Experimental Study on Spontaneous Imbibition under Confining

Feb 14, 2018 - Influencing Factors and Selection of CH4 and CO2 Adsorption on Silurian Shale in Yibin, Sichuan Province of China. Energy & Fuels. Niu,...
0 downloads 0 Views 2MB Size
Subscriber access provided by UNIV OF DURHAM

Article

Experimental Study on Spontaneous Imbibition under Confining Pressure in Tight Sandstone Cores Based on Low-Field Nuclear Magnetic Resonance Measurements Yun Jiang, Yang Shi, Guoqing Xu, Chen Jia, Zhan Meng, Xianyou Yang, Hanqing Zhu, and Bin Ding Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03776 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 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

Energy & Fuels

4

Experimental Study on Spontaneous Imbibition under Confining Pressure in Tight Sandstone Cores Based on Low-Field Nuclear Magnetic Resonance Measurements

5

Yun Jiang†, Yang Shi†, Guoqing Xu†, Chen Jia†, Zhan Meng†, Xianyou Yang†,

6

Hanqing Zhu† and Bin Ding*†

1 2 3



8

Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, P.R. China

9

ABSTRACT: Spontaneous imbibition (SI) is an important method to improve oil recovery in tight

10

sandstone reservoirs. Commonly, the physical simulation of SI is performed at atmospheric pressure

11

but the characteristics of spontaneous imbibition under confining pressure (SIUCP) is often

12

neglected. In this study, oil distribution in tight cores was obtained in combination of high pressure

13

mercury intrusion (HPMI) measurements and low-field nuclear magnetic (LF-NMR) measurements.

14

After that, oil recovery for SI and SIUCP of tight core samples with all faces open (AFO) were

15

obtained using LF-NMR measurements. Finally, a new scaling law for SIUCP was proposed to

16

predict shut-in time in field scale. The results showed that 95.94 - 98.12 wt% of the oil was

17

distributed in nano-pores (0.1 ms < T2 < 100 ms) of core samples, and the average amount of oil in

18

nano-micro-pores, nano-meso-pores and nano-macro-pores were 34.04 wt%, 40.15 wt% and 22.75

19

wt%, respectively. Ultimate oil recovery forcore samples were 22.41 wt%, 44.41 wt%, 57.27 wt%,

20

61.84 wt % and 62.82 wt%, respectively, as confining pressure increased from 0 psi to 2175 psi. The

21

improved oil recovery for SIUCP was associated with the decline of effective pore radius as a

22

function of confining pressure, which results in the effect of enhanced SI and compaction. Finally, a

23

modified dimensionless time model was proposed in combination of Mason’s dimensionless time

7

ACS Paragon Plus Environment

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

24

model and effective pore radius as a function confining pressure.

25

KEYWORDS: LF-NMR; oil distribution; oil recovery for SIUCP; effective pore radius;

26

dimensionless time model;

27

1. INTRODUCTION

28

Tight reservoir in Ordos Basin is characterized with ultralow permeability (less than 1 mD),

29

porosity (less than 10%) and narrow flow channels (abundant nano-micro pores) 1. A series of

30

problems, including low formation pressure coefficient (0.6-0.8), fast decline of production and

31

low oil recovery, are difficult to solve. One effective method to economically produce oil and gas

32

is hydraulic fracturing stimulation, which is completed by injecting massive fracturing fluid to

33

create complex fracture networks, to increase contact area with reservoirs and to improve pressure

34

coefficient of reservoirs2. After hydraulic fracturing, the well is commonly shut down for a period

35

to promote water uptake into shale/sandstone rocks, which has been proved to be a driving force

36

for enhanced oil recovery, especially for water wet reservoirs3-8. In several simulation studies,

37

extended shut-in time might contribute to early-time production but decrease load recovery and

38

late-time production9-11. Appropriated shut-in time becomes critical for understanding enhanced

39

oil recovery from water imbibition.

