Polymeric microsphere injection in large pore-size porous media

Dongqing Co,Ming Hn,,*,Jinxun Wng,Amr J.Alshehri

a Beijing Research Center,Aramco Asia,Beijing,100102,China

b EXPEC Advanced Research Center,Saudi Aramco,Dhahran,31311,Saudi Arabia

ABSTRACT High water-cut has become a worldwide challenge for oil production.It requires extensive efforts to process and dispose.This entails expanding water handling facilities and incurring high power consumption costs.Polymeric microsphere injection is a cost-effective way to deal with excessive water production from subterranean formations.This study reports a laboratory investigation on polymeric microsphere injection in a large volume to identify its in-depth fluid diversion capacity in a porous media with large pore/particle size ratio.The performance of polymeric microsphere injection was evaluated using etched glass micromodels based on the pore network of a natural carbonate rock,which were treated as water-wet or oil-wet micromodels.Waterflooding was conducted to displace oil at reservoir temperature of 95 °C,followed by one pore volume of polymeric microsphere injection.Three polymeric microsphere samples with median particle size of 0.05,0.3,and 20 μm were used to investigate the impact of particle size of the polymeric microspheres on incremental oil production capacity.Although the polymeric microspheres were much smaller than the pores,additional oil production was observed.The incremental oil production increased with increasing polymeric microsphere concentration and particle size.As a comparison,polymeric microsphere solutions were injected into oil-wet and water-wet micromodels after waterflooding.It was observed that the oil production in oil-wet micromodel was much higher than that in water-wet micromodel.The wettability of micromodels affected the distribution patterns of the remaining oil after waterflooding and further dominated the performance of the microsphere injection.The study supports the applicability of microsphere injection in oil-wet heterogeneous carbonates.

Keywords:High water-cut Polymeric microsphere injection Etched glass micromodel Pore/particle size ratio Wettability Heterogeneous carbonate reservoir Conformance control Oil production

1.Introduction

High water-cut has become a worldwide challenge for oil production.Large amount of associated water production result in not only reduced oil production,but also increased operational cost due to disposal facility expansion and power consumption [1].Both polymer flooding and conformance control technologies have been used in the fields to increase oil production and reduce water cut.Polymer flooding has been successfully applied in the field for heterogeneous reservoir matrix [2,3].Different from polymer flooding,conformance control technologies are used in the reservoirs with dominant flow networks composed of high-permeability zones and/or fractures [4,5].The materials used in conformance control technologies are usually solid or semi-solid materials like solid particles,resins,in-situ bulk gels,preformed gels [1].

The bulk gel usually occupies the pore spaces with controllable gelation time,adjustable strength and good injectivity [6,7].We have conducted systematic studies on bulk gels for potential applications in high temperature and high salinity carbonate oil reservoirs [8-14].Han et al.[15]also presented an extensive review on in-depth gel-based fluid diversion technologies,including weak gels,sequential injection for in-situ gels,colloidal dispersion gels,microgels and preformed particle gels.

Unlike bulk gels,the pre-formed particle gels are synthesized before injection into the reservoirs [16-24].Among the pre-formed particle gels for waterflooding conformance control,polymeric microspheres(or micron-size crosslinked polymer particles)have been attracted great attention owing to the unique properties for in-depth fluid diversion.Synthesized by inverse emulation polymerization method [18],polymeric microsphere presents uniform spherical shape.With the crosslinking structure,the thermal and mechanical stability are better than the corresponding linear polymer [25,26].The main principle of polymeric microsphere injection is to sterically block the flow channel by adsorption or strain and divert the flow to the uninvaded areas.Associate with strong physical interactions,the polymeric microsphere injection is a feasibly applied technology under various reservoir conditions [27-31].

