Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-10-26
2001-01-30
Szekely, Peter A. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S004000, C524S005000, C525S057000, C525S058000, C525S059000, C525S060000, C525S061000
Reexamination Certificate
active
06180689
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention concerns the field of petroleum service and supply industries, in particular that of cementing the annulus surrounding the casing in an oil, gas, water, geothermal or analogous well. More precisely, the invention relates to cement slurry formulations which can prevent the passage of gas during the whole cement setting phase.
BACKGROUND
The main aim of cementing an oil (or analogous) well is to isolate the different formation layers traversed by the well to prevent migration between the different geological layers or between the layers and the surface. In particular, it is essential from the point of view of safety to prevent any gas from rising through the annulus between the well wall and the casing.
When the cement has set, it is impermeable to gas. Because of the hydraulic pressure of the height of the cement column, the injected slurry is also perfectly capable of preventing such migration. However, between the two states, there is a critical phase which lasts several hours during which the cement slurry no longer behaves as a liquid but also does not yet behave as an impermeable solid. For this reason, the industry has developed a series of additives which are intended to maintain a gas tight seal during the whole of the cement setting period.
PRIOR ART
Among such (numerous) additives are those which also tend to reduce fluid loss, i.e., prevent dehydration of the petroleum industry fluid when the latter comes into contact with a naturally porous or fissured formation. In general cement slurries with a fluid loss of less than 40 ml over thirty minutes, measured in accordance with API (American Petroleum Institute) standards, are also impermeable to gas, although the correlation between the two phenomena is not systematic, and other parameters such as the almost complete absence of free water formation must also be considered, especially in a non vertical slanted well since the supernatant free water can create a path for the migration of gas. An additive based on a styrene-butadiene latex is the most frequently used additive for preventing gas migration. Such a product is highly effective but relatively expensive and in particular, requires a stabilizer which is known to be toxic and is thus prohibited in certain drilling zones.
Further, a cement slurry comprises in practice a whole series of additives, almost systematically among them an agent which encourages dispersion of the cement particles. The dispersing agents used vary depending on the type of well bored or, more exactly, depending on the temperature to which the cement is subjected. Argillaceous minerals such as bentonite are also frequently used as they can reduce the density of the cement slurry, an essential point when cementing in zones where the formation pressure is low. The question of the compatibility of each new additive with current additives, over a wide range of working temperatures and pressures, is thus fundamental, it being understood that no additive is genuinely universal.
There is thus a need for additives which can render a cement gas tight during setting of the cement, which additives are effective, non toxic and compatible with the majority of additives in current use.
French patent FR-A-2 704 219 describes a novel type of fluid loss control agent for petroleum industry fluids, in particular for cementing slurries. Such fluid loss control agents are constituted by micro-gels obtained by chemically cross-linking a polyvinyl alcohol (PVA) in the presence of a cross-linking agent which polycondenses hydroxy groups. The most preferred product is obtained by cross-linking, in aqueous solution, a polyvinyl alcohol with a molecular weight which is in the range 30000 to 250000, with glutaraldehyde, the solution of polyvinyl alcohol and glutaraldehyde being acidified with vigorous stirring to obtain a micro-gel with a theoretical degree of cross-lining which is in the range 0.1% to 0.5% (i.e., in the range 0.001 moles to 0.005 moles of glutaraldehyde per mole of monomer unit). Such a product has remarkable fluid loss control properties but loses any effectiveness when used in the presence of certain other additives in current use, in particular dispersing agents based on polynaphthalene sulphonate. The dispersing agents which are recommended for use with this fluid loss control agent are polyrnelamine sulphonate type dispersing agents. However, under those conditions, the cement slurries formed usually tend to form free water, albeit in very small amounts (typically of the order of several milliliters measured under API conditions) but nevertheless judged to be too high for wells which are subject to gas migration problems.
SUMMARY OF THE INVENTION
The authors of the present invention have shown that, remarkably, it is possible to enlarge considerably the compatibility range of micro-gel type fluid loss control agents based on chemically cross-linked polyvinyl alcohol by the addition of an agent which can block any damaging interaction between the micro-gel and the dispersing agent.
The invention thus provides a fluid loss control agent for a cement slurry, comprising a surfactant and a micro-gel obtained by chemical cross-linking of a polyvinyl alcohol, by reacting the polyvinyl alcohol in solution with agents which can condense with at least two alcohol functions at a pH of less than 10, the molar concentration of the cross-linking agent with respect to the monomer units of the PVA being in the range about 0.1% to 0.5%.
The surfactant preferably contains polar groups which modify the electrostatic environment of the polyvinyl alcohol particles. Non ionic surfactants are preferred. The surfactant of the invention is preferably selected among the group consisting of polyvinylpyrrolidone, styryl phenol derivatives, N-alkyl pyrrolidones with an alkyl chain of less than 12, alkoxylated alcohols with an alkyl chain less or equal to 14 and water-soluble copolymers of vinyl pyrrolidone such as vinyl acetate with a vinyl acetate content of less than 50%.
The best results were obtained with a polyvinylpyrrolidone-type surfactant. With a C10-C12 alcoholate, the compatibility with polynaphthalene sulphonate type dispersing agents was also improved but without reducing the quantities of free water.
The surfactant/micro-gel weight ratio is generally in the range 0.5 to 1.5, typically of the order of 1, this ratio being calculated on the basis of the active mass of cross-linked polyvinyl alcohol.
The condensing agent used to synthesize the micro-gel is preferably selected from glyoxal, glutaraldehyde, maleic acid, oxalic acid, dimethylurea, polyacroleins, diisocyanates, divinylsulphate, and chlorides of diacids. Glutaraldehyde is particularly preferred. The micro-gel is typically prepared in aqueous solution comprising 2% to 5% (by weight), preferably on the order of 3.5% of cross-linked polyvinyl alcohol.
The fluid loss control agent of the invention is used in cement slurries in amounts of 0.5 to 2 gps (gallons per sack of cement) and is not only compatible with polymelamine sulphonate type dispersing agents but also with polynaphthalene sulphonate type dispersing agents, as well as with various other additives, in particular bentonite.
Using these dispersing agents and other additives, it is possible to obtain fluid losses of less than 40 ml, with free water volumes of less than 1 ml. Advantageously, these formulations are gas tight.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In a particularly preferred embodiment of the invention, the surfactant is added to the micro-gel before its use but the two components can also be added independently on preparing the cement slurry.
Micro-gel synthesis
A commercial polyvinyl alcohol was used which had a degree of hydrolysis of about 88 mole % and a molecular weight of about 160000 (Ubbelohde viscosity determination). A dilute aqueous polyvinyl alcohol solution was prepared. The solution was heated to 80° C. for 24 hours, with stirring. The temperature was then reduced to about 50° C. for addition of a 25% glutaraldehy
Nava Robin C.
Schlumberger Technology Corporation
Szekely Peter A.
Waggett Gordon G.
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