This chapter summarizes the current understanding of the state of in-situ stress in the Western Canada Sedimentary Basin. The information compiled here includes specific in-situ stress measurements, interpretations of well logs, inferences from hydraulic fracture records, inferences from drilling procedures and well production behavior, and measurements of geological indicators. These data combine to give a coherent regional picture of principal stress orientations, but are not as successful in providing a comprehensive appreciation of stress magnitudes.
We use the term in-situ stress to refer to the present-day active natural stresses within the basin - in other words, how much and in what directions are the rocks being compressed at a specific location today? We are not concerned here with any stress regimes of the past, or paleostresses, which might be indicated by permanent changes in rock fabric (e.g., Hobbs and Talbot, 1966).
Buried sediments respond to pressure and temperature and to fluids that pass through them. For example, the effects of temperature in terms of organic metamorphism are widely recognized and studied; the changes initiated by migrating fluids underpin the fields of diagenesis and metamorphic petrology. Less attention has been paid to the effects of pressure. The importance of fluid pressures in determining the effective strength of rocks is addressed in structural geology and its role in the entrapment and production of hydrocarbons is recognized. However, rock pressure (stress) has often been regarded as a vertically-acting phenomenon, synonymous with the overburden load. Horizontal stresses are commonly tacitly assumed to be equal to the overburden stress, yet most rocks in sedimentary basins are not compressed isotropically. Geological evidence of past conditions, together with numerous contemporary measurements, clearly document that buried rocks are generally under anisotropic stress conditions, where the principal stresses are of different magnitudes. As this situation exists in the Western Canada Sedimentary Basin, many geological and petroleum engineering investigations would benefit from considering the contemporary stress regime.
Appreciation of the stress regime can help us understand the recent tectonic evolution of a basin. From an engineering standpoint, how a rock behaves mechanically is determined by its own inherent physical properties and by the stress field to which it is subject. In-situ stress information can predict in which directions production-enhancing hydraulic fractures will propagate, can suggest within which formations they are likely to be contained, and can determine the optimum orientation for inclined and horizontal wells. Thus, stress data are valuable for plannning how best to recover hydrocarbons from reservoirs.
Last modified: August 20, 2008
