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In offshore drilling, the riser serves to move fluids from the sea floor to the mobile offshore drilling unit (MODU) on the surface. For the initial segment of the well, the riser may not be in place, so drilling fluid composition is important.
Drilling without a riser
Until the riser is in place, all drilling fluid returns to the seabed. A variety of drilling-fluid systems are available for drilling the top-hole riserless interval, including:
- 1:1, 2:1, and 3:1 seawater/base-fluid blends
- Ballast-storable fluids
- Precisely engineered “pad muds” for maintaining wellbore stability while running casing
Typically, drilling fluid must be mixed and treated at flow rates of up to 1,000 U.S. gal/min (1,400 bbl/hr). A three-inlet, high-performance eductor is used to blend riserless drilling fluids, which allows a third stream of CaCl2 or an alternate inhibitive brine to be mixed with standard seawater and base-fluid streams.
Using a specialized, polymer-free base fluid facilitates efficient pumpoff from supply boats. The ideal fluid is thin enough to be transported by a standard centrifugal pump, yet retains the desired rheological properties when blended with seawater.
For remote locations that do not have continuous workboat support, using a ballast-storable fluid system helps ensure that the operator has access to the required volume of drilling fluid during inclement weather. The fluid should be virtually solids-free to help avoid settling in the tanks. This type of system has been used for the riserless portion in several deepwater operations. 
Accurate modeling helps when formulating an optimal pad mud for each well. A properly designed pad mud helps to ensure that the wellbore will remain stable while surge pressures are minimized. Modeling-anticipated hole conditions allow the drilling-fluids engineer to predict the effects of interdependent parameters such as:
- The maximum recommended rate of penetration (ROP) at various flow rates and cuttings concentrations
- The equivalent circulating density (ECD) on the bottom and the annular cuttings concentration at different ROPs and flow rates
- Transport efficiencies and annular shear rates at different flow rates
- A comparison of proposed casing running speed with surge-pressure tolerances
The pad mud should exhibit controllable fluid-loss characteristics to minimize filter-cake buildup across permeable sands.
- Morales, L. 1999. Drill Fluid Stored in Semi Ballast Tanks Controls Shallow Water Flow. World Oil 220 (7): 59.
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