GB2294693A - Cementig compositions and application thereof to cementing oil wells and the like - Google Patents

Abstract

The invention relates to a cementing composition and to the use of said composition in cementing wells for oil, gas, water, geothermal purposes, and the like. The composition comprises a liquid phase containing a binder which, after polymerization, is suitable for maintaining cohesion between solid particles, and a solid phase that is constituted by particulate additives that are insoluble in said liquid phase. There are at least two such additives, and relative to one another they present grain size distributions that are disjoint, with the choice of proportions for the solid materials relative to the liquid and relative to one another being such that the resulting mixture is put into conditions of hindered settling and has a PVF close to the maximum possible PVF. The composition may comprise monomers and cross-linking agents. The polymerisation may be initiated by irradiation.

Description

CEMENTING COMPOSITIONS AND APPLICATION THEREOF TO CEMENTING OIL WELLS AND THE LIKE The present invention relates to the techniques of drilling wells for oil, gas, water, geothermal purposes, or the like. More precisely, the invention relates to cementing compositions and to applying them to cementing such wells.

After a well has been drilled, it is cased by means of tubular ducts that are generally made of steel and known as casing. To provide mechanical support for the casing, and above all to avoid interchanges between fluids coming from different levels underground, and in particular to prevent water from water tables mixing with zones that produce oil and/or to prevent gas rising to the surface through the annular gap or "annulus" between the casing and the rock wall of the well, the annular gap is usually cemented.

Very briefly, the operation of cementing consists in pumping a cement slurry from the surface down the well inside the casing so as to expel the liquid or drilling mud which rises to the surface via the annulus. The slurry is itself propelled by an inert fluid so that as pumping progresses, the slurry replaces the drilling mud in the annulus while the inside of the tubular duct is filled with inert fluid.

It will readily be understood that in this type of operation, the setting and hardening time of the cement is a factor of great importance: if setting takes too quickly, then the cement cannot be put into place and it is necessary to redrill the well, which operation is both difficult and expensive, and delays bringing the well into production.

If the slurry sets too slowly, then the operator is again faced with long delays before being able to proceed with subsequent operations. In addition, it becomes difficult to control key parameters such as fluid loss which tends to dry out the slurry by depriving it of a fraction of its water, and can indeed give rise to settling, to the formation of free water, etc.

In addition, temperature and pressure conditions downhole can be very different from those at the surface. Unfortunately, it is well known that the properties of a cement slurry are largely dependent on those conditions, such that the cycle to which the slurry is subjected - surface, bottom, surface - makes it particularly difficult to control several key parameters.

It is also necessary to adjust the density of the slurry very accurately as a function of the characteristics of the rocks through which the hole passes in order to balance the pressures exerted by the rocks and by the slurry: a slurry that is too dense runs the risk of damaging the rocks by acting as a fracturing fluid; while a slurry that is not dense enough runs the risk of having external fluids intruding therein. In well known manner, density is adjusted by acting on the quantity of water and by adding appropriate quantities of special additives such as silica, sand, industrial waste, clays, in particular barite, etc., for example.Naturally, the quantity and the nature of the additives must not be such as to harm the cementing operation itself, with the first and major requirement being that the slurry composition must be capable of being pumped with the tools commonly used for that type of operation.

Paradoxically, the cements commonly used, e.g. Portland cements, cenainly do not constitute the best response to the problem posed, even if a very great deal of work has been done to make it possible to overcome certain difficulties to a large extent by proposing additives such as setting retarders and accelerators, to mention but one example amongst the multitude of additives made available to the person skilled in the art. In addition, conditions downhole are not always known with the desirable degree of accuracy, such that formulating a slurry is always a difficult operation, encumbered by high financial risk if it should turn out that the formulation is at least partially unsuitable. Nevertheless, the relatively cheap cost of cement has, until now, prevented cement being replaced by any other substance.

An object of the present invention is to develop cement-free cementing compositions that are of reasonable cost and that are capable of presenting propenies that are better than those of usual compositions. The term "cement-free" is used to designate any formulation that includes a binder which is at least for the most part not of the Portland cement type nor of any other analogous type of hydraulic binder.