40

Qing. L et al.12 calculated shut-in time in field scale based on Ma’s dimensionless time (tD)

41

model 13, 14 and SI experiments of Horn River shale samples with AFO. B. Roychaudhuri et al. and

42

K. Makhanov et al.15, 16 provided references for determining shut-in time by calculating water loss

43

in both lab scale (SI of Marcellus shale samples and Horn River shale samples, respectively) and

44

field scale (similarity criterion). In their studies, experiments of SI in lab scale were performed at

45

atmospheric pressure and the corresponding scaling law considering confining pressure was

ACS Paragon Plus Environment

Page 2 of 33

Page 3 of 33 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

Energy & Fuels

46

neglected.

47

One of the main difficulties in simulating SIUCP of tight cores is to record the amount of

48

imbibed fluid since the cores are set in the closed and pressurized system and the imbibed amount

49

is always limited. In SIUCP measurements, big error may occur by monitoring imbibed fluid

50

volume using an Amott imbibition cell17 or weighting the core mass using a precision balance18.

51

LF-NMR T2 distribution is a fast and non-destructive method to measure fluid distribution in

52

pores and to provide information for pore size distribution (PSD) combined with HPMI

53

measurements19-23. Furthermore, T2 distribution was also used to monitor uptake of liquid in shale

54

samples and tight sandstone samples24, 25. Scaling equations for SI have been proposed in different

55

forms based on capillary models26, 27, Buckley–Leverett equations13, 28-30. On the basis of these

56

theoretical equations, factors influencing SI including viscosity ratios, initial water saturation,

57

gravity, wettability, boundary conditions and tortuosity were discussed5,

58

dimensionless time model considering the effect of confining pressure has not been presented.

7, 8, 31-35

. However,

59

Gas flow in tight cores is usually affected by gas slippage effects as effective pore radius is

60

ten times larger than the mean free path length of gas molecules. Then gas slippage factor and

61

corresponding pore radius can be calculated according to Klinkenberg procedure36. As confining

62

pressure increases, a series of parameters including porosity, permeability and pore radius, etc. are

63

influenced, which is related to stress sensitivity phenomenon37-39.

64

The objective of this study is to study the characteristics of SIUCP and construct a new

65

scaling law for SIUCP. This study consisted of three sections. In the first section, PSD of core

66

samples selected form Ordos Basin was obtained by combining HPMI and LF-NMR

67

measurements and then oil distribution in tight cores was discussed. In the second section,

ACS Paragon Plus Environment

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

68

recovery of SIUCP based on LF-NMR T2 distribution measurements was obtained and the result

69

was compared with SI. Finally, a new scaling law for SPUCP based on Mason’s dimensionless

70

time model32 was proposed to provide reference for calculating shut-in time in field scale.

71

2. MATERIALS

72

2.1. Core samples.

73

Tight sandstone core samples were taken from Upper Triassic Yanchang formation in Ordos

74

Basin, China. Core plugs (3-7 cm in length and 2.5 cm in diameter) were cut from larger materials

75

and the fragments collected during cutting process were used for X-ray diffraction (XRD)

76

analysis.

77

The core samples were cleaned by exposing them to alternating vapor of toluene and

78

methanol for 30 days to remove all residual oil in the core samples, and then dried under vacuum

79

at 105 °C for 48 hours prior to measurements. Effective porosity was determined using a helium

80

porosimeter and oil saturation method (measuring difference between oil saturated weight and dry

81

weight), respectively. Helium gas permeability was determined using a pulse-decay permeametry,

82

which was conducted at net confining pressure of 300 psi at 20 ºC.

83

Permeability and porosity may differ significantly among tight cores as they are abundant

84

with micro and nano pores. In this study, core samples with similar physical properties were

85

carefully classified so that the effect of difference of permeability and porosity on imbibition can

86

be minimized. More specifically, the prepared cores were classified into three sets (Table 1). Every

87

core sample in the first set was cut into two slices and then all the cores were divided into two

88

groups. They were used for HPMI measurements (A11, A12, A13, A14 and A15) and SIUCP

89

measurements (A21, A22, A23, A24 and A25). Core samples used for SIUCP measurements were

ACS Paragon Plus Environment

Page 4 of 33

Page 5 of 33 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

Energy & Fuels

90

vacuumed at 20 to 50 mbar for 2 days and then saturated with oil at confining pressure of 4000 psi

