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Further Investigation of Effects of Injection Pressure and Imbibition Water on CO2 Huff-n-Puff Performance in Liquid-rich Shale Reservoirs Lei Li, James J. Sheng, Yuliang Su, and Shiyuan Zhan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00536 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Valve 2 Valve 3

Three way valve 1

Saturated with crude oil

Crude oil Piston Water Valve 4

Pedestal

Vacuum Pump

Saturation vessel

Quizix QX Pump

Accumulator

Schematic of the set up for the core saturation experiments. Pressure regulator Valve 1

Valve 2 Temperature controlled oven Pressure gauge Valve 3

Valve 4

CO2

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Backpressure regulator Pedestal huff-n-puff vessel Gas Cylinder

Syringe Pump

Schematic of the set up for the CO2 huff-n-puff experiments.

ACS Paragon Plus Environment

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Further Investigation of Effects of Injection Pressure and Imbibition Water on CO2 Huff-n-Puff Performance in Liquid-rich Shale Reservoirs Lei Li, *a James J. Sheng, b Yuliang Su, a Shiyuan Zhan a a

Department of Petroleum Engineering, China University of Petroleum (East China),

Qingdao, China b

Department of Petroleum Engineering, Texas Tech University, Lubbock, TX, 79409, United

States *Corresponding author: Tel: +86 156.1001.2697, Email: [email protected] / [email protected] ABSTRACT Shale oil production has increased rapidly in the last decades, especially in United States; and results in a revolution in the energy landscape. However, one main problem existing in the shale reservoir development is the sharp decline of liquids production in all the hydraulically fractured wells. In recent years, CO2 huff-n-puff injection has been proved to be a potential method to enhance the oil recovery. In this study, the effects of injection pressure and imbibition water on CO2 huff-n-puff performance were further investigated. Eagle Ford core samples and Wolfcamp dead oil were used in this experimental study. The microscopic pore characteristics of Eagle Ford shale core samples were analyzed and the results show that 98.08% of the pore sizes are distributed between 3nm to 50nm. The experimental results demonstrate the great potential of CO2 huff-n-puff EOR. The cumulative oil recovery can reach 68% after 7 huff-n-puff cycles. The oil recovered in each cycle deceases as injection cycle number increases due to the permeability damage caused by asphaltene precipitation, oil saturation reduction, and low injected CO2 sweep efficiency. The effect of injection pressure was studied by injecting CO2 at both immiscible and miscible conditions. CO2 huff-n-puff has better performance (more than 9.1% EOR) under miscible conditions than immiscible conditions. After that, a novel experiment was designed to saturate the core samples with both water (15 wt% KCl) and Wolfcamp dead oil to investigate the influence of imbibition water on CO2 huff-n-puff EOR performance. The existence of imbibition water impedes oil production in shale core samples. The oil recovery decreased about 45.3% after seven huff-n-puff cycles compared to the condition without water. A simulation study provides a better understanding of CO2 huff-n-puff application in liquid-rich shale reservoirs, which is fundamentally important for applying and optimizing CO2 huff-n-puff in field production.

1. INTRODUCTION The combination of horizontal drilling and multistage hydraulic fracturing technologies has made it economical to unlock oil and gas from unconventional reservoirs.1-3 Several giant unconventional plays such as Permian Basin, Eagle Ford, Bakken have had great success in shale oil and gas production and revolutionized the energy landscape in the United States.4-9 However, the liquid production has a steep decline curve in all the unconventional wells; and recovery factors are estimated to be less than 10% of OOIP.10, 11 Many wells were initially produced at too high of a drawdown resulting in acceleration of pressure-dependent permeability loss, early pressure drop below the bubble point and preferential gas phase


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mobility (increasing GOR), all resulting in low recovery factors.12-14 Therefore, effective enhanced oil recovery techniques are needed to displace oil from nanoscale shale matrix, maintain profitable oil production rates, and increase the ultimate oil recovery.1, 5, 15, 16 Several EOR methods such as water flooding, chemical flooding, gas injection including gas flooding and gas huff-n-puff were evaluated. As the permeability and porosity in shale reservoirs are extremely low, it’s difficult to conduct water and chemical flooding. Gas flooding is eliminated due to the severe fingering phenomenon in fractured shale formations. Gas huff-n-puff, using the same well as both an injection well and a production well to avoid gas fingering, becomes the most suitable EOR method in shale reservoir development.17, 18 CO2 injection has been a successful EOR method applied in conventional reservoirs.15, 19 For conventional reservoirs, after reaching the minimum miscible pressure (MMP), the injected gas CO2 and crude oil would reach a miscible condition through multi-contact processes at reservoir pressure. CO2 helps increase the oil sweep volume and displacement efficiency, which results in a high oil recovery.20 By 2012, CO2 miscible and immiscible flooding produced 308,564 bbl/d and 43,675 bbl/d, respectively, which accounts for more than 40% of the total US EOR daily production.21, 22 Microscopic experiment of CO2 huff-n-puff has been done on conventional oil reservoirs.23 Typical results are shown in Figure 1. After water flooding, there is still lots of oil remaining in the core. Then the injected CO2 flows from the big channels to the small conduits to contact with oil. During the soaking period (Figure 1 e), the injected CO2 diffuses into the oil, swelling the oil, reducing oil viscosity and inducing oil in the small pores flowing into the fractures or big channels. Then during the puff period, more oil can be produced, resulting in less oil remaining in the core.

