The problems associated with the quality of well cementing are described. The main negative consequences of poor zonal isolation are highlighted. One of the solutions to this problem is the use of expanding cementing compositions to increase the tightness of the well casing and, consequently, to improve the quality of isolation between productive and water-bearing formations. The mechanisms of expansion processes in cementing compositions are considered. A review of the main types of expanding additives for cement slurries is carried out. It has been established that the greatest magnitude of linear expansion is achieved with the oxide expansion mechanism through the introduction of calcium oxide and magnesium oxide additives.
Oxide expansion is caused by the formation of hydroxides of the corresponding metals, whose oxides occupy a smaller volume compared to the hydration products. The main problem limiting the widespread application of oxide-type expansion is the high hydration rate of the initial substances, which leads to the formation of hydroxides in the mobile cement slurry, thereby preventing the expansion of the cement stone. In order to obtain expansion in a plastic cement stone that has not yet gained high strength, studies were conducted and the effect of certain chemical reagents on the hydration rate of calcium oxide was determined. The most optimal component compositions of calcium oxide–based expanding additives for cement slurries were identified.
Requirements for the technological parameters of the base cement slurry were established. The component composition was determined and a formulation of the base cement slurry based on Portland cement grade PCT-I-G-CC-I was developed, which serves as the basis for the expanding cementing composition. To regulate water retention and rheological properties, the cement slurry was modified with additives. Hydroxyethyl cellulose is proposed as a water-retaining and structure-forming additive, while a silicone defoamer is recommended as an antifoaming agent. The introduction of a polycarboxylate plasticizer is allowed to increase the mobility and flowability of the cement slurry. A 2% aqueous solution of polyacrylamide is proposed for use as a buffer fluid.
The scientific novelty of the work consists in establishing the dependence of the rate and magnitude of cement stone expansion on temperature conditions, the concentration of the expanding additive, and inhibitors of the calcium oxide hydration reaction; in determining the dependence of the structural, rheological, and mechanical properties of cement slurries and cement stone on the composition and chemical nature of their constituent components; as well as in determining the dependence of well cementing quality on the type of buffer fluid.
For conditions of abnormally high formation pressures, expanding additives based on calcium oxide combined with lignosulfonates or sodium silicates are effective, while for conditions of abnormally low formation pressures, additives based on ferrite and calcium oxide are effective, providing linear expansion of the cement stone in the range of 0.2–22% at an expanding additive concentration of 3–8%.
Improvement of well casing tightness is ensured by the use of a stabilized cement slurry based on PCT-I-G-CC1 containing expanding additives and modifying reagents, which enhance isolation characteristics due to the expansion effect (up to 8%), increased adhesion (up to 1.8 times), and low fluid loss (up to 35 cm³/30 min).
Based on the modern level of analytical and experimental studies, the reproducibility of the obtained experimental data and field test results, the practical significance of the work has been established:
The causes of this phenomenon are complex and depend on many factors. One of the main reasons is insufficient cleaning of deviated wellbores from drilling fluid and from “tongues” of solid phase deposited on the walls of inclined and horizontal wells from the drilling fluid.
For effective cleaning of deviated wellbores from drilling fluid and solid-phase tongues, the dynamic shear stress of the buffer fluid must exceed 15 Pa. However, the technical water or aqueous solutions of surfactants currently used in the Western Kazakhstan fields as cleaning buffer systems do not possess sufficient displacement properties. Therefore, it can be stated with a certain degree of confidence that these systems do not allow for effective cleaning of inclined wellbores of oil and gas wells from drilling fluid and, especially, from the formed solid-phase tongues. For this purpose, it is necessary to develop special compositions of structured buffer fluids.
Studies were carried out to develop effective compositions of structured buffer fluids that ensure a comprehensive solution to the problems of preparing wellbores for cementing, namely: separation of cement and drilling fluids, complete displacement of the drilling fluid from the annular space, cleaning of the wellbore from hard-to-displace solid-phase tongues, and colmatation of permeable formation walls. In addition, the developed buffer fluid compositions are required to meet additional criteria such as ease of preparation, availability, and relative low cost of the components used.
