The oil samples are the degassed crude oil from the M and H super-heavy oil reservoir of Venezuela. The super-heavy oil samples with specific gas-oil ratio were compounded by the high-pressure and high-temperature sampler under the initial conditions of two reservoirs. The reservoirs temperature, pressure and dissolved gas-oil ratio used in this experiment are shown in Table 1.

The artificial long-cores of two reservoirs were prepared according to the reservoir properties. The core parameters are shown in Table 2.

A novel visualization microscopic flow experimental apparatus with long-core displacement for foamy oil flow was established, which is shown in Figure 1. The experimental apparatus consists of five parts: fluid injection system, super-heavy oil reservoir simulation system, microscopic visual monitoring system, oil-gas separating and measuring system, and data acquisition and control system. The novelty of this experimental apparatus is the connection between the long-core holder and the microscopic visualization monitoring device, which can indirectly observe the flow characteristic of foamy oil in the cores.

(1) Fluid injection system: The keys of the fluid injection system are the pump (FY-HSHY-80), two intermediate containers, and a heating sleeve. Oil samples and formation water are both prepared in the intermediate containers. Two intermediate containers are connected to the pumps to saturate cores with formation water or oil.

The objective of the experiments is to investigate the reasons for the differences of production performance in foamy oil flow stage between M and H reservoirs. The experimental procedures are as follows:

(1) Experimental preparations: The oil samples, formation water and the long cores are prepared before the experiment. The experimental apparatus is installed as described in Section 2.2, and check for air leak.

The experimental results show that the bubble point pressure is 5.10 MPa and the pseudo bubble point pressure is 2.12 MPa in M reservoir, while the bubble point pressure is 3.54 MPa and the pseudo bubble point pressure is 0.41 MPa in H reservoir. The production performance of M reservoir is shown in Figures 2–4.

Three production stages of DCP are divided: stage I (the pressure greater than bubble point pressure), stage II (the pressure ranging from pseudo bubble point pressure to bubble point pressure), and stage III (the pressure lower than pseudo bubble point pressure). The production performances in each production stage are quite different. Furthermore, the production performances between two reservoirs are also different. The analyses of production performance in M reservoir are as follows:

Stage I: During this stage, the oil rate is very low, and the cumulative gas production and GOR are 0. The pressure at each pressure tap decreased linearly with time and at the same pace due to the high permeability.

Stage II: During this stage, the oil rate, cumulative oil production all increased quickly, but the instantaneous gas oil ratio remained low, ranging from 11.36 to 20.16 m³/m³. The pressures at each point tap decreased linearly with time when the pressure is lower than 5.10 MPa, and started to deviate from the pressure of the back-pressure valve. The reason is that the oil viscosity lowered and the oil volume expanded due to the foamy oil flow, which effectively maintain the reservoir pressure. In addition, the continuous gas phase has not been formed so that the instantaneous gas-oil ratio is retain low.

Stage III: During this stage, the oil rate decreased rapidly while the gas rate increased rapidly, due to the formation of continuous gas phase. Furthermore, the pressure fluctuation near the inlet is small while which near the outlet is large, indicating foamy oil flow at the inlet and continuous gas flow at the outlet.

The production performance of M reservoir is shown in Figures 5–7, which are as follows:

Stage I: During this stage, the production performance between M and H reservoirs is the same. The oil rate is very low, and cumulative gas production and GOR are 0.

Stage II: This stage can be further divided into two distinct periods. At the early period of stage II (pressure ranging from 3.54 MPa to 2.20 MPa), the instantaneous gas-oil ratio was very low, the oil production rate and accumulative oil production increased slowly. The deviation of pressures at each point tap from the pressure of back-pressure valves is small. At the later period of stage II (pressure ranging from 2.20 MPa to 0.41 MPa), the oil rate and cumulative oil production increased rapidly. The instantaneous GOR is slightly higher than dissolved GOR, ranging from 8.25 to 14.84 m³/m³. The deviation of pressures at each point tap from the pressure of back-pressure valves gradually increased.

Stage III: During this stage, the increase of oil rate, cumulative oil production was reduced, but the oil production capacity can still be maintained. The instantaneous GOR increased sharply in a brief period before decreasing sharply. Due to the formation of the continuous gas phase, the pressure near the inlet remained high, and the differential pressure gradually increased.

