Reverse circulation cementing system and method

A system and method of cementing in an annulus formed between a casing and the wall of a well bore, wherein a foamed cement is introduced into the upper portion of the annulus, directed downwardly through the annulus and back up into the casing, and then displaced back down the casing and into the annulus.

BACKGROUND

In oil field recovery operations, a casing, in the form of a steel pipe, or the like, is often placed in an oil and gas well to stabilize the well bore. In these installations, a cement sheath is formed in the annulus between the casing and the wall of the well bore to support the casing, to prevent migration of fluids in the annulus, and to protect the casing from corrosive formation fluids.

In accordance with conventional cementing operations, the sheath is formed by introducing a cement slurry into the upper end portion of the casing at the ground surface and allowing the cement to flow through the casing to the bottom of the well and reverse direction into the annulus. The cement then flows into and through the annulus between the casing and the wall forming the well, and circulates back to the ground surface. The flow of cement is then terminated and the cement allowed to set to form the sheath.

Numerous challenges can be present in these types of cementing operations. For example, it is often difficult to obtain the proper circulation of cement inside the annulus due to a weak formation around the well. Also, the hydrostatic weight of the cement exerts significant pressure against the formation, especially when additional pressure is applied to the formation due to the friction of the cement slurry that must be overcome.

One technique utilized to overcome these deficiencies and reduce the formation pressure employs reverse circulation in which the cement slurry is pumped down the annulus and back up the casing. While this greatly reduces the total pressure applied to the formation, it has several drawbacks. For example, it is impossible for the operator to determine exactly when the cement completely fills the annulus without the use of some type of tool which is expensive and time consuming. Thus, the operator runs the risk of either not completely filling the annulus with the cement or of filling the cement back up inside the casing string, thus covering potential productive areas and/or requiring additional time and expense to drill out this cement.

Another challenge to reverse circulation is that the heavier cement tends to flow inside the casing due to “U-Tubing.” Since typical float equipment used to prevent this cannot be used in reverse circulations, pressure must be held on the annulus until the cement has sufficiently set to prevent the U-Tubing. This can cause a micro-annulus to form between the cement sheath and casing. A micro-annulus can make it difficult to bond log the casing to evaluate the quality of the cementing operation and determine if the annulus is properly sealed. A micro-annulus can also allow unwanted flow of gas, brine, etc., behind the casing.

Still further, since the cement will not vary much in density throughout the height of the well bore, the benefit of reverse circulating a conventional cement is minimal since the total hydrostatic pressure of the cement column acts on the formation at the end of the operation.

Therefore, what is needed is a system and method that eliminates the problems with conventional circulation, yet avoids the problems associated with reverse circulation.

DETAILED DESCRIPTION

Referring toFIG. 1, the reference numeral10refers to an underground, substantially vertically-extending, well bore. A casing12extends from the ground surface (not shown) into the well bore10and terminates at a predetermined depth in the well bore10. The outer wall of the casing12is slightly spaced from the inner wall of the well bore10to form an annulus14.

In order to prevent migration of fluids in the annulus14, to support the casing12or liner string, and to protect of the casing12from corrosive formation fluids, a cement sheath is formed in the annulus14. To form a sheath in accordance with most conventional, prior-art methods, a fluid cement16is introduced from a source at the ground surface into the upper end of the casing12and flows downwardly through the bottom end of the casing12as shown in FIG.1. The fluid cement16then flows to the bottom of the well bore10, or to a plug in the annulus14below the lower end of the casing12, where it reverses direction and flows up the annulus14. The flow of the fluid cement16is terminated and the fluid cement16is allowed to set, thus forming a sheath.

As indicated above, according to this conventional cementing technique, it is often difficult to obtain the proper circulation of the fluid cement16inside the annulus14. Also, the hydrostatic weight of the fluid cement16exerts a significant pressure against the formation surrounding the well bore10, especially when additional pressure is applied to the formation due to the friction of the cement16that must be overcome.

The system and method according to an embodiment of the present invention is illustrated inFIG. 2which also operates in connection with the well bore10, the casing12, and the annulus14of FIG.1. In particular, a foamed cement18, which contains a compressible gas phase, is initially formed in any conventional manner and is introduced from a source at the ground surface into the upper end of the annulus14. The foamed cement18thus flows downwardly through the bottom end of the annulus14, and to the bottom of the well bore10, or to a plug in the annulus14below the lower end of the casing12. The foamed cement18thus reverses direction before entering the lower end of the casing12and flowing into the casing12and preferably back to the ground surface.

