Compositions and methods for well completions

Abstract

Well-cementing compositions for use in high-pressure, high-temperature (HPHT) wells are often densified, and contain weighting agents such as hematite, ilmenite, barite and hausmannite. The weighting agents are usually finely divided to help keep them suspended in the cement slurry. At high temperatures, finely divided weighting agents based on metal oxides react with the calcium-silicate-hydrate binder in set Portland cement, leading to cement deterioration. Finely divided weighting agents based on metal sulfates are inert with respect to calcium silicate hydrate; consequently, set-cement stability is preserved.

Claims

1. A method for maintaining the compressive strength of a well-cementing composition, comprising: (i) providing a cement slurry comprising water, Portland cement, silica and a weighting agent comprising one or more metal sulfates selected from the group consisting of barite, celestine and anglesite, wherein the average particle size of the weighting agent is smaller than 10 m; and (ii) curing the cement slurry at a temperature higher than or equal to 200 C. and a pressure higher than or equal to 69 MPa, such that the cement slurry sets and forms xonotlite as a main binding phase of a set cement, wherein the one or more metal sulfates are present in at least two median particle-size ranges such that particle packing is optimized, wherein, after 500 hr of curing, xonotlite remains as the main binding phase, and a water permeability of the set cement does not exceed 0.1 mD.

2. The method of claim 1, wherein the density of the composition is higher than 2035 kg/m.sup.3.

3. The method of claim 1, wherein the weighting agent concentration is between 1% and 150% by weight of cement.

4. The method of claim 1, wherein the composition further comprises one or more additives selected from the group consisting of accelerators, retarders, extenders, fluid-loss additives, dispersants, gas-generating agents, antifoam agents, chemical-expansion agents, flexible additives, pozzolans and fibers.

5. The method of claim 1, wherein the cement slurry has a viscosity lower than 1000 mPa-s at a shear rate of 100 s.sup.1.

6. The method of claim 1, wherein the median particle size of the weighting agent is smaller than 5 m.

7. A method for cementing a subterranean well, comprising: (i) providing a cement slurry comprising water, Portland cement, silica and a weighting agent comprising one or more metal sulfates selected from the group consisting of barite, celestine and anglesite, wherein the average particle size of the weighting agent is smaller than 10 m; and (ii) placing the slurry into the well, wherein, a bottomhole temperature in the well is higher than or equal to 200 C. and a pressure is higher than or equal to 69 MPa, such that the cement slurry sets and forms xonotlite as a main binding phase of a set cement, wherein the one or more metal sulfates are present in at least two median particle-size ranges such that particle packing is optimized, wherein, alter 500 hr of exposure to the bottomhole temperature and bottomhole pressure, xonotlite remains as the main binding phase, and a water permeability of the set cement does not exceed 0.1 mD.

8. The method of claim 7, wherein the density of the composition is higher than 2035 kg/m.sup.3.

9. The method of claim 7, wherein the weighting agent concentration is between 1% and 150% by weight of cement.

10. The method of claim 7, wherein the composition further comprises one or more additives selected from the group consisting of accelerators, retarders, extenders, fluid-loss additives, dispersants, gas-generating agents, antifoam agents, chemical-expansion agents, flexible additives, pozzolans and fibers.

11. The method of claim 7, wherein the cement slurry has a viscosity lower than 1000 mPa-s at a shear rate of 100 s.sup.1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a HPHT strength-development curve for a high-density Portland cement system containing hematite and hausmannite.

(2) FIG. 2 shows a HPHT strength-development curve for a high-density Portland cement system containing hematite with two particle-size distributions.

(3) FIG. 3 shows a HPHT strength-development curve for a high-density Portland cement system containing hematite and titanium oxide (rutile).

(4) FIG. 4 shows a HPHT strength-development curve for a high-density Portland cement system containing barite with two particle-size distributions.

DETAILED DESCRIPTION

(5) At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. In addition, the composition used/disclosed herein can also comprise some components other than those cited. In the summary and this detailed description, each numerical value should be read once as modified by the term about (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration within the range, including the end points, is to be considered as having been stated. For example, a range of from 1 to 10 is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors possessed knowledge of the entire range and all points within the range. All ratios or percentages described here after are by weight unless otherwise stated.

