Parallel sand-filled pipe model with inter-pipe fluid streaming function as well as preparation method and application of parallel sand-filled pipe model

Abstract

The invention belongs to the technical field of water drive reservoir development, and particularly relates to a parallel sand-filled pipe model with an inter-pipe fluid streaming function as well as a preparation method and application of the parallel sand-filled pipe model. The invention provides a parallel sand-filled pipe model with an inter-pipe fluid streaming function. The model comprises a plurality of sand-filled pipes with different permeability and interlayer connecting pipes 4, each sand-filled pipe comprises a sand-filled pipe body 1 with inter-pipe interconnection holes 3 and a filler 2 filled in the sand-filled pipe body 1; and the plurality of sand-filled pipes with different permeability are connected in parallel by using interlayer connecting pipes 4 through the pipe interconnection holes 3. The parallel sand-filled pipe model has an inter-pipe fluid streaming function and can simulate the seepage condition of a heterogeneous reservoir in a layer, and the simulation result is close to the actual condition.

Description

Technical Field

[0001] The invention belongs to the technical field of water drive oil reservoir development, and in particular relates to a parallel sand filling pipe model with an inter-pipe fluid bypass function, a preparation method and an application thereof. Background Art

[0002] At present, the oil reservoir has entered the high water content and high recovery (double high) development stage. The long-term water injection and polymer injection flushing has caused the rock structure of the high permeability part of the reservoir to be destroyed, the reservoir heterogeneity has been enhanced, and the inefficient and ineffective circulation of the oil displacement working fluid in the reservoir has become increasingly serious. It is urgent to take measures to reduce the ineffective circulation of the oil displacement working fluid and expand the swept volume. However, before these measures are applied in actual operations, physical simulation is needed to predict and evaluate their effects.

[0003] Physical simulation is an important technical means for the optimization of working fluid types for chemical flooding, profile control and water plugging, and the optimization of injection process parameters in mines. Cores are consumable materials that must be used in physical simulation. Due to the limited sources and high prices of natural cores, as well as the limitations of their own size and macroscopic heterogeneity, artificial cores have become a substitute for natural cores. At present, artificial cores include cemented and filled cores. Among them, filled cores (also known as sand-filled tubes) have the characteristics of relatively simple process, low price and strong pressure resistance, and are widely used in petroleum scientific research.

[0004] The heterogeneity of actual reservoirs can be divided into two categories: interlayer heterogeneity and intralayer heterogeneity. There are interlayers between each small layer in the interlayer heterogeneous reservoir, and there is no fluid exchange between the small layers, that is, no bypass phenomenon. In contrast to the interlayer heterogeneous reservoir, fluid exchange can occur between the permeable parts (small layers) of the intralayer heterogeneous reservoir. During the water flooding and chemical flooding development process, part or even all of the oil displacement working fluid entering the low permeability part will turn to enter the medium and high permeability part. Due to the small inner diameter and long length of the sand filling pipe, it is difficult to fill it with different materials and form different permeability areas. The existing parallel sand filling pipe model is used to simulate the water flooding, chemical flooding and profile control and flooding process and measures of the intralayer heterogeneous reservoir, but it not only exaggerates the oil increase and water reduction effect of chemical flooding or profile control and flooding, but also has a large difference between the dynamic characteristics and the dynamic characteristics of the actual reservoir. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a parallel sand-filled pipe model with the function of fluid bypass between pipes, and a preparation method and application thereof. The parallel sand-filled pipe model has the function of fluid bypass between pipes, can simulate the seepage conditions of heterogeneous reservoirs within the layer, and the simulation results are close to the actual situation.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a parallel sand-filled pipe model with the function of fluid bypassing between pipes, comprising: a plurality of sand-filled pipes with different permeabilities and an interlayer connecting pipe 4; each sand-filled pipe comprises a sand-filled pipe body 1 with an inter-tube interconnecting hole 3 and a filler 2 filled in the sand-filled pipe body 1;

[0008] The plurality of sand-filled pipes with different permeabilities are connected in parallel by interlayer connecting pipes 4 through inter-tube interconnecting holes 3 .

[0009] Preferably, the number of the sand-filled tubes is 2 to 6; the inner diameter of each sand-filled tube body 1 with the inter-tube interconnecting holes 3 is 3 to 6 cm, and the tube length is 30 to 120 cm.

[0010] Preferably, the filler 2 includes mineral particles, or mineral particles and an adhesive; and the adhesive includes an epoxy resin adhesive.

[0011] Preferably, the mineral particles include one or more of quartz sand, albite, potassium feldspar, dolomite, calcite, montmorillonite, illite, chlorite, kaolinite and natural core crushed oil sand of oil fields.

[0012] Preferably, the length of the interlayer connecting tube 4 is 10 to 30 cm.

[0013] The present invention also provides a method for preparing the parallel sand-filled pipe model with the inter-pipe fluid flow function described in the above technical solution, comprising the following steps:

[0014] According to the vertical permeability of the target oil reservoir, the stratification of the target oil reservoir and the average permeability of each reservoir are obtained, and according to the stratification and the average permeability of each reservoir, the number of sand filling pipes and the permeability of each sand filling pipe are determined, the number of stratification is the number of sand filling pipes in the parallel sand filling pipe model, and the average permeability of each reservoir layer corresponds to the permeability of each sand filling pipe;

[0015] The sand-filled pipes are connected in sequence by using interlayer connecting pipes 4 through the inter-pipe interconnecting holes 3 according to the average permeability of each reservoir layer, so as to obtain the parallel sand-filled pipe model with the inter-pipe fluid bypass function.

[0016] Preferably, the permeability of each sand-filled pipe is formed by the filler filled in the sand-filled pipe, and the filling is: filling the filler 2 in each sand-filled pipe body 1, and by adjusting the type, particle size and compaction degree of the filler 2 in each sand-filled pipe body 1, a sand-filled pipe corresponding to the average permeability of each reservoir layer is obtained.

[0017] Preferably, before filling the filler 2, the method further includes: roughening the inner surface of the sand-filled tube body 1; the roughening treatment is: pickling the inner surface of the sand-filled tube body 1, coating an adhesive, and then filling quartz sand, and after the adhesive is cured, removing excess quartz sand in the tube.

