The oil well cement is Portland cement. Mixing the limestone, clay and some iron ore together according to suitable percentage, and then grind and get the raw material. The raw material burned under high temperature can produce a kind of clinker which is with calcium silicate as the main ingredients. The oil well cement is produced by grinding the clinker and some gypsum.
The cement clinker is constituted by the following four minerals.
1.Tricalcium silicate, 3CaO·SiO2(C3S). Take 40%-60% in the cement. It generates the strength of set cement. And it has the main effect to the cement early strength. The content of the C3S is high in the cement which has good early strength.
2. Dicalcium silicate, 2CaO·SiO2(C2S). Take 15％~35％ in the cement. It is slow hydration mineral. It can make the cement strength improving slowly and lasting a long time.
3. Tricalcium aluminate, 3CaO·Al2O3(C3A). Take 6％~15％ in the cement. It can promote the cement hydrating quickly. And it is the main ingredient which can decide the thickening time and the initial thickening time.
4. Tetracalcium aluminoferrite, 4CaO·Al2O3·Fe2O3(C4AF). Take 8％~10％ in the cement. If the content of the C4AF increasing, the cement strength will decrease.
There are nine classes in the API. They are A, B, C, D, E, F, G, H, and J.
Class A cement belongs to Portland cement series. It have only one type--the ordinary type (similar with the I type in ASTM standard). The usage depth of it changes from the surface of the earth to 1830m well depth. It is only can be used as the general cement.
Class B cements’s suitable well depth is similar with class A, but it also has salt resistant property. The class B cement is divided into middle sulfate-resistant and high sulfate-resistant two kinds. And the suitable well depth changes from the surface of the earth to 1830m depth. The middle sulfate-resistant class B cement is similar with the Ⅱtype in ASTM standard. Compared to class A cement, the class B cement just need lower C3A content, in order to satisfied the sulfate-resistant requirement.
Class C cement is suit from the surface of the earth to the 1830m depth. It is divided into ordinary type, middle sulfate-resistant and high sulfate-resistant three kinds. The content of C3A is higher and the specific surface area of the cement is larger. So it has the early strength property. Class C cement is similar with the Ⅲ type in ASTM standard. In order to satisfy the requirements of early strength and sulfate-resistant, class C cement is produced by grinding the specific composition clinker.
The main factors that influence the class C cement properties including clinker, gypsum dosage and the finesses etc.
Different class C cement has different property requirements. The main different is the sulfate resistant property. From the angle of clinker, adjusting the C3A content can control the resistance of the sulfate-resistant performance and also can control the clinker strength. Enough C3S has significant effect to the cement 24h compressive strength.
2. Gypsum dosage
The cypsum dosage has significant effect to the cement physical properties. Increasing the gypsum, the thinkening time will extend and the compressive strength will increase. Research show that the thinkening time will extend about 60% when the content of SO3 increasing from 1.5% to 3.0%. The compress strength increases about 10%-30%, especially the 8h compressive strength. So the SO3 content should be controlled in 2.5%-3.0%.
3. The finesses of the cement
In order to ensure the cement having a higher early strength, the class C cement should be grinded very fine, and the specific surface area should be controlled in 440~460m2/kg. It is should emphasize that the grinding aid is necessary, when grinding. And if having not grinding aid, the grinding efficiency will decrease and the powder consumption will increase sharply. Mixing a amount of triethanolamine into the cement as grinding aid, can shorten the time grinding and improve the compressive strength of the cement.
Class D, E, F cement is suit for middle depth well and explore well. They are all divided into middle sulfate-resistant type and high sulfate resistant type. They are called retard cement and suit for deeper well. This kind cement is produced by decreasing sharply the content of C3S and C3A and increasing the cement particle size for the purpose of retarding. But the retarder has a great improvement now, so the usage amount of them begins to decrease.
Class D cement is used under middle temperature and middle pressure condition in 1830-3050m depth well and also can be used as MSR and HSR type cement.
Class E cement is used under high temperature and high pressure condition in 3050~4270m depth well and also can be used as MSR and HSR type cement.
Class F cement is used under super high temperature and high pressure condition in 3050~4880m depth well and also can be used as MSR and HSR type cement.
Class D, E, F cement is different from other kind cement. They are used in middle-deep and deep well operation under higher temperature and pressure. As mentioned before, the general cement slurry thinkening time will shorten and compressive strength will decay if they are used under high temperature and high pressure condition. This is because, the high temperature and pressure changes the slurry hydrating process and the hydration products composition and morphology structure. In order to ensure the quality of cementing under high temperature and pressure condition, specific composition clinker is needed. But this method is complex, produce control is hard and cost is high. So most of the countries use the following method to satisfy the cementing requirements. Using one or two cement as foundation and adding different additives to adjust the cement properties. The additives using in the United States are the most. And the well cement additives reach more than 1000. According to the API standard, class G and H cement is the basic varieties of cement. Class D, E, F cement can be produced by adding some retarders to the class G or H cement.
Class G and H cement is two basic oil well cement. The so called basic oil well cement has two contents. The one is this kind cement is not allowed to added other additives except some gypsum. And the other one is this kind basic cement use with many kinds additives and can satisfy larger well depth and temperature. Both class G and H cement can be divided into two types which is middle sulfate-resistant and high sulfate-resistant. If they are used alone, the suitable well depth is from the earth surface to the 2440m depth.
1. The main technical performance requirements of class G and H cement
They have a lower initial consistency, and the thickening time and compressive strength are all good and completely meet the API Spec 10 requirements. The water cement ratio of class H is 0.38 which is different from the class G. Except that, the other technical properties are all the same with the class G cement.
2. The influencing factors of high sulfate-resistant G cement properties
In practice, the dosage of raw gypsum and the cement fineness have significant effect to the cement physical properties. The raw gypsum dosage affects the slurry thickening time and 8h compressive strength. The compressive strength increases quicker with the gypsum dosage increasing. The initial consistency and 8h compressive strength increase and the thickening time become short when the fineness of the cement is increasing. So there is a strict requirement on the cement fineness and gypsum dosage in practice.
3.The production and properties of middle sulfate-resistant class H cement
The middle sulfate-resistant class H cement is the main oil well cement produced in American. This kind cement has loose requirements to the quality of raw material and fuel and production process. The specific surface area is only 270~300m2/g, and it is beneficial for factory production organization and cost reduction.
4.The application of class G and H cement
Class G and H cement is the most widely used cement at present. They are used in the general cementing from the surface of the earth to 2440m well depth. When the accelerator or retarder is added to the cement, there are will have a wider suitable well depth and temperature. Class G and H cement is used as middle sulfate-resistant and high sulfate-resistant cement.