Study on a Cement Slurry for Cementing Uncemented Shallow Section of Old Casing String by Secondary Displacement
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摘要: 胜利油田经过了二十多年的开发，早期的开发井油层套管水泥面以上的套管腐蚀破坏严重，严重干扰正常生产。为此，拟对浅部地层自由套管段进行二次固井，可在修复套管密封失效的同时防止套管进一步腐蚀。针对环空二次固井注浆施工工艺特点，结合浅部地层松软的特点，在明确浅部地层自由套管外二次固井性能需求的基础上，对水泥浆体系进行开发。通过对胶凝材料体系、低黏触变剂、降失水剂和缓凝剂的研选，构建出一种低密度-低黏度-强触变-长稠化时间的固井水泥浆体系，其配方为：40%G级油井水泥+30%粉煤灰+30%矿渣+（20%～25%）低黏触变剂L-TA+2.5%聚乙烯醇降失水剂+（0.16%～0.32%）氧化锌+1%分散剂+0.5%消泡剂，水固比为0.56，水泥浆密度低至1.5～1.6 g/cm3，黏度小于50 mPa·s，稠化时间大于15 h，平均静切力差值为17.885 N/m2、且触变具有可重复性，满足东部老区浅部地层自由套管外二次固井作业的要求。Abstract: In Shengli Oilfield, after more than 20 years’ development, the production casing strings above the top of the set cement in some early development wells have been seriously damaged by corrosion, and this badly hinders the normal production of the oil and gas wells. To deal with this problem, it was proposed to conduct secondary well cementing to the free section of the production casing string in the shallow formations. This secondary well cementing not only repairs the integrity of the casing string, it also protects the casing string from further corrosion. Several factors were considered in developing the required cement slurry, such as the technical characteristics of cement slurry injection into the annulus, the nature of the shallow formations being soft, and the requirements of the secondary cementing operation outside the free section of the production casing string. A low-density low-viscosity strong-thixotropy long-thickening time cement slurry was formulated with carefully selected gel agent, low-viscosity thixotropic agent, fluid loss additive and retarder. The composition of the cement slurry is as follows: 40% class G oil well cement + 30% fly ash + 30% slag + (20% – 25%) low-viscosity thixotropic agent L-TA + 2.5% polyvinyl alcohol fluid loss additive+ (0.16% – 0.32%) ZnO + 1% dispersant + 0.5% defoamer. Water/solids ratio of this cement slurry is 0.56. This cement slurry has a density that is as low as 1.5 – 1.6 g/cm3, viscosity of less than 50 mPa∙s, thickening time of longer than 15 h, difference of average gel strengths of 17.885 N/m2, as well as repeatable thixotropy. Field application proved that this cement slurry satisfied the requirements of the secondary cementing outside the free section of the casing string in the shallow formations in Shengli Oilfield.
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表 1 不同加量球形L-TA触变剂水泥浆触变性测试
L-TA/% φ600 φ300 φ200 φ100 φ6 φ3 φ3' △φ3 静切力差值/
N·m−110 135 109 89 78 47 39 40 1 0.511 15 117 101 83 75 44 36 51 15 7.665 20 109 85 81 70 41 33 88 55 28.105 25 89 61 56 49 30 20 55 35 17.885 30 63 52 46 43 27 25 57 32 16.352 注：基础配方为40%G级油井水泥+30%矿渣+30%粉煤灰+L-TA，水固比为0.56；φ3'为静置10 min后测得φ3读值 
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表 2 降失水剂的降失水性能测试及对水泥浆触变性影响
降失水剂 φ600 φ300 φ200 φ100 φ6 φ3 φ3' 静切力差值/
N·m−1FL/
mL0.5% BCG 105 65 46 25 0.5 0 3 1.533 47 0.5% BXF-200L 75 41 29 15 1.0 0 4 2.044 51 2.5% G60S 37 18 12 6 0 0 0 0 49 2.5% PVA 135 79 56 33 11.0 11 56 28.616 50 注：配方为40%G级油井水泥+30%矿渣+30%粉煤灰+20%L-TA，水固比为0.56；φ3'为静置10 min后测得φ3读值
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表 3 缓凝剂的缓凝性能测试及对水泥浆触变性影响
降失水剂 φ600 φ300 φ200 φ100 φ6 φ3 φ3' 静切力差值/
N·m−1t稠化/
h2.0%GH-9 35.0 17 12 6.0 1 1 3 1.533 1.5% GH-9 48.0 34 28 23.0 22 21 75 38.325 7 0.16%氧化锌 76.5 59 55 48.0 35 29 75 38.325 16 0.32%氧化锌 65.0 49 45 38.5 26 23 69 35.259 ＞20 注：配方为40%G级油井水泥+30%矿渣+30%粉煤灰+20%L-TA+2.5%聚乙烯醇降失水剂，水固比为0.56；φ3'为静置10 min后测得φ3读值
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