The cement used for oil well cementing differs from concrete or masonry work in that it consists of a thin slurry of primarily cement and water. The cement used in oil wells must possess three primary properties. They must possess a proper water-to-cement ratio, a sufficient fluid time to allow placement, and must develop enough strength in a minimum time to bond the pipe to the formation.
HYDRATION OF CEMENT
Hydration (reaction with water) begins when water is added to cement. The cement slurry gradually sets to a solid as hydration continues. After hydration begins, which initiates the setting, the process slows, and the strength of the set cement continues to increase for many days.
ACCELERATION AND RETARDATION
Anything that will have an effect on the chemical reaction influences the degree of acceleration. Factors that could influence the reaction rate are: pressure, temperature, concentration of each chemical or ionized particles present, and the chemical nature of the combined chemicals present.
Cement setting is retarded by use of an additive either at the time of manufacture or at the time of use. The retardation of slow set cements is due chiefly to the addition of chemical retarders such as borax and starch, which are added at the time of manufacture. Another method is to adjust the particle size (grind) of the cement. The cement will set more slowly if it is coarsely ground.
Cement is accelerated about the same way that it is retarded by the manufacturer. It is performed by composition adjustment, particle size adjustment, and by the addition of a chemical accelerator. The addition of a chemical accelerator is the most effective way to accelerate the setting of
cement.
Calcium is the most effective and economical accelerator for Portland cement. Calcium Chloride slightly reduces the viscosity of Portland cement slurries. The more calcium chloride added to cement, the more pronounced the acceleration. Optimum concentration of calcium chloride for early strength is reached between 2% and 4% by weight of the dry cement. More or less will not produce higher strength. Any strength greater than 5% lowers strength. The usual effect of 2% Calcium Chloride is to reduce the thickening time by one half and to double the twenty-four hour strength.
COMMONLY MEASURED CEMENT PROPERTIES
There are several properties of Portland cement, which are commonly measured. These properties are thickening time, compressive strength, slurry volume, free water separation, and hydraulic flow properties. The thickening time is the amount of time necessary for the slurry to reach a consistency of 100 poises at different well temperature, depth, and pressure conditions. The compressive strength is the force per unit internal cross-sectional area in psi necessary to crush the cement specimen. The free water separation is the measurement of the water loss of the cement expressed in volume per unit time. It is directly proportional to the water to cement ratio. The hydraulic flow properties are the rheological properties of the cement necessary to make critical velocity calculations.
The calculation of slurry volumes will usually be based upon water to cement ratios. The water to cement ratio is the ratio of the weight of water to a unit weight of dry cement. The ratio of water to cement is important because cementing material must contain sufficient water to make it pumpable, yet not have setting with free water separation. The two terms used for water ratios are maximum and minimum. Maximum water is the greatest amount that can be mixed with cement and produce a set volume equal to the slurry volume. Minimum water is the least amount that can produce a pumpable slurry. The specific gravity of the cement is calculated to be 3.15.
A commonly used water to cement ratio is 0.46, which means 46 grams of
water to 100 grams of dry cement.
The results of the thickening time tests on the Consistometer. The Consistometer readings are plotted on common graph paper with the consistency as the ordinate and time as the abscissa. The general shape of the consistency time curve plotted as previously described presents a picture of a particular cement as far as it’s setting characteristics are concerned. There are certain features common to all consistency-time curves. As the cement is introduced, it generally has a fairly low consistency. This value usually drops a little as the stirring is begun. It then begins to increase at a very gradual rate. As time goes on, the rate of increase of the consistency is accelerated to such an extent that the latter part of the curve is very steep. This acceleration varies with different cements and with different temperature of testing.
The two main flow properties of a cement slurry are shear rate and shear stress. These properties permit the determination of two slurry properties: (1) Flow Behavior Index, n’, and (2) Consistency Index, K’. These factors will then allow estimation of the pumping rate for turbulence of slurry in the annuals, frictional pressure drop of slurry in the annulus and pipe, and hydraulic horsepower necessary to overcome friction losses for non-Newtonian fluids. The flow curve, which is constructed to obtain the Flow Behavior Index and the Consistency Index, is prepared using a Fann V-G meter by plotting shear stress (pounds force/square foot) on the ordinate and the shear rate (sec -1) on the abscissa on logarithmic coordinate paper.
Shear Stress = Dial Reading of Viscometer x N x 1.066÷100
where N = range extension factor of the torque spring
Shear Rate = (1.703)x (RPM of Viscometer)
When the data points do not form a straight line flow curve, the best straight line through these values should be drawn and extrapolated to the shear stress axis. The Flow Behavior Index is equal to the slope of the flow curve and is dimensionless. The Consistency Index is equal to the intercept of the flow curve at the unity rate of shear with the units (lb-secn’ /ft²).
