Abstract: Spacer fluids providing an effective buffer between drilling fluids and cement slurries during well completion. The spacer fluids comprise a sulfonated styrene copolymer dispersant and one or more additional components such as surfactants, viscosifiers and weighting materials to form a rheologically compatible fluid between the drilling fluid and the cement slurry.

Abstract: An aqueous spacer composition, compatible with drilling muds and cement slurries including saline slurries, and application there of drilling oil and gas wells.Preferred spacer bases in accordance with the invention comprise a biopolymer of the type produced by Azotobacter indicus, a high molecular weight sulfonated styrene or toluene-styrene polymer, and a dispersing agent. This base fluid has no solid particles. The final spacer composition additionally contains a weighting agent (barite) and water. Total compatibility is provided with muds and/or cement slurries whether or not they are saline, together with excellent fluid loss controlling, rheological, and anti-settling properties. A wetting agent may be added to provide compatibility with oil-based muds.

Abstract: In accordance with illustrative embodiments of the present invention, a ball injecting apparatus for use in launching cementing plugs into a well casing includes a tubular body having an inlet port and one or more cylinders on the sides thereof adapted to be loaded with the balls, gates on the cylinders to prevent and permit injections of the balls, drive members extending into the upper and lower ends of the body and being rotatable relative thereto, and a sleeve slidably splined to the upper drive member and movable downward in the body in response to fluid pressure to transmit rotation of the upper drive member to the lower drive member. A set of uniquely arranged upper and lower plug assemblies are releasable attached to one another and are arranged to be sequentially released in response to seating of a ball, bomb or a dart therein.

Abstract: This invention relates to a method of changing the direction of drilling a well bore wherein a cement plug is formed in the well bore at a location therein where the direction of drilling is to be changed and a pilot hole is drilled in the plug. The cement plug is formed using a slurry containing hydraulic cement consisting of discrete particles having a particle size not greater than about 30 microns. The use of such finely divided hydraulic cement significantly reduces the waiting time required for the slurry to develop sufficient compressive strength.

Abstract: A method and a treating mixture for controlling lost circulation of fluid from a wellbore to a formation wherein the treating mixture is flowed down the wellbore to block flow into the formation. The mixture is comprised of a non-aqueous carrier liquid (e.g. diesel) and particles of compressed sponge) which expands upon contact with an aqueous liquid in the formation. The mixture may also contain a second lost circulation material (e.g. unhydrolyzed polyacrylamide) which hydrates to bridge and plug any voids or spaces left between the expanded sponge particles. When used to control lost of an aqueous liquid, e.g. water-based mud, a non-aqueous liquid (e.g. diesel) is used ahead and behind the treating mixture to separate it from aqueous liquids. When used to control loss of an oil-based liquid, water is pumped behind the treating mixture to provide the aqueous liquid necessary for expanding the sponge particles once they are in place.

Abstract: The present invention provides an apparatus for cementing a liner string in a well bore. The apparatus comprises an operating string having a smaller diameter than the internal diameter of the liner string which is insertable therein. The operating string is connected within the internal diameter of the liner string and may be operated by rotation of the operating string to release the string from the liner. A plug is connected to the lower end of the operating string and has a top plug which is releasably supported from the plug carrying body together with a longitudinally spaced bottom plug which is releasably supported from the top plug. The top plug is pressure releasable relative to the plug carrying body, as is the bottom plug to the top plug, and a retainer on the bottom plug, respectively. A shift is provided for locking the top plug to the plug carrying body until the bottom plug is released by application of pressure.

Abstract: A system for cementing a casing string in a slant well includes a slant drilling rig including a slanted mast having a pair of spaced rails with an elevator spanning between the rails and reciprocable along a length thereof for reciprocating a casing string with the casing string lying generally between and parallel to the rails. The mast has an open generally upward facing side. A cement plug container apparatus has a cementing manifold and a release plunger assembly attached thereto and positioned about the container relative to each other such that an angle of substantially less than 180.degree., and most preferably approximately 980.degree., is defined therebetween. Thus, the container is substantially free of any radial protrusions outside of that angle and can lie generally flat within the confines of the space between the rails of the tilted mast.

Abstract: A sequential remote control plug release system allows actuation of plug release plungers and manifold valves associated with a cementing plug container only in a predetermined sequence. With regard to a plug container having two plugs, and thus two plug release plungers, and three manifold valves, the middle and upper manifold valves and the upper plug release plunger can be opened and retracted only in a predetermined sequence. This is implemented through interconnected actuation valves and sequencing valves contained in a housing which can be located remotely from the plug container. An improvement in plug release plungers is also disclosed.

