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Nazarov Azizbek Ulugbekovich, Principal specialist-PhD., JV «Gazprom International», Tashkent, Uzbekistan E-mail: nazizbek@yahoo.com Tulyaganov Shuhrat Dilshatovich, Principal specialist-PhD., JV LLC «UzLITI Engineering, Tashkent, Uzbekistan E-mail: shtulyaganov@mail.ru
TECHNOLOGICAL PROSPECTS OF OPTIMIZING UNDERGROUND GAS STORAGE SYSTEMS
Abstract: Drilling of a new, repairs of the existing wells in 2008-2011, and transporting part of buffer volume to active storage volume significantly increased gas extraction power of Gazli underground gas storage system (UGS). Decrease of reservoir pressure due to previously mentioned actions made it necessary to develop and employ actions to increase efficiency of existing storage powers for medium- and long-term perspectives. Technical solutions for all active wells are developed which guarantee the decrease of spillage area of gas bubble, created during the injection process, through formation of local repressive swirl with a zone of increased reservoir pressure around well. Implementation of this technology provided significant increase of well flow rates due to rational usage of energy of inj ected gas.
Keywords: underground gas storage, reservoir pressure, porosity, relative permeability, regulation of gas bank
pumping.
Results and discussion. Demand for natural gas in world markets is constantly increasing. This tendency causes fast development and implementation of energy saving program and optimization of gas consumption.
Demand for gas is characterized with seasonal fluctuation. Decrease of consumption of natural gas in industrial and socio-economic economic sectors in countries around the word happens during spring-summer periods. During winter periods, consumption stays at a high level.
Accounting for consistent tendency of gas consumption fluctuation during summer and winter periods, and accounting for ever-increasing demand for gas during winter periods, the development and improvement of underground storage of gas is becoming economically prevalent.
In this aspect it is important to give attention to goals of optimization of gas storage, which should account for maximum effective usage of underground reservoirs like UGS, created at water-bearing beds as well as at facilities based on developed gas reservoirs.
Analysis of cyclic exploitation of UGS "Gazli", created on the facilities of the largest gas reservoir, opened in 1956, shows the possibility of increasing the gas storage volumes in UGS station without an increase of gas-saturated volume of interstitial space by the means of including part of the buffer storage volumes of natural gas into circulation. Gazli UGS
is a unique facility where gas is stored under anomalous low reservoir pressures, and it has high potential for development due to functional significance and location.
In the period of 2008-2011 the power of this storage significantly increased, which resulted in increase of natural gas withdrawal during winter time from 2.38 billion to 3.10 billion cubic meters.
Due to prepared and accomplished solutions related to drilling of 30 wellsites, recovery of productivity of 46 wellsites along IX, X-XII horizons, additional 6.5 billion cubic meters of gas is withdrawn, which is directed to Republic's consumers and to export.
Regulation of spillage front. As a research has shown, gas injection along 80 exploited wells (2010) caused a significant decrease of their productivity under gas withdrawal out of255 wells. Project solutions related to gas injection into 102 injection wells and withdrawal from 232 wells did not allow to maintain consistent withdrawals along all wells. Every well, being at disproportionally big gas saturated porous volume (under low reservoir pressure of 1.0-1.14 MPa), has its own drainage area, and absence of pumping of gas along these wells does not allow to increase their productivity during withdrawals.
This area of drainage determines production possibilities of each well.
Due to that the goal of optimization of exploited characteristic of all the wells is the decrease of spillage area of gas bubble created during pumping process due to local repressive swirl with a zone of high reservoir pressure around wells.
Solution is developed which ensures gas injection into wells with the control of spillage front of gas bubble and
which allows ensuring maintenance of injected gas volume at near-well zone by creation of local zones of high pressure. This happens during the increase of wells' productivity in the process of gas withdrawal during winter periods under the decrease flow of reservoir water to well (Figure 1).
Figure1. Hypothetical process of gas spillage
This solution requires thorough repairs of wells with the recovery of cement ring and substitution of exploited column (Figure 2).
Implementation of the following technology allows to implement:
- rational usage of energy of pumped gas with the 1.52.5 times increase of gas production from wells during winter periods;
- most effective usage of installed compressor stations at gas withdrawal with maximum load;
- decrease of nonproductive buffer gas storage;
Figure 2. Thorough repairs of wells of IX horizon with the recovery of cement ring and substitution of exploitation column
Optimization of operating wells. Development of pre-project modernization solutions of underground gas storage (UGS) at IX horizon of Gazli reservoir showed that option, which uses horizontal wells for UGS exploitation, allows to optimize all wells and technological regime of their work.
In order to increase the active storage volume to up to 10 billion cubic meters a year, it was calculated that 117 new horizontal wells would be needed. This compared to traditional option of drilling 260 vertical wells for achieving planned volume will allow a decrease in investments for drilling and field facilities installation.
