Operating envelopes are used in many industries, including the oil industry, to define appropriate operating boundaries and limits for production systems. In the oil industry, the boundaries generally include minimum and maximum operating rates, as well as minimum and maximum operating pressures and temperatures for the different components of the production system. Today, the use of operating envelopes has been extended to the subsurface (specifically hydrocarbon-bearing reservoirs) with the aim of identifying optimum limits for production rates and reservoir pressure depletion.
The Sofad field is one of the biggest and most structurally complex oil fields in the Western Niger Delta straddling land, swamp and shallow offshore. The high offtake rates from the reservoirs, coupled with the high fault density, have inhibited the response of the otherwise active aquifer especially in the key producing reservoirs. As a result, the key reservoirs have suffered severe pressure depletion over the years.
Analyses of the historical production/pressure trends of these reservoirs clearly indicate that the reservoir pressures are generally sensitive to the rates of production from the reservoirs. Thus, it became essential to evaluate the maximum production rates for these reservoirs in order to manage the reservoir pressure depletion and consequently maximise oil recovery (pending the execution of the water injection project for pressure maintenance).
This paper presents how the operating envelopes for the reservoirs were designed. It also showcases how the operating envelopes are being used in ensuring the maintenance of reservoir pressures, which has led to an improvement in the recovery from these reservoirs over the last 3 years (2010 – 2012).