WO2017181444A1 - Method and device for measuring diffusion coefficient of carbon dioxide in crude oil - Google Patents

Abstract

A method and a device for measuring the diffusion coefficient of carbon dioxide in crude oil. The method comprises: step 1, filling a closed container of V_C with crude oil of V₀, where V₀<V_C; step 2, keeping the closed container at an constant experimental temperature; step 3, filling the closed container with carbon dioxide gas, measuring the pressure P(t) of the carbon dioxide gas once at every time interval t from the starting time t₀ until a balanced pressure is reached, the initial pressure of the carbon dioxide gas at the starting time t₀ being P₀(t); and step 4, obtaining the diffusion coefficient D_AB of carbon dioxide in crude oil according to the expansion coefficient β of crude oil with carbon dioxide dissolved therein and by using the relation between the diffusion time t and the pressure P(t). The device comprises: a closed container (15) for testing, a closed carbon dioxide container (14) provided upstream of the closed container (15) for testing, and a closed crude oil container (29) provided downstream of the closed container for testing.

Description

Method and device for determining diffusion coefficient of carbon dioxide in crude oil Technical field

The present invention relates to the field of oil and gas field development, and more particularly to a method and apparatus for determining the diffusion coefficient of carbon dioxide in crude oil.

Background technique

In the development of oil and gas fields, the use of carbon dioxide flooding can improve the recovery of crude oil, so carbon dioxide production technology has been widely used. The steps of using carbon dioxide to produce oil are as follows: first, a certain amount of carbon dioxide gas is injected into the oil layer, and then the well is closed to balance the diffusion of carbon dioxide in the crude oil, and finally the well is opened for oil recovery. In order to predict the time when carbon dioxide reaches equilibrium in crude oil, it is necessary to know the diffusion coefficient of carbon dioxide in crude oil. If the diffusion coefficient is not accurate, the equilibrium time cannot be accurately confirmed, which has an adverse effect on oil recovery.

In the prior art, methods for measuring the diffusion coefficient of carbon dioxide in a liquid can be classified into two types: direct method and indirect method. Among them, the direct method is: sampling the fluid at different times and different diffusion distances, and then analyzing the samples to obtain the concentration data of carbon dioxide, and then deducing the diffusion coefficient of carbon dioxide in the fluid. However, the sampling process can interfere with the flow field in the system, causing experimental errors. The indirect method does not require sampling, and the NMR (nuclear magnetic resonance) and PVT (pressure-volume-temperature) methods are the two most widely used methods. The NMR method can directly measure the concentration of carbon dioxide in a closed system by nuclear magnetic resonance spectroscopy, but it is expensive and costly. The PVT method is a relatively conventional method. Taking the pressure exhaustion method as an example, it measures the pressure change of the gas-liquid diffusion system, and then calculates the diffusion coefficient by the curve of the pressure versus time. However, the direct or indirect measurement cannot directly measure the diffusion coefficient of carbon dioxide gas. In addition, these measurement methods are unable to obtain accurate diffusion coefficients of carbon dioxide gas, thereby failing to accurately confirm the equilibrium time of carbon dioxide gas in crude oil, which adversely affects oil recovery.

Summary of the invention

In view of the above technical problems existing in the prior art, the present invention proposes a method for determining the diffusion coefficient of carbon dioxide in crude oil. By the method of the present invention, the diffusion coefficient of carbon dioxide in crude oil can be accurately determined to accurately predict the time when carbon dioxide reaches equilibrium in crude oil. The present invention also proposes a device for determining the diffusion coefficient of carbon dioxide in crude oil.

According to a first aspect of the invention, a method for determining a diffusion coefficient of carbon dioxide in crude oil is provided, comprising the steps of

Step 1: Filling a closed container of volume V _C with V ₀ volume of crude oil, where V ₀ <V _C ;

Step two: making the closed container after step one constant at the experimental temperature;

Step 3: Filling the sealed container after the second step with carbon dioxide gas, and measuring the pressure P(t) of the carbon dioxide gas every t time from the start time t ₀ until the equilibrium pressure is reached, wherein at the initial time t ₀ , The pressure of the carbon dioxide gas is P ₀ (t);

Step 4: According to the expansion coefficient β of the crude oil in which carbon dioxide is dissolved and using the relationship between the diffusion time t and the pressure P(t), the diffusion coefficient D _{AB of} carbon dioxide in the crude oil is obtained.