For the development of structured buffer fluid compositions, it is proposed to use 2% polymer reagents based on oxyethyl cellulose, aluminum sulfate as a structuring additive, and the colmatant IKKARB-75. During the studies, regularities in the influence of aluminum sulfate additives on the structural and rheological properties of aqueous polymer reagent solutions were established.
The buffer fluid was prepared as follows: an aqueous solution of aluminum sulfate was prepared in a laboratory mixer by dissolving it in technical water at a temperature of 50–60°C with stirring until complete dissolution for 10 minutes. Then the polymer reagent was introduced, and the resulting mixture was stirred for 15 minutes. After that, while maintaining constant stirring, the IKKARB-75 reagent was added. After 5–10 minutes of stirring and achieving uniform distribution of the IKKARB-75 reagent throughout the volume, the properties of the resulting buffer fluid were measured.
To preserve the reservoir properties of productive formations during cementing, the colmatating filler IKKARB-75 was used as a filter cake–forming additive for buffer fluids. Due to the formation of a dense impermeable carbonate filter cake on the borehole walls, the IKKARB-75 additive prevents fluid filtration both from the buffer mixture and from the cement slurry. The carbonate-containing filter cake adheres well to the cement stone and is easily removed due to good solubility during hydrochloric acid treatments performed in the process of secondary opening of the productive formation.
When using a structured buffer fluid, its required volume, in accordance with regulatory requirements, corresponds to 15% of the volume of the cement slurry used for well cementing when a bottom wiper plug is applied. For example, for wells with depths of 2700–3000 m during casing cementing at fields in the Western Kazakhstan region, the buffer fluid volume is taken as 6.0 m³. The required volumes of structured buffer fluids depend on the borehole and casing diameters and should be specified in each individual case.
For well cementing under conditions of abnormally low formation pressures, it is advisable to use a lightweight polymer cementing mixture containing vermiculite with a density of 1.1–1.2 g/cm³ and a spread of 21–23 cm. In this case, the flexural strength of the formed stone is 7.2–8.7 MPa. With an increase in the percentage of hardener addition, for example 0.08% by weight of the calculated resin volume, the setting start time decreases.
The start of setting time depends on the amount of hardener added to the polymer cementing mixture at different temperatures.
To prepare 1 m³ of polymer cementing mixture, the following materials are required:
The newly developed polymer cementing mixture with a density of ρ = 1.1–1.2 g/cm³ is recommended for oil and gas well cementing as well as for water inflow isolation during pilot industrial tests at fields in Western Kazakhstan.
At a spread of 21 cm and a density of ρ = 1.22 g/cm³, the start of setting at a 1.0% hardener addition is 225 minutes. The strength of the formed stone is 7.0 MPa. When the hardener content is increased to 4.0% of the total resin volume, the setting start time sharply decreases to 140 minutes.
A new polymer cementing mixture with fillers (sawdust) and hardeners was studied at temperatures of 60–120°C. Based on the analysis, the following formulation of the polymer cementing mixture was obtained: urea-formaldehyde resin with a density of ρ = 1.27–1.29 g/cm³, hardener (technical copper sulfate), and wood sawdust. A 30% solution of the KPA-5 reagent was used as a hardener.
The studies show that as the temperature increases and the percentage of hardener addition rises, the strength of the proposed cementing mixture also increases.
Subsequently, laboratory studies of the same polymer cementing mixture with the addition of 5% sulfur were continued. The results showed that the thickening time of the mixture increased while maintaining unchanged mechanical strength of the stone. In this case, sulfur acts as a stabilizer of the polymerization process, ensuring the required thickening onset time.
Of considerable interest is the study of the composition of the cementing mixture consisting of 150 g of resin, 7.5 g of wood sawdust, and 2% hardener. The results show that at a temperature of 100°C, the expansion of the proposed resin increases with temperature growth, which in turn ensures the strength of the stone.
A cementing mixture containing 150 g of resin, 7.5 g of wood sawdust, and 2% hardener exhibits an expansion of 60%, while a mixture containing 0.7% hardener at a temperature of 100°C shows an expansion of 75%.
The developed cementing compositions and buffer fluid were used during casing cementing operations.