Justification of drilling parameters of a typical well in the conditions of the Samskoye field

In the Republic of Kazakhstan, there is a noticeable shortage of water resources, which is a consequence of the natural features of its territory and climate. In particular, there are big problems in the water supply to the Mangystau region. The main source of water in the region is the Samskoye groundwater field. In this work, the conditions of the Samskoye field are typified, the method and technological parameters for drilling a typical well are selected and substantiated. It has been established that rotary drilling with reverse circulation in the conditions of the Samskoye field has significant advantages. The maximum possible production rate of drilling with reverse circulation, taking into account the limited thickness of the aquifer, is 4.3 times higher than with rotary drilling with direct circulation and 2.5 times higher than with percussion drilling. With the same filter pipe diameter, the greatest possible thickness of the gravel pack layer in reverse circulation drilling is 15 times greater than for conventional rotary drilling and 3.7 times greater than for percussion drilling. Thus, the use of rotary drilling with reverse circulation will solve an important problem – the provision of domestic and drinking water supply to the area.


Introduction
Water resources play a crucial role in the economy of any country.An important resource is groundwater extracted from boreholes.The problem of the development and protection of groundwater of the Planet Earth is in the focus of attention of special UN organizations [1].In the Republic of Kazakhstan, there is a noticeable shortage of water resources, which is a consequence of the natural features of its territory and climate.A significant part of its vast territory, including the center, south and west, belong to the zones of deserts and semi-deserts, characterized by rare precipitation and underdeveloped river networks.The earth's surface is often covered with salt marshes, and the permeable layers close to the surface contain waters with high mineralization and cannot be used for drinking purposes.
Since 2002, the country has been consistently implementing the programs "Drinking Water" for 2003-2010, "Ak-Bulak" for 2011-2020 and the State Program for the Development of Regions for 2020-2025.
The "Drinking Water" Program started in 2002.The goal of this program was the complete provision of drinking water to more than 7,000 settlements, including the installation of water 1254 (2023) 012052 IOP Publishing doi:10.1088/1755-1315/1254/1/012052 2 supply systems in 174 villages and 86 towns.It lasted 8 years for its implementation, 195 billion tenge was allocated from the budget [2].
The second program, which was supposed to help provide Kazakhstan with clean drinking water, was launched in 2011 for a period of 9 years.The program was supposed to provide by 2020 with high-quality drinking water from centralized water supply systems the rural population of Kazakhstan by 85% and the urban population by 100%.For these purposes, it was planned to allocate a total of 1.3 trillion tenge.In 2011-2018, within the framework of the state program for the development of regions, 2015 projects were implemented from the republican budget for the development of water supply and sanitation systems [2].
Unfortunately, for a number of reasons, the implementation of these programs did not allow solving the problems of water supply in certain regions.
Mangystau is an industrial region.The basis of the economy is the oil and gas sector.In the structure of industry, the main share is occupied by the mining industry and quarrying, the share of which at the end of 2020 amounted to 85%.The industry employs about 25% of the population of the region, the share of the industry in the gross regional product is about 50%.Regional enterprises annually produce more than 10% of the industrial output of the country [3].
Based on the current demographic situation and the development of the region as a whole, the need for water supply in the region is growing every year.The water supply of the region is carried out from the Astrakhan-Mangystau water conduit and desalinated sea water, since there are few sources of natural groundwater.To date, drinking water consumption is 149 thousand m 3 /day.There is a deficit in the region's water supply in the amount of 51 thousand m 3 , and given the development of the region by 2025, the need will be 250-260 thousand m 3 , and the deficit will be 100-110 thousand m 3 [4].
The implementation of the 2nd stage of bringing the capacity of the desalination plant "Kaspiy" to 40 thousand m 3 /day has begun.JSC NC KazMunayGas is building a plant with a capacity of 17 thousand m 3 /day at the Karazhanbas field.
To supply the city of Zhanaozen in the area of Kenderli and the village of Kuryk, it is planned to build desalination plants with a capacity of 50 thousand m 3 /day, and on the territory of MAEC-Kazatomprom LLP with a capacity of 24 thousand m 3 /day.It is planned to build a desalination plant with a capacity of 5 thousand m 3 /day in the city of Fort Shevchenko [3].
Drinking water supply is provided by three sources and their share in the total volume of water consumption has the ratio: • sea water -52.4%;

