GB2299078A - Cementing composition for wells - Google Patents

Abstract

A composition for cementing an oil, gas, water, geothermal, or analogous well includes as its dispersing agent a salt of polyvinyl sulfonic acid in which 0% to 20% of the monomer groups are replaced by acrylic or methyacrylic monomers. The composition is particularly suitable for use in cementing wells in which the temperature does not exceed 85{C. The molecular weight of the salt is in the range 1,500 to 10,000 and it may be used as an aqueous solution.

Description

A CEMENTING COMPOSITION INCLUDING A DISPERSANT AGENT, AND USE THEREOF IN CEMENTING WELLS The present invention relates to the technical field of cementing oil, gas, water, geothermal, or analogous wells. More precisely, the invention relates to cementing compositions for such wells.

After a well has been drilled, casing is lowered down the hole and, in most cases, a cement slip is injected into the annular gap that extends between the casing and the underground wall of the well, both to consolidate the well and to isolate various zones so as to control the flow of fluids present at different levels in the formations.

The cement slip is injected into the casing and is pumped up via the annular gap so that it rises up inside the gap. Once it has been put into position, the cement is allowed to set. A cement slip is a highly concentrated solution of solid particles. The aqueous base is generally constituted by sea water or brine, and as a result it includes all sorts of ionic species to which there are added various organic or inorganic additives in order to modify properties such as setting time, fluid loss due to filtration into certain types of drilled formation, cement density, etc. These numerous species, to which it is naturally necessary to add the cement proper, are subjected to very numerous ;nteractions of the electrostatic or Van der Waals type. As a result, the rheological behavior of the cement slip is very far from that of a Newtonian fluid.In particular a flow threshold is often observed, thus impeding displacement at low speed, and giving rise to sudden irregularities of flow.

To mitigate such difficulties, cement slip compositions generally include a dispersant agent having the function of modifying the distribution of electrical charge on the surface of cement particles, and more generally of modifying surface interactions between particles.The dispersants most commonly used in this industry are polyelectrolytes, i.e. charged polymers that are soluble in water, and in particular polyanions including sulfonate groups fixed on a skeleton constituted by a polymer having a high branching rate. In this context, mention may be made of polymelamine sulfonate (PMS), polynaphthalene sulfonate (PNS), the condensation product of melamine and formaldehyde having sulfonate groups, polystyrene sulfonate, or lignosulfonates.Another large class of ionic dispersants is constituted by low molecular mass polyacrylic acids, which dispersants are used widely in the building industry, but are not widely used in oil industry cements because they have a very considerable retarding effect.

Other dispersants that are non-ionic, such as polysaccharides having low molecular weight, cellulose derivatives, polyvinyl alcohol, etc., are also known. Finally, there also exist nonpolymer dispersant agents, such as citric acid, for example. In general, dispersants that are non-ionic or non-polymer are little used, at least as dispersants, because they have a very considerable delaying effect on the setting of the concrete.

Although highly effective, polymers based on polynaphthalene sulfonate (PNS) suffer from the major drawback of being somewhat toxic, which is particularly inconvenient in off-shore drilling where it is difficult to recover any reject from an operation.

The present invention seeks to provide a novel cementing composition suitable for cementing oil, gas, water, geothermal, or analogous wells, the composition being essentially constituted by an aqueous base and a Portland type cement, and including a novel type of dispersant agent that does not suffer from the drawbacks of prior art substances, in particular with respect to toxicity, and that is suitable, in particular, for being dumped in the sea.

Cementing compositions of the invention include, as a dispersant agent, a salt of polyvinyl sulfonate acid, in which 0% to 20% of the monomer groups may be replaced by acrylic or methacrylic monomers. The salt is generally provided in an aqueous solution. Polymers of the invention, regardless of whether they are the non-substituted homopolymer or copolymers having acrylic or methacrylic structural units, present no special toxicity problems and, from the environmental point of view, they satisfy the stiffest criteria currently in force in certain countries.

The cation is advantageously selected from alkali metals, and in particular sodium. The chains of the polymers of the invention are preferably relatively short, having molecular weights that preferably do not exceed 10,000 and preferably not exceed 4,000 and more preferably still lie in the range 1,500 to 3,000.

Dispersants of the invention are advantageously soluble in water and can therefore be used as a liquid additive, thereby simplifying implementation thereof, particularly when it comes to measuring them out. Good results have been obtained with aqueous solutions including 35% dry matter, for volume concentrations lying in the range 9% to 13% of the volume of the cement.

In addition, they are compatible with most of the additives commonly used in preparing oil industry cement compositions, for example retarders or accelerators of cement setting, weighting agents, filtrate control additives, antifoaming agents, and other additives well known in the art. It has been found that salts of polyvinylsulfonic acid have a retarding effect at temperatures less than or equal to IX5 F (X5 C). Measurements have not been performed at higher temperatures.

