Numerical Simulation of Reservoir Structures, Part II: Propagation of a Pressurized Fracture in a Sequence of Rock Layers
We use finite element simulations to study the effect of local geologic conditions during hydraulic fracturing of rock layers with damage rheology. This work is part of our study on structural processes in reservoir rocks using numerical simulations with the code Abaqus. Part I covers rock rheology and benchmark simulations, and Part III studies the role of visco-plastic rheology on ramp induced thrust-folding (Busetti et al.; Heesakkers et al., this meeting).
Hydraulic fractures frequently propagate through multiple layers of naturally fractured rock, each with distinct stress state and material properties. Thus, we examine fracture propagation as a function of mechanical properties of the host and neighboring layers, layer dimensions, tectonic stress state, and internal pressure. We model a wellbore-scale section of layers with frictional contacts located away from near-borehole effects. Beds of high elastic modulus and yield strength reflect potential “fracture barriers”. Rheology is for elastic-plastic damaged rock based on experiments of Berea Sandstone, Indiana Limestone, and Barnett Shale (see Part I). We first investigated the up-section propagation of a vertical hydrofracture 0.25 m tall, embedded in a 0.3 m host layer, overlain by 1 to 8 horizontal layers from 0.125 to 1 m thick. We establish tectonic stresses for depths of ~2.5 km and then apply increasing pressure (0 - 100 MPa) to viscous fluid in the fracture to simulate a single injection stage.
The results suggest that the model parameters are interrelated. We found the following parameters reduce the tendency to propagate fractures: (1) thinner layers; (2) lower inter-layer friction; (3) higher vertical stress; (4) higher elastic modulus ratio between the host and overlying layers. Higher stress ratio (Sv > Sh) increased the tendency for longer fractures; lower stress ratio (Sv ≈ Sh) increased the tendency for multiple sub-vertical fractures. The models indicate that interlayer slip is a strong mechanism to locally accommodate pressurization strain. We anticipate that slip along preexisting fractures and bedding planes could redirect flow along diffuse fracture patterns. The simulations indicate that to predict propagation of hydrofractures, one should consider fracture interaction with preexisting structures and their local stress. Future models will explore the effect of fracture inclination as well as growth in three dimensions.
This work is supported by funds from ConocoPhillips
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009