Significant challenges exist when drilling through and below salt formations. The thief zone at the base of the salt can introduce severe lost circulation and well control problems, often resulting in loss of the interval or the entire well. In many cases, operators choose to drill the salt section with water-based mud systems rather than risk costly losses of oil- or synthetic-based fluids. The slower drilling rates and poor hole quality obtained by drilling with water-based mud are offset by the potential losses of the invert emulsion drilling fluid. The thief zone immediately below the salt formation is typically a thin zone of highly fractured rock, usually shale. Controlling losses in this zone has proved to be extremely difficult. Very few effective lost circulation remedies have been successful, especially when drilling with invert emulsion fluids. The producing formations of the Hassi Messaoud field are located below a 900 meter (3000 ft) zone of mixed salt, which introduces a severe thief zone into the well construction process. The severe losses encountered drilling these wells result in significant non-productive time, massive fluid loss, borehole instability and loss of the drill string. A severe pressure inversion also contributed to the lost circulation problems, with pore pressure reductions from +/- 2.01 sg (16.8 ppg) in the salt to +/- 0.93 sg (7.8 ppg) in the thief zone. Over 800 wells have been drilled in the Hassi Messaoud field, with more than half of these wells encountering total loss of circulation once the salt section is drilled. The degree of severity of the losses varies between sub blocks as a function of local stresses, which provide additional forces for rock destabilization. Through the application of specialized loss circulation materials, these wells can now be drilled without significant fluid losses. The drilling time for the troublesome intervals has been reduced by 50% in most cases. This paper reviews the methods applied to avoid lost circulation in the sub-salt thief zone and discuss the time and cost savings obtained
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Success Recipe To Migrate Severe Sub-Salt Losses In The Algerian Sahara
Paper presented at the Offshore Mediterranean Conference and Exhibition, Ravenna, Italy, March 2005.
Paper Number: OMC-2005-032
Published: March 16 2005
Ferras, M., and M. Galal. "Success Recipe To Migrate Severe Sub-Salt Losses In The Algerian Sahara." Paper presented at the Offshore Mediterranean Conference and Exhibition, Ravenna, Italy, March 2005.
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