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Abstract Coring operations were conducted with a mineral oil based mud to obtain low invasion cores from the Kuparuk sandstone in the Point McIntyre field on Alaska's North Slope. These cores were taken to provide oil saturation, porosity, and permeability data for calculation of oil reserve ownership for the three major oil companies participating in the field. This paper describes the development, testing and application of a very low invasion, low water content, mineral oil based coring fluid. This fluid, in combination with specially designed low invasion coring bits and high penetration rates, consistently produced 4-inch cores with filtrate invasions limited to the outer 0.25 to 0.50 inches of the cores in sandstone with permeabilities ranging from a few millidarcies to greater than 1.0 Darcy. A new oil soluble tracer was identified, which when present at 30 parts per million could reliably quantify mud filtrate invasion. Introduction A mineral oil base mud was designed to core eight wells in the Point McIntyre field to provide data for distribution of oil reserves. These cores were taken to obtain data on connate water saturation, porosity, permeability and geology for the Kuparuk sandstone. This is a companion paper to SPE 26353, which describes the coring operations and core bit development for this project. The starting point for the new mud design was the oil base mud used in the Prudhoe Bay field some years ago for a similar equity coring program. The Prudhoe fluid was primarily designed so that the filtrate would not significantly after the water saturation of the cores. The coring done in the Prudhoe Bay Sadlerochit sandstone provided some intervals of core with uninvaded centers. Advances in low invasion coring technology, safety and health considerations, and lack of availability of a number of products used in the Prudhoe fluid made R necessary to design a new fluid for coring at Point McIntyre. The design criteria are described below, but the goal was to formulate a fluid that would;consistently provide core with an uninvaded center,have a filtrate that would not significantly alter connate water saturations and rock wettabilities, andhave all of the rheological properties, etc., which would make the fluid easy to build and maintain in the field. The low invasion character of the fluid was achieved by using organophilic clay, asphaltic fluid loss additives, and large quantities of fine bridging solids to rapidly build a very tight filter cake on the core. Minimizing changes in water saturation and rock wettability was addressed by choosing an emulsifier that did not generate low interfacial tensions compared to those normally found between live crude and brine, but was still able to emulsify any water introduced into the mud from cuttings, etc. during coring operations. Some commercial organophilic clays are very slow to develop viscosity and gel strength in mineral oils under normal mixing plant conditions and are very sensitive to water. This creates a situation where an all-oil fluid has trouble suspending weighting material during transportation to the rig, but upon contamination with small amounts of water becomes so viscous that it is not easily pumpable. P. 117^ |