Taking account in particular of price differences between firstly binders constituted by synthetic polymer materials such as adhesives and secondly ordinary oil industry cements which are essentially silico-calcium or silico-sodium cements, it has been essential to find a formulation that makes it possible to achieve a major reduction in the quantity of binder used in the formulation while minimizing loss of binder through the pores of the rock.

In European patent application No. 94400836.6 filed on April 18, 1994, a new concept is proposed for formulating cement slurries whereby special insoluble additives are incorporated in the slurry, in proportions and quantities selected in such a manner that: the formulation includes at least three special materials that are insoluble in the liquid medium, preferably an aqueous medium, which materials are preferably inorganic, and at least one of them is an oil industry cement or a comparable hydraulic binder; the grain size ranges of the particles are disjoint, i.e. they do not overlap to any large extent, which criterion can generally be considered as being satisfied if the grain-size curves are shifted by not less than a distance corresponding to half the peak;; the proportions of the solids relative to the liquid (or mixing fluid of the slurry) is such that the resulting slurry is placed under hindered settling conditions, conditions in which the solid particles behave collectively like a porous solid material, with the percentages of the various fractions being the same from top to bottom of the column, whereas otherwise, under other conditions, particles of different sizes or of different densities settle out separately at different speeds.This threshold, which is a function of the selected grain sizes, corresponds in practice to much higher concentrations of solid mater than those accepted in the prior art for cementing slurry compositions; and the choice of proportions for the solids relative to one another and as a function of their respective grain sizes is such as to maximize the packing volume fraction (PVF), or at least to achieve conditions close to maximum PVF.

This "multimodal" concept leads to slurries being obtained which have rheological properties that are quite remarkable and which are easy to mix and to pump, even with very low quantities of mixing fluid.

Starting from the above novel compositions, the present inventors now propose formulations in which the particulate phase constituted by the cement is replaced by a different particulate material, in particular an inorganic material, and the aqueous mixing fluid is replaced by a binder or more generally by a binder in aqueous solution or possibly in other solvents such as oils or emulsions, for example, the binder being such that after initially providing fluidity, it polymerizes like an adhesive and ensures that the various solid particles cohere.

The formulation of the invention is preferably a "tetramodal" combination including at least four types of particles having different grain size ranges and more preferably still, a "pentamodal" composition, thereby making it possible to further reduce the quantity of liquid required, or even combinations that are even more complex, which compositions are nevertheless generally more expensive. It should be emphasized that regardless of the number of "modes" used, it is important to satisfy the characteristic of disjoint grain size ranges and there can be no question of making a formulation with a continuous range of grain sizes.

It is possible, for example, while satisfying the criteria specified above, to use combinations of the following non-limiting categories: "very coarse" (average size greater than 1 mm) e.g. coarse sand or crushed waste; "coarse" (average size 100 microns to 800 microns, preferably 100 microns to 300 microns, and more preferably still about 200 microns) selected, for example, from sand, silica, carbonate, barite, hematite or other iron oxides, carbon, sulfur, and certain crushed plastics or other wastes; "medium" (average size 10 microns to 20 microns) e.g. silica or crushed waste, this category being the category to which oil industry cements belong, which cements are absent according to the invention;; "fine" (average size 0.5 microns to 10 microns, and preferably about 1 micron) selected, for example, from carbonates (marble, chalk, calcite, ...), barite, hematite or other iron oxides, silica, carbon, sulfur, and certain residues such as fine fly ash (or "micro fly ash", "micro slag"); "very fine" (average size 0.05 microns to 0.5 microns) e.g. latex, a silica condensate of the silica soot type, a condensate of manganese oxides in pigment soot, or indeed certain polymer microgels such as a filtrate-controlling agent; and "ultrafine" (average size 7 nanometers to 50 nanometers) e.g. dispersed colloidal silicas or aluminas.

Without going beyond the ambit of the invention, it is naturally possible, where appropriate, to use materials of different kinds but having the same range of grain sizes, in which case those materials together constitute a single "mode".

In practice, compositions including all six types of categories specified above will rarely be used, with particles of the smallest grain sizes, and in particular so-called "ultrafine" particles, being reserved for the most extreme cases, and in particular plugging cracks ("squeeze cementing"). Under such circumstances, it will generally be appropriate to avoid using particles that are "very coarse" or even particles that are "coarse".

