Local rock expansion in non-homogeneous productive gas/oil fields

In situ reservoir compaction measurements by radioactive markers represent a promising technique for a most realistic evaluation of the actual mechanical properties of producing gas/oil fields. As is at present implemented by Schlumberger and Western Atlas the marker technique spans depth intervals which are 10.5 m in size. At such a vertical resolution the reservoir may exhibit significant changes of sediment lithology with fine or very fine grained interbedded layers which may cause a delay of the local rock depletion, and hence a retarded compaction, or even an expansion at the early stage of gas/oil pumping. This may create serious difficulties for the correct interpretation of early marker data when a prediction of the expected field compaction is of paramount importance for a reliable modeling of the related land subsidence. A numerical study by a coupled 3-D finite element (FE) axisymmetric model is performed to address the mechanical behavior of the stressed rock at the marker resolution scale with special reference to Plio-Pleistocene gas reservoirs of the Northern Adriatic sedimentary basin. Preliminary results show that with particular layering configuration within the recorded depth interval some low permeable levels may expand after the inception of fluid withdrawal thus providing a measurement where the overall compaction may be partially or also totally offset by the local expansion. If such a measurement is used to estimate the in situ rock vertical compressibility large errors may arise which are not due to the instrument inaccuracy as at a first glance it might appear. Our analysis indicates that a great care must be paid to the detailed lithology of each monitored depth before the marker data may be used to evaluate the actual stiffness of a reservoir.