Literature review
The Samskoye field is the main source of groundwater for the city of Zhanaozen.The field has been in operation since 1970.In 1979, the established total water withdrawal was only 0.02 thousand m 3 /day, mainly due to private wells dug by the local population.Currently, the withdrawal of groundwater in the city of Zhanaozen has increased to 6.4 thousand m 3 /day, which is 18% of the resources of the Samskoye field, although the problem of high-quality water supply to the city is still acute [6].
This problem can be solved only by significantly increasing the number of water wells and obtaining the maximum flow rate of groundwater of standard quality at the lowest cost.
Established according to the report [7] and approved for category B, the operational groundwater reserves are 21.2 thousand m 3 /day for fresh water and 14.3 thousand m 3 /day for slightly brackish water.The same values appear in modern documents [6].
For successful drilling of a well in the conditions of the Samskoye field, it is necessary to justify the drilling method, select drilling equipment, composition and parameters of the drilling fluid, rock cutting tools and drilling mode parameters.
As a rule, the main directions of scientific research are carried out in two main directions: solving issues related to the technology of cleaning a well from cuttings [8] and developing optimal parameters for the operation of a rock cutting tool [9].
Most often, drilling with direct circulation of drilling fluid is used for water wells.This technology is simple to organize, does not require additional equipment, and allows efficient use of the energy of the drilling fluid for the destruction of rocks [10].
However, with this circulation method, low drilling fluid flow rate, poor particle retention, low drilling efficiency, and severe wear of the drill bit are observed [11].
Another huge problem with this method is the high time and cost involved in combating fluid losses [12].
Usually, to eliminate this complication, the installation of casings or plugging of the absorbing layer in various ways is used [13].However, when drilling wells for water, the use of these methods will only lead to unjustified expenditures of time and money.
The use of reverse circulation effectively solves the problem of drilling fluid losses in the well [14].
Reverse circulation drilling has proven to be highly effective in drilling wells for various purposes.
Thus, this method was successfully applied for the extraction of uranium ore by underground borehole leaching at operating technological wells of Volkovgeologiya, with an average total depth of 300-500 m [15].
There are examples of its use even in mine exploration instead of traditional core drilling, where reverse circulation drilling has high drilling efficiency and low cost [16].
According to [17], compared with traditional core drilling, drilling with reverse air circulation increased drilling efficiency by 70-90% while reducing costs by 30-50%, the number of accidents during drilling decreased by 60-70%.
It should be noted that the reduction of accidents is the most important factor in improving the efficiency of drilling wells, since the cost of repairs significantly increases the cost of well construction [18].
Airlift reverse circulation drilling showed high efficiency when drilling geothermal wells [19].The application of airlift reverse circulation drilling technology is possible even in the construction of well with a depth of 4200 m, which is the deepest geothermal well in China [14].
Note that another possible application of reverse circulation with the help of an airlift is not drilling a well, but expanding it with the help of jet jets [20].
The experience of using this technology in drilling wells for gas hydrates is also known [21].Thus, in recent years there has been a steady trend towards expanding the scope of drilling wells with reverse circulation of drilling fluid.This is due to the fact that this method has a number of significant advantages.
Thus, the use of reverse circulation drilling allows drilling wells with a diameter of up to 1500 mm [22].
Reverse circulation drilling technology is much more efficient, has better technical support and will play an increasingly important role in water well drilling in the future [23].
When using this method, the well production rate increases by about 30% compared to direct circulation drilling [24].
The key parameters of the drilling technology with reverse circulation and airlift gas injection are the volume of gas injection and displacement of the drilling fluid, the change of which regulates the bottomhole pressure [25].
It should be noted that reasonable recommendations do not include a choice of parameters for reverse circulation mechanisms and a large number of design flaws hinder the wide practical application of this drilling method [26].
Thus, the purpose of the article is to typify the conditions of the Samskoye field, choose the drilling method and justify the technological parameters of drilling a typical well, which will solve an important problem -providing household and drinking water supply to this area.