[0018] The present invention also provides the use of the parallel sand-filled tube model with inter-tube fluid flow function described in the above technical solution or the parallel sand-filled tube model with inter-tube fluid flow function prepared by the preparation method described in the above technical solution in simulating water-driven oil seepage in heterogeneous reservoirs within the layer.

[0019] The present invention also provides a method for simulating water-driven oil seepage in a heterogeneous reservoir, comprising the following steps:

[0020] Inject water into the injection end of the parallel sand-filled pipe model with the function of fluid bypass between pipes, perform the first water drive until the water content of the parallel sand-filled pipe model as a whole reaches 98±1%, collect the oil production of each sand-filled pipe, and obtain the water drive recovery factor of each sand-filled pipe and the water drive recovery factor of the parallel sand-filled pipe model as a whole;

[0021] After the first water drive to the water content of the whole parallel sand-filled pipe model is 98±1%, a water plugging working fluid is injected into the production end of the sand-filled pipe with the highest permeability in the parallel sand-filled pipe model, and then water is injected from the injection end of the parallel sand-filled pipe model to perform a second water drive to the water content of the whole parallel sand-filled pipe model is 98±1%, and the produced oil volume of each sand-filled pipe is collected to obtain the water plugging recovery rate of each sand-filled pipe and the water plugging recovery rate of the whole parallel sand-filled pipe model;

[0022] The parallel sand-filled tube model with the function of fluid flow around the tubes is the parallel sand-filled tube model with the function of fluid flow around the tubes described in the above technical solution or the parallel sand-filled tube model with the function of fluid flow around the tubes prepared by the preparation method described in the above technical solution.

[0023] The present invention provides a parallel sand-filled pipe model with an inter-tube fluid bypass function, comprising: a plurality of sand-filled pipes with different permeabilities and an interlayer connecting pipe 4; each sand-filled pipe comprises a sand-filled pipe body 1 with an inter-tube interconnection hole 3 and a filler 2 filled in the sand-filled pipe body 1; the plurality of sand-filled pipes with different permeabilities are connected in parallel by the interlayer connecting pipe 4 through the inter-tube interconnection hole 3. If there is no bypass function between the heterogeneous sand-filled pipes, during the water flooding process, the flow direction of the injected water in the high and low permeability layers is parallel; after the water plugging operation is performed at the production end of the high permeability layer, the seepage resistance of the high permeability layer increases; during the subsequent water flooding process, the fluids in the high and low permeability layers can only flow in parallel. However, under actual reservoir conditions, the injected water has bypass and cross-seepage phenomena in the reservoir. In order to simulate the cross-seepage between the high permeability layer and the low permeability layer of the actual reservoir, the parallel sand-filled pipe model with the inter-tube fluid bypass function provided by the present invention has an interaction between the layers in the water drive stage, and the liquid flow can cross-flow between the high and low permeability layers in addition to the parallel flow in the high permeability layer and the low permeability layer; after the water plugging operation, the seepage resistance in the high permeability layer increases; in the subsequent water drive process, in addition to the parallel flow, due to the presence of water plugging working fluid in the high permeability layer, the seepage resistance in the high permeability layer increases, and after the injected fluid enters the high permeability layer, the liquid flow turns in the middle part of the sand-filled pipe in the high permeability layer, so as to achieve the purpose of expanding the swept volume. Compared with the parallel sand-filled pipe without the inter-tube fluid bypass function, the sand-filled pipe with the inter-layer bypass function is similar to the seepage law of the heterogeneous reservoir in the actual layer, and can better simulate the bypass and expansion of the swept volume in the heterogeneous reservoir in the layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of a parallel sand-filled tube model with the function of fluid bypassing between tubes in Example 1 of the present invention, wherein 1-sand-filled tube body, 2-filler, 3-inter-tube interconnecting hole, 4-inter-layer connecting tube, 5-pressure measuring point, 6-end cover, 7-injection end pressure measuring point;

[0025] Figure 2 This is a physical picture of a single sand-filled pipe;

[0026] Figure 3 It is a diagram of the sand-filled pipe water plugging experiment equipment, where 1-beaker, 2-advection pump, 3-simulated oil, 4-valve, 5-wastewater, 6-intermediate container, 7-four-way, 8-pressure gauge, 9-produced liquid collector, 10-sand-filled pipe model, 11-tee, 12-piston intermediate container, 13-piston, 14-isolation fluid, 15-water plugging working fluid A, 16-water plugging working fluid B, 17-hand pump, 18-distilled water;

[0027] Figure 4 This is the relationship between the injection pressure and PV number of the "inorganic gel / polymer fine powder" working fluid after water blocking in a single sand-filled pipe;

[0028] Figure 5 This is the relationship between the injection pressure and the injection PV number of the combined working fluid of "inorganic gel / fly ash / polymer fine powder / resin beads" after water blocking in a single sand-filled pipe;

[0029] Figure 6 This is the relationship diagram between the injection pressure and the injection PV number of the "inorganic gel / polymer fine powder" working fluid after water plugging in the parallel sand-filled pipe model with the function of fluid bypass between pipes;

[0030] Figure 7 This is the relationship between the water content and the injected PV number of the "inorganic gel / polymer fine powder" working fluid after water plugging in the parallel sand-filled pipe model with the function of fluid bypass between pipes;

[0031] Figure 8 This is the relationship between the recovery rate and the injected PV number after water plugging of the "inorganic gel / polymer fine powder" working fluid in the parallel sand-filled pipe model with the function of fluid bypass between pipes;

[0032] Fig. 9 This is the relationship between the diversion rate and the injected PV number of the "inorganic gel / polymer fine powder" working fluid after water plugging in the parallel sand-filled pipe model with the function of fluid bypass between pipes. DETAILED DESCRIPTION

[0033] The present invention provides a parallel sand-filled pipe model with the function of fluid bypassing between pipes, comprising: a plurality of sand-filled pipes with different permeabilities and an interlayer connecting pipe 4; each sand-filled pipe comprises a sand-filled pipe body 1 with an inter-tube interconnecting hole 3 and a filler 2 filled in the sand-filled pipe body 1;

[0034] The plurality of sand-filled pipes with different permeabilities are connected in parallel by interlayer connecting pipes 4 through inter-tube interconnecting holes 3 .

[0035] Unless otherwise specified, the present invention has no special requirements on the sources of the raw materials used, and commercially available products known to those skilled in the art may be used.