91

for 5 days prior to measurements. After that, they were equilibrated to 0 psi which lasted for 24

92

hours. Similarly, core samples in the second set were also cut and divided into two groups and

93

they were used for contact angle (CA) measurements (B11, B12 and B13) and oil mass calibration

94

(OMC) measurements (B21, B22 and B23). The core samples used for OMC measurements were

95

also saturated with oil (the step was same as core samples used for SIUCP measurements) prior to

96

measurements. The last set of core samples (A1, A2, A3 and A4) were used to determine

97

pulse-decay permeability (PDP), gas slippage factor and effective pore radius at different range of

98

net confining pressures using a pulse-decay permeametry.

99

2.2. Fluid properties.

100

Oil (No.3 Jet Kerosene), purchased from Beijing Unicorn Company Limited, with a purity of

101

99% was used in oil-saturation process. Deionized (DI) water, purchased from Cambridge Isotope

102

Laboratories, with a purity of 99.9% was used in imbibition measurements. The physicochemical

103

properties of the fluids used for imbibition experiments were listed in Table 2.

104

3. METHODS

105

3.1. XRD analysis.

106

Bulk and clay mineralogies of core samples were investigated using an X’Pert X-ray

107

diffractometer. The experimental procedure was following the Chinese Oil and Gas Industry

108

Standard SY/T5163-2010 (Analysis Method for Clay minerals and Ordinary Non-clay Minerals in

109

Sedimentary Rocks by the X-ray diffraction).

110

3.2. HPMI measurements.

111

PSD was determined using a mercury porosimeter (AutoPore IV 9520). Core samples were

ACS Paragon Plus Environment

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

Page 6 of 33

112

dried at 200 °C to remove all the water prior to measurements. After that, mercury was injected by

113

applying uniform level of pressure, ranging from atmospheric pressure to 60,000 psi. PSD was

114

calculated using Washburn’s equation26 based on the assumption of a bundle of connected

115

capillary tubes.

116

3.3. LF-NMR measurements.

117

LF-NMR signal was measured using inversion and Carr-Purcell-Meiboom-Gill (CPMG)

118

pulse sequences. T2 distributions was generated using SIRT (Simultaneous Iterative

119

Reconstruction Technique) inversion algorithms. The measurements were performed using a

120

LF-NMR core analysis system (MesoMR-060H-HTHP-I) with a magnetic field intensity of 0.5 T.

121

The major testing parameters included dominant frequency (21.326 MHz), echo spacing (TE = 0.2

122

ms), polarization time (TW = 3000 ms) and echo number (NECH = 8000).

123 124

125

LF-NMR transverse relaxation time (T2) in porous media is generally governed using the following equation:

1 1 1 1 = + + T2 T2,bulk T2,surface T2,diffusion

(1)

126

Where the subscript “bulk”, “surface” and “diffusion” indicate bulk relaxation, surface

127

relaxation and diffusion relaxation, respectively. Diffusion relaxation can be neglected when

128

assuming a uniform magnetic field and a small field gradient. Moreover, bulk relaxation can also

129

be neglected compared with surface-relaxation (bulk is commonly larger than 3000 ms). Thus, T2

130

can be converted into pore size in the following equation:

131

132

S C 1 1 = =ρ =ρ T2 T2, surface V R

(2)

Where, S is core surface area, cm2; V is pore volume, cm3; R is pore radius, µm; C is a

ACS Paragon Plus Environment

Page 7 of 33 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

Energy & Fuels

133

constant parameter, C=1, 2 or 3 for planar, cylindrical and spherical model, respectively. ρ is

134

surface relaxivity, which can be calculated by combining HPMI and LF-NMR measurements

135

using the following equation40:

ρ=

136

T2 LM × Rp

(3)

C 200

∑ ln(T

2i

T2LM = exp(

137

) × Ai

i =1

200

∑A

)

(4)

i

i =1

138

Rp =

139

2V A

(5)

140

Where ρ is surface relaxivity, µm/ms; T2LM is the logarithmic mean value of T2 , ms; Ai is

141

amplitude of LF-NMR T2 distribution spectrum at T2i, a.u.; Rp is average pore radius, µm; V is