a. Dry core

d. CO2 injection finished

b. Saturated core

c. After water flooding

e. Soaking finished

f. Puff period finished

Figure 1. Microscopic CO2 huff-n-puff process in conventional reservoirs.

In unconventional reservoirs, the pore sizes are nanometer in scale. The reservoir permeability and porosity are ultra-low. How to improve oil production in unconventional reservoirs using CO2 huff-n-puff is of particular interest to both scientists and engineers.16, 24


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In recent years, core scale experiments are applied to investigate CO2 huff-n-puff EOR performance in unconventional reservoirs.25-27 Table 1 summarizes some representative experiment work on gas huff-n-puff.20, 28-39 Song and Yang28, 29 studied the CO2 huff-n-puff process in tight cores with the permeability range of 0.27-0.83 mD and found that both near-miscible and miscible CO2 huff-n-puff achieve superior recovery performance than immiscible CO2 huff-n-puff process. Zhang38 conducted miscible core flooding experiments on a Bakken core sample with an average porosity of 7.5% and permeability of 1.8 µD. The results showed that about 70% of the oil was produced. However, limited experimental studies have been performed on the evaluation of CO2 huff-n-puff in shale cores with an ultra-low matrix permeability (less than 1 µD). Moreover, the fluid behavior in nanopore media would be different from that in conventional pore systems,40, 41 so it is necessary to study the performance of miscible / immiscible CO2 huff-n-puff in nanopore reservoirs. The previous core scale experiments (e.g. Gamada et al. 2013, 2014) have studied the effects of the operating parameters of injection pressure, soaking time, pressure depletion rate, and number of cycles on the recovery performance of CO2 huff-n-puff process in shale core plugs.34, 35 However, those core samples were saturated with mineral oil rather than shale oil. Furthermore, in the previous huff-n-puff experiments with ultra-low permeability core samples, nearly all the core samples were only saturated with oil, few CO2 huff-n-puff experiments were done on core samples saturated with both water and oil.20, 28-38, 42, 43 Shayegi et al.30 explored both pure CO2 and CO2/CH4/N2 mixture huff-n-puff performances on sandstone cores. Zhang et al.31 investigated the EOR potential of CO2 huff-n-puff in a medium-gravity oil reservoir in Saskatchewan waterflood residual oil systems. Both Wang et al.33 and Song & Yang28, 29 studied the CO2 huff-n-puff potential on cores in diverse reservoir conditions such as immiscible, near-miscible and miscible conditions. There was water saturated in their cores, but the cores were not ultra-low permeability shale cores. Large permeability core samples may perform rather differently from the shale cores.44, 45 Some use the preserved core samples from tight oil reservoirs to conduct the experiments. For example, Tovar et al.43 conducted huff-n-puff experiments on preserved sidewall cores in tight oil formation at pressures of 3000 psi and 1600 psi. Their results showed that the oil recoveries could vary from 18% to 51% for the 3000 psi case and 19% to 55% for the 1600 psi. But they did not observe water production from the preserved sidewall cores, and they were not sure if water exists in the cores. Jin et al.8, 39 did CO2 bathing (totally 24 hours) experiments at reservoir conditions on core samples collected from Bakken wells. The water saturation in the core samples were in the ranges of 0.07 to 0.58. The permeability of the core samples was among 0.001 to 0.058 md. Their results showed that 28% to 95% of the present mature hydrocarbon was produced at the end of the 24-hour exposure. Their experiments were operated in a constant-pressure mode to determine the role of pure molecular-diffusion-dominated flow by eliminating the pressure-gradient-dependent flow. In this study, we want to investigate the CO2 huff-n-puff potential with all the oil recovery mechanisms. Thus, systematic CO2 huff-n-puff experiments should be conducted under miscible / immiscible conditions and with imbibition water existing condition. In this study, the core samples used are Eagle Ford core samples. Wolfcamp dead oil was used as saturation oil and its components were analyzed. The microscopic pore characteristics of Eagle Ford shale core samples were investigated. The potential of CO2 huff-n-puff in


