AC.TA MECHANICA SINICA, Vol.5, No.l, February 1989 Science Press, Beijing, China Allerton Press, INC., New York, U. S. A. ISSN 0567-7718 POROUS FLOW WITH PHYSICO-CHEMICAL PROCESSES MICROSCOPIC STUDY Guo Shangping Huang Yanzhang Hu Yaren Zhou Juan Cheng Yongmin (Institute of Porous Flow arm Fluid Mechanics under China National Petroleum Co. and Chinese Academy of Sciences ) AllSTRACIF : This paper describes the results of microscopic study on fluid flows through porous media. The oil-water two-phase flow, the oil-air-water tri-phase flow. the Foam-surfactant-oil-water-air multi-phase flow, the microemulsion-oil-water multi-phase flow and flow with neutralization reaction are introduced. The micromodels, the technology of fabricating micromodels and the method of their application are also described. KEY WORld: porous flow, micromodels, physic.o-chemical fluid flow. I. INTRODUCTION Fluid flow through porous media in underground reservoir and industrial installations are often accompanied by complicated physical processes and chemical reactions. Physical simulation is one of the effective approaches for investigating such problems. The most efficacious physical simulation should be the combination of macroscopic with microscopic simulation. The term "microscopic simulation" or "micro-research" in the area of porous flow means investigation on the pore-level. A series of flow details and important flow mechanisms may be obtained by means of microsimulation, while macroscopic research seems to be powerless in this respect. Nevertheless, little significant progress has ever been obtained in the domain of microresearch on porous flow because of the difficulties in fabricating micromodds and acquiring experimental data. Three types of micromodels and relevant measuring apparatuses have been successfully developed in our laboratory of microsimulation in recent years. All these models are transparent. The first kind of micromodels has regular geometry and topology of pore systems, which may be called "idealized models "(Fig. 1). The second kind of them imitates real pore structure of given porous media in nature, industrial installations or engineering materials, for example, imitates actual pore structure of given slices of a reservoir sand stone. These models are the so called "imitating models"( Fig. 2). The third kind of micromodels belongs to ball-packed models. The technique of making the first and second types of micromodds essentially consists in etching network in glass by using mechanical and photo-chemical method. The technique of making the third type of micromodds lies in packing one mono-layered tiny beads into the fissure between two sheets of glass. Received\26 September 1988.
2 ACTA MECHANICA SINICA ~,989 Fig. 1 Idealized model Fig. 2 Imitating model Wettability of pore surfaces in these micromodels can be controlled, including three kinds of typical wettabilities, that is- strongly water-wet (Fig. 3), strongly oil, wet (Fig. 4) and medium ones (Fig. 5). Fig. 3 Oil-water interface (oil-vet porous media) Fig. 4 Oil-water interface (oil-wet porous media) Fig. 5 Oil-water interface (medium porous media) The roughness of pore surfaces also can be controlled, including three kinds of typical roughness, namely, smooth surfaces (Fig. 6), rough surfaces (Fig. 7) and medium ones (Fig.8). Fig. 6 Smooth pore surfaces Fig. 7 Rough pore surfaces Fig. 8 Medium pore surfaces Micromodels, having been used for experiments, can be regenerated so that they return to their original wettability. Moreover, the surface wettability of a model can be changed several times according to the requirements of researchers, while the pore structure, surface roughness and other characteristics of the models remain unchanged. Thus, repetitiveness, precision and reliability of the laboratory data are guaranteed, while they are not ensured in the usual experiments of porous flow. In order to form a complete set of laboratory techniques, special measuring
Vol.5, No.1 Guo Shangping et al.:porous Flow with Physico-Chemical Processes 3 apparatuses were developed in our institute, among them are flowmeters capable of measuring very tiny flow rate of either phase for two-phase flows (Fig.9), and of any phase for multi-phase flows (Fig. 10). Fig. 9 Flowmeter for two-phase flow Fig. 10 Flowmeter for multi-phase flow The authors of this paper have successfully utilized the above-mentioned models and apparatuses for micro-study of a series of important problems on porous flow. Some significant flow mechanisms and flow details have been discovered. II. OIL-WATER TWO-PHASE FLOW Mechanism of oil-water two-phase flow was studied in imitating and idealized models. 1. Bound-water (connate-water) Three patterns of existence of bound-water were discovered in water-wet models, namely, (a) the bound-water exists in very tiny pores su.n'ounded by larger pores (Fig. 11), (b) it is present in the pores with one dead end (Fig. 12), where there is some times a wide space (Fig. 13), and (c), it is held on the pore surfaces in the form of thin water films, which is found especially on the rough surfaces (Fig. 14). Fig. 11 Patterns of existence of Fig. 12 Patterns of existence of bound-water bound-water Fig. 13 Patterns of existence of bound-water Fig. 14 Patterns of existence of bound-water
