Additive for oil well cement, and cement composition and cement slurry both including said additive for oil well cement

Abstract

Provided is a PVA-containing additive for oil well cement superior in fluid loss-reducing property. Specifically, provided is an additive for oil well cement, the additive containing polyvinyl alcohol having a saponification value of 75 to 85 mol % and a viscosity-average polymerization degree of 2,800 to 4,500. Also provided are a cement composition containing the additive for oil well cement in an amount of 0.01 to 30% bwoc and a cement slurry containing the additive for oil well cement in an amount of 0.01 to 30% bwoc.

Claims

1. An additive for oil well cement comprising polyvinyl alcohol particles containing a polyvinyl alcohol having a saponification value of 75 to 85 mol % and a viscosity-average polymerization degree of 2,800 to 4,500, wherein the polyvinyl alcohol particles comprise particles having a particle diameter in a range greater than 0 μm and not more than 75 μm in an amount of a range greater than 0 and not more than 25 mass % in 100 mass % of the particles, particles having a particle diameter of 500 μm or more in an amount of a range equal to or greater than 0 and not more than 10 mass % in 100 mass % of the particles, and particles having a particle diameter in a range greater than 75 μm to less than 500 μm in an amount of balance in 100 mass % of the particles.

2. The additive for oil well cement according to claim 1, wherein the viscosity-average polymerization degree is 2,800 to 3,800.

3. A cement composition containing an additive for oil well cement in an amount of 0.01 to 30% bwoc, wherein the additive comprises polyvinyl alcohol particles containing polyvinyl alcohol having a saponification value of 75 to 85 mol % and a viscosity-average polymerization degree of 2,800 to 4,500, and wherein the polyvinyl alcohol particles comprise particles having a particle diameter in a range greater than 0 and not more than 75 μm in an amount of a range greater than 0 and not more than 25 mass % in 100 mass % of the particles, particles having a particle diameter of 500 μm or more in an amount of a range equal to or greater than 0 and not more than 10 mass % in 100 mass % of the particles, and particles having a particle diameter in a range greater than 75 μm to less than 500 μm in an amount of balance in 100 mass % of the particles.

4. The cement composition according to claim 3, wherein the additive for oil well cement is added in an amount of 0.01 to 5% bwoc.

5. A cement slurry containing an additive for oil well cement in an amount of 0.01 to 30% bwoc, wherein the additive comprises polyvinyl alcohol particles containing polyvinyl alcohol having a saponification value of 75 to 85 mol % and a viscosity-average polymerization degree of 2,800 to 4,500, and wherein the polyvinyl alcohol particles comprise particles having a particle diameter in a range greater than 0 and not more than 75 μm in an amount of a range greater than 0 and not more than 25 mass % in 100 mass % of the particles, particles having a particle diameter of 500 μm or more in an amount of a range equal to or greater than 0 and not more than 10 mass % in 100 mass % of the particles, and particles having a particle diameter in a range greater than 75 μm to less than 500 μm in an amount of balance in 100 mass % of the particles.

Description

EXAMPLES

(1) Hereinafter, the present invention will be described more in detail with reference to Examples. The Examples described below are only some examples of the typical Examples of the present invention and it should be understood that the scope of the present invention shall not be restricted by these Examples. The “part” and “%” below respectively mean “part by mass” and “mass %,” unless specified otherwise.

(2) <Preparation of PVA>

Example 1

(3) 100 parts by mass of vinyl acetate, 5.3 parts by mass of methanol, and 0.02 mol % of azobisisobutyronitrile were placed in a polymerization tank equipped with a reflux condenser, a dropping funnel, and a stirrer and the mixture was allowed to polymerize, as it was stirred at its boiling temperature for 4.0 hours under a nitrogen gas stream. Unreacted vinyl acetate monomer was then discharged out of the polymerization system, to give a methanol solution of a polyvinyl acetate having a polymerization degree of 3,500.

(4) A methanol solution of sodium hydroxide (0.004 mole of sodium hydroxide to vinyl acetate) was added to the polyvinyl acetate methanol solution thus obtained and the mixture was subjected to saponification reaction at 40° C. for 150 minutes. The reaction solution obtained was dried under heat, to give a PVA of Example 1 having a saponification value of 80 mol %.

(5) After drying, the PVA particles were sieved with a screen having an opening of 500 μm. The PVA particles remaining on the screen were pulverized in a pulverizer and mixed thoroughly with the PVA particles previously sieved, to give a PVA containing particles of a particle diameter of 75 μm or less in an amount of 22%.

Examples 2 to 6 and Comparative Examples 1 to 71

(6) PVAs of Examples 2 to 6 and Comparative Examples 1 to 7 were obtained in a manner similar to Example 1, except that the amount of methanol during polymerization, the amount of sodium hydroxide during saponification, and the opening of the screen used during particle diameter adjustment were changed to those shown in Table 1 below.