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extremely low permeability shale core samples were explored. The effects of injection pressure and imbibition water were analyzed. Table 1. Representative Experiment Work of Gas Huff-n-Puff to Recover Oil Authors

Injected gas CH4/N2/ CO2

Permeability

Oil

Water

EOR extent

139-319 md

C15

with water

4.6%-31.3%

CO2/flue

66 md

light oil

water flooding

about 50%

CO2

0.02-0.13 md

live oil / dead oil

none

18-25%

CO2

100-150 md

heavy oil

water injection

about 16%

N2

N/A

mineral oil

none

10-50%

CO2

N/A

mineral oil

none

33-85%

CO2

0.27-0.83 md

dead oil

water saturation

about 63.0%

N2

300-500 nd

Wolfcamp dead oil

none

about 50.5%

Zhang (2016)38

CO2

tight oil

none

about 70%

Jin et al (2017)39

CO2

1.8 ud 0.001-0.058 md

reservoir oil

0.07 to 0.58

28% to 95%

CO2

300-500 nd

Wolfcamp dead oil

none

around 70%

Shayegi et al. (1996)30 Zhang et al. (2006)31 Kovscek et al. (2008)32 Wang et al. (2013)33 Gamadi et al. (2013)34 Gamadi et al. (2014)35 Song and Yang (2013, 2017)28, 29

Yu and Sheng (2015, 2016)36, 37

Li et al. (2017) 20

2. EXPERIMENTAL SECTION 2.1. Experimental Materials. A dead oil sample from the Wolfcamp formation in Apache's Lin field was used in the study for core saturation. The dead oil density is 0.796 g/ml, and oil viscosity is 3.58 cp at temperature of 72°F and pressure of 14.7 psi. The oil API is 46.7. Water was applied to saturate the core samples. Since shales and clays swell in the presence of fresh water, clay stabilizer is necessary to add to the saturation fluid. Potassium chloride (KCl) helps control clay swelling in the presence of water and helps minimize fines migration.46 Typical KCl concentrations for fracturing applications are 2% to 7% by the weight of the base fluid, depending on the clay content of the formation.46 Based upon our lab tests, 15 wt% of KCl solution is selected as the water saturation fluid. Shale core plugs from Eagle Ford outcrop were used in this study, as shown in Figure 2. Table 2 gives the dimensions and measured helium porosities of the core samples. The nitrogen permeability of the core plugs varies from 200 nD to 350 nD. The permeability was determined using a complex transient measurement system (AutoLab 1000) developed by NER Inc., USA. The injection gas in these huff-n-puff experiments is industrial CO2 gas with purity of 99.999% from Airgas Company. The original pressure of the CO2 gas is 850 psi.


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Figure 2. Eagle Ford core samples used in this study.

Table 2. Core Samples Specifications Core No.

Dry weight, g

Length, mm

Diameter, mm

Porosity

EF_1

125.4418

38.1

50.8

7.50%

EF_2

123.2952

37.9

50.7

7.23%

EF_3

124.4882

38.3

50.4

7.65%

2.2. Experimental Apparatus. The experimental work included oil component analysis, core sample microstructure exploration, shale core plug saturation with Wolfcamp dead oil, shale core plug saturation with both water (15 wt% KCl) and Wolfcamp dead oil, and CO2 huff-n-puff injection. The experimental setup used in this work were designed and modified based on previous studies.35 The dead oil component analysis system mainly consists of the Thermo Scientific Trace Ultra Gas Chromatograph and Weight/Quadruple ISQ Mass Spectrometer (GC/MS). The produced chromatogram was analyzed by Xcalibur software from Thermo Scientific. The experimental flow for shale core saturation with both oil and water is shown in Figure 3. This setup mainly consists of a vacuum pump, a saturation vessel, an accumulator, a pressure gauge and Quizix QX pump. The Quizix QX pump is a syringe pump with two cylinders, and it can pump fluids continuously. An accumulator with piston cooperating with the QX pump pushes crude oil to the core container and maintains a high saturation pressure at the same time. The piston separates the accumulator into two separately sides: water and oil. A pedestal is placed under the core plug to ensure that the saturation fluid can be infused into the core plug from all sides of the core. The schematic of the CO2 huff-n-puff setup is shown in Figure 4. The setup mainly consists of a syringe pump, a temperature controlled oven, and a pressure vessel. A pressure gauge was placed directly on the top of the stainless huff-n-puff vessel to monitor the pressure during injection stage, soaking period and pressure depletion time. The syringe pump is used as a pressure boost to increase the injected gas pressure to the designed pressure. As CO2 was able to move freely around the sample rather than being forced through the sample (in contrast to a conventional coreflooding experiment), the core huff-n-puff process was designed to be a bath experiment. The core sample was put in the center of the huff-n-puff vessel and injected gas could move freely during the huff-n-puff process.


