Smart Technique in Water Shutoff Treatment for a Layered Reservoir

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Smart Technique in Water shutoff treatment for Layered Reservoir through engineered injection/production scheme Elham Sharifpour, Masoud Riazi, and Shahab Ayatollahi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02191 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 22, 2015

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Industrial & Engineering Chemistry Research

Smart Technique in Water shutoff treatment for Layered Reservoir through engineered injection/production scheme Elham Sharifpour, Masoud Riazi, Shahab Ayatollahi1* EOR Research Center, School of Chemical and Petroleum Engineering, Shiraz University. Shiraz, I.R. Iran. Email: [email protected]

ABSTRACT

The challenge of excess water production from low permeable layer of a stratified gas reservoir is investigated in this study. Considering the fluid flow behavior in heterogeneous porous media, placing a blocking agent within the lower permeable layer and protecting the adjacent higher permeable layer from damage is practically impossible. To overcome such challenge, a novel cation sensitive smart bio-polymer is utilized in a wise sequential polymer injection/gas production scenario that includes a protective gas flow. To evaluate the conceptual proposition, a series of flow tests have been conducted in a stratified micromodel. it is shown that the proposed treatment efficiently restrains saline water in the lower permeable layer and protects the higher permeable layer from undesired damage. The results indicate that being smart in water shutoff 1

Currently with Sharif University of technology

ACS Paragon Plus Environment

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treatment is not only employing smart materials; but the engineered application is the key for the success. INTRODUCTION Excess water production is a common problem in most mature oil and gas reservoirs that decreases well productivity and increases the costs of production, lifting and separation.1-3 It is estimated that three barrels of water are produced per barrel of oil worldwide that imposes an annual disposal cost of $40 billion to the industry.4 Management of this huge amount of contaminated water is also a major environmental concern.5,6 Moreover, increasing water cut stimulates some other costly problems such as fine migration, sand production, corrosion and scale formation.2,7 Therefore, it is necessary to prevent or at least reduce the amount of excess water production. Among various water shut off methods, special attention is paid to polymer gel treatment, during which a solution of polymer and cross linker is injected into the formation (this combination is called gelant). Gelant is supposed to form a gel network within the porous media. The efficiency of water shut off treatment depends on the compatibility of the gelant with harsh reservoir conditions as well as the proper placement of the gelant in water pathways; because the gel network blindly blocks any path, which is occupied by the gelant.8,9 Different gelant systems have been evaluated for this purpose, through micromodel10-12, packed beds13-15and core flooding experiments.16,17 Studies show that the excess water is mostly produced from conductive fractures, thief zones and high permeable layers.18 Therefore, gel treatments have been usually aimed to block the high permeable zones and fractures. In such circumstances the permeability contrast between the layers is considered as a beneficial prospect of the formation that restricts the penetration of the


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gelant into the hydrocarbon rich low permeable layers.9,19 However, the gel invasion into the low permeable layers is still probable, since many parameters other than permeability contrast (such as injection rate, wettability and porosity) control the gelant flow behavior.20 As a result, water shut off treatment sometimes leads to undesirable reduction in hydrocarbon production.21,22 Several techniques have been proposed to improve the selective injection of the gelant into the high permeable layers. Controlling the injection pressure of the gelant23, increasing the concentration and molecular weight of the polymer in gelant solution8, micro gel treatment24-26 and using micro spherical particles13 are some of these techniques. The high permeable layer is usually considered as the excess water producer; however, in specific gas producing formation, excessive saline water is surprisingly produced from nonisolatable low permeable lenses, that are usually fully water saturated due to the capillary effects.27 Severe challenges due to the excess saline water production from such lenses have been reported recently in Kangan gas reservoir (KGR).28-31 KGR is a volumetric wet gas reservoir located at the south of Iran, suffering from saline water production in approximately half of the wells.28-31 The saline water production ceases by decreasing the gas production rate to almost one sixth of its normal aptitude. Detailed studies showed that by increasing the gas production rate, the pressure gradient in the vicinity of the well exceeds the capillary resistance and induces a water flow within these lenses.28-31 In this case, most of currently proved technologies that are based on blocking the HP layer, lose their applicability. Considering the fluid flow behavior in heterogeneous porous media; it is stated that the selective injection of the gelant into the lower permeable lenses in competition with the adjacent HP layers is practically impossible9,19 and consequently the challenge requires innovative treatments. In this research based on the difference of dominant phase within the layers (e.g. saline water in the lower permeable layer