[0036] The parallel sand-filled pipe model with the function of fluid bypassing between pipes provided by the present invention comprises: a plurality of sand-filled pipes with different permeabilities; each sand-filled pipe comprises a sand-filled pipe body 1 with inter-pipe interconnecting holes 3 and a filler 2 filled in the sand-filled pipe body 1.

[0037] As an implementation mode, the number of the sand-filled tubes is 2 to 6, and the specific embodiment is 2 to 3; the inner diameter of the sand-filled tube body 1 with interconnecting holes 3 between tubes is 3 to 6 cm, and the specific embodiment is 3 cm, the outer diameter is 4 to 7 cm, and the specific embodiment is 4.0 cm, and the tube length is 30 to 120 cm, and the specific embodiment is 50 to 100 cm; the material of the sand-filled tube body 1 with interconnecting holes 3 between tubes is stainless steel; both ends of the sand-filled tube body 1 with interconnecting holes 3 between tubes are processed with threads and sealing end covers 6; the end cover 6 is a gate.

[0038] The more interconnected holes there are between pipes in the present invention, the closer it is to the actual geological reservoir.

[0039] As an embodiment, the filler 2 includes mineral particles, or mineral particles and a binder.

[0040] As an embodiment, the mineral particles include one or more of quartz sand, oil field natural core crushed oil sand, albite, potassium feldspar, dolomite, calcite, montmorillonite, illite, chlorite, kaolinite and oil field natural core crushed oil sand. In a specific embodiment, it is a mixed sand of quartz sand and oil field natural core crushed oil sand; the mass proportion of oil field natural core crushed oil sand in the mixed sand is 15%; the particle size of the mineral particles is 20 to 400 meshes, and in a specific embodiment, it is 100 to 270 meshes.

[0041] In one embodiment, the adhesive comprises an epoxy resin adhesive.

[0042] The parallel sand-filled pipe model with the inter-pipe fluid bypass function provided by the present invention comprises: an interlayer connecting pipe 4; the plurality of sand-filled pipes with different permeabilities are connected in parallel by the interlayer connecting pipe 4 through the inter-pipe interconnecting holes 3.

[0043] As an implementation mode, the length of the interlayer connecting tube 4 is 10 to 30 cm, 10 to 20 cm in a specific embodiment, the inner diameter is 0.03 mm, and the outer diameter is 0.07 mm; the material of the interlayer connecting tube 4 is stainless steel; the number of the interlayer connecting tubes 4 is 1 to 9, 9 in a specific embodiment; the number of the inter-tube interconnecting holes 3 is determined according to the number of the inter-tube interconnecting holes 4; the spacing between two adjacent inter-tube interconnecting holes 3 is 10 to 15 cm, 10 cm in a specific embodiment.

[0044] As an implementation mode, one end of the plurality of sand filling pipes with different permeabilities is connected in parallel as an injection end, and the other end is used as a production end; the injection end is provided with an injection end pressure measuring point 7, and a pressure gauge is installed at the injection end pressure measuring point 7.

[0045] As an implementation mode, the parallel sand-filled pipe model with the inter-pipe fluid bypass function is provided with a pressure measuring point 5; the number of the pressure measuring points 5 is 1 to 3, and 3 in the specific embodiment; the pressure measuring point 5 is a pressure gauge provided in the middle of the interlayer connecting pipe 4.

[0046] The parallel sand-filled pipe model with the function of fluid bypass between pipes provided by the present invention can increase the interaction between the high permeability layer and the low permeability layer, and can simulate the bypass of liquid flow in the middle of the core in the actual reservoir through the interconnected holes between pipes, thereby increasing the similarity between the fluid flow direction of the indoor core simulation and the actual geological reservoir.

[0047] The present invention also provides a method for preparing the parallel sand-filled pipe model with the inter-pipe fluid flow function described in the above technical solution, comprising the following steps:

[0048] According to the vertical permeability of the target oil reservoir, the stratification of the target oil reservoir and the average permeability of each reservoir are obtained, and according to the stratification and the average permeability of each reservoir, the number of sand filling pipes and the permeability of each sand filling pipe are determined, the number of stratification is the number of sand filling pipes in the parallel sand filling pipe model, and the average permeability of each reservoir layer corresponds to the permeability of each sand filling pipe;

[0049] The sand-filled pipes are connected in sequence by using interlayer connecting pipes 4 through the inter-pipe interconnecting holes 3 according to the average permeability of each reservoir layer, so as to obtain the parallel sand-filled pipe model with the inter-pipe fluid bypass function.

[0050] The present invention obtains the stratification of the target oil reservoir and the average permeability of each reservoir layer according to the longitudinal permeability of the target oil reservoir reservoir, and determines the number of sand filling pipes and the permeability of each sand filling pipe according to the stratification and the average permeability of each reservoir layer. The number of stratification is the number of sand filling pipes in the parallel sand filling pipe model, and the average permeability of each reservoir layer corresponds to the permeability of each sand filling pipe one by one.

[0051] The present invention analyzes the target oil reservoir geological characteristics, well logging and coring test data to clarify the target oil reservoir reservoir heterogeneity, mineral composition and physical parameters (permeability and porosity), and stratifies the target oil reservoir reservoir according to the longitudinal permeability. The number of stratifications is the number of sand filling tubes in the parallel sand filling tube model. The average permeability of each reservoir layer corresponds to the permeability of each sand filling tube in turn, and the average permeability of each reservoir layer is obtained through the well logging analysis curve provided by the mine.

[0052] As an embodiment, the permeability of each sand-filled pipe is formed by the filler filled in the sand-filled pipe, and the filling is: filling the filler 2 in each sand-filled pipe body 1, and by adjusting the type, particle size and compaction degree of the filler 2 in each sand-filled pipe body 1, a sand-filled pipe corresponding to the average permeability of each reservoir layer is obtained.

[0053] As an implementation mode, the compaction degree is achieved by adjusting the pressurized pressure of the filler 2 in the sand-filled tube body 1; the process of adjusting the pressurized pressure of the filler 2 in the sand-filled tube body 1 is: after filling the filler 2, the tube wall of the sand-filled tube body 1 is hit with a metal hammer to reduce the intergranular voids of the filler 2, and then a cylindrical metal block is attached to the top of the sand-filled tube body 1, and the pressure of the cylindrical metal block is increased by the metal hammer. The present invention adjusts the compaction degree of the filler in the sand-filled tube by adjusting the pressurized pressure, thereby adjusting the permeability of the filler in the sand-filled tube.

