The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Mechanical friction during drilling operations is a known problem which is enhanced in long narrow, highly inclined, directional, and “horizontal,” boreholes. A directional well is typically drilled to position a reservoir entry point. Directional wells are initially drilled straight down to a predetermined depth and then gradually curved at one or more different points to penetrate one or more given target reservoirs. A horizontal well is commonly defined as any well in which the lower part of the well bore parallels the oil zone. Horizontal drilling allows a single wellbore at the surface to penetrate oil or gas-bearing reservoir strata at horizontal or near horizontal angles to the dip of the strata. The angle of inclination used to drill the well does not have to reach 90° for the well to be considered a “horizontal” well. Horizontal drilling has become one of the most common and popular forms of drilling for recovery of hydrocarbons from subterranean formations used today. Extended-Reach Drilling has evolved from simple directional drilling to horizontal, lateral, and multi-lateral step-outs. Since extended-reach drilling employs both directional and horizontal drilling techniques, the utility of the present invention is applicable to extended-reach drilling as well.
Friction reducers are typically either liquids or solid particles. Liquids form a film between two surfaces minimizing contact and consequently friction. Liquid additives include among others glycol, oil, ester, and fatty acid ester-based lubricants. The efficiency of such liquids as friction reducers depends heavily on the type of drilling fluid in which the liquids are used. The effectiveness of liquids may be lost altogether in high-solids drilling fluids.
Solid particles are less dependent on the type of drilling fluid used and various forms have been used. One example of a solid lubricant is glass beads, which were originally thought to act like small ball bearings downhole. Later, the beads were considered by some to slide instead of act like a ball bearing, but in either event, the beads were found to reduce friction. Another example of a solid lubricant is polymer beads, which, like glass beads, form a slippery lubricant between the borehole and the drill string. Still another similar example is plastic beads.
As used herein, the term “beads,” includes, besides preferably spherical particles, rotational elliptic particles, egg shaped particles, e.g., particles which are rotationally symmetric to one axis, droplet shaped particles, pellets, and also less symmetrical particles like spheres with protrusions. Most preferably, the beads are in the shape of substantially perfect spheres.
The size of beads has been considered important, as larger beads (for example, greater than about 75 μm) tend to be filtered out in solids control equipment. On the other hand, fine beads (for example, about 44-88 μm) pass most shale shaker screens but tend to be removed by desilters. Generally, a size in the range of about 40 to 60 mesh is considered most preferred. Beads typically have a density comparable to the desired density of the drilling fluid, or may have a lower density to lighten the fluid column.
With more and more drilling for the recovery of hydrocarbons involving or including horizontal drilling, the use of beads as lubricants has shown the problem of gravity migration and settling of beads on the lower side of the borehole. However, beads also reduce drill pipe and casing wear in high angle directional and horizontal wells by preventing metal to metal contact. The beads form a layer or sheath between such metals to reduce friction.
Moreover, beads do not typically influence drilling fluid properties, except for a negligible increase in viscosity and possibly a slight decrease in 10 minute gel strength. Consequently, there is continued interest in new and better beads for use as drilling fluid lubricants, particularly in directional and horizontal wells.