(6) As stated earlier, there is a need for weighting agents that are inert with respect to calcium-silicate-hydrate cement minerals under HPHT conditions. The inventors have surprisingly discovered that metal sulfates, including (but not limited to) barium sulfate (barite), strontium sulfate (celestine) and lead sulfate (anglesite), do not react with xonotlite, and do not cause a loss of cement compressive strength or increased cement permeability. Such sulfates are essentially insoluble in water.

(7) In an aspect, embodiments relate to well-cementing compositions that comprise water and solids comprising Portland cement, silica, and an additive comprising one or more members of the list comprising barite, celestine and anglesite. The composition is preferably pumpable. Those skilled in the art will recognize that a pumpable cement slurry usually has a viscosity lower than 1000 mPa-s at a shear rate of 100 s.sup.1. Metal sulfates with a very fine particle-size distribution are preferred. The median particle size is preferably smaller than about 10 m, more preferably smaller than about 5 m and most preferably equal or smaller than about 3 m.

(8) In a further aspect, embodiments relate to methods for maintaining the compressive strength of a well-cementing composition. A cement slurry is provided that comprises water, Portland cement and silica. An additive is incorporated into the slurry that comprises one or more members of the list comprising barite, celestine and anglesite. The slurry containing the additive is then cured at a temperature higher than or equal to about 200 C. Metal sulfates with a very fine particle-size distribution are preferred. The median particle size is preferably smaller than about 10 m, more preferably smaller than about 5 m and most preferably equal or smaller than about 3 m.

(9) In yet a further aspect, embodiments relate to methods for cementing subterranean wells. A cement slurry is provided that comprises water, Portland cement and silica. An additive is incorporated into the slurry that comprises one or more members of the list comprising barite, celestine and anglesite. The slurry containing the additive is then placed into the well, wherein the bottomhole temperature is higher than or equal to about 200 C. Metal sulfates with a very fine particle-size distribution are preferred. The median particle size is preferably smaller than about 10 m, more preferably smaller than about 5 m and most preferably equal or smaller than about 3 m. Those skilled in the art will recognize that the methods may pertain to both primary and remedial cementing operations.

(10) For all embodiments, the slurry density is preferably higher than about 2035 kg/m.sup.3 (17.0 lbm/gal). The additive concentration is preferably between about 1% and about 150% by weight of cement (BWOC). The slurry may further comprise one or more additives in the list comprising: accelerators, retarders, extenders, fluid-loss additives, dispersants, gas-generating agents, antifoam agents, chemical-expansion agents, flexible additives, pozzolans and fibers. Accelerators may be required in slurries that are pumped in thermal-recovery wells. Such wells are usually shallow and are cemented at a low temperature. During production, the wells may be heated to temperatures exceeding 200 C.

(11) Furthermore, for all embodiments, the solids in the slurry (cement+silica+metal-sulfate additive+additional solid additives) may be present in at least two particle-size ranges. Such designs are engineered-particle-size systems in which particle packing is optimized. A thorough description of these systems may be found in the following publication. Nelson E B, Drochon B and Michaux M: Special Cement Systems, in Nelson E B and Guillot D (eds.) Well Cementing2.sup.nd Edition, Houston, Schlumberger (2006) 233-268.

EXAMPLES

(12) The following examples serve to further illustrate the disclosure.

(13) For all examples, cement-slurry preparation and strength measurements were performed according to procedures published in ISO Publication 10426-2. Strength measurements were performed in an Ultrasonic Cement Analyzer (UCA).

Example 1

(14) A solid blend was prepared with the following composition: 35% by volume of blend (BVOB) Dyckerhoff Black Label Class G cement (median particle size 15 m), 40% BVOB silica sand (median particle size 315 m), 10% BVOB silica flour (median particle size 3 m), 5% BVOB hematite (median particle size 32 m) and 10% BVOB Micromax hausmannite (median particle size 2 m). To this mixture, 1.5% by weight of blend (BWOB) bentonite was added.

(15) A fluid was prepared with the following composition: 4.17 L/tonne of blend silicone antifoam agent, 66.8 L/tonne retarder (a blend of sodium pentaborate and pentasodium ethylenediamine tetramethylene phosphonate [EDTMP]weight ratio: 9.3), 0.75 BWOB styrene sulfonate-maleic anhydride copolymer dispersant (NARLEX D72, available from Akzo Nobel), 0.8% BWOB fluid-loss additive (UNIFLAC, available from Schlumberger), and sufficient water to prepare a slurry with a solid-volume fraction (SVF) of 0.61. The slurry density was 2277 kg/m.sup.3 (19.0 lbm/gal).