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Figure 3. Schematic of the set up for the core saturation experiments.

Figure 4. Schematic of the set up for the CO2 huff-n-puff experiments.

2.3. Experimental Procedures. 2.3.1 Core saturation with dead oil. The core saturation with dead oil procedures were: (1) The core plugs were put into an oven at a temperature of 274°F for 24 h to remove any residual fluids inside the core. Then the core plugs were named and dry weighted as Wdry. (2) The core plug was placed in the saturation vessel and vacuumed for 48 h. (3) Turn off the vacuum pump and the Quizix QX pump was used to displace water into the accumulator. The dead oil on the top of the piston was pushed into the saturation vessel by the force transient from the water. (4) After the vessel was full of crude oil, the pressure was built up to 4000 psi slowly. The pressure in the vessel was maintained for one week. Then the saturated oil volume was calculated and compared with the core porosity to make sure the core plugs fully saturated with oil. (5) After the saturation period, the pressure in the vessel was bled to atmosphere pressure slowly to avoid


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damage to the core plugs caused by the pressure change. (6) The weight of the core plug after being saturated with oil was measured and recorded as Wsat. 2.3.2 Core saturation with water (15 wt% KCl) and dead oil. The core saturation with water and Wolfcamp dead oil procedures are similar to the above procedures. The first two steps were exactly the same as the core saturation with dead oil. In the third step, the injection fluid, dead oil, was replaced by water with 15 wt% KCl. After the saturation vessel was full, the pressure was kept at atmosphere pressure for 24 h to allow the water to imbibe into the core. Then the core was removed from the saturation vessel and weighed to get the plug weight (Wr+w) with water imbibition. Then water was removed and the core plug was placed into the saturation vessel and dead oil was injected into the vessel through the accumulator until the vessel was full. The pressure was built up to 4000 psi again and the pressure was maintained for one week. After the dead oil saturation period, the pressure was released to atmosphere pressure slowly. The core plugs were then removed from the saturation vessel and weighed again to get the saturation weight with water and oil (Wr+o+w). The weight of total saturated fluid in the core plugs were calculated using the difference between the dry weight (Wdry) and saturated weight (Wsat or Wr+w+o). Then the CO2 huff-n-puff process was conducted. 2.3.3 CO2 huff-n-puff injection. The procedures of CO2 huff-n-puff were: (1) Prior to the CO2 huff-n-puff injection, the core plug was put into the huff-n-puff vessel. All the valves in Figure 4 were open, and CO2 gas was flowed through the huff-n-puff vessel at low pressure to displace air in the vessel.47 (2) Valve 4 was closed and CO2 was injected into the core container. If the gas pressure was lower than our designed pressure, the Syringe pump was used to boost the pressure to the operation pressure. (3) After the gas injection stage, value 3 was closed and a certain period was given for CO2 to soak with the crude oil inside the core plugs. (4) After the soaking period, the pressure in the huff-n-puff vessel was bled to atmosphere pressure. The core sample was weighed (Wi) after being kept in a sealed container under the atmosphere pressure for a certain time. (5) After finishing one injection stage, soaking period and pressure depletion period, one huff-n-puff cycle was finished. More cycles were repeated until the designed number of cycles was reached. During this process, the core plug was placed on a pedestal which allows all sides of the core to be directly in contact with the injected CO2 gas. Each core plug was weighed and recorded after each cycle to calculate oil recovery or liquid recovery with material balance using the equation (1) and (2). The oven was controlled at a temperature of 104 °F during all the experiments.

=

− × 100 −

=

+!+ − × 100 +!+ −

(1) (2)

Wsat is the weight of the shale core plug fully saturated with Wolfcamp shale oil or n-decane, Wdry is the weight of dry shale core plug, Wi is the weight of the shale core plug after each huff-n-puff cycle, Wr+w+o is the weight of the shale core plug saturated with water (15 wt% salinity) and n-decane.