By the method of the present invention, the factor of the volume expansion of the crude oil after the carbon dioxide is diffused into the crude oil is considered, so that the diffusion coefficient of carbon dioxide in the crude oil can be accurately determined.

In one embodiment, in step four, according to

get

Thereby obtaining the diffusion coefficient D _{AB of} carbon dioxide in the crude oil,

Wherein, Z _g is a compression factor of carbon dioxide, and Z _{g is} approximately equal to 0.81774 in a pressure range of 5-7 MPa, and R is a general gas constant, that is, 8.314 cm ³ ·MPa·mol ^-1 ·K ^-1 , and T is an experimental temperature. x _eq is the equilibrium interface concentration of carbon dioxide at the oil and gas interface, determined by experimental conditions, and A is the cross-sectional area of the core, determined by experimental conditions.

In one embodiment, in step four, different amount of carbon dioxide dissolved in the crude liquid volume V _L measured at the bubble point pressure P, the via _{V L / V d = βP +} 1 to give crude expansion coefficient β, Where V _d is the volume of crude oil without carbon dioxide. Through the method, the volume expansion of the crude oil due to the dissolution of carbon dioxide can be accurately measured to obtain the precise diffusion coefficient D _{AB of} carbon dioxide in the crude oil.

In one embodiment, in step three, the time interval t of the pressure P(t) of the carbon dioxide gas is measured to be 1 minute. This uniform time interval helps to automatically control the measurement process through the software, facilitating the operator's operation.

In one embodiment, in step two, the test temperature is 97.53 °C. In the third step, the initial pressure P ₀ (t) of the carbon dioxide gas is 7 MPa. These parameters are consistent with the conditions of the actual oil recovery work, and the measured diffusion coefficient of carbon dioxide in crude oil can accurately reflect the actual situation.

In one embodiment, in step one, the pressure in the closed vessel is maintained at a vacuum of 0.1 MPa prior to charging the closed vessel with crude oil. This lower pressure minimizes the effect of residual gas in the closed vessel on the measurement results.

According to a second aspect of the invention, there is provided a device for determining the diffusion coefficient of carbon dioxide in crude oil, comprising a closed container for testing, upstream of a closed container for testing and a closed container for testing Carbon dioxide a closed container, a crude oil sealed container connected to the closed container for testing downstream of the closed container for testing, wherein the carbon dioxide sealed container and the closed container are disposed in the incubator to thermostat the sealed container for testing at the experimental temperature .

In one embodiment, a vacuum pump that evacuates the closed container for testing is also included.

In this application, the same parameters are used for the same parameters.

An advantage of the present invention over the prior art is that the method of the present invention takes into account the effect of volume expansion of the crude oil after diffusion of carbon dioxide into the crude oil, thereby enabling accurate determination of the diffusion coefficient of carbon dioxide in the crude oil, and thus in carbon dioxide Oil production can accurately predict the balance time, which is conducive to the oil production work. The method of the present invention uses a uniform time interval to measure pressure, which facilitates automatic control and facilitates operator operation.

DRAWINGS

The invention will be described in more detail hereinafter based on the embodiments and with reference to the accompanying drawings. among them:

Figure 1 is a schematic illustration of an experimental setup in accordance with the present invention;

Figure 2 is a schematic illustration of the steps in accordance with the present invention;

Figure 3 is a graph showing the relationship between the volume expansion ratio and the pressure of the carbon dioxide-dissolved crude oil obtained according to the present invention;

Figure 4 is a graph showing the relationship between the pressure correction term and the square root of the diffusion time in accordance with the present invention;

Figure 5 is a graph of the relationship between the pressure term and the square root of the diffusion time in accordance with the prior art.

