Wells – Processes – Cementing – plugging or consolidating
Reexamination Certificate
2000-12-08
2002-06-18
Marcantoni, Paul (Department: 1755)
Wells
Processes
Cementing, plugging or consolidating
C166S294000
Reexamination Certificate
active
06405801
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to subterranean well cementing operations, and more particularly, to environmentally acceptable well cement fluid loss control additives, well cement compositions containing the additives and methods of using the compositions.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein which supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also utilized in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks or holes in pipe strings and the like.
Fluid loss control agents are used in well cement compositions to reduce fluid loss from the cement compositions to the permeable formations or zones into or through which the cement compositions are pumped. In primary cementing, the loss of fluid, i.e., water, from the cement compositions to a permeable subterranean formation or zone can result in premature gelation of the cement composition whereby bridging of the annular space between the permeable formation or zone and the pipe string being cemented prevents the cement composition from being placed over the entire length of the annulus.
Heretofore, environmentally acceptable hydroxyethylcellulose having an average molecular weight of about 90,000 and higher has been utilized as a well cement composition fluid loss control agent. However, the use of such hydroxyethylcellulose as a cement composition fluid loss additive has involved a number of disadvantages. For example, the ability of the hydroxyethylcellulose to prevent fluid loss in a cement composition decreases rapidly at temperatures above about 230° F. In addition, the hydroxyethylcellulose has a limited salt tolerance. That is, if the salt content of the water in the cement composition is above about 5% by weight of the water, the ability of the hydroxyethylcellulose to prevent fluid loss is reduced. In offshore cementing operations, the hydroxyethylcellulose has been utilized in an aqueous solution so that it can be readily added to the mix water or the cement slurry. In order to prevent the aqueous solution of the hydroxyethylcellulose from having too high a viscosity, the hydroxyethylcellulose must be dissolved in relatively large quantities of water making its mixing and handling more difficult than other fluid loss control agents.
Thus, there is a need for an improved environmentally acceptable hydroxyethylcellulose fluid loss control agent which can be utilized at temperatures higher than 230° F., which can be dissolved in relatively small quantities of water without greatly increasing the viscosity of the solution and which has a higher salt tolerance while maintaining the same fluid loss control.
SUMMARY OF THE INVENTION
The present invention provides environmentally acceptable well cement fluid loss control additives, well cement compositions and methods of using the compositions which meet the needs described above and overcome the deficiencies of the prior art.
The environmentally acceptable well cement fluid loss control additives of this invention are basically comprised of hydroxyethylcellulose having a reduced average molecular weight of about 60,000 ethoxylated in an amount in the range of from about 1 to about 4 moles of ethylene oxide per anhydroglucose unit, i.e., an average molecular substitution of ethylene oxide of 2.5 per anhydroglucose unit. The fluid loss additives can also include a temperature stability increasing agent to increase the temperature at which the additives remain stable and continue to prevent fluid loss. The temperature stability increasing agent can be selected from synthetic hectorite, magnesium oxide and sodium thiosulfate. Of these, magnesium oxide is preferred.
The improved well cement compositions of this invention are basically comprised of a hydraulic cement, water present in an amount sufficient to form a pumpable slurry and the environmentally acceptable well cement fluid loss control additive of this invention, i.e., hydroxyethylcellulose having a reduced average molecular weight of about 60,000 ethoxylated in an amount in the range of from about 1 to about 4 moles of ethylene oxide per anhydroglucose unit. The fluid loss control additive can also include a temperature stability increasing agent selected from the group of synthetic hectorite, magnesium oxide and sodium thiosulfate. Synthetic hectorite and sodium thiosulfate can be added in particulate solid form directly to the hydraulic cement or mix water utilized, or it can be dissolved in water whereby a storable aqueous solution results which can be readily combined with the mix water or cement slurry. Magnesium oxide can only be added to the mix water of the cement slurry.
The methods of this invention for cementing a zone in a subterranean formation penetrated by a well bore are basically comprised of the steps of preparing a cement composition of this invention including the above described environmentally acceptable well cement fluid loss control additive, placing the cement composition in the zone to be cemented and allowing the cement composition to set into an impermeable solid mass therein.
It is, therefore, a general object of the present invention to provide improved environmentally acceptable well cement fluid loss control additives, improved well cement compositions including the additives and methods of using the cement compositions.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.
DESCRIPTION OF PREFERRED EMBODIMENTS
As mentioned, the environmentally acceptable well cement fluid loss control additive of this invention is basically comprised of hydroxyethylcellulose having a reduced molecular weight in the range of from about 30,000 to about 90,000, i.e., an average molecular weight of about 60,000, ethoxylated in an amount in the range of from about 1 to about 4 moles of ethylene oxide per anhydroglucose unit, i.e. an average molecular substitution in the range of from about 2.2 to about 2.7 per anhydroglucose unit, most preferably 2.5 per anhydroglucose unit. The average molecular weight of commercially available hydroxyethylcellulose is about 90,000 or higher. While such hydroxyethylcellulose can be and has been utilized in well cement compositions as an environmentally acceptable well cement fluid loss control additive, it suffers from the disadvantages that it is unstable and its fluid loss control properties are significantly reduced at temperatures above about 230° F. and/or when it is in contact with salt water containing salt in an amount above about 5% by weight of the water. In addition, when the commercially available hydroxyethylcellulose is dissolved in water so that it can be added to a cement slurry or its mix water as a liquid, the resulting solution has a high viscosity. As a result, a liquid hydroxyethylcellulose solution containing only about 7.5% by weight commercial hydroxyethylcellulose can be used.
The above is contrasted with the ethoxylated hydroxyethylcellulose of reduced molecular weight of the present invention which is stable and effectively controls fluid loss from cement compositions at temperatures above 230° F. to at least about 260° F., can be utilized in contact with salt water c
Dao Bach
Melbouci Mohand
Vijn Jan Pieter
Doughery, Jr. C. Clark
Halliburton Energy Service,s Inc.
Marcantoni Paul
Roddy Craig W.
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