142

total intrusion volume, mL/g; A is total pore area, m2/g;

143

3.4. CA measurements.

144

CA measurement is the most widely used method to determine wettability between a single

145

phase and a core sample. In this study, a sessile drop method using an automated goniometer

146

(Drop shape analyzer, DSA-100) was performed to measure CA41. Prior to measurements, core

147

samples were polished with 600-mesh diamond polisher and then aged in oil for at least 100 h at

148

80 °C. After that, the core samples were dried in an oven at 80 °C for 20 min. After that, the core

149

sample was set on the goniometer and a drop (10 - 20 µL) of DI water was dripped on the surface

150

of it. Finally, the contact angle was calculated from a camera screenshot.

151

3.5. OMC measurements.

152

When the core sample is saturated with oil and DI water, LN-NMR T2 signal can reflect the

153

statistical distribution of oil in pores as DI water has no response to hydrogen proton42-46. The

ACS Paragon Plus Environment

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

Page 8 of 33

154

integral area of T2 distribution along the axis of T2 is proportional to oil mass. Therefore, the main

155

purpose of OMC is to transfer LF-NMR T2 signal into oil mass so that we can calculate oil mass

156

in pores, especially initial oil mass. After that, oil recovery can be calculated and compared with

157

previous studies. Moreover, overall T2 distribution is obtained using SIRT (Simultaneous Iterative

158

Reconstruction Technique) inversion algorithms which implies that the integral area of T2

159

distribution equals to the sum of amplitude of T2 distribution. Finally, a correlation between oil

160

mass in pores and cumulative amplitude of T2 distribution can be built.

161

Two methods were used for OMC in this study. The first method was to measure the T2

162

cumulative amplitude of several bottles (without response to hydrogen proton) filled with different

163

amount of oil (Figure 1a), and then the curve between T2 cumulative amplitude and oil mass was

164

fitted. The second method was to measure T2 cumulative amplitude of tight core samples (Figure

165

1b) before and after several centrifuge tests which were conducted to obtain a wide range of oil

166

mass in pores to minimize the errors when using the fitting correlation to calculate oil mass. The

167

oil mass was simultaneously measured using a precision balance (A & D GF-1000). The curve

168

between T2 cumulative amplitude and oil mass was fitted. Centrifuge tests were performed using a

169

high speed refrigerated centrifuge (CSC-12(S), Lu Xiangyi Centrifuge Company, Shanghai,

170

China). The rotating speed were 3000 rpm, 4000rpm, 5000rpm, 6000rpm, 7000rpm, 8000rpm,

171

respectively. Finally, oil mass was calculated using the following equation: 200

m = a + b ⋅ ∑ Ai

172

(6)

i =1

Where m is oil mass in pores, g; Ai is amplitude of LF-NMR T2 distribution spectrum at T2i,

173 174

a.u.;

ACS Paragon Plus Environment

Page 9 of 33 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

Energy & Fuels

175

3.6. SI/ SIUCP measurements.

176

SIUCP for oil-saturated core samples with all faces open (AFO) was simulated in a sealed

177

and pressurized system (Figure 2). In the following, we briefly explained the experimental

178

procedure.

179

(1) T2 distribution of oil-saturated the core samples was measured;

180

(2) The core sample and 100 mL DI water were set in the floating piston accumulator, and

181 182 183

valves in the upstream and downstream were kept open; (3) Distilled water was pumped to the bottom of floating piston accumulator continuously until all the gas in the accumulator was removed;

184

(4) The valve in the upstream was closed and the ISCO pump started to work in the mode of

185

constant pressure. The confining pressure in each accumulator was kept 0 psi, 362.5 psi,

186

725 psi, 1450 psi and 2175 psi, respectively;

187 188 189 190

(5) Pressure in the accumulator was removed at selected time interval and the core sample was taken out; (6) Liquid in the surface of the core was dried with cotton yarn and then T2 distribution was measured;

191

(7) Step 2 to 6 were repeated until the end of the experiment, which lasted for 25 days;

192

(8) Oil recovery for SIUCP at different time interval was calculated using the following

193

194

195 196

equation:

Roil =

m0 − mi ×100% m0

(7)

