The Seepage Mechanism of Micro Remaining Oil During Cyclic Waterflooding in Low-permeability Reservoir

To explore the flow mechanism of micro-remaining oil during cyclic waterflooding in low-permeability reservoir, the displacement effect of micro-remaining oil under conventional waterflooding and different rounds of cyclic waterflooding was investigated by using a micro-visual displacement experiment based on microfluidics model. The produced ratio of different types of micro remaining oil during cyclic waterflooding was quantified, and major types of micro remaining oils that can be applied effectively during cyclic waterflooding were analyzed. Moreover, the flow mechanism of micro remaining oil during cyclic waterflooding was investigated. Research results demonstrated that clustering type remaining oil and columnar type remaining oil were major types that were applied during cyclic waterflooding. After three rounds of cyclic waterflooding, clustering-type remaining oil decreased by 2.50% on average, and columnar-type remaining oil decreased by 0.81% on average. The major mechanism of cyclic waterflooding is introduced as follows: at intervals of cyclic waterflooding, clustering type remaining oil and columnar type remaining oil exchange with water in pores, throats, and spaces between big and small pores due to the fluctuation of pressure field and effect of capillary force. As a result, oil and water redistribute. Clustering-type remaining oil and columnar-type remaining oil become movable oils.


Introduction
Fuyang Reservoir is the major exploitation layer in the X oilfield, where the proven geological reserves account for about 88% of total reserves.Fuyang Reservoir has dense strata, a small radius at the throat [1] , great percolating resistance, and a large start-up pressure gradient [2][3] .Oil wells mainly apply fracturing production, which is characteristic of low productivity, quick decreasing, and fast rising of aquifers after encountering water.Hence, it is very difficult to implement the waterflooding development.According to practices in mines in China and foreign countries, cyclic waterflooding is one of the effective means to improve the exploitation effect of low-permeability oilfields [4], and it is advantages for a small investment, quick effects and simple operation [5][6] .It can relieve the rising rate of the aquifer in the low-permeability reservoir and increase the final waterflooding recovery efficiency to some extent [7] .At present, there are many studies on reasonable working systems of cyclic waterflooding [8][9] , cyclic waterflooding parameter optimization [10] , nonlinear percolation of cyclic waterflooding [11] , and macroscopic yield-increasing mechanism of cyclic waterflooding [12] in China and abroad.However, there's no clear understanding of the flow mechanism of micro remaining oil during cyclic waterflooding as well as relevant remaining oil types.An explicit flow mechanism of micro remaining oil during cyclic waterflooding in low-permeability reservoirs can provide theoretical support to the implementation of the field scheme and the optimal selection of the reasonable working system.For this reason, this study investigated the flow of micro remaining oil during cyclic waterflooding and produced ratios of different oil types and flow mechanisms.The flow mechanism of micro remaining oil during cyclic waterflooding was investigated, and the applied oil types were determined.

Experimental materials
The simulated oil was prepared by mixing the crude oil and kerosene in Daqing Oil Field.The viscosity was 4.0 mPa•s (under 45℃).The microfluidics model used in this experiment was formed by processing of natural core pore structure in a low-permeability reservoir in Daqing Oil Field, and permeability was about 5×10 -3 μm 2 , 15×10 -3 μm 2 , and 25×10 -3 μm 2 , respectively.The degrees of mineralization of injected water and formation water were 485 mg/L and 4767 mg/L, respectively.

Experimental equipment
The experiment mainly used the ISCO pump, Welch1402 vacuum pump, and ZJ-WG micro-visual displacement analysis system.

Experimental steps
Experimental steps under 45℃ were introduced as follows: (a) After oil saturation in the microfluidics model, water was injected into the model at the rate of 0.2 mL/h to displace the crude oil to the drainage end until the water cut reached 100%; (b) After waterflooding was stopped for 3h, water was injected into the model at the rate of 0.2 mL/h again to displace crude oil; (c) Step (b) was repeated and changes of remaining oil after 3 rounds of cyclic waterflooding were observed.The area and quantity of different types of remaining oil at the end of cyclic waterflooding were calculated.

The flow of micro remaining oil during cyclic waterflooding
To study the exploitation effect of low-permeability reservoirs during cyclic waterflooding, microvisual displacement experiments under different rounds of cyclic waterflooding were carried out.Experimental results are shown in Figure 1.The displacement efficiencies of conventional waterflooding and different rounds of cyclic waterflooding are shown in Figure 2. Clearly, since percolating resistance at large pore throats is small during conventional waterflooding, the injected water flows along the large pore throat, forming a fingering phenomenon.When the water cut at the drainage end is 100%, there is still abundant remaining oil in regions beyond the reach, which cannot be applied by conventional waterflooding.After cyclic waterflooding, the pressure field and oil-water saturation field in the model redistribute at intervals of cyclic waterflooding.After the second waterflooding, on the one hand, the injected water entered into some small pore throats which haven't been affected yet.On the other hand, crude oil in some regions beyond the reach entered into the waterflooding channel, thus decreasing the micro remaining oil after cyclic waterflooding.Moreover, the displacement efficiency increases gradually, while the remaining oil content decreases gradually with the increase of waterflooding rounds.In view of different permeability, the recovery rate of conventional waterflooding increases with the increase of permeability, while the recovery ratio growths of cyclic waterflooding compared to that of conventional waterflooding decrease.When the permeability is 5×10 -3 μm 2 , 15×10 -3 μm 2, and 25×10 -3 μm 2 , the recovery ratio of cyclic waterflooding is increased by 7.50%, 5.83%, and 5.67% than that of conventional waterflooding.For the microfluidics model with the same permeability, recovery ratio growth decreases with the increase of waterflooding rounds.When the permeability of the microfluidics model is 15×10 -3 μm 2 , the recovery ratio of the first-round increases by 4.04% compared to that of conventional waterflooding.The recovery ratio of the second-round increases by 1.18% than that of the first round.The recovery ratio of the third-round increases by 0.61% than that of the second round.