The injectivity of particle gels into reservoir matrix is a critical issue in the applications.Bai et al.[16]investigated pre-formed particle gel(PPG)transport mechanisms through porous media by etched-glass micromodels and concluded that the largest diameter ratio of a PPG particle and a pore throat that the PPG particle could pass through depended on the swollen PPG strength and driving pressure gradient.Almohsin et al.[32]reported that the particle could not penetrate the rock below certain permeability and the residual resistance factor of the high-permeability rock (more than 1 μm2)was relatively small.Yao et al.[33]showed that micron sized microsphere entered into core plug high permeability layer 3.6 μm2and did not clog low-permeability layer of 0.5 μm2.Lenchenkov et al.[34]studied the microsphere propagation in outcrops by coreflooding.They presented that the nano-sized sphere propagation in porous media was highly dependent on the brine salinity and flow rate.Tian et al.[35]and Sun et al.[36]reported a pore size and particle size matching 1/2 and 1/3 bridge law in the polymeric microsphere injection in field applications.Meanwhile,field applications were usually conducted with small amount of pre-formed gels that could effectively reduce water productions and increase oil productions[16,21,27].

Unlike the small slug injection with the matching between particle size and pore size,this work presents a study to use large quantity of small-sized polymeric microspheres that ensure both adequate injectivity into the formations and effective in-depth fluid diversion.We studied the behaviors of polymeric microsphere injection in the micromodels with very large pore size to particle size ratio,which allows direct visualization of the fluid flow in the pore scale,as well as the dynamic variations in oil/water saturation.The studies also emphasized on the microsphere size,concentration,and micromodel wettability to investigate the feasibility of polymeric microsphere injection.

2.Experimental

2.1.Crude oil and brine

The crude oil was a degassed crude oil with a viscosity of 5 mPa·s at 90 °C.Table 1 shows the ion compositions of the synthetic brines used in this study.The total dissolved solids (TDS)of injection water and connate water are 57,670 mg/L and 213,734 mg/L,respectively.The synthetic connate water was used to saturate the micromodel,and the synthetic injection water was used to prepare the chemical solutions.

2.2.Polymeric microspheres

The polymeric microsphere samples were in the form of water/oil emulsion.The microspheres were in the water phase.Three polymeric microsphere samples were used in this work: MSP-A,MSP-B and MSP-C.The median particle sizes of MSP-A,MSP-B and MSP-C in injection water were 0.05,0.3,and 20 μm,respectively.

2.3.Polymer

Fig.1.Micromodel with one injector and one producer at the two opposite corners.

The polymer used in these experiments is a sulfonated polyacrylamide with a molecular weight of 14 million g/mol and hydrolysis degree of 4.2%.

2.4.Micromodel displacement system

Using micromodels for fluid-flow experiments has been recognized as an effective method for studying effects of various parameters on oil displacement [37,38].The pore network in the micromodel was extracted from a thin section of a natural carbonate rock.The network pattern was duplicated to form a 4 cm × 4 cm pore structure,as shown on Fig.1.The model had one injector and one producer at two opposite corners,simulating a quarter of an inverse 5-spot pattern.The glass micromodel is naturally water-wet.To modify the wettability to oil wet,the micromodel was treated by silicon oil.The porosity of the micromodel is 39.44%.Pore diameter ranges from 120 to 480 μm.The micromodel was cleaned after each test using dichromic acid to guarantee experiment repeatability.

The micromodel was cleaned after each test using dichromic acid to guarantee experiment repeatability.The acid was injected into the micromodel manually by a syringe.After half hour,the acid was displaced out by plenty of distilled water.The above steps was repeated for three times in order to fully clean the micromodel for future use.

High Temperature and High Pressure (HTHP)Micromodel Displacement System was used in the micromodel tests.Fig.2 shows the schematic of the micromodel displacement system set-up.

2.5.Micromodel displacement procedures

The procedures of a micromodel displacement test is as following.(1)Saturate micromodel with connate water by vacuum.

Table 1 Composition of injection water and connate water.

