Актуальность исследования буферных жидкостей для скважин, пробуренных инвертно-эмульсионными буровыми растворами, и их взаимосвязи с загрязнением продуктивного пласта.

Полный текст

Hydrocarbon drilling activities in Kazakhstan have increased with the discovery of new hydrocarbon deposits (e.g., Klimene and Khalel Uzbekgaliyev) and the expansion of exploration and appraisal operations, which has consequently led to a growth in well construction and cementing activities [1]. However, several production challenges in many oil fields of Kazakhstan—such as behind-casing hydrocarbon migration and water coning—remain unresolved. These problems are closely related to the technical condition of wells and the quality of well completion [2]. According to various studies, approximately one quarter of oil and gas wells worldwide experience inter-casing pressure of varying intensity [3, 4]. These data highlight the importance and relevance of improving well completion quality and developing advanced cementing materials to ensure reliable casing integrity in oil and gas wells.

A considerable number of studies have been conducted to address the problem of inter-casing pressure and fluid crossflows in oil and gas wells. However, this issue remains relevant not only in Kazakhstan but also worldwide [5, 6]. Measures aimed at eliminating poor zonal isolation associated with inter-casing flows typically involve shutting in the operating well and using kill fluids, which may negatively affect well productivity. In addition, remedial operations require significant time and financial resources [5, 6–8].

Gas migration, inter-casing pressure, and behind-casing fluid flows are often the result of poor-quality cementing, which requires careful attention during the selection and design of cement slurry systems. The proper formulation of cement slurries is therefore critically important.The primary objective of well cementing is to create a hermetic wellbore barrier that maintains its integrity throughout the entire operational life of the well. If the integrity of the wellbore is compromised, particularly in the productive formation interval, subsequent well stimulation methods aimed at improving well productivity may become ineffective.

Unfortunately, after hardening, the cement sheath is subjected to various mechanical and thermal loads that may lead to cracking and, consequently, to the loss of wellbore integrity. Such loads may occur during drilling operations when drill bits and drilling tools impact the casing walls while drilling out the cement plug and during subsequent deepening of the well; during casing running; during casing pressure testing; during well perforation; and during hydraulic fracturing operations. In addition, thermal loads may arise during cement hydration, thermal cycling of formations, and during the injection of steam, hot water, or cold water into the reservoir [12]. Undoubtedly, these loads can lead to the formation of cracks in the cement sheath, particularly in the productive formation interval. The consequences of this problem include gas migration, inter-casing pressure, and a decrease in the effectiveness of enhanced oil and gas recovery methods. It is estimated that inter-casing pressure occurs in more than 30% of wells worldwide. Furthermore, the complexity of this issue is associated with the inaccessibility of such cracks, which significantly complicates subsequent well repair operations or secondary cementing.

 

Figure 1. Schematic representation of potential leakage pathways along the wellbore: (a) between the cement and the outer surface of the casing; (b) between the cement and the inner surface of the casing; (c) through the cement sheath; (d) through the casing; (e) through cracks in the cement; (f) between the cement and the formation.

 

Many researchers focus in their studies on the proper selection of cement slurry formulations and on the cementing process after slurry placement. However, it is also important to emphasize the significance of efficient displacement and pre-circulation prior to cement slurry placement (Figure 2) [10].

During pre-cementing circulation, drilling fluid remains in the annular space and is subsequently displaced by spacer fluids and cement slurry. However, two main problems may arise during this circulation process. The first problem is related to complications caused by the incompatibility of technological fluids, which can lead to loss of circulation [10]. The second problem is the incomplete displacement of drilling fluid, which may result in the absence of cement sheath behind the casing and the formation of channels between the cement and the casing or between the cement sheath and the formation.

Figure 2. Onset of cement slurry circulation loss [10].

 

If cement slurries are incompatible with drilling fluids [11], contact between the cement slurry and the drilling fluid may lead to the formation of a highly viscous mass that prevents further displacement of fluids in the annular space. As a result, operational problems may occur, including accidents or incomplete cement slurry placement in the annulus up to the required height. To prevent such problems, preliminary flushing using intermediate fluids, commonly referred to as spacer fluids, is applied [12]. As intermediate fluids, chemical solutions that do not contain suspended solids may be used, as well as spacer fluids containing solid additives mixed to achieve different densities.