[0054] As an embodiment, before filling the filler 2, it also includes: roughening the inner surface of the sand-filled tube body 1; the roughening treatment is: pickling the inner surface of the sand-filled tube body 1, coating the adhesive, filling the quartz sand, and removing the excess quartz sand in the tube after the adhesive is cured; the acid used for pickling is hydrochloric acid; the mass concentration of the hydrochloric acid is 3-15%, and 5-10% in the specific embodiment; the adhesive is an epoxy resin adhesive; the particle size of the quartz sand is 200 mesh. The role of the roughening treatment in the present invention is to keep the inner wall of the sand-filled tube body rough and prevent the filler in the sand-filled tube body from sliding as a whole.

[0055] After obtaining the sand-filled pipes, the present invention sequentially connects the sand-filled pipes with interlayer connecting pipes 4 through the inter-pipe interconnection holes 3 according to the average permeability of each reservoir layer to obtain the parallel sand-filled pipe model with the inter-pipe fluid bypass function.

[0056] As an implementation method, it also includes: connecting one end of each filling sand pipe in parallel as an injection end and the other end as a production end.

[0057] The present invention also provides the use of the parallel sand-filled tube model with inter-tube fluid flow function described in the above technical solution or the parallel sand-filled tube model with inter-tube fluid flow function prepared by the preparation method described in the above technical solution in simulating water-driven oil seepage in heterogeneous reservoirs within the layer.

[0058] The present invention also provides a method for simulating water-driven oil seepage in a heterogeneous reservoir, comprising the following steps:

[0059] Inject water into the injection end of the parallel sand-filled pipe model with the function of fluid bypass between pipes, perform the first water drive until the water content of the parallel sand-filled pipe model as a whole reaches 98±1%, collect the oil production of each sand-filled pipe, and obtain the water drive recovery factor of each sand-filled pipe and the water drive recovery factor of the parallel sand-filled pipe model as a whole;

[0060] After the first water drive to the water content of the whole parallel sand-filled pipe model is 98±1%, a water plugging working fluid is injected into the production end of the sand-filled pipe with the highest permeability in the parallel sand-filled pipe model, and then water is injected from the injection end to perform a second water drive to the water content of the whole parallel sand-filled pipe model is 98±1%, and the produced oil volume of each sand-filled pipe is collected to obtain the water plugging recovery rate of each sand-filled pipe and the water plugging recovery rate of the whole parallel sand-filled pipe model;

[0061] The parallel sand-filled tube model with the function of fluid flow around the tubes is the parallel sand-filled tube model with the function of fluid flow around the tubes described in the above technical solution or the parallel sand-filled tube model with the function of fluid flow around the tubes prepared by the preparation method described in the above technical solution.

[0062] The present invention injects water into the injection end of the parallel sand-filled pipe model with the function of fluid bypassing between pipes, performs a first water drive until the water content of the entire parallel sand-filled pipe model reaches 98±1%, collects the produced oil volume of each sand-filled pipe, and obtains the water drive recovery factor of each sand-filled pipe and the water drive recovery factor of the entire parallel sand-filled pipe model.

[0063] As an embodiment, the present invention connects the parallel sand-filled pipe model with the function of fluid bypass between pipes to a water drive system and a water plugging working fluid injection system; the water drive system includes a displacement pump and an intermediate container; the water plugging working fluid injection system includes a displacement pump, an intermediate container, a pressure sensor and a stainless steel pipeline.

[0064] As an implementation mode, the water drive recovery rate of each sand-filled pipe is the produced oil volume of each sand-filled pipe divided by the corresponding saturated oil volume of each sand-filled pipe; the water drive recovery rate of the overall parallel sand-filled pipe model is the sum of the produced oil volumes of all sand-filled pipes in the parallel sand-filled pipe model divided by the sum of the saturated oil volumes of all sand-filled pipes in the parallel sand-filled pipe model.

[0065] In the present invention, the main purpose of the above water flooding process is to simulate the water flooding stage of the primary development process of the oil reservoir.

[0066] In the present invention, the saturated water volume (pore volume), porosity, and permeability K of each sand-filled tube in the parallel sand-filled tube model with the function of fluid bypass between tubes are w , saturated oil volume and oil saturation tests are as follows:

[0067] After each sand-filled tube in the parallel sand-filled tube model with the function of fluid bypass between tubes is evacuated, each sand-filled tube is saturated with water. The saturated water volume of each sand-filled tube, that is, the pore volume of each sand-filled tube, is obtained by subtracting the weight of each sand-filled tube after saturation with water from the weight before saturation with water. The saturated water volume of each sand-filled tube is divided by the volume of each sand-filled tube to obtain the porosity of each sand-filled tube. Then, each sand-filled tube is water-driven until the pressure is stable, and the permeability K of each sand-filled tube is obtained by Darcy's formula.w ;

[0068] Use simulated oil to displace the water in each sand filling tube until no water is produced at the production end. Collect the produced water volume of each sand filling tube. The produced water volume is the saturated oil volume. The saturated oil volume of each sand filling tube is divided by the corresponding saturated water volume of each sand filling tube to obtain the oil saturation of each sand filling tube. This process is called saturated oil.

[0069] As an implementation manner, the vacuum pressure is -0.1 MPa.

[0070] As an implementation method, the basic form of the Darcy formula is:

[0071]

[0072] Where Q represents the penetration rate of the liquid (unit: cm 3 / s), K W Indicates permeability (unit: D or mD, 1mD = 10 -3 μm 2 ), A represents the permeable area (unit: cm 2 ), ΔP represents the pressure difference of the liquid in the porous medium (unit: MPa), μ represents the viscosity of the aqueous solution (unit: mPa·s); L represents the flow distance of the liquid in the porous medium (unit: cm).

[0073] The present invention obtains the oil saturation of the parallel sand-filled pipe model as a whole and each sand-filled pipe according to this method. The main purpose of the above two processes of saturation with water and saturation with oil is to simulate the environment of the oil reservoir before it is mined.

