Reservoir Development Planning For The Forties Field
Authors: Hillier Gordon R.K , Cobb Robin M. , Dimmock Philip A. 
Published: December 2007

The initial development plan and subsequent development of the Forties Field are compared. The paper shows how the geological model was modified by results of development drilling, and a reservoir description is included. A prediction of future performance is presented.

DISCOVERY, INITIAL APPRAISAL AND CONCEPTUAL PLANS

Forties Field, located primarily in the U.K. Licence Block 21/10, is situated nearly 180 km ENE of Aberdeen (Figure 1). It was discovered in October 1970, when oil-bearing Palaeocene sandstones, (in what has subsequently been named the Forties Formation, Ref. 1) were encountered in an anticline at 2100m subsea. Four appraisal wells, drilled during 1971–1972, proved the existence of a major oilfield with an area of some 90 sq km and a first estimate STOOIP of 4600 million bbl.

Pre-development stratigraphical and structural knowledge of the field has already been published (Ref. 2 and 3). The Forties Formation can be split into two lithologically distinct sequences - an upper predominantly sandy sequence and a lower interbedded sandstone and shale sequence. These are now referred to as the Sandstone and Shale Members of the Forties Formation (Figure 2). The Sandstone Member contains most of the Forties oil.

A model of sand distribution was constructed from available appraisal well and seismic data (Figure 3). Three significantly different areas of sandstone development were identified within the Field. These were:Central and Northern area - Very good reservoir characteristics and potential aquifer support.Western area - Good reservoir development, but subject to more variability.Southern and southeastern - Very poor sand development.

The reservoir rock and fluid data were quite favourable to oil production although the reservoir crude was highly undersaturated. The initial values of the more important parameters are listed below:

Forties Field Reservoir and Fluid Properties

Average Porosity 27% Average Water Saturation 23% Average Permeability 400 md Reservoir Pressure 3215 psig at 2175 mss Reservoir Temperature 195 °F at 2175 mss Saturation Pressure 1090 psig Oil Gravity GOR 37 ° API GOR 340 SCF/STB FVF 1.23 Formation Water Salinity 101,900 mg/l Oil Water Contact 2217 mss Thickest Oil Column 150 m gross Reservoir Oil Viscosity 0.8 cp

Inspection of the initial data indicated that this inherently low-energy reservoir required some form of external energy, either in the form of natural water influx or fluid injection, or both, for optimum recovery of its reserves.

Conceptual Plans

During the period 1972–74, many studies were performed to assess the probable reservoir performance. Most of these involved computer simulation models with varying degrees of refinement. Included were both single phase and multiphase two and three-dimensional layered models. They were used to investigate pressure behaviour under various offtake profiles (centred around a 400 MBD plateau), movement of water vertically and horizontally and the effect of the wide range in permeabilities. A radial model checked the adequacy of peripheral water injection.

Forties Field Reservoir and Fluid Properties

Average Porosity 27% Average Water Saturation 23% Average Permeability 400 md Reservoir Pressure 3215 psig at 2175 mss Reservoir Temperature 195 °F at 2175 mss Saturation Pressure 1090 psig Oil Gravity GOR 37 ° API GOR 340 SCF/STB FVF 1.23 Formation Water Salinity 101,900 mg/l Oil Water Contact 2217 mss Thickest Oil Column 150 m gross Reservoir Oil Viscosity 0.8 cp

Inspection of the initial data indicated that this inherently low-energy reservoir required some form of external energy, either in the form of natural water influx or fluid injection, or both, for optimum recovery of its reserves.

Conceptual Plans

During the period 1972–74, many studies were performed to assess the probable reservoir performance. Most of these involved computer simulation models with varying degrees of refinement. Included were both single phase and multiphase two and three-dimensional layered models. They were used to investigate pressure behaviour under various offtake profiles (centred around a 400 MBD plateau), movement of water vertically and horizontally and the effect of the wide range in permeabilities. A radial model checked the adequacy of peripheral water injection.