The drawings are not drawn to scale.

detailed description

The invention will now be further described with reference to the accompanying drawings.

Figure 1 shows schematically an experimental setup 10 according to the invention. The experimental apparatus 10 includes a sealed container 15 for testing, a carbon dioxide sealed container 14 that communicates with the closed container 15 upstream of the sealed container 15, and a crude oil sealed container 29 that communicates with the closed container 15 downstream of the sealed container 15. Among them, the carbon dioxide sealed container 14 and the closed container 15 are provided in the incubator 16 so that the closed container 15 can be thermostated at the experimental temperature. In one embodiment, a temperature sensor 18 is provided on the oven 16 to ensure a constant test temperature.

A pressure sensor 17 is provided on the pipe 19 that connects the carbon dioxide container 14 and the hermetic container 15 to measure the pressure of carbon dioxide in the pipe 19. Since the conduit 19 is always in communication with the closed vessel 15, the pressure sensor 17 actually indicates the pressure of the carbon dioxide-crude oil system within the closed vessel 15. In one embodiment, in order to be able to evacuate the closed container 15, a vacuum pump 21 is also provided. As shown in Fig. 1, the vacuum pump 21 is connected to the pipe 19 through a pipe 25, a valve 26 is provided between the junction of the pipe 25 and the pipe 19, and the outlet of the carbon dioxide container 14, and a valve 27 is provided downstream of the hermetic container 15, so that After the valves 26 and 27 are closed, the vacuum pump 21 is actually only in communication with the hermetic container 15, so that the hermetic container 15 can be evacuated. In one embodiment, in order to reduce the influence of the residual gas in the closed container 15 on the measurement result, the pressure in the closed container 15 is maintained at a vacuum degree before the sealed container 15 is filled with the crude oil. 0.1 MPa. In addition, the experimental apparatus 10 further includes a carbon dioxide supply unit such as a pump 22, a crude oil supply unit such as a pump 23, and a data processing unit 24, which are well known to those skilled in the art and will not be further described herein for the sake of brevity.

Figure 2 shows the steps of the method according to the invention,

Step 31: filling the sealed container 15 with a predetermined volume of V ₀ crude oil;

Step 32: Constantly confining the closed container 15 to the experimental temperature;

Step 33: charging the sealed container 15 with carbon dioxide gas, and measuring the pressure P(t) of the carbon dioxide gas every t time from the start time t ₀ until the equilibrium pressure is reached;

Step 34: treating the measured pressure P(t) according to the expansion coefficient β of the crude oil in which the carbon dioxide is dissolved;

Step 35: Obtain a diffusion coefficient D _{AB of} carbon dioxide in the crude oil.

In the present application, the term "starting time" is defined as the time at which the carbon dioxide inflation ends.

In order to be able to accurately determine the expansion coefficient β, in one embodiment, the liquid phase volume V _L is measured at a bubble point pressure P in a crude oil in which different amounts of carbon dioxide are dissolved, and the expansion of the crude oil is obtained by V _L /V _d =βP+1 Coefficient β, where V _d is the volume of crude oil free of carbon dioxide.

The invention will be described in detail below with reference to FIG.

Example 1:

The volume V _c of the hermetic container 15 was 174 ml. After the experimental apparatus 10 was connected, it was washed with petroleum ether, dried with hot air, and then checked for airtightness of the entire apparatus to ensure airtightness. The valve 26 and the valve 27 were closed, and the vacuum pump 21 was started to evacuate the closed container 15 for 16 hours. After the end of the pumping, the valve 27 is opened while V ₀ = 100 ml of crude oil is supplied into the hermetic container 15 through the pump 23, and the valve 27 is closed. The crude oil had a density of 0.87 g/cm ³ and a viscosity of 20.1 mPa·s. The closed vessel 15 and the carbon dioxide sealed vessel 14 were thermostated to an experimental temperature of 97.53 °C. The valve 26 is opened, and the sealed container 15 is filled with carbon dioxide gas by the pump 22 until the pressure sensor 17 indicates 7 MPa, and the valve 26 is closed. The pressure P(t) indicated by the pressure sensor 17 is recorded every 1 minute, where t is the time point at which the pressure is measured until the equilibrium pressure is reached. The measurement results are shown in Table 1.

