Subscriber access provided by NEW YORK UNIV
Article
Impact of Secondary and Tertiary Floods on Microscopic Residual Oil Distribution in Medium-to-High Permeability Cores with NMR Technique Hui Gao, Yueliang Liu, Zhang Zhang, Baolun Niu, and Huazhou Li Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2015 Downloaded from http://pubs.acs.org on June 24, 2015
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 free 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 accessible to all readers and 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 34
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
Impact of Secondary and Tertiary Floods on Microscopic Residual Oil Distribution in Medium-to-High Permeability Cores with NMR Technique
Hui Gao*, Yueliang Liu**, Zhang Zhang***, Baolun Niu***, and Huazhou Li** *School of Petroleum Engineering, Xi’an Shiyou University, Xi’an, China 710065 **School of Mining and Petroleum Engineering, Faculty of Engineering, University of Alberta, Edmonton, Canada T6G 2W2 ***Sinopec Production Engineering and Technology Institute, Zhongyuan Oilfield Branch Company, Sinopec, Puyang, China 457001
Corresponding Author: Huazhou Li Assistant Professor, Petroleum Engineering University of Alberta Phone: 1-780-492-1738 Fax: 1-780-492-0249 Email:
[email protected] 1 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
Abstract In order to explore the impact of various flood schemes and pore throat heterogeneity on oil recovery efficiency in porous media, coreflood experiments and nuclear magnetic resonance (NMR) tests are conducted to quantitatively determine the initial oil distribution and the residual oil distribution in medium-to-high permeability cores subjected to these various flood schemes. Multiple experimental runs are conducted with 4 field core samples to cover the various flood schemes: the secondary water flood, CO2-foam flood, and water-alternating-CO2 flood (WAG). Experimental results show that, relatively speaking, at the initial oil saturation condition, the moderate pore throats contain the highest amount of oil. Water flood recovery degree is higher from larger pore throats (average recovery degree of 98.57%) than that from moderate pore throats (average recovery degree of 78.29%). The water flood efficiency in different cores is found to be dependent of the degree of heterogeneity in pore throat distribution. After water flood, the residual oil is mainly located in smaller pore throats. CO2-foam flood shows a good performance in tapping the residual oil contained in smaller pore throats, while the WAG can recover more oil from larger pore throats. Furthermore, it is found that the combination of CO2-foam flood and WAG provides the highest recovery efficiency since it is effective in reducing the oil saturation in pore throats with varied sizes. Based on this investigation on the residual oil saturation in pore throats subjected to secondary and tertiary floods, it is possible to design an optimum flood scheme which suits the microscope pore throat characteristics Keywords: Residual for a given oil distribution; reservoir. NMR; CO2-foam flood; Water flood; WAG 2 ACS Paragon Plus Environment
Page 2 of 34
Page 3 of 34
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
1 Introduction Many medium-to-high-permeability reservoirs have entered the production phase with a high water cut due to a long time of secondary water injection. The potential of enhanced oil recovery (EOR) methods, e.g., CO2 flood, water-alternating-gas injection (WAG), and CO2-foam flood, are often explored in order to improve the ultimate recovery efficiency from these reservoirs. Many previous studies have been focused on evaluating the overall oil recovery efficiency of various EOR methods based on coreflood experiments, while few of them1-5 has examined the oil recovery efficiency from different pore throats as well as the resulting residual oil saturation at a microscopic level following the floods. When evaluating these various EOR methods, it is extremely important to have a good microscopic understanding in the residual oil saturation in cores subjected to secondary or EOR floods. At present, various methods have been used to investigate the microscopic residual oil distribution, such as the photo-etching model1, micro-displacement model built with true sandstone2, and X-CT scanning technique, etc3. It is noted that these methods their own advantages as well as disadvantages. For instance, the photo-etching model differs from the true cores considerably, especially, in that it cannot imitate the of clay minerals in true cores. What is more, residual oil distribution in the photo-etching model or sandstone micro-displacement model is observed by a microscope, which renders the accurate quantification of residual oil distribution difficult. Although X-CT scanning technique can quantitatively evaluate the residual oil distribution in different pore throats, its accuracy highly depends on the image processing techniques4,5. Different from X-CT scanning technique, the nuclear magnetic resonance (NMR) signals do not respond to rock matrix materials, but only correlate with the fluid distribution in pore throats. Meanwhile, variation in T2 3 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
obtained from NMR reflects the fluid distribution in pore throats6-8. Many scholars that there exists a positive correlation between T2 value and the radius of pore throat many sandstones and some carbonate
9,10
. According to mercury intrusion data, T2
can be converted into pore throat radius, such that the fluids distribution in different pore throat can be obtained11,12. On the basis of the variations in the amplitude of the pore throat distribution curve, the extent of fluid change in different pore throats can compared quantitatively. This makes it possible to quantitatively evaluate the residual oil distribution and the total injection volume in pore throats13,14. The major purpose of this study is to elucidate the dependence of residue oil saturation in a microscopic level on the following factors: 1) secondary and tertiary flood schemes; 2) microscopic pore throat heterogeneity. In this study, four core samples are obtained from Zhongyuan oilfield in central China. The reservoirs in this oilfield exhibit characteristics of thick pay zone, high temperature and high salinity. Multiple coreflood tests, NMR tests, and mercury intrusion measurements are carried out to examine the recovery degree and residual oil saturation in different pore throats subjected to secondary water flood and various EOR floods.