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and gas within the HP one), it is suggested that the clever utilization of smart salinity sensitive materials have the potential of solving the problem. The efficiency of the salinity sensitive Gellan biopolymer is evaluated during a sequential polymer injection/gas production scenario in a dual permeable micromodel. The tests are conducted in transparent micromodel in order to provide complete visual information about the fluid flow, displacement mechanisms and phase distribution during the proposed water shut off treatment. This research is one of the first to study the applicability of blocking agents with the aim of blocking the lower permeable layer while the least effect on HP layer is desired. The objectives of this work are to: (1) investigate the effects of permeability contrast on miscible and immiscible displacement in a stratified porous media; (2) evaluate the aptitude of a salt sensitive agent for blocking a certain zone of a heterogeneous porous media (3) evaluate the capability of the proposed sequential injection/production scenario for protecting the HP layer in a dual permeable porous media.

MATERIALS AND METHODS MATERIALS Distilled de-ionized water and analytical grade NaCl with the purity of 99% (Kimia Mavad Co.) were used for preparing synthetic saline water. Industrial nitrogen (Pars Balloon Co.) was used as the injection gas and analytical grade Gellan biopolymer with the purity of 99.9% (Sigma Alderich) was utilized as the smart salinity sensitive gelling agent. Gellan gum (bacterial polysaccharide S-60) is a linear extracellular anionic biopolymer that was initially discovered in 1978 and is currently allowed to use as a natural food additive. When the Gellan bio-polymer is solved in the distilled water, it produces a low viscous fluid (6.55 cp at 2,000 ppm, room


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temperature) however if the cations are added to the solution the bio-polymer aggregates and forms gel structures.32 The thermal stability of the produced depends on the number of the acyl group in the molecular structure of the Gellan bio-polymer, which can be tuned through bioengineering of the bio-polymer.33 Extensive information on the physical and chemical properties of the bio-polymer have been recently published.32 MICROMODEL TESTS (FLOW TESTS FACILITIES) Micromodel: to prepare a micromodel, initially a flat glass was carefully covered with thin adhesive plastic layer that works as a protective coating, then the desired pattern was laser printed on the coated glass, the energy and intensity of the laser beam was adjusted in order to only engrave the coating, not the glass. After that the glass is acid-washed and the uncovered glass surface (the laser printed pattern) is chemically etched as contacted with HF acid.34 The depth of the etched pattern depends on the time of acid washing and the purity of the acid solution. The coating is then completely washed out by appropriate solvent. Finally the model was completed by fusing a flat glass over the etched glass in an accurately temperature controlled furnace.35 Table 1.Basic characteristics of the micromodel

Length (cm)

Width (cm)

PV (cc)

HP Porosity (%)

LP Porosity (%)

Micromodel Depth (mm)

HP pore Diameter (mm)

LP pore Diameter (mm)

7.5

3.6

0.3

58

29

0.25

0.95

0.45

The designed pattern includes two different parallel layers with cross flow (Figure 1). Each layer is occupying one-half of the micromodel width. The characteristics of the micromodel are


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shown in Table 1. In this table, PV, HP and LP stand for pore volume of the micromodel, HP layer and lower permeable layer of the model, respectively. LP layer

1 cm

Port

Port A

HP layer

Figure 1.Micromodel pattern Experimental setup: A schematic diagram of the experimental setup is shown in Figure 2. During the experiments, a syringe pump was used for injection of aqueous phase (brine and biopolymer solution). Gas injection was applied with an accurate mass flow controller (AliCatMC-1slpm) that establishes the injection rate with the accuracy of 5 Scm3min-1 (standard cubic centimeter per minute) in the range of 5-1000 Scm3min-1. Besides, it measures the inlet gas pressure up to 147 psia, with the accuracy of 0.01 psi. The micromodel was mounted horizontally over a uniform light source and the visual changes of the fluid saturations and distributions in the micromodel were recorded using a digital microscope (Dino-413T, 1.3 Megapixel). The process was recorded with the rate of 30 frames per second. The same micromodel was used for the whole experiments and after each test the micromodel was cleaned carefully by flushing more than 500 PVs of hot water (about 90oC) through it. Usually the main assessment during micromodel tests is qualitative; however, through careful image processing, quantitative results can also be obtained.36 In this research, the images were analyzed using Adobe Photoshop© CS5 image analysis package, and phase saturations were evaluated by pixel counting. The bio-polymer solution was utilized without any dyed additives,


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since the primary bottle tests proved that colored bio-polymer produces weaker gel. Therefore, the bio-polymer gel structure and gas phase could not be distinguished from each other through automatic expert systems without careful human intervention. An original image of the micromodel and its analyzed illustration is shown in Figure 3. During the image analysis, the grain boundaries were initially defined on the image. The distribution and portion of different phases within the pore spaces were carefully detected and marked using different colors. Finally, the resolution of the images was improved to obtain better illustration for visual interpretations. The quantitative phase saturations were determined based on the assumption of uniform pore depth throughout the micromodel.36