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Three main purposes of this study are: 1) to investigate the CO2 huff-n-puff EOR potential in shale oil reservoirs; 2) to compare the huff-n-puff injection performance at both miscible and immiscible condition; 3) to analyze the EOR ability with/without imbibition water existing in the core samples. Based on these objectives, two groups of experiments were conducted in this study shown in Table 3. The main purpose of Test 1 (T1) was used to analyze the CO2 huff-n-puff potential. Then the effects of huff-n-puff cycle number and puff time were also investigated. Tests 2 to 5 were applied on the same core samples to investigate the injection pressure effect. Using the slimtube experiments (the slimtube was packed with sand particles), the MMP of CO2-Wolfcamp dead oil system was measured to be 1620 psi at temperature of 104 °F in our previous work.20 For shale core samples, the effective MMP was about 200 psi higher than the MMP measured from slimtube experiments.20 Thus in Group 1, we chose injection pressures above and below the effective MMP for CO2 huff-n-puff tests. The purpose of Group 2 was to analyze the effect of imbibition water. The core plugs in Tests 5 and 6 (T5, T6) were saturated with both water (15 wt% KCl) and Wolfcamp dead oil to analyze the effect of imbibition water on liquid / oil recovery during the CO2 huff-n-puff process in shale cores. Table 3. CO2 Huff-n-Puff Tests Design Group No.

1

1

Huff pressure, psi 2000

2 3

1800 1600

Test No.

Core sample

EF_1

4 2

Soaking time

6 hrs

Puff time

No. of cycles

6 hrs

7

Saturated fluid

Condition

Wolfcamp dead oil

Miscible Immiscible

1200

5

EF_2

2000

6

EF_3

2000

6 hrs

6 hrs

water (15 wt% KCl) and Wolfcmap dead oil

7

Miscible

3. OIL AND CORE ANALYSIS RESULTS 3.1. Oil Components Analysis. The crude oil mixture covered the components with boiling range from n-C3 to n-C41+. The GC result for Wolfcamp dead oil is shown in Figure 5. The mole percent of different components in the dead oil is presented in Table 4. pA 180 180

C14

C8

C12

C15

C18

C42

C40

C42

C40

C28

C36

2020

C16

C14

4040

C36

C24

C24

6060

C28

C20

C18

C9

8080

C17

C9

C16 C17

C10

C10

100100

C20

120120

C12

C11

C11

140140

C15

C7

C6 C7

C6

160160

C8

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Energy & Fuels

00 10

0

10

20

20

30

40

30

40

Figure 5. GC result for Wolfcamp dead oil.


min

min

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Table 4. Mole Percent Data of Different Components in Wolfcamp Dead Oil Component

Percent

Component

Percent

Component

Percent

C3H8

0.009%

FC9

8.339%

FC21-22

2.268%

IC4

0.004%

FC10

8.336%

FC23-24

1.040%

NC4

0.005%

FC11-12

11.786%

FC25-26

1.727%

IC5

1.349%

FC13-14

9.413%

FC27-28

1.050%

NC5

1.349%

FC15-16

6.787%

FC29-30

0.501%

FC6

4.588%

FC17-18

4.940%

FC31-36

0.952%

FC7

10.684%

FC19

2.148%

FC37-40

0.937%

FC8

12.295%

FC20

1.284%

FC41+

8.210%

3.2. Core Sample Analysis. In order to understand the mineral composition of the rock, X-ray Diffranction (XRD) technique was employed to identify and quantify the mineral phases present in the sample.37 A Rigaku Ultima III powder X-ray diffractometer was used for quantitative analysis. The mineral composition of the plug samples is shown in Table 5.37 Quartz (24.9 wt%) and calcite (58.3 wt%) are the main mineral compositions in these core samples. The total content of clay mineral, kaolinite and illite is 7.4 wt%. High-pressure mercury injection was performed to study the pore size distribution. The result is presented in Figure 6. It illustrates that 98.08% of the pore sizes are distributed between 3nm to 50nm. 0.53% of the pore sizes vary from 1µm to 3um. Also 1.39% of the pore sizes are between 9µm to 10µm, which are believed to be micro-fractures in shale cores. Table 5. Mineral Compositions of the Eagle Ford Core Samples Phase

Quartz

Calcite

Anhydrite

Kaolinite

Gypsum

Acmite

Illite

Wt%

24.9

58.3

2.3

4.0

3.6

3.5

3.4

1

10

100

1000

10000

0.14 0.12

Pore size distribution

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

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0.10 0.08 0.06 0.04 0.02 0.00

Diameter, nm

Figure 6. Pore size distribution results of Eagle Ford shale core.


