Experimental Investigation on CO2 Injection in Block M

Hubei Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University, Wuhan, Hubei 430100, China College of Earth Science, Yangtze University, Wuhan, Hubei 430100, China MLR Key Laboratory of Saline Lake Resources and Environments, Institute of Mineral Resources, CAGS, Beijing 100037, China CBM Company, PetroChina, Beijing 100028, China Well Testing Company, Bohai Drilling Company, PetroChina, Langfang 065000, China Production Logging Center of Logging Co. Ltd., PetroChina, Xi’an 710201, China


Introduction
Located in the abdomen of the Junggar Basin, Block M has a reservoir depth of approximately 3900 m, porosity of 13.52%, and permeability of 15.27 mD, and it can be categorized as a low porosity and low permeability reservoir. e formation experiences a long period of sedimentary cycles with strong planar and longitudinal heterogeneity, which occur initially for the delta front sediments and later for the former delta sediments [1,2]. e oil reservoir is developed using water injection; however, the effects of water flooding are poor, and there are significant problems in the development process. Firstly, certain oil wells produce water rapidly after water injection, and the water content rises quickly, thus resulting in several wells being shut down because of high-water content. Secondly, the plane heterogeneity is strong, the state of each well is extremely different, and significant water channeling occurs. Lastly, the overall development effects are poor, the current recovery rate is only 15.6%, and the remaining reserves of the reservoir are large. CO 2 -enhanced oil recovery (EOR) has been used worldwide for more than 50 years [3,4]. e most commonly used methods for EOR by CO 2 injection are CO 2water alternate flooding, immiscible flooding, miscible flooding, and cyclic CO 2 injection [5,6]. e mechanism of EOR by injecting CO 2 primarily includes a reduction in the viscosity of crude oil and expansion, vaporization, extraction, and improvement in the oil removal efficiency [7,8]. Based on the production date of the CO 2 EOR project in the Jilin oilfield, reservoir pressure and wellhead injection pressure are the primary factors used to control the performance of wells, whereas gravity extraction and light/medium distillation extraction are the key mechanisms for enhancing oil recovery in tight reservoirs [9]. Yu et al. analyzed the EOR mechanism in tight oil reservoirs using a CO 2 huff and puff technology and verified its feasibility to increase the recovery in volume fracturing horizontal wells in the Bakken oilfield [10].
To prevent CO 2 from channeling in the high-permeability layer during a multilayer combined CO 2 injection, a reasonable plugging agent has been developed [11]. CO 2 -water alternate flooding can reduce the gas breakthrough, thus improving the displacement efficiency. Furthermore, several field application results demonstrate that it can enhance recovery by 5%-10% [12]. By integrating an orthogonal array and Tabu technology to a genetic algorithm, the optimization of key parameters in CO 2 flooding, such as injection speed, slug size, and cycle time, is formed, and good results have been achieved in the field [13]. CO 2 -EOR has been widely studied and proven successful in numerous oilfields. Additionally, it is a mature method to EOR technology, and it can be used as a technique to improve the development effects of the M reservoir to study certain mechanisms.
Nuclear magnetic resonance is widely applied to the study of pore throat distribution and flow characteristics of fluids in porous media [14][15][16]. It is difficult to evaluate the permeability of a tight reservoir, and thus, certain studies combine the pore throat characteristics with the relaxation time T 2 of the nuclear magnetic resonance to solve this problem [17]. To evaluate the residual oil distribution and effects of imbibition on oil recovery after water flooding, nuclear magnetic resonance can be used to monitor the distribution of the remaining oil in pore throat after various flooding [18]. e application of nuclear magnetic resonance is less in the CO 2 -EOR [19,20]. Nuclear magnetic resonance and core displacement are combined to study the change in oil saturation in different pore throats, and further revealed the EOR mechanism under CO 2 injection [6].
Based on a slim tube test, PVT experiment, and long core displacement experiment, this study analyzes the minimum miscibility pressure and physical property changes of oil and evaluates the effects of continuous CO 2 flooding and CO 2water alternate flooding after water flooding. Using nuclear magnetic resonance, the oil content of different pore throats is observed during flooding, which can clarify the flow mechanisms of different pore throats. Furthermore, a basis for improving oil recovery by CO 2 injection is provided, which can also result in references for other development methods.