Table 1

Time/min	Pressure / MPa	Time/min	Pressure / MPa	Time/min	Pressure / MPa	Time/min	Pressure / MPa
0	7.00145	1	7.00145	2	6.98685	3	6.99048
4	6.98065	5	6.97187	6	6.97225	7	6.96566
8	6.9636	9	6.95921	10	6.95119	11	6.94254
12	6.94642	13	6.94229	14	6.93659	15	6.92756
16	6.93595	17	6.92097	18	6.91283	19	6.90494
20	6.9065	twenty one	6.90844	twenty two	6.89203	twenty three	6.88363

twenty four	6.87549	25	6.87937	26	6.8689	27	6.85831
28	6.84848	29	6.85016	30	6.84615	31	6.84615
32	6.83117	33	6.81645	34	6.80868	35	6.81282
36	6.81256	37	6.80856	38	6.7902	39	6.78788
40	6.77509	41	6.76889	42	6.76669	43	6.77314
44	6.77057	45	6.76889	46	6.76061	47	6.75403
48	6.74614	49	6.73386	50	6.71694	51	6.71293
52	6.70246	53	6.70208	54	6.70208	55	6.70027
56	6.7066	57	6.70014	58	6.70221	59	6.69807
60	6.70221	61	6.6982	62	6.69161	63	6.68773
64	6.68993	65	6.68579	66	6.68153	67	6.67469
68	6.6774	69	6.67545	70	6.67325	71	6.6646
72	6.65401	73	6.65363	74	6.65013	75	6.64561
76	6.63991	77	6.63759	78	6.62544	79	6.62118
80	6.61717	81	6.61459	82	6.60425	83	6.60413
84	6.604	85	6.59585	86	6.59159	87	6.5859
88	6.57918	89	6.57711	90	6.58151	91	6.58526
92	6.59185	93	6.58965	94	6.58745	95	6.58552
96	6.58061	97	6.57518	98	6.57066	99	6.56872
100	6.56458	101	6.56213	102	6.55981	103	6.55799
104	6.55605	105	6.55399	106	6.55192	107	6.54753
108	6.54546	109	6.54546	110	6.53744	111	6.53474
112	6.53254	113	6.53461	114	6.53009	115	6.52634
116	6.52014	117	6.52026	118	6.51419	119	6.51355
120	6.50798	121	6.50773	122	6.50334	123	6.50153
124	6.49958	125	6.49262	126	6.49068	127	6.4868
128	6.47852	129	6.48227	130	6.47219	131	6.46987
132	6.46573	133	6.46172	134	6.45966	135	6.45138
136	6.45138	137	6.44919	138	6.44505	139	6.4369
140	6.43458	141	6.43057	142	6.42851	143	6.42825
144	6.42192	145	6.41791	146	6.41597	147	6.41597
148	6.41184	149	6.40706	150	6.40757	151	6.40486
152	6.39905	153	6.39711	154	6.39478	155	6.39246
156	6.38832	157	6.3847	158	6.3763	159	6.38056
160	6.37681	161	6.36803	162	6.36402	163	6.36195
164	6.35743	165	6.35317	166	6.34528	167	6.34127
168	6.33455	169	6.333	170	6.32654	171	6.32046
172	6.31826	173	6.31413	174	6.30573	175	6.30573
176	6.29927	177	6.29784	178	6.29306	179	6.28272
180	6.28272	181	6.27639	182	6.27264	183	6.26567
184	6.26204	185	6.25817	186	6.25454	187	6.2477
188	6.24162	189	6.23942	190	6.2336	191	6.23335