If drilling fluids were fully compatible with cement slurries, the use of spacer fluids could potentially be avoided [11]. Studies conducted by several authors in this area have reported positive results [13–15]. However, drilling fluids may be either water-based or hydrocarbon-based, which are often incompatible with cement slurries and conventional spacer fluids.Despite their advantages in preserving the reservoir properties of productive formations and their promising application in drilling operations, oil-based drilling fluids present additional challenges. In particular, they tend to form a film or filter cake on the wellbore surface, which reduces the adhesion of the cement sheath to the surrounding surfaces. As a result, the use of effective spacer fluids becomes essential when oil-based drilling fluids are employed [16–17].

Oil-based drilling fluids consist of oil as the continuous phase and water as the dispersed phase, combined with emulsifiers, wetting agents, and gelling agents. Various hydrocarbon liquids may be used as the oil phase, including diesel fuel, kerosene, fuel oil, selected crude oil, or mineral oil; however, in practice diesel or kerosene are most commonly used [12]. Such drilling fluids are characterized by high drilling rates, reduced torque and drag on the drill string, and a lower risk of differential sticking. This type of drilling fluid can also be used as a completion and workover fluid, as a spotting fluid for freeing stuck pipe, and as a packer or casing fluid. Oil-based drilling fluids are particularly effective when drilling reactive shale formations, such as “gumbo” shales.

The density of the drilling fluid can be adjusted within the range of approximately 7–22 lb/gal. Although these fluids are sensitive to temperature, they do not undergo dehydration as water-based drilling fluids do. They also do not have strict limitations on the concentration of drilled solids. The water phase should be maintained at a pH above 7, and the stability of the emulsion depends on the alkalinity of the system [18]. Oil-based drilling fluids are also commonly referred to as invert emulsion drilling fluids. In such systems, the dispersed phase typically consists of an aqueous solution containing various salts to maintain wellbore stability during drilling. Surfactants are used to ensure the stability of the emulsion.

The aqueous phase typically contains highly concentrated salts, such as calcium chloride or calcium hydroxide. When in contact with cement slurry, the salts present in the aqueous phase of the drilling fluid may accelerate the cement setting process. In contrast, emulsifiers may have the opposite effect, as they can adsorb onto the surface of cement particles and thereby prolong the hydration process [20]. The compatibility of invert emulsion drilling fluids with cement slurries may also depend on the composition of the cement system itself [21]. Therefore, analysis of these factors indicates that invert emulsion drilling fluids exhibit more complex and generally less effective compatibility with cement slurries compared with water-based drilling fluids [12].

Another advantage of oil-based drilling fluids is the lower friction within the wellbore. Therefore, they are often used in extended-reach wells, where friction becomes a critical parameter. In contrast to water-based fluids, significant deterioration of drilling fluid properties over time is typically not observed when oil-based systems are used. In addition, capillary pressure prevents the penetration of oil into water-wet formations [16].

In addition to the excellent filter-cake–forming properties of oil-based drilling fluids, when properly formulated their use can minimize the risk of disturbing the natural inflow conditions of productive formations.

Oil-based drilling fluids are particularly effective when drilling through formations such as highly reactive shales, where water wetting can cause significant operational difficulties. They also allow core samples to be obtained without contamination by drilling water.These fluids act as the liquid medium of the system, control viscosity, contribute to the development of initial and final gel strengths, provide gel structure and stability to prevent particle settling, and can serve as a weighting medium in the drilling fluid system [17].

Another challenge is poor displacement efficiency. For example, polymer-based drilling fluids are commonly used to reduce fluid loss; however, these fluids are more difficult to displace from the annular space compared with bentonite-based drilling fluids. Invert emulsion drilling fluids contain emulsifiers and surfactants that can adsorb onto the surfaces of minerals and form a coating over the entire surface. If poor displacement leads to various problems associated with inadequate cementing, the situation becomes even more complicated when invert emulsion drilling fluids are used, since the cement may not properly adhere to the casing surface [19]. Therefore, the study and development of effective spacer fluids for wells drilled with invert emulsion drilling fluids is of great importance and remains a highly relevant research topic.

