Subsurface Re-assessment of a Mature Nigerian Field Leading to New Well Opportunities: A Case Study

Parkins, Steve ¹; Nowak, Marius ¹
(1) Nigeria Business Unit, OML-123, Addax Petroleum, Geneva, Switzerland.

The Greater Ebughu field is a mature asset in the OML-123 license (Nigeria Delta) which has been producing for 22 years. Over the last few years, the field has been out performing all expectations and it was decided to perform a detailed review of the field to assess additional in-fill opportunities.

A full multi-disciplinary approach was taken starting with a new seismic interpretation and depth conversion of the reprocessed seismic survey. A new geo-model was constructed incorporating recently drilled wells. The existing logs were normalized to increase confidence in facies identification. Cross-plotting methods further aided in identifying flow units that were previously thought to be tight. The level of reservoir compartmentalisation was assessed by material balance work and review of the PVT. The grid was differentially upscaled by refining the grid in the oil column and coarsening it in the gas and water leg optimising simulation speed and results accuracy.

This review led to a successful history match with the incorporation of:

a) net reservoir rock previously modelled as shale

b) different equilibration regions and fluid contacts across the blocks

c) a differential grid upscaling in the oil, gas and water columns

d) faults transmissibilities calculated using a shale-gauge-ratio method

e) an edge drive aquifer, according to the depositional system

f) depth shifting of wells due to the deviation survey uncertainty, supported by comparing contacts between the simulation and actual logged data

As a result of this study, the full field STOIIP was increased by 36% and the GIIP by 29%, primarily obtained through an increase in net to gross and property distribution via facies modelling. The Proved Developed Producing (PDP) forecast showed a 25% recovery factor for a depletion mechanism with gas cap and aquifer drive. The combination of low recovery factor, low drainage radius of existing wells (due to gas and water coning) and an increase in STOIIP led to plan additional infill wells. Three wells have been identified with potential recoveries of a least 1.5 MMstb oil each. Their reserves were maximised by positioning the wells below the original mid-oil rim because of the movement of the gas-oil contact revealed by the model.

Finally, an appraisal well was identified which could prove up to 20 MMstb of additional STOIIP in an undrained block and a campaign of four work-overs was proposed as an outcome of this new study.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.