1. Technical Field of the Invention
This invention broadly relates to cementing. The invention further relates to a cement composition for supporting pipe in a borehole which penetrates one or more subsurface earth formations. The invention still further relates to an additive included in a hydraulic cementing composition. The invention more specifically relates to a copolymer additive useful to reduce the loss of water from a slurry of hydraulic cement in water.
2. Description of the Prior Art and Problems Solved
It is known in the art of well cementing to form a sheath of hardened cement in the annular space between a well pipe, such as a casing, and the walls of a well bore which penetrates a subterranean earth formation. The purpose of the sheath is to support the casing in the well bore and to prevent undesirable movement of formation fluids, such as oil, gas and water, within the annular space between subsurface formations and/or to the surface of the earth. The process of forming the sheath is referred to in the art as primary cementing.
In the art of primary cementing, a slurry of hydraulic cement in water is pumped down the interior of the casing and caused to circulate up from the bottom of the casing in the annulus to a desired location therein, and then permitted to remain undisturbed in the annulus for a time sufficient to enable the hydraulic cement to react with the water in the slurry, i.e., set, to produce the sheath of hardened cement.
A slurry of hydraulic cement in water, when first placed in the annulus, and for a period of time thereafter, acts as a true liquid and can transmit hydrostatic pressure. Loss of water from the slurry to the formation, referred to as fluid loss, causes a reduction in slurry volume which can cause pressure loss. Gas migration within the setting slurry can occur if pressure loss occurs at a time when the slurry has gelled to degree which prevents full transmission of hydrostatic pressure.
A slurry of hydraulic cement, over a period of time, sets into a hardened mass having compressive strength. It is believed that the hardening process experiences three phases.
During the first phase of the hardening process, it is believed that the setting slurry retains liquid sufficient to enable it to transmit full hydrostatic pressure in the well bore through the column of cement slurry. It is believed that gas migration will not occur if there is sufficient transmitted pressure to oppose formation gas pressure. The first phase ends when the developed static gel strength attains a first critical value which is believed to be about 100 lb-force/100 sq. ft. The period of time required for a slurry of hydraulic cement to reach the first critical value is referred to as zero gel time. Zero gel time is thus defined as the time required for a slurry to develop a static gel strength of about 100 lb-force/100 sq. ft. During this time, it is believed that the volume of fluid lost to the formation will not result in loss of pressure across a gas zone in an amount sufficient to permit gas migration.
During the second phase of the hardening process, the developed static gel strength exceeds the first critical value. The setting slurry loses the ability to transmit full hydrostatic pressure, but fluid loss continues. As a setting slurry passes from a fluid state to a gelled state, defined as the transition period, hydrostatic pressure cannot be fully transmitted. Accordingly, any loss of fluid volume during the transition period will cause loss of pressure across a gas zone, which could result in gas migration. The second phase ends when the developed gel strength attains a second critical value which is sufficient to resist formation gas pressure. It is believed that the second critical value is about 500 lb-force/100 sq. ft. The purpose of a fluid loss additive is to provide fluid loss control during the transition period. It is desired that the transition time be as short as possible.
During the third phase of the hardening process gas migration is prevented if a gas channel has not been previously formed, because developed gel strength is greater than the second critical value and is sufficient to resist formation gas pressure.
It is desirable to extend zero gel time and to reduce transition time. The loss of fluid from a slurry of hydraulic cement in water increases with increase in bottom hole circulating temperature. Accordingly, a material to be added to a slurry of hydraulic cement to extend zero gel time and to reduce transition time, while reducing fluid loss rate at high temperatures, is a problem addressed herein.
Laramay et al (U.S. Pat. No. 6,089,318) discuss problems caused by fluid loss. Such problems include gas migration, the development of inadequate static gel strength, and the formation of channels in the sheath of cement. The discussion is included herein by reference.
Laramay et al disclose a composition which reduces fluid loss from a slurry of hydraulic cement in water at temperatures up to 400° F. and particularly above 200° F. The composition disclosed by Laramay et al is the copolymerization reaction product of a vinylamide morpholine derivative with a styrene sulfonic acid salt, when performed in the presence of a humate. The preferred vinylamide morpholine derivative is acryloylmorpholine. The preferred styrene sulfonic acid is sodium styrene sulfonate. The humate employed is potassium humate.
Laramay et al further disclose that acrylamide and derivatives of acrylamide can be employed along with the previously mentioned components to produce the composition of their invention. Examples of such optional compounds mentioned by Laramay et al include acrylamide, the sodium salt of 2-acrylamido-2-methylpropanesulfonic acid and N,N-dimethylacrylamide.
It is known in the art that a slurry of hydraulic cement is comprised of particles of a hydraulic cement suspended or dispersed in water. The slurry can also include various added materials, for example, fluid loss additives, dispersants, suspension agents, strength stabilizing agents, set time retarders, set time retarder intensifiers and defoamers.
A slurry of hydraulic cement in water can be caused to flow, such as with pumping equipment, down a well pipe and then upwardly into an annular space where it is permitted to remain static while it sets into a hardened sheath. The study of the ability of a slurry of hydraulic cement in water to flow is included in a term broadly referred to as rheology, which, in large part, is concerned with the flow of matter in the liquid phase. Such liquids can be classed as either Newtonian fluids or as non-Newtonian fluids. A non-Newtonian fluid exhibits an apparent change in viscosity with rate of change of strain (strain rate). A slurry of hydraulic cement in water is a non-Newtonian fluid.
A rheological property of a slurry of hydraulic cement in water which is of particular interest to persons skilled in the art of well cementing is the ability of the slurry to suspend the particles of cement in the fluid. The rheological properties must be adequate to suspend solids at surface conditions during mixing, and also at down hole temperature and pressure. The goal is to prevent solids settling and bridging while pumping. Instruments employed to measure this settling property provide numerical readings at various shear rates (measured in rpm). As a general rule, higher numerical readings indicate satisfactory suspension and lower readings indicate less satisfactory to unacceptable suspension. A combination of materials added to a slurry can be necessary to provide a slurry having low water loss, as well as satisfactory solids suspension and acceptable pumping time.
Consistency is a rheological property of a fluid which is related to cohesion of individual particles of a material in the fluid, such as the particles of cement in the slurry, the ability of the fluid to deform and its resistance to flow. The consistency of a cement slurry is determined by thickening time tests and is a measure of the ability of the slurry to be pumped. Consistency is, accordingly, a measure of the pumpability of the slurry.
It is clear that the slurry must contain a sufficient quantity of water to enable sufficient hydration of the cement particles. In addition, there must be a sufficient quantity of water in the slurry to enable it to be pumped, but not so much that the consistency (sometimes known as apparent viscosity) of the slurry is not sufficient to retain the particles of cement in suspension during the hardening process.
Rheology, including consistency, is effected by fluid loss. Fluid loss can be adjusted by materials added to a cementing composition. Such materials include fluid loss additives, suspending agents and dispersants. This invention, accordingly, further addresses cementing compositions comprised of fluid loss additives, dispersants and suspending agents.