Background

Summary

Model updating in history matching is a difficult and non-unique process. Normally, the model update depends on the insight of reservoir engineers. The updating of the model is done so that the historical production data can be reproduced. 3D seismic data in general are used to constrain geostatistical model building. In this study, we propose an approach to update the reservoir model using 3D seismic data as a constraint in the process of history matching. This technique is applied to an offshore field producing for a year. 3D seismic data were acquired before that production. The rock model is calibrated using logs. Then, the rock model is applied to the reservoir model to generate synthetic seismic which is compared to the observed. By scanning the seismic mismatch, the reservoir model quality is diagnosed. The area in the model with poor seismic matching is modified to generate a reasonable seismic match first. Subsequently, reservoir simulation is performed to check if the production is matched. It is found that the updated model from the seismic matching is able to generate a better history match.

Many offshore field developments are carefully planned. Generally, 3D seismic is acquired. Geological and reservoir models are built using geostatistical techniques. For this study, the reservoir model has a size of 117x43x50 grid cells. Twenty six wells are drilled in the field. The field is a fluvial depositional system. The porosity is high, ranging from 22% to 27%. The permeability is also high with an average value of 2,000 md. The wells are produced according to the designed schedule.

In general, seismic history matching is used for 4D seismic (Huang, 1998, 2000). For 4D seismic history matching, the seismic is treated as dynamic data. Even though the reservoir model is built using a seismic-constrained geostatistical technique, the model generally is not re-evaluated to see if the seismic response can be reproduced. Also, in the process of history matching, the model update may generate inconsistencies with the seismic. This work is to develop a workflow and tools to check the model consistency with the seismic and constrain the model updating with seismic in the process of history matching.

Workflow

Reservoir engineers generally work on the model in the depth domain and with irregular grids. On the other hand, geophysicists work on a regular grid with seismic data in the time domain. To use seismic as a constraint in history matching, several techniques need to be merged. We developed a workflow as shown in Figure 1. The workflow starts with a re-gridding of the seismic, rock model calibration, acoustic property simulation, synthetic seismic generation, comparison of the synthetic with the observed and model updating.

Seismic re-gridding and model loading

Generally, the simulation grid is coarser than the seismic grid laterally. In order to constrain the model updating in the history matching process, the seismic traces can be resampled into the simulation grid. The model loading will include static and dynamic properties such as porosity, net-gross ratio, permeability, saturation and pressure.

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