Height Growth in Layered Unconventional Reservoirs: The Impact of Formation Moduli, Interfaces, and In-Situ Stress

Abstract

In recent years, much research effort has focused on hydraulic fracture (HF) height growth because height containment is needed to ensure effective stimulation of target zones. In many cases, fracture height growth determines the success or failure of a hydraulic stimulation. For layered rock systems, material properties, interface mechanical characteristics and permeability, and in-situ stresses influence both the lateral and height growth of HFs. It is generally believed that stress contrast is a dominant factor that directly controls the fracture height. The influence of Young's modulus contrast on height growth is usually ignored. Simplified “average methods” are often proposed and used to homogenize layered modulus. Also, it is commonly assumed that the layer interfaces are perfectly bonded without slippage even when high stress contrast exits. The above assumptions are made partially due to the difficulty in handling all the factors (e.g., layered modulus and stress contrast between adjacent layers) involved in simulations. In this study, a fully coupled 3D HF simulator that is based on the finite element method (FEM) is used to investigate the above factors and study how they impact HF propagation and height growth. The influence of modulus contrast, interface conditions, and in-situ stress on hydraulic fracturing and especially on fracture height growth is analyzed. The numerical approach is a 3D FEM with a special zero-thickness interface element based on the cohesive zone model (CZM) to simulate the fracture propagation and fluid flow in fractures. A local traction-separation law with strain-softening is used to capture tensile cracking. The nonlinear mechanical behavior of frictional sliding along interface surfaces is also considered. Because discontinuities are explicitly simulated through using the interface elements, details of the deformation processes are captured and revealed. For example, information related to aperture opening/sliding and stress distribution along the discontinuities is obtained in the simulations. After verification and validation of the numerical model, it is used to simulate height growth in layered rock of practical interest. The numerical model is evaluated through a commonly used crossing/arrest criterion. Laboratory experiments on fracture-discontinuity interaction under triaxial stress conditions are also studied. Numerical results match well with predictions from theoretical formulations and with laboratory observations. Typical processes associated with fracture-discontinuity interaction are revealed. The recorded injection pressure increases when the HF reaches a bedding interface (or other discontinuities). Continuous opening and/or sliding along the interface requires higher injection pressure. With the existence of a horizontal interface, the influence of modulus contrast and stress contrast on HF height growth is analyzed. The combined effects of rock properties, mechanical properties of the interfaces, and in-situ stress can effectively inhibit HF height growth. Analyzing pressure and aperture responses during fracture/interface interaction, it is revealed that the injection net pressure in the case where an offset exists along the interface is larger than that required if the HF could propagate continuously across the interface (no offset from the intersection point to the flaw along the interface). Whenever the free propagation of fracture tips is limited, either by horizontal formation interface or displacement constraints set intentionally to prevent fracture propagation, the injection net pressure and aperture at the injection point exhibit increase.