Abstract: A cement method and composition for cementing a well penetrating subterranean formations and aqueous based drilling fluid containing at least one cement retarder characterized by a major proportion of the drilling fluid from the well as it was drilled; water; a lesser proportion of dry cementitious material; a minor amount of a dispersant that does not effect satisfactory set cement within an acceptable time interval; and an accelerator selected from the class consisting of acetic acid; the first 4 carbon esters thereof; acetamide; monoethanolamine; and diethanolamine. The first 4 carbon esters are the methyl; ethyl; propyl; both normal and isopropyl; and butyl ester, normal, isobutyl and tertiary butyl.

Abstract: Spacer fluids providing an effective buffer between drilling fluids and cement slurries during well completion. The spacer fluids comprise a sulfonated styrene copolymer dispersant and one or more additional components such as surfactants, viscosifiers and weighting materials to form a rheologically compatible fluid between the drilling fluid and the cement slurry.

Abstract: A method of cementing wells to minimize gas intrusion by applying pressures on the subsurface formations substantially equal to the natural formation pressures.

Abstract: An operating string has upper and lower wipers with external seals thereon releasably connected to the operating string. A lock arrangement prevents premature release of the upper and/or lower wiper by mechanical force, but is responsive to fluid pressure in the operating string to first release the lower wiper from the operating string prior to release of the upper wiper. The upper wiper cannot be released from the operating string until the lower wiper has released from the operating string. Seals are provided to close off communication between the operating string bore and the external wiper seals to confine the pressure present in the operating string in a manner to assure release of the wipers as and when desired and in a sequence as may be desired.

Abstract: A well bore operating string has upper and lower wipers thereon with the wipers releasably connected by a frangible member for releasing the wipers in response to pressure in the operating string. Seals are provided to close off communication from the operating string bore to confine the pressure present in the operating string in a manner to assure release of the wipers as and when desired and in a sequence as may be desired. Upper and lower pump down plugs sealably seat, respectively, in the upper and lower wipers to form a movable, mechanical barrier to isolate or wipe ahead of and behind a substance for movement through the operating string and subsequent discharge into the well bore.The wipers are each connected to the operating string to prevent relative rotation therebetween to further reduce the possibility of malfunction or improper functioning in releasing the wipers.

Abstract: What is disclosed is a method of accelerating the set of a retarded cement. The cement may range from a drilling fluid containing dispersants or retarders that will slow down the set of the drilling fluid when cementitious fluid is added thereto, through overwashing, or treating a retarded primary or remedial cement that may have been injected into fractures or areas within a well to overwashing a plug or any volume of retarded cement that needs to be set in a well.The specific uses are discussed.

Abstract: A method of accelerating the set of retarded cement characterized by contacting the retarded cement slurry with a solution containing an effective amount of a compatible accelerator. The compatible accelerator is selected from the group consisting of triethanolamine, diethanolamine and monoethanolamine. This may be employed in overwashing operations, treating retarded cement that is injected into open areas in the well or treating the set of a plug of retarded cement in a well to effect acceleration and/or viscosification of the cement. Also disclosed are specific uses.

Abstract: An operating string O is provided for conducting cementing operations in a well bore.The operating string O is shown sealably and releasably connected with a liner L and extends through upper liner wiper means 45 and terminates in overlapping relationship to lower liner wiper means 26, both of which wiper means 26 and 45 are releasably secured to the operating string O with the lower liner wiper means 26 releasably secured below the upper liner wiper means 45 so that its bore 28a originates adjacent and extends away from the lower end of the upper liner wiper means 45.The operating string O includes positioning means H to hang the liner L on well bore casting C, and means 145, 146 and 148 to rotate and reciprocate the liner L before hanging it and/or to rotate the liner L during and after cementing.

Abstract: A method for plugging the areas of lost circulation occurred during the drilling of a well with the use of a drilling mud, and a lost circulation material to be used therefor. The lost circulation material comprising 1 to 90 parts by weight of a water-soluble solvent containing no active hydrogen atoms and hardly miscible with mineral oils and 10 to 99 parts by weight of a hydrophlic urethane prepolymer, and the method comprises injecting through the drilling pipe the lost circulation material sandwiched by a spacer of mineral oils, so as to allow the lost circulation material to penetrate into the lost circulation areas, to react with water which is present at the areas, and to plug the areas through its polymerization.

