The quality of oil well cement is very important in maximizing the rate of production from an oil well. Good bonding between cement and casing also between cement and rock formation are essential for effective zone isolation, hence usually the additive should be added in order to improve the characteristics of standard cement.
The purposes of this research are to find the effect of adding expanding material, burnt pure-MgO, on the value of compressive strength and shear bond strength of API class G in high pressure and high temperature condition.
The method that has been used in this research is evaluating data from simulator, which was set up similar to field condition, 100 - 200°C and 2000 psi.
According to the research, some conclusion can be drawn: adding expanding additives, in fact, will increase shear bond strength but will reduce compressive strength although generally still higher than recommended minimum value. The Appearance of mud cake will decrease shear bond strength. For compositions without expanding additives, shear bond strength was dropped drastically. MgO shows excellent performance, as expanding additive, in increasing the shear bond strength.
For low conditioning temperature, 100 - 135°C, MgO that are burnt at 1000°C are reliable, whereas for higher conditioning temperature up to 200°C, the pure MgO which are burnt at 1200°C, are more convenient. The concentration of expanding additives, which could be used to increase the shearbond strength of cements, are around 5%.
In cementing an oil well, the cement slurry is pumped through the casing and then back up through the annular space between the bore hole wall and the casing. Generally, the purpose of cementing job is to14,19,20:
Support the casing.
Protect casing against underground environment effect, i.e. high pressure and temperature, and formation fluids.
Shut off lost circulation zone and over pressure zone.
Isolate space beyond casing to avoid interlines communication, especially at producing zone.
Prevent gas or high-pressure formation fluids movement into annulus casing-wellbore that caused trouble at the surface, such as fire.
Reduce gas oil ratio, water oil ratio, and water gas ratio.
Minimize casing wear.
Close an abandoned portion of the well.
Squeeze perforated zone, etc.
The main ingredients in almost all drilling cements is Portland Cement and some cement additives to provide performance that are needed and also water as a mixing fluid.
Portland cement made of limestone, that is consists of carbonate, marl and oysters shell. Beside of them, it is made of argillaceous materials such as clay, shale and iron ore. Also some chemical additives are used to provide acceptable cement characteristics.
For several years the construction industry has fought the problem of drying shrinkage in concrete. Standard concrete presents a reduction in volume as they dry after setting. This volume reduction can produce due to the development of tensile strengths. Expansive cement is manufactured to compensate for this drying shrinkage by expanding during this critical period.
The oil industry has recognized the benefits of expanding cement that could generate a better bond between casing-cement and cement-formation. These expansive cements undergo expansions that, as previously reported, are substantially larger than the expansions presented by conventional oil well cements at shallow depths.