(7) <Calculation of Viscosity-Average Polymerization Degree of PVA>

(8) The limiting viscosity [η] (g/dL) of the PVAs obtained in Examples 1 to 6 and Comparative Examples 1 to 7, was determined and the viscosity-average polymerization degree was calculated according to the Formula (1) above.

(9) <Evaluation of Fluid Loss-Reducing Property>

(10) The fluid loss-reducing property of PVA was determined according to the fluid loss test method of a specification of American Petroleum Institute (API) 10B-2 (April, 2013) and the fluid loss obtained was expressed by cc. In the following Examples, it was determined at a slurry density of 1,900 kg/m.sup.3. The amount of PVA added was determined under a condition of 0.25% bwoc at an evaluation temperature of 20° C., 0.4% bwoc at 40° C., or 0.6% bwoc at 60° C.

(11) <Results>

(12) Results are summarized in the following Table 1.

(13) TABLE-US-00001 TABLE 1 Example Comparative Example 1 2 3 4 5 6 7 1 2 3 4 5 6 Polyvinyl acetate polymerization condition 5.3 5.3 5.3 2.3 10 5.3 10 15 15 15 15 20 10 Methanol (parts) Saponification condition 0.004 0.008 0.002 0.004 0.004 0.004 0.010 0.015 0.010 0.004 0.012 0.016 0.0010 Sodium hydroxide (mole)*1 Particle diameter adjustment condition 300 300 300 300 300 500 500 300 300 300 180 500 300 Screen opening (μm) Properties Saponification value (mol %) 80 84 76 80 80 80 80 96 88 80 91 99 88 of PVA Viscosity-average 3500 3500 3500 3800 2800 3500 2800 2400 2400 2400 2400 1700 2800 polymerization degree Particle 75 μm or less (%) 0 0 0 0 0 1 0 0 0 0 0 0 0 diameter 500 μm or more (%) 22 23 22 22 23 10 45 22 23 24 44 8 22 Fluid 20° C. (cc) 12 14 13 11 12 10 19 18 14 18 24 460 14 loss 40° C. (cc) 12 17 14 12 22 11 45 48 26 48 590 *2 14 60° C. (cc) 10 30 15 11 50 11 90 900 690 140 *2 *2 1070 *1 moles with respect to vinyl acetate *2 not determined

(14) As shown in Table 1, the cement slurries of Examples 1 to 7, which contained a PVA having a saponification value of 75 to 85 mol % and a viscosity-average polymerization degree of 2,800 to 4,500, gave a fluid loss smaller than those of Comparative Examples 1 to 6 which contained a PVA having a saponification value and/or a viscosity-average polymerization degree outside the scope of the present invention. More specifically, for example when Example 1 and Comparative Example 3, wherein a PVA having a saponification value of 80 mol % was used, are compared, the cement slurry containing a PVA having a viscosity-average polymerization degree of less than 2,800 of Comparative Example 3 gave a larger fluid loss, compared to the cement slurry of Example 1 containing a PVA having a viscosity-average polymerization degree in the range of 2,800 to 4,500. Alternatively when Example 5 and Comparative Example 6, wherein the a PVA having a viscosity-average polymerization degree of 2,800 was used, are compared, the cement slurry of Comparative Example 6 containing a PVA having a saponification value of more than 85 mol % gave a significantly larger fluid loss at 60° C., compared to the cement slurry of Example 5 containing a PVA having a saponification value in the range of 75 to 85 mol %.

(15) These results suggest that use of a PVA having a saponification value of 75 to 85 mol % and a viscosity-average polymerization degree of 2,800 to 4,500 leads to improvement in fluid loss-reducing property.

(16) When the cement slurries of Examples are compared, the slurry of Example 1 containing a PVA having a saponification value in the range of 78 to 82 mol % gave a smaller fluid loss, compared to the cement slurries of Examples 2 and 3 respectively containing a PVA having a saponification value outside the range above. The result suggests that the saponification value of the PVA contained in the additive for oil well cement according to the present invention is more preferably controlled to 78 to 82 mol %.

(17) Further, the cement slurry of Example 1 containing a PVA having a viscosity-average polymerization degree of 3,000 or more gave a smaller fluid loss, compared to the cement slurry of Example 5 containing a PVA having a viscosity-average polymerization degree of less than 3,000. The result suggests that the viscosity-average polymerization degree of the PVA contained in the additive for oil well cement according to the present invention is more preferably controlled to 3,000 or more.

(18) Further when the cement slurries of Examples 5 and 7, which contained a PVA having the same saponification value and viscosity-average polymerization degree, are compared, the cement slurry of Example 5 containing PVA particles having a PVA particle diameter of 75 μm or less in an amount of 30% or less gave a smaller fluid loss. Alternatively when the cement slurries of Examples 1 and 6, which contained a PVA having the same saponification value and viscosity-average polymerization degree, are compared, the cement slurry of Example 6 containing PVA particles having a particle diameter of 75 μm or less in an amount of 15% or less gave a smaller fluid loss. The result suggests that the content of PVA particles having a particle diameter of 75 μm or less in the additive for oil well cement according to the present invention is preferably 30% or less, more preferably 15% or less.