stress tensor is important in the case of solution mining and geothermal hot-dry rock energy extraction, as the man-made fracture constitutes a crucial link between the injection and withdrawal wells. Although the pre-existing in-situ stress field is only one of the parameters controlling the geometry of such fractures, it generally plays the most important role. A similar argument may easily be developed when considering large underground openings for mining, civil or military purposes, as their relative orientation with respect to the stress tensor will affect the overall stability of the structure. The method described in this paper relies upon complete strain relief because it considers the strains a rock core undergoes after removal from its underground environment.

2. PRINCIPLE

As early as 1923, Adams and Williamson noted that rocks containing cracks are more compressible than the same rocks would be without cracks. Based on this basic observation, and assuming that the microcracks present in rock samples are related to the applied stresses, several authors predicted that the crack spectrum could be used to evaluate the past history of a particular formation (Simmons and Richter, 1976; Richter et al., 1976; Batzle and Simmons, 1976). Correlation of the time-dependent strain induced in rocks after removal from the ground and the pre-existing stress is not new (Emery, 1962; Voight and St. Pierre, 1974). Appreciation of these concepts led to initial attempts at stress prediction using Differential Strain Analysis resulting solely from core relaxation (Siegfried and Simmons, 1978; Simmons and Richter, 1976; Simmons et al., 1974; Simmons et al 1978a; Strickland et al 1979). However, the data generated from such strain relaxation analyses were far from convincing, even when stringent efforts were made to monitor strain immediately upon core recovery (Strickland, personal communication, 1980). To overcome this difficulty, a modified testing philosophy was adopted. When a core is removed from its underground environment, the drilling or sawing operation constitutes a stress relief mechanism. The physical detachment of the specimen from the rock mass allows it to undergo differential relaxation.

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