Stress Determination from Logs. Why the Simple Uniaxial Strain Model is Physically Flawed but Still Gives Relatively Good Matches to High Quality Stress Measurements Performed on Several Fields Offshore Norway

Determination of in-situ stress is important for many engineering applications such as wellbore stability, fracture injection and stimulation, sand production as well as for considering processes of hydrocarbon migration and leakage. Profiles of minimum horizontal stress have been generated for several fields on the Norwegian Continental Shelf based on the operator's database of high-quality stress measurements. Standard methods are used to (straight-line) interpolate between measured points to provide a “continuous” stress curve through the subsurface. A best practice is described on how to “transfer” stress measurements from nearby infill wells on each structure to the well of interest. An alternative approach uses the sonic log to estimate complete profiles based on assumptions of elastic burial with uniaxial strain conditions. These profiles are then compared to the measured values. Despite there being many arguments against using the simple elastic uniaxial strain model for estimating stresses that have developed over the course of many millions of years in geological processes that are often non-elastic in nature, a relatively good match is obtained by this method, especially in younger sediments. Potential mechanisms to explain this surprising result are discussed. An interesting suggestion is that it isn't the quasi static mechanical properties (Poisson's ratio) dictating the in-situ stresses but it is rather the in-situ stresses that dictate the undrained rock's (small strain) response. As such the Poisson's ratio (as determined from small strain, dynamic value typical during sonic logging and not during lab testing) is “forced” by the in-situ stresses present in the subsurface. The work presents a rare investigation when the veracity of the sonic stress method is compared to a database of many high-quality stress measurements. Even though a relatively good match for minimum stress is obtained in young sediments, some discrepancies are locally observed. This method should not replace dedicated stress measurements, but it may help fill the gaps between the depths where such tests are normally performed (usually in competent claystone) and identify potential weak zones.