192	6.22081	193	6.22107	194	6.21461	195	6.21215
196	6.20853	197	6.20181	198	6.1938	199	6.19599
200	6.18759	201	6.18527	202	6.18126	203	6.1748
204	6.17273	205	6.16665	206	6.16446	207	6.16007
208	6.15813	209	6.15036	210	6.14792	211	6.14158
212	6.1399	213	6.1293	214	6.12723	215	6.12529
216	6.12103	217	6.11495	218	6.11237	219	6.10837
220	6.10811	221	6.09802	222	6.09984	223	6.0935
224	6.0935	225	6.08704	226	6.0873	227	6.07916
228	6.07282	229	6.07295	230	6.06712	231	6.0621
232	6.06003	233	6.0586	234	6.05382	235	6.0515
236	6.04813	237	6.04374	238	6.03741	239	6.03352
240	6.02525	241	6.02913	242	6.02293	243	6.01672
244	6.01272	245	6.01078	246	6.01065	247	6.00431
248	5.99979	249	5.99578	250	5.99152	251	5.99165
252	5.9857	253	5.98325	254	5.97756	255	5.97768
256	5.96928	257	5.97148	258	5.96695	259	5.95868
260	5.95493	261	5.95041	262	5.95041	263	5.95222
264	5.94239	265	5.93994	266	5.93386	267	5.93606
268	5.92947	269	5.9296	270	5.92766	271	5.92133
272	5.91512	273	5.91305	274	5.91137	275	5.91073
276	5.90504	277	5.90297	278	5.90039	279	5.89819
280	5.8903	281	5.88772	282	5.88604	283	5.89017
284	5.87958	285	5.87996	286	5.87338	287	5.87338
288	5.86548	289	5.86924	290	5.86278	291	5.86122
292	5.85683	293	5.85501	294	5.85488	295	5.84855
296	5.84442	297	5.84247	298	5.84002	299	5.83614
300	5.83821	301	5.82955	302	5.82981	303	5.82529
304	5.82373	305	5.82154	306	5.81921	307	5.81702
308	5.81301	309	5.8068	310	5.80706	311	5.80486
312	5.79827	313	5.79814	314	5.79878	315	5.79608
316	5.79245	317	5.79452	318	5.79207	319	5.78379
320	5.78405	321	5.78198	322	5.77966	323	5.77126
324	5.77164	325	5.771	326	5.7688	327	5.76518
328	5.76324	329	5.76337	330	5.75691	331	5.75665
332	5.75458	333	5.75096	334	5.7507	335	5.7436
336	5.74669	337	5.73985	338	5.74192	339	5.7401
340	5.73377	341	5.73377	342	5.73208	343	5.72524
344	5.72705	345	5.7255	346	5.71929	347	5.72123
348	5.71929	349	5.71477	350	5.71528	351	5.71296
352	5.71309	353	5.70895	354	5.70468	355	5.7008
356	5.70016	357	5.70055	358	5.69797	359	5.69615

360	5.69008	361	5.69408	362	5.69421	363	5.68995
364	5.68594	365	5.68167	366	5.68142	367	5.67922
368	5.67715	369	5.6756	370	5.67327	371	5.67133
372	5.67314	373	5.66914	374	5.6672	375	5.66332
376	5.66525	377	5.66099	378	5.66061	379	5.65918
380	5.65647	381	5.65284	382	5.65401	383	5.65052
384	5.64871	385	5.64664	386	5.64871	387	5.64638
388	5.64419	389	5.64212	390	5.64031	391	5.63604
392	5.63591	393	5.63397	394	5.63591	395	5.62777
396	5.62971	397	5.62958	398	5.62557	399	5.62519
400	5.62544

The expansion coefficient β of the crude oil in which carbon dioxide is dissolved is measured. First, the volume V _L of the liquid phase is measured at the bubble point pressure of the crude oil in which different amounts of carbon dioxide are dissolved. Such measurement methods are well known to those skilled in the art and will not be described again for the sake of brevity. Then, the expansion coefficient β is obtained from V _L /V _d =βP+1, where V _d is the volume of the crude oil containing no carbon dioxide. As shown in Table 2, the measurement was carried out under the conditions of a molar ratio of carbon dioxide to crude oil of 0.412, 1.813, 3.057, 3.806, respectively.