Experimental Materials
Oil samples and sandstone core samples are collected from the Block M. e properties of the core samples used in the experiments are listed in Table 1. To characterize the heterogeneity of the reservoir, long cores are assembled with different properties of the core samples. Based on the physical distribution between the injection wells and the production wells, the permeability of the long cores is gradually improved from the inlet to the outlet in this experiment. Table 2 lists the components of the crude oil sample. e crude oil used in this experiment is dehydrated crude oil, and its viscosity of crude oil is 0.708 MPa·s under the reservoir conditions of 104°C and 32 MPa. e type of formation water used is NaHCO 3 . e synthetic brine has a salinity of 16253 mg/L and viscosity of 0.457 MPa·s under the reservoir conditions, and its mineralization composition is listed in Table 3. CO 2 is an industrial gas with a purity of 99.99%.

Slim Tube Test.
e minimum miscibility pressure is the most important parameter used to determine miscible displacement in CO 2 flooding. e slim tube test is commonly used to determine the minimum miscibility   Journal of Chemistry pressure [21]. e experiment is conducted at a temperature of 104°C. e length of the slim tube is 12.5 m with an inner diameter of 0.47 mm, and it is lled with quartz with a diameter of 140-230 µm. e porosity of the filling tube is 35%, the permeability is 5D, and the tube is saturated with crude oil, as indicated in Table 2. In this experiment, 1.2 PV of CO 2 is injected into the slim tube at a rate of 1 cm/min at 8 di erent pressures, and the oil recovery at di erent pressures is depicted in Figure 1. As the injection pressure increases, the oil recovery increases, and there is no apparent in ection point in this process. However, there is a gradual process, and the transition point from immiscible to miscible is at the near-miscible point. Based on the characteristics of the test points, the minimum miscibility pressure of Block M is 32.6 MPa, and the CO 2 injection point under the current reservoir condition occurs in the near-miscible ooding region.

PVT Experiment.
e PVT characteristics of crude oil are closely related to the development e ect in the CO 2 ooding [22,23]. e PVT experiment is performed to understand the change in properties of the crude oil at the reservoir temperature. A CO 2 solubility experiment, including the viscosity of crude oil, is carried out to understand the change in properties of crude oil. During the CO 2 injection process, CO 2 dissolves in crude oil, oil volume increases, and viscosity decreases, which will make oil to ow out of pores. It is necessary to understand the solubility of CO 2 in crude oil and the change of viscosity, which will help to clarify the mechanism of CO 2 -enhanced oil recovery. e solubility of CO 2 in crude oil is an important parameter for immiscible ooding. A high-temperature and high-pressure sample (PY-1 type) is used, and the ash method is used to measure the solubility of CO 2 in the range of 2-30 MPa at 104°C. e result is shown in Figure 2. As the pressure increases, the solubility of CO 2 increases; however, it increases slowly after reaching a certain pressure. At 104°C, the solubility of CO 2 is 15.12 m 3 /m 3 for 2 MPa and 195.27 m 3 /m 3 for 30 MPa (Figure 2). is result indicates that the solubility in crude oil is large, and crude oil has a strong expansion ability during the CO 2 injection. Expansion of crude oil will cause some residual oil to be converted into mobile oil and increases the recovery rate.
When CO 2 is dissolved in the crude oil, the volume of the crude oil expands, whereas the viscosity decreases. To evaluate the change in viscosity during the CO 2 injection, the viscosity of the crude oil is tested at 104°C. e results indicate that the viscosity decreases signi cantly from 0.708 to 0.489 after dissolving CO 2 as the pressure increases from 0 MPa to 30 MPa (Table 4). e decrease in the viscosity of the crude oil reduces the mobility ratio between the uids, and it can reduce the gas cone, improve the utilization efciency of the injected gas, and enhance the oil recovery.