Figure 2.Schematic diagram of the flow test setup Experimental Procedure: In general, a six step experimental procedure is proposed as describe in Table 2 and a series of four tests are designed. Depending on the objective of each test, some of these steps are omitted. Brief description of the micromodel flow tests and their objectives are as follows:


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Test1: Base Case (No Treatment). In this test, only steps 1 and 6 of Table 2 are followed. This test represents the effect of gas flow rate on phase distributions and saturations within a layered system. The results are used as a reference for evaluation of the treatment efficiency. Test 2: Bio-Polymer Treatment. In this test de-ionized water is used instead of the saline brine (step 1), and the other steps of the procedure are chased as explained in Table 2. Through this test, the effect of biopolymer without cations is examined. Test 3: Smart Gel Treatment. The whole stages of the procedure are conducted in this test. The performance of the purposed smart water shut off treatment is evaluated by comparing the results of this test to Test 1. Test 4: Conventional Gel Treatment. During this test, step 4 of the general experimental procedure is omitted. The effect of the protective gas flow in the performance of the water shutoff treatment is determined by comparing the results of this test to Test 3. 1 cm

Gas Gas

Gas Water

Water

Gas

Water

Water

a: Original image

b: Processed image

Figure 3.An image of the micromodel during gas flow and its analyzed illustration. In Figure a, grains were almost yellow, water was blue and the remainder was gas; and in Figure b, grains were illustrated in white, water was shown in blue and gas shown in red (gas was injected from left to right)


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Table 2.General experimental procedure

Process

Objective

Rate

Direction of injection

1

Brine Injection (75,000 ppm NaCl brine)

Saturate the model with water

High flow rate

From formation to well

2

Gas Injection

Establish initial saturations to mimic the near well conditions of a layered gas reservoir

Controlled low flow rate


3

Gellan biopolymer Injection (2,000 ppm solution)

Injected from reverse side of the model

1 cc/min

From well to formation

4

Protective Gas Injection

Displace gelant from HP before gel formation

Controlled low flow rate


5

Aging

Allow the gel structure to be formed in-situ

--

--

6

Gas Injection

Investigate the gas production behavior

Gradually increasing flow rates


The 2,000 ppm bio-polymer solution and 75,000 ppm NaCl brine were utilized through the experiments. These concentrations were chosen with respect to the results of primary bottle tests. During step 6 of the procedure, the gas flow rates were increased from 5 to 600 Scm3min-1 in 9 successive steps (i.e. 5, 10, 50, 100, 200, 300, 400, 500, 600 Scm3min-1). This wide range of flow rates is conducted because the reports of the KGR challenge elucidated that increasing the gas production rate, enhances the excess water production.28-31 Besides, the challenge is mainly related to the near wellbore region in which the gas flow rates are much higher than normal reservoir rates; therefore, high range of gas flow rate is performed. Gas injection at each flow rate was continued until no more water displacement was observed and the injection pressure reached its steady state condition (i.e. no more changes in phase saturations and pressure were


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observed). All flow tests are performed at room temperature, and the outlet port of the model is always kept open to the atmosphere. Treatment satisfaction criteria: The general target of water shutoff treatments is to reduce the water permeability with minimum decrease in hydrocarbon permeability (either oil or gas). Since in the intended case, the production of the connate saline water from the LP lenses caused the problem; the efficiency of the proposed water shut off treatment is evaluated through considering the alteration of water saturation of the LP layer. In this study, an advantageous treatment should restrain more water in the LP zone at high gas flow rates (compared to the base case). Besides, the effect of suggested treatment on gas permeability is analyzed through the wellknown permeability reduction factor (PRF) or residual resistance factor.11,13,18 This parameter is defined as:

PRFg

K   K 

rg before treatment rg after treatment

Qavg 

 Qavg     P  before treatment   Qavg     P  after treatment

2Q1 P1 2Q2 P2  P1  P2  P1  P2 

Eq.1

Eq.2

Where, Krg is the relative permeability of the gas phase, Qavg is the average volumetric gas flow rate with respect to the compressibility of the gas37, and ΔP is the total pressure difference occurring over the model. For driving this equation the gas phase viscosity, cross sectional area and length are assumed constant. It was stated that small PRFg values (near unity), is needed for satisfactory water shut off treatment.12,13 Therefore, a successful treatment is defined as the one that trap more water within the LP layer and simultaneously has a small PRFg values.


