Abstract

This paper presents laboratory experiments designed to explore the interaction between hydraulic fractures and preexisting natural fractures that are strongly cemented but over only a portion of the natural fracture. The results show that a hydraulic fracture penetrates directly through a fully and strongly cemented pre-existing natural fracture. We then vary the proportion of the natural fracture that is strongly cemented. In most of these partially-cemented cases, the hydraulic fracture is observed to persist through the entire height of the specimen both before and after the interface. However, the fracture path persists directly through strongly-cemented portions while there are offsets at uncemented portions. A fully debonded result is obtained when the cemented region is 5 mm in height, that is, 10% the height of the interface and about 13% of the hydraulic fracture halflength at the time of intersection. The initial results of this seldom-considered but almost certainly realistic configuration of partial bonding suggest that hydraulic fracture path is strongly influenced by the size of the bonded region of the natural fracture, and, perhaps more importantly, that strong bonding over only a portion of the natural fracture can be sufficient to promote hydraulic fracture crossing. Four patterns are observed for the interaction between the hydraulic fracture and the strongly cemented natural fracture: (1) complete crossing, (2) crossing and offset, (3) crossing, partial debonding and offset, (4) complete debonding.

1. INTRODUCTION

Hydraulic fracturing is a widely used well-stimulation technique for enhancing the productivity of oil and natural gas in unconventional reservoirs. The interaction between hydraulic fractures and pre-existing natural fractures in a reservoir can strongly influence the fracture network geometries and is widely recognized as one of the main issues for understanding hydraulic fracture propagation in unconventional, and some conventional, reservoirs. The mechanics of the interaction of hydraulic fractures with natural fractures is often understood through some now-classical analytical solutions and mechanical models [1-4]. Work on this subject is ongoing, with recent contributions aimed at experimental evaluation of these criteria and/or development of more generalized approaches [5]. Extensive numerical and experimental studies have also been conducted to interpret and predict the mode of interaction [6-10].

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