Abstract: What is disclosed is a method of accelerating the set of a retarded cement. The cement may range from a drilling fluid containing dispersants or retarders that will slow down the set of the drilling fluid when cementitious fluid is added thereto, through overwashing, or treating a retarded primary or remedial cement that may have been injected into fractures or areas within a well to overwashing a plug or any volume of retarded cement that needs to be set in a well.The specific uses are discussed.

Abstract: A method for staging a fracturing treatment in a horizontal wellbore where solidified gel is used as a diverting medium. A desired section of the horizontal wellbore farthest removed from the angle of deviation from vertical of the wellbore is perforated. Via perforations contained in the horizontal section, the desired interval is fractured hydraulically. The gel is displaced with a "wiper plug" and the gel confined to the fractured interval and wellbore area adjacent the fractured interval. Here the gel forms a solid gel in the interval and a gel plug in the wellbore. Afterwards, another section of the horizontal well is perforated. Thereafter, a second desired interval is fractured. After completion of the fracturing process, the gel plug breaks and the "wiper plug" is pumped to the farthest end of the horizontal wellbore.

Abstract: A sub-surface release plug assembly for use in cementing an outer casing annulus around a well casing. The assembly includes a top plug releasably connected to a drill string by a collet mechanism and including a swivel connector and a vent thereabove. A sleeve assembly extends through the top plug, and a bottom plug is releasably attached thereto. A vent is provided in the sleeve assembly between the top and bottom plugs, and this vent may be closed prior to releasing the bottom plug. The construction of the inner sleeve assembly provides that no high pressure from the drill string is applied to inner portions of either the top or bottom plugs. Thus, the plugs may be made of relatively soft, easily drillable material, even including plastic. When the lower plug is released and pumped to the bottom of the well casing, additional pressure opens a valve so that a flow passageway is provided therethrough.

Abstract: A thixotropic preflush fluid is employed in a well drilling operation to prevent loss of cement slurry to an air zone penetrated during the drilling operation. The preflush fluid comprises water, a cement, calcium sulfate, and a quick-hardening accelerator. Optionally, the preflush fluid also comprises a thickening agent.

Abstract: A cement head and casing assembly includes a cement head body having a cement passage defined therethrough. The body is mounted on the well casing and has a lower portion extending down into the casing, with an upper portion extending a relatively short distance above an upper end of the casing. A cement plug is closely received within a bore of the casing below the upper end thereof. The cement plug is releasably connected to the lower portion of the cement head body so that the plug is suspended from the lower portion of the cement head body within the casing. The cement plug has a plug passage defined therethrough which is communicated with a cement passage in the cement head body so that cement can be pumped down through the cement head body and the cement plug into the casing. The passage through the cement plug can be closed and the cement plug can be released from the cement plug body after the cement has been pumped into the casing.

Abstract: Subterranean oil and gas producing formations are fractured at vertically spaced intervals utilizing perforating guns forced down the well casing in tailing off "trains" comprising tamping and spacing water columns respectively positioned above and below each perforating gun, a perforation plugging slurry positioned below the spacing water column, and a concentrated proppant slurry column positioned below the perforation plugging slurry column. The perforation plugging and proppant slurry columns are sealed at their upper and lower ends by specially designed gel plug seals each comprising a short column of strong gel solution in which a spaced pair of casing size rock salt balls are disposed to provide the seal with structural reinforcement.

Abstract: Wellbores are cemented with a cement composition formed from drilling fluid occupying the wellbore by circulating a quantity of drilling fluid mixed with a cement material and a dispersant such as a sulfonated styrene copolymer with or without an organic acid into the wellbore to displace the drilling fluid and to continuously convert the drilling fluid into the cement composition. The cement composition may be recirculated through the space in the wellbore to completely evacuate any drilling fluid not converted to cement so as to minimize any faults in the cement composition occupying the wellbore. A preflush composition containing the dispersant and being rheologoically compatible with both the drilling fluid and the drilling fluid converted to cement slurry may be used as a displacement fluid.

