A method of asynchronous water injection in ultra-low permeability reservoirs

Abstract

The present invention relates to the technical field of oil and gas development in unconventional oil reservoirs, and specifically to a stratified asynchronous water injection method for ultra-low permeability oil reservoirs, comprising the following steps: step one, determining the asynchronous water injection development strata by static division and dynamic division; step two, normalizing the multiphase seepage characteristics of the cores of different strata in the coring wells in the well area according to the seepage mechanism research, combining the imbibition characteristics and the results of the start-up pressure gradient experiment, clarifying the stratum oil displacement mechanism; step three, determining the asynchronous water injection development mode and water injection parameters of different strata on the basis of stratum division and oil displacement mechanism research. The present invention effectively solves the problems of low water injection utilization rate, failure to inject, failure to produce, and poor water injection development benefits in ultra-low permeability oil reservoirs, and can enable oil wells to produce continuously without stopping the wells when the layers are stopped, can give full play to the imbibition effect, maintain good formation energy, improve the single well production capacity, improve the water injection development effect, and achieve economical and efficient development of ultra-low permeability oil reservoirs.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas development in unconventional oil reservoirs, and in particular to a stratified asynchronous water injection method for ultra-low permeability oil reservoirs. Background Art

[0002] Low permeability reservoirs are reservoirs with low matrix permeability. It is a relative concept and has no fixed and unified standard boundary. It changes according to the resource status and technical and economic conditions in different regions and periods. According to the average permeability of the oil layer, my country divides low permeability reservoirs into three types: general low permeability reservoirs, ultra-low permeability reservoirs, and ultra-low permeability reservoirs. Ultra-low permeability reservoirs have geological characteristics such as low permeability, dense formations, strong heterogeneity, small pore throats, poor physical properties, and large seepage resistance.

[0003] Asynchronous injection and production means shutting down the oil well when the water injection well is injecting water to prevent the injected water from flowing along the cracks. Under the dual effects of water injection pressure and capillary pressure, the injected water enters the oil-bearing pores in the deeper part of the matrix rock. After the formation pressure is restored, the water injection well is stopped to balance the pressure field and redistribute the oil and water in the whole area. After a period of oil and water separation, the oil well resumes production. The pressure difference between the cracks and the matrix rock accelerates the oil drainage effect of capillary imbibition. Under the displacement pressure difference and capillary imbibition, more injected water entering the matrix rock is retained, thereby replacing an equal amount of crude oil into the fracture system. Its essence is to give full play to the favorable factors of the fracture system as a water supply and oil flow channel, and use the displacement pressure difference, matrix rock compression and expansion, capillary imbibition, etc. to promote crude oil to flow from the matrix rock to the fracture system, thereby expanding the injection swept volume of the matrix rock and improving the oil recovery efficiency.

[0004] For ultra-low permeability reservoirs, during the development process, the greater the drop in formation pressure, the greater the reduction in porosity, the deterioration of oil layer properties, the greater the reduction in permeability, the increase in seepage resistance, and ultimately the acceleration of production decline. Moreover, when the formation pressure drops below the saturation pressure, the formation crude oil degasses, which increases the viscosity of the crude oil and reduces the seepage capacity, resulting in a decrease in oil production from the producing wells, or even no oil production. After depressurization, due to the differences in reservoir properties near the oil wells, regional deficits will occur. The deficits in sections with better porosity and permeability are large, while the deficits in sections with poorer porosity and permeability are relatively small. After water injection, the water line will be unevenly advanced. Sections with large deficits will attract more water flow, and areas with small deficits will have less water flow. It is difficult for oil to be displaced, and dead oil zones will be formed for a long time.

[0005] Experts at home and abroad have conducted a lot of research on this type of reservoir and put forward some new ideas and new technical methods, such as advanced water injection, synchronous water injection, single well huff and puff, cyclic water injection, etc. The application of these new methods has improved the development effect of ultra-low permeability reservoirs to a certain extent, but the overall development effect is still not ideal and the economic benefits are poor. Summary of the invention

[0006] The purpose of the present invention is to provide a stratified asynchronous water injection method for ultra-low permeability reservoirs, so as to solve the problems of low water injection utilization rate and poor water injection development efficiency under the existing water injection development mode.

[0007] To achieve the above purpose, the ultra-low permeability reservoir stratified asynchronous water injection method of the present invention adopts the following technical scheme:

[0008] A method for asynchronous water injection in ultra-low permeability reservoirs comprising the following steps:

[0009] Step 1: statically and dynamically divide the strata for asynchronous water injection development, and determine the strata for asynchronous water injection development based on the division results;

[0010] Step 2: Based on the study of the seepage mechanism of cores from different layers in the coring wells in the well area, the multiphase seepage characteristics of cores from different single layers are normalized, and the oil displacement mechanism of the layers is clarified by combining the imbibition characteristics and the results of the start-up pressure gradient experiment, that is, whether the oil displacement mechanism is mainly displacement, the combined effect of displacement and imbibition, or the imbibition effect;

[0011] Step three: Based on the stratum division and oil recovery mechanism research, determine the asynchronous water injection development method and injection parameters for different strata.

