Dynamic reservoir characterization from overburden time-lapse strains
Garcia Azuero, Alejandro
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Accurate reservoir depletion or pressure change patterns are of great value when planning infill drilling programs for field development, as well as when monitoring injection wells and swept/unswept areas. In addition, a precise dynamic geomechanical description of the reservoir and overburden stress state could prevent costly undesired effects on the production infrastructure such as sea floor subsidence, casing shear and well failure. Dynamic characterization of reservoirs, until recently, had only well data to rely on, which apart from the inherent uncertainties (e.g. due to formation damage), provides no direct information on what is taking place between the wells. The advent of time-lapse seismic at the end of the 1990s meant that this gap could be bridged, providing measurements of the changes taking place in the subsurface. In its origins, time-lapse seismic was conceived as a tool to image intra-reservoir fluid movements via the dependency of reflection amplitudes on acoustic impedance, which is affected by fluid saturation changes in the porous reservoir rocks. However, depletion induced velocity changes are also non negligible. Furthermore, the reflectors may undergo deformation and displacement where compaction and subsidence are involved. As a consequence, analysis of amplitude changes is not straightforward, since in most cases, amplitudes have been shifted by a non negligible time difference or time-shift, presenting not only challenges, but also new possibilities. It is in the possibilities of these time-shifts that the present study is based. This research presents a novel method which numerically solves the static field problem in a multilayered heterogeneous media, relating overburden strain to reservoir depletion. It builds up on previous works based on Geertsma type solutions requiring a homogeneous half-space. This technique makes it possible to estimate the reservoir’s stress state, strain and pressure changes from measured overburden strain by considering the earth as a linear filter with reservoir compaction and overburden strain as parameters. However, some a priori knowledge is needed in the form of a rough subsurface model and a preliminary geomechanics simulation in order to approximate the transfer functions as Wiener filters. In this thesis, the Wiener filter concept has been applied to three real North Sea fields. First, to the Elgin field, an HP/HT shallow marine Upper Jurassic sandstone reservoir located in the UK sector of the North Sea. Then, to the Ekofisk and South Arne fields, both compacting chalk reservoirs in the Norwegian and Danish sector of the North Sea respectively. Additionally, by using a synthetic example the method has been validated and compared with a linear inversion approach using a Geertsma type Green’s function achieving higher accuracy. The project involved not only the development and application of the method itself, but the calculation of time-strains from the measured seismic and the construction and implementation of full field geomechanical models.