Table 1. - Forecast of technological indicators of UGS exploitation when horizontal wells are drilled and used to increase gas storage volumes to 10 billion cubic meters
Year Horizon IX (UGS)
Yearly withdrawal (injection), bln cub m Amount of wells
Existing New Total
2018-2019 2500 255 255
2019-2020 3273 255 4 259
2020-2021 4651 217 28 245
2021-2022 7500 184 75 259
2022-2023 8789 157 97 254
2023-2024 10000 133 117 250
2024-2025 10000 113 117 230
2025-2026 10000 96 117 213
2026-2027 10000 82 117 199
2027-2028 10000 69 117 186
2028-2029 10000 59 117 176
2029-2030 10000 50 117 167
2030-2031 10000 43 117 160
2031-2032 10000 36 117 153
2032-2033 10000 31 117 148
Table 2 - Indicators of horizontal and vertical wells productivity when 10 bln cub m of active gas storage is achieved
Injection of 10 bln cub m of active gas (182 days) Withdrawal of 10 bln cubic m of active gas (182
Indicators
april may june july august september october november november december january february march april
Amount of days 18 31 30 31 31 30 31 5 15 31 31 29 31 10
Gas volumes, mln cub m 301 1329 1714 1771 1771 1714 1329 71 793 2436 2436 2279 1771 285
Production of
one vertical/ horizontal o NO ON 8 3 3 OO 8 7 OO 3 2 ON 7 6 ON 01 0 4 0 0 2 5 OO 2 9 31 5 7 NO 2 3 NO 5 2 N(3
well, thou- <N OO OO 8 OO 9 OO 0 2 OO 21 5/ 2 6/ 2 91 OO 7 uo 6 <N 5 <N 14 141
sand cub m 2 2
a day
Amount of vertical/horizontal wells 98/22 240/55 304/69 6 6 OO 8 2 Z9/SLZ 262/60 190/43 64/15 3 6 NO 7 2 441/100 476/108 515/117 400/91 202/46
Besides already mentioned advantage of investment decrease, exploitation of horizontal wells will allow with high probability to prevent losses of make up gas into payzone. This will happen due to creation of gas bubble at bottomhole, and
this won't let cut out or trapping of gas with reservoir water during cyclic process of production activity (Figure 3).
Figure 3. Scheme of production of vertical and horizontal wells during UGS regime (IX horizon) and during gas production from X-XIII horizons
Exploitation of X-XIII gas reservoir horizons is more effective when horizontal wells are used. This is due to bigger coverage of drainage, inclusion into development of earlier unavailable sublayers, and production ofwells at lower depressions which allows to increase non-water period of natural gas production.
Optimization of withdrawal regimes. Due to this the maintenance of technologic regime of gas pumping at consistent volumes along all wells of UGS is a necessary condition for creating high reservoir pressures at drainage zones of wells.
With the goal of increase of gas withdrawals during winter periods we developed and implemented a new system of research work and processing of informative materials.
Current material includes exploited parameters of objects during the pumping and withdrawal of a gas, in the sectional view of each well, which guaranteed optimization of technological regimes of withdrawal of natural gas from UGS at the level of 21.0-25.5 millions of cubic meters per day.
For operating control and monitoring of exploited characteristics of UGS and each separate well we developed and implemented info-analytical system (IAS) Gasanalitic.
Usage of IAS Gasanalitic allows optimization works of geologo-technological service of gas storage with the development of operative solutions for accomplishing controlling effects on gas storage facilities.
The goal of this is the optimal usage of installed technology for pumping and withdrawal in connection with energetic possibilities of reservoirs [1] (Figure 3).
The development of this product solves the following goals:
- infological structure is created and system of analysis of exploited characteristics of wells is made;
- algorithms and program products for analysis of technical condition of wells and program products of research results of geo-industrial parameters of exploitation of UGS are developed;
- principles of solutions of operative goals of analysis of exploited characteristics of UGS on the level of geologic services of gas storage and research organization are determined;
-3D working geological model of underground storage for operative estimation of gas saturation of reservoirs is built;
- methods of operative forecast of indicators of UGS exploitation depending on the planned levels of pumping and withdrawal are developed;
- software for formulation and usage of information, and documentation about condition of UGS exploitation with database is created.
The basis of info-technological solutions during the creation of program products were the methods and procedures of analysis of complex systems, to which system of underground gas storage belonged [1] (Figure 4). Systematic usage of methodological practices of analysis of exploitation of UGS with regulatory procedures of collection of geological-geophysical information about wells at the real scale allows creating wholesome infological structure, which maintains monitoring of exploited characteristics among gas storage facilities [2].
Conclusion
Practical implementation of the offered technical solutions for optimization of storage of natural gas at underground
- increase the exploitation efficiency of all wells up to 7% due to decrease of water saturation of well production to the middle and the end of gas withdrawal;
- decrease the volume of trapped gas at gas-water zones of reservoirs up to 3% due to improvement of the system of installation of rational technological regimes ofwell exploitation during pumping and withdrawal of gas.
References:
1. Butovsky A. G. Methods of system control with distributed parameters.- M: Science, - 1975. - 568 p.
2. Nazarov U. S. Informative analysis system of oil gas deposits development // Izvestiya AN RUz Series "Informatica". -Tashkent. - 1993. - No. 6.
reservoirs of Gazli UGS, and control and monitoring of exploitation storage objects allowed
- guarantee the 5% decrease of energetic expenses by decreasing repression on reservoir during pumping of gas and depression under gas withdrawal;