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RESULTS AND DISCUSSIONSUNCERTAINTY OF THE EXPERIMENTAL RESULTS One of the most important concerns of experimental studies is the reliability of the results. In order to ensure the reliability, appropriate calibrated instruments with acceptable accuracies are used. The practical accuracy of the obtained data such as pressure and phase saturations is evaluated through repeating Test 1 for three times and comparing the results. The results showed that the pressure data are adequately meeting the required precision. Uncertainty of the saturation measurements is also evaluated in comparison of the repeated experimental data with the average values (Figure 4). Based on the obtained results, it is concluded that the maximum error of the saturation measurement occurred at the early stage of the gas injection (flow rates less than 50 Scm3min-1). However, the overall accuracy is generally acceptable for the flow rate domain of the experiments. 0.7 High Permeable Media

0.6

Gas Flow rate (SLPM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Low Permeable Media

0.5 0.4 0.3 0.2 0.1 0 0

20

40

60

80

100

Water Saturation (%)

Figure 4.Error analysis of water de-saturation for HP and LP layers during gas flow of Test 1 GAS WATER DISPLACEMENT IN DUAL PERMEABLE MICROMODEL Figure 5 illustrates the fluid distribution in the micromodel during gradual gas injection of Test 1. These results show the flow behavior of immiscible displacement within a layered porous media. Wise understanding of this phenomenon and affecting mechanisms is crucially important not only in the design of appropriate water shutoff treatment, but also in many other processes such as water flooding, solution gas drive, water alternating gas injection (WAG), and geological


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CO2 sequestration.,38,39 The results illustrate that the heterogeneity of the micromodel leads to non-uniform displacement front. Since gas is the non-wetting fluid and brine is the wetting fluid, gas injecting in the model is a drainage process. During drainage, gas preferentially advances through the HP layer and larger pores, especially at low flow rates (Figure 5-a, b).When the gas flow rate is increased, the capillary resistance of the LP layer is eventually overcome, the gas enters into the smaller pores27,39,40 and gradually displaces the wetting phase (Figure 5-c). Figure 5 show that when the gas flow rate is equal to 100 Scm3min-1, the cross flow between the layers is ceased and the boundary between the layers is mainly filled with water. The established finger of the gas through the LP media is the sign of capillary dominant process (Figure 5-c). It was previously stated that, when a non-wetting fluid is injected into a stratified porous media, it flows in the LP layer with lower rate than the HP layer. As a result the displacement is capillary dominated within the LP layer; while in the HP layer the flow is mainly viscous.39 By increasing the gas flow rate, the contribution of the viscous flow is increased, the boundary between the layers drain and the flow within the LP layer turns into viscous dominated as well. Therefore, high gas flow rates lead to depletion of the LP layer from water (Figure 5-d, e, and f). The results of this work are generally confirming these expressions. The de-saturation results of test 1 are presented in Figure 6. De-saturation data is usually reported versus capillary number; however, true definition of the capillary number is impractical in this work; because determination of the correct interfacial tension is practically impossible due to the uneven distribution of the biopolymer, gel and brine within the model. Therefore, dimensionless Reynolds number is used for scaling the results and evaluating the gas flow rate’s effect on the flow behavior within a dual permeable porous media. Re 

vd 

Eq.4


12

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In this equation, Re is the Reynolds number, ρ, v, µ are the density, velocity and viscosity of the displacing phase respectively. d is the hydraulic diameter of the media that was determined with respect to the volume of the micromodel, and its surface area. 1 cm

a: injection rate 5 cc/min

b: injection rate 50 cc/min

c: injection rate 100 cc/min

d: injection rate 300 cc/min

e: injection rate 400 cc/min

f: injection rate 600 cc/min

Figure 5.Fluid distribution within the micromodel during gas flow of Test 1; gas is injected from left to right, gas is shown in red and brine in blue The results are clearly indicating that increasing gas rate reduces the water saturation in both layers. However, the reduction is considerably greater in the HP layer, especially at low flow rates. As it is shown, at 50 cc/min gas flow, water saturation in the HP layer decreased to almost 20%, while that of the LP layer was still above 95%. However, at elevated gas flow rate (500 cc/min), the water saturation of both layers were almost the same (about 1%). It is concluded that increasing the gas flow rate, amplifies the ability of the gas for overcoming the entry capillary pressure of the LP layer and induces a pressure gradient (hydraulic potential) that can mobilize the initially immobile water within the LP layer. This observation is a phenomenological explanation of the KGR challenge.28-31


13

700

80

600

70

500

60 50

400

40

300

30

200

20

100

10 0

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inlet gas flow rate (cm3/min)

