Enhancing the quality of the initial discovery of carbonate gas deposits in the Zahoryanska field zone by improving the drilling mud

The results of studies of the flushing fluids influence on the capacity-filtration characteristics of carbonate rocks are presented. It was found that hydrogel-magnesium fluids using desulfurized bischofite are the optimal flushing fluid for opening carbonate-type formations. Solutions of the hydrogel-magnesium type, with a relatively small depth of penetration into the formation, are distinguished, among others, by a high coefficient of recovery of permeability. After the application of such solutions and subsequent acid treatment, the pore space of the formation can be almost completely restored. The effect of the presence of sulfate ions in the bischophyte on the permeability and recovery coefficient of the carbonate core was investigated. It was established that in the presence of sulfate ions in bischophyte, the permeability and formation recovery in the presence of formation water decreases by 11.53 times, and when using pure bischophyte – only by 1.29 times. An improved method of preparing bischofite for use in washing liquids has been developed. The results of industrial use of the developed method are given.


Introduction
Ukraine has significant deposits of developed mineral deposits.Nowadays, 90 types of minerals have been explored, which are concentrated in almost 8,000 deposits.For sustainable development, it is necessary to increase the efficiency of the natural resources using [1][2][3].
Provision of own energy resources is a guarantee of not only energy, but also state independence of Ukraine.Unfortunately, in recent years, gas production in Ukraine has begun to steadily decrease.In 2020, 20.23 billion cubic meters of gas were produced, which is 2.2% less than in 2021 (19.79 billion cubic meters), in 2022 Ukraine is going to produce 18.5 billion cubic meters.m of gas, which is 6.6% (1.3 billion cubic meters) less than in 2021.The bloody war between Russia and Ukraine in 2022 radically changed the national gas sector: there was a significant decrease in both consumption and production due to Russian bombing of our gas fields.
Also, the reasons that prevent the realization of the domestic hydrocarbon potential are the depletion of deposits (by 65 -70%), which causes a sharp drop in production and difficult access (the average depth of the deposits exceeds 3500 m) and the dispersion of a large number of small fields -reserves 89% of deposits do not exceed 5 billion m 3 .At the same time, in order 1254 (2023) 012011 IOP Publishing doi:10.1088/1755-1315/1254/1/012011 2 to ensure the sustainable development of the oil and gas sector, the annual growth of the raw material base should be 2 -3 times higher than the production level [4].
The drilling of oil and gas wells leads to a destruction of the natural balance of fluid saturation of productive layers.As a result of the penetration of leachate and the solid phase of washing or process fluids, the character of the saturation of the pore space of the rocks in the bottomhole formation zone of the formation changes, processes occur that are caused by the physical and chemical interaction of drilling muds with the rock and formation fluids, which significantly worsen the filtration properties of the rocks [5].
The influence of flushing fluids on productive formations is due to penetration into the pore space of the filtrate collector and dispersed phase, swelling of clay cement, formation of solid sediments and emulsions during interaction with formation fluids, reduction of effective pore volume [5,6].All this leads to deterioration of productive characteristics of wells.Therefore, the question arises of preserving the natural productive properties of formations during the construction, repair and restoration of wells.An equally important task is the choice of means and methods for restoring to the initial level and improving the filtration properties of rocks contaminated with drilling fluids.