[0074] After the first water drive to the parallel sand-filled pipe model as a whole reaches a water content of 98±1%, the present invention injects a water plugging working fluid into the production end of the sand-filled pipe with the highest permeability in the parallel sand-filled pipe model, and then injects water from the injection end to perform a second water drive to the parallel sand-filled pipe model as a whole reaches a water content of 98±1%, collects the produced oil volume of each sand-filled pipe, and obtains the water plugging recovery rate of each sand-filled pipe and the water plugging recovery rate of the parallel sand-filled pipe model as a whole.

[0075] As an implementation mode, the water plugging recovery rate of each sand-filled pipe is the produced oil volume of each sand-filled pipe divided by the corresponding saturated oil volume of each sand-filled pipe; the water plugging recovery rate of the overall parallel sand-filled pipe model is the sum of the produced oil volumes of all sand-filled pipes in the parallel sand-filled pipe model divided by the sum of the saturated oil volumes of all sand-filled pipes in the parallel sand-filled pipe model.

[0076] The main purpose of the present invention through the above process is to simulate water flooding, take water blocking measures, and then water flood again to obtain the result of improving the recovery rate.

[0077] The present invention evaluates the oil-increasing and water-reducing effects of different water-plugging working fluids in the sand-filled pipe by the increase in recovery rate and the decrease in water content before and after water plugging. A larger increase in recovery rate means a stronger oil-increasing and water-reducing effect, while a lower increase in recovery rate means a poorer oil-increasing and water-reducing effect.

[0078] As an implementation mode, the water plugging working fluid comprises one or more of an inorganic gel water plugging working fluid, a polymer gel water plugging working fluid, a polymer-containing oil displacement agent and a particle water plugging working fluid, and in a specific embodiment, the water plugging working fluid is an inorganic gel water plugging working fluid.

[0079] As an embodiment, the inorganic gel water plugging working fluid comprises an auxiliary agent, an isolation fluid and a main agent; the auxiliary agent is a calcium chloride solution; the mass concentration of the calcium chloride solution is 0.5-15.0%; the isolation fluid is softened water or clean water; the Ca 2+ +Mg 2+ <20mg / L; the main agent is sodium silicate solution; the concentration of the sodium silicate solution is 0.5-15.0%. The inorganic gel water plugging working fluid is injected into the parallel sand filling pipe model with the function of fluid bypass between pipes through a single round or multiple rounds. The inorganic gel water plugging working fluid is mainly used for water plugging operations in high-temperature reservoirs.

[0080] As an implementation mode, the polymer gel water plugging working fluid comprises anionic polyacrylamide and a cross-linking agent; the mass concentration of anionic polyacrylamide in the polymer gel water plugging working fluid is 0.05-0.50%, and the mass concentration of the cross-linking agent is 0.01-0.5%; the cross-linking agent is organic chromium or phenolic resin. The polymer gel water plugging working fluid is mainly used for water plugging operations in large pores.

[0081] As an embodiment, the polymer-containing oil-displacing agent includes a polymer solution, or a binary mixed solution of a polymer and a surfactant, or a ternary mixed solution of a polymer, a surfactant and an alkali, or a ternary mixed solution of a polymer, a surfactant and a salt; the polymer is polyacrylamide with a hydrolysis degree of 15 to 30%; the surfactant is one or more of anionic surfactants, cationic surfactants, nonionic surfactants and zwitterionic surfactants; the salt is one or more of sodium chloride, calcium chloride, magnesium chloride and sodium sulfate; the alkali is sodium carbonate and / or sodium hydroxide. The polymer-containing oil-displacing agent is mainly used for deep water plugging operations in high permeability reservoirs.

[0082] As an embodiment, the particle-based water plugging working fluid includes polymer microspheres, polymer fine powders and inorganic particles; the particle sizes of the polymer microspheres and polymer fine powders are independently nanometer-level, micrometer-level or millimeter-level; the inorganic particles are one or more of quartz sand, fly ash, artificial zeolite and cement. The present invention does not specifically limit the particle size of the inorganic particles, and the particle size can be selected according to the pore throat size of the reservoir.

[0083] The inorganic particles in the particle-based water plugging working fluid can be carried into the reservoir through the polymer and are mainly used for water plugging operations in the near-wellbore area of ​​the reservoir.

[0084] The present invention has no special limitation on the process of water flooding, simulated oil displacement, injection end injection of chemical flooding working fluid or profile control and flooding working fluid and injection of water plugging working fluid, and any experimental process known in the art may be used.

[0085] The present invention utilizes a parallel sand-filled pipe model with an inter-pipe fluid bypass function to simulate a water-driven oil seepage simulation method in a heterogeneous reservoir within a layer. The method can not only monitor the bypass conditions of the heterogeneous reservoir within the layer, but also monitor the pressure changes at different positions between different heterogeneous reservoirs, and has a corrective effect on the results of ordinary sand-filled pipe oil displacement experiments that do not have a bypass function.

[0086] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention, but they should not be construed as limiting the protection scope of the present invention.

[0087] Example 1

[0088] like Figure 1 As shown, the filler 2 is a mixture of quartz sand (270 mesh) and crushed oil sand (270 mesh) from a natural core of an oil field, wherein the mass of crushed oil sand from a natural core of an oil field accounts for 15%, the number of sand filling tubes is 2, the inner diameter of the sand filling tube body 1 with the inter-tube interconnection hole 3 is 3 cm, the outer diameter is 4 cm, the tube length is 100 cm, and the material is stainless steel; threads and sealing end covers 7 (gates) are processed at both ends of the sand filling tube body 1 with the inter-tube interconnection hole 3, the number of inter-layer connecting tubes 4 is 9, the number of inter-tube interconnection holes 3 is determined according to the number of inter-layer connecting tubes 4, the spacing between two adjacent inter-tube interconnection holes 3 is 10 cm, the length of the inter-layer connecting tube 4 is 15 cm, the inner diameter is 0.03 mm, the outer diameter is 0.07 mm, and the material is stainless steel;

[0089] The preparation method of the parallel sand-filled pipe model with the inter-pipe fluid flow function is as follows:

