Development of environmentally friendly lost circulation materials for riserless drilling
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The recent scientific drilling operations have proved that wells as deep as 6000-ft. from ocean bottom can be safely drilled with small riserless drill ships. However, the problem is that theweight of the drilling fluid must beoccasionally increased from the sea water density to 9-11-ppg. The problems encountered while using weighted polymer drilling fluids are wellbore instability, stuck pipe induced by lost circulation and insufficient cutting transport problems. Polyglycolic Acid (PGA) and Polylactic Acid (PLA) have been widely used for medical operations,since the products left in human body dissolve with time. One of the properties desired for lost circulation materials is that the materials prevent lost circulation problems during application, while they disappear when they are disposed to ocean bottom duringriserlessdrilling. The lost circulation resistance tests are conducted using the standard API slot tester, mixing fibers, grains and powders, all made of PGAor PLA in a Xanthan Gum polymer drill-in fluid. Ratio of fibers, grains and powder is optimized to maximizethe lost circulation resistance for 0.04 and 0.12-inch slots. Then, the changes of particle size and fiber strength with time are measured. Cutting retention tests with the fibrous LCM are also performed to check the cutting retention capability when the circulation is ceased during the connection of drill pipes. The cutting fall rate is measured changing fiber concentration, pipe diameter and cutting volumes. The results show that the capability of preventing lost circulation is maintained withina reasonable time. PGA and PLA lost circulation materials are applicable when the temperature is less than 60C. The fibrous LCM can also have a capability of cutting retention so that the cutting fall rate is trivial even if the fluid circulation is stopped. The conclusion shows that the PGA and PLA LCM are capable of sealing fractures for drilling low temperature wells up to 60C and the fibrous PGA and PLA also help to transport cuttings from well bottom. The merit of PGA is the high strength during the first several hours and the merit of PLA is the pro-longed strength although its initial strength is low. The dissolution time is several days for PGA LCM while it is several weeks for PLA LCM. The strength and dissolution time can be adjusted by the grain sizes and mixture ratio of these two polymers. This paper emphasizes that when the mud weight is increased, mixing lost circulation materials with enhancing cutting transport capability is important to conserve the mud volume since, for riserless drilling, the weighted mud is not circulated back to the mud pit. Copyright 2012, SPE/APPEA International Conference on Health, Safety, and Environment in Oil and Gas Exploration and Production.
Society of Petroleum Engineers - SPE/APPEA Int. Conference on Health, Safety and Environment in Oil and Gas Exploration and Production 2012: Protecting People and the Environment - Evolving Challenges
author list (cited authors)
Matsui, H., Daitoh, M., Yoshimura, K., & Morita, N.
