The field under study is a fractured carbonate of low matrix permeability (∼15mD) and modest fracture permeability (<1200mD). The structure is relatively flat with a maximum movable oil column of 60m and a median thickness of 30m and is underlain by a strong aquifer. The live oil viscosity is 7cp. To date the field has recovered a small fraction of its STOIIP, mostly under natural depletion and WOGD. The field is scheduled for re-development by Cold Gas-Oil Gravity Drainage (CGOGD). The feasibility of a later phase of development by Steam-Assisted GOGD (SAGOGD) was evaluated and is the subject of this paper.
The field's reservoir properties present a unique set of challenges for an SAGOGD development. Firstly, the relatively low oil viscosity at initial conditions is unfavourable for a large viscosity reduction by heating. Also, the extent of the structure together with the modest fracture permeabilities complicates the spread-out of steam over the field. Finally, heat transfer efficiency to the matrix is problematic given the small thickness of the reservoir and the large matrix blocks.
The SAGOGD development strategy calls for simultaneous gas and steam injection by injectors that are spread out over the field, which stabilises the oil rim, spreads out the steam and reduces aquifer water influx. The incremental recovery over CGOGD was demonstrated to be substantial (16% of STOIIP) with an acceptable oil-steam ratio. Analysis of physical processes contributing towards recovery showed that viscosity reduction was still the main contributor to oil recovery.
Significant uncertainties remain to be reduced before committing to an SAGOGD development. Besides the wide range of uncertainty in incremental recovery (+/-60% from the base case), water supply and disposal requirements are major challenges for the steam project. Whilst these latter are being evaluated, the initial phase of CGOGD will narrow subsurface uncertainties for SAGOGD development substantially. In this paper the subsurface elements of the study will be discussed.