4 ACTA MEEHANICA SMNICA 1989 The bound-water saturation was measured and it had the average value of 16.4%. 2. Flow behavior and oil-water distribution Flow behavior and oil-water distribution in water-wet porous media are different from those in oil-wet media. Oil-water interface Oil-water interfaces in water-wet models are concave toward the oil phase (Fig.3,15), those of the oil-wet ones are concave toward the water phase (Fig.4,16), and those of the neutral ones are a plane (Fig.5). Flow behavior The primary flow behavior in water-wet models is that water creeps along the pore walls, and the injected water might fill or occupy whole pore volume of small pores. In the case of larger pores, the pore walls only might be covered with a film of water in the beginning, and the injected water couldn't occupy whole pore volume at once. Therefore, in the first stage of water injection, a lotof smaller pores are occupied with water, while a part of larger pores are surrounded by the smaller pores occupied with water. In these larger pores an amount ofoil remains and cannot be removed, so that they form "isolated island" (Fig. 15). Oil entrapped in the isolated pores has great difficulty in moving in cases of small driving rate. Nevertheless, It can be still removed when driving conditions are changed and the driving rate is increased. Fig. 15 Flow behavior (water-wet media) Fig. 16 Flow behavior (oil-wet media) In cases of oil-wet models, on the ~ontrary to the cases of water-wet models, injected water moves primarily along the axial part of the pores. For this reason, water in large pores flows more easily than in samller ones, so that injected water first of all moves towards these larger pores and then occupies them (Fig. 16). It is discovered, that "fingering breakthrough" (Fig. 17) of water flow takes place, namely, water flows with a larger rate along some narrow pathes consisting of continual large pores. This phenomenon will decrease oil recovery. Fig. 17 Flow behavior (oil-wet media) Fig. 18 Pattern of existence of residual oil (water-wet media)
Vol.5, No.1 Guo Shangping et al.:porous Flow with Physico-Chemical Processes 5 3. Residual oil entrapped in the porous media after water-flooding process in the case of water-wet models appears mainly in l/re forms of irregular strings, cakes and beads, a great part of which is entrapped in larger pores (Fig. 18). In the cases of oil-wet models oil is distributed mainly in groups of larger pores sorrounded by smaller pores (Fig. 19), and, secondly, it is entrapped in small pores and pores with a dead end. Sometimes it adheres to the pore walls, especially to the rough pore surfaces in the form of oil film or oil drops (Fig.20). Fig. 19 Pattern of existence of residual oil (oil-wet media) Fig. 20 Pattern of existence of residual oil (oil-wet media) 4. Water-flooding efficiency and oil recovery The experiments performed in the imitating models show that water-flooding efficiency and oil recovery in the cases of water-wet models have the higher value (45.3%), while the oil-wet models have the lower value (35.4%), when 1.5 PV, of water has been injected (Fig.21). 8O f,. water-wet. oil-wet 40 O Fig. 21 0.4 0.8 1.2 1.6 PV Oil recovery from imitating micromodels m. OIL-AIR-WATER TRI-PHASE FLOW Flow mechanism of oil displacement by water-air mixture was studied. It is d~scovered that coexistence of three phases and their simultaneous flow in water-wet porous media, appears under the conditions of suitable air-water ratio. Water creeps near pore walls, air flows along pore axis and oil moves between the surfaces of flowing water and flowing air (Fig. 22). The residual oil, which originally adhered to the pore walls and couldn't move, starts moving forward now due to reduced resistance to flow, that is, due to the fact that adhesion of oil to water surfaces and to air surfaces is much lower than that to pore walls, that just explains why the oil recovery might be increased by water-air mixture flooding.
6 ACTA MECHANICA SINICA 1989 Fig. 22 Oil-air-water [low IV.. FOAM. SURFACTANT- OIL- WATER - AIR MULTIPHASE FLOW Our experiments on mechanism of such multi-phase flow have shown that it is easy to produce foam when foaming agent and air coexist in porous media. Po- rous media can promote the formation of foam. The concentration of foaming agent and the air-foaming agent ratio are important factors for formation, development and breakup of foam in porous media. When air gets into the pores, small pearl-like foam, which is rather scattered and regular, forms in the beginning (Fig.23), it becomes bigger polyhedrons with increasing air-foaming agent ratio (Fig.24), and then breaks up, when air content increases to much higher value (Fig.25). Fig. 23 Foam formation Fig. 24 Foam development Fig. 25 Foam break up It is clear that in engineering practice,in order to produce the expected foam in reservoir to raise oil recovery, such factors as foaming agent concentration and air-foaming agent ratio must be controlled. Our experiments have shown that foam has a tendency to occupy the bigger pores. For this reason, we can make use of this mechanism to develop a technology of foam-flooding, namely, we can use foam to block up large pores, so as to lead injected water to displace the oil remaining in small pores. Thus, oil recovery will be raised. V. MICROEMULSION-OIL-WATER MULTIPHASE FLOW Mechamism of residual oil displacement by microemulsion-micellar solution was studied in imitating models, idealized models and ball-packed models. Flooding with different microemulsion has distinct flow mechanisms. Here we introduce only flow mechanism of medium-phase microemulsion flooding as an example. 1. Starting and movement of oil