Thus, it is preferable to use tetramodal formulations of the "very coarse", "coarse", "medium", and "fine" type, or of the "coarse", "medium", "fine", and "very fine" type. Also preferably, pentamodal formulations are used that are analogous to the tetramodal compositions but that further include a phase of "ultrafine" particles with particles having an average size of 7 nm to 50 nm, or a composition comprising "very coarse", "coarse", "medium", "fine", and "very fine" particles.

It should also be observed that the specified grain size ranges are given purely by way of indication, and other materials could be used providing they satisfy the criteria listed above. It should be observed that calculating PVF is a conventional procedure and it is merely recalled that it makes use of "subdivisions" of each range of grain sizes for each component of the mixture of particles, with satisfactory results being obtained using subdivisions comprising thirty or more fractions, for example.

In addition to their particulate phases, formulations of the invention include a liquid phase which serves to perform three functions: to provide the binder base (in other words the "adhesive") that serves to fix the various solid particles to one another in definitive manner; to ensure that the mixture is mixable and pumpable; and finally, where appropriate, to adjust the density of the composition, even though this function may optionally be performed by choosing particles of different densities for one of the grain size ranges.

In conventional oil-industry cement compositions, the liquid phase represents at least 50% by volume of the formulation. In contrast, with formulations as proposed above, it is possible to use formulations suitable for being pumped from the inside to the outside of the casing that have a liquid phase whose volume is less than 20% of the total volume, for example.

Under such conditions, it is possible under economically realistic conditions to consider using materials that are much more expensive, e.g. polymer materials in liquid form or in solution. Depending on circumstances, it is possible to directly inject a reactive solution directly containing the monomer(s) and the cross-linking agent(s) or, and this naturally gives far greater flexibility, to initiate cross-linking once the composition is in place in the annulus, e.g. by forming radicals in situ that are themselves generated, for example, by bombardment using slow neutrons, gamma rays, electrons, protons, etc., with those various techniques being well known specialists in plastics materials.

Preferably, the chosen one of those techniques, such as bombardment using slow neutrons, enables polymerization to be initiated from the inside volume of the casing with the bombardment then passing through the casing.

It is preferable for the monomers used to be capable of polymerizing in the presence of water in order to ensure good compatibility with the thin layer of mud that covers the walls of the annulus after the drilling mud has been displaced (insofar as the drilling mud used is a mud having an aqueous base). In addition, it is preferable to select materials that are soluble in water or that are capable of forming an emulsion in water. It is also possible to envisage using bases that are not aqueous, in particular oils or emulsions of water in oil such as those used in particular for preparing drilling muds; again, this possibility is available essentially because the quantity of liquid used is very small and it may optionally be formed using a material that is more expensive than water.

The cementing formulations of the invention are particularly suitable for cementing operations in wells for oil, gas, water, geothermal purposes, or the like, regardless of whether the cementing operations are so-called "primary" operations during installation of the casing, or "secondary" operations, in panicular for the purpose of plugging cracks in the cemented annulus. They may also be used in all ordinary applications for cements, and in particular in all cases where it is desirable to use compositions that are "dry", containing no or practically no water.

Even in the absence of any filtrate control agent, or in the presence of very small quantities thereof, the multimode formulations of the invention provide very good resistance to fluid losses into the rock. This point is particularly pertinent in the present case insofar as the fluid is not essentially constituted by water as is the case for conventional compositions, but on the contrary contains a cross-linkable binder.

Although it is well known that large fluid losses are undesirable for ordinary compositions insofar as they dry out the cement and consequently reduce the quality of the cementing, nevertheless such losses do not damage the rock and the operator can improve the cementing by "secondary" cementing operations.

However, in the present invention, the fluid is not an inert fluid, so if it penetrates into the underground formation it will plug the pores or the microcracks therein which is certainly not the looked-for objective in zones that produce oil, gas, etc It will thus be better understood that this factor is particularly important, and independently of the price factor, it should be emphasized that, for this reason alone, a binder of the present invention would be unsuitable for use as the mixing fluid in a conventional composition. It should also be observed that although the multimode formulations contain very little liquid, they can nevertheless tolerate larger fluid losses before they cease to be capable of flowing than can conventional formulations, which is quite surprising.

