A new method of obtaining improved zonal isolation using drilling fluid solidification technology has been developed. A water-base drilling fluid is converted into a cement using a hydraulic blast furnace slag. Hydraulic blast furnace slag is a unique material which has low impact on rheological and fluid loss properties of drilling fluids, can be activated to set in drilling fluids which are difficult to convert to cements with other solidification technologies, is a more uniform and consistent quality product than Portland well cements, and is available in large quantity from multiple sources.
Because of its low impact on drilling fluid properties, blast furnace slag may be added to a drilling fluid at low concentrations during drilling operations. The filter cake and drilling fluid in washed out sections thereby contain a hydraulic material. After reaching casing point, a mixture of drilling fluid containing chemical activators and higher concentrations of slag may be used to cement the casing string. Chemical activators from this mixture cause the slag in the filter cake and any bypassed drilling fluid to set. The result is a more complete seal for the annulus.
Fluid and hardened solid properties of blast furnace slag and drilling fluid mixtures used for cementing operations are comparable to properties of conventional Portland cement compositions. The design, testing and field application for this technology are similar to conventional cementing methods. Fluids with densities between about 1198 kg/m3 (10 lb/gal) and 2397 kg/m3 (20 lb/gal) may be prepared. The mixtures have been applied in wells where temperatures range from about 4 degrees C (40 degrees F) to 315 degrees C (600 degrees F).
This new solidification method provides the proper combination of fluid and solid properties, simplicity of design and application, improved zonal isolation, and broad applicability to bring drilling fluid solidification technology into widespread use.
Conversion of drilling fluids (muds) into cements suitable for well cementing operations has been an area of interest within the petroleum industry for over fifty years. Improved zonal isolation in the annular space between a casing and borehole has been and continues to be the primary reason for pursuing this technology.
For nearly a century, zonal isolation has been attempted largely by the placement of Portland cement formulations in the annular space. The widely practiced cementing process is plagued by variables which are often difficult to predict or control yet have a critical impact upon the quality of seal achieved. Two of these variables include effective removal of the drilling fluid occupying the annulus and the effects of mud or drilling fluid contamination on cement formulation properties in the liquid (slurry) and solid states.
A fundamental weakness of the conventional cementing process is the uncertainty of establishing a true, reliable seal at the borehole wall and cement interface.
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