Table 2

	Molar ratio 0.412	Molar ratio 1.813	Moore ratio 3.057	Moore ratio 3.806
Pressure P/MPa	4.169	11.790	18.507	24.995
V d /mL	97.929	96.657	95.852	95.201
V L /mL	102.283	105.199	112.961	116.243

According to V _L /V _d =βP+1, the pressure P is plotted on the abscissa and V _L /V _d is plotted on the ordinate in the plane direct coordinate system, as shown in Fig. 3. Fitting

It follows that the expansion coefficient β is equal to 0.0089.

The data in Table 1 was processed according to the expansion coefficient β measured above. Square root of diffusion time

For the abscissa, the pressure correction term

For the ordinate, plot in the plane Cartesian coordinate system, as shown in Figure 4. The fit yields y = 0.8457 x - 26.512, where y represents the pressure correction term and x represents the square root of the diffusion time, resulting in a slope k equal to 0.8457. according to

(hereinafter referred to as formula 1),

Therefore, it is concluded that the diffusion coefficient D _{AB of} carbon dioxide in crude oil is equal to 0.000772233 cm ² /s.

In the present embodiment, the parameters used were as follows: Z _g carbon dioxide compression factor. Although those skilled in the art with a known amount _g is a change in pressure change Z, the inventors found that in a pressure range of 5-8MPa the Z _g may be considered to be approximately constant, and the inventors used this art The method well known to the skilled person was measured under the experimental conditions, and the result was 0.81774; R was a general gas constant, 8.314 cm ³ · MPa·mol ^-1 · K ^-1 ; T was an experimental temperature of 97.53 ° C; x _eq equilibrium state The amount of carbon dioxide in a unit volume under the pressure of P(t) in the closed vessel 15 , wherein

A is the core cross-sectional area, which is 4.71 cm ² in this embodiment; t is the time point at which the pressure is measured; t ₀ is the time at which the test is started, and in the present embodiment, t ₀ =0.

Comparative example 1:

The experimental apparatus and experimental procedure were the same as in Example 1, except that in step 34, the data in Table 1 was processed using the methods of the prior art. In the prior art, the formula on which it is based is:

The meanings of P(t), P ₀ (t), t, t ₀ , Z _g , R, T, x _eq , and A are the same as those in Embodiment 1, and are not described herein again.

Square root of diffusion time

For the abscissa, the pressure difference P(t)-P ₀ (t) is plotted on the ordinate and plotted in a plane Cartesian coordinate system, as shown in Figure 5. The fit yields y = 0.0124x - 0.3833, where y represents the pressure term and x represents the square root of the diffusion time and the slope k' is equal to 0.0124. according to

It is concluded that the diffusion coefficient D' _{AB of} carbon dioxide in crude oil is equal to 0.001033 cm ² /s.

In the actual oil recovery process, the diffusion coefficient D _AB = 0.000772233 cm ² /s obtained by the method of the invention predicts that the time for the carbon dioxide to reach equilibrium in the crude oil is 450 days, and the method according to the prior art is used. The diffusion coefficient D' _AB = 0.001033 predicts that the time for carbon dioxide to reach equilibrium in crude oil is 352 days. However, it has been verified that the time for carbon dioxide to reach equilibrium in crude oil is 465 days, and it can be seen that the diffusion coefficient obtained by the method of the present invention is more accurate.

The process of obtaining Equation 1 used in the present invention will be described below:

According to Fick's second law

Where x: concentration of carbon dioxide in crude oil; D _AB : diffusion coefficient of carbon dioxide in crude oil; t: diffusion time, z: diffusion distance. At the beginning of the experiment, no carbon dioxide entered the crude oil, so the initial conditions were: ₀ ≤ z ≤ z ₀ , t = 0, x = 0

The boundary conditions are:

z=z ₀ , t>0, x=x _eq ,

z=0, all t,

Wherein, the amount of carbon dioxide per unit volume in the x _eq equilibrium state.

Solve Fick's second law and get

Only the diffusion distance of carbon dioxide in crude oil is considered here, so y is used instead of z.