(2)Load the micromodel into the HTHP holder.

(3)Set the confining pressure as 0.3 MPa.Heat the holder by a heating jacket to 95 °C.

(4)Inject crude oil into the model to setup oil saturation at 2 cc/min.

(5)Clean the injection line with injection water through the bypass line at flow rate 0.5 cc/min.

(6)Inject the injection water and chemicals into the micromodel to displace the oil by the syringe pump at flow rate 0.0025 cc/min and collect the real time image.The injection volume was controlled by the injection time at the fixed injection rate.Before each slug,prefill the injection line with the injected solution through the bypass line at flow rate 0.5 cc/min to minimize the dead volume.

3.Results and discussions

3.1.Polymeric microsphere injection in etched glass micromodels

Fig.3.Oil production vs injection volume in the micromodel displacement test.

A series of micromodel displacement tests were conducted using various concentrations of polymeric microspheres suspended in 2000 mg/L polymer.Fig.3 shows oil production versus injection volumes in an oil-wet micromodel.Water was injected into the micromodel until a plateau oil production was achieved.The test was then switched to microsphere injection at the same flow rate.After 1 pore volume (PV)of polymeric microsphere injection at a microsphere concentration of 5 vol% of active MSP-B.The real-time image of the displacement process was captured by a camera and oil production was calculated by image segregation analysis.A significant oil production of 21% was obtained by the polymeric microsphere injection.

The matching diameter ratio of particle and pore has been studied extensively.It was generally concluded that the particles flow through the porous media when pore size is 10 times larger than the particles[16].In our experiment,the pore size was much larger than the particle size by 2-3 orders of magnitude (pore/particle size ratio is ranged from 400 to 1600).Additional oil production increased was stilled observed in the micromodel tests.This is attributed to the aggregation of the particles during the microsphere propagation.This phenomenon can be described by “l(fā)og-jamming” that causes mobilizing capillary-trapped oil by a slow blocking of pore channels [39].The blocking is realized by the mass difference between the particle and the solvent.Water molecules accelerate the heavier particles at the throat with abrupt velocity change,leaving the particles accumulating and blocking the pores.In addition,partially attractive particles could form multi-layer adsorption on pore walls,and eventually completely plug pores to achieve effective blocking [40].The blocking made a redistribution of the pressures and diverted the flows at macroscopic and microscopic levels,leading to better fluid injection performance.

3.2.Microsphere injection in oil-wet and water-wet micromodels

3.2.1.Oil production in oil-wet and water-wet micromodels

Fig.4.Oil production of 1 PV MSP-B injection at oil-wet and water-wet conditions.

The wettability of carbonate and sandstone reservoirs are different owing to the natures of the rock lithology.To identify the performance of microsphere injection in different kinds of reservoirs,micromodels with different wettability were used.Fig.4 compares the results of increased oil production by 1 PV of MSP-B injection in oil-wet and water-wet micromodels.For the oil-wet condition,the microsphere injection improved oil production in the whole microsphere concentration range,and the incremental oil production increased with increasing microsphere concentration.A plateau was reached in oil production with a polymeric microsphere concentration higher than 2%.It is worth noting that for the water-wet conditions,the polymeric microsphere did not increase oil production until its concentration reached above 2%.It appears a critical polymeric microsphere concentration,above which the remaining oil after water flooding could be produced.The result that microsphere injection performed better in the oil-wet condition than in the water-wet condition was confirmed by coreflooding tests [41].

3.2.2.Flow in oil-wet and water-wet micromodels

Displacement process can be captured from the images collected during the flow in the micromodel.In the micromodel displacement tests at oil-wet conditions,waterflooding swept the diagonal area between the injector and producer,leaving other two corners upswept.The polymeric microsphere injection swept the oil in the two corners,resulting in higher oil production.The reason is that the polymeric microsphere injection primarily invaded the water swept zones with lower flow resistance and then diverted subsequent fluid into adjacent upswept zones.This resulted in increasing macroscopic sweep effi-ciency and improving oil production.In the water swept areas,there was some remaining oil trapped by capillary forces.The polymeric microsphere injection recovered some of these remaining oil blobs.Fig.5 shows the view of the flow at different injection pore volumes for water flooding and microsphere flooding.

