Abstract

Since the distribution of in-situ stress is not symmetric with respect tothe wellbore, the hydraulically induced fracture from a horizontal wellborefavors the growth of one wing and/or along the wellbore axis. Furthermore, whenthe wellbore axis is not in the direction of the principal horizontal in-situstress, the action of the off-plane shear stresses tends to turn and to twistthe fracture plane aligning the plane perpendicular to the direction of theminimum in-situ stress. Several numerical examples are presented to illustratethe phenomena.

Introduction

Consider a horizontal wellbore as sketched in Fig. 1. Since the distributionof in-situ stress is not symmetric with respect to the wellbore axis, thepropagation characteristics of a hydraulic fracture from the horizontalwellbore is very different from its vertical counterpart. In a verticalwellbore, the distribution of in-situ stresses and the growth of ahydraulically induced fracture have been tactically assumed to be symmetricwith respect to the wellbore axis. However, the hydraulic fracture from ahorizontal wellbore, due to a non-symmetric distribution of in-situ stresses, favors the growth of one wing and/or along the axis of the wellbore.Furthermore, when the wellbore axis is not in the direction of the principalhorizontal in-situ stress, the action of the off-plane shear stresses tends toturn and to twist the fracture plane aligning the plane perpendicular to thedirection of the minimum in-situ stress. These are the problems to be addressedin the paper.

For an understanding of the mechanics of non-symmetric growth of a hydraulicfracture and setting stage for the discussion of a 3D fracture model, thesolution from a modified Geertsma-de Klerk (GdK) model will be examined first.Our solution has demonstrated that the non-symmetric growth of fracture isdetermined by the stress intensity factor at the fracture tips and by theentire length of the fracture. And, due to the non-symmetric distribution ofin-situ stresses, the stress intensity factors are not equal and have a stronginfluence to the geometry of the fracture opening and the fluid pressuredistribution inside the fracture. Our numerical results hive demonstrated thatan in-situ stress contrast of 50 psi can retard the growth of one wing of ahydraulic fracture.

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