90

0 20

40 60 80 water saturation (%)

100 0

100

LP water saturation 1000 0 (%) HPwater watersaturation saturation 0

50 100

inle t…

0

Re ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Re (Dimensionless)


Figure 6.De-saturation curves during gas flow of Test 1 As it is mentioned previously, the other important parameter that should be considered during the treatment is the gas phase mobility. Commonly, this parameter is defined at certain saturation such as residual hydrocarbon saturation or irreducible water saturation. However, it is shown that the irreducible water saturation is a function of gas flow rate. Therefore, the gas phase mobility of Test 1 as a function of gas flow rate is presented in Figure 7. At low gas flow rates (less than100 Scm3min-1), increasing the rate leads to a sharp increase in the gas phase mobility and at the same time a sharp decrease in water saturation of the HP layer, with no significant change in the LP water saturation. At this state, increasing the gas phase mobility is mainly attributed to the enlargement of the opened cross sectional area in the HP layer due to decreased water saturation. When the injection flow rate increased gradually to 300 Scm3min-1, the trend of gas phase mobility is still increasing; however, the slope is considerably decreased as well as the rate of saturation change of the HP layer. This observation corresponds to the start of gas fingering into the LP layer. The fact that the gas mobility modification is proportional and coincident with the alteration of the HP water saturation, is clarified the dominancy of the HP layer on gas mobility. Finally, when the flow rate exceeded 300 Scm3min-1, gradual rise of gas flow rate reduces the


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gas mobility. The negative slope of gas mobility is initiated exactly when the water saturation of the HP layer decreased to almost zero. The reduction of gas mobility may be either related to the transfer of the flow regime from laminar to turbulent (non-Darcy flow), or to the decreased lubricating effect of water film in the HP media. Detailed explanation of the involved mechanisms is not the interest of this paper and requires more studies.

Gas mobility (m2/pa.s)

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6.E-06 5.E-06 4.E-06 3.E-06 2.E-06 1.E-06 0.E+00 0

50

100

Re (Dimensionless)

Figure 7.Gas mobility in micromodel during gas flow in Test 1 THE CHALLENGE OF PROPER GEL PLACEMENT In General, fluid displacement through horizontal micromodel (without gravity effect) is governed by capillary and viscous forces. When the polymer solution is injected into the dual permeable micromodel, the viscous resistance of the HP layer is considerably less than that of the LP layer. Therefore, most of the polymer invasion occurs through the HP layer, which is the most challenging practical aspect of bio-polymer injection into a stratified media in this study. The saturation distribution of different phases just before polymer injection is shown in Figure 8a, which resembles the real conditions around a gas well in a stratified formation. The Figure shows that the HP layer is mainly filled with the non-wetting gas phase. Therefore, the biopolymer penetration into the HP layer is also accelerated by the aid of capillary imbibitions. While the LP layer is totally water saturated and the bio- polymer cannot imbibe. As a result, both viscous and capillary forces drive the bio-polymer solution through the HP layer. The


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polymer entrance into the LP layer occurs only when more than half of the HP layer is filled by the bio-polymer solution (Figure 8-b).

1 cm

a: micromodel just befor polymer injection

b: 0.4 PV polymer was injected

c: 1 PV polymer was injected

d: 1.5 PV polymer was injected

Figure 8.Fluid distribution in the micromodel during polymer injection. The polymer is injected from right to left, aqueous phase is shown in blue and bio-polymer and gas phases are transparent. The stable polymer front within the HP layer is shown in Figure 8-b, which is due to its favorable mobility ratio. Figures 8-c and 8-d show the fluid distribution in micromodel after injection of 1 and 1.5 PV of the bio-polymer respectively. Obviously, less than half of the LP layer is occupied by the bio-polymer solution. Figure 9 shows a magnified section of the micromodel. It is shown that polymer and gas phases have almost similar colors which cannot be distinguished easily.


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It should be mentioned that most of the previous studies on water shutoff treatment were focused on the advantages of this preferential invasion characteristic of polymer that improves the potential of blocking fractures and high permeable thief zones and protect the hydrocarbon rich LP layer.9,26 It was stated that increasing permeability contrast, enhances this charactristic.19,41,42 According to this fact, it is evident that proper gel placement in this work is a challenge that requires wise effort. 0.5 cm

polymer Gas

Figure 9.Magnified section of the micromodel during bio-polymer injection. Water is shown in blue, polymer and gas have approximately the same color and their differentiation requires carful tracing of interfaces that illustrated in the circles. EFFECT OF PROTECTIVE GAS INJECTION It is shown that after bio-polymer injection, the HP layer is almost completely filled with the injected solution, while the LP layer is only partially affected. Therefore, it is expected that if the model aged without further treatment, the gel would form mainly through the HP layer and hinder the gas flow considerably. To prevent this challenge, a controlled gas flow (production) is proposed that aims to wash out the chemicals from the main gas routes before gel formation.


