Sweep pills in hole cleanup for cementing
The most important factor in a sweep program is to carry it out in a proactive manner. It is much easier to keep the hole clean than it is to try to clean it up after solids buildup has occurred.
Hole cleaning depends on fluid type. When wells are drilled with invert oil emulsion systems, cuttings tend to be harder, more competent, and better defined than in water-based mud (WBM). This method allows the cuttings to be removed from the wellbore more readily. Even highly inhibitive, high-performance WBM systems do not generate cuttings of the same high level of integrity as inverts. Hole cleaning may also be compromised by the effect of WBM fluids on the nature of the borehole, which will often exhibit rugosity or out-of-gauge hole, thereby reducing annular flow velocities. Large washouts tend to require more frequent hole-conditioning trips. Silicate, CaCl2 , and some cationic polymer WBM systems produce near-gauge holes in formations of moderate or low chemical reactivity, but in the Gulf of Mexico (GOM), even these WBM types can fall short, and poor hole cleaning and packing off are very real risks. This problem is also manifested at the shakers, which usually require extra attention to keep the screens clean and handle the larger dilution volumes needed to maintain WBM properly.
Cuttings transport in deviated wellbores is more challenging than in vertical wells. Correct well planning, drilling practices, and sweep procedures can have a positive influence on “on-bottom” drilling times. Primary factors contributing to hole-cleaning challenges include:
- Drillpipe eccentricity
- The need for sliding while maintaining hole direction
- The resultant flow-path changes in the annulus
A factor that compounds the situation is that cuttings settle toward the low side of the deviated hole. This situation, shown in Fig. 1, is known as the Boycott Effect.
Fig. 1—Boycott effect: the tendency for cuttings to settle along the low side of a deviated well.
Regardless of drilling-fluid rheology, it is almost impossible to clean a highly deviated wellbore without drillpipe rotation. Drillpipe rotation agitates the settled cuttings back into the flow stream, so they can be transported to the surface.
- Calvert, D.G., Heathman, J.F., and Griffith, J.E. 1995. Plug Cementing: Horizontal to Vertical Conditions. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 22-25 October. SPE-30514-MS. http://dx.doi.org/10.2118/30514-MS.