Results
On the territory of the Mangystau peninsula, surface water is practically absent.Since the sixties, exploration work has been carried out here, which made it possible to discover a number of groundwater deposits suitable for development.
A typical field is the Samskoye field.In the course of exploration work for 1968-1969, according to the report [7], this field has the following features.It has a total area of 1500 km 2 and is composed of Quaternary field of the North Ustyurt trough.Water-bearing formations have the form of lenses of various shapes and sizes.They are represented by fine-grained sands with small admixtures of medium and fine-grained sands.
The field has been in operation since 1970.A number of wells have been drilled, mainly by hand, as well as by UGB-50 drilling rigs.These wells are characterized by a low flow rate; their depth does not exceed 50 m, and their diameter is 150 mm.
According to the degree of water mineralization, the deposits are divided into two groups.The first group includes fresh waters with salinity up to 1 g/l (in fact, mineralization from 0.2 to 1 g/l occurs).These waters belong to the hydrocarbonate-chloride-sodium type.The second group includes slightly brackish waters with salinity up to 3 g/l.Waters of the first type are suitable for drinking needs of the population, waters of the second type can be used for technical needs and for the needs of cattle breeding.
Water-bearing rocks are mainly represented by fine-grained sands with small admixtures of medium-and fine-grained sands.Aquifers are generally highly homogeneous.The values of the filtration coefficient in general are in the range from 1 to 12 m/day, but in most cases they do not go beyond the range of 5.6-7.0 m/day.Waters are characterized by low pressure.
Roof and bottom rocks for aquifers are usually loams, less often they are represented by sandy loams, sandstones on lime cement and clays.
The depth of occurrence of fresh waters established by the mentioned studies is in the range from 1.5 to 44 m, the thickness of aquifers, according to modern data, reaches 39 m with an average value of 14 m.Aquifers of weakly brackish waters are located below the aquifers of fresh waters and are separated from them by layers of aquicludes.
In the article [27] based on the study of the geological and technical conditions of the Samskoye field, it was substantiated that the use of a rotary drilling method with reverse circulation makes it possible to multiply the well flow rate; reduce their required number; improve the quality of produced water; drastically reduce the well completion time; significantly lengthen the time of operation of wells; provide high rate of penetration (ROP); reduce the cost per cubic meter of produced water.Below, in support of this proposal, mathematical algorithms are given that allow obtaining the necessary numerical characteristics.
In order for the mathematical apparatus to be focused on the specific geological and hydrogeological conditions of the considered groundwater deposit, a basic model of a water well was built, which must meet the requirements of the following factors.
Economic factors: • the maximum possible production rate of water; • the maximum degree of its purification from mechanical impurities; • minimum costs for the equipment of the water intake and for experimental pumping; • minimum costs for current and workover of the well; • maximum ROP; • minimal time spent on round trips and drill string extensions; • minimum cost of the drill string; • the possibility of exploration and development of productive layers located below those that are known at the moment.

Geological and hydrogeological factors:
• depth of the productive aquifer; • productive formation capacity; • its mineralogical composition; • filtration coefficient; • reservoir pressure; • types of rocks included in the well section; • drillability of rocks.

Technological factors:
• ensuring the speed of the upward fluid flow, which guarantees the cleaning of the bottomhole from cuttings and a high ROP; • using an airlift method to create a reverse circulation; • ensuring the efficient operation of the airlift at the greatest planned depth of the well; • taking into account the fact that the maximum height of the updraft corresponds to the upper position of the swivel on the mast.