Vol.5, No.1 Guo Shangping et al.:porous Flow with Physico-Chemical Processes 7 Contact of injected microemulsion with residual oil in the porou media leads to miscibility action of these two kinds of fluids, regardless of that the models are water-wet or oil-wet. The originally red oil becomes light-red due to the influence of miscibility. The originally unremoved residual oil starts and moves now as a result of the so called "miscibly stripping", namely, miscibly stripping the pore wails of oil (Fig. 26). The miscible fluids of this kind are easy to start and move owing to reduced interfacial tension and capillary pressure. In addition, in part of the pores emulsification occurs. Being carried by water, the emulsified oil beads move forward also (Fig. 27). Fig. 26 Oil was miscibly stripped Fig. 27 Oil was emulsifyed and from pore surfaces and starts to move starts to move 2. Enrichment of oil Miscible fluids flow forward in the form of cakes, drops, beads and thread, continuously stripping miscibly the pore wails of residual oil which they meet in the process of motion. The stripped oil drops get together with each other, so that the flowing fluids are oil-enriched gradually. At the same time, the emulsified oil continuously get together in the process of flow forward. 3. Formation, development and vanishing of oil-bank Continuous and miscible stripping, emulsificating and gathering of the oil pearls promote unceasing get-together of flowing oil, thereby a zone enriched with oil is gradually formed in the front part of the flow. This zone may be called as an "oil-bank" (Fig.28), which develops and moves from the inlet to the outlet of the model. When the oil-bank reaches the outlet, oil output jumps to a certain value, and from this time on, the size of the bank reduces until it disappears (Fig.29 a--f). Fig. 28 Get-together of oil and formation of "oil-bank ~
8 ACTA MECHANICA SINICA 1989 Fig. 29 Fig. 30 A drop of oil is trapped within a pore
Vol.5, No.1 Guo Shangping et al.:porous Flow with Physico-Chemical Processes 9 To this time, oil originally entrapped in the porous media is recovered almost 100%, only a few oil drops and small oil cakes remain (Fig.30). 4. Displacement efficiency and oil recovery The oil re overy of water-flooding only reaclies 53.0% and 32.2%, respectively for water-wet and oil-wet models, and that of the microemulsion flooding comes up to 96.8% and 92.7%, respectively for water-wet and oil-wet models (Fig.31). In other words, the microemulsion flooding will greatly increase oil recovery. water-wet model 1 2 [ Injection of F~'~ o_.o~ 80J- micellar solution ~,,.~'~oc~e \ O k`" i i I i I I I I I I I I I I I _1 Q 1.0 2.0 3.0 PV Fig. 31 Oil recovery by micehar solution flooding VI. POROUS FLOW WITH NEUTRALIZATION REACTION A new technology for enhancing oil recovery is presently under study. The method is based on the fact,that organic acids originally occurring in crude oil, will react with alkaline water, which is injected into the reservoir, to produce soaps at the oil-water interfaces. This surfactant produced in-situ will promote movement of residual oil in water-flooded reservior. The mechanism of such multi-phase flow with neutralization reaction has been studied by using micromodels in our laboratory. 1. Starting and movement of oil The suffactant produced in-situ leads to the emulsification of oil and water, changes fluid-pore wall and fluid-fluid interfacial relation, reduces interfacial tension, adhesive force and capillary pressure. For this reason, the residual oil starts and begins to flow now. Fig. 32 Oil in water emulsion kvater-wet model)
10 ACTA MECHANICA SINICA 1989 The details of mechanisms are quite different for different conditions. For example, in the case of water-wet models and alkaline water free of sodium chloride, oil-in-water emulsion is formed (Fig. 32), while water-in-oil emulsion will be produced if sodium chloride is contained in alkaine water. In the case of oil-wet model, the water-in-oil emulsion appears (Fig.33), whether the alkaline water is or isn't free of sodium chloride. Emulsification stronly promotes the residual oil entrapped in pores to start' and move, regardless of the types of emulsion formed in-situ. Fig. 33 Water in oil emulsion (oil-wet model) 2. Enrichment of oil and formation of oil-bank. The flowing fluid is enriched in oil gradually with the development of flow process, and therefore an oil-bank forms and develops also. It's size begins to reduce when the leading edge of the oil-bank arrives at th~ outlet of the model, and then it disappears at last. The oil recovery is increased considerably corresponding to oil content of the oil-bank. REFERENCES [ 1 l Guo Shangping, Huang Yanzhang, Zhou Juan & Kuang Peiqiong, Microscopic research of physico-chemical fluid flow through porous media,acta Mechanica Sinica, 4,1(1985), [ 2 ] Guo Shangping, Huang Yanzhang, Ma Xiaowu & Zhou Juan,. Microscopic and Macroscopic Research on Multi-phase Flow through Porous Media, Proceeding of the 2nd Asian Congress on Fluid Mechanics, Beijing (1983). [ 3 ] Guo Shangping, Huang Yanzhang, Ma Xiaowu & Zhou Juan, Microscopic experimental study of multi-phase fluid llow through porous media, Chinese Petroleum Journal, 5,1(1984) (In Chinese).