Case Study: SILDRIL Has Found Its Place in Cuulong Basin, Vietnam


In offshore Vietnam, the use of SBM to drill multiple wells from a single platform is prohibited due to environmental restrictions. To reach the fractured basement reservoir of the Cuulong Basin, there are two time sensitive shale formations that require drilling through. These are the Bach Ho shale at the upper part of the hole, and Oligocene shale at the lower part of the hole. Highly permeable sands also exist in this interval.

Traditionally the Bach Ho shale was drilled with a conventional KCl/PHPA system (17½”), then cased off with 13⅜” casing. The Oligocene shale was then drilled with KCl/PHPA/Glycol system (12¼”) and cased with 9⅝”. Both casing strings are required due to the reactive/dispersive and time-dependent nature of the shales which have a propensity to cave-in causing hole enlargement and other problems commonly associated with hole instability.


SILDRIL was chosen to drill the well so as to provide the necessary shale inhibition and well-bore stability normally associated with SBM, but unlike SBM also comply with the strict environmental requirements. In doing so a longer hole section could be drilled to alleviate a string of casing. Good hole cleaning was also considered critical for this long section of deviated well-bore.

The SILDRIL system was therefore rheologically engineered to provide elevated LSRV for optimum hole cleaning. This was attained using premium xanthan FLOVIS PLUS in combination with the modified starch FLOTROL. M-I Pac additions where restricted to 1.0 lb/bbl


The extended 12¼” section was drilled trouble free with higher ROP’s being experienced when compared to offset wells using KCI/PHPA/Glycol. Drill cuttings were well defined on the shale shakers. A core was cut from 2215 to 2243m without any problems prior to continuing drilling to interval total depth without incident. The hole was logged successfully and proved to be in gauge. The 9⅝” casing was run and cemented with full returns to surface. The success of using SILDRIL in the Cuulong basin has made it the fluid of choice for this operator.

Well Information
Location Cuulong Basin, Vietnam
Spud / Completion October 2, 2001 / December 4, 2001
Total Depth 5204m
Interval Drilled 12¼”, 2059m in length
Maximum Density 1.3sg
Hole Angle Held at 26°
Max BHST (12.25”) ± 270°C
Total days on Well 64

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“These swelling, dispersible, and time sensitive shales were drilled successfully using SILDRIL producing very well-defined cuttings at the shakers, which traditionally could only been found when using oil based mud.”
M. Nakajima
GM of Operation Department