Problem statement
Drilling muds play an important role in ensuring the growth of drilling volumes and hydrocarbon production.Contamination of the bottomhole formation zone of the formation during primary and secondary opening and the associated deterioration of natural reservoir properties can lead to a significant loss of well productivity and, conversely, minimization of such contamination can allow obtaining industrial products from deposits, the production of which, even recently, was impossible for technical or economic reasons [5][6][7][8][9].
A significant contribution to research of the fundamental physico-chemical processes occurring in the wellbore and in the area around it, in relation to the issues of the stability of the shaft, the hydrodynamics of flushing and cleaning of the outcrop, and the qualitative opening of productive horizons were made by number of researchers.The development of these provisions regarding the conditions of drilling wells in the deposits of Ukraine is reflected in [7,8].These scientists have developed and implemented a large number of various formulations of drilling muds aimed at effective drilling of wells and high-quality opening of productive horizons in difficult mining and geological conditions.
According to modern ideas, clay-free biopolymer systems provide the highest quality of opening productive horizons among water-based drilling muds [10][11][12].This is due to the biodegradable polysaccharide composition and acid-soluble solid phase, as well as unique rheological, structural-mechanical and filtration properties.In addition, clay-free biopolymer drilling muds create almost no man-made load on the environment, ensure successful passage of horizontal and inclined wells, uncomplicated opening of zones with abnormally low formation pressures, avoidance of scree, absorption and differential adhesions.
For carbonate-type collectors, which are typical for the gas condensate field of the Zahoryanska area of the Dnieper-Donetsk rift, it is advisable to use a polymer-potassium lowclay type, a clay-free polymer-magnesium type liquid on a polyacrylamide basis, and a hydrogelmagnesium type liquid using bischofite.
The use of bischofite solutions as part of flushing fluids is promising.Estimated resources of bischophite only within the Orchykiv depression of the Dnieper-Donetsk rift amount to about 10 billion tons.In practice, the use of bischofite at gas production enterprises is often limited due to its irrational use.Salt deposits, when bischophite enters the formation, can form in the pores of the rocks of the bottomhole formation zone, reducing their permeability and porosity [13,14].The chemical composition of inorganic deposits is represented mainly by calcium sulfates and carbonates, oxides, carbonates, oxides, and iron sulfide [10,11].
The main reasons for the formation of salts are: • a decrease in pressure and an increase in the temperature of mining fluids, which leads to the release of dissolved carbon dioxide into the gas phase: • mixing of incompatible waters: • supersaturation of fluids is limited by soluble salts and their salting out.
Considering the above, there is a need to study the influence of flushing fluids on the capacityfiltration characteristics of carbonate rocks and to improve the method of preparing bischofite solution for use as part of flushing fluids when drilling wells.
The main direction of research was to determine the type of liquid that will be used as a flushing agent in carbonate-type rocks and at the same time will maximally preserve the capacity-filtration properties of the pore space.

Materials and methods
The following drilling muds were studied: polymer-potassium low-clay type fluids, clay-free polymer-magnesium type fluids on a polyacrylamide basis, and hydrogel-magnesium type fluids.
The influence of the flushing fluid on the properties of carbonate rocks was evaluated according to the standard methodology at the UDPK-1M installation (figure 1).
The research was carried out on carbonate core material taken from the drilling interval 4935 -5090 m of well №1 of the Zahoryanska area, which corresponds to the interval of well №3, which needs to be opened.Before conducting the research, the core was wetted with formation water of the chlorcalcium type ρ=1.18 g/cm 3 , typical for deposits B24-25 of the Zahoryanska field.Repression on the formation during the research was 250 -300 atm, that is, the conditions of maximum repression on the formation under dynamic loads during the drilling process were modeled.
According to some data, under the influence of process fluids based on bischofite, the filtration properties of the reservoir often deteriorate due to the formation of insoluble sulfate deposits in the pore space as a result of the interaction of filtrates and reservoir fluids [2].In the second series of experiments, the effect of the presence of sulfate ions in bischophyte on the properties of carbonate collectors was studied.In order to detect the process of pore blocking by chemically formed gypsum, the core was wetted with formation water of chlorcalcium type ρ=1.18 g/cm 3 .Up to 20 volumes of the pore space of purified and untreated bischofite ions SO 2- 4 were passed through the core.
Bischofite solutions with a mass fraction of MgCl 2 of 24% from well №1 of the Zahoryanska area were used for research.The chemical macro-composition of bischofite solutions from well №1 of the Zahoryanska field is presented in table 1.
In order to find components that accelerate the precipitation of gypsum deposits, studies were conducted to determine the effect of surface-active substances on the process of calcium sulfate deposition.
Before conducting the experiment, Na 2 SO 4 •10 H 2 O was added to the bischofite solution with a mass fraction of MgCl 2 of 24% at the rate of 28.38 g/dm 3 .In a measuring cylinder with a volume  of 100 ml, the necessary amount of the prepared solution of bischofite with a mass fraction of MgCl 2 of 24% and with an additive of surfactant concentration of 0.1% was placed.The calculated amount of calcium chloride dried to a constant mass was added to the solution.Some time, after the initial turbidity, massive spontaneous formation of solid phase nuclei occurred.At this stage, the contents of the measuring cylinder were mixed.After the onset of sedimentation, stirring was stopped and further sedimentation took place under static conditions.The resulting solutions were kept in a thermostat at a temperature of 20 ℃ for 168 hours.The content of sulfates in the sediment was determined by the gravimetric method.After 7 days, the sediment was filtered through a "blue tape" filter, decanted, washed with hot distilled water until there was a negative reaction to the chloride ion.After that, the filter was dried on a tile in a crucible, charred in a crucible furnace at a temperature of 500 ℃, fired at a temperature of 800 ℃ and weighed after cooling.The degree of deposition of calcium sulfate X in % was determined by formula 1: where m -is the mass of calcium sulfate in the sediment, g; m 0 -is the mass of calcium sulfate, which corresponds to the initial concentration of sulfate ions in the solution, g.At the same time, the nature of the deposition was visually determined by the change in the volume of the sediment and the structure of the crystals under an Olympus BX 41 microscope (2005, Japan) with a magnification of 100 times.

