Measuring Crustal Stress: Core-Based Methods

Abstract

In-situ stresses are important for many processes in tight gas reservoirs, ranging from siting the well, to drilling, completion and long-term production. The orientation of the stress field, which controls the azimuth of the hydraulic fracture (Sect. 7.2), is important for field development as optimum drainage in a tight reservoir will depend on the drainage from a hydraulic fracture. If wells are not sited properly, drainage patterns will overlap and production may be uneconomic. Stress magnitudes are important for wellbore stability and hydraulic fracturing. For fracturing, in particular, the stress difference between lithologies in a geological sequence is the major control on fracture-height growth. The focus of this chapter is shifted from borehole methods to core-based methods for in-situ stress estimates. As pointed out in Sect. 7.1, the crucial assumption in core-based stress estimates is that the dominating portion of the stress-relief cracks observed in deep drill cores is caused by the relief of present-day stresses. Only with this assumption, the azimuth of relief-crack populations can be related to in-situ stress azimuth, and the closing pressure of relief cracks to in-situ stress magnitudes.