Basin-scale compressibility of the northern Adriatic by the radioactive marker technique

The paper presents a constitutive model for the one-dimensional vertical compressibility, m_v, of the northern Adriatic sedimentary basin based on in-situ deformation measurements by the radioactive marker technique. Gas reservoir compaction and expansion occurring during the field production life and after its abandonment have been measured in three deep boreholes of the northern Adriatic by FSMT (formation subsidence monitoring tool) and CMI (compaction monitoring instrument) developed by Schlumberger and Western Atlas respectively. The compressibility values, as obtained by relating the shortening of the distance between several pairs of adjacent markers to the pore pressure decline experienced by the productive levels during the monitored time interval, are statistically processed to reduce the influence of operational and instrumental errors. The method of moving weighted averages is used over clustered depth intervals. The compressibility thus derived is related to the in-situ vertical effective stress to provide basin-scale constitutive relationships for the expected m_v in both virgin loading and unloading–reloading conditions, together with the 68% and 95% confidence intervals characterising the law uncertainties. The results indicate that for virgin loading m_v ranges between 7 × 10⁻⁴ MPa⁻¹ and 5 × 10⁻⁵ MPa⁻¹ over the effective vertical stress interval 10< p′ < 80 MPa—that is, for a depth between 900 m and 7000 m in undisturbed conditions—and decreases by two to three times in unloading–reloading conditions.