Estimated Fluid Contact Using Material Balance Technique and Volumetric Calculation Improves Reservoir Management Plan

The Gryphon Field located in block 9/18b of North Sea was discovered in 1987 and originally comprised of a ~200ft thick oil rim under a gas cap with a strong aquifer. It is connected to another nearby field via the gas cap and has been producing since October 1993. One of the biggest challenges in placing future wells is that production has significantly thinned the original thickness of oil column above the oil-water contact (OWC) and the gas-oil contact (GOC) has moved up due to gas movement and production.

A methodology has been developed to estimate current water (OWC) and gas (GOC) oil contacts in the field from initial fluid in place, production and rock and fluid properties. The methodology is based on the volume of remaining fluid in the reservoir using material balance techniques and calculation of the fluid contacts assuming the whole reservoir as a single tank, and with best estimates of initial contacts and residual saturations.

Here, the contact movements have been calculated versus time. The calculated fluid contacts show a good agreement for two direct measurements of GOCs from two pilot holes. There is a 50 ft difference between OWCs from these two pilot-holes measurements; the calculated OWC is almost the average of these two values - providing confidence in the approach.

The calculated fluid contacts were also compared with recently obtained 4D seismic and up to date reservoir simulation model for different producing wells that show a good match.

The completion interval of all wells has been mapped versus the contact movements. The results show that in order to increase oil production from the current the wells, gas injection could be increased in a way that takes into account the effects of fluid movement. The wells need to be operated as much as possible at low gas oil ratio that is possible due to strong water drive. It is also shown that the datum of the new infill wells should be at a shallower depth compared to previous wells.