The authors of study [22] identified, in addition to compatibility and the degree of casing and wellbore cleaning, several key indicators of spacer fluid performance. These include the stability of rheological properties, pumping rate and contact time, and the volume of the spacer fluid (Figure 3).

Figure 3. Main criteria for the improvement of spacer fluids.

 

Various approaches have been proposed for the development of effective spacer fluids. However, among these approaches, the investigation of the cleaning and filter-cake removal properties of developed spacer systems remains insufficiently studied. Therefore, the development of effective spacer fluids for cementing wells drilled with oil-based drilling fluids, based on studying the mechanisms of interaction between the hydrocarbon components of the drilling fluid and both the formation and casing surfaces, as well as investigating the cleaning and filter-cake removal capabilities of the developed spacer systems, represents a novel research direction.

However, reservoir contamination remains a significant concern. For example, the study of reservoir permeability recovery after interaction with mixtures of technological fluids is very important during primary cementing. Primary cementing is the process of placing cement slurry in the annular space between the casing and the wellbore wall. After placement, the cement slurry hardens into a solid cement sheath that prevents migration of formation fluids and provides reliable zonal isolation. Therefore, primary cementing is a critically important stage in well construction, as there is typically only one opportunity to perform this operation successfully [23]. High-quality well cementing, particularly during well completion, is critically important, since in addition to ensuring reliable wellbore isolation it is also necessary to prevent contamination of the productive formation. Reservoir contamination refers to the sequential invasion of technological fluids such as drilling fluids, spacer fluids, and cement slurries into the formation. Many researchers indicate that drilling fluids are the primary type of technological fluid responsible for reservoir contamination and have the most significant negative impact on the filtration and reservoir properties of productive formations [24]. However, it should also be noted that the fluid loss of cement slurries can be up to ten times greater than that of drilling fluids [25, 26]. The filtrate of cement slurry can penetrate into the productive formation to depths of up to 10 meters. Consequently, cement slurry filtrate can affect not only the efficiency of subsequent operations such as well perforation and well clean-up, but also the overall well productivity. For example, earlier studies conducted by N.Kh. Karimov and F.A. Agzamov on core samples demonstrated that the permeability recovery coefficient of the cores did not exceed 60% [25, 26].

Undoubtedly, the productive formation may also be damaged during secondary reservoir exposure due to the contaminating effects of filtrates from perforation fluids [24–27]. However, reducing the impact of cement slurry filtrate on reservoir contamination is also important for several reasons. The fluid loss of cement slurries is generally higher than that of drilling fluids, and the filtrate of cement slurry can significantly affect the permeability of the near-wellbore zone as well as the effectiveness of secondary reservoir exposure operations.

The main causes of reservoir impairment are considered to be the following [28, 29]:

• swelling of clay particles;
• formation of emulsions;
• high interfacial tension at the filtrate–formation fluid interface;
• chemical interactions between the filtrate and formation fluids, as well as between different filtrates.

The latter leads to pore plugging in reservoir rocks as a result of the supersaturation of formation water with salts originating from the filtrate [30], as well as to changes in rheological properties. Pore blockage may also occur due to the penetration of solid particles from the cement slurry into the pore space [30]. In addition, cement slurries may contain insoluble salts such as CaCO₃ and CaSO₄. Cement slurries typically exhibit a higher pH compared with drilling fluids. Under such conditions, clay and other fine rock particles may detach from the formation matrix and migrate through the reservoir, potentially causing pore plugging [31]. This phenomenon may influence experimental results, which necessitates the use of core samples that have been pre-cleaned of clay particles. A review of the literature shows that many researchers have conducted experiments in this area [28–31]. However, the relevance and necessity of further investigation are increasing due to the fact that modern cement slurry formulations include a wide range of additives such as expanding agents, plasticizers, accelerators and retarders, fluid-loss reducers, defoamers, anti-settling additives for lightweight materials, fibers, and other chemical reagents that serve as regulators of technological properties [31]. Nevertheless, many developed cement slurry systems also exhibit undesirable effects that may lead to contamination of productive formations.

