Fracture pressure, leak-off tests and Poisson's ratio

Fracture pressure models are used to create pre-drill fracture pressure/depth plots essential to the design of drilling and casing programs in oil and gas wells, and future development of carbon capture and underground storage. Some of these models include an empirical term (stress ratio) which relates effective horizontal stress to effective vertical stress. Based on the literature, stress ratio is assumed to vary with compaction and can be calculated from Poisson's ratio (nu ). An alternative to models which utilize the stress ratio term is a model which relates fracture pressure to a constant fraction of the vertical stress. This paper demonstrates that a constant fraction of vertical stress is equivalent to a stress ratio which increases slightly with depth. Estimation of how the nu changes with compaction is complicated by the multiple methods which are commonly employed to measure nu . The available static and dynamic nu data suggest a substantial decrease in nu with porosity reduction. Evaluation of in situ nu data from leak-off tests (LOTS) and associated fluid pressure data in Tertiary Basins indicates that in situ nu does not decline as rapidly as indicated by the other methods of nu determination. In situ nu data indicate that calibration of stress ratio from traditional nu methods is not appropriate and may substantially underestimate fracture pressure (Fp). Fracture pressure models which utilize a constant fraction of vertical stress or estimate stress ratio empirically from LOT data should be employed for pre-drill estimation of fracture pressure.