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This is entitled “protective gas” stage in this text. The basic idea for employment of protective fluid injection during water shutoff treatment has been recommended previously to protect near wellbore region from possible damages;8,25 however, the effect of layering and permeability contrast has not considered. The performance of the protective gas flow is evaluated with comparison of Test 3 (smart gel treatment) to Test 4. The phase distributions of the micromodel as a function of gas flow rate are illustrated in Figure 10. Comparing test 1 to test 4, it is illustrated that after conventional gel treatment the gas enters into the LP layer at lower injection rate (Figure 10-c: 5 cc/min vs. Figure 10-a: 50 cc/min) and its flow through the HP layer is decreased (Figure 10-c: 5 cc/min vs. Figure 10-a: 5 cc/min). This is a clear indication of the undesired gel blockade within the HP layer in Test 4. The Gellan gel is almost shear thinning, therefore, increasing the gas flow rate decreases the blockade of the HP layer and its aqueous phase saturation. The results of smart gel treatment (Test 3) show a considerable increase in water saturation of the LP layer compared to Test 1 (Figure 10- a,b), while the changes of the water saturation in the HP layer is negligible specially at high gas flow rates (Figure 10- a,b: 400 and 600 cc/min). The superiority of Test 3 over Test 4 is mainly attributed to the effect of the protective gas flow in the smart gel treatment. It is concluded that the protective gas washes the Gellan biopolymer from some pathways within the HP layer before gel formation and consequently protects the HP layer from sever sealing. The phase distributions in Test 3 also illustrate that a portion of aqueous phase in the LP layer is remained almost immobile (Figure 10-b: 400 cc/min), in a part of the micromodel that the polymer was not expected to reach during the 1.5 PV chemical injection (Figure 8-d). This is also attributed to the protective gas effect. It is found that the protective gas conducts a portion of


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the HP polymer content into the LP layer and as a result improves the performance of the smart

Injection Rate

treatment in sealing the LP layer. Test 1 a

Test 3 b

1 cm

1 cm

400 cc/min

100 cc/min

5 cc/min

1 cm

Test 4 c

600 cc/min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10.Fluid distribution in the micromodel during gas flow for Tests 1, 3 and 4. Gas is injected from left to right; gas is shown in red and water in blue


19

700 600 500 400 300 200 100 0 50 100 LP water saturation (%)

150

1000 Test 1: without treatment Test 2: polymer treatment Test 3: smart gel treatment 0 0 4: conventional gel 0 LpTest 50 100 150 water saturation (%)treatment

Re

inlet gas flow rate (SLPM)

100

Page 20 of 33

Inlet gas flow rate (cm3/min)

90 80 70 60 50 40 30 20 10 0 0

Figure11.De-saturation curve of LP layer during gas flow 700 600 500 400 300 200 100 0 50 Hp water saturation (%)

100

1000 Test 1: without treatment

Re

0

Test 2: polymer treatment

0

Test 3: smart gel treatment 0

inlet gas flow rate (cm3/min)

90 80 70 60 50 40 30 20 10 0 100

inlet gas flow rate (SLPM)

Re

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Re


gel treatment 0 LpTest 504: conventional 100 150(%) water saturation

Figure 12. De-saturation curve of HP layer during gas flow The other advantage of the smart gel treatment is illustrated quantitatively in Figures 11 to 13. Figure 11 shows the de-saturation curves of the LP layer during gas injection for different tests. As it can be seen, the smart gel treatment (Test 3) leads to the maximum increase in the irreducible water saturation of the LP layer. However, conventional gel treatment results in undesirable raise of the HP layer water saturation (Figure 12, Test 4). As it was stated previously, the HP layer is the main gas flow path. Therefore, maintaining more aqueous phase in this layer is considered as the hostile blocking of the gas flow path which leads to reduction of


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the gas conductivity. This effect of water shut off treatment is more discussed based on gas mobility curve and PRF (Figure 13). The gas mobility curves and PRFs versus Re number are illustrated in Figures 13. The PRF of Test 1 is equal to unity, since it is the basis of the comparison. As it is expected, the conventional gel treatment (Test 4) which restrains more aqueous phase in the HP layer, decreases the gas mobility more than the others (Figure 13-a), consequently it results in higher PRF (Figure 13-b). On the other side, in the smart gel treatment (Test 3) the gas mobility and PRF curve are less affected and the unfavorable effects vanished at high flow rates (Figure 13-a,b). 2

