Clay Mineralogy and Solutions to The Clay Problems in Norway

Abstract

If you think that gate, trough, and ditch, cow grate, and possum belly all belong down on the farm, think again. They have a lot to do with getting the gumbo out, and that has a lot to do with faster, cheaper drilling. Introduction One of the major problems confronting operators developing the Norwegian sector of the northern North Sea is the presence of soft, sticky, gumbo clays. These clays, of Recent to Tertiary origin, are encountered downwards from the sea bed to the Lower Eocene. A gradual transition from well defined clay to shale occurs in the Lower Eocene. Instantaneous penetration rate through these intervals is high. Drilling rates penetration rate through these intervals is high. Drilling rates for the 17 1/2 -in. hole often exceed 2 to 4 minutes per joint (to 5,000 ft), and the 12 1/4-in. hole can be drilled in 6 to 12 minutes per joint (to 9,000 ft). Hole making itself is rapid, but problems associated with the drilled clay cuttings are serious and time consuming. For example, as shown in Fig. 1, actual rotating tie (including time spent opening the surface hole) for Well B to a depth of approximately 8,500 ft was 101 hours. The time spent for other operations related to drilling amounted to 190 hours, or almost twice the time spent rotating. Most of the 190 hours was spent circulating and conditioning, pumping out clay rings, cleaning mud ditches loaded with clay balls, and cleaning blinded shaker screens. This paper assesses the gumbo problems, explains the steps-that were taken to overcome those problems, and tells what equipment is recommended problems, and tells what equipment is recommended and what mud characteristics are desirable. Occurrence of Swelling Clay as a Function Of Clay Density In a recent study by Burst on Gulf Coast clayey sediments it was proposed that water migrates from the clay in three separate stages of dehydration during the geological development of the clay. The data given in Fig. 6 of Burst's paper have been replotted to show the percentage of swelling clay at any stage of its geological development and are presented in Fig. 2. This illustration shows the percentage of swelling clays, nonswelling clays, pore water, and interlayer water present in a typical clay or shale during the process of diagenesis. The total water content reflects both pore water and interlayer water. The total water plus the sum of clayey and nonclayey material at any stage of development constitute 100 percent. percent. The percentage of swelling clay reaches a maximum of about 40 at the end of the first stage of dehydration when most of the pore water and interlayer water has been removed by overburden pressure. Porosity and total water decrease as density pressure. Porosity and total water decrease as density increases. Percentagewise, the interlayer water is also at a maximum at the end of the first stage of dehydration. The clay has a density of about 1.96 gm/cc and a total water content of about 30 percent. Swelling clays in excess of 35 percent may be expected for clays with an over-all density of 1.7 to 2.1 gm/cc. The density of clays observed in Norway has consistently been within this range for the interval between 1,500 and 8,500 ft. We should expect to find a large proportion of swelling clays when the clay densities are less than 2 gm/cc. JPT P. 25