Three aspects of rock mechanics have been of great Trois aspects de la mécanique de la roche ont largevalue in developing hydraulic fracturing theories, viz.: ment contribué au développement des théories de la (1) Elastic behavior. (2) Shear failure criteria. (3) Con- fracturation causée par une force hydraulique.... cepts of fracture propagation. (poussées d'eau). c.à.d.: (i) Le comportement Within the framework of these concepts, the follow- élastique. (2) Les critères des efforts de cisaillement qui ing topics will be discussed: (i) The relationship causent la faille. (3) Les concepts de l'extension de la between rock matrix stress and total stress. (2) Esti- fracture. mates and significance of in situ earth stresses. (3) Fac- C'est dans le cadre de ces concepts que les sujets qui tors influencing fracture orientation. (4) Fracture suivent vont être discutés: (1) Les rapports entre les extension pressures. (5) Apparent rock surface ener- efforts de la matrice de la roche et la somme des efforts gies. (6) Fracture widths. (2) Evaluation et signification des efforts terrestres sur les lieux ("in situ"). (3) Les facteurs qui influencent l'orientation de la fracture. (4) Les pressions qui résultent de l'extension de la fracture. (5) Les energies évidentes à la surface de la roche. (6) Les largeurs de la fracture.
departure from idealized linear behavior, and for many problems the linear elastic theory must be augmented One of the significant advances in petroleum tech- with additional concepts or approximations. nology within the past twenty years has been the development of hydraulic fracturing processes. Within this same period, understanding of the mechanics of Limits to idealized behavior rock behavior has increased. It is the purpose of this paper to show how knowledge of rock mechanics has There is clearly a limit to elastic behavior if a been applied in the understanding and further develop- sufficiently large uniaxial tensile stress is applied (the ment of hydraulic fracturing processes. tensile strength). Also, if a combination of stresses is Those aspects of rock behavior that will be of interest applied, there is a limit beyond which shear failure will are briefly summarized, and then in situ earth stresses occur. For present purposes, it is sufficient to describe and factors influencing hydraulic fracture initiation this envelope of shear failure by the well known Mohr and extension are discussed. failure theory.' If stresses are not too large, the shear limit can be written in a simple manner as shown by equation (i).
BEHAVIOR OF ROCKS r = C'+un tan q5 (1) = shear stress at failure Many investigators have treated rocks as linear, C = unit cohesive strength elastic materials. Such an assumption permits the powerful mathematics of the linear elastic theory to be un = stress normal to the plane of failure appl
