INVESTIGATION OF THE COMPOSITION AND PHYSICO- MECHANICAL PROPERTIES OF CEMENT–MICROSILICA COMPOSITIONS BASED ON INORGANIC INGREDIENTS FOR SOIL WALL STABILIZATION IN OIL AND GAS WELLS

ABSTRACT

The article presents the results of a study on the influence of microsilica on the properties of composite cementing materials. It was found that the introduction of a certain mass fraction of microsilica into Portland cement enhances the physicochemical and strength properties of the resulting composite cementing materials, reduces their bulk density, and improves their technological characteristics used in the process of securing the walls of oil and gas wells.

Based on the conducted research, it was determined that the introduction of microsilica into the cement mixture and the production of low-density composite cementing slurries positively affect the physico-chemical and physico-mechanical properties of both the composite paste and the bulk density of the materials based on it.

АННОТАЦИЯ

В статье приведены результаты исследования влияние микрокремнезема на свойства композиционных тампонажных (цементирующих) материалов. Установлено, что введение определенной массовой части микрокремнезема на состав портландцемента повышаются физико-химические и прочностные свойства получаемых на их основе композиционных тампонажных материалов и облегчается их объемная масса, а также улучшаются их технологические характеристики, применяющие в процессе закрепления грунтовых нефтегазовых скважин.

На основании проведенных исследований установлено, что введение микрокремнезема в цементную смесь и получение низкоплотных композиционных тампонажных растворов положительно влияет на физико-химические и физико-механические свойства как композиционной пасты, так и насыпной плотности материалов на ее основе.

Keywords: microsilica, Portland cement, density, composite binders, matrix, cementing material, oil and gas wells, drilling process.

Ключевые слова: микрокремнезем, портландцемент, плотность, композиционных вяжущих, матрица, тампонажный материал, нефтегазовых скважин, процесс бурения.

Introduction. Currently, the effective development of oil and gas fields is achieved through the construction of wells that allow for increased reservoir oil recovery, reduced operational costs for infrastructure, drilling of boreholes into oil-bearing formations located beneath salt domes, avoidance of complicated rock zones, development of hard-to-reach oil and gas fields, and enhancement of hydrocarbon extraction efficiency in fields at the late stages of development, etc. [1].

One of the ways to address this issue is the further development and broader application of efficient, lightweight, and water-resistant composite materials. These materials are high-performance cement-based plugging materials produced using finely dispersed industrial waste, which enable the formation of a strong structure during the hardening of the mortar mixtures. One such material is composite binding materials obtained with the use of finely dispersed fillers [1-4].

In most cases, the construction of the aforementioned wells is associated with numerous technical and technological challenges. Approximately 25% of oil and gas wells experience interlayer fluid migration, which is caused by poor-quality cementing. An important direction in ensuring the reliable casing of wells is the development of cementing fluid formulations that improve the cleaning of the wellbore from clay deposits and enhance the sealing of the annular space.

Due to the depletion of existing easily accessible deposits, an increasing amount of well construction work must be carried out under various geological conditions. In this regard, the development of composite cementing slurry formulations using low-density fillers that meet the requirements for well casing in different geological conditions is a highly relevant task.

The reduction in the cost of cementing materials is achieved through the use of composite binding materials based on cement and microsilica, which is a finely dispersed industrial waste from the metallurgical industry.

Objects and Methods of Research. To obtain the composite cementing material and the slurry based on it, the following materials were used: Portland cement grade M400DO from the “Kizilkumcement” plant; finely dispersed microsilica waste from JSC “Uzmetkombinat” was used as a micro-filler; and as a modifier, fibrous chemical waste materials based on polyacrylonitrile (PAN), soapstock, caustic soda, and soda ash were used [5, 6].

It should be noted that microsilica (silica fume) is an ultrafine material consisting of spherical particles, obtained during the gas cleaning of electric arc furnaces in the production of silicon and ferrosilicon. The main component of the material is amorphous silicon dioxide. Microsilica is a crucial component in the production of cementing materials with high performance properties. Microsilica is ultrafine silicon dioxide in an amorphous state (specific surface area: S_уд = 16–22 m²/g for micro-silica, 30–300 m²/g for nano-silica). The chemical composition of microsilica, as shown in Table 1, consists mainly (more than 90%) of silicon dioxide (SiO₂). Microsilica is available in the following grades: MK-65, MK-85, MK-95, MKU-65, MKU-85, MKU-95. MKU denotes condensed, densified microsilica. The bulk density of MK grades should be in the range of 150–300 kg/m³, while MKU grades range from 310–600 kg/m³. The numbers indicate the minimum content of silicon dioxide (SiO₂) in percent. Microsilica exhibits pozzolanic activity and is therefore used as an effective additive in cements and concretes. According to GOST R 56196-2014, it belongs to technogenic mineral additives. The requirements for microsilica are specified in State Standard R 56178-2014 [2–5].