2 Experimental Section 2.1 Materials The crude oil used in this experiment is a dehydrated crude oil with viscosity of 1.82 mPa.s and density of 0.750 g/cm3 at reservoir conditions of 70°C and 12 MPa. The formation water has a salinity of 24×104 mg/L, a salt type of CaCl2, a chloride content of 16×104 mg/L, viscosity of 0.5 mPa.s at 70ºC and 12 MPa. Manually prepared water 4 ACS Paragon Plus Environment
Page 4 of 34
Page 5 of 34
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
is used in the experiments to simulate the formation water. The ZYH-0860 foam agent (Zhongye Company, Tianjin) with a concentration of 0.5 wt% is used to generate in the experiment. CO2 with 99.9% purity is supplied by Xi’an Specific Standard Coal Gas Supply Station (China). The cores used in the experiments are true cores retrieved from Zhongyuan oilfield of China. Table 1 shows the properties of the core samples as well as the corresponding flood schemes conducted on for these core samples. It is noted that these core samples are water-wet sandstone.
2.2 Experimental Setup Figure 1 shows the coreflood apparatus which mainly comprises of a syringe pump (ISCO-500D, USA), a hand pump (Huaxing Company, China), an oven (Huaxing Company, China), transfer cylinders (Huaxing Company, China), and core holders (Niumag, Shanghai). Figure 2 shows the schematic of the NMR test apparatus (Mini-MR, Niumag, China), while Figure 3 provides the digital images showing the coreholder placed inside the probe coil for the NMR test.
During the coreflood tests, fluid injection is maintained at a constant injection rate of 0.05 mL/min. The core holder is made of one type of non-magnetic material PEEK can withstand pressure and temperature up to 20 MPa and 80ºC, respectively. The confining pressure of the core holder is applied by immersing the core in the fluorocarbon oil with the manual pump. The oven is used to maintain a constant temperature for the coreflood. The injection fluids, including brine, oil, foam and CO2, are placed in the transfer cylinders. The NMR apparatus, used to measure the core’s 5 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
spectrum at different experimental stages, has a magnetic field intensity of 0.5 T, a gradient value of 0.025 T/m in X, Y, Z direction, and a radio frequency range of 1 - 30 MHz with a control precision of 0.01 MHz. In the experimental process, fluorocarbon oil is used to apply confining pressure on the core holder. The fluorocarbon oil, free of hydrogen signals, does not affect the NMR response, which enables one to carry out the T2 spectrum test directly under the experimental temperature and pressure conditions without dismantling cores from the core holder. When the flood experiments are completed, the non-magnetic core holder is quickly moved to the NMR instrument for testing after closing the inlet and outlet. The time needed for the moving and NMR testing process is quite short (about one minute), that the fluid shrinkage in the core can be ignored. This obviously makes the measurements much more representative of the in situ conditions. A constant speed mercury injection setup (ASPE-730, Core corp. USA) is used for conducting the mercury intrusion measurements. The maximum injection pressure is 900 psi corresponding to a pore throat radius of 0.12 µm and a mercury injection rate of 0.00005 mL/min. The mercury intrusion measurements can be used to measure pore throat radius distribution of the core. 2.3 Experimental Procedure The experimental procedure is briefly described as follows. First, the core is cleaned with benzene and dried. Subsequently, the core is saturated with the simulated brine 12 hours under the pressure of 10 MPa. Then the porosity of the core is determined based on brine density, together with the weight difference before and after brine 6 ACS Paragon Plus Environment
Page 6 of 34
Page 7 of 34
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
saturation, while its permeability is measured with a gas permeameter. To eliminate hydrogen signals of water in the core, the core sample is first displaced with 1.5 times of pore volume (PV) of Mn2+ solution (15000 mg/L); then the core is placed inside core holder; and the oven temperature is set at the formation temperature of 70°C and confining pressure of 12 MPa is applied to the core with the fluorocarbon oil. Three PVs of crude oil is then injected to displace the water for establishing the initial oil saturation. Following the oil injection, the NMR test is initiated to measure the T2 spectrum of the core while maintaining the confining pressure on the core. The measured T2 spectrum can be used to infer the initial oil saturation distribution since NMR signal from connate water has been suppressed. Next, the core is flooded with simulated formation water with Mn2+ concentration of 15000 mg/L until the effluent water cut reaches 100%. NMR test is again used to measure the T2 spectrum of the core samples. After water flood experiment for Core #1 and Core #2, additional CO2-foam and WAG floods are continued until the effluent water cut reaches 100% every time (See Table 1 for the detailed flood schemes). In the CO2-foam flood, slugs of CO2 and foam are injected in an alternative manner. Only water flood is executed for Core #3 and Core #4. NMR test is conducted every time the overall flood scheme is altered.
Finally, upon finishing the experiment, the core samples are cleaned with benzene and dried; the mercury injection test is conducted under the room temperature of 25°C and an injection rate of 0.00005 mL/min. During the test, the mercury saturations 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
Page 8 of 34
corresponding to different mercury injection pressures are recorded to infer the pore and throat radius distribution of the core sample.
2.4 Relationship between T2 Response and Pore Throat Radius The total T2 response of fluids in porous media can be described with the following equation15,16, 1 1 1 1 = + + T2 T2 S T2 D T2 B
(1)
Where T2S is the transverse relaxation time due to surface relaxation (ms), T2D is the transverse relaxation time due to diffusion in magnetic gradients (ms), and T2B is the transverse relaxation time due to bulk relaxation (ms). When applying NMR technique to flow in porous media, the last two terms in Equation (1) could be normally neglected, and consequently T2 relaxation of fluids depends mainly on the rock-surface relaxation term15,17. The rock-surface relaxation is strongly related to rock’s specific area (i.e., the ratio of pore’s surface area to the pore volume): a larger rock’s specific area results in a stronger T2 relaxation, but a smaller T2 relaxation time. The rock surface relaxation can be expressed as follows18,19, 1 S = ρ2 T2 S V
(2)
where ρ 2 is the relaxation rate (µm/ms), and S / V is the specific area of pore throat (1/µm). The specific area is related to the pore throat radius as follows,
S FS = V r
(3)
where Fs is the dimensionless shape factor of pore throat and r is the pore throat radius (µm). Eventually, the T2 response can be calculated by18,19,
8 ACS Paragon Plus Environment
Page 9 of 34
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
T2 = Letting
1 r ρ 2 FS
(4)
1 be C (µm/ms), Equation (4) can be expressed as, ρ 2 FS
r = CT2
(5)
As for a given core sample, ρ2 and FS can be approximately regarded as constants, leading to that the coefficient factor C can be regarded as a constant. It can be seen from Equation (5) that the pore throat radius is directly proportional to the T2 response. After obtaining the constant C in Equation (5), the T2 spectrum of NMR can be eventually converted into distribution curve of pore throat radius.
3 Results and Discussion 3.1 T2 Spectrum Distributions Figures 4-7 illustrate the T2 spectrum distributions for Cores #1-#4 that have been flooded with different schemes. By dividing the T2 spectrum into four intervals, i.e., ms, 1-10 ms, 10-100 ms, and 100-1000 ms, Table 2 compares the T2 distributions of four intervals for each core sample. As can be found from Table 2, with the core samples having the initial oil saturation, the T2 spectrum of all these four experimental cores is mainly distributed in the range of 10-100 ms, then followed by the ranges of 1-10 ms, 10 µm radius than those with 100 µm pore throats. Table shows the residual oil distributions of these four core samples after water flood. From Table 7, in general, it is noted that, after water flood, the residual oil is mainly distributed in pore throats with radius smaller than 100 µm. It is noted that the oil distributed in pore throats with a diameter of