[0090] According to the vertical permeability of the target reservoir, it is divided into two layers, of which the average permeability of the low permeability layer is 20650×10 -3 μm 2The average permeability of the high permeability layer is 520×10 -3 μm 2 , corresponding to the permeability of sand-filled tube I and sand-filled tube II, the difference between the high permeability layer and the low permeability layer is 39.7; the inner surface of each sand-filled tube is pickled with 10wt% hydrochloric acid and then coated with epoxy resin adhesive, the amount of adhesive is 5g / cm 2 , and then fill with 200 mesh quartz sand. After the adhesive is cured, remove the excess quartz sand in the tube, and then fill the sand filling tube I and sand filling tube II with the mixed sand. After filling, use a metal hammer to knock the tube wall of the sand filling tube to reduce the empty pores between the mixed sands. Then, a cylindrical metal block is attached to the top of the sand filling tube, and the pressure of the cylindrical metal block is increased by a metal hammer to adjust the compaction degree of the filled mixed sand, so that the permeability of the sand filling tube I and the sand filling tube II corresponds to the average permeability of the low permeability layer and the average permeability of the high permeability layer, respectively, to obtain two sand filling tubes with different permeabilities; through the tubes The interconnected hole 3 uses an interlayer connecting pipe 4 to connect the above two sand-filled pipes in sequence according to the average permeability of the low permeability layer and the high permeability layer. The same end of each sand-filled pipe is connected in parallel as the injection end, and the other end is used as the production end. Both ends of each sand-filled pipe are processed with threads and sealed end covers 6 (gates). A pressure gauge is provided at the injection end as the injection end pressure measuring point 7. The 9 interlayer connecting pipes are arranged in sequence from the injection end to the production end. A pressure gauge is provided in the middle of the interlayer connecting pipes numbered 3, 6, and 9 as the pressure measuring point 5, thereby obtaining the parallel sand-filled pipe model with the function of fluid bypass between pipes.

[0091] Example 2

[0092] The difference from Example 1 is that the number of sand-filled tubes in the parallel sand-filled tube model with the function of fluid bypass between tubes is 3, and the permeability of each sand-filled tube is 510×10 -3 μm 2 , 10270×10 -3 μm 2 , 20400×10 -3 μm 2 , respectively simulating the high permeability layer, medium permeability layer and low permeability layer of the target oil reservoir, and the rest of the content is the same as Example 1.

[0093] Example 3

[0094] The difference from Example 1 is that the level difference between the high permeability layer and the low permeability layer in the parallel sand-filled pipe model with the inter-pipe fluid bypass function is 10.5, and the rest of the contents are the same as Example 1.

[0095] Comparative Example 1

[0096] The difference from Example 1 is that the sand-filling pipes do not have interconnecting holes between pipes, and are ordinary parallel sand-filling pipes that are connected in parallel without interlayer connecting pipes through the interconnecting holes between pipes. The pressure measuring points of the ordinary parallel sand-filling pipes are set to 4 for each sand-filling pipe, and the positions are respectively set at 20 cm, 40 cm, 60 cm, and 80 cm from the injection end of each sand-filling pipe.

[0097] Comparative Example 2

[0098] The difference from Example 1 is that the permeability of the high permeability layer (520×10 -3 μm 2 ) The consistent sand filling tube II is used as a single sand filling tube.

[0099] Performance Testing

[0100] 1. Experimental conditions

[0101] 1.1 Experimental Materials

[0102] The water plugging working fluid includes inorganic gel (generated by mixing a 10wt% calcium chloride solution and a 5wt% sodium silicate solution in a volume ratio of 1:1), polymer fine powder (Shandong Noer Biotechnology Co., Ltd., model HPAM), artificial zeolite (modified fly ash from the Daqing Oilfield self-contained power plant) and epoxy resin beads (Wuxi Resin Factory, model E44).

[0103] The simulated oil was prepared with crude oil exported from an oil field. The viscosity was 14.3 mPa·s at a reservoir temperature of 55°C. The simulated oil consisted of crude oil and white oil. The water was produced wastewater and softened water (without divalent metal ions in the produced wastewater) from an oil field. The ion composition of the wastewater and softened water is shown in Table 1.

[0104] Table 1 Ion composition of sewage and softened water (mg / L)

[0105]

[0106] 1.2 Experimental process and evaluation

[0107] 1.2.1 Water plugging effect evaluation

[0108] Sand filling pipe water blocking experimental equipment Figure 3 , which includes a water drive system consisting of a displacement pump and an intermediate container, and a water plugging working fluid injection system consisting of a hand pump and a piston container, wherein the sand filling pipe model is a single sand filling pipe of comparative example 2, wherein four pressure measuring points are arranged at the injection end and the middle of the single sand filling pipe, "pressure measuring point 1, pressure measuring point 2, pressure measuring point 3 and pressure measuring point 4" are connected to the gate seat, and a pressure gauge is installed on the gate seat, such as Figure 2 shown. Figure 3The isolation fluid is softened water, the water blocking working fluid A is sodium silicate solution, the working fluid B is calcium chloride solution, and distilled water is used for pressurization in the pump.

[0109] Test steps:

[0110] (1) Sand filling pipe water drive, test permeability K w The injection rate was 1.0 mL / min. After the water drive was stable, the pressure values ​​at both ends of the injection and production were recorded. The pressure difference △P was obtained by subtraction. The length L of the sand filling pipe was measured, and the permeability was calculated according to the Darcy formula.

[0111] (2) Use a hand pump to inject water plugging working fluid from the production end of the sand filling pipe according to the content of various water plugging working fluids and wait for solidification.

[0112] Water plugging working fluid includes the following two types:

[0113] 1) Inorganic gel / polymer fine powder combination

[0114] Inject reagents from the production end (the gate at the injection end is open): the first round, 0.050PV (5.00wt% sodium silicate solution, softened water) + 0.01PV (isolation fluid, softened water); the second round, 0.05PV (10wt% calcium chloride aqueous solution + 3wt% polymer fine powder (200-300 mesh, the same below), softened water) + 0.01PV (isolation fluid, softened water); the third round, 0.050PV (5.00wt% sodium silicate solution, softened water) + 0.01PV (isolation fluid, softened water).

[0115] After waiting for 4 hours to solidify, subsequent water drive (wastewater) is carried out from the injection end.

[0116] Note: PV refers to the injected volume divided by the pore volume, which is used to describe the relationship between the amount of water injected and the pore space of the oil layer.

