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Chemistry and Technology of Fuels and Oils, Vol. 47, No.6, January, 2012 (Russian Original No.6, November-December, 2011)

RESEARCH

PHYSICAL SIMULATION OF THE MECHANISM FOR OPERATIONOF WATER-ENCROACHED (FLOODED) UNDERGROUND GASSTORAGE FACILITIES

Lei Shi,1 Shu-Sheng Gao,2 and Wei Xiong2

____________________________________________________________________________________________________1 Institute of Porous Fluid Mechanics, Chinese Academy of Sciences. 2 Langfang Branch, PetroChina

Research Institute of Petroleum Exploration and Development. Translated from Khimiya i Tekhnologiya Topliv iMasel, No.6, pp. 11 – 15, November – December, 2011.

0009-3092/12/4705–0426 2012 Springer Science+Business Media, Inc.

We have developed a physical simulation model for gas injection/withdrawal consistent with thecharacteristics of water-encroached (flooded) underground storage facilities and we have simulatedthe construction and operation of the storage facility. Based on the operating schedule for the floodedunderground gas storage and the mechanism for gas and water percolation in the flooded reservoir, wedetermined the important characteristics of flooded storage facilities including porosity, degree offlooding, and degree of heterogeneity of the reservoir as well as the operating scheme. We found that thedegree of flooding and the heterogeneity of the reservoir are the crucial parameters for flooded storagefacilities. We show that homogeneous reservoirs should be used as the main target layer of an undergroundgas storage facility. The water content in the reservoir must be taken into account during injection andwithdrawal of gas from the storage facility.Key words: flooded (water-encroached) gas reservoir, underground gas storage, physical simulationmodeling, percolation mechanism, heterogeneity.

Underground gas storage (UGS) is an effective method for storing natural gas with the aim of controllingthe imbalance between its supply and demand [1]. Flooded UGS facilities can include underground gas storage inaquifers and underground storage in depleted gas reservoirs. Although theoretical studies of UGS facilities havegradually matured, not much is known about flooded UGS facilities. It has been assumed that the gas/waterdistribution in the pores of the reservoir varies as a result of interaction between the gas and water during constructionand operation of the flooded UGS. Initially the reservoir is saturated with water, which is displaced by the gas as it is injected.As the water is replaced by gas, the water is pushed out from small pores through the pore throat into large pores. As thenumber of gas injection/withdrawal cycles increases, flow in the pores is enhanced, the gas saturation of the reservoirincreases, and accordingly the capacity of the flooded UGS increases [2].

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Table 1

Cycle Capacity, 108 m3 Volume, 108 m3 Operating

pressure, MPa injected gas recovered gas recovered water

1 11.47 1.59 0.94 0.19 21.45-13.80

2 15.07 4.54 3.89 1.88 30.80-12.23

3 16.35 5.17 1.18 0.05 31.08-24.65

4 17.86 2.69 3.3 1.72 29.81-17.67

5 19.09 4.53 5.18 0.92 30.65-14.22 6 19.98 6.07 5.38 0.98 31.35-15.26

The operation of flooded UGS facilities is affected by the reservoir characteristics, the presence of bottomwater, the heterogeneity of the reservoir, and the UGS operating schedule [3, 4]. During operation offlooded UGS cacilities, as the number of gas injection/withdrawal cycles increases, the gas/water distributionbecomes more complicated, the gas mobility is reduced [5-7], and it becomes impossible to predict the variation inthe gas in storage (the inventory) and the volume of gas that can be withdrawn [8, 9]. In this connection, designinga system for assessing the feasibility of operation of flooded UGS facilities is of practical importance. The aim ofthis work was to study the mechanism for operation of flooded USG facilities using a physical simulation model,which will become the basis for a feasibility study for construction of flooded UGS facilities.

Fig. 1 shows a simplified model for a flooded UGS. The operating pressure in the UGS was 13-30.5 MPa,remaining reserves before construction of the UGS were 9.88·108 m3, formation water salinity was 8000 ppm.After 6 injection/withdrawal cycles, the total gas injection reached 24.59·108 m3, the cumulative gas recoveryreached 19.87·108 m3; and the gas recovery rate reached 81%. Dynamic operating data for the UGS are givenin Table 1.

Dur ing deve lopment o f the model , we took in to cons idera t ion the opera t ing condi t ionsduring UGS operation: the presence of bottom water, the reservoir heterogeneity, operating pressurevariation [10, 11], and the duration and number of gas injection/withdrawal cycles. Based on the characteristics ofthe flooded USG under construction and in operation, we developed a physical simulation model forinjection/withdrawal (Fig. 2).