6.E-06

1.8 5.E-06

1.6 1.4

4.E-06

1.2

PRF

gas mobility (m2/pa.s)

3.E-06

1 0.8

2.E-06

0.6 0.4

1.E-06

0.2 0.E+00

0 20

40 Re

60

80

0

20

40

60

80

Re

100 Test 1: without treatment 0 smart gel 0Test 3: 20 40treatment 60 80 100 Lp water saturation (%)

1000 Test 2: polymer treatment 0 inlet gas flow rate (SLP M)

0

Re

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 Test 4: conventional gel treatment

Figure 13.Gas mobility and PRFg during gas flow

In addition to the general fact that the conventional gel treatment decreases the gas mobility, the odd shape of the PRF curve in Test 4 demonstrates another drawback of this scenario (Figure 13-b). The initial value of the PRF is well above unity (more than 10) which is interpreted to the large entry pressure. Such increase of the threshold pressure has the potential to induce terrible challenges in field applications through complete loss of well productivity. This effect is also vanished by injection of the protective fluid, as the PRF is initiated with small values (smaller than one) in Test 3.


21


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Brief comparison of the results is presented in Table 3. Based on these results, it is concluded that the conventional gel treatment (Test 4), is not appropriate for the mentioned case. Moreover, it is verified that the protective gas injection not only decreases the damages of the high permeable zone and maintains the gas mobility, but it also increases the sealing of the LP layer. The results clearly indicate that being smart in water shutoff treatment is not only employing of smart materials; but the engineered application is the main key for a successful treatment. Table 3.The brief comparison of the results of different cases at elevated injection rates Test 1

Test 3

Test 4

Injection rate LP water saturation (%) LP water saturation (%) PRF LP water saturation (%) PRF 400 cc/min

2

56

1.02

25

1.56

600 cc/min

0

36

1

16

1.5

THE EFFECT OF GEL FORMATION Polymer injection had been recommended for controlling the invasion of water to the production wells through increasing water viscosity and decreasing its mobility; 16,43 therefore, it is important to differentiate the individual effect of the bio-polymer solution from that of the formed gel in the purposed treatment. This is achieved through the comparison of the results of the polymer treated case (Test 2) with Tests 1 and 3. Primary tests indicated that the bio-polymer cannot form gel in de-ionized water. Therefore, when the initial aqueous phase within the micromodel is salt free, the treatment is recognized as polymer treatment (Test 2). The qualitative results show that the polymer treatment (Test 2) cannot affect the fluid distribution and irreducible water saturation within the micromodel (Figure 14). The quantitative results of Test 2 also have a noticeable consistency with the results of Test 1 (Figure 11 to 13). It is found that the Gellan bio-polymer cannot be considered as a


22

Page 23 of 33

treatment without the presence of appropriate cations (the cross linkers) in the aqueous phase. The noticeable difference between the irreducible water saturation of the LP layer of Test 2 and Test 3 indicates that the gel formation is essential for gaining appropriate result from the

Injection Rate

proposed water shut off treatment. Test 1

Test 2

Test 3

a

b

c

1 cm

1 cm

400 cc/min

100 cc/min

5 cc/min

1 cm

600 cc/min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 14.Fluid distribution in the micromodel during gas flow for Tests 1, 2 and 3, gas is injected from left to right; gas is shown in red and water in blue AN INSIGHT TO FIELD SCALE APPLICATION It is found that saline water production from the low permeable lenses (that are located within the pay zone of high gas flow rate wells), ceases by decreasing the gas production rate to almost one sixth of its normal value.28-31 High gas flow rates impose large pressure gradient in the


23


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Page 24 of 33

vicinity of the wellbore, which mobilizes the naturally immobile saline water content of the low permeable lenses28-31 as shown schematically in Figure 15-a.

a: elevated gas flow draws saline water from the LP layer

b: Gellan biopolymer is injected into the formation

c: protective gas is conducted with controlled flow rate toward the well and ejects the bio-polymer from the HP layer

d: after the aging, the treated part of the LP layer is blocked and elevated gas is conducted without excess saline water production

Figure 15.Shematic explanation of the proposed treatment. The gas producing HP layer is shown in red, the water saturated LP in blue and the polymer in green. The color of the flashes indicate the phases and the directions are appropriate A novel smart gel treatment scenario is proposed to overcome this challenge. The treatment begins with the injection of the cation sensitive Gellan bio-polymer solution into the well, as schematically shown in Figure 15-b. To prevent the blockade of the gas producing layer (HP layer) and maintain the productivity of the well, a controlled flow of a gas toward the well should be conducted immediately after bio-polymer injection. This step that is recognized as the


