Field Application of a Novel Core-Based In-Situ Stress Estimation Technique

In this paper, we present the first field application of a novel method to estimate in-situ rock stress. This technique, which we have baptized "PKM (Pestman-Kenter-vanMunster) method", uses Acoustic Emission-controlled polyaxial reloading of core in the laboratory. The method works analogous to the Kaiser effect that is observed when reloading along a single stress axis. Contrary to other Acoustic Emission methods, our novel approach is likely to measure present-day stresses, even if the core has been subjected to paleo-stresses that are higher than the present day stresses¹ . Proof of concept of the method has been provided using extensive experimental work on outcrop sandstone²  (which was subjected to a stress path simulating burial, coring and core retrieval to surface), a simple analytic model³  and numerical simulations using a 3D discrete particle method⁴ .

Recently, the method was applied for the first time on real core material, a preserved sandstone core from the Munnekezijl field (onshore gas) of NAM (Nederlandsche Aardolie Maatschappij) in The Netherlands. An advantage of first application of the technique to this field was that there was availability of good quality stress estimates (for the same layer as where our core was taken) obtained from other methods. A study including hydraulic frac data and borehole breakout shape had been performed in the past to estimate the magnitudes of the major and minor horizontal effective stress. Hence, the results of our method could be compared to other good quality data.

Three samples have been cut from core taken at a depth of some 4000 m [13000 ft] in the Munnekezijl field, for application of our PKM method. Only the weakest sample showed a clear stress memory effect, and was used for stress estimation using our method. The results compared very satisfactorily with the estimates from the hydraulic frac data and borehole breakout analysis. Although this is only the first application, and further validation is needed, these results show a clear promise of the PKM method.