Producing well (for gas withdrawal).

Low permeability layerHigh permeability layer

WaterReservoir

Fig. 1 Schematic drawing of a flooded UGS

Radial heterogeneity

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Gas accumulation, preliminary work, buildup of the UGS capacity, and 6 injection/withdrawal cycles werecompleted before development of the model. The major parameters to be recorded during the simulation were theinjected and withdrawn gas volumes, the volume of recovered water, and the operating pressure. The physicalsimulation included the following steps:· measurement of core permeability· gas accumulation, displacement of water by gas to the bound water state, then displacement of gas by waterto residual gas· buildup of the UGS capacity and gas injection: water was displaced by gas with a constant gas flow rate untilgas appeared at the outlet, then the outlet valve was closed and the injected gas flow rate was held constant;when the pressure at the outlet reached the upper limit, the inlet valve was closed;· gas withdrawal (gas production): the inlet valve was opened, the flow rate of the gas to be withdrawn washeld constant until the pressure reached the lower limit, and then the inlet valve was closed;· gas injection and withdrawal: 6 cycles.

Based on comparison of the actual UGS under study (the prototype) and its model, we developed asimplified model of the producing wells (for gas withdrawal) in order to determine the gas withdrawal rate in themodel. The horizontal distance between wells was a, the vertical distance was b, the reservoir thickness was h, thearea per well was A = ab (Fig. 3). In the physical model, there was a small radial flow and a linear flow

Fig. 2 Physical model for gas injection/withdrawal into/from the UGS (simulationflowsheet). 1) intermediate vessel with gas; 2) gas flow controller; 3) high-pressure drytube; 4) gas mass flowmeter; 5) pressure regulator; 6) ISCO piston pump; 7) intermediatevessel with liquid.

Highpermeabil i ty

Lowpermeabil i ty

Fig. 3 Simplified model of the wells.

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near the bottom of the well , and also straight flow (Fig. 4). As a simplified model of the flow, weselected a rectangle with width a, length b, thickness h, and production of the central well equal to Q. Accordingto symmetry, the experimental model was equivalent to 1/2 the simplified actual model. Thus we studied 1/2 theactual model (Figs. 5, 6). The matching relationship between the experimental model and the actualprototype UGS is represented by the following equations:

2

2

1

1

Aq

Aq

111

111 LA

Tqη

222

222 LA

Tqη

211

221 η

LLη

Fig. 4 Flows in the physical model.

Fig. 5 Simplified actual model of the flows.

Fig. 6 Simplified experimental model of the flows.

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Table 2

Cycle Gas in storage in UGS, mL

Available gas in storage, mL

Gas saturation, %

Available gas saturation, %

Gas pore space utilization, %

Available gas pore space

utilization, % 1 16024.91 11408.03 34.3 18.3 57.6 30.8

2 16398.09 12131.98 34.9 19.5 58.6 32.7

3 17045.01 13331.73 35.9 21.4 60.2 35.9

4 17760.48 14163.33 36.9 22.7 62.1 38.2

5 18650.9 15183.14 38.3 24.4 64.4 40.9 6 19589.41 16828.66 39.7 27.0 66.8 45.4

Note. Initial gas saturation: 59.5%. Gas saturation of flooded UGS: 27.4%.

where the subscript 1 corresponds to the experimental model; 2 corresponds to the actual prototype UGS; is a simple timevariable; q is the production of the well; T is the production time (duration of withdrawal); is the porosity of the reservoir.

Based on the indicated matching relationship, the gas withdrawal flow rate in the physical simulationwas 7.207 mL/s, while the gas injection flow rate was 4.118 mL/s.

We obtained the dynamic data during physical simulation of gas injection/withdrawal. Based on the equilibriumequation for a volumetric gas reservoir, we calculated the primary parameters of the physical simulation model.

The available gas in storage for the UGS is:

cccmaxmaxmaxigigigig TZPTZPTZPQG ////

where Gig is the available gas in storage for the UGS; Pig is the static pressure in the UGS for production of the centralwell Q; Zig is the gas deviation factor for production of the central well Q; Tig is the temperature in the USG for production of thecentral well Q; Pmax is the upstream static pressure (inlet pressure) for the UGS; Zmax is the gas deviation factor forpressure Pmax; Tmax is the temperature in the UGS for static pressure Pmax for the UGS; Zc is the gas deviation factor forpressure Pc; Tc is the temperature in the UGS for static pressure Pc.

