Wells – Processes – Cementing – plugging or consolidating
Reexamination Certificate
2000-03-17
2001-08-28
Bagnell, David (Department: 3672)
Wells
Processes
Cementing, plugging or consolidating
C166S300000, C106S694000, C106S718000
Reexamination Certificate
active
06279655
ABSTRACT:
FIELD OF INVENTION
This invention relates to thixotropic materials, particularly thixotropic settable materials such as thixotropic cements.
BACKGROUND TO THE INVENTION
Thixotropic materials have the property of being fluid under shear but developing a gel structure and becoming self-supporting when at rest. The process is reversible. Thixotropic cements, which are thin and fluid during mixing and placement but reversibly form a rigid gel structure when pumping ceases, are useful in various oilwell applications. For example, thixotropic cement systems are used to plug lost circulation zones during both drilling and cementing operations, to repair damaged or corroded casing, as grouts, and to limit annular gas migration in some situations. Such systems have also been used in situations where weak formations are exposed and would otherwise fracture under the hydrostatic pressure of a cement column: with a thixotropic cement the hydrostatic pressure of the column diminishes as the cement gels.
Various thixotropic cement systems are known in the prior art, including the following:
1. Clay-based systems. These typically comprise Portland cement and bentonite clay.
2. Calcium sulphate-based systems. The material most widely used for this purpose is calcium sulphate hemihydrate. See, for example, U.S. Pat. No. 3,847,635.
3. Aluminium sulphate/iron (II) sulphate systems. See, for example, U.S. Pat. No. 4,415,367.
4. Crosslinked cellulose polymer systems. See, for example, U.S. Pat. Nos. 3,959,003, 3,804,174 and 4,524,828.
5. Mixed metal hydroxide systems. See, for example, U.S. Pat. No. 4,822,421.
The known thixotropic cement systems have various limitations, one of which is that a significant time is taken for a gel structure to develop on removal of shear: at best this is of the order of several minutes and can be substantially longer, possibly approaching the timescale over which the cement sets. This can present problems in certain situations.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a settable thixotropic material, comprising a thixotrope and a settable substance, the material being capable of rapidly and reversibly gelling.
The term “rapid” is used in this context to mean the material gels in a gelling time of less than 60 seconds, preferably less than 30 seconds, more preferably less than 10 seconds.
A gel shear yield stress of at least 100 Pascal (Pa), typically 150 to 300 Pa, and possibly up to 500 Pa or more is desirably developed in the gelling time.
The material preferably reaches substantially its maximum gel strength (i.e. at least about 90% of the maximum value) within the gelling time, and maintains this, value (i.e. staying within about 20% of this value) for an extended period of time (i.e. at least 2 hours) until setting has started.
The material desirably has a low viscosity so as to be readily pumpable, conveniently having a viscosity of less than 30 Bearden units as measured in a standard oilfield consistometer.
Preferably the material is mixable in standard oilfield cement mixing equipment.
The time taken for the material to set (the setting time) is substantially longer than the gelling time, typically at least 2 hours and possibly up to 8 hours or more. Thus, if material flow stops, even for an extended time, it will still be possible to resume pumping. The gel is preferably reversible until setting has occurred.
The gelling time and the setting time of the material are preferably separately controllable so that it is possible to produce a material having desired combinations of gelling and setting times, e.g. a fast gelling/slow setting material, a fast gelling/fast setting material etc. The setting time is typically controlled by use of retarders in a manner well known to those skilled in the art.
The properties of the material when set, including strength, porosity, interfacial bonding to rock and steel/plastics, can be tailored to suit the intended use of the material.
The material preferably has the characteristics and performance specified above under down-hole conditions. These typically include temperatures in the range 50 to 150° C. and possibly higher, and pressures of up to 1000 bar and possibly higher. The material should also be able to cope with environmental factors such as the variable and sometimes high salinity and hardness of wellbore fluids, and the presence of hydrocarbons and particulate matter.
The thixotrope may be selected from a number of known thixotropic substances, particularly strongly interacting particulate and molecular species. In the first case, the thixotrope conveniently comprises a fine grained (having a mean particle size with maximum dimension of less than 1 micron) inorganic colloid, particularly fine grained clays, especially smectite clays, e.g. hectorites. It is preferred to use synthetic smectite type clay colloids, and good results have been obtained with the synthetic clay known as Laponite RDS from Laporte (Laponite is a Trade Mark of Laporte Industries Limited). Laponite is a synthetic trioctahedral smectite similar to the natural clay hectorite. Laponite RDS is a layered hydrous sodium lithium magnesium silicate modified with tetra sodium pyrophosphate. It is in the form of a free-flowing powder which is easily dispersed in water. At concentrations below approximately 10% by weight in water, it forms a stable sol. The individual clay platelets in the sol are about 250 Å in diameter and about 10 Å thick with a negative face charge and a positive edge charge.
Other strongly interacting colloids such as latexes and other materials, e.g. as used in the paint industry and the pharmaceutical industry, may also be useful for this purpose. Similarly, mixed metal hydroxides may be useful. Associative polymers and self-assembling surfactant systems may also act as suitable thixotropes.
Appropriate mixtures of thixotropes may be used.
The settable material may be selected from a range of known settable materials, including the following:
1. Cementitious materials, e.g. cements, particularly Portland cements, blast furnace slag, fly ash/lime mixes and mixtures of these materials.
2. Other ceramic-forming materials.
3. Polymeric materials, e.g. thermosetting polymers etc.
Appropriate mixtures of settable materials may be used.
The thixotropes and settable materials should be selected to be compatible with each other, in known manner.
The material may include other compatible ingredients, such as additives conventionally used in oilfield-cements. Where the settable material is a cement, a cement retarder will generally be included. Suitable retarders are known to those skilled in the art and include, for example, the sodium or calcium salts of lignosulphonic acids. Further, a surfactant may be included to act as a dispersant and/or cement retarder. Anti-foaming agents may also be included. Depending on the intended use of the material, other materials may be included as fillers. Other conventional additives may also be included provided they do not interfere with the gel forming properties of the material.
Good results have been obtained with mixtures of Laponite RDS and Portland cement, particularly of classes A and G. These cements are predominantly calcium oxide and silicon dioxide with minor amounts of iron oxide, aluminium oxide, sulphur trioxide and other trace level compounds. The chemical compounds included in the set anhydrous cements include tricalcium aluminate, dicalcium silicate and tetracalcium alumino ferrite. Typical compositions comprise 3 to 6% Laponite RDS by weight of water, with a water/cement ratio of about 50%.
In order to produce thixotropic slurries, it is necessary to prehydrate the Laponite RDS with water, thus forming a sol. Once cement powder or other fine solid is added to this fluid, or when the electrolyte concentration is increased to within an appropriate range, a thixotropic gel is rapidly formed. The fluid may be easily pumped but when shearing ceases the cement rapidly gels (within a matter of a few seconds) and becomes immob
Davies Stephen Nigel
Maitland Geoffrey Colin
Pafitis Demosthenis Georgeou
Bagnell David
Mitchell Thomas O.
Nava Robin C.
Schlather Stephen
Schlumberger Technology Corporation
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