For the micromodel tests at water-wet conditions,the distribution of the remaining oil was different from that at oil-wet condition.The remaining oil was more dispersive.Fig.6 shows many small oil blobs left behind trapped by capillary forces.To recover the remaining oil,sufficient polymeric microspheres were required to initiate the fluid diversion.This could be the reason why there exists a threshold microsphere concentration at water-wet condition.

3.2.3.Flow at pore scale

From the previous discussion,the pattern of the remaining oil after waterflooding affected the performance of microsphere flooding.The local view of the flow displayed more details of the pore-scale flow.In the oil-wet micromodel,when water was injected,water and oil had very clear interface and meniscus as shown on Fig.7.The displacement process was more than a percolation process.Water moved through the centers of the larger pores with low capillary pressure,left oil in the smaller pores.This indicates that the flow was dominated by capillary forces.Water flow along the diagonal line from injector to the producer,left the two corners with large oil clusters.

In the water-wet micromodel,unlike the oil-wet conditions,the injected water spontaneously imbibed and flowed along the pore surfaces.Fig.8 shows that the oil was easily snapped off and became dispersive oil blobs.Because there was no meniscus,the capillary pressure cannot confine the flow as it did in oil-wet conditions.Therefore,the injected water invaded into the pores without much restriction,not necessarily moving along with the diagram line so that the remaining oil was disordered.

Fig.5.MSP-B injection at oil-wet condition.

Fig.6.MSP-B injection at water-wet condition.

Fig.7.Pore scale water flow at oil-wet condition.

3.3.Impact of polymeric microsphere particle size on oil production

Three polymeric microsphere samples (MSP-A,MSP-B and MSP-C)with median particle size of 0.05,0.3,and 20 μm,respectively,were used to study the effect of microsphere particle size on the large volume injection.Table 2 shows the pore/particle size ratios of the microsphere samples.

Fig.9 presents the oil production by 1 PV polymeric microsphere injection in an oil-wet micromodel.Although the size of the polymeric microspheres was much smaller than that of the pores,additional oil production was observed,that increased with increasing particle size and concentration.In general,increasing the particle size and concentration enhanced fluid diversion capacity,resulting in higher oil production.It was found that the larger polymeric microsphere size,the better performance in incremental oil production.It is also worth highlighting the different responses between “small” size microsphere(MSP-A and MSP-B)and “l(fā)arge” size microsphere (MSP-C).As the particle size close to the pore size,the “l(fā)arge” microspheres tend to retain inside the porous media in the course of deep propagation.A balance between deep displacement and oil production performance should be taken into account for microsphere injection applications.

Fig.8.Pore scale water flow at water-wet condition.

Table 2 Pore/particle size ratios of polymeric microsphere samples.

Fig.9.Production of polymeric microsphere injection with different particle sizes at oil-wet condition.

4.Conclusions

(1)Polymeric microsphere injection produced incremental oil from the etched glass micromodel that has been waterflooded.The pore size in the micromodel is in the range of 120-480 μm,which is 400-1600 times as the polymeric microsphere size of MSP-B suspended in injection water.

(2)Polymeric microsphere injection performed better in the oil-wet micromodel than in the water-wet micromodel.This could be attributed to the wettability that affects the distribution patterns of the remaining oil after water flooding.

(3)Oil production of polymeric microsphere injection increased with increasing the result of particle size and concentration.Large amount injection of micro-sized particles could be effective in waterflooded porous media.

(4)The study suggests the applicability of polymeric microsphere injection in heterogeneous oil-wet porous media after waterflooding.