In-Situ Stress
In-situ stresses are one of the most important factors controlling hydraulic fracture behavior. These stresses exist naturally in the subsurface due to the weight of overlying rock layers and tectonic forces. The stress state of a reservoir determines how fractures initiate, propagate, and orient within the formation.
Hydraulic fractures typically propagate perpendicular to the minimum principal stress, following the path of least resistance. As a result, understanding the in-situ stress field is essential for predicting fracture geometry and designing effective hydraulic fracturing treatments.
The primary source of in-situ stress is the overburden weight, which results from the accumulation of rock layers above the reservoir. This weight produces the vertical stress acting on the formation.
In addition to vertical stress, horizontal stresses develop because the deeper rock layers tend to expand laterally under compression from the overburden. However, this lateral deformation is often constrained by surrounding rock formations, resulting in the development of horizontal stresses.
The magnitude of horizontal stress is influenced by rock mechanical properties such as Poisson’s ratio, which describes how a material deforms laterally when compressed vertically.
In the subsurface, rocks are subjected to three principal stresses:
- \(\boldsymbol{\sigma_v}\): vertical stress (overburden stress)
- \(\boldsymbol{\sigma_H}\): maximum horizontal stress
- \(\boldsymbol{\sigma_h}\): minimum horizontal stress
These stresses define the mechanical environment in which fractures form and propagate.
Accurate measurement of the in-situ stress field is challenging but essential for fracture design. One commonly used method is the Diagnostic Fracture Injection Test (DFIT), which analyzes pressure behavior during controlled fluid injection to estimate formation stresses.
