This paper discusses the differences between fully coupled and uncoupled formulations of models of production and subsidence. For highly compacting hydrocarbon reservoirs, production can cause compaction of the reservoir and subsidence of the overburden, and in turn, compaction and subsidence can affect the productivity of the reservoir by increasing the reservoir pressure (i.e., the so-called compaction drive). Intuitively, analyses of production and subsidence should be done in a fully-coupled fashion. However, most, if not all, of the analyses done so far on compacting reservoirs are uncoupled where production and subsidence are calculated in a staggered manner.
The results of the numerical analyses using an uncoupled reservoir simulation, and a fully-coupled finite element simulation based on Biot's formulation (both coupled and uncoupled simulations are based on single phase fluid) of a typical compacting reservoir are presented and compared. Different pore pressure response were obtained depending on whether an uncoupled or a fully-coupled analysis was performed, and also depending on whether there is arching of the overburden or not. The results of fully-coupled analyses of compaction and subsidence showed that the generation of additional pore pressure due to compaction cannot be correctly analyzed by simply adjusting the rock compressibilities in reservoir simulation. The most pronounced effect of coupling, obtained from the numerical simulations, is on the possibility of pore pressure increase close to the reservoir flanks even during production. The implications of the differences in the results of fully-coupled and uncoupled simulations are discussed.
Reservoir compaction and overburden subsidence are of major concern in hydrocarbon reservoirs with weak formations. Pressure depletion due to hydrocarbon production can cause significant deformation and shear failure in weak reservoir rocks. As the reservoir pressure drops, the portion of the load carried by the reservoir rock matrix (i.e., the effective stresses) increases which could induce significant compaction of the reservoir. Compaction is then transferred to the seabed as subsidence which can have financial and environmental impacts However, compaction and subsidence are not entirely detrimental.