[0012] The beneficial effect of the above technical scheme is that: the present invention determines the asynchronous water injection development layer system by statically and dynamically dividing the layer system for asynchronous water injection development, and then normalizes the multiphase seepage characteristics of the cores of different layer systems according to the seepage mechanism research of the core wells in the well area, and combines the imbibition characteristics and the start-up pressure gradient experiment results to clarify the layer system oil recovery mechanism, that is, whether the oil recovery mechanism is mainly displacement, the combined effect of displacement and imbibition, or the main effect of imbibition. Finally, on the basis of the layer system division and the oil recovery mechanism research, the asynchronous water injection development mode and water injection parameters of different layer systems are determined. The present invention adopts a stratified asynchronous water injection method to effectively solve the problems of low water injection utilization rate, failure to inject, failure to produce and poor water injection development efficiency in ultra-low permeability oil reservoirs, in view of the differences in reservoir properties of target blocks, differences in oil recovery mechanisms of different strata and the important role of imbibition in the development process of ultra-low permeability oil reservoirs. The method can enable oil wells to produce continuously without stopping the layers or the wells, can give full play to the imbibition effect, maintain good formation energy, increase the production capacity of a single well, improve the water injection development effect, and achieve economical and efficient development of ultra-low permeability oil reservoirs.

[0013] Furthermore, in order to facilitate dynamic division, in step one, production data and test data are applied, and reservoir engineering methods are used to perform dynamic analysis of oil and water wells, and dynamically divide the strata for asynchronous water injection development.

[0014] Furthermore, in order to ensure the comprehensiveness of the production data used, the production data includes the production of fluid, oil, water, oil pressure, casing pressure of the oil well, as well as the injection volume and pressure of the water well, and the water content variation pattern of the single layer.

[0015] Furthermore, the test data includes a liquid production profile and a water absorption profile.

[0016] Furthermore, in order to facilitate static division, in step one, the strata for asynchronous water injection development are statically divided based on static data, which include differences in oil layer characteristics, reserves of a single development stratum, barriers and pressure-temperature systems between strata, and physical properties of crude oil.

[0017] Furthermore, in order to ensure that the data is comprehensive, the oil layer characteristics include sedimentary conditions, sedimentary environment, reservoir lithology, permeability, geometry and distribution of major oil sand bodies.

[0018] Furthermore, in order to facilitate data acquisition and ensure data accuracy, before proceeding to step two, the cores of the coring wells in the well area are first returned, sampled, and indoor experiments are conducted to test the multiphase seepage characteristics, imbibition characteristics, and start-up pressure gradient characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 In the embodiment of the method of the present invention, 3 3 Isopach map of layers and sand bodies;

[0020] Figure 2 In the embodiment of the method of the present invention, 3 2 Isopach map of layers and sand bodies;

[0021] Figure 3 In the embodiment of the method of the present invention, 3 1 Isopach map of layers and sand bodies;

[0022] Figure 4 This is a sand body connectivity diagram of the 2-28-5 well group in the embodiment of the method of the present invention;

[0023] Figure 5 This is a sand body connectivity diagram of the 2-28-8 well group in the embodiment of the method of the present invention;

[0024] Figure 6 In the embodiment of the method of the present invention, 3 1Contour map of effective oil layer thickness;

[0025] Figure 7 for Figure 6 A partial enlarged view of the

[0026] Figure 8 In the embodiment of the method of the present invention, 3 2 Contour map of effective oil layer thickness;

[0027] Fig. 9 for Figure 8 A partial enlarged view of the

[0028] Fig.10 This is a cross-sectional view of water absorption of well 2-28-5 in the embodiment of the method of the present invention;

[0029] Fig.11 This is a cross-sectional view of water absorption of well 2-28-8 in the embodiment of the method of the present invention;

[0030] Fig.12 This is the injection-production connection diagram of the 2-28-5 well in the embodiment of the method of the present invention;

[0031] Fig.13 This is the injection-production connection diagram of the 2-28-8 well in the embodiment of the method of the present invention;

[0032] Fig.14 This is a morphological diagram of the oil-water relative permeability curve of the main channel reservoir in the study area in the embodiment of the method of the present invention;

[0033] Fig.15 This is a morphological diagram of the oil-water relative permeability curve of the secondary river reservoir in the embodiment of the method of the present invention;

[0034] Fig.16 This is a graph showing the relationship between the imbibition time and the imbibition recovery rate in an embodiment of the method of the present invention;

[0035] Fig.17 This is a graph showing the relationship between the imbibition time and the imbibition speed of different samples in the embodiment of the method of the present invention;

[0036] Fig.18 Graph showing the relationship between gas permeability and starting pressure gradient of 3 reservoirs in the embodiment of the method of the present invention;

[0037] Fig.19 It is a schematic diagram of the layered asynchronous water injection in the embodiment of the method of the present invention. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0039] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0040] It should be noted that relational terms such as "first" and "second" that may appear are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, terms such as "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device that includes a series of elements includes not only those elements, but also other elements that are not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by a sentence such as "comprises a..." does not exclude the existence of other identical elements in the process, method, article or device that includes the element.

[0041] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0042] An embodiment of the stratified asynchronous water injection method for ultra-low permeability oil reservoirs of the present invention: Taking a low permeability block as the research object, the oil reservoir type is an ultra-low pressure, low porosity and ultra-low permeability lithologic oil reservoir, and the stratified asynchronous water injection method for the oil reservoir specifically comprises the following steps:

[0043] Step 1: statically and dynamically divide the strata for asynchronous water injection development, and determine the strata for asynchronous water injection development based on the division results.

[0044] (I) Static division of the strata for asynchronous water injection development based on static data.

[0045] Static data include differences in oil layer characteristics, reserves of a single development layer system, interlayers and pressure-temperature systems between layers, and crude oil properties. Among them, oil layer characteristics include: sedimentary conditions, sedimentary environment, reservoir lithology, permeability, heterogeneity within the layer, geometric morphology and distribution of major oil sand bodies; