DEVELOPMENT OF POLYMINERAL COMPOSITIONS FROM CLAYS OF THE UZBEKISTAN DEPOSIT FOR THE PURPOSE OF OBTAINING THERMOSTABLE DRILLING FLUID

ABSTRACT

In conditions of high temperatures and pressures, reagents-stabilizers widely used in practice undergo chemical change and lose their stabilizing properties, as a result of which the stability and thixotropic functions of clay drilling muds deteriorate, their yield increases and the separation of the suspension occurs with the release of the dispersed phase.

АННОТАЦИЯ

В условиях высоких температур и давлений широко применяемые на практике реагенты-стабилизаторы испытывают химические изменения и теряют свои стабилизирующие свойства, в результате чего устойчивость и тиксотропные функции глинистых буровых растворов ухудшаются, повышается их водоотдача и происходит расслоение суспензии с выделением дисперсной фазы.

Keywords: drilling mud, salt formation, clay mineral, hydrothermal regime, polymineral clay compositions, water loss, salt tolerance, clay, additives, bentonite, palygorskite, dispersion, coagulation.

Ключевые слова: буровой раствор, глинистый минерал, гидротермальный режим, полиминеральные композиции глин, водоотдача, глина, добавки, бентонит, палыгорскит, дисперсия, коагуляция.

Introduction. Currently, in the Bukhara-Khiva and Ustyurt regions of Uzbekistan, drilling of oil and gas wells is mainly carried out in saline formations, where for such purposes it is necessary to use drilling fluids obtained mainly using palygorskite clays (atapulgite), which are rich in CaO.

For drilling deep wells in difficult geological conditions, heat-resistant drilling fluids are needed, obtained using palygorskite clays and chemical reagents.

The development of heat-resistant drilling fluids can be carried out in two main directions, partially complementing each other;

• creation of high-quality dispersed systems based on heat-resistant natural mineral raw materials;
• based on heat-resistant stabilizer reagents [1].

It is known that the use of even high-quality montmorillonites (bentonites) does not allow efficient drilling in mineralized environments without treatment with chemical reagents, which is many times higher than the consumption of clays for drilling fluids, and often leads to the impossibility of their operation due to coagulation of the clay suspension with electrolytes [2]. The use of palygorskite clays when drilling continuous salt-bearing strata makes it possible in some cases to do without expensive chemical reagents, which reduces the cost of drilling fluid tenfold.

Objective. The purpose of this study is to study the thermal stability of drilling fluids obtained from the developed compositions of Navbakhor clays.

Results and discussion. In practice, in particular in laboratory conditions, the method of hydrothermal treatment of clay solutions is more often used, simulating the conditions of deep and superdeep wells, as well as changes in the nature of the formation of coagulation structures of clay minerals [3].

The study of the processes of formation of the latter under conditions of elevated temperatures is of particular importance in connection with the need to develop methods for obtaining clay solutions for complicated drilling [4].

We carried out hydrothermal treatment in a laboratory autoclave AV-2 under isothermal conditions at a temperature of 350⁰C and a pressure of 169 for 3 hours. The concentration of the studied aqueous dispersions of clay minerals in all cases was 10% [5,6].

Carbonate palygorskite Navbakhor deposit (Navoi region) is an aqueous magnesium aluminosilicate with the ideal formula R₅ [Si₈O₂] (OH)₂ ∙ (OH₂)₄, 4H₂O. The crystal structure of this palygorskite, which has a layered - ribbon structure, resembles the structure of an amphibole and has zeolite - like minerals 6.4-3.7 Å in size [7]. Two types of water are located in them: molecules freely located and associated with the electronegative surface of the bases of tetrahedra, and molecules that have a bond with octahedral cations on side walls of the channels. These molecules are removed from the latter at higher temperatures, just like water molecules from zeolites. The density of carbonate palygorskite (depending on the place of sampling) varies between 2.3÷2.5 g/cm³.

Multiple chemical analyzes revealed that in the carbonate palygorskite, the ratio of SiO₂: RO ranges from 2.1÷2.5 (where RO is the content of MgO, FeO, Al₂ O₃ , Fe₂O₃ expressed in equivalents of the amount of MgO) [8]. For carbonate palygorskite Navbakhor deposit, the total exchange capacity is 20–30 meq per 100 g of sample.

Table 1.