According to the state equation PV=nRT of the ideal gas, the amount of the substance reduced by the gas phase as the time t elapses during gas diffusion is:

According to Fick's second law, the amount of material in the gas phase that enters the liquid phase through the gas-liquid interface is:

According to the well-known principle that the amount of the substance reduced in the gas phase is equal to the amount of the substance in the gas phase entering the liquid phase through the gas-liquid interface, it is concluded that:

Here V is the volume of the gas phase reduction. Because carbon dioxide diffuses into the crude oil, it causes expansion of the liquid phase volume, while the light components in the crude oil are also extracted by carbon dioxide. According to the formula V _L =V _d [1+βP(t)] of the expansion coefficient β, since the original volume of the crude oil is V ₀ , the formula according to the expansion coefficient β can be rewritten as:

V _L =V ₀ [1+βP(t)], (Equation 4)

Where V _L is the volume of the oil after absorption of carbon dioxide. When the container volume is V _C ,

V=V _C -V _L =V _C -V ₀ -βV ₀ P(t), (Equation 5)

Equation 2-5 is derived in parallel to obtain Equation 1 used in the present invention:

It should be noted that the expansion coefficient β can also be measured separately.

Although the present invention has been described with reference to the preferred embodiments thereof, various modifications may be made without departing from the scope of the invention. In particular, the technical features mentioned in the various embodiments can be combined in any manner as long as there is no structural conflict. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims (3)

1. A method for determining a diffusion coefficient of carbon dioxide in crude oil, comprising the following steps,

   Step 1: Filling a closed container of volume V _C with V ₀ volume of crude oil, where V ₀ <V _C ;

   Step two: making the closed container after step one constant at the experimental temperature;

   Step 3: Filling the sealed container after the second step with carbon dioxide gas, and measuring the pressure P(t) of the carbon dioxide gas every t time from the start time t ₀ until the equilibrium pressure is reached, wherein at the initial time t ₀ , The initial pressure of carbon dioxide gas is P ₀ (t);

   Step 4: According to the expansion coefficient β of the crude oil in which carbon dioxide is dissolved and using the relationship between the diffusion time t and the pressure P(t), the diffusion coefficient D _{AB of the} carbon dioxide in the crude oil is obtained, in the step 4, according to

   

   get

   

   Thereby obtaining the diffusion coefficient D _{AB of} carbon dioxide in the crude oil,

   Wherein, Z _g is a compression factor of carbon dioxide, and Z _{g is} approximately equal to 0.81774 in a pressure range of 5-7 MPa, and R is a general gas constant, that is, 8.314 cm ³ ·MPa·mol ^-1 ·K ^-1 , and T is an experimental temperature. x _eq is the equilibrium interface concentration of carbon dioxide at the oil and gas interface, determined by the experimental conditions, A is the core cross-sectional area, determined by the experimental conditions. In the fourth step, the crude oil with different amounts of carbon dioxide is determined at the bubble point pressure P. The liquid phase volume V _L , and the expansion coefficient β of the crude oil is obtained by V _L /V _d =βP+1, wherein V _d is the volume of the crude oil without carbon dioxide, and in the third step, the pressure P of the carbon dioxide gas is measured. The time interval t of (t) is 1 minute, and in the second step, the experimental temperature is 97.53 ° C. In the third step, the initial pressure P ₀ (t) of the carbon dioxide gas is 7 MPa, In the first step, the pressure in the sealed container is maintained at a degree of vacuum of 0.1 MPa before the crude oil is filled into the sealed container.
2. A device for determining the diffusion coefficient of carbon dioxide in crude oil using the method of claim 1, comprising a closed container for testing, communicating with said closed container for testing upstream of said closed container for testing a carbon dioxide sealed container, a crude oil sealed container in communication with the closed container for testing downstream of the closed container for testing, wherein the carbon dioxide sealed container and the closed container are disposed in an incubator to be used for The sealed container tested was thermostated at the experimental temperature.
3. The apparatus of claim 2 further comprising a vacuum pump for evacuating said sealed container for testing.

Images