Shell Development Co. has introduced a downhole cementing system that does not contain conventional Portland cement but instead consists of blast furnace slag and drilling mud.
Shell said it can save operators thousands of dollars per well in drilling costs.
Shell Offshore Inc. has used the drilling mud conversion technology in more than 70 wells and almost exclusively in its Auger field development project in Garden Banks Blocks 426 and 471 in the Gulf of Mexico.
Shell presented details of the mud conversion technology at the American Association of Drilling Engineers local meeting in Houston this month, and later in a paper by K.M. Cowan, A.H. Hale, and J.J. Nahm at the Society of Petroleum Engineers annual technical conference and exhibition in Washington, D.C.
S-MIX USE
The slag/drilling mud mixture (S-mix) has been used in 31 Auger cementing jobs, saving about $500,000/well.
Shell plans to install the Auger tension leg platform in 2,862 ft of water on Garden Banks Block 426. Auger will be Shell's first production platform in water deeper than 1,500 ft. Development wells are being drilled through a seabed template.
The savings flow from elimination of stage cementing tools, a reduction in rig time, and reduction in the cost of cementing materials. Substituting drilling mud for cement also has reduced excess drilling fluid discharges by about 70%.
Cost of the slag is about the same as that of Portland cement, except transportation costs are slightly higher because of a less developed distribution network. The cost per cubic foot is usually less than that of a comparable conventional cement.
Slag is a nonmetallic substance produced with iron in a blast furnace. It essentially consists of silicates and alu-minosilicates of calcium, mangesium, and other bases.
ADVANTAGES CITED
Shell said S-mix has better fluid loss control, improved solids suspension, lower free water, and lower hydration volume reduction than conventional cement with an equal cost.
S-mix can improve zonal isolation in applications in which contamination of the cement by the mud may reduce job success. S-mix usually requires no spacers between the mud and cement because mud forms the base for the new cementing material.
The mixture also minimizes the effects of mud contamination on compressive strength, interfacial sealing, and shear bond strength.
S-mix cementing fluid densities can range from 10 ppg to 20 ppg. The mixture has been used successfully in low temperature deepwater wells at 40 F. and in high temperature thermal wells at 600 F.
Shell said slag has better Theological compatibility than conventional cement with drilling fluid, can be activated in a wide range of water base drilling fluids, has a more uniform quality than Portland cement, and is widely available.
BATCH MIXING
Shell has pumped S-mix on two operations in batch mixes using standard rig equipment and pumps.
Blast furnace slag is typically added at concentrations from 40 lb/bbl to 500 lb/bbl.
Common alkalis, such as sodium or potassium hydroxide and sodium carbonate, are added to the mixture to adjust thickening time and compressive strength.
Blast furnace slag may be added to the drilling fluid during drilling because the slag has only a small effect on drilling fluid properties. The drilling fluid in washed out sections of the hole and the mud filter cake will therefore contain slag.
At casing point, a mixture of drilling fluid with a higher concentration of slag and chemical activators is pumped. The activators react with the slag in the mud cake and in washed out areas, resulting in a more complete seal in the annulus.
Shell said fluid properties and solid properties of S-mix are comparable to properties of conventional Portland cement. Design, testing, and field applications are similar to conventional cementing methods.
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