After comparative analysis, it can be found that when the pressure is higher than the bubble point pressure (Stage I) or lower than the pseudo bubble point pressure (Stage III), the production performance of M reservoir is the same as that of H reservoir. However, when the pressure decreased from the bubble point pressure to the pseudo bubble point pressure (Stage II), there is an obvious difference in production performance between M and H reservoir during DCP.

The images of foamy oil flow in M reservoir were shown in Figure 8, and the morphology descriptions were shown in Table 3. The flow characteristics of foamy oil in M reservoir during each stage of DCP are as follows:

Stage I: During this stage, there is primarily single oil flow, no dissolved gas separation, and the bubble quantities are 0.

Stage II: During this stage, the oil phase contains a lot of widely dispersed bubbles due to the dissolved gas separating from the oil phase. The foam shape is basically regular round, the foam size is small. As the pressure drops from 5 MPa to 2.25 MPa, the bubble quantity gradually increased from 6 to 789, and the bubble density gradually increased from 1.33/cm² to 174.85/cm², which demonstrate a strong foamy oil flow in M reservoir. In addition, the maximum diameter and average diameter of bubbles generally increase, indicating that the small bubbles gradually grow and become larger with the decrease of pressure at this stage.

Stage III: During this stage, the bubble quantity and bubble density respectively decreased from 456 to 101.05/cm² to 103 and 22.16/cm² when the pressure dropped from 2.02 MPa to 0.10 MPa, while the maximum bubble diameter and mean bubble diameter increased from 1.41 mm to 0.30 mm to 6.25 m and 0.48 mm, respectively. In, addition, the bubble shape also changed from the regular round to the irregular round, and finally to the long strip. These phenomena demonstrate that small bubbles progressively converge into larger ones, and form the continuous gas phase. Finally, the foamy oil flow changes into oil-gas two-phase flow.

The images of foamy oil flow in H reservoir were shown in Figure 9, and the morphology descriptions were shown in Table 4. The flow characteristics of foamy oil in H reservoir during each stage of DCP are as follows:

Stage I: During this stage, the microscopic flow characteristic of H reservoir is the same as which of M reservoir. It is the single oil flow, and the bubble quantity is 0.

Stage II: During this stage, the microscopic flow characteristic of H reservoir is quite different from which of M reservoir. The bubble quantity and the bubble density are 9 and 1.99/cm² at the pressure of 3.39 MPa, respectively. It is difficult to form foamy oil flow due to the small bubble quantity and large bubble diameter. As the pressure drops from 1.71 MPa to 0.58 MPa, the bubble quantity and bubble density increased from 24 to 5.32/cm² to 397 and 87.98/cm² respectively, which indicating the formation of foamy oil. However, the bubble quantity and bubble density of H reservoir are lower than those of M reservoir. All these phenomena demonstrate a weak foamy oil flow in H reservoir.

Stage III: During this stage, the bubble quantity and bubble density respectively decreased from 397 to 87.98/cm² to 86 and 19.06/cm² when the pressure dropped from 0.58 MPa to 0.18 MPa, while the maximum bubble diameter and the mean bubble diameter increased from 1.04 mm to 0.30 mm–1.82 mm and 0.40 mm, respectively. In, addition, the bubble shape also changed from the regular round to the irregular round, and finally to the long strip. Therefore, the flow characteristics of H reservoir at this stage are similar to those of M reservoir.

The experimental results show that the oil recovery, cumulative oil production, cumulative gas production and maximum oil rate of M reservoir are respectively 19.81%, 30.95 g, 1498 cm³ and 0.34 g/min, which are 9.38%, 15.29 g, 1,168.31 cm³ and 0.10 g/min higher than those of H reservoir during DCP. The comparison results of production performance between M and H reservoir are shown in Table 5.

The difference in production performance between the two reservoirs occurs in stage II, that is, the difference in foamy oil flow characteristics between the two reservoirs. The flow characteristic of M reservoir in stage II is a strong foamy oil flow, while that of H reservoir is a weak foamy oil flow. The main causes are as follows:

(1) The initial dissolved GOR of M reservoir is 15.86 m³/m³, higher than that of H reservoir (8.6 m³/m³). The formation of bubbles is affected by the initial GOR. More gas will disperse in super-heavy oil during DCP due to larger dissolved GOR of M reservoir, which could form the foamy oil flow. In addition, he higher the original dissolved gas-oil ratio, the increase of critical gas saturation, which is conducive to the reduction of gas phase fluidity.