Flow of different remaining oil types during cyclic waterflooding
The distribution area of remaining oil was calculated by recognizing the distribution morphology, thus getting distribution ratios of different types of micro remaining oil after different rounds of cyclic waterflooding.On this basis, the flow of different types of micro remaining oil during cyclic waterflooding was analyzed, and major oil types that can be applied during cyclic waterflooding were recognized.Experimental results of the saturation degree of different remaining oil types in the microfluidics model with different permeability after conventional waterflooding and different rounds of cyclic waterflooding are shown in Figure 3.In view of different types of remaining oil, saturations of clustering type remaining oil and columnar type remaining oil decreased mostly after cyclic waterflooding.When the permeability of the microfluidics model was 5×10 -3 μm 2 , clustering type remaining oil decreased by 2.44%, and columnar type remaining oil decreased by 1.07% after three rounds of cyclic waterflooding.The amplitudes of variations of oil droplet type remaining oil, oil film type remaining oil, and the remaining oil in disconnected pores were relatively low.These three types of remaining oil decreased by 0.58%, 0.23%, and 0.32%, respectively.When the permeability of the microfluidics model was 15×10 -3 μm 2 , clustering type remaining oil decreased by 2.22%, and columnar type remaining oil decreased by 0.91% after three rounds of cyclic waterflooding.When the permeability of the microfluidics model was 25×10 -3 μm 2 , clustering type remaining oil decreased by 2.83%, and columnar type remaining oil decreased by 0.45% after three rounds of cyclic waterflooding.This proved that in low-permeability microfluidics models, cyclic waterflooding mainly applies clustering type remaining oil and columnar type remaining oil.Still, oil droplet type remaining oil, oil film type remaining oil, and the remaining oil in disconnected pores were used slightly.

Flow mechanism of micro remaining oil during cyclic waterflooding
According to experimental displacement results of microfluidics models and morphological changes of different remaining oil types after cyclic waterflooding, the flow mechanism was analyzed.It was summarized into the following two major aspects: Firstly, the clustering type remaining oil enters into the mainstream throat channel due to fluctuation of the pressure field during cyclic waterflooding.In the late stage of conventional waterflooding, the injected water flew along the large pore throats, which absorbed abundant water and had high pressure.By contrast, water absorption of small pore throats was low or zero, with low pressure.Small pore throats with zero or low water absorption formed clustering type remaining oil.At intervals of cyclic waterflooding, large pore throats with high pressure produced abundant liquids, and pressure declined quickly.However, pressure in the small pore throats with a lot of remaining oil decreased slowly, and there is high pressure.Since there is a pressure difference between small and large pore throats, it forced oil-water exchanges between large and small pore throats (Figures 4 and 5 Secondly, columnar-type remaining oil entered into the mainstream throat channel again due to capillary force.During conventional waterflooding, columnar-type remaining oil was kept static due to the collaborative effect of waterflooding pressure gap (∆P =P 1 -P 2 ) and the large capillary force (∆P c = P c1 -P c2 ).Under this circumstance, ∆P=∆P c .Results are shown in Figures 6 and 7.At intervals of cyclic waterflooding, P 1 =0.At this moment, columnar-type remaining oil entered into the mainstream throat under the effect of capillary force and became movable oil.Influenced by follow-up waterflooding, columnar-type remaining oil was displaced gradually.
(2) Clustering type remaining oil and columnar type remaining oil are two major oil types that are applied during cyclic waterflooding.In contrast, the contents of oil droplet type, oil film type remaining oil, and remaining oil in disconnected pores change slightly.After three rounds of waterflooding, clustering-type remaining oil decreases by 2.44%, and columnar-type remaining oil decreases by 1.07%.
(3) The flow mechanism of micro remaining oil during cyclic waterflooding is introduced as follows: at intervals of cyclic waterflooding, clustering type remaining oil and columnar type remaining oil exchange with water in pores, throats and spaces between big and small pores due to the fluctuation of pressure field and effect of capillary force.As a result, oil and water redistribute.Clustering-type remaining oil and columnar-type remaining oil become movable oils.
). Clustering type remaining oil in small pore throats flows into large pore throats.During secondary waterflooding, the injected water displaced clustering type remaining oil partially along large pore throats.a. Conventional waterflooding b.Cyclic waterflooding Figure 4. Flow mechanism of clustering type remaining oil during cyclic waterflooding.a. Conventional waterflooding b.Cyclic waterflooding Figure 5.The flow of clustering type remaining oil during cyclic waterflooding.
a. Conventional waterflooding b.Cyclic waterflooding Figure6.Flow mechanism of columnar type remaining oil during cyclic waterflooding.
waterflooding b.Cyclic waterflooding Figure 7.The flow of columnar type remaining oil during cyclic waterflooding.