He prepared samples based on the cement–microsilica composition were tested for flexural and compressive strength using a PGM-500 MG4A hydraulic press. The specific surface area of the composite cementing materials was determined using a PSKh-4 device. To study the effect of microsilica on the strength characteristics, standard samples of 4×4×16 cm were prepared from normal consistency paste of the developed composite cementing compositions.

Results and Analysis. The chemical properties of the raw materials and the physicochemical properties of the developed cement–microsilica composition were investigated. The chemical composition of the raw materials is presented in Table 1. The physicomechanical characteristics of Portland cement are shown in Table 2.

Table 1.

Chemical Composition of Selected Raw Materials for the Production of Composite Cementing Compositions

Table 2.

Physico-mechanical Characteristics of Portland Cement from the “Kizilkumcement” Plant

During the laboratory studies, high-performance composite cementing slurry formulations with low density were developed for the cementation of casing strings under various geological conditions. The composition includes hydraulic binders, microsilica as a finely dispersed filler, lime, and other additives. The main physical characteristics of microsilica are presented in Table 3.

Table 3.

Main Physical Characteristics of Microsilica (JSC “Uzmetkombinat” according to Ts 00186200-12:2019)

Lime was added to the composite cementing compositions to improve the plastic properties of the slurry, reduce shrinkage deformations, act as a water-retaining additive, and increase the working life of the slurry. Condensed microsilica is produced during the smelting of ferrosilicon and its alloys at “Uzmetkombinat” (Bekabad Metallurgical Plant). A large amount of amorphous silicon dioxide forms spherical particles as a very fine product from a portion of silicon monoxide after oxidation and condensation. In the technological process, some of the silicon monoxide (SiO) forms an extremely fine product resembling ultrafine powder, with particles of amorphous silicon dioxide having an average specific surface area of about 18–20 m²/g. The average particle size is approximately 0.1 μm, which is a hundred times smaller than the average cement grain.

Cement–microsilica compositions were prepared by mixing the components in various ratios in a laboratory mixer for 15 minutes until a homogeneous composition was obtained. In developing a low-density cement composition, the effect of adding up to 12% microsilica on the specific surface area, water demand, setting times, and strength properties of the composite cementing slurries, consisting of Portland cement, microsilica, and lime, was investigated.

The study of changes in the specific surface area of the composite binders showed that with an increase in the content of finely dispersed microsilica up to 12%, the specific surface area reaches up to 7600 cm²/g. The results of the specific surface area changes of the composite cementing compositions are shown in Fig. 1.

Figure 1. Dependence of the Specific Surface Area of Composite Cementing Materials on Microsilica Content

The water demand of cement paste is the amount of water required to obtain a paste of normal consistency, i.e., the quantity of water that ensures a specific consistency [6]. Laboratory studies showed that with an increase in the microsilica content in the composite cementing compositions, the water demand rises from 25.0% to 37.0%. The results of the study are shown in Figure 2.

The studies also demonstrated that due to the high fineness of microsilica, more water is required to wet its particles compared to the original Portland cement. The average particle size of microsilica is 0.65–0.75 μm, which is significantly smaller than the average cement particle size of 6.0 μm.

Figure 2. Change in the Normal Consistency of Composite Cementing Materials with Microsilica Content

It is known that the setting times of cements depend on the normal consistency, fineness of grinding, and various additives. In this regard, studies were conducted to investigate the effect of microsilica content on the setting times of composite cementing materials designed for producing lightweight cementing slurries.

To determine the setting times of the developed composite cementing materials, a Vicat apparatus was used [5-7]. The results of the setting time measurements of the composite cementing materials are shown in Figure 3. The study showed that with an increase in microsilica content, both the initial and final setting times of the slurry increase.

Figure 3. Dependence of the Setting Time Duration of Composite Cementing Materials on Microsilica Content (I.S. – Initial Setting, F.S. – Final Setting)

It was established that with an increase in water demand and microsilica content, the hydration process of the cementing material slows down, thereby prolonging the setting time of the composite cementing slurry.

The study showed that the presence of 2–5% fine microsilica in the cement mixture leads to densification of the transition zone structure by filling the voids. As a result, the crystal size decreases, which strengthens this weak zone of the hardened cellular structure. This process promotes the recovery of self-released water, improves the bonding between the cement matrix and the fine filler, and facilitates the formation of a porous structure. The studies also demonstrated that the pozzolanic reactions of fine microsilica, as a chemical factor, further enhance the strength and durability of composite cementing materials. Examination of the hardening process of composite cementing materials showed that the presence of microsilica in the composite binder positively affects early strength development. The interaction of microsilica particles with hydration products of cement monominerals begins at the early stages of slurry hardening and continues up to 28 days.

Conclusion. The compositions and physico-chemical properties of microsilica waste from AO “Uzmetkombinat” and M400DO Portland cement from the “Kizilkumcement” plant were studied. Based on the conducted research, it was determined that the introduction of microsilica into the cement mixture and the production of low-density composite cementing slurries positively affect the physico-chemical and physico-mechanical properties of both the composite paste and the bulk density of the materials based on it. This allows for the improvement of cementing material characteristics used for reinforcing the walls of oil and gas wellbores.

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