1. Field of the Invention
This invention pertains to the manipulation of ferrofluids. More specifically, this invention is related to a magnetic device for the manipulation of ferrofluids in the cementing of oil wells.
2. Technology Review
U.S. Pat. No. 4,356,098 (Chagnon) describes certain stable ferrofluid compositions and a method of making same. Chagnon indicates that ferrofluids are ferromagnetic liquids which typically comprise colloidal dispersion of finely-divided magnetic particles, such as iron, gamma-Fe.sub.2 O.sub.3 (maghemite), Fe.sub.3 O.sub.4 (magnetite) and combinations thereof, of subdomain size, such as, for example, 10 to 800 Angstroms, and more particularly 50 to 500 Angstroms, dispersed in a liquid through the use of a surfactant-type material. Chagnon states that typically ferrofluids are remarkably unaffected by the presence of applied magnetic fields or by other force fields in that the magnetic particles remain uniformly dispersed throughout the liquid carrier. Such ferrofluid compositions are widely known, and typical ferrofluid compositions are described, for example, in U.S. Pat. No. 3,764,540 and a process for making such materials is described in U.S. Pat. Nos. 3,917,538 and 4,019,994. Chagnon and the recited references therefore describe ferrofluids in which the only suspended particles are magnetic particles and the liquid medium is generally organic rather than aqueous.
The well cementing technology is replete with references to hydraulic cements and methods of using such cements in cementing wells. In this technology, cement slurries are used to fill the void space between the casing or pipe and the walls of the borehole penetrating a subterranean formation; a process called "well cementing" in the industry. In using such cement slurries, a line or string of pipe is inserted into the borehole and a cement slurry is pumped downwardly through the pipe into the bottom of the borehole and then upwardly along the outside of the casing or pipe displacing drilling mud from the annular space. The cement slurry is then displaced from the interior of the pipe before it hardens; this is normally accomplished by injecting a liquid medium behind the cement slurry and using it as a "hydraulic fluid" to force the remaining cement out of the pipe and into the annulus. In another but less common technique, the borehole is filled with the cement slurry and the pipe or casing (normally with the end sealed) is lowered into the hole. Cementing techniques are more fully described by D. K. Smith in "Cementing", Monograph Vol. IV, Henry L. Doherty Series, Society of Petroleum Engineers of AIME, New York (1976). A problem can occur if the cement slurry does not form a good bond with the casing and the formation wall. If drilling mud is not uniformly and completely displaced from the annulus, a "microannulus" will form around the casing when the slurry fails to bond properly. Various techniques have been used but the most common commercial techniques used to combat this problem involve chemical washes or spacers and/or expansive cements. See, for example, U.S. Pat. No. 4,207,194 (Sharpe et al.) which illustrates the use of chemical washes and spacers. Such chemical washes are injected as a preflush ahead of the cement slurry and are thereby used to displace the drilling mud and "wash" the walls of the formation and casing before being contacted by the cement slurry. See also U.S. Pat. No. 4,328,036 (Nelson et al.) which illustrates the use of expansive cements. Another method utilizes sonic or energy-carrying waves to displace the drilling mud from the walls of the casing and formation. See U.S. Pat. No. 4,093,028 (Brandon). These methods work to a greater or less degree, but a need still exists for a method of removing the drilling mud from the casing and formation walls so as to promote better bonding of the cement slurry thereto. The drilling muds and cement slurries are typically incompatible, particularly when the drilling mud is an oil-base drilling mud.
U.S. Pat. No. 4,691,774 (Nelson) discloses a novel stable cementitious ferrofluid suitable for use in cementing wells, and a process for its use. The present invention provides a device for manipulating cementitious or non-cementitious ferrofluids used in a process for cementing wells.