Well Design - PE 413 Chapter 3: Cements

Introduction Overview The basic principle of oil well cementing involves displacing cement slurry down the casing to a predetermined point in the well. The slurry is formed by mixing water with Portland cement, or with cement blended with additives. The casing must be cemented to exclude water and other unwanted fluids. Cement slurry is forced into the annular space between the casing and the wall of the hole, where the cement can set and form a permanent barrier against water and other fluids.

Introduction Overview Cement that is pumped down into the annulus to hold the casing in place and is used as a sealant to help protect: Casing and wellbore from external pressure that could collapse the pipe or cause a blowout Oil- and gas-producing strata from undesired fluids Casing from possible corrosion and electrolysis caused by formation waters and physical contact with various strata Downhole production and drilling equipment Pipe from the stresses of formation movement

Introduction Overview The cement composition and placement technique for each job must be chosen so that the cement will achieve an adequate strength soon after being placed in the desired location. This minimizes the waiting period after cementing. However, the cement must remain pumpable along enough to allow placement to the desired location. The main ingredient in almost all drilling cements is Portland cement, artificial cement made by burning a blend of limestone and clay. This is the same basic type of cement used in making concrete.

Introduction Cement Cement is composed principally of a blend of anhydrous metallic oxides. The addition of water to this material converts these compounds to their hydrated form. After a period of time, the hydrates form an interlocking crystalline structure which is responsible for the set cement's strength and impermeability.

Composition of Portland Cement The main components of common Portland cement are 50% tricalcium silicate (3CaO·SiO2) - C3S 25% dicalcium silicate (2CaO·SiO2) – C2S 10% tricalcium aluminate (3CaO·Al2O3) - C3A 10% tetracalcium aluminoferrite (4CaO·Al2O3·Fe2O3) - C4AF 5% other oxides

Composition of Portland Cement 2(3CaO.SiO2) + 6H2O --> 3CaO.2SiO2.3H2O + 3Ca(OH)2 2(2CaO.SiO2) + 4H2O --> (slow)3CaO.2SiO2.3H2O + Ca(OH)2 4CaO.Al2O3.Fe2O3 + 10H2O + 2Ca(OH)2 --> (slow)6CaO.Al2O3.Fe2O3.12H2O + Ca(OH)2 3CaO.Al2O3 + 12H2O + Ca(OH)2 --> (fast)3CaO.Al2O3.Ca(OH)2.12H2O 3CaO.Al2O3 + 10H2O + CaSO4.2H2O --> 3CaO.Al2O3.CaSO4.12H2O Oxide Lime (CaO or C) Silica (SiO2 or S) Alumina (Al2O3 or A) Ferric Oxide (Fe2O3 or F) Magnesia (MgO) Sulfur Trioxide (SO3) Ignition loss

Composition of Portland Cement API uses the following equations for calculating the weight percent of the crystalline compounds from the weight percent of the oxides present. C3S = 4.07C – 7.6S – 6.72A – 1.43F – 2.85SO3 C2S = 2.87S – 0.754C3S C3A = 2.65A – 1.69F C4AF = 3.04F These equations are valid as long as the weight ratio of Al2O3 to Fe2O3 present is greater than 0.64

Example Example: Calculate the percentages of C3S, C2S, C3A, and C4AF from the following oxide analysis of a standard Portland cement. Oxide Weight Percent Lime (CaO or C) Silica (SiO2 or S) Alumina (Al2O3 or A) Ferric Oxide (Fe2O3 or F) Magnesia (MgO) Sulfur Trioxide (SO3) Ignition loss 65.6 22.2 5.8 2.8 1.9 1.8 0.7

Example The A/F ratio is 5.8/2.8 = 2.07. C3S = 4.07C – 7.6S – 6.72A – 1.43F – 2.85SO3 C3S = 4.07(65.6) – 7.6(22.2) – 6.72(5.8) – 1.43(2.8) – 2.85(1.8) = 50.16% C2S = 2.87S – 0.754C3S C2S = 2.87(22.2) – 0.754(50.16) = 25.89% C3A = 2.65A – 1.69F C3A = 2.65(5.8) – 1.69(2.8) = 10.64% C4AF = 3.04F C4AF = 3.04(2.8) = 8.51%

API Tests for Cementing Cement specifications almost always are stated in terms of these API standard tests. The test equipment needed to perform the API tests includes: A mud balance for determining the slurry density, A filter press for determining the filtration rate of the slurry, A rotational viscometer for determining the rheological properties of the slurry, A consistometer for determining thickening rate characteristics of the slurry Specimen molds and strength testing machines for determining the tensile and compressive strength of the cement A cement permeameter for determining permeability of the set cement,

API Tests for Cementing Mud Balance – Slurry Density Test The test consists essentially of filling the cup with a mud sample and determining the rider position required for balance. Water is usually used for the calibration fluid. The density of fresh water is 8.33 lbm/gal.

