COMPRESSIVE AND SHEAR BOND STRENGTH OF OIL WELL CEMENT WITH CALCIUM CARBONATE AND SILICA FUME Arnoldus

One of the critical factors of cementing process in oil drilling of off-shore-project is designing the cement slurry. For this reason, the slurry properties which have been classified by American Petroleum Institute (API) should be changed so it will match with the requirement of reservoir condition. Changing the slurry properties can be done by adding the additive material into the cement slurry such as Calcium Carbonate and Silica Fume. The research objective is to study the effect of calcium carbonate and silica fume to the compressive and shear bond strength of oil well cement. Fourty five cylinder specimens with the size of (75 x 150) mm were made for compressive strength testing and fourty five cylinder specimens with the size of (25.4 x 50.8) mm were made for shear bond strength testing. Five variants of the specimen were made in this study. The variant were cement slurry with (0% Calcium Carbonate + 0 % Silica Fume) as a reference specimen; (5% Calcium Carbonate + 5 % Silica Fume); (10% Calcium Carbonate + 10 % Silica Fume); (15% Calcium Carbonate + 15 % Silica Fume); (20% Calcium Carbonate + 20 % Silica Fume). The oil well cement specimens were tested in 7, 14, and 28 days. The experimental results show that the compressive strength of oil well cement will decrease when it is added with calcium carbonate and silica fume. The shear bond strength of the oil well cement increases for the specimen with 5 % Calcium Carbonate + 5 % Silica Fume. However, the shear bond strength will decrease when content of the Calcium Carbonate + Silica Fume more than 5 %. Based on the result of this research, the optimum amount of calcium carbonate and silica fume that can be use is 5%, because with 5% of calcium carbonate and 5% of silica fume, the reducing of compressive strength is the smallest and the shear bond strength is increased compare to the others specimen with 10%, 15%, and 20% calcium carbonate and silica fume.


INTRODUCTION
When a certain section of the depth of an oil or gas well has been drilled successfully, the drilling fluid cannot permanently prevent the well bore from collapsing.Therefore, oil well cementing was introduced in the late 1920s (Joshi and Lohita, 1997) with a number of objectives: (i) protecting oil producing zones from salt water flow, (ii) protecting the well casing from collapse under pressure, (iii) protecting well casings from corrosion, (iv) reducing the risk of ground water contamination by oil, gas or saltwater, (v) bonding and supporting the casing, and (vi) providing zonal isolation of different subterranean formations in order to prevent exchange of gas or fluids among different geological formations.
Oil well cementing is the process of placing cement slurry in the annulus space between the well casing and the geological formations surrounding to the well bore in order to provide zonal isolation in oil, gas, and water wells (Shahriar, 2011).The goal of the well cementing is to exclude fluids such as water or gas to move from one zone to another zone in the well.Incomplete zonal isolation and/or a weak hydraulic seal between the casing and the cement and between the cement and the formations, may cause oil spills and the well may never run at its full producing potential (Calvert, 2006).The appropriate cement slurry design for well cementing is a function of various parameters, those are physical and chemical characteristic.Both physical and chemical characteristic of oil well cement (OWC) slurries must be optimized to achieve an effective well cementing operation.The schematic of a cemented well is depicted in Figure 1.   , 2014).There is also another additive that can improve the mechanical properties of cement, and that is silica fume.By adding silica fume to the cement slurry, it will improve the compressive strength (from 28.61 MPa to 31.64 MPa).It can be said also that the silica fume can be used as an additive to improve the compressive strength of the cement (Souza et al., 2012).
So it can be seen from the discussion above that to improve the shear bond strength of the cement slurry is adding with the calcium carbonate and to improve the compressive strength of the cement slurry is adding with silica fume.Therefore, to improve both the shear bond and the compressive strength of the cement slurry is adding the cement slurry with calcium carbonate and silica fume.

Oil well cement stability
According to American Petroleum Institute (API) Specification 10A ( 2002), compressive strength for oil well cement is defined as the capability to restrain the forces that come either from the formation or the casing.The minimum allowable compressive strength for oil well cement is 500 psi (3.447 MPa).The compressive strength of the cement will be calculated by using the following equation: (1) where: f'c = compressive strength, kgf/cm 2 P = maximum load, kgf A = specimen cross section area, cm 2 According to API Specification 10A ( 2002), the shear bond strength for oil well cement is defined as the capability to restrain the forces from the weight of the casing.The minimum allowable shear bond strength for oil well cement is 100 psi (0.689 MPa).
The shear bond strength of cement will be calculated by using the following equation (API Specification 10A, 2002): (2) where: SBS = compressive strength, kgf/cm 2 P = maximum load, kgf D = specimen diameter, cm h = specimen height, cm

Expanding Additive
Expanding of cements means expansion of cement relative volume (an external part of cement visually becomes bigger) due to cement bulk expansion (Danjuschewskij, 1983).This occurrence is caused by: a.Chemical contraction that formed another hydrated products on liquid phase condition, i.e. crystallizing of dissolved salt at high temperature.b.The presence of expanding materials in cement slurry before hardened condition, i.e.CaO, MgO, CaSO4, etc. c.The presence of electrolyte around the cement bulk after the hardened condition.Rubiandini et al. (2005) stated that the second item (b) is the merit condition that might bring to increase the shear bond strength of oil well cement.

Materials and specimens
The cement slurry made of portland cement, water, calcium carbonate and silica fume.Fourty five cylinder specimens with the size of (75 x 150) mm were made for compressive strength testing and fourty five cylinder specimens with the size of (25.4 x 50.8) mm were made for shear bond strength testing.Five variants of the specimen were made in this study.The variant were cement slurry with (0% Calcium Carbonate + 0 % Silica Fume) as a reference specimen; (5% Calcium Carbonate + 5 % Silica Fume); (10% Calcium Carbonate + 10 % Silica Fume); (15% Calcium Carbonate + 15 % Silica Fume); (20% Calcium Carbonate + 20 % Silica Fume).

Specimens testing
The compressive strength and the shear bond strength of the cement slurry were tested at 7, 14, and 28 days.In each testing, three specimens of each variant were tested to find the average compressive strength and three specimens of each variant were also tested to find the average shear bond strength of the slurry cement.The Universal Testing Machine with the capacity of 30,000 kgf was used to conduct compression and shear bond test of cylinder specimens.The compressive strength and the shear bond strength were tested following the American Petrolium Institute (API) Specification 10A (see Figure 2 and 3).

Compressive strength
Compressive strength was tested at the age of 7, 14, and 28 days.The test was conducted using Shimadzu Universal Testing Machine (UTM) and it was following the API Specification 10A (2002) as the reference for the standard of the test.The compressive strength value of the cement with the size 75 mm x 150 mm can be seen in Table 1.

Shear bond strength
Shear bond strength test is done at the age of 7, 14, and 28 days.The test was conducted using Shimadzu Universal Testing Machine (UTM) and it was following the API Specification 10A (2002) as the reference for the standard of the test.The shear bond strength value of the cement with the size 25.4 mm x 50.8 mm (1 inch x 2 inch) can be seen in Table 2. Table 2 shows that the shear bond strength for the specimen with variant 5 % SF + 5 % CC increased compared to the reference specimen (0 % SF + 0 % CC), however after that the shear bond strength was decreasing.So it can be said that the optimum portion to improve the the shear bond strength is 5 % SF + 5 % CC.