oa The Role of Rock Physics Modelling for the Enfield 4D Seismic

The Enfield rock physics model was constructed to enable interpretation of the 2007 Enfield 4D monitor. The rock physics model links reservoir static and dynamic parameters to impedances. The model was based on a combination of log data from 5 wells in the field, laboratory core measurements taken from core on Enfield and neighbouring fields, and theoretical rock models from the literature. The reservoir is modelled by a sandshale mix. Sand properties are described using a modified critical porosity model whereas shale properties are generated from log data averaging. The dynamic properties in the model include saturation and pressure. Saturation is modelled using Gassmann’s formula assuming a homogeneous mixing. The reservoir sand velocity-pressure relationship was described by an empirical model fitted to dry core plug measurements. An assessment of the effect of uncertainty was included for both the saturation and pressure elements of the model. The resultant rock physics model was used prior to the acquisition of the monitor to assess the likelihood of measuring a 4D signal only 7 months after production start-up. Our modelling results indicated that the strong pressure build-up around the water injectors would result in a measurable 4D signal and this prediction was fulfilled by the successful 4D survey acquired in 2007. The rock physics model has been validated against the 4D monitor data and is being used to quantify the 4D interpretation, linking the observed 4D response back to predicted pressure and saturations in the field.