(16) The slurry was placed in a UCA instrument, and cured at a final temperature of 302 C. (575 F.) and pressure of 122 MPa (17,700 psi). The heat-up time to reach 274 C. (525 F.) was 100 min, and the total heat-up time to reach 302 C. was 240 min. The time to reach 122 MPa was 100 min. The UCA chart is shown in FIG. 1.

(17) The strength reached a maximum value after about 100 hr. Then the strength began to decrease, and reached a plateau after about 400 hr. The UCA test was terminated after 500 hr. At that time the strength had stabilized.

(18) The cement sample was removed from the UCA and cored for measurement of actual compressive strength and water permeability. The compressive-strength result was 20.6 MPa (2990 psi). The water permeability was 0.77 mD, which those skilled in the art would recognize as being too high. For proper zonal isolation, the maximum allowable permeability value is generally considered to be 0.1 mD.

(19) Next, the sample was ground to a fine powder and dried first with acetone and then with ethyl ether. The crystalline composition of the powder was analyzed by x-ray diffraction. The cement matrix was mainly composed of johannsenite. A small amount of xonotlite (the expected cement mineral at this temperature) was detected. The presence of hausmannite (Mn.sub.3O.sub.4) was not noted.

Example 2

(20) A solid blend was prepared with the following composition: 35% by volume of blend (BVOB) Dyckerhoff Black Label Class G cement (median particle size 15 m), 40% BVOB silica sand (median particle size 315 m), 10% BVOB silica flour (median particle size 3 m), 5% BVOB hematite (median particle size 32 m) and 10% BVOB hematite (median particle size 3 m). To this mixture, 1.5% by weight of blend (BWOB) bentonite was added. The difference between this blend and the one of Example 1 is the replacement of 10% BVOB Micromax with the same volume of very fine hematite.

(21) A fluid was prepared with the following composition: 4.17 L/tonne of blend silicone antifoam agent, 66.8 L/tonne retarder (a blend of sodium pentaborate and pentasodium EDTMPweight ratio: 9.3), 0.75% BWOB styrene sulfonate-maleic anhydride copolymer dispersant (NARLEX D72, available from Akzo Nobel), 0.8% BWOB fluid-loss additive (UNIFLAC) and sufficient water to prepare a slurry with a solid-volume fraction (SVF) of 0.61. The slurry density was 2280 kg/m.sup.3 (19.03 lbm/gal).

(22) The slurry was placed in a UCA instrument, and cured at a final temperature of 302 C. (575 F.) and pressure of 122 MPa (17,700 psi). The heat-up time to reach 274 C. (525 F.) was 100 min, and the total heat-up time to reach 302 C. was 240 min. The time to reach 122 MPa was 100 min. The UCA chart is shown in FIG. 2.

(23) The strength reached a maximum value after about 150 hr. Then the strength began to decrease, and was still decreasing after 1260 hr when the test was terminated.

(24) The cement sample was removed from the UCA and cored for measurement of actual compressive strength and water permeability. The compressive-strength result was 12.2 MPa (1770 psi). The water permeability was 0.15 mD, which those skilled in the art would recognize as being too high. For proper zonal isolation, the maximum allowable permeability value is generally considered to be 0.1 mD.

(25) Next, the sample was ground to a fine powder and dried first with acetone and then with ethyl ether. The crystalline composition of the powder was analyzed by x-ray diffraction. The cement matrix was mainly composed of andradite and quartz. Small amounts of xonotlite and hematite were detected.

(26) Another UCA test was performed with this cement formulation. In this case, the test was terminated after only 216 hr. The compressive strength of the cement core was 27.4 MPa (3975 psi), and the water permeability was below 0.007 mD (the detection limit of the equipment). The cement matrix was mostly composed of xonotlite, quartz and hematite. This result shows that xonotlite was initially the principal binding phase but, with time, was consumed by reacting with hematite.

Example 3

(27) Next, titanium oxide (TiO.sub.2, also known as rutile) was used. Its specific gravity is 4.15.

