Abstract:

A key technical challenge in successful hydraulic fracturing (HF) of gas and oil-rich shales is to understand fracture growth through layered systems from a horizontal wellbore. This understanding enables an improved HF design and selection of the horizontal wellbore landing depth. In the absence of the clear stress barriers for fracture height growth it is important to account for fracture toughness variations. Since fracture toughness is relatively difficult to measure routinely we attempt to establish good correlations between it and other more readily measurable physical and mechanical rock properties. This paper presents the measurements of fracture toughness for a liquid-rich shale done using the Cracked Chevron Notch Brazilian Disk (CCNBD) geometry following the International Society for Rock Mechanics Suggested Method. The data do not confirm strong correlations for fracture toughness previously reported in the literature. Only weak correlations with sample bulk density, compressional and shear velocities were observed. The contrast of measured toughness values are used to define the upscaled toughness profile. Its impact on the fracture geometry is demonstrated through numerical simulations for slick water and cross-linked gel stimulations.

Introduction

Efficient hydraulic fracturing (HF) is one of the most important technologies to the current unconventional gas/liquid rich shale plays in the oil and gas industry (see, e.g., King, 2010). A key technical challenge in successful HF treatments is to understand HF growth through layered systems from a horizontal wellbore.

A conventional target for stimulation with HF would be a sandstone reservoir bounded by the non-reservoir units, typically shales. In such cases a clear lithological characterization aids to reduce uncertainty about the fracture height growth, which is mostly controlled by the stress contrast between sand and shale layers (e.g., Smith and Shlyapobersky, 2000). Important distinctive attributes of unconventional plays are an uncertainty about the vertical extent of the target zone, the significant height of the induced hydraulic fractures as interpreted from the microseismic data, and uncertainty about the control parameters in a thick package of geological layers with subtly varying lithology.

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