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4. RESULTS AND DISCUSSION 4.1. EOR Potential of CO2 Huff-n-Puff. CO2 huff-n-puff (Test No.1) was conducted on core sample of EF_1 at a pressure of 2000 psi. The weights of the dry core and oil saturated core are 125.4418g and 128.8005g, respectively. After each huff-n-puff cycle, oil was produced to the core surface. We wiped the produced oil on the surface of the core. The core weight (Wi) was measured and recorded. The oil recovery factor (RF) was calculated based upon Equation (1) and presented in Table 6. After 7 huff-n-puff cycles, the cumulative oil recovery can reach 68%, while the primary depletion is less than 10% according to the field data.10, 11 Cumulative oil recovery was recorded at different depletion times in the 7th cycle. The result is displayed in Figure 7. It shows that the cumulative oil recovery increased fast in the first depletion hour and became stable after 3 hours. This phenomenon illustrates that during the puff period, the dissolved CO2 gas will be produced and bring oil out of the core. Even when the surrounding pressure is already released to the atmosphere pressure, the pressure inside the core is still higher than the atmosphere pressure and some oil will continue be produced out of the core. Table 6. The Core Weight and Cumulative Oil Recovery after Each Huff-n-Puff Cycle Cycle No.

1

2

3

4

5

6

7

Wi, g

128.1624

127.7616

127.4444

127.1466

126.8752

126.6588

126.5175

19.00%

30.93%

40.38%

49.24%

57.32%

63.77%

67.97%

Cumulative oil recovery

0.70

0.68


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0.66

0.64

0.62

0.60

0.58 0

200

400

600

800

1000

1200

Depletion Time (min)

Figure 7. Cumulative oil recovery at different depletion time at 7th huff-n-puff cycle (Test No.1). The amount of oil recovered in each huff-n-puff cycle is determined based upon the data in Table 6 and the result is presented in Figure 8. The first huff-n-puff cycle has the most oil recovered, then the oil recovered in each cycle decreases as more cycles are conducted. The phenomena have been observed by many researches,30, 33, 34, 48 but have not been explained clearly. Gamadi et al.34 found that the oil recovery factors peaked at the first cycle. Wang et al.33 claimed that this decline trend of recovery factor may result from the gradual reduction


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of remaining oil saturation in subsequent cycles. Shayegi et al.30 observed that injection pressure difference had no discernable effect on the oil recovery decline when they applied a higher injection pressure in the second cycle. In this study, we summarize the reasons into three main aspects. CO2 huff-n-puff process will cause asphaltene precipitation, which will reduce the core plug permeability. This has been proved in the publication of our colleague that 26.8% permeability reduction was observed after the first cycle and 48.5% permeability reduction was observed after 6 cycles of CO2 huff-n-puff injection.38 In addition, as more oil is produced from the core, the oil saturation becomes lower. Even at the same pressure gradient, the amount of produced oil will become less as the oil saturation gradient declines. The sweep area is another main reason. As more huff-n-puff cycles are conducted, the injected CO2 will enter the relatively high permeability region of the core, which has already been huffed and puffed several times. Most of the oil in the easy accessible region has been produced, thus the oil recovered in the latter cycle becomes less than the former cycle. 0.20

0.19

0.18

Oil recovered in one cycle

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

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0.16 0.14

0.119

0.12 0.10

0.094

0.089

0.08

0.081 0.064

0.06

0.042

0.04 0.02 0.00 1

2

3

4

5

6

7

Number of huff-n-puff cycles

Figure 8. Oil recovered at different huff-n-puff cycles.

4.2. Effect of Injection Pressure. From our previous slimtube experiments and CO2 huff-n-puff study, the measured effective MMP of CO2-Wolfcamp dead oil system in shale core samples during huff-n-puff process is around 1800 psi at the temperature of 104 °F.20 Thus, the other three tests of injection pressure (1200 psi, 1600 psi and 1800 psi) were tested in this study to investigate the CO2 huff-n-puff EOR performance during immiscible and miscible conditions. The results are shown in Table 7 and Figures 9-10. Tests 1-2 were conducted under miscible condition and tests 3-4 were conducted under immiscible condition. Figure 9 shows the cumulative oil recovery at different huff pressures. The total oil recoveries for all seven cycles were 40.7%, 58%, 67.1% and 68% when the huff pressures were 1200, 1600, 1800, and 2000 psi, respectively. The oil recovery increases 9.1% when the injection pressure increase from 1600 psi to 1800 psi. CO2 huff-n-puff EOR performance is better at miscible condition. When the injection pressure increases from 1800 psi to 2000 psi, the cumulative oil recovery has a very small amount of increase (about 0.9%), which indicates that after reach miscible condition, the further increase in pressure does not significantly enhance oil recovery in shale oil reservoirs during CO2 huff-n-puff process. The


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abnormally high cumulative oil recovery of cycle 3 in the case with injection pressure of 1800 psi is caused by manual measurement error, thus it does not follow the trend of the other data. Figure 10 presents cumulative oil recovery at different huff-n-puff cycles. It illustrates that the cumulative oil recovery under miscible condition is higher than that under immiscible condition at all the huff-n-puff cycles. Table 7. Oil Recovery Data for Group 1 Tests Test No.

