1. Field of the Invention
The present invention relates to cementing operations, and more particularly, to cement compositions comprising an improved fluid loss control additive, and methods of using such compositions in surface and subterranean applications.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the 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 that substantially supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
In order for such well cementing operations to be successful, the cement compositions utilized must include a fluid loss control additive to reduce the loss of fluid, e.g., water, from the cement compositions when they contact permeable subterranean formations and zones. Excessive fluid loss, inter alia, causes a cement composition to be prematurely dehydrated, which limits the amount of cement composition that can be pumped, decreases the compressive strength of the cement composition, and prevents or reduces bond strength between the set cement composition and the subterranean zone, the walls of pipe, and/or the walls of the well bore. Fluid loss control agents may also be used in surface cement compositions.
Conventional contemporary synthetic fluid loss control additives are large, water-soluble polymers that are capable of functioning at a wider range of temperatures. An example of such synthetic fluid loss control additive is a fluid loss additive consisting of hydrolyzed copolymers of acrylamide (“AA”) and 2-acrylamido, 2-methyl propane sulfonic acid (“AMPS”). However, certain of these AA/AMPS copolymers are useful only in operations where the bottom hole circulating temperature (“BHCT”) ranges from about 90° F. to about 125° F., whereas BHCT ranges encountered in such operations are often outside such a range. Still further, certain of these copolymers have a salt tolerance of only up to about 10%.
The temperature limitations of certain of the AA/AMPS copolymers, e.g., ineffectiveness at temperatures above about 125° F. BHCT, are believed to be the result of hydrolysis of the amide groups. The carboxylate groups formed by such hydrolysis convert the copolymers to materials which function to retard the setting of the cement and to reduce the compressive strength of the set cement. Further, in the lower portion of the above-mentioned temperature range (between about 90° F. to about 100° F.) certain of the AA/AMPS copolymers are less effective as a fluid loss additive, requiring inclusion of larger amounts of such additive than at higher temperatures. The inclusion of a sufficiently large amount of a fluid loss control additive, to create a cement composition with acceptable fluid loss often creates viscosity and pumpability problems, since the addition of such copolymer directly affects the resultant slurry rheology. Certain copolymers of acrylamide and AMPS exhibit high viscosity and poor mixability, resulting in cement slurries having poor pumpability characteristics during cementing operations. Mixability is a subjective term used to describe how well the components in the cement composition wet and mix with each other, as well as the energy required to create a generally homogeneous slurry.