Where Roil is oil recovery, %; m0 is oil mass in core samples before SI/ SIUCP measurements, g; mi is oil mass in pores measured for i th. time water at selected time, g;

ACS Paragon Plus Environment

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

197

Page 10 of 33

3.7. PDP measurements.

198

PDP at different net confining pressures, ranging from 362.5 psi to 2175 psi, was measured

199

using a PDP system (PDP-200) at 20 ºC. Slippage factor was calculated from a linear regression

200

performed on the 1/Pp and PDP, which referred as Klinkenberg procedure. PDP, slippage factor

201

and effective pore radius were calculated using the following equations47:

ka =

202

αµ g Lcg 1 1 A( + ) Vu Vd

203

ka = k∞ (1 +

204

b= λ=

205

b ) Pp

4cλPp

r

µ

RgT π

Pp

2M

(8)

(9)

(10)

(11)

206

where ka is PDP, mD; α is slope of pressure decay curve in the semi-logarithmic plot; µg is

207

gas viscosity, mpa·s; cg is gas compressibility, psi-1; L is sample length, cm; A is sample cross

208

sectional area, cm2; Vu/d are upstream and downstream gas storage reservoirs volumes,

209

respectively, mL; k∞ is Klinkenberg gas permeability, mD; Pp is mean value of inlet pressure and

210

outlet pressure, psi; b is gas slippage factor, psi; λ is mean free path length of gas molecules, µm; c

211

is a constant parameter, in the order of 1; r is effective pore radius, µm; Rg is gas constant, J/K·mol;

212

T is absolute temperature, K; M is molar mass of gas, mol-1;

213

4. RESULTS

214

4.1. Mineralogy.

215

XRD analysis of bulk-rock mineralogy (Figure 3a) showed that twelve core samples mainly

216

consisted of feldspar (42.0 - 53.3 wt%), quartz (28.1 - 33.9 wt%), and contained moderate amount

ACS Paragon Plus Environment

Page 11 of 33 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

Energy & Fuels

217

of dolomite (11.0 - 15.0 wt%) and clays (10.8 - 17.2 wt%). Clay mineralogy (Figure 3b) of the

218

core samples was categorized into three types: illite (9.2 - 18.5 wt%), chlorite (42.1 - 60.6%) and

219

mixed layer illite/smectite (25.2 - 48.7 wt%).

220

4.2. PSD.

221

Pore diameter determined by HPMI (Figure 4a) was categorized into four groups, 1 - 10 nm,

222

10 - 100 nm, 100 - 1000 nm and > 1000 nm. According to pore size classification proposed by

223

Loucks48, nano-pores ranged from 1 nm to 1 µm and micro-pores ranged from 1 µm to 62.5 µm. In

224

this study, average mercury intrusion saturation of nano-pores (Figure 4b) and micro-pores were

225

86.76 wt% and 13.24 wt%, respectively.

226

On the other hand, a wide range of T2 distribution (Figure 4b) was also observed

227

simultaneously, including ≤ 0.1 ms, 0.1 - 1 ms, 1 - 10 ms, 10 - 100 ms and ≥ 100 ms. The

228

correlation between T2 and pore radius can be built by calculating surface relaxivity using Eq.3 –

229

Eq.5.

230

The calculated surface relaxivity (Table 3) showed that surface relaxivity varied among core

231

samples with similar petrophysical properties, ranging from 2.75 µm/s to 10.68 µm/s. The result

232

was in accordance with Milad Saidian’s40 observation that a linear correlation (Eq. 11) between

233

surface relaxivity and illite content (Table 4) was satisfied. ρ = A × f illite + B

234 235 236

(12)

Where, ρ is surface relaxivity, µm/s; fillite is illite content, wt%; A, B are constant parameters, µm/s;

237

The calculated PSD using calculated relaxivity and measured HPMI PSD (Figure 5) showed

238

a good correlation with each other. Thus, using measured T2 distribution to obtain PSD is also an

ACS Paragon Plus Environment

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

239

efficient method. The correlation between pore diameter and T2 can be roughly constructed in

240

Table 5.