Taking into account the above requirements, the following typical model for drilling a water well was adopted
The depth of a typical well is assumed to be 200 m.This is justified by the following arguments: • Established by earlier studies, the depth of fresh water (with salinity below 1 g/l) at the Samskoye field reaches 44 m, and the thickness of aquifer lenses reaches 39 m.Summing up these two figures, we obtain the depth of the base of the aquifer 83 m.The fact that the aquifers are composed, as a rule, of fine-grained sands, we take the length of the settling tank to be 15 m.Thus, the maximum depth of a well drilling into fresh water that exists at the moment can be 98 m; • In addition to fresh water, it is also planned to use waters with a higher (up to 5 g/l) salinity.
Such waters are used for technical and agricultural needs.According to the exploration work carried out, the brackish water aquifers are located below the fresh water aquifers and are separated from them by layers of aquicludes.Thus, the maximum required depth may be significantly greater; • The given parameters of water wells refer to exploration work carried out before 2012 [6].
The possibility of discovering exploitable aquifers when drilling wells to great depths should hardly be ruled out; • The study of the features of drilling water wells by the proposed method to a depth of 200 m (and possibly even more) will allow a more complete assessment of the proposed method of drilling.The depth of the roof of the productive aquifer is assumed to be 170 m.
Based on the previous paragraph, in which, according to the considerations given there, it was decided to take the depth of a typical well equal to 200 m, the location of the aquifer was accordingly shifted down.
The aquifer has a thickness of 14 m.
According to available materials, this is the average thickness of productive aquifers containing fresh water.
The filtration coefficient of permeable rocks of the aquifer is 6.3 m/day.The aquifers of the Samskoe field are composed of homogeneous fine-grained sands and are characterized by the indicated average value of the filtration coefficient.
The static head of a productive formation penetrated by a typical well is assumed to be 100 m.This is justified as follows: • the available materials do not allow to establish the average value of the static head for the drilled wells of the Samskoye field; • since the depth of a typical well is assumed to be 200 m, which is at least 100 m higher than the depth of water wells drilled to date, then, accordingly, the water in this well may have a static head of the order of 100 m.
The diameter of a typical well is assumed to be 800 mm.This is justified as follows: • according to the literature data [28], the diameters of water wells drilled by the rotary method with reverse circulation can be in the range from 300 to 1500 mm; • most often such wells have a diameter of 500 or 800 mm; • with an increase in the diameter, the possible flow rate of the well increases; • the required length of the receiving part of the well is reduced; • the quality of mechanical water purification is improved due to the creation of a powerful layer of gravel; • simplifies the technology of creating gravel; • the required time of experimental pumping is reduced; • at the same time, with an increase in the diameter of the well and, accordingly, the volume of the destroyed rock, the required amount of circulating drilling fluid increases; • when passing through permeable layers, there is a high degree of absorption of drilling fluid in the walls of the well, which requires the continuous supply of new volumes of water; • the ROP decreases; • taking into account the above, and also, taking into account the low filtration coefficient, in the water-bearing rocks of the Samskoye field, which causes a relatively small loss of circulation, the largest of the two most commonly used diameters is 800 mm.

