Present-day in-situ stresses versus paleo-stresses for locating sweet spots in unconventional reservoirs

Abstract

The effect of stresses on permeability is a combination of external stress and pore pressure. The authors examine if and how present-day in-situ stresses and the spatial distribution of permeable locations in the Moomba-Big Lake fields in the Cooper Basin are correlated. Image logs, well logs, and formation tests are analysed and the orientation and magnitudes of the three principal stresses are calculated. A 3D model was constructed and the calculated stress magnitudes and orientations were applied to the model using the software Poly3D. The resulting stress distribution in the present-day stress state showed a potential sweet spot in the Big Lake field, which is presently the location of a gas pool that forms, with the Moomba field, one-third of the gas reserve in SA. No potential sweet spots, however, are located in the Moomba area. The authors also used the finite element method (FEM) and the boundary element method (BEM) for modelling the behaviour of folds, fractures, and faults and for mimicking the tectonic history of the basin. Software codes Dynel3D and Traptester were used to examine the validity of geomechanical restoration techniques for locating sweet spots in the Moomba-Big Lake fields. The methodology attempts to reconstruct the present-day structural and geometrical placement and to predict fractures generated due to stresses released during past tectonic events. Predicted stresses succeeded in mapping the same sweet spot in the Big Lake field using both software codes. Accordingly, the present permeability and production rate is controlled by a combination of present-day and stored stresses.