Further, the density of cementing compositions of the invention can be adjusted very accurately without excessively increasing the viscosity of the composition and thus without compromising its capacity for being mixed and cast, even in the absence of any dispersing agents, or merely in the presence of very small quantities thereof.

The following examples illustrate the invention and show up other advantageous details and characteristics thereof, while nevertheless not limiting the scope, and in the examples reference is made to the accompanying sheets of drawings, in which: Figure 1 shows a trimodal distribution of grain sizes that is suitable for implementing the invention; Figure 2 shows comparable curves representative of the viscosity of a mixture as a function of its solid volume fraction, for different values of PVF; Figure 3 shows comparable curves representative of the viscosity of a mixture as a function of the porosity of the mixture, for different values of PVF; and Figure 4 is a curve showing the porosity required to obtain a viscosity of 200 cP, as a function of the value of the PVF.

The invention is illustrated by compositions in which the particle fill is made up of "coarse" particles (sand), "medium" particles (silica flour), and "fine" particles (fine silica) to which the following are added for compositions having further or more modes: "very fine" particles (latex), and for pentamodal compositions "ultrafine" particles (dispersed colloidal silica).

The materials mentioned are given merely by way of example. However, their grain size characteristics, or more particularly the way they are spaced apart relative to one another constitutes an essential point. In the present case, the grain size curves for the "coarse", "medium", and "fine" particles are drawn in Figure 1 respectively as a solid line (mean diameter about 100 microns), a chain-dotted line (mean diameter about 10 microns), and a dashed line (mean diameter about 1 micron). It will be observed that there is a factor of about 10 between adjacent average sizes of the particles and very little overlap between the various curves which are highly disjoint. It is also important to select materials that are compatible with one another and with the adhesive used for binding the particles together.It should also be observed that the particles chosen are advantageously spherical, or in any event non-anisotropic, fibrous materials thus being generally avoided.

Initially, a composition was prepared comprising liquid and silica flour. With such a composition having a single solid component, the packing volume fraction or PVF is 0.8 (in other words 20% of the volume which could theoretically be occupied by the particles if they were disposed adjacent to one another in optimum manner to minimize their steric hindrance is in fact inaccessible since the particles do not organize themselves naturally in this way).

A bimodal second formulation was prepared using a mixture of two solid components, with sand replacing a fraction of the silica flour. Because of the respective grain size ranges of these two components, it was possible to obtain a higher PVF value by an appropriate choice of ratio between the two components. In this case, a mixture comprising 40% sand and 60% silica flour had a PVF of 0.828.

The trimodal third formulation further included fine silica in addition to silica flour and sand, in the following proportions (still in terms of solid matter volume): sand 55%, silica flour 30%, and fine silica 15%. The PVF was 0.873.

The above three formulations were used to prepare slurries, varying the quantity of the solid volume fraction in the slurries. Figure 2 is a plot of the plastic viscosity of the slurries (in centipoise or m.Pascal.s) for various solid fraction values. The diamond-shapes, the crosses, and the squares correspond respectively to the monomodal formulation, the bimodal formulation, and the trimodal formulation. In expected manner, the greater the solid fraction in the silica flour and fluid mixture, the greater its viscosity. In contrast, the greater the PVF, the further the curves are shifted towards high concentrations of solid matter.

Thus, with the first composition, the curve is substantially asymptotic for a solid fraction of about 50% which, in practice, means that it is not possible to use mixtures that have less than 50% liquid. With the second formulation, the steep increase in viscosity of the mixture does not appear until the concentration is about 55%. For the third formulation, concentrations of about 63% are still sufficiently fluid to enable them to be pumped.

This is also illustrated in Figure 3 which, for different values of PVF, is a plot of viscosity obtained as a function of the porosity of the mixture. It should be observed that the selected limit value of 200 cP (or 200 mPascal.s) is a relatively low value, since mixtures of higher viscosity can be pumped without difficulty. PVF values lying in the range 0.9 to 0.95 can be obtained from tetramodal mixtures. To obtain the value 0.98, it is necessary to use a pentamodal mixture.

The curve of Figure 4 is derived from Figure 3 and it is a plot of porosity as a function of the PVF value for a viscosity of 200 cP. It should be observed that the resulting curve is practically linear, with an improvement of about 20% in the PVF value (as can be obtained by an appropriate selection of grain size ranges and of the proportions of the various solid ingredients) which makes it possible to reduce the quantity of fluid required by nearly as much, thereby making it possible to use fluids that are more expensive.