API Tests for Cementing Filter Press – Fluid Loss Test The rate at which a cement slurry loses the water required for its fluidity through a permeable barrier is called filtration rate or fluid-loss rate. The standard API filter press has an area of 45 cm2 and is operated at a pressure of 100 psig (6.8 atm). The filtrate volume collected in a 30-min time period is reported as the standard water loss.

API Tests for Cementing Rotational Viscometer The mud is sheared at a constant rate between an inner bob and an outer rotating sleeve. Six standard speeds plus a variable speed setting are available with the rotational viscometer.

API Tests for Cementing Rotational Viscometer Flow curves of time-independent fluids Newtonian fluids: Power law fluids: Bingham fluids: Herschel-Bulkley (Yield power law fluids)

API Tests for Cementing Rotational Viscometer where: ma(cp) - apparent viscosity, qN - dial reading in degrees, N(RPM) - rotor speed, mp(cp) - plastic viscosity, and ty (lbf/100ft2) - shear stress, and yield stress g(1/s) - shear rate, m and mp(cp) - fluid viscosity and Bingham viscosity, K (lbfxsn/100ft2) - consistency index, n - flow behavior index. For Bingham fluid:

API Tests for Cementing Rotational Viscometer The data below are obtained from a rotational viscometer. Determine type of fluid and the rheological model of this fluid. RPM Dial Reading 3 10 6 12 100 35 200 48 300 60 600 75

API Tests for Cementing Cement Consistometer – Thickening Time Test A device used to determine the thickening time of cement slurries under simulated downhole pressure and temperature conditions. The thickening time is a measurement of the time during which cement slurry remain in a fluid state and is capable of being pumped. Thickening time is assessed under simulated downhole conditions using a consistometer that plots the consistency of a slurry over time at the anticipated temperature and pressure conditions. The end of the thickening time is considered to be 50 or 70 Bc for most applications.

API Tests for Cementing Cement Consistometer – Thickening Time Test The thickening time of a slurry under realistic conditions must be established to ensure adequate pumping time for slurry placement. Excessive thickening time must be avoided to prevent: Delays in resuming drilling operations Settling and separation of slurry components

API Tests for Cementing Cement Consistometer The apparatus consists of a rotating cylindrical slurry container equipped with a stationary paddle assembly, all enclosed in a pressure chamber capable of withstanding temperatures and pressures encountered in well cementing operations. The cylindrical slurry chamber is rotated at 150 rpm during the test. The slurry consistency is defined in terms of the torque exerted on the paddle by the cement slurry.

API Tests for Cementing Cement Consistometer The relation between torque and slurry consistency is given by T is the torque on the paddle in cm-g (centimeter x gram force) Bc is the slurry consistency in API consistency units designated by Bc. The thickening time of the slurry is defined as the time required to reach a consistency of 100 Bc. This value is felt to be representative of the upper limit of pumpability.

API Tests for Cementing Cement Consistometer Consistometer for simulating down-hole conditions Consistometer for simulating atmosphere conditions

Typical thickening time test output API Tests for Cementing Cement Consistometer Typical thickening time test output

API Tests for Cementing Cement Consistometer The torque required to hold the paddle assembly stationary in a cement consistometer rotating at 150 rpm is 520 cm-g. Compute the slurry consistency.

API Tests for Cementing Cement Permeameter Cement permeameter is an apparatus for measuring the permeability of a core sample. The permeability of a set cement core to water is determined by measuring the flow rate through the core at a given pressure differential across the length of the core. The permeability then is computed using an appropriate form of Darcy’s law

API Tests for Cementing Cement Permeameter Darcy’s law equation: K(mD) is the permeability; q(mL/s) is the flow rate; m(cp) is the water viscosity; L(cm) is the sample length; A(cm2) is the sample cross-sectional area; DP(psi) is the differential pressure.

API Tests for Cementing Cement Permeameter

API Tests for Cementing Cement Permeameter A class E cement core having a length of 2.54 cm and a diameter of 2.865 cm allows a water flow rate of 0.0345 mL/s when placed under a pressure differential of 20 psi. A second core containing 40% silica cured in a similar manner allows only 0.00345 mL/s of water to flow under a pressure differential of 200 psi. Compute the permeability of the two cement samples.

API Tests for Cementing Cement Permeameter

API Tests for Cementing Strength Testing Machine – Compressive Strength Test The compressive strength of the set cement is the compressional force required to crush the cement divided by the cross- sectional area of the sample.