Huff pressure, psi

cycle 1

cycle 2

cycle 3

cycle 4

cycle 5

cycle 6

cycle 7

1 2 3 4

2000 1800 1600 1200

19.00% 17.98% 14.35% 9.96%

30.93% 30.38% 23.99% 16.40%

40.38% 42.44% 33.04% 23.73%

49.24% 52.12% 41.07% 29.14%

57.32% 58.07% 49.16% 34.86%

63.77% 63.51% 54.35% 39.02%

67.97% 67.10% 57.97% 40.73%

0.7


0.6 0.5 0.4 0.3

1200 psi 1600 psi 1800 psi 2000 psi

0.2 0.1 1

2

3

4

5

6

7

No. of huff-n-puff cycle

Figure 9. Cumulative oil recovery at different injection pressure. 0.7 0.6


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

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0.5

Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 Cycle 7

0.4 0.3 0.2 0.1 0.0 1000

1200

1400

1600

1800

Huff Pressure (psi)

Figure 10. Cumulative oil recovery at different huff-n-puff cycles.


2000

2200

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4.3. Effect of Imbibition Water. In this part, the effect of imbibition water was studied. All the ultra-low permeability core sample experiments in the tests we’ve done above were conducted on core samples only saturated with oil. In this study, the shale core samples were saturated with both water and Wolfcamp dead oil. Two tests were conducted as shown in Table 3. The core samples in T5 and T6 were saturated with water (15 wt% KCl) firstly, and then saturated with Wolfcamp dead oil. For T5 and T6, the recovery result is liquid recovery factor as it is calculated by the weight of produced liquid divided by the weight of total saturated liquid. The experiment results are presented in Table 8. With imbibition water existing in the core sample, the liquid recovery decreases by about 41% after seven huff-n-puff cycles, compared with oil recovery. The properties of core samples EF_2 and EF_3 are similar as shown in Table 2. Thus, the similar results of T5 and T6 indicate that the experiment can be repeatable. Figure 11 compares the results of cumulative liquid recovery with/without water exists. Tests 1, 5, and 6 were all conducted at huff pressure of 2000 psi. In T1, the core sample was only saturated with Wolfcamp dead oil, which has the highest oil recovery, about 67.97% after seven huff-n-puff cycles. This kind of extremely high oil recovery cannot be reached in the field study. In this lab experiment, all sides of the core samples were exposed to the injected gas. Also, the sizes of the core samples are much smaller than the matrix scale in a reservoir. With a greater surface area contacted with the injected gas and a higher pressure gradient, miscible conditions are easy to reach and larger oil recovery will be achieved from lab experiments. T5 and T6 are the core samples saturated with both water and Wolfcamp dead oil. The liquid recovery decrement in the first cycle is the most (about 53%) and declines in the following cycles. After three cycles, the decrement becomes stable, about 41% in every cycle compared with the oil recovery in that of T1. The reduction of liquid recovery can be caused by two aspects. On one hand, the existence of water will reduce the relative oil permeability to impede the oil flow efficiency. On the other hand, the imbibed water might cause clay swelling, which results in permeability damage. Table 8. Liquid / Oil Recovery Data for Comparison of Water Effects on Oil Recovery Test No.

Liquid / Oil Recovery Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Cycle 6

Cycle 7

T1 (EF_1)

19.00%

30.93%

40.38%

49.24%

57.32%

63.77%

67.97%

T5 (EF_2)

8.75%

16.23%

22.95%

28.87%

33.45%

37.22%

39.51%

RF decreased Percentage

53.95%

47.53%

43.16%

41.37%

41.65%

41.63%

41.87%

T6 (EF_3)

8.91%

16.94%

23.26%

29.04%

33.79%

37.01%

39.63%

RF decreased Percentage

53.10%

45.24%

42.39%

41.03%

41.05%

41.96%

41.70%


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1.0

T1 saturated with Wolfcamp dead oil (EF_1) T5 saturated with water and Wolfcamp dead oil (EF_2) T6 saturated with water and Wolfcamp dead oil (EF_3)

0.9 0.8

Liquid / Oil recovery

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

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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

5

6

7

8

9

Number of huff-n-puff cycles

Figure 11. Liquid / oil recovery results with different saturation fluids at the same huff pressure of 2000 psi.

