Abstract:

Hydraulic fracturing is often necessary in unconventional oil and gas reservoirs to enhance formation permeability by creating a conductive pathway for fluid flow to one or multiple wellbores. Reservoir development often requires many wellbores consisting of several fracture stages. Wellbore and fracture stage placement and orientation are of key importance when planning full reservoir development. Fractures are often induced parallel to one another along a horizontal well oriented in the minimum principal stress direction. Laboratory investigations of fracture spacing and the interaction between two parallel hydraulic fractures were performed. Laboratory hydraulic fracture testing was performed through two parallel wellbores in single-block granite samples with dimensions of 15 × 15 × 25 cm3 while being subjected to true-triaxial stress conditions. Acoustic emissions (AE) were monitored throughout stimulation of both Wells A and B to better understand the extent of microcrack damage induced, coalesced hydraulic fracture orientation, and width of the damage zone. Wellhead pressure, pump pressure, and flow were monitored for each wellbore throughout the fracturing processes. AE density imaging showed progressive fracture growth from an openhole wellbore for each well. Longer-term monitoring of the multi-wellbore system for several weeks revealed AE events residing within the densest regions of microcracks. These events, termed relaxation events, provided a clear map of the hydraulic fracture network and helped shed light on the continuously changing induced fracture networks. Typically, AE monitoring in the field for hydraulic fracture development is used to create a map of induced hydraulic fractures. This map is held constant for future analysis. Results from these laboratory tests show the necessity for continuous monitoring and treatment of the induced hydraulic fracture networks as an evolving system of fractures, particularly when pore fluids are being extracted.

Introduction

Reservoir rock permeability enhancement through hydraulic fracturing is a common practice in oil- and gas-bearing formations, as well as enhanced geothermal systems (EGS) reservoirs. Reservoirs requiring hydraulic fracture stimulation typically have very low permeability and many are naturally fractured and heterogeneous in composition and stress. Introducing hydraulic fractures in a dense naturally fractured reservoir serves to connect the natural network to the wellbore and expose new reservoir material to allow flow from micro and nano pores. With the past implementation of horizontal drilling, many hydraulic fractures are now able to be performed along a single wellbore. The spacing between these fracture stages varies wildly in industry, from the order of tens of meters to hundreds of meters. In many cases, iso-spacing of fracture stages irrespective of material properties or well log measurements is performed.

Performing multiple fracture stages along a horizontal wellbore in a naturally fractured network introduces multiple complexities in terms of understanding the wellbore connected network. These include the effects of fracture dimension alteration and stress shadowing effects on induced fracture directions, and the opening/closure of existing natural fractures. Studying the problem of hydraulic fracture interaction, laboratory testing is necessary to visualize the processes.

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