It should be observed that dispersants of the invention differ from dispersants commonly used in compositions for oil industry cements, in particular by having a linear, non-branching structure. The electrostatic interactions that take place between the sulfonate groups and optionally the acid groups of the polymer chains tend to stiffen the chains. Under such circumstances, the adsorption of polyanionic chains of the invention on the surface of grains of cement is not, a priori, enhanced. The effectiveness of salts of polysulfonic acid as a dispersant agent in oil industry cement compositions is, as a result, all the more surprising.

Advantageously, dispersants of the invention enable the quantity of free water to be reduced.

This is all the more surprising in that prior art dispersants are known, on the contrary, to enhance the phenomenon of free water formation, such that dispersants are usually associated with suspension-maintaining additives.

Other advantageous details and characteristics of the invention appear from the following description made with reference to the accompanying drawings, in which: Figure 1 is a graph of curves representative of the rheology of a cementing composition with and without a dispersant of the invention; Figure 2 is a graph of curves representative of the rheology of a cementing composition for various concentrations of dispersant of the invention; Figure 3 is a graph of a curve showing how plastic viscosity varies as a function of dispersant concentration; Figure 4 is a graph of curves showing how the shear threshold varies as a function of dispersant concentration; Figure 5 is a graph of a curve showing the influence of dispersant concentration on the volume of free water in the composition; ; Figure 6 is a graph of curves showing the influence of temperature on the rheology of the cementing composition for different dispersant concentrations; and Figure 7 is a graph of curves representative of the rheology of the cementing composition with different dispersants of the invention.

In all tests, a cementing base was prepared by mixing Portland cement (trademark Cemoil, class G) with water. This gave a density of 1.89 g/cm3 (15.8 ppg). After 20 minutes preparation at measurement temperature, rheological measuie nents were performed using a rotary viscosiometer including a rotary outside cylinder in compliance with API standards, Section 10, Appendix H (Chan 35 model).

The units and symbols used were as follows (units in square brackets corresponding to the unit recommended by the API (American Petroleum Institute).

Volume concentration [gps = US gallons per sack].

PV: plastic viscosity (mPa.s) [cP].

Ty: shear threshold (Pa) [lb./100 sq.ft.].

Gel strength (kg/100 m2) [lb./100 sq.ft.].

EXAMPLE 1 A sodium salt of polyvinylsulfonic acid having the formula

in aqueous solution was added to the base, at a dry matter concentration of about 35%. This salt is sold by Rhone Poulenc, France, under the name Bevaloid XB 16/20. The manufacturer gives a molecular weight lying in the range 3,500 to 4,000.

Rheological tests were performed at 100 F (37.7X C). The results were as follows:

Test Dispersant Plastic Shear (;el strength No. volume % viscositp threshold after 10 s.

1 0 39.5 [4').X] X7.X [1X] 2 5.57 [0.2] 24.3 531 [11.1] 63.4 [13] 3 13.93 [0.5] 18.4 101(2.1] 63.4 [13 The dispersant percentages correspond to a solution concentrated to 35%. A concentration of 5.57% thus corresponding in fact to a volume of "active material" of about 1.95%. The correspond rheological curves are marked on Figure 1, where the torque to be exerted (in terms of apparatus deflection) is plotted as a function of the speed imparted to the bowl, given in revolutions per minute (rpm).

EXAMPLE 2 Example 1 was repeated for different concentrations of dispersant (in solution at 35%). Figure 2 shows the resulting rheological curves. A very clear improvement in rheological characteristics can be observed when the concentration passes from 0.2 gps (5.57% by volume, i.e. a concentration of 0.27% relative to the weight of cement) to 0.25 gps (6.96% by volume) and then to 0.35 gps (9.75% by volume). However the rheological conditions of slurry were not significantly improved when the concentration was as great as 0.5 gps (13.93% by volume).

For tests at 85"C, a cement setting retarder of the refined lignosulfate type was added at a concentration of 0.3% relative to the weight of the cement.

Figure 3 shows curves representing how the plastic viscosity (in centipoise) vanes as a function of dispersant concentration (in solution), and at a temperature of 100"F (37.7X C) or I X5 F (X5"C). Figure 4 gives the shear thresholds for the same compositions.

The dispersant of the invention is entirely compatible with the usual additives and does not give rise to any restrictions as to which other additives can be selected for inclusion in the cement slurry.

A small decrease in plastic viscosity is observed, however the shear threshold decreases very significantly, and in practically linear manner, for concentrations lying in the range 0 to about 0.35 gps. At even higher concentrations, the shear threshold changes little, and for the test at X5 C, it even tends to rise slightly for concentrations of 0.5 gps.

The various curves show that a concentration of dispersant solution close to 0.35 gps (9.75arc by volume or about 0.5% by weight of cement) constitutes the best compromise and is therefore preferred in the invention.