A drill string is accepted from commercially available pipes with a diameter of 146 mm (inner diameter 136 mm).
This is justified as follows: • according to literature data [28] for rotary drilling with reverse circulation, drill pipes with a diameter of 127, 146, 168 and 219 mm can be used; • with an increase in the bore section of drill pipes, the hydraulic resistance to the upward flow decreases sharply; • the content of sludge in it decreases; • at the same time, the required water supply is increased; • sharply increase the time spent on round trips and build-up; • commercially available pipes cannot be used in airlift drilling without significant modification, and their threaded connections are replaced with flanged ones, which drastically slows down the process of connecting pipes into a string and increases the total cost of work; • the minimum diameter of the drill string, when implementing the proposed new solution, is 146 mm.
The height of the sludge-containing water-air mixture above the earth's surface is assumed to be 10 m.This is justified as follows: • the maximum lifting height of the cuttings-containing mixture corresponds to the height of the swivel at the moment of resumption of drilling after the drill string is built up; • this height is equal to the height of the mast of the drilling rig, minus the dimensions of the blocks, hook, shackle, etc., as well as the distance that ensures the necessary maneuver (for example, raising the drill string above the clamp holding it before lowering it into the well); • height of masts of rigs intended for rotary drilling with reverse circulation varies from 8.2 to 18.5 m [29] and averages 13 m; from installations of a similar purpose, the UGB3UK-OP installation has a mast height close to the indicated average value (14.2 m); • thus, minus the average height of the mast 3-4 m, we get the average height of the swivel in its uppermost position, equal to 10 m.
The speed of the upward flow of water is 2.5 m/s.This is justified as follows: • the main advantage of rotary drilling with reverse circulation is the high speed of the upward flow, which ensures effective cleaning of the bottomhole from destruction products; • this advantage is achieved due to a multiple decrease in the cross-sectional area of the channel through which the ascending flow moves; • if during direct circulation speed of the upward flow rarely reaches 1 m/s, then according to the literature [28] during reverse circulation, it should be in the range from 1.5 to 3.5 m/s; • at upstream speeds that do not reach the lower limit of this interval, the advantages of drilling with reverse circulation are lost, and in order to reduce the amount of cuttings formed per unit time, it is necessary to resort to reducing the diameter of the wellbore to normal sizes; • when the upper limit of the specified interval is reached, the removal of large particles is ensured, the size of which is close to the inner diameter of the drill pipes and the discharge hose; • aquifers of the Samskoe field are homogeneous in their mineralogical composition and, as a rule, are composed of fine-grained sands [7]; there is no information on the presence of large pebbles and, especially, boulders; • for the indicated reasons, when drilling a typical well, the upward flow rate will be maintained at the level of the average value over the given interval -i.e.2.5 m/s [30].
The value of the average ROP in the enclosing rocks is assumed to be 15 m/h.This is justified as follows: • when drilling in aquifers composed of fine-grained sands, the penetration rate may exceed 100 m/h; • the limiting factor is the supersaturation of the upward flow with sludge and an unacceptable increase in its total density; • a sharp increase in the density of the upward flow can negate the effect of aeration -the speed of the upward flow will drop to zero and the bit will be stuck with cuttings; • in connection with the indicated risks, the ROP during the passage of the aquifer should be restrained to values that are typical for the host rocks; • the rocks hosting the aquifers in the field under consideration are clays and loams, therefore the accepted ROP of 15 m/h is quite progressive for these rocks when drilling with reverse circulation and, at the same time, quite achievable; • the calculations below will show that at this ROP there will be no unacceptable increase in the density of the updraft.
The depth of the mixer is assumed to be 2 m less than the depth of the well.This is justified as follows: • fluid circulation in the airlift method is carried out due to the fact that at the mixer level a difference in hydrostatic pressures is created in the annulus filled with water and in the drill string filled with water-air mixture; • at an extremely low (which corresponds to the stated ratio) location of the mixer, the maximum height of the mixture column is achieved, and hence the maximum value of the difference mentioned above; • due to this difference, the maximum speed of the upward flow is created and, accordingly, the best cleaning of the bottomhole from cuttings; • starting from a certain drilling depth, due to the limited pressure of the compressor, the resumption of airlift circulation after technologically necessary stops (for example, when building up the drill string) becomes impossible; • to resume circulation, the immersion depth of the mixer has to be reduced; • at the same time, the height of the mixture column and the difference in hydrostatic pressures inside and outside the drill string decrease with a corresponding decrease in the circulation intensity; • the existing methods of separating the operating conditions of the compressor during the resumption of circulation and during the normal drilling process are reduced to significant complications in the design of the drill string.
Summarizing, we will present the main parameters of drilling a typical well in a summary (table 1).
Below are calculations of possible results of the practical application of the above typical model of rotary drilling with reverse circulation.
The well flow rate is determined by the possibility of creating the necessary drawdown on the productive formation.During pumping, this drawdown (i.e.pressure reduction) is created by lowering the water level in the pay zone near the wellbore.This reduction is: where h D is the dynamic level (i.e. the distance from the surface to the water in the well during pumping), h S is the static level (the same level before pumping).The static level is also defined as: where h P is the formation top depth, H S is the static head.During pumping, the flow rate corresponding to the decrease (in m 3 /h) is determined by the formula [31]: where m is the thickness of the aquifer in m; K F is the filtration coefficient in m/day.In confined aquifers, according to formula (2), the largest possible decrease is a decrease by the value of the static head.
S max = h P − h S In unconfined aquifers, this formula is reduced to: where h F is the depth of the base of the productive formation.When trying to achieve the maximum flow rate from this particular well, an important limitation is to prevent catastrophic production of reservoir material from the walls of the well, caused by an excessively high rate of water withdrawal.
The criterion here is the maximum allowable rate of water filtration through the formation pores in the receiving part of the well.The maximum filtration rate (in m/h) is determined by the formula: where K F is the filtration coefficient in m/day.From formula (6) it can be seen that the maximum allowable velocity drops sharply (in cubic dependence) with a decrease in the filtration capacity of the productive formation.
Taking into account the above limitation when pumping water from a given well, the maximum allowable flow rate in m 3 /h is determined by the formula: where F is the surface area of the well in m 2 through which water enters (corresponding to the area of the filter).Subject to the condition of not exceeding the allowable filtration rate, the highest allowable well flow rate is achieved by varying the filter parameters.
Given the cylindrical shape of the well, formula (7) can be written as where L is the length of the receiving part in m; D is the diameter of the receiving part in m.Formula (8) shows that the surface F can be reached in three ways (or using their combination): (i) By increasing the length of the filter -the most practical and least expensive way.The required length of the receiving part is equal to: However, the application of this method is limited by the thickness of the productive formation m. Taking into account the capacity, the maximum possible well flow rate is determined as follows: (ii) By increasing the diameter of the receiving part, which is usually associated with complex additional operations to expand it: (iii) By drilling a well with a large diameter, which is ensured by the use of rotary drilling with reverse circulation.
We use the results of the methodology presented in formulas ( 1)-( 11) to evaluate the effectiveness of the method of rotary drilling of wells with reverse circulation in comparison with two other drilling methods used in drilling water wells -a rotary method with direct circulation and a shock-rope method.
In a comparative assessment of drilling methods, the main role is played by the maximum possible diameters of the bits when using them (without taking into account the possibility of expanding the receiving part).
For rotary drilling with reverse circulation we accept the diameter value equal to 800 mm (table 1).
For rotary drilling with direct circulation in the case of using the widespread installation 1BA-15V, the final diameter (i.e., the one with which the productive aquifer is opened) can be a diameter of 190 mm [32].
When percussion drilling with a rig UKS-22 the size of the first casing cannot exceed 600 mm (i.e.22 inches in inner diameter).When designing a casing telescope specific to percussion drilling, each subsequent casing must be at least 2 inches smaller than the previous one, with the protrusion of one casing from under the other increasing with decreasing diameter.
From table 2 it follows that when building a well with a depth of 200 m and a diameter of the first casing of 22 inches, the required depth can be achieved, taking into account the above conditions, by creating a telescope of 6 casings.In this case, the diameter of the receiving part of the well is equal to the outer diameter of the last (sixth) casing -i.e.columns with an outer diameter of 12 inches (324 mm).
The initial data given in lines 1-5 of table 1 were taken as the basis for comparative calculations.
Given in table 3 the maximum theoretical (excluding restrictions) flow rate of each of the 3 wells is determined by formula (3).The maximum possible value, namely the static head H S , is taken as the reduction in S.
According to formula (6), the admissible filtration rate U F = 5.01 m/h was obtained.The highest production rate (at reservoir thickness m = 14 m), Q max , m 3 /h 41 71 176

