ABSTRACT

In 1997 MRIL-C was run in multi-acquisition mode to improve reservoir characterisation in a North Sea gas field. In the absence of core data, reservoir characterisation based on conventional logs only is extremely difficult in this field, due to the variability of matrix properties. NMR logs, which provide "matrix independent" measurements, improve the interpretation of porosity and fluid content compared to conventional logs. The reservoir quality in this clastic play is controlled mainly by the degree of diagenesis. Halite plugging in particular deteriorates reservoir quality considerably. Porosities in the field vary typically between 2 and 30% BV, salt contents range between 0 and 40% BV. With no quantitative information on halite content, the use of conventional logs in a standard deterministic approach for reservoir characterisation, will provide highly unreliable results. MRIL-C was run in a well drilled with OBM over the gas zone and across the GWC. Dual wait time data, diffusion- and BVI-data were acquired in three passes. Shell's proprietary processing software provided, independent from any other log data, porosity, permeability, fluid saturation, the volume of irreducible water (BVI) and indicated the location of the FWL estimate. The presence of the various fluids in the pore space (water, OBM filtrate and gas) and the effects of the variable matrix properties on reservoir quality could clearly be seen in the NMR results. Based on the NMR evaluation, different reservoir intervals and theft properties were identified:

  • High permeability and porosity in line with the evaluation from conventional logs were calculated in the halite free gas interval.

  • The large variations in porosity and permeability, related to variations in halite content were quantified in the cemented interval.

  • A GWC in line with the FWL identified from pressure measurements could be determined, with significant amounts of free water and no gas in the bottom part of the well.

  • In line with core descriptions from other wells in the field across the GWC, the NMR results also suggested that the diagenesis is different in the water zone as compared to the gas zone.

  • Significant amounts of invaded OBM filtrate were only observed in the lower permeability gas zones and in the water bearing interval.

The absence of invasion in the high permeability gas interval can be explained by the high mobility of the gas.

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