Abstract: A stage valve for stage cementing of casings in a well bore including a tubular collar and a telescopic tubular sleeve where the sleeve has a piston portion disposed in a collar chamber and the piston portion separates a sleeve port from a collar port in a lower position. In the lower position, the sleeve mechanically engages the collar and is movable to an upper position by a differential hydraulic pressure across the piston portion to an upper position placing the sleeve port and the collar port in fluid communication with one another. In moving to the upper position, the sleeve removes a barrier from a ratchet ring in the collar so that when the sleeve is moved downwardly from an upper position, a ratchet portion on the sleeve engages the ratchet ring in a lower position of the sleeve to lock the sleeve in a closed position.

Abstract: Thread leaks, small holes, and leaks around packers in well casings and production tubing are repaired by applying a compressed foam mixture which includes discrete solid particles of various sizes for forcing the suspended particles into the opening to provide a high friction seal. The foam mixture is moved along the inside of the conduit sandwiched between fluid bodies for maintaining the foam mixture intact. Back pressure is applied to the foam mixture for aiding the mixture to be forced through the openings.

Abstract: The present invention relates to a method and apparatus for cementing a liner in a well bore having a casing wherein the liner is connected to a running tool and includes hanger means for securing the liner to the casing in the well bore. The liner includes an upper and lower liner portion each having port means therein and means to lock the telescoping liner portions against relative longitudinal movement to prevent premature actuation or operation of the port means.The liner portions provide a mechanically actuated port arrangement to be employed with first and second liner wiper means and first and second pump down plug means for cementing above a packer on the lower liner portion, or for providing dual stage cementing.

Abstract: A manifold arrangement for use with a top drive power unit to enable fluid circulation to be conducted to a well string for well bore conditioning to be effected with cementing plugs in place in the manifold arrangement and for conducting a pumpable substance such as cement to the well string without first circulating it through the top drive power unit.

Abstract: An aqueous fluid for treating underground, carbon dioxide-containing formations penetrated by casing regions of a wellbore, prior to the cementing of casings into the wellbore, comprises: (i) water, (ii) an agent which substantially increases (preferably by at least about 50 percent) the solubility of lime in water, and (iii) lime in an amount at least about saturating the water in the fluid. The lime in the treatment fluid reacts with carbon dioxide in penetrated formations to produce insoluble calcium carbonate, which blocks pores in the formations. Insoluble calcium hydroxide, formed by reaction of the lime with the water in the treatment fluid, deposits as an insoluble filter cake on the walls of the borehole where the fluid penetrates formations. The calcium hydroxide filter cake may react with the carbon dioxide to form additional calcium carbonate which helps the filter cake form a tight, impermeable barrier at the borehole.

Abstract: An apparatus and method are shown for cementing a liner in a well bore. An operating string of smaller diameter is inserted within the internal diameter of the liner. A setting tool initially connects the operating string within the liner and includes a plug carrying body at the lowermost extent. A top plug is releasably supported from the plug carrying body and a longitudinally spaced bottom plug is releasably supported from the top plug. A collet latch secures the top plug to the plug carrying body and a collet latch secures the bottom plug to the top plug. A shiftable sleeve positively locks the top plug to the plug carrying body until the collet latch securing the bottom plug is released.

Abstract: The invention concerns a new process for plugging and/or consolidating a permeable geological formation, notably when traversed by a borehole.The process, according to this invention, comprises the following steps: (a) insertion into the formation of an overdelayed gelling compound, composed of a gelable material and a hardener, for which the setting time is considerably longer than the suitable positioning time for the composition, in such a way that this permeates the formation over a significant distance. (b) insertion, after a mechanical or fluid plug, of a rapid-action gelling agent, which causes the instantaneous gelling of that part of the overdelayed gelling compound in the immediate neighborhood of the interface between the zone to be treated and the borehole. Thus preventing displacement of the overdelayed gelling compound toward the interior of the formation. The resultant gelling being both resistant and correctly placed.The invention application is to oil, gas, water or geothermal drillings.

Abstract: The plug set of the present invention comprises an upper plug having a non-rotation insert retained therein and a lower plug having longitudinal splines therein and a non-rotation insert on the bottom thereof.

Abstract: A method and apparatus for indicating the position of a cement wiper plug in a wellbore during liner or casing cementing operations. The arrival of the cement wiper plug at a shearable, temporary restriction means in a pipe string is sensed by an increase in pipe string pressure at the surface. Using the knowledge of the location of this shearable, temporary restriction means, a determination of whether or not it is desirable to displace the cement wiper plug to a landing assembly at the bottom of the pipe string can be made. A predetermined amount of displacing fluid, sufficient to either completely or partially displace the cement wiper plug to the landing assembly, can then be introduced into the pipe string.