[0117] 2) Inorganic gel / fly ash / polymer fine powder / resin beads combination

[0118] Inject reagents from the production end (the injection end gate is open): the first round, 0.050PV (5.00wt% calcium chloride solution, softened water) + 0.01PV (spacer fluid, softened water); the second round, 0.05PV (5.00wt% sodium silicate solution + 0.2wt% polymer dry powder + 50wt% fly ash, softened water, fully matured and then added with fly ash) + 0.01PV (spacer fluid, softened water); the third round, 0.05PV (10wt% calcium chloride aqueous solution + 3wt% polymer fine powder) + 0.01PV (isolation liquid, softened water)); the fourth round, 0.050PV (10.00wt% sodium silicate, softened water) + 0.01PV (isolation liquid, softened water); the fifth round, 0.01PV (mixed liquid (epoxy resin liquid (100g epoxy resin + 20g dibutyl ester + 30g ethylenediamine) + polymer solution (200g sewage + 3g polymer fine powder + 30g fly ash)) + 0.01PV (isolation liquid, softened water).

[0119] Wait for 24 hours for subsequent water drive (wastewater) from the injection end.

[0120] (3) Subsequent water drive was carried out from the injection end of the sand filling pipe until the pressure was stable. The injection rate was 1.0 mL / min, and the pressure at each pressure measuring point was recorded.

[0121] 1.2.2 Analysis of the effects of water plugging fluid types and combinations on plugging efficiency

[0122] (1) Inorganic gel / polymer fine powder combination

[0123] The sand filling pipe production end was plugged with "inorganic gel / polymer fine powder" working fluid, and then "constant rate (1.0mL / min)" water flooding was performed until the pressure was stable. The injection pressure, injection pressure gradient and plugging rate data at the end of water flooding are shown in Table 2, and the relationship between injection pressure and injection PV number is shown in Figure 4 .

[0124] Table 2 Plugging rate experimental data

[0125]

[0126] From Table 2 and Figure 4 It can be seen that the permeability of the sand-filled pipe was significantly reduced after plugging with the "inorganic gel / polymer fine powder" working fluid, and the subsequent water flooding injection pressure increased significantly, with a plugging rate of up to 91.2%.

[0127] (2) Inorganic gel / fly ash / polymer powder / resin beads combination

[0128] The sand filling pipe production end is plugged with a combined working fluid of "inorganic gel / fly ash / polymer fine powder / resin beads", and then water flooding is carried out at a constant rate (1 mL / min) until the pressure is stable. The injection pressure, injection pressure gradient and plugging rate data at the end of water flooding are shown in Table 3, and the relationship between injection pressure and injection PV number is shown in Table 3. Figure 5 .

[0129] Table 3 Plugging rate experimental data

[0130]

[0131] From Table 3 and Figure 5 It can be seen that the permeability was significantly reduced after the combined plugging of "inorganic gel / fly ash / polymer fine powder / resin balls" was implemented at the production end of the sand filling pipe, and the subsequent water drive pressure increased significantly. After the breakthrough pressure reached 7MPa, it dropped to 3MPa and stabilized. The plugging rate can reach 99.91%.

[0132] 1.2.3 Evaluation of water plugging and flow diversion effects

[0133] The experimental equipment for evaluating the water blocking and flow diversion effect of the parallel sand-filled pipe model is shown in Figure 3 , wherein the single sand-filled tube is replaced by the parallel sand-filled tube model with the inter-tube fluid bypass function of Example 1 and the ordinary parallel sand-filled tube of Comparative Example 1.

[0134] Experimental steps:

[0135] (1) The parallel sand-filled pipe model with the function of fluid bypassing between pipes of Example 1 was evacuated and saturated with water. The water permeability K was measured. w ;

[0136] (2) The model is saturated with oil, i.e. oil drives water. The produced water volume is collected, i.e. the saturated oil volume, and the oil saturation is calculated;

[0137] (3) Figure 3 The single sand-filled tubes in the embodiment are replaced by the parallel sand-filled tube model with the function of fluid bypass between tubes in Example 1 (see Figure 1 ), in the ordinary parallel sand-filled pipes of comparative example 1, the model is water-flooded to an overall water content of 98±1%, the produced fluid is collected, the oil-water volume is measured, and the water-flooding recovery factor of each sand-filled pipe and the water-flooding recovery factor of the overall model are calculated;

[0138] (4) Water plugging was carried out in the model high permeability sand-filled pipe. The water plugging working fluid was "inorganic gel / polymer fine powder". The formula of the water plugging working fluid was the same as that of the single sand-filled pipe plugging experiment mentioned above;

[0139] (5) After the model is water-blocked, the injection end continues to water drive until the overall water content reaches 98±1%. The produced fluid is collected, the oil and water volume is measured, and the water-blocking recovery factor of each sand-filled pipe and the water-blocking recovery factor of the entire model are calculated.

[0140] The injection speed was 0.6 mL / min, the pressure was recorded every 30 minutes, and the amount of liquid collected was measured once.

[0141] 1.2.4 Effect of water plugging working fluid type and combination on flow diversion and oil increase effect

[0142] (1) Recovery rate

[0143] The parallel sand-filled pipe model with the function of fluid bypassing between pipes of Example 1 and the ordinary parallel sand-filled pipe model of Comparative Example 1 were subjected to water plugging operation after water flooding. The experimental data of the recovery rate are shown in Table 4.

[0144] Table 4 Recovery factor experimental data

[0145]

[0146] As can be seen from Table 4, the water drive recovery rate of the parallel sand-filled pipe model with the function of fluid bypass between pipes provided by the present invention is 47.04%, the recovery rate after water plugging is 58.17%, and the recovery rate increase is 11.13%. From the recovery rate of each sand-filled pipe, the recovery rate of the low permeability layer (low permeability sand-filled pipe) after water plugging is as high as 21.79%, and the liquid flow diversion and oil increase effects are very obvious. Compared with the ordinary parallel sand-filled pipe model, although the recovery rate of this method is lower, it is closer to the actual heterogeneous reservoir situation in the layer, so it has more reference value.

[0147] (2) Recovery rate

[0148] The parallel sand-filled pipe models with the function of bypassing fluid between pipes of Example 1 and Example 2 were subjected to water plugging after water flooding. The experimental data of the recovery factor are shown in Table 5.

[0149] Table 5 Recovery factor experimental data

[0150]

[0151] It can be seen from Table 5 that as the number of sand-filled pipes in the parallel sand-filled pipe model with the function of fluid bypass between pipes increases, the average permeability of the parallel sand-filled pipes increases, the injection pressure decreases, the effect of expanding the swept volume weakens, and the recovery factor increase decreases.