241

4.3. CA.

242

E.J. Peters41 proposed that a system was defined as water-wet when CA was between 0 º to

243

60 º - 75 º. The system was defined as oil-wet when CA ranged from 105 º - 120 º to 180 º, and

244

neutral-wet was defined when CA was in the range of 75 º - 105 º. In this study, CA (Figure 6) was

245

22.7 º, 27.7 º and 25.8 º, respectively. Thus, the system was determined to be water-wet.

246

4.4. Oil mass calculation.

247

Two fitting curves (Figure 7) using different methods both showed strong positive linear

248

correlations between oil mass and T2 cumulative amplitude. But the calculated oil mass (Figure 8)

249

using method 2 was more close to authentic oil mass as the average error between calculated oil

250

mass and authentic oil mass was smaller (Table 6). Finally, the correlation between cumulative T2

251

distribution and oil mass was written as m = 0.125 × 10 × ∑ − 0.300   = 0.954

252

4.5. Oil recovery for SI and SIUCP.

253

Oil recovery with a wide range of imbibition time and different range of confining pressure was

254

obtained (Figure 9) through converting measured cumulative T2 distribution into oil mass using Eq.6 and

255

Eq.7. Clearly, oil recovery as a function imbibition time (Figure 9a) was divided into two phases for both

256

SI and SIUCP, each with its own slope. In phase 1, water uptake occurred rapidly and oil recovery

257

increased quickly with imbibition time. After that, little water was imbibed into core samples and oil

258

recovery tended to stabilize correspondingly. Two dash lines were placed on phase 1 and phase 2,

259

respectively, to represent for rate of water uptake. As confining pressure increased, the intersection of

260

two dash lines in the horizontal axis of Cartesian coordinate were 15 days, 10 days, 7 days, 5 days and 3

ACS Paragon Plus Environment

Page 12 of 33

Page 13 of 33 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

Energy & Fuels

261

days, respectively.

262

Similarly, oil recovery as a function of square root of imbibition time (Figure 9b) was also divided

263

into two phase. In phase 1, oil recovery for SIUCP was exponential to square root of imbibition time,

264

which was significantly from that of SI (linear correlation was observed). In phase 2, oil recovery for

265

both SI and SIUCP were linear to square root of imbibition time.

266

Furthermore, the ultimate oil recovery (Figure 10) as a function of confining pressure can also be

267

divided into two stages. Oil recovery increases significantly within confining pressure of 725 psi, and

268

then it slowly increased. The ultimate oil recovery for SIUCP enhanced by 21.98 wt%, 34.86 wt%, 39.43

269

wt% and 40.41 wt%, respectively, than that of SI.

270

4.6. Gas slippage factor and effective pore radius.

271

PDP measurements at different range of net confining pressure were performed on four tight core

272

samples, and then slippage factors were calculated from linear regression expressions. PDP as a function

273

of 1/Pp (reciprocal of mean value of inlet pressure and outlet pressure) of core sample C1 was plotted in

274

Figure 11 and results for three other core samples were obtained simultaneously. Gas slippage factor and

275

effective pore radius of four core samples were calculated and listed in Table 6.

276

Effective pore radius as a function of net confining pressure was plotted in Figure 11. Clearly, the

277

drop of effective pore radius occurred in two different periods, each with its own distinct slope. It

278

decreased significantly in the first period with respect to net confining pressure, and then it slowly

279

decreased until it reaches a fixed value in the second period. The effective pore radius as a function of net

280

confining pressure can be expressed in the following equation:

281 282

r = 0.834 ⋅ exp( − P / 393.212) + 0.0792 Where, r is effective pore radius, µm; P is net confining pressure, psi.

ACS Paragon Plus Environment

(13)

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

283

5. DISCUSSION

284

5.1. Oil distribution in tight cores.

285

Oil distribution in the core sample was obtained according to measured T2 distribution (Figure 4b)

286

before imbibition experiments. During the measurements of CPMG sequence, TE is required to bigger

287

than 6*P2 (width of 180 degree radio frequency pulse). In this study, TE = 0.2 ms was optimized as the

288

measured P2 was 24 µs. The optimized parameter may inevitably result in partially lost of authentic

289

information from small pores less than T2=0.2 ms. In this study, the area of T2