Abstract: A sub-surface release plug assembly for use in cementing an outer casing annulus around a well casing. The assembly includes a top plug releasably connected to a drill string by a collet mechanism and including a swivel connector and a vent thereabove. A sleeve assembly extends through the top plug and a bottom plug is releasably attached thereto. A vent is provided in the sleeve assembly between the top and bottom plugs, and this vent may be closed prior to releasing the bottom plug. The construction of the inner sleeve assembly provides that no high pressure from the drill string is applied to inner portions of either the top or bottom plugs. Thus, the plugs may be made of relatively soft, easily drillable material, even including plastic. When the lower plug is released and pumped to the bottom of the well casing, additional pressure opens a valve so that a flow passageway is provided therethrough.

Abstract: A composition for use in agglomerating and removing solid contaminants in well fluids is disclosed. The composition comprises a carrier and the amide/glyceride reaction product obtained by heating a secondary amine in the presence of a fat or oil. The composition is used in processes to agglomerate the solids in contaminated well fluid and in which the agent is diluted and used as a preflush prior to replacing drilling mud with fresh well fluid.

Abstract: A wiper for a wellbore plug which is rigid for breaking off breakaway members and wellbore plug having a first wiper flexible enough to bypass breakaway members and a second wiper rigid enough for breaking off the bypassed members.

Abstract: Novel cementitious and non-cementitious ferrofluids are disclosed which have properties making them suitable for use in cementing wells. The cementitious fluids comprise: (a) a hydraulic cement, (b) finely divided magnetic particles, (c) a bonding agent or a surfactant, and (d) a liquid medium. The binding agent or surfactant is present in the compositions in an amount sufficient to render the compositions stable in a magnetic field. When used in cementing wells, the cementitious ferrofluids are implaced in the annulus separating a casing from the formation in a wellbore by conventional techniques and the slurry is subsequently subjected to an activating magnetic field that causes movement of the magnetic particles in the slurry. The non-cementitious ferrofluids comprise: (a) finely divided magnetic particles, (b) stabilizing agent, and (c) a liquid medium.

Abstract: Aqueous chemical washes suitable for use in conjunction with well cementing operations are disclosed which comprise water, an anionic or a non-ionic surfactant (or a combination thereof) and a fluid loss control agent of the type described in U.S. Pat. No. 3,891,566.

Abstract: A plug catcher assembly comprising an elongated adapter sub having threads on its upper end for connecting to the pipe string, a reverse circulation or jet sub threadedly connected to the adapter sub, and a catcher sub threadedly connected to the jet sub for catching plugs.The catcher sub and the jet sub have a plurality of upwardly and outwardly directed apertures in its wall which are arranged to promote turbulent flow and slurry mixing and to initially direct such slurry flow in the upward direction. The apertures are vertically spaced in the catcher sub so as to prevent pressure locking and to assure that the sub stays free of cement slurry.The catcher sub can contain and catch as many as eight plugs to save rig time by eliminating tripping of the string between plugging operations.Additionally, a plug launching apparatus is disclosed for use in launching a plurality of plugs during a cementing operation.

Abstract: A method for cementing a casing into a borehole. The borehole is drilled to the desired depth in the usual manner. Before removing the drill string and without substantially interrupting the flow of fluids in the borehole, the mud therein is displaced with a fluid substantially free of undissolved solids from the portion of the borehole into which it is desired to cement the casing. When the solids-free fluid is in place, the drill string is removed, the casing suspended in the borehole, and a cement slurry placed in the annulus where it is allowed to set. Preferably, the solids-free fluid is more viscous and dense than the mud, and less viscous and dense than the cement slurry. Optionally, mutually compatible spacer fluids or cementing plugs are used to separate the mud from the solids-free fluid and the solids-free fluid from the cement slurry.

Abstract: A cementing system using a tubular member 43 coupled to a string of tubing 14 where upper and lower tubular liner wiper plugs (20, 21) are independently attached to and separately releasable from the tubular member. The upper tubular liner wiper plug 20 has outer liner wiper seal members, an outer latching mechanism (95) and an inner latching mechanism (52). A fluid bypass (59) is located in the liner wiper plug (20) below the wiper seal members with a check valve structure for bypassing fluid from below the seal members to the bore (45) of the tubular member in response to a pressure differential. A second check valve structure (46) is provided in the tubular member to provide a fluid bypass from the exterior of the tubular member above the seal members to the interior of the tubular member in response to a pressure differential.

