Application of MP-geostatistics to modeling and history matching of ...

Application of MP-geostatistics to modeling and history matching of a volcanic reservoir

Tomomi Yamada and Hiroyuki Okano (JAPEX)

A gas field is modeled and history-matched by applying recent SCRF approach. In deforming MP-based realization, this method updates CPDF to account for dynamic data by assuming relative increment of its contribution to be constant.

The formation of interest comprises stacked rhyolitic lava domes with cemented hyaloclastite enclosing each of them, as per typical consequence of submarine eruptions. Gas drainage system is further complicated by Intrusion of impermeable basaltic lava sheet and occasional sedimentation of mudstone. In view of stratigraphic correlation, multiple reservoirs identified at development stage seem either vertically or laterally separated from each other. However, more than 20 years of production has uncovered pressure communication between them. Quality of seismic record has been prohibitive to raise any suggestion on this event. Therefore, a stochastic modeling is launched to resolve the nature of the pressure system from its behavior observed at wells.

MP technique is employed to reproduce twisted surfaces of each facies. Hierarchical modeling with five different scales is practiced in COSNESIM to distribute facies having different connectivity. Servo system parameter for global PDF reproduction is tuned to allow OGIP to vary over deformations. Training image is inferred from geologist’s knowledge and literature because no outcrop has been available to retrieve stacked pattern of multiple lava domes. Therefore, it is extensively adjusted in volumetric fraction of each facies and in distribution of lava-body size until well pressures are simulated within reasonable number of deformations. Eventually, acceptable history match is obtained after 15-20 iterations, each of which comprises up to 7 trials to update CPDF. A number of semi-compartments, due either to confinement by hyaloclastite and to limited reservoir connection are visible on resulting realizations. 20 of them are generated and variability in OGIP and COP over prescribed period of time is quantified.

Building proper hierarchy of different scales and careful preparation of training image is the key to successful modeling and history matching. It is also noted that deformation process shall allow key reservoir values, such as OGIP to vary over iterations to accommodate dynamic information. Application of multi-point geostatistics is one viable solution to modeling volcanic reservoir with unknown reservoir connectivity.