Results of analyzes of endo- and exothermic effects by DTA methods of bentonites and palygorskite Navbakhor field (NF)

Table 1 presents the results of DTA of three types of clays from the Navbakhor deposit. Table 1 shows that the bentonites (montmorillonites) of the Navbakhor deposit have differences in the temperature of the observed endothermic effects, which shows the difference in their crystal structure. In alkaline bentonite, the first maximum is observed at 150-180 ⁰C, when the main amount of hygroscopic water is released;

At 550-600 ⁰C, moisture is removed from the crystal lattice of the mineral of hydroxyl groups; at 720-760 ⁰C, the remains of hydroxyl groups are removed with the recrystallization of montmorillonite [9]. When the type of bentonite changes from alkaline to alkaline earth, approximately the same phenomena are observed, with some difference in the temperatures of the maxima.

Features of the crystal structure and chemical composition of carbonate palygorskite containing more than 16% CaO play a significant role in obtaining heat-resistant drilling fluids [10]. At the same time, it should be noted that carbonate palygorskite is characterized by a relatively greater dispersion than other types of clays.

Figura 1. Change in the effective viscosity (η_eff) of the clay solution depending on the temperature of its hydrothermal treatment: 1-for AEB NF; 2 - for CP NF, 3 - for the composition of CP: AEB NF = 50:50

Figure 1 shows changes in the effective viscosity of aqueous solutions of Navbakhor clays and their compositions. In this case, the effective viscosity of the solutions was measured on a high-temperature rotational viscometer - replastometer VSN-2, manufactured by KF VNIIK Azneftegaz [11].

It can be seen from figure 1 that with individual use of AEB (alkaline-earth bentonite) NF and CP NF in a 10% aqueous suspension, their effective viscosity (η_eff) increases up to 150 ⁰С and further, strongly decreases. This is explained by the fact that, during the hydrothermal treatment of alkaline nanomaterials, some increase in its fineness is observed and, simultaneously, a change occurs in the submicroscopic region (from 69 to 105 Å) of inhomogeneity. Above 150 ⁰C, the number of crystals involved in the formation of coagulation structures decreases, which is reflected in the drop in the effective viscosity of the solution [12]. In the case of CP NF- elasticity and plasticity increase, as well as the coefficient of stability of the coagulation structure. The results obtained are presented in table 2.

Table 2.

Values of cation exchange of clays of the Navbakhor deposit

From the data of table 2 it can be seen that the lowest value of cation exchange belongs to carbonate palygorskite Navbakhor deposit (27.8 mg equiv per 100 g of clay), which is consistent with the salt tolerance of drilling fluids obtained on its basis. The replacement of the exchange complex of carbonate palygorskite with various ions slightly affects its hydrophilicity.

In practice, “bentonite–kaolin”, “bentonite–palygorskite”, “bentonite–hydromicaceous” clay mixtures are used for drilling wells in various conditions.

However, mixtures i.e. Compositions of these types of clays from the Navbokhors deposit for drilling fluids have not been studied enough.

In order to fill this gap, we studied a number of compositions obtained on the basis of clays from the Navbakhor deposit, etc.

In drilling fluids obtained from compositions of clay minerals, attention is drawn to the relative decrease in the minimum concentration of the formation of a coagulation structure in all basic mixtures [13]. All these features of the formation of coagulation structures in solutions of polymineral clay compositions, determined by the possibility of the occurrence of contacts of certain types, the most effective under the conditions of the existence of suspensions, and their distribution in the volume of the system, i.e. scheme for constructing the framework of the solution, are coagular confirmation of the advantage of carbonate palygorskite over other types of clays [14].

Therefore, we can conclude that, in terms of physicochemical and technological parameters, drilling fluids obtained from compositions of CP NF with other clay minerals are superior to the solution obtained from other individual clays.

Carbonate palygorskite rather slowly increases its viscosity in comparison with bentonites in the drilling fluid. Therefore, some time is required to increase the viscosity of the solution after the addition of carbonate palygorskite (CP NF).

Conclusions

1. The use of CP NF in the composition of clays for the production of drilling fluids increases their thermal stability, which is very important for deep and complicated drilling of oil and gas wells;
2. CP NF increases the resistance of drilling fluids to salt and hydrogen sulfide aggression;
3. The addition of CP NF to the composition of the clay composition reduces the consumption of clays in drilling fluids.

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