In this study, experimental data is further analyzed by simulation method as the oil and water recovered from the experiments cannot be split. 4.3.1 Simulation method. The composition modeling of the crude oil sample used in this study was based on our previous work.20, 49 A fully compositional model with radial coordinates was built to mimic the CO2 huff-n-puff process using Computer Modelling Group’s GEM reservoir simulator (Figure 12). In the model, the shale core was centralized inside the vessel, which is surrounded by an empty space for CO2 injection. All faces of the shale core sample were exposed to CO2 during the gas injection, soaking, and production stages. Properties of Wolfcamp dead oil were used as input data to the simulation model. The Jossi-Stiel-Thodos (JST) viscosity correlation is used in this model. The validation of the model was established by accurately reproducing the results performed in the laboratory. Figure 13 shows the comparison of experimental and simulation results for the EF_1 sample. The simulation model was able to illustrate the CO2 huff-n-puff process with imbibition water exists. Figure 14 presents the history matching result of liquid recovery for T6 with imbibition water. The basic rock and fluid properties used in the model can be found in Table 9. Water saturation here used is 0.4. Figure 14 describes the calculated oil recovery and water recovery which cannot be measured in the experiments. In the first three cycles, the oil recovery was higher than the water recovery. After that, the water recovery became higher than the oil recovery. It indicates that oil is easier to flow and be produced when oil saturation is high. The oil recovery and water recovery after seven huff-n-puff cycles were 37.16% and 45.04%, respectively. The oil recovery decreased about 45.3% compared to the condition without water. Figure 15 shows the produced oil and water volumes, and remaining oil and water volumes in the core.


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Figure 12. Model build up process and radial simulation model with logarithmic refinement (legend is initial oil saturation).20, 49 80

GEM simulation results Experimental results

Oil recovery factor, %

70 60 50 40 30 20 10 0 0

1

2

3

4

5

Operation time, days

Figure 13. Reproduction of the CO2 huff-n-puff in Eagle Ford core sample (EF_1) using a simulation model. 50

Oil / Water recovery factor, %

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

Page 16 of 21

40

30

20

Oil recovery factor - Simulation Water recovery factor - Simulation Liquid recovery factor - Simulation Liquid recovery factor - Experiment

10

0 0

1

2

3

4

5

Time, days

Figure 14. History matching for liquid oil recovery of CO2 huff-n-puff with imbibition water in Eagle Ford core sample (EF_3).


Page 17 of 21

3.0

Oil produced Water produced Oil remaining in the core water remaining in the core

2.5

Oil / Water volume, cm3

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

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2.0

1.5

1.0

0.5

0.0 0

1

2

3

4

5

Time, days

Figure 15. The volumes of produced oil and water, and remaining oil and water in the core.

Table 9. Reservoir and Fluid Properties Used in The Simulation Model Parameter

Value

Unit

Matrix permeability

0.00025

mD

Fracture permeability

1000

mD

Porosity

7.65

%

Total compressibility

1×10-6

psi-1

Initial core pressure

14.7

psi

CO2 injection pressure

2000

psi

Water saturation

0.4

/

Temperature

104

̊F

5. CONCLUSIONS In this study, systematic experiments were conducted to investigate the potential for extracting hydrocarbons from the shale samples using CO2 huff-n-puff. The experiment results demonstrate great potential of CO2 huff-n-puff EOR ability. The cumulative oil recovery can reach 68% after 7 huff-n-puff cycles. The reasons for the recovered oil decline in the subsequent cycles is caused by decline of oil saturation gradient, limited sweep area, and the reduction of absolute permeability due to asphaltene precipitation. The effect of injection pressure was further investigated. The cumulative oil recovery could increase about 9.1% when the injection condition changed from immiscible to miscible. At the miscible condition, cumulative oil recovery for the total seven cycles was around 68%. After reaching the miscible condition, the further increase in pressuredid not significantly increase oil recovery. A novel experiment design was proposed to saturate the core samples with both imbibed water (15 wt% KCl) and Wolfcamp dead oil. The oil recovery decreased about 45.3% after seven huff-n-puff cycles with water existed. This work provides operators with valuable laboratory data on the CO2 huff-n-puff EOR potential and a better understanding of CO2 huff-n-puff mechanisms in liquid-rich shale reservoirs.


Energy & Fuels 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

ACKNOWLEDGMENTS Wolfcamp crude oil component and Wolfcamp core plugs were provided by Apache Corporation. The work presented in this paper is supported by the Department of Energy (DE-FE0024311), the Fundamental Research Funds for the Central Universities (18CX02170A), the Province Natural Science Foundation of Shandong (ZR2018BEE018), and the Funding for Scientific Research of China University of Petroleum East China (2017010922).

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