Interpretation of radioactive marker measurements to evaluate compaction in the Northern Adriatic gas fields

When the field consists of multi-pay reservoirs with the heterogeneity scale of the porous medium smaller than the marker spacing (10.5 m) a great care must be exercised in the interpretation of a radioactive marker survey. The rock may expand, or, if the monitored depth interval incorporates thin low permeable layers or the measurement is made in an active well, the measured compaction can occur in non-oedometric conditions. In the present paper a set of radioactive marker measurements made by ENI-Divisione Agip over the last decade in 3 deep boreholes of the Northern Adriatic basin are simulated with the aid of a FE coupled poroelastic model. The modeling results show that the measurements can be reproduced satisfactorily in several case, both from a qualitative and also quantitative viewpoint, and indicate that an efficient marker installation requires that the monitored depth interval be made mostly of sand entirely depleted, with the markers placed far from a producing well and approximately 10.5 m apart. The cM constitutive law used in the numerical analysis appears to be realistically accurate for a depth larger than 2500 m and generally underestimated between 800 and 1500 m. The measured expansions due to a natural pore pressure recovery allow for the assessment also of the reservoir cM under unloading/reloading conditions.