[0152] (3) The parallel sand-filled pipe models with the function of bypassing fluid between pipes of Example 1 and Example 3 were subjected to water plugging after water flooding. The experimental data of the recovery factor are shown in Table 6.

[0153] Table 6 Recovery factor experimental data

[0154]

[0155] It can be seen from Table 6 that the permeability difference of the parallel sand filling pipe has an impact on the water plugging recovery rate. As the permeability difference increases, the penetration rate of the injected water in the sand filling pipe increases. During the water flooding process, the recovery rate of the high permeability layer increases, and the recovery rate of the low permeability layer decreases. After the oil well is plugged with water, as the difference increases, the recovery rate of the low permeability layer increases, and the effect of expanding the swept volume is significant.

[0156] (4) Dynamic features

[0157] During the experiment, the relationship between the injection pressure, water content, recovery rate and diversion rate of the parallel sand-filled pipe model with the function of fluid bypass between pipes prepared in Example 1 and the injection PV number is shown in Figure 1. Figures 6 to 9 .

[0158] from Figures 6 to 9 It can be seen that the injection pressure of the parallel sand-filled pipe model with the function of fluid bypass between pipes during water flooding shows an overall "reducing-stabilizing" trend, the recovery rate and water content show an increasing trend, the diversion rate of the high permeability layer (high permeability sand-filled pipe) gradually increases, and the low permeability layer (low permeability sand-filled pipe) gradually decreases. After water plugging, the liquid absorption capacity of the high permeability layer in the model is greatly reduced. Under the "constant rate" injection method, the injection pressure is greatly increased, the suction pressure difference increases, the liquid absorption amount of the low permeability layer increases, the swept volume increases, and the recovery rate increases.

[0159] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention rather than all the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A parallel sand-filled pipe model with the function of fluid bypass between pipes, characterized in that: include: A plurality of sand-filled pipes and interlayer connecting pipes (4) with different permeabilities; each sand-filled pipe comprises a sand-filled pipe body (1) with inter-tube interconnecting holes (3) and a filler (2) filled in the sand-filled pipe body (1); The plurality of sand-filled tubes with different permeabilities are connected in parallel by interlayer connecting tubes (4) through inter-tube interconnecting holes (3).

2. The parallel sand filling tube model according to claim 1, characterized in that: The number of the sand-filled tubes is 2 to 6; the inner diameter of each sand-filled tube body (1) with inter-tube interconnecting holes (3) is 3 to 6 cm, and the tube length is 30 to 120 cm.

3. The parallel sand filling tube model according to claim 1, characterized in that: The filler (2) comprises mineral particles, or mineral particles and a binder; the binder comprises an epoxy resin binder.

4. The parallel sand-filled tube model according to claim 3, characterized in that: The mineral particles include one or more of quartz sand, albite, potassium feldspar, dolomite, calcite, montmorillonite, illite, chlorite, kaolinite and crushed oil sand from natural core of oil field.

5. The parallel sand-filled tube model according to claim 1, characterized in that: The length of the interlayer connecting tube (4) is 10 to 30 cm.

6. The method for preparing the parallel sand-filled tube model with the function of fluid bypassing between tubes according to any one of claims 1 to 5, characterized in that: The following steps are involved: According to the vertical permeability of the target oil reservoir, the stratification of the target oil reservoir and the average permeability of each reservoir are obtained, and according to the stratification and the average permeability of each reservoir, the number of sand filling pipes and the permeability of each sand filling pipe are determined, the number of stratification is the number of sand filling pipes in the parallel sand filling pipe model, and the average permeability of each reservoir layer corresponds to the permeability of each sand filling pipe; The sand-filled pipes are connected in sequence by using interlayer connecting pipes (4) through inter-pipe interconnecting holes (3) according to the average permeability of each reservoir layer, thereby obtaining a parallel sand-filled pipe model with the function of fluid bypassing between pipes.

7. The preparation method according to claim 6, characterized in that: The permeability of each sand-filled pipe is formed by the filler filled in the sand-filled pipe, and the filling is: filling the filler (2) in the pipe body (1) of each sand-filled pipe, and by adjusting the type, particle size and compaction degree of the filler (2) in the pipe body (1) of each sand-filled pipe, a sand-filled pipe corresponding to the average permeability of each reservoir layer is obtained.

8. The preparation method according to claim 7, characterized in that: Before filling the filler (2), the method further comprises: roughening the inner surface of the sand-filled tube body (1); the roughening treatment comprises: pickling the inner surface of the sand-filled tube body (1), coating it with an adhesive, and then filling it with quartz sand, and after the adhesive is cured, removing excess quartz sand from the tube.

9. Application of the parallel sand-filled tube model with inter-tube fluid bypass function as described in any one of claims 1 to 5 or the parallel sand-filled tube model with inter-tube fluid bypass function prepared by the preparation method described in any one of claims 6 to 8 in simulating water-driven oil seepage in heterogeneous reservoirs within a layer.

10. A method for simulating water-driven oil seepage in a heterogeneous reservoir, characterized in that: The following steps are involved: Inject water into the injection end of the parallel sand-filled pipe model with the function of fluid bypass between pipes, perform the first water drive until the water content of the parallel sand-filled pipe model as a whole reaches 98±1%, collect the oil production of each sand-filled pipe, and obtain the water drive recovery factor of each sand-filled pipe and the water drive recovery factor of the parallel sand-filled pipe model as a whole; After the first water drive to the water content of the whole parallel sand-filled pipe model is 98±1%, a water plugging working fluid is injected into the production end of the sand-filled pipe with the highest permeability in the parallel sand-filled pipe model, and then water is injected from the injection end of the parallel sand-filled pipe model to perform a second water drive to the water content of the whole parallel sand-filled pipe model is 98±1%, and the produced oil volume of each sand-filled pipe is collected to obtain the water plugging recovery rate of each sand-filled pipe and the water plugging recovery rate of the whole parallel sand-filled pipe model; The parallel sand-filled tube model with the function of fluid bypassing between tubes is the parallel sand-filled tube model with the function of fluid bypassing between tubes as described in any one of claims 1 to 5 or the parallel sand-filled tube model with the function of fluid bypassing between tubes prepared by the preparation method described in any one of claims 6 to 8.

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