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

The radioactive marker technique (RMT) appears to be a promising tool to evaluate the in-situ uniaxial vertical compressibility c(M) of deep producing gas/oil reservoirs. However, when the field consists of multipay layers with the porous-medium heterogeneity scale smaller than the marker spacing (10.5 m), great care must be exercised in the interpretation of an RMT survey. If the monitored depth interval incorporates thin clayey layers or the measurement is made in an active pumping well, c(M) can be grossly underestimated. Moreover, rock may expand, with the expansion correctly recorded by RMT. In the present paper, a set of RMT measurements made by Eni-E&P over the past decade in three deep boreholes of the Northern Adriatic basin are simulated with the aid of a 3D poroelastic model solved by finite elements (FEs). Use is made of the much detailed lithostratigraphies of the test holes and the c(M) constitutive equation derived from a previous statistical analysis of the marker data. The modeling results show that the measurements also can be reproduced satisfactorily from a quantitative viewpoint, and they indicate that an efficient marker installation requires that the monitored depth interval be made mostly from an entirely depleted sandy unit, with the markers placed possibly far from a producing well and approximately 10.5 m apart (i.e., the distance between the gamma ray detectors that monitor the marker position). The c(M) constitutive law used in the numerical analysis is realistically accurate from 2500 to 3000 m depth, while it appears to be underestimated by a factor of 2 between 800 and 1500 in. The measured expansions allow for the assessment of the reservoir c(M) under unloading conditions. This turns out to be 2.5 times smaller than c(M) in virgin loading conditions.