Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Issues in Estimating Horizontal Stress with Poroelastic Models
(1) Hess Corporation, Katy, TX.
Hydraulic fracturing is a key technology for developing shale reservoirs in particular and tight reservoirs in general. Horizontal stress is a primary control on propagation of hydraulic fractures, so it is important to understand how horizontal stress varies from formation to formation. The most common way to estimate horizontal stress is to use a poroelastic model that relates stress to elastic strain. Poroelastic properties are typically derived from compressional and shear sonic logs and from sparse core measurements.
In anisotropic formations such as shales the poroelastic model is very complex. It involves some parameters that are very hard to measure accurately and some that are almost never measured. So this model has usually been applied in practice with simplifying assumptions. Some assumptions are valid in specific tectonic settings, but not generally. Many assumptions are questionable, if not demonstrably false. Some of the parameters are estimated through empirical correlations that are highly uncertain. Nevertheless, in practice the simplified models work in that they seem to match data. This talk suggests that, because the model is so complex, combinations of incorrect assumptions and parameters can sometimes be chosen so that errors largely cancel.
If that is true, then applications of the poroelastic model do not really tell us much about how stress and strain are related in the earth. We might be able to estimate stresses just as well by using a purely empirical model to relate measured stress to lithology or porosity, rather than to elastic properties.