Unloading-reloading uniaxial compressibility of deep reservoirs by marker measurements

The radioactive marker technique appears to be a promising tool for the in situ measurement of the uniaxial vertical compressibility cM of deep gas/oil reservoirs. A constitutive model for cM,unload in the Northern Adriatic sedimentary basin in unloading-reloading (II cycle) conditions is derived. This relies on marker measurements obtained from an instrumented borehole located in depleted gas reservoirs that had experienced a pore pressure recovery over the period 1997-1999. Record analysis indicates that cM,unload is on the order of 5-6·10-5 MPa-1 over the effective stress interval 35 ≤ σz ≤ 65 MPa, i.e. for a burial depth between 3200 and 5800 m in undisturbed conditions. The ratio of virgin loading cM,load to cM,unload varies between 1.5 and 3 with an average value equal to 2.2. The cM,unload constitutive law derived for rock expansions is implemented into a coupled finite element consolidation model in an attempt to reproduce, also quantitatively, the vertical deformation recorded i the marker borehoole after field abandonment.