Abstract

Oseberg Sør field, operated by Hydro Oil & Energy, is situated 130km west of the Norwegian coast on the eastern flank of the Viking Graben structure. It consists of fault bounded structural units of varying geological complexity. Within these units the reservoir intervals are of moderate to poor quality and can exhibit strong contrasts in permeability and formation water composition. Reservoir pressure support is provided by combined injection of gas and Utsira aquifer/produced water. The wells are a combination of platform and subsea and include extended reach horizontals with complex geometry and lesser numbers of vertical well.

Coreflooding has been undertaken on material from the major producing intervals (Ness, Middle & Upper Tarbet and Upper Jurassic). This coreflooding has been used to assess potential formation damage resulting from application of oil based mud or scale inhibitor squeeze treatment. In all instances, the major cause of formation damage was the mobilisation of kaolinite that can form up to 20wt% of the bulk rock and occurs in all producing intervals. Coreflooding demonstrated that even at very low flow rates (30ml/h) permeability was reduced by up to 99% during initial plug saturation and before the onset of treatment. Modification of the coreholder allowed multi-port permeability measurements to be made that gave an indication of the depth of damage.

Field evidence only gives an indirect indication of the effect of kaolinite mobilization on well productivity. For example, a gradual reduction in well productivity has reversed after squeeze treatment. This gradual decline is possibly a result of the migration of kaolinite during well inflow with consequent reduction in permeability. Squeezing could have caused the dispersion of kaolinite away from the near wellbore with an increase in well productivity.

Given the adverse results obtained from coreflooding due to kaolinite mobilization, significant effort has been directed towards identify methods by which to reduce its impact, through incorporation of clay stabilizers in scale inhibitor squeeze treatments and remedial treatments using mud acid stimulation. Laboratory results of these evaluations will be presented (mud acid coreflooding results were not available at the time of writing).

While the occurrence of kaolinite on Oseberg Sør has potentially reduced well inflow performance, it has had a positive influence on scale inhibitor squeeze lifetimes. A field example will be given to show this.

Introduction

This paper will demonstrate the approach taken on Oseberg Sør in the identification of the magnitude of kaolinite mobilization from different producing intervals along with the preventative and remedial measures that have been considered and implemented. Mobilisation of kaolinite during well production is not a new phenomenom and has been reported from many fields globally 1,2,3. The initiation of kaolinite mobilization can result from introduction of a liquid of different salinity to the formation water (salinity shock) or where the critical velocity for fines mobilization has been exceeded. Regardless of the initiation mode, the result is potential productivity impairment due to pore blockage within the near wellbore region.

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