A small area of carbonate field in North Oman was tested to monitor steam injection. The reservoir is a fractured carbonate and is overlain by a 250 m thick shale. Models are required to translate indirect observations or extrapolate point measurements of wells to give us a three dimensional view on field response.
The microseismic monitoring system for the steam injection pilot consists of 6 dedicated observation wells. The objectives of the system are to detect any generation of new fractures or re-activation of existing fractures due to steam injection and monitor cap-rock integrity.
800 events were recorded by the micro-seismic system so far. The events occur mainly in a clustered fashion and are attributed to cooling- and heating, and potentially to re-activation of a fault zone close to updip injectors. Micro-seismic activity has expressed the extent of this fault zone. Finally, some of the events were attributed to well related noise.
Microseismic events in the overburden could represent a continuum between two end members. One being that they represent failure of critically stressed asperities along pre-existing lineaments in which the faults and fractures are not critically stressed over a majority of surface area. The other end of the spectrum is that the minor events are indicative of incipient larger scale failure.
To gauge where the subsurface is on this spectrum, 3D geomechanical simulations were performed to estimate the proximity to large scale failure in the reservoir and overburden.
The base case model results suggest that the micro-seismic events are indicative of the first end member state, and represent asperities along pre-existing lineaments. Exploration of the full range of uncertainty in rock material showed that no large-scale fault re-activation is to be expected.