The available pore volume is:

sssigscscp PTZGTPV //

where Vp is the available pore volume; Psc is normal atmospheric pressure; Tsc is the air temperature; Ps is the static pressure inthe UGS; Zs is the gas deviation factor for UGS pressure Ps; T is the temperature in the UGS for pressure Ps.

The available capacity is:

TZPVPTG maxmaxpscscmax ///

where Gmax is the available gas in storage in the UGS for static pressure Pmax.The available cushion gas is:

( TZPVPTG ccpscscc ///

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Fig. 7 Simulation results for gas in storage (inventory) (I = injection; W = withdrawal):1) total inventory 2) available inventory.

Fig. 8 Simulation of available gas saturation. 1, 2 = respectively the unavailable porespace utilization and the available pore space utilization .

Gas

in s

tora

ge, m

L

Cycle1I 1W 2I 2W 3I 3W 4I 4W 5I 5W 6I 6W

Gas

satu

ratio

n, %

Pore

spa

ce u

tiliz

atio

n, %

CycleInitial

where Gc is the available gas in storage in the UGS for static pressure Pc.The available working gas is:

ccmaxmaxpscscwm ZZPTVPTG ////

where Gwm is the available gas withdrawal from the UGS.Taking into consideration the actual operating conditions for a flooded UGS [12], we simulated 6 gas

injection/withdrawal cycles. Table 2 gives the gas injection and withdrawal simulation results.We studied the operating schedule of a flooded UGS in accordance with the physical simulation results. When we

compared our results with the results from actual UGS operation, we observed a good match between the physical simulationand the actual operation.

Gas in storage (inventory). During the physical simulation, in response to displacement of water by gas, the spacefilled with gas steadily increased and accordingly the gas in storage (inventory) increased. As the inventory and the numberof injection/withdrawal cycles increased, the available pore space and the available capacity increased. Once the gas bubbleformed, the inventory stopped increasing. After 6 injection/withdrawal cycles, the cumulative increment in inventoryreached 22.4% and the cumulative increment in available inventory reached 47.5% (Fig. 7).

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Gas saturation. Because of capillary absorption (imbibition), water spills over from small pores into large pores, andtherefore the unavailable pore space occupied by water gradually decreases with repeated gas injection/withdrawal operations.These pores become available for the gas. The available gas saturation of the reservoir and consequently the availablecapacity increase. After 6 gas injection/withdrawal cycles, the available gas saturation increased from 18.3% to 27% andthe available pore space utilization increased from 30.8% to 45.4%, the unavailable gas saturation of the reservoir wasreduced from 16% to 12.7%, and the unavailable pore space utilization (for pores occupied by water) was reducedfrom 26.9% to 21.4% (Fig. 8).

Available pore space utilization. The reservoir heterogeneity determines the available pore space utilization. In thesimulation, after 6 injection/withdrawal cycles, the available gas saturation of a low permeability reservoir increased from11.2% to 17.7% and the available pore space utilization increased from 22.3% to 35.4%. At the same time, for a high permeabilityreservoir with higher available capacity, the available gas saturation increased from 22.2% to 30.4% and the available porespace utilization increased from 34.5% to 47% (Fig. 9). Because of the heterogeneity of the reservoir and the limited drivingpressure differential, the increase in available pore space is limited and the available pore space utilization cannotreach 100%.

From our studies we can draw the following conclusions:· the physical model we developed can be used in simulation modeling of construction and operation offlooded UGS facilities and to provide a basis for studying these USG facilities;· the physical simulation results match the results of study of the actual operation of a UGS facility quite well;· due to the presence of basal water and reservoir heterogeneity, the capacity and the working and cushion gas volumesincrease gradually during the physical simulation, the proportion of the working gas in the total UGS capacity increases whilethe proportion of the cushion gas is reduced; once the gas bubble forms, the variation in the indicated parameters becomesless marked;· the performance of the UGS is better in a high permeability layer than in a low permeability layer;· the performance of a flooded SGS is determined by the porosity of the reservoir, its degree of flooding, the heterogeneityof the layers, and the operating schedule;· before constructing an underground gas storage facility, the geological characteristics, reservoir parameters, and floodingconditions must be thoroughly studied and in addition, the operating schedule for the UGS must be rationally designed.

CycleInitial

Avai

labl

e ga

s sa

tura

tion,

%

Avai

labl

e po

re s

pace

util

izat

ion,

%

Fig. 9 Effect of reservoir heterogeneity on available gas saturation and available pore spaceutilization (1, 2 = respectively high permeability and low permeability layers).

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