Abstract: In drilling a well, sections of casing are run down a borehole, preferably with a float shoe at the lower end which is equipped with a double valve enabling the casing to fill with drilling mud both while the casing is moving down and also while it is stationary. Within the casing is a baffle collar which defines a socket for a latching dart carried by a plug. The plug and dart are driven down to the collar, when the pumping of cement into the casing has been completed, by a launching dart which also closes the passageway through the plug.

Abstract: A composition designed for dissolution or dispersion in water to give an aqueous treatment fluid having a yield point of zero, at a temperature between about 85.degree. and about 160.degree. F., which is compatible with cement slurries and with drilling muds and a method of using same to displace a drilling mud from a borehole which penetrates a subterranean formation, particularly immediately preceding the injection of a fluid such as a cement slurry, and which also exhibits good fluid loss control and may be adapted for use with either water-based or oil-based drilling muds.

Abstract: A squeeze cementing method wherein the well tubing does not have to be pulled from the well so no workover rig is needed; a smaller volume of squeeze cement is required; the pressures involved in the squeeze operation can be precisely controlled with no risk of fracturing the formation; and there is no need for any drilling operations to remove hardened cement remaining in the well casing. More specifically, the present invention provides a squeeze cement operation that is carried out with a coiled tubing. The wellbore is isolated adjacent the perforations by flowing a packing fluid having a density greater than the cement through the coiled tubing to fill the lower part of the wellbore. An incompressible loading fluid is then flowed onto the packing liquid and this is followed by pumping the squeeze cement down the coiled tubing. After the cement is placed, a contaminating liquid is flowed down the coiled tubing to mix with the cement left in the casing and prevent it from setting.

Abstract: A well drilled with air or gas as a circulating medium traversing pay zones with greater than normal pressures or pressures such that hydrocarbons would permeate drilling fluids and cement is drilled and completed while permitting a continuous flow of hydrocarbons from the well until it is desired to shut the well in at the surface. The intermediate casing string and the production casing string cooperate with the tubing so that the cement for the production string may be positioned without contact with the hydrocarbons.

Abstract: A method for the cementing in of a well so that the cement is not contacted by deleterious materials, such as gas, is disclosed.

Abstract: The present invention relates to an aqueous composition and method for substantially preventing contact between non-compatible hydrocarbonaceous and aqueous fluids employing an aqueous spacer composition. The composition comprises an aqueous fluid, an admixture of nonylphenols and, preferably, at least one member selected from the group of a sulfonated linear C.sub.8 to C.sub.18 straight chain alcohol ethoxylated with an average of from about 2 to about 20 moles of ethylene oxide, a low molecular weight aliphatic glycol ether containing from about 3 to about 12 carbon atoms and an alcohol containing from about 3 to about 8 carbon atoms ethoxylated with an average of 2 to 4 moles of ethylene oxide.

Abstract: Improved bonding in a well system between cement, well tubular members and the formation about the well/bore at a pay zone. Drilling fluid materials (mud, solids, filter cake, etc.) are displaced at turbulent flow conditions by a slug of concentrate consisting essentially of a water-free mixture of surfactant and alcohol. The aqueous fluid cement immediately follows the slug. Residual filter cake and tubular member are water-wetted. The cement on hardening has an improved bonding to both tubular members and formation at the pay zone.

Abstract: A method for determining true fracture pressure of earth formations disposed below a liner or casing cemented in place comprising the location of an elastomer sealing element cemented in place against the borehole wall at the bottom of the liner or casing and just above the casing shoe so as to prevent annular migration of liquids in the seal interface with the earth formations and to permit true fracture pressure of the formations to be determined.

Abstract: The present invention is a method for improving well cementing. A fluid such as drilling mud or preflush fluid is oscillated in the annulus prior to introduction of cement slurry into the annular interval to be cemented. The fluid is oscillated by changing its direction of flow at least twice. The oscillatory flow of the fluid flushes gelled drilling mud and filter cake from the annulus more effectively than conventional unidirectional flow. Following the oscillatory phase, cement slurry is pumped into the annular interval to be cemented, thus displacing the fluid, and is allowed to set. The fluid can be oscillated in the annulus either before or after the cement slurry is introduced into the casing. A selective check valve is also described which permits all fluids to flow in the forward direction out of the casing and into the annulus and which permits all fluids other than cement slurry to flow in the reverse direction out of the annulus and into the casing.