Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1996-06-12
2001-11-06
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S724000, C106S737000, C106S823000, C106SDIG001
Reexamination Certificate
active
06312515
ABSTRACT:
The present invention concerns drilling techniques in oil, gas, water, geothermal and analogous wells. More precisely, the invention concerns cementing compositions and their application to cementing such wells.
Before an oil well goes into production, a casing or coiled tubing is lowered into the borehole and cemented over all or a part of its length. Cementing primarily prevents fluid exchange between the various formation strata around the borehole and stops gas from rising through the annular space surrounding the casing, or limits the ingress of water into the production well.
In some cases, it appears that the cemented annular space does not fulfill its primary function and fluids find a path through pores, micro-cracks or other cavities in the cement or through a “micro-annular” space at the interface between the cement and the casing or between the cement and the formation. Holes or cracks may also have been made deliberately, for example at the beginning of well life, but may become undesirable as exploitation of the well continues.
In order to overcome such faults in sealing, repair or squeeze cementing must be carried out. During such operations, a slurry of fine cement is injected under pressure into the cracks, micro-cracks or other openings which are to be blocked.
The principal technical difficulties connected with squeeze cementing are also encountered during other operations, in particular positioning cement plugs, for example to isolate a zone in the well for specific treatment or for cementing gravel packing. This packing is used to filter the sand in unconsolidated formations while allowing the oil to flow through. However, after exploitation of a well, the productive zones of the formations can be invaded by brine and it becomes necessary to remove the gravel packing—with all its ensuing problems—or to plug it by cementing through the gravel.
A cement slurry is a dispersion of solid particles, assumed to be spherical, in a fluid. The slurry can only penetrate into a crack if the largest particles are smaller than the crack. This is quite clear, but a feature which is well known to all specialists in the flow of suspensions must be taken into account, namely that a slurry can penetrate deeply into a crack only if the diameter of its particles is three or more times smaller than the diameter of the crack opening.
An ordinary Portland cement contains particles with an average diameter of the order of 20 micrometers (&mgr;m), the largest particles having a diameter of the order of 70 &mgr;m to 90 &mgr;m. Under such conditions, the cement slurry ought to be able to penetrate easily into crevices that are 300 &mgr;m across, for example. This is certainly not the case in practice.
Fine or ultra fine cements have thus been proposed which have an average diameter of less than 10 &mgr;m, or just a few micrometers, with the largest particles not exceeding 30 &mgr;m, for example. Those cements, usually termed “microcements”, have relatively disappointing performance as regards ability to penetrate, even when they are thoroughly dispersed using the usual dispersing additives such as polyanions containing sulfonate groups.
Further, since the reactivity of a cement slurry increases with the specific surface area of its particles, i.e., with its degree of fineness, it is not always wise to reduce the size of the cement grains too much, as there is a risk of the cement setting too quickly, before the positioning operation is complete.
The aim of the present invention is to provide novel compositions for squeeze cements, in particular for cementing operations in oil, gas, water, geothermal and analogous wells, which compositions have improved penetration properties compared with known prior art compositions.
We have discovered that the poor results obtained with prior art compositions are usually attributable to an excessive increase in the viscosity of the cement slurry due to fluid loss.
The cracks or other openings which are to be blocked by squeeze cementing are all at least in part delimited by porous walls (the formation around the borehole or hardened cement from a primary cementation) which constitutes a major difference compared with conventional flow in a conduit. The slightly porous medium tends to dry out the cement slurry by removing a portion of its aqueous phase, resulting in an increase in the viscosity of the slurry, thereby increasing friction with the walls and hindering further progress of the slurry into the crack. This increase in friction tends to encourage exchange with the porous medium thus generating more fluid loss. The cement slurry must be very stable since injection into a narrow crevice encourages settling and the formation of free water which can lead to pseudochromatography, namely the densest particles of the slurry being deposited at the entrance to the crack—which, of course, blocks deeper penetration of the remaining slurry to be injected into the crack. Further, it is clear that the small amount of cement slurry which succeeds in penetrating into the crack will not produce a good quality cement, in particular since its strength is very low.
It should be emphasized that squeeze cementing operations are always in a less favorable position as regards fluid loss since exchange areas with formations or other porous media are much larger. For primary cementation (cementation of the annular space around a casing), the area-to-volume ratio is generally less than 1 and often of the order of 0.4. In contrast, ratios of the order of 25 are normal for squeeze cementing operations.
A first object of the present invention is thus constituted by cementing compositions for injection under pressure having, at well temperature, fluid loss of less than 30 ml, preferably less than 20 ml and more preferably less than 15 ml, the values being measured using the API (American Petroleum Institute) standard, Spec. 10, Appendix F.
It should be noted that the prior art has largely underestimated the importance of fluid loss for squeeze cementing compositions. Current recommendations, based mainly of the work of Hook, F. E. and Ernst, E. A. [SPE 15104, 1969, “The Effects of Low-Water-Loss Additives, Squeeze Pressure, and Formation Permeability on the Dehydration Rate of a Squeeze Cementing Slurry”], are not to go beyond 200 ml API, for example, in formations of extremely low permeability, 100 ml to 200 ml API in slightly permeable formations, and 35 ml to 100 ml API in highly permeable formations (permeability greater than 100 millidarcy). Further, the majority of low fluid loss additives increase the plastic viscosity of the cement slurry while the prime criterion of the prior art is the need for plastic viscosity which is as low as possible to facilitate penetration of the slurry into the cracks.
Insofar as fluid loss can be greatly reduced but not completely avoided, it is advantageous for the rheology of the cement slurry to be as low as possible. The term “rheology” covers not only plastic viscosity, the importance of which is recognized in the prior art as indicated above, but also, and to a certain extent mainly, the yield point of the slurry. Relatively large fluid losses are tolerated better when the initial rheology of the formulation is low.
High rheology increases the pressure drop, and thus the pressure which must be exerted on the fluid to force it into the crack. However, an increase in this pressure contributes to an increase in fluid loss which, as already seen, is highly deleterious. High yield points also cause fingering in the crevice to be filled.
Preferred compositions of the invention have a yield point of less than 10 lbf/100 ft
2
(478.8 Pa), preferably less than 5 lbf/100 ft
2
(239.40 Pa), and more preferably less than 2 lbf/100 ft
2
(95.76 Pa). Plastic viscosity is preferably less than 100 cP (0.1 Pa.s).
To this end, the compositions of the invention include a dispersing agent which reduces the rheology of the cement slurry. Normal dispersing agents can be used. Examples are polyelectrolytes, i.e., charged water-soluble poly
Barlet-Gouedard Veronique
Dargaud Bernard
Garnier Andre
Maroy Pierre
Marcantoni Paul
Mitchell Thomas O.
Nava Robin C.
Ryberg John J.
Schlumberger Technology Corporation
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