Conclusions
According to the results of the work, rotary drilling with reverse circulation in the conditions of the Samskoye field has such advantages.The maximum possible production rate of drilling with reverse circulation, taking into account the limited thickness of the aquifer, is 4.3 times higher than with rotary drilling with direct circulation and 2.5 times higher than with percussion drilling.These results are ratios of accepted diameters.For this reason, the maximum achievable, according to formula (10), flow rate may increase or decrease depending on the thickness of the reservoir m, but the indicated ratio of maximum flow rates will remain the same.
If the reservoir thickness is not a limiting factor, then with the same flow rate (formula (3)), the indicated ratio will be valid for the required lengths of the receiving part calculated by formula (9).The larger the diameter, the smaller the required length (line 4 of the table), with a decrease in which the costs of both the equipment of the receiving part and its repair are reduced.
With the same diameter of the filter pipe (line 2 of the table), the maximum possible thickness of the gravel pack layer during reverse circulation drilling is 15 times greater than for conventional rotary drilling and 3.7 times greater than for cable percussion drilling.
Powerful gravel sprinkling provides the best quality of mechanical cleaning of the sampled water.The consequence of this is also the minimum time required for experimental pumping and the minimum length of the well sump.Such sprinkling also provides the maximum duration of overhaul operation.
It should be noted that with rotary drilling of large diameter wells with reverse circulation, labor costs for the manufacture of a gravel pack are minimized.As a rule, gravel is filled manually through the wellhead, since the large area of the annular space eliminates the possibility of plugging and failure of the filled material to reach the receiving part.

Table 1 .
Basic drilling parameters of a typical well.

Table 2 .
The results of comparative calculations are given in table4.1254(2023)012052 Typical design of a percussion-rope drilling well with a depth of 200 m.

Table 3 .
General parameters used in the comparative analysis of three drilling methods.

Table 4 .
Main indicators of drilling water wells.