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№ 3 (March 2008)

New Oil-Well Cement Improves Integrity of Well Annular Space

One of the reasons for well failures, gas seeps, and water encroachment is leakage of the cemented annular space caused by corrosion of the cement stone by salts in the formation water

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This study examines the corrosion resistance of barium-containing oil-well cement. Tests were performed at gas wells in the Urengoi field.

Formation Water Aggressiveness

Formation fluids often contain large amounts of chemical compounds corrosive to the cement stone. They include sulfates, sodium, calcium and magnesium chlorides, as well as carbonates and sulfur compounds. Reactions between the aqueous medium and cement stone were classified into three types:
– Processes taking place in the cement stone due to the water having low temporary hardness, so-called soft water. Cement stone components are dissolved and transported through the concrete layer in the process of filtration.
– Exchange reactions between the components of water and concrete, producing soluble products or products having no cementing properties, which weaken the stone structure.
– Accumulation and crystallization of salts in concrete cracks, pores and capillaries, which can destroy cement stone and concrete (salt corrosion).
By chemical composition, waters of oil and gas fields can be classified as sodium sulfate, sodium bicarbonate, magnesium chlorine, or calcium chlorine. Fluids of the alkaline-chloride and sulphate-alkaline-chloride subgroups are common in Russian oil fields.

According to data from numerous researchers, oil fields contain formation waters of different chemical composition and various concentrations of aggressive ions. Most often, the concentrations of magnesium and sulfate-ions significantly exceed the allowable limit for corrosion safety of oil-well cements. Special corrosion-resistant cements are required in these cases.

At present, Russia produces sulfate-resistant Portland cement according to GOST 22266-94, which is used mostly in the construction industry.

Requirements for sulfate-resistant oil-well cements are regulated by GOST 1581-96 “Oil-Well Portland Cements Specifications” uniform with regard to cements of class I-G and 1-H with АРI Specification 10A “Specifications for Cements and Materials for Well Cementing.” According to this standard, Portland clinker for oil-well cements of high sulfate resistance (PTzТI–G-CC-1 and PTzТ–H-CC-1) must comply with the following requirements (mass percent): С3S content between 48 and 65, C3A not more than 3, and the sum C3A + C3AF not more than 24. Technological peculiarities, as well as the lack of appropriate raw materials at the plants, complicate production of clinker with this mineralogical composition. Besides, this type of cement is sulfate-resistant only in environments with relatively low concentrations of aggressive ions (2.7 g/l). Higher concentrations require cements with higher sulfate resistance.

Study of Corrosion Resistance of Barium-Containing Cement

To produce this cement (2), the following raw materials were used: limestone, barite ore, ash from thermal power plants, and pyrite drosses. Mineralogical composition is shown in Table 1. Sulfate-resistant Portland cement (1) as per GOST 22266-94 without mineral additives was used for comparison.
The fineness of grinding was 350 m2/kg for all types of cement. Analysis of corrosion resistance of the cements consisted of determining the strength characteristics of samples immersed in corrosive medium during a certain hardening time.
A five-percent solution of Na2SO4 and formation water was used as the corrosive medium. Selection of aggressive media was based on the fact that formation waters contain significant amount of sulfates, and a five-percent sodium sulfate solution is always used in research of corrosion resistance of binding materials.
The main characteristics of oil-well cements include rheological properties of cement slurries, plugging capability, strength of cement stone contact with limiting surfaces, water permeability, water loss, and corrosion resistance, which together determine the reliability and longevity of the isolation layer.

Rheological Properties

Rheological properties of cement slurries characterize their pumpability (thickening time) and penetrability. They are necessary to calculate the parameters of pumping and movement of slurries in the pipes and rock fractures.

Rheological properties are determined by various methods. In particular, structural viscosity and yield point are determined using the Reotest-2 rotational viscosimeter. The easiest method is determining the spreadability of cement-water solution as well as its thickening ability, which is measured with consistometers KTs-5 and KTs-3. The KTs-5 consistometer, working at normal atmospheric pressure, is used to test cements intended for low, normal and moderate temperatures. The KTs-3 consistometer working at higher pressure is used for high-temperature cements. Measurement results are given in Table 2.
Analysis of experimental data (Table 2) shows that barium-containing cements with the same water-cement ratio (W/C) and similar spreadability as oil-well Portland cement have denser structure and are characterized by a shorter thickening time, still within the required parameters.

Plugging Capability

This parameter characterizes the ability of cement slurry to resist water filtration at differential pressure. Sedimentation stability of the cement slurry is an indirect measure of this capability. According to GOST 1581-96, it is determined as water loss. Experimental results are given in Table 3.
Experimental data (Table 3) show that with an increase in spreadability (W/C), water loss of barium-containing cement is much lower than that of oil-well Portland cement. It indirectly contributes to reduced formation of channels and fractures in the course of further cement stone hardening.

Cement Strength

Strength properties reflect the efficiency of well cementing. The results of strength determination for samples of barium-containing cements and a check sample are given in Table 4. The samples were formed at W/C 0.45.
Determination of strength characteristics showed that the kinetics of barium-containing cement stone strength increase is similar to that of the oil-well Portland cement, though absolute strength values are higher.
Contact strength was determined as shear resistance of the cement stone cylinder sample pressing out of the metal holder. Results of the determination of cement stone contact with a limiting surface for samples under study are given in Table 5.

A comparison of the results shows that values of the adhesive strength of both types of cement are similar. This parameter of the barium-containing cement even exceeds the adhesive strength of the common oil-well cement after both two and seven days of hardening.

Corrosion resistance of barium-containing cement was studied in laboratories of the Russian Chemical-Engineering University named after Mendeleev and the Research Institute for Reinforced Concrete Studies. It was determined that the resistance factor (Fr) of Portland cement after 12 months of hardening is 0.68 in the sodium sulfate solution and 0.51 in formation water. In the same period of hardening, barium-containing cement has the following characteristics: Fr= 0.86 and 0.85 (in sulfate medium and formation water respectively).

In a sulfate medium, as a result of hydration, calcium aluminate in Portland cement interacts with the calcium hydroxide produced by hydration of С3S and sodium and calcium sulfate to form ettringite:
3СаО•Al2O3 + Ca(OH)2 + Na2SO4 + H2O —  3СаО•Al2O3•3CaSO431H2O
Barium-containing cement contains much less tri-calcium aluminate than Portland cement and has a higher corrosion resistance than sulfate-resistant Portland cement.
After the studies, pilot batches of barium-containing cement were produced and successfully tested in the drilling of gas wells at the Urengoi field.

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