The Yufutsu field is a fractured granite, gas condensate reservoir, locatedon Hokkaido island, in the north of Japan. There is minimal primary porosityand secondary porosity consists of fractures. Fractures are categorised intofour types according to size (mega, major, minor and halo fractures). Megafractures govern the flow, while micro fractures govern gas storage. Gasmigrated into the reservoir after the fractures were formed.

The field shows abnormal production behaviour, and this paper attempts to characterise production, by the use of 3D-stochastic geological modelling and reservoir simulation. Incorporating all of the effects that were observed fromthe Yufutsu field into a reservoir model proved to be an extremely complexprocess.

A three-stage reservoir study was undertaken:

  1. To build a 3D stochastic geological model. Initially small-scale models were built, with both mega and micro fractures modelled stochastically asobjects.

  2. Effective permeability (matrix + fracture) was calculated across a 200,000 cell single porosity simulation model, along with tracer efflux rates forrelative permeability calculations.

  3. The above model behaviour was replicated in a 6 cell dual porosity model, which was substituted into the coarser scale single well and full field models.

The resultant methodology used provided accurate and realistic description of the fracture system, which was then used for well planning and production forecasting.

Background Geology

The Yufutsu field is made up of early Cretaceous granitic plutons, covered with Tertiary and Quaternary siliclastic sediments up to 4000m thick.

Basal units of the sediments are Eocene in age and are comprised of the Ishikari Group and overlying Poronai Formation. The Ishikari group is composed of non-marine/paralic thick conglomerate beds, grading into sandstone-shale units, interspersed with coal bands. The overlying Poronai Formation consists of massive marine shales.

These Eocene strata are overlain unconformably by the Upper Oligocene Minami-Naganuma Formation. The Miocene Takinoue Formation in the Yufutsu Fieldis characterised by thick basic volcanic rocks, with relatively minor marinemudstone beds. The volcanic complex can reach a thickness of up to 1100m. The overlying Biratori/Karumai Formations and Nina Formation are dominated by diatomaceous shales and mudstones, intercalated with conglomerates and sandstones.

The basement granitoid complex of the Yufutsu Field is deformed heavily by north-south trending normal faults. As a result of the faulting, the basementEocene-Oligocene strata and part of the lower Miocene section make up a large scale horst running north-south. Seismic and stratigraphical data suggests that the normal faulting had its main active phase during the Late Oligocene - Early Miocene. At present the Yufutsu Field is located in the frontal area of a thrust deformation front, which has propagated west ward from the Hidaki Mountain range, since initiating in the Late Miocene. The structures in the field remain unaffected by this contractional pulse.

Geological Modelling

The single well simulation field is defined around the current main producing well MY-1, towards the south-east corner of the Yufutsu field. Wells MY-2D andMY-3D are also covered within the area of interest (AOI). Faults curtail the extent of the AOI to the east and west. An arbitrary limit is inferred to the north.

Top granite and top conglome rate surfaces are identified from seismic interpretation. A pseudo base granite horizon is added at a depth of 1000m below the top granite. The horizon is essentially a transposed duplicate of the top granite surface, which is adjusted to the inclined faults in order to ensure consistency. The new base granite depth was chosen to exceed the maximum well depth penetrated in the AOI and adjacent blocks, where a number of well spenetrate much deeper.

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