Claims (13)

1. A cementing composition comprising: (a) a liquid phase; (b) a solid phase, wherein said solid phase comprises substantially particulate materials that are insoluble in said liquid phase; (c) wherein the particulate materials of the solid phase are of disjointed grain size distributions relative to one another, the particulate materials being formed such that the proportions of solid materials relative to liquid materials facilitate a resulting mixture under conditions of hindered settling; (d) wherein the packing volume fraction of the solid particles is maximized by choice of proportions for the solid materials relative to one another, and as a function of their respective grain size ranges; (e) the liquid phase containing a binder that serves to maintain cohesion between the solid particles after polymerization.

2. The composition of claim 1 wherein the grain size ranges of the solid particulate additives are selected from one or more of the following categories: (a) very coarse grain sizes having average sizes greater than about 1 mm; (b) coarse grain sizes having average sizes between about 100 microns and about 800 microns; (c) medium grain sizes having average sizes between about 10 microns and 20 microns; (d) fine grain sizes having average sizes between about 0.5 microns and 10 microns; (e) very fine grain sizes having average sizes between about 0.05 microns and about 0.5 microns; and (f) ultra fine grain sizes having average grain sizes between about 7 nanometers and about 50 nanometers.

3. The composition of claim 2 wherein the liquid phase is a reactive solution including monomers and crosslinking agents.

4. The composition of claim 3 comprising solid particulate additives of very coarse, coarse, medium, and fine grain sizes.

5. The composition of claim 3 comprising solid particulate additives of coarse, medium, fine and ultrafine.

6. The composition of claim 3 wherein the liquid phase further comprises a solution including monomers whose reticulation is induced by in situ formation of radicals.

7. The composition of claim 6 wherein the radical formation is generated by bombardment using slow neutrons, gamma rays, electrons, or protons.

8. The composition of claim 7 wherein said binder is in aqueous solution.

9. The composition of claim 7 wherein said binder is dispersed in the form of an emulsion in a continuous phase.

10. A method of treating a subterranean formation comprising the steps of: (a) providing a wellbore; (b) providing a cementing composition comprising: (i) a liquid phase; (ii) a solid phase, wherein said solid phase comprises substantially particulate materials that are insoluble in said liquid phase, wherein the grain size ranges of the solid particulate additives are selected from one or more categories of sizes comprising very coarse grain sizes having average sizes greater than about 1 mm, coarse grain sizes having average sizes between about 100 microns and about 800 microns, medium grain sizes having average sizes between about 10 microns and 20 microns, fine grain sizes having average sizes between about 0.5 microns and 10 microns, very fine grain sizes having average sizes of about 0.05 microns to about 0.5 microns, and ultra fine grain sizes having average grain sizes of about 7 nanometers to about 50 nanometers; (iii) wherein the particulate materials of the solid phase are of disjointed grain size distributions relative to one another, the particulate materials being formed such that the proportions of solid materials relative to liquid materials facilitate a resulting mixture under conditions of hindered settling; (iv) wherein the packing volume fraction of the solid particles is maximized by choice of proportions for the solid materials relative to one another, and as a function of their respective grain size ranges; (v) the liquid phase containing a binder that serves to maintain cohesion between the solid particles after polymerization; and (c) pumping the cementing composition into the wellbore.

11. The method as set forth in claim 10 wherein the treating comprises drilling into the subterranean formation and the step of pumping comprises pumping the cementing composition through tubing and through a drill bit located at an end of the tubing in the subterranean formation.

12. A method of treating a subterranean formation comprising the steps of: (a) providing a wellbore; (b) providing a cementing composition comprising a liquid phase and a solid phase; (c) generating radicals, wherein the liquid phase comprises a solution including monomers whose reticulation is induced by formation of radicals, the radicals being formed by bombardment using one or more of the following: slow neutrons, gamma rays, electrons, and protons; and (c) pumping the cementing composition into the wellbore.

13. The method of claim 12 additionally comprising the step of cementing an annular gap, the annular gap comprising the spacing between a casing and the rock through which a well for oil, gas, water, or geothermal purposes passes, further wherein bombardment is performed through the casing.