Experimental laboratory results
From the conducted research, it can be concluded that solutions of clay-free polymer-magnesium type on polyacrylamides have the greatest depth of penetration into the formation.At the same time, they are characterized by a low coefficient of reservoir recovery.In addition, after processing the core with an acid solution, insoluble polymer structures are formed, which almost completely calm the formation.
Solutions of the polymer-potassium type, although they have a small penetration depth, but this effect is achieved by calming the pores with a solid insoluble clay phase, which is part of these solutions.Due to this, the formation recovery rate after using such solutions is also low.
Solutions of the hydrogel-magnesium type, with a relatively small depth of penetration into the formation, are distinguished, among others, by a high coefficient of recovery of permeability.After the application of such solutions and subsequent acid treatment, the pore space of the formation is almost completely restored.They do not contain an introduced solid phase that would irreversibly calm the pore space.Carbonate blockers are formed during the preparation of this flushing fluid and do not calm the pore space.
Table 2 presents the results of studies of the influence of flushing fluid on the capacity-filtration characteristics of carbonate rocks.
Bischophyte is a component of the investigated solutions.A study of the influence of the presence of sulfate ions in bischophyte on the properties of carbonate reservoirs was carried out.The influence of sulfate ions on the permeability and recovery coefficient of the carbonate core is shown in Table 3.It is noted that in the presence of sulfate ions in bischofite, the permeability and recovery of the formation in the presence of formation water decreases by 11.53, and when using pure bischofite -by only 1.29 times.Thus, as the research results showed, it is not necessary to use desulfated bischofite for the preparation of polymer-magnesium solutions.To solve this problem, it is necessary to use precipitation catalysts, which make it possible to simultaneously carry out rapid desulfation of bischofite and prevent salting out of chlorides.The crystallization of calcium sulfate from an aqueous solution of bischofite with a mass fraction of MgCl 2 of 24% in the presence of surface-active substances was investigated.The processes of calcium sulfate crystallization in the presence of various surface-active substances differ significantly (figure 2) [15,16].
In figure 3 shows unwashed and washed from chloride precipitates on the filter.Precipitates with the addition of amphoteric surfactants have a visually smaller volume compared to the control experiment, and those with the addition of cationic surfactants have a more fragile structure and large crystals.
In the unwashed sediment formed from pure bischofite, without the addition of surfactant, a group of closely spaced crystals of various shapes is formed.Addition of cationic surfactants (KI-1M, St) to the initial solution leads to a decrease in extraneous formations and the precipitation of needle-like crystals of different lengths.
The structure of calcium sulfate crystals without the addition of surfactants and with the addition of cationic surfactants is approximately the same.Crystals are needle-shaped and vary in length.However, the addition of amphoteric surfactants (EM and CAPB) and cationic surfactant SRK leads to the formation of larger lamellar crystals, the diameter and thickness of which increase significantly, and the length decreases (figure 4).After calcination of the precipitate at a temperature of 800 ℃, the structure of the crystals with different additives of surfactant and the control experiment is approximately the same.
The positive effect of surfactant additives is obvious, since in the presence of these reagents the sediment is formed in the form of a fine dispersion, which, unlike amorphous sediment, can be easily removed from the container where bischofite desulfation takes place.
As can be seen from table 4, the degree of precipitation of calcium sulfate from an aqueous solution of bischofite with a mass fraction of MgCl 2 of 24% in the presence of surface-active substances (w = 0.1%) decreases in the following order: CAPB > EM > SRK > St > KI-1M, and the sediment volume, on the contrary, increases in this order.
The lower value of the calcium precipitation degree of sulfate relative to the volume of sediment in the presence of St and KI-1M can be explained by the fact that the process of salting out chlorides prevails over the chemical reaction of precipitation of sulfates.
Analysis of the influence of the amount of precipitant on the rate of separation of solid and liquid phases showed that EM, CAPB and SKR surfactants also have close results in terms of the rate of precipitation.Phase separation takes place within 0.5-1 h, the solid phase settles to the bottom of the vessel, a clear separation boundary between the phases is visible.Over time, the volume of sediment does not increase, and after 5 days its compaction is observed.In the presence of cationic surfactants KI-1M and St, phase separation occurs very slowly, the sediment is loose, occupies 2/3 of the cell volume.
The results of the study showed that the excess introduction of precipitant CaCl 2 by more than 4 times in the presence of all surface-active substances leads to a decrease in the degree of precipitation of CaSO 4 , while the volume of the precipitate does not change.The results of the experiments show that among the investigated surface-active substances EM, CAPB and SKR are effective catalysts of sulfate precipitation and inhibitors of salting out chlorides.Therefore, in order to accelerate the process of precipitation of CaSO 4 during the desulfation of bischofite intended for the preparation of the flushing fluid of the primary opening of the carbonate deposits of the Zahoryanska area of well № 3, the catalyst cocamidpropylbentaine was used.Due to the addition of this reagent in a volume of 0.1% to bishofite at the time of introduction of calcium chloride, the precipitation process was accelerated four times.Thus, the precipitate of CaSO 4 in bischofite without the addition of a catalyst was formed in a vertical container with a volume of 36 m 3 for 20 days, and with a catalyst -for 5 days.
In addition, in order to fully use the CaCl 2 introduced into the first portion of bischofite, at the in bischofite, the second portion of desulfurization of bischoffite was carried out by introducing 0.1% into non-desulphated bischoffite cocamide-probilbentaine and CaSO 4 sediment, which, according to chemical analysis, contained up to 100 g/l of ions Ca 2+ , which made it possible to speed up the desulfation process and ensure maximum purification of bischofite from sulfate ions.
Thus, in industrial conditions, for the first time, the catalyst for the precipitation of CaSO 4 during the desulfation of bischofite was tested and the technology of preparation of bischoffite was applied by reusing CaSO 4 sediment with an excess content of CaCl 2 , which made it possible to reduce the cost of the preparation process and accelerate the precipitation by 4 times.