A preferred foamed cement18for use in the present invention comprises a hydraulic cement, sufficient water to form a slurry, sufficient gas to foam the slurry, and foaming additives present in an amount sufficient to facilitate the formation of the foamed cement18. A variety of hydraulic cements can be utilized in accordance with the present invention including those comprised of calcium, aluminum, silicon, oxygen, and/or sulfur which set and harden by reaction with water. Such hydraulic cements include Portland cements, slag cements, pozzolana cements, gypsum cements, aluminous cements and silica cements. Portland cements or their equivalents are generally preferred for use in accordance with the present invention. Portland cements of the types defined and described in the API Specification For Materials And Testing For Well Cements, API Specification 10, 5th Edition, dated Jul. 1, 1990 of the American Petroleum Institute are particularly suitable. Preferred such API Portland cements include classes A, B, C, G, and H, with API classes G and H being preferred. Suitable foaming additives are described in U.S. Pat. Nos. 5,133,409; 5,147,565; 5,897,699; 6,063,738 and 6,367,550, the entire disclosures of which are incorporated herein by reference.

As shown inFIG. 3, a wiper plug20is then introduced into the casing12, and is attached to a wire line, piping, or coiled tubing22. The plug20can be of any known type, such as that disclosed in U.S. Pat. No. 6,196,311, which is incorporated herein by reference, assigned to the assignee of the present invention, and, includes two axially spaced wiper blades24aand24bextending from its outer surface that engage the inner wall of the casing12.

The plug20can be forced downwardly in the casing12in any conventional manner, such as by the use of a displacement fluid, or the like, acting on its upper end. As the plug20moves down in the casing12, it compresses the foamed cement18in the casing12and forces it back down the casing12, while the wiper blades24aand24bwipe the inner wall of the casing12. The plug20is shown in an intermediate position inFIG. 3for the purposes of illustration, it being understood that it is displaced to the bottom end of the casing12to compress and force all of the foamed cement18out of the casing12and into the annulus14. In this context, it is understood that a latching device, or the like (not shown), can be provided in the lower end portion of the casing12to latch on to the plug20and release the pressure on the casing12after the foamed cement18is compressed and displaced. Three and five wiper casing latchdown plugs are available from Halliburton Energy Services in Duncan, Okla. These latchdown plugs have a top portion with either three or five wipers and a lower portion with a latch-in nose to latch into a baffle installed in the casing12. The latchdown plug, when landed and latched in its seat, can prevent backflow of the cement18into the casing12.

The compression of the foamed cement18and the forcing of it back down the casing12and into the annulus14by the plug20in the above manner, results in several advantages as follows:The density of the foamed cement18is relatively low as it is circulated in the above manner resulting in a relatively low pressure on the formation and relatively little increase, or a possible reduction, in the pressure in the annulus14throughout the entire operation. This reduces the risk of fracturing weak formations and losing foamed cement18into the formation.The foamed cement18is of relatively good quality, and any accumulation of the foamed cement18above the desired depth inside the casing12is eliminated. As a result, no additional rig time is needed to drill out unwanted foamed cement18inside the casing12.The need for logging tools or special techniques used to determine when foamed cement18has reached the bottom of the well bore10is eliminated.The pressure on the casing12can be released after the foamed cement18is displaced which reduces the chance of a detrimental micro-annulus forming and insures that foamed cement18does not flow back inside the casing12.The flow of the foamed cement18out of the well bore10can be stopped once uncontaminated foamed cement18flows back at the surface thus eliminating, or at least minimizing, the amount of waste foamed cement18that must be circulated out of the well into a pit or other container.

It is understood that variations may be made in the foregoing without departing from the scope of the invention. Examples of these variations are as follows:A non-foamed cement can be initially introduced into the annulus14, followed by the introduction of the foamed cement18, if higher strength cement is desired and/or if foamed fluids cannot be handled or are not desired at the surface. In this case, the foamed cement18in the annulus14can still be compressed to allow the displacement of the cement back down inside the casing12.The type of wiper plug20, and the number of wiper blades, can be varied within the scope of the invention.The term “casing” is meant to cover any type of tubular member, including a conduit, pipe, liner, liner string, etc.Although the well bore in the illustrative example above was shown and described as being substantially vertical, it is understood that it can deviate from vertical within the scope of the invention. In the latter context, expressions such as “down” and “up,” were used for the purpose of illustration only.Still further, the relative sizes of the various components, as well as the annulus14and well bore10, can be varied within the scope of the invention.