24

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protective gas flow is considered as the essential key for the success of the proposed scenario. The protective gas flow takes the advantage of natural gas flow of the reservoir to washout the polymer from the HP layer and protects the well from possible damages. This process is schematically illustrated in Figure 15-c. Then the treated well should be allowed to age for a definite period of time, which is sufficient for formation of gel within the saline water bearing formation. Finally the treated well will be ready for gas production at high flow rates, while the water producing layer is practically disinfected, as shown schematically in Figure 15-d. For implementing this conceptual proposition to a real case, extensive experimental and theoretical studies should be conducted in advanced. Some of the subjects that should be essentially investigated follow: bio-engineering of the Gellan bio-polymer to tolerate harsh reservoir conditions (ref), determining the fluid flow behavior of different phases (bio-polymer, gas) through the real porous media (reservoir rock), determining the optimum injection amount, concentration and flow rate of the bio-polymer solution as well as the rate of protective gas flow, based on the physical and chemical properties of the real porous media CONCLUSIONS The water shut off treatment of excess saline water production from the LP lenses of a layered gas producing system is visually investigated in this study. The application of a novel smart biopolymer gel that is salinity sensitive trough an innovative treatment scenario is proposed. The main concluding remarks of this conceptual work are as below: 

The utilization of salinity sensitive Gellan bio-polymer in the proposed smart treatment scenario, lead to successful water shut off treatment for the intended case. The treatment decreases the saline water production from the LP layer, while the gas productivity of the HP layer remains almost unaffected.


25


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

Page 26 of 33

Permeability contrast between the layers has a significant effect on immiscible fluid displacement that impose practical challenges to the selective injection of the polymer solution into the LP layer, while it eases the polymer flow through the HP layer.



The protective gas flow not only protects the HP layer from undesirable gel blockade to maintain the gas productivity, but also improves the efficiency of the treatment for increasing the irreducible water saturation of the LP layer.



The protective gas flow critically decreases the inherent large pressure gradient that is normally required for bringing a polymer/gel treated well into the service. Such increase of the threshold pressure has the potential to induce terrible challenges in field applications through complete loss of well productivity.



It is found that being smart in water shutoff treatment is not only employing smart materials, but their engineered application is the main key for successful treatment.

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Selective Placement of a Polymer Gel Treatment to Improve Water Injection Profiles and Sweep Efficiency in the Lagomar Field Western Venezuela. SPE International Petroleum Conference, Mexico. 2004, 7-9 Nov. SPE 92025. List of Figures: Figure1.Micromodel pattern Figure2.Schematic diagram of the flow test setup Figure3.An image of the micromodel during gas flow and its analyzed illustration. In Figure a, grains were almost yellow, water was blue and the remainder was gas; and in Figure b, grains were illustrated in white, water was shown in blue and gas shown in red (gas was injected from left to right) Figure 4.Error analysis of water de-saturation for HP and LP layers during gas flow of Test 1


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Figure5. Fluid distribution within the micromodel during gas flow of Test 1; gas is injected from left to right, gas is shown in red and brine in blue Figure6.De-saturation curves during gas flow of Test 1 Figure7.Gas mobility in micromodel during gas flow in Test 1 Figure8.Fluid distribution in the micromodel during polymer injection. The polymer is injected from right to left, aqueous phase is shown in blue and bio-polymer and gas phases are transparent Figure9.A magnified section of the micromodel during bio-polymer injection. Water is shown in blue, polymer and gas have approximately the same color and their differentiation requires carful tracing of interfaces that illustrated in the circles. Figure10.Fluid distribution in the micromodel during gas flow for Tests 1, 3 and 4. Gas is injected from left to right; gas is shown in red and water in blue Figure11.De-saturation curve of LP layer during gas flow Figure12.De-saturation curve of HP layer during gas flow Figure13.Gas mobility and PRFg during gas flow Figure14.Fluid distribution in the micromodel during gas flow for Tests 1, 2 and 3, gas is injected from left to right; gas is shown in red and water in blue Figure 15.Schematic explanation of the proposed treatment. The gas producing HP layer is shown in red, the water saturated LP in blue and the polymer in green. The color of the flashes indicate the phases and the directions are appropriate List of Tables: Table1.Basic characteristics of the micromodel Table2.General experimental procedure Table3.The brief comparison of the results of different cases at high gas flow rates

TOC graphic:


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Smart Technique in Water Shutoff Treatment for a Layered Reservoir

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