ABSTRACT

This paper presents a laboratory study of hydraulic fracture/natural fracture (HF/NF) interaction using a set of sandstone blocks from the Keshen sandstone formation, Tarim Oil Field. Simulated natural fractures were created in each block with strike and dip directions similar to those commonly encountered in the Keshen sandstone reservoirs. Each block was loaded in a strike-slip stress state in a polyaxial stress frame and hydraulically fractured. Many of the results of this test are consistent with previous experimental and numerical studies from the literature. However, whereas most other experimental or numerical studies of this type pose the problem in such a way that a 2D approximation is justified, both the dipping natural fractures and the strike-slip stress state resulted in 3D effects that have not been reported previously. When a hydraulic fracture crosses a dipping natural fracture, rather than reinitiating as a planar fracture, the fracture may instead split into a series of en-echelon fractures. Test results also show that different perforation orientations greatly affect the treating pressure and the final fracture network. These effects have important implications for practical stimulation design in strike-slip environments, and/or environments with dipping natural fractures.

Introduction

Hydraulic fracturing has become a critical aspect of economic production of oil and gas in many fields. As petroleum exploration and development expanded into lower and lower permeability reservoirs, the success of hydraulic stimulation treatments became more and more critical. At the same time, improved imaging and diagnostic techniques has led to a greater appreciation that hydraulic fractures were often complex 3D networks rather than simple planar structures. This has led to a large number of theoretical and experimental studies on the interaction between hydraulic fractures (HF) and natural fractures (NF), which has significantly improved our understanding of the mechanisms involved. These studies have shown that generally, larger differences between the principal stresses diminish the effect of natural fractures (Blanton, 1986) (C. & Pollard, 1995), as does higher viscosity fluids and higher pumping rates (Chuprakov, Melchaeva, & Prioul, 2014). While these studies have led to a significantly improved understanding of how and why complex hydraulic fracture networks develop, virtually all studies to date, both experimental and theoretical, have been essentially limited to 2D plane strain approximations. This is a good approximation to many field cases, and is a necessary simplification in many cases both to preserve the mathematical tractability of the problem and because of computational resources. This paper will present the results of a laboratory study of HF/NF interaction where this 2D plane strain approximation is inadequate. It will be shown that the geomechanical conditions of this field lead to some HF/NF interaction mechanisms that are fundamentally 3D and therefore are not captured by existing HF/NF interaction models, and yet are likely to have important implication for successful petroleum production.

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