Conclusion
Based on the above, the following conclusions can be drawn: 1. Hydrogel-magnesium fluids using desulfurized bischofite are the optimal flushing fluid for opening carbonate-type formations.2. The opening of productive layers in depressions using a solution of desulfurized bischofite allows to preserve the natural filtering properties of the collectors and increase the efficiency of extraction of hydrocarbons from the subsoil.In the presence of sulfate ions in bischophyte, the permeability and formation recovery in the presence of reservoir water decreases by 11.53 times, and when using pure bischophyte -only by 1.29 times.3. When preparing bischofite, it is recommended to use the amphoteric surfactant CAPB as a catalyst for precipitation of sulfates and an inhibitor of salting out chlorides at a mass fraction of 0.1%.This makes it possible to increase the degree of precipitation of CaSO 4 by 76.86%, to reduce the volume of the formed sediment by 9.4 times.4. Industrial tests showed that the preparation of bischofite solution using CAPB ensures a 4-fold reduction in bischofite preparation time, a 7.5-fold increase in the degree of calcium sulfate removal, and a 3-fold decrease in sediment volume.

Table 1 .
Chemical macro-composition of bischophyte solutions from well №1 of the Zatyryno field.

Table 2 .
Results of studies of the influence of flushing fluid on the capacity-filtration characteristics of carbonate rocks.

Table 3 .
The influence of sulfate ions on permeability and recovery indicator of carbonate core.

Table 4 .
The influence of the amount of precipitant on the degree of calcium sulfate of precipitatio in the presence of surfactants (w=0.1%).
3, i.e. in a fourfold excess in relation to the content SO2- 4