(28) A solid blend was prepared with the following composition: 35% by volume of blend (BVOB) Dyckerhoff Black Label Class G cement (median particle size 15 m), 40% BVOB silica sand (median particle size 315 m), 10% BVOB silica flour (median particle size 3 m), 5% BVOB hematite (median particle size 32 m) and 10% BVOB rutile (Ti-Pure R-902, available from DuPont Titanium Technologiesmedian particle size 0.6 m). To this mixture, 1.5% by weight of blend (BWOB) bentonite was added. The difference between this blend and the one of Example 1 is the replacement of 10% BVOB Micromax with the same volume of titanium oxide.

(29) A fluid was prepared with the following composition: 4.17 L/tonne of blend silicone antifoam agent, 66.8 L/tonne retarder (a blend of sodium pentaborate and pentasodium EDTMPweight ratio: 9.3), 0.75% BWOB styrene sulfonate-maleic anhydride copolymer (NARLEX D72, available from Akzo Nobel), 0.8% BWOB fluid-loss additive (UNIFLAC) and sufficient water to prepare a slurry with a solid-volume (SVF) of 0.61. The slurry density was 2235 kg/m.sup.3 (18.65 lbm/gal).

(30) The slurry was placed in a UCA instrument, and cured at a final temperature of 302 C. (575 F.) and pressure of 122 MPa (17,700 psi). The heat-up time to reach 274 C. (525 F.) was 100 min, and the total heat-up time to reach 302 C. was 240 min. The time to reach 122 MPa was 100 min. The UCA chart is shown in FIG. 3.

(31) The strength reached a maximum value after about 200 hr. Then the strength began to decrease and reached a plateau after about 900 hr. XRD analysis revealed that the cement matrix was mainly composed of titanite (CaTiSiO.sub.5) and schorlomite [Ca.sub.3(Fe,Ti).sub.2((Si,Ti)O.sub.4).sub.3]. Very small amounts of xonotlite and rutile were detected.

Example 4

(32) A solid blend was prepared with the following composition: 35% by volume of blend (BVOB) Dyckerhoff Black Label Class G cement (median particle size 15 m), 40% BVOB silica sand (median particle size 315 m), 10% BVOB silica flour (median particle size 3 m), 5% BVOB barite (median particle size 17 m) and 10% BVOB barite (median particle size 1.5 m). To this mixture, 1.5% by weight of blend (BWOB) bentonite was added. The difference between this blend and the one of Example 1 is the replacement of 10% BVOB Micromax with the same volume of very fine barite, and the replacement of 5% BVOB hematite with the same volume of barite with a larger median particle size.

(33) A fluid was prepared with the following composition: 4.17 L/tonne of blend silicone antifoam agent, 66.8 L/tonne retarder (a blend of sodium pentaborate and pentasodium EDTMPweight ratio: 9.3), 0.75% BWOB styrene sulfonate-maleic anhydride copolymer (NARLEX D72, available from Akzo Nobel), 0.8% BWOB fluid-loss additive (UNIFLAC) and sufficient water to prepare a slurry with a solid-volume (SVF) of 0.6. The slurry density was 2222 kg/m.sup.3 (18.54 lbm/gal).

(34) The slurry was placed in a UCA instrument, and cured at a final temperature of 302 C. (575 F.) and pressure of 122 MPa (17,700 psi). The heat-up time to reach 274 C. (525 F.) was 100 min, and the total heat-up time to reach 302 C. was 240 min. The time to reach 122 MPa was 100 min. The UCA chart is shown in FIG. 4.

(35) The strength reached a maximum value after about 150 hr. Then the strength began to slowly decrease, and reached a plateau after about 500 hr. The UCA test was terminated after 600 hr.

(36) The cement sample was removed from the UCA and cored for measurement of actual compressive strength and water permeability. The compressive-strength result was 26 MPa (3770 psi). The water permeability was 0.008 mD. Unlike the previous tests, these results were acceptable.

(37) Next, the sample was ground to a fine powder and dried first with acetone and then with ethyl ether. The crystalline composition of the powder was analyzed by x-ray diffraction. The cement matrix was mainly composed of xonotlite, quartz and barite, indicating the barite behaves as a chemically inert filler under HPHT conditions.