Unconventional reservoirs associated with the presence of fractures have been identified in the Jurassic horizons in the Raudhatain field in North Kuwait. These reservoirs contain high-quality light oil and gas under HPHT conditions with associated H2S and CO2. The operator in this field is presently developing these gas/light oil reserves under the ‘North Kuwait Gas Development Project, The well discussed in this paper is one of a number of new production wells.

Historically, Jurassic wells in this field have been drilled using conventional barite-weighted, oil-based mud (OBM) with densities up to 18.5 ppg. However, the challenge to maximize the amount and quality of reservoir data collected and to reduce any impact on the formation caused by OBMs led to a search for an optimized water-based mud (WBM) system as an alternative.

A WBM formulated with saturated potassium-formate brine, weighted with manganese tetra-oxide, was selected for the following characteristics: higher base-fluid density, non-damaging nature, robust behavior in HPHT environments, and the possibility of better quality image logs.

The well reached 16,530 ft., in a 6–in. hole, with bottomhole temperatures around 280°F. The fluid was thermally resistant, exhibiting very low HPHT filtrate loss and highly stable rheological properties. The maximum fluid density was 16.2 ppg, with no evidence of settling of weight material, even after three days of static conditions.

A complete suite of logs was run successfully. The elemental capture spectroscopy (ECS), along with sonic and neutron porosity logs provided good data quality, and the image logs showed excellent resolution.

This paper will present all aspects of the planning, design, and utilization of this new drilling fluid. It will focus on lessons learned and conclude with recommendations for further optimization of the design of these drilling fluids.

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