A secondary effect that is often reserved with prior art dispersants is a tendency to encourage the formation of density gradients from the top to the bottom of a cement column. Depending on circumstances, such gradients represent more or less pronounced settling phenomena with, at one extreme, genuine segregation of the particles in the composition according to density, or with a layer of "free water" being formed at the top of the column.

In a manner that is most surprising, and so far as the inventors are aware, unique, and as shown in Figure 5, far from encouraging this phenomenon, the dispersant of the invention contributes to reducing the quantity of free water. The curve marked in Figure 5 gives the volume of free water (in milliliters) observed on a 250 ml cement column as a function of dispersant concentration. The test was performed using the procedure laid down in API Spec 10 (Section 6, Appendix M), at a temperature of 85 C. The dispersant of the invention may advantageously be used on its own, without any need to add a suspension-maintaining agent such as bentonite, or cement compatible polysaccharides such as wallanes, or metal salts, and in particular magnesium salts.

In addition, it may be observed that the retarding effect on cement setting, which is observed with all dispersants, is of the same order of magnitude as that obtained using the commonly employed PNS or PMS type polymers.

EXAMPLE 3 The formulation of Example 1 was used, and a cement setting retarder of the refined lignosulfate type was again added thereto at a concentration of 0.3% relative to the weight of cement.

The rheological curves of Figure 6 show the influence of temperature and dispersant concentration (in solution). It may be observed that the curves at 100"F (37.7X C) and 145 F (62.78"C) practically coincide, and that constitutes a significant advantage insofar as the downhole temperature in a well is not always known very accurately and, in addition. even whenthe well is not considered as being particularly deep, and therefore as being particularly hot, there always exist a fairly large temperature difference between the surface and the bottom of a well, and it is always simpler to proceed with a well cementing operation using a slurry whose rheological properties remain constant throughout the cement displacement cycle.

At 1X5"F (X5 C), the dispersant of the invention no longer enables the same rheology to be conserved, but it nevertheless remains highly effective when compared with the reference composition having no dispersant.

For this test, the concentration of the dispersant was caused to vary over the range 0.35 gps (0.47% relative to the weight of cement) to 0.8 gps (1.06% relative to the weight of cement). It can again be seen that any improvement obtained by increasing the concentration of the dispersant is relatively small in this range of concentrations, and indeed at high stresses, some results are even inverted. At this temperature, the best results are obtained for the dispersant solution having a concentration of 0.45 gps (0.60% relative to the weight of cement).

EXAMPLE 4 As mentioned above, 0 to 20% vinylsulfonic monomer can be substituted by acrylic or methacrylic monomers.

The test was performed again, comparing the rheological conditions obtained with nonsubstituted polymer A as used in the preceding examples and with copolymers B and C respectively including 10% and 20% acrylic monomer. These substances are sold by Rhone Poulenc France, under the name Bevaloid XB16/31D and Bevaloid XBl6/31C. Their molecular weights lie in the range 1,500 to 2,000. As before, they are in the form of an aqueous solution at a concentration of 35%; all of the concentrations indicated above correspond to concentration in terms of solution and not in terms of active material.

The corresponding curves are shown in Figure 7. The dispersant effect of these copolymers can be seen even though the homopolymer provides significantly better performance at given concentration.

It has thus been shown that the dispersant of the invention can replace dispersants known in the art because of its high effectiveness as a dispersing agent, because of a retarding effect comparable to that of prior art substances, and because of its effect in reducing the formation of free water. In addition, it should be emphasized that the substance of the invention has no special toxicity and satisfies the severest standards concerning the environment.

Claims (7)

1. A composition for cementing an oil, gas, water, geothermal or analogous well, the composition comprising an aqueous base, a Portland type cement, a dispersant, and possibly other additives, the composition being characterized in tha e dispersant is a salt of polyvinylsulfonic acid, having a molecular weight lying in the range 1,500 to 10,000 ; 0% to 20% of its monomer groups replaced by acrylic or methacrylic monomers.

2. A composition according to claim 1, characterized in that the salt is a sodium salt.

3. A composition according to either preceding claim, characterized in that the polymer has a molecular weight lying in the range 1,500 to 4,000.

4. A composition according to any preceding claim, characterized in that it further includes at least one additive of the type for accelerating or retarding cement setting, a weighting agent, a filtrate control agent, an antifoaming agent, etc.

5. A composition according to any preceding claim, characterized in that the polymer is used in an aqueous solution, at a dry matter concentration of 35%.

6. A composition according to claim 5, characterized in that the dispersant solution is added to a concentration by volume lying in the range 9% to 13% of the volume of cement.

7. The use of the composition according to any preceding claim in cementing inside wells in which the temperature does not exceed X5 C.

X. A use according to claim 7 for controlling free water during cementing inside an oil well.