Experimental Method.
Ideally, the ooding experiment should use a natural long core; however, it is not feasibly obtained from the current coring technique. Currently, a natural short core is commonly used to assemble the long core in a certain arrangement. e permeability of the long core can be given as follows: where L is the length of the long core, K is the permeability of the long core, L i is the length of the short core i, and K i is the permeability of the short core i. e experimental device is primarily composed of the injection system, core holder system, and output measurement system (Figure 3). e injection system consists of   an injection pump, uid tube, and temperature control device. e core holder system includes a core holder, thermostat, and pressure control device. e output measurement system is comprised of a three-phase separator and gauging device. e CO 2 ooding experiment and CO 2 -water alternate ooding experiment are designed to evaluate the e ects of CO 2 EOR at Block M. In this experiment, the temperature is controlled at 104°C, and the displacement rate is 0.125 cm 3 /min. e experimental procedure can be brie y described as follows: (1) certain basic parameters of the short cores are measured; (2) the long core is assembled, vacuumed for 12 h, and saturated with synthetic brine water under 10 MPa for 24 h; (3) the irreducible water is established through oil ooding, the irreducible water saturation is calculated, and the sample rests for 24 h; (4) the sample is displaced by water until the water is reduced by 98%, CO 2 is then used for displacement, and the experiment is terminated when the oil production no longer increases; and (5) the other sample repeats the above process, and CO 2 -water alternate ooding is conducted after the water ooding. e volume ratio of water to gas is 1 : 1.

Relationship between the Relaxation Time T 2 and the Pore
roat Radius. Nuclear magnetic resonance is primarily used as a signal to detect the hydrogen nuclear atom, and it analyzes the distribution of uid in di erent pore throats. Using heavy water as the water phase, the signal is not generated when the relaxation time T 2 is measured. e T 2 spectrum is T 2 of the oil phase in the core. e T 2 spectrum before displacement re ects the occurrence of crude oil in the original state, and the T 2 spectrum after displacement re ects the remaining oil. e change in the T 2 spectrum can quantitatively analyze the degree of oil production, remaining oil occurrence, and displacement potential, and it can provide guidance for development of the potential evaluation and program adjustment. e distribution curves of the relation time T 2 can reveal the distribution of the pore throat, and a correlation exists. Due to the complexity and irregularity of the pore throat, its surface area and volume ratio are avoided. In the actual calculation, the surface relaxation rate of ρ 2 and the structure factor F S is used as the conversion factor. e expression of the relaxation time T 2 and the pore throat radius r can be written as follows [24]: 1 e relaxation time T 2 distribution represents a complex geometrical arrangement including small to large pore size domains [25,26]. e relaxation time T 2 is less than 10 ms, which is equivalent to the pore size of clay. From 10 ms to 100 ms, the pore is mesoporous. When T 2 is greater than 100 ms, the pore is macroporous [6,27]. e relationship between the percentage of volume distribution and the relaxation time T 2 is used to reveal the degree of oil production in di erent pore throats and describe the displacement mechanism and the remaining oil distribution.