In field practice, various technologies are applied to reduce or prevent contamination of the near-wellbore zone of the reservoir, such as underbalanced drilling and the use of oil-based drilling fluids. However, underbalanced drilling is a relatively complex process to implement, and well cementing operations are still performed using conventional methods. The use of oil-based drilling fluids is considered a less damaging technology for reservoir protection; however, their high cost, increased operational risks, and other factors impose certain limitations on their application [28]. Therefore, it is necessary to take into account the composition and compatibility of technological fluids with formation water and the reservoir rock of productive formations.

Based on the above-mentioned materials, it can be concluded that the improvement of spacer fluids should be guided by several key criteria [12] (Figure 3):

• compatibility of the spacer fluid with various drilling fluids;
• compatibility of the spacer fluid with different cement slurries;
• complete displacement of drilling fluids and ensuring tight contact between the cement sheath and the confining surface;
• tolerance to variations in cement slurry density;
• adaptability to different temperatures and pressures;
• ease of preparation under drilling conditions;
• low fluid loss;
• stability of rheological properties [22];
• pumping rate and contact time [22];
• spacer fluid volume [22].

The relevance of studying spacer fluids for wells drilled with invert emulsion drilling fluids and their relationship with reservoir contamination lies in the possibility of utilizing the anomalous behavior of dispersed systems to prevent filtrate penetration into productive formations. This involves identifying non-Newtonian and filtration anomalies of filtrates from drilling, spacer, and cementing fluids in porous media containing microscale channels, with the aim of limiting fluid loss and predicting contamination of the near-wellbore zone of productive formations. Such studies are based on investigating the mechanisms of interaction between filtrates of technological fluids, rock-forming minerals, and polymeric reagents, as well as examining the conditions of filtrate flow through pores of different sizes. In addition, predictive evaluation of the depth of filtrate penetration into rocks with different permeability characteristics is performed.

Conclusions

1. The primary potential leakage pathways along the wellbore are gaps between the cement sheath and the surrounding confining surfaces.
2. When assessing the compatibility of technological fluids, it is essential to consider the compatibility of their filtrates in order to prevent pore plugging in the near-wellbore zone.
3. The onset of cement slurry circulation loss most commonly occurs during pre-cementing circulation.
4. In addition to compatibility and the degree of casing and wellbore cleaning, key indicators of spacer fluid performance include the stability of rheological properties, pumping rate, contact time, and spacer fluid volume.

5. The results of the literature analysis confirm that the development of effective spacer fluids is particularly important for wells drilled with invert emulsion drilling fluids due to their complex interaction with cement systems and wellbore surfaces.
6. Further studies should focus on investigating the mechanisms of interaction between technological fluid filtrates and reservoir rocks in order to minimize formation damage and improve the quality of primary cementing.

 

Об авторах

Дина Исаева

Казахстанско-Британский технический университет

Автор, ответственный за переписку.
Email: isaevadina86@gmail.com
ORCID iD: 0009-0005-0384-0122

Докторант Школы Энергетики и нефтегазовой индустрии

Казахстан, Республика Казахстан, Алматы, ул. Толе би, 59

Арман Арстангалиевич Кабдушев

Таразский региональный университет им. М.Х. Дулати

Email: arman-kz@mail.ru
ORCID iD: 0000-0003-3579-9054
Scopus Author ID: 57194217536
Казахстан, Тараз

Фарит Акрамович Агзамов

Уфимский государственный нефтяной технический университет

Email: faritag@yandex.ru
ORCID iD: 0000-0001-5850-5261

докт. техн. наук

Россия, Уфа

Абдулахат Абдукаримович Исмаилов

Казахстанско-Британский Технический Университет

Email: a.ismailov@kbtu.kz
ORCID iD: 0000-0002-1957-5168
Scopus Author ID: 57202758242
ResearcherId: JON-3767-2023

Докторант Школы Энергетики и нефтегазовой индустрии

Казахстан, Алматы

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