Rock expansion and compaction at the marker scale in gas producing reservoirs

One of the main difficulties in the accurate prediction of land subsidence due to the exploitation of subsurface fluids is the quality and reliability of the geomechanical data used to represent the field. A most fundamental parameter for the simulation of the porous medium deformation is the vertical uniaxial rock compressibility, which can be determined by monitoring the on-going in situ compaction with the aid of the so-called radioactive marker technique. Successfully implemented in several gas fields all over the world, the radioactive marker technique has been recently used by ENI-Divisione Agip, the Italian national oil company, for a most reliable assessment of rock mechanical properties in the Northern Adriatic basin. However, the outcome of the Northern Adriatic surveys does not frequently allow for a straightforward interpretation because of the unexpected behavior of several markers, which provide expansion rather than compaction, or a deformation although no pore pressure change was measured. The present work studies the rock deformations at the marker scale induced by the depletion of producing reservoirs in heterogeneous porous media, in order to provide suggestions as to the most correct marker measurement interpretation. The study is carried out with the aid of an original finite element coupled consolidation model. Chapter 1 provides new results on the numerical issues involved for developing a fast, accurate, and robust mathematical tool. A particular attention is paid to the ill-conditioning caused by the integration in time of consolidation differential equations, and to the applicability of efficient iterative schemes to the problem solution. The finite element model is used to simulate the expected marker response in the geological and geomechanical setting of the Northern Adriatic basin. The radioactive marker technology and the use of related deformation measurements are reviewed in detail in Chapter 2, which provides also some examples of unexpected data recorded in several Northen Adriatic boreholes. Analytical solutions are used at this stage to gain a first insight into the basic deformation mechanisms in producing reservoirs and in the surrounding medium. Chapter 3 gives a detailed and up-to-date description of the main geological, mechanical, and hydraulic properties of the Northern Adriatic basin. In particular, an original constitutive model for uniaxial compressibility is derived at the basin scale both in virgin loading and in unloading-reloading conditions. The numerical study is performed in Chapter 4 and 5. The finite element model is first used to simulate the vertical deformation recorded by an ideal marker tool in a realistic heterogeneous setting. This study gives indications as to the main factors influencing the marker response, the layering configurations which are most likely to provide useful data, and the theoretical marker reliability for the compressibility estimate. Then, the behavior recorded by radioactive markers in three deep boreholes of the Northern Adriatic is simulated numerically. It is shown that the model can reproduce satisfactorily several records, providing suggestions for a more efficient marker installation and supporting the reliability of the proposed basin-scale constitutive law derived in the study area.