Results and Discussion
In order to evaluate the e ect of CO 2 ooding and CO 2 -water alternate ooding, they are used to continue to displace the core after water ooding. e T 2 spectrum is T 2 of the oil phase in the core, it shows the amount of remaining oil in the pores of di erent sizes, and the T 2 spectrum frequency of the pores decreases when oil is extracted from the pores. If the frequency distribution of the T 2 spectrum is obtained after water ooding, CO 2 ooding, and CO 2 -water alternate ooding, the degree of recovery of oil in di erent pores can be analyzed under these ooding methods. Two sets of core displacement experiments are conducted, CO 2 ooding and CO 2 -water alternate ooding are used, respectively, after water ooding, and the frequency distribution of T 2 spectrum is monitored after these experiments.
When the rate of water content is increased to 98%, the injection of water in sample no. 1 is terminated. e recovery factor is 50.17%, and the sample is scanned using nuclear magnetic resonance. en, CO 2 ooding is performed, and the displacement is terminated without apparent oil recovery, resulting in a recovery rate of 58.91%, en, the core is rescanned, and the recovery rate of the CO 2 ooding increases by 8.74%. At the beginning of the water ooding process, the sample is purely oil-extracted, and the recovery of oil signi cantly increases and then decreases with an increase in the water content. During the CO 2 ooding, the volume of oil expands after dissolving CO 2 and part of immovable oil becomes movable, oil production increases, and the water content decreases signi cantly and then increases slowly (Figure 4). Because the oil in some channels is produced after ooding, the frequency of the T 2 spectrum decreases in these channels. e curve of the relation time T 2 shows that the oil in the large and medium pores has a high degree of recovery, and the crude oil in the small pores has not been used during the water ooding process. After the CO 2 ooding, the crude oil production in the large and medium pores is further improved, whereas the oil recovery in the small pores remained nearly unchanged ( Figure 5). is may be due to the relatively small ow resistance in large and medium pores, which forms a dominant percolation channel.
Water is injected to a water content of 98%, and the water ooding process is then stopped in sample no. 2, resulting in a recovery factor of 50.66%. en, the sample is scanned. Next, CO 2 -water alternate ooding is performed, and the ooding is terminated when there is no signi cant oil recovery, resulting in a recovery factor of 62.88%. Lastly, the core is scanned again. e CO 2 -water ooding enhances the recovery rate by 12.22%. In the displacement process, the recovery of oil is almost similar to that of sample no. 1, and the water content increases in a wave-like manner during the CO 2 -water alternating ooding process ( Figure 6). In the process of water ooding, the degree of oil recovery in the large-medium pores is high, the oil in the large pores is basically produced, and the oil in the small pores is not used. e large-medium pores form a dominant percolation channel during the water ooding, and capillary force makes it di cult to ow in the small pores. After the CO 2 -water alternating ooding occurs, the capillary force between two phases of gas and water is larger, displacement pressure increases at both ends of the pores, oil begins to ow in small pores, and the oil in the small pores significantly increases (Figure 7). In the process of CO 2 flooding and CO 2 -water alternating flooding, the difference in the displacement mechanism may cause a significant difference in the oil recovery rate in the small pores. e EOR of CO 2 flooding in this experiment occurs primarily due to these facts: CO 2 is dissolved in crude oil to cause expansion of crude oil, which increases the saturation of the oil phase in the pores, and part of the irreversible oil becomes movable oil. e interfacial tension between oil and CO 2 is small in nearmixed conditions, and gas can displace crude oil in the small pores. Additionally, extraction can improve oil recovery to a certain extent. CO 2 -water alternate flooding involves the EOR mechanism of CO 2 flooding. Furthermore, the interfacial tension between the gas phase and the water phase is large, thus resulting in a large capillary force and avoiding the formation of gas flow advantage channels, which can improve the oil output in the small pores and enhance oil recovery.

Conclusion
Based on the slim tube test, PVT experiment, and flooding experiment of the long core, the miscible pressure of Block M is determined, the feasibility of CO 2 injection to enhance oil recovery is evaluated, and nuclear magnetic resonance is combined to analyze the recovery of oil in different sizes of pore throats. A few conclusions can be drawn as follows: the miscible pressure of Block M is 32.6 MPa, and the CO 2 flooding approaches miscible flooding under the current formation condition. e oil recovery can reach 58.9% when using CO 2 flooding after water flooding, which is an improvement of 8.74%. e recovery using CO 2 -water alternate flooding after water flooding reaches 62.88%, which is an increase of 12.22%. is result demonstrates that the effects of CO 2 -water alternate flooding are superior. Medium and large pores have a high displacement efficiency, and the efficiency of the small pore displacement is extremely low during the water flooding process. CO 2 flooding primarily increases the displacement efficiency in macropores and mesopores; however, CO 2 -water alternate flooding can also increase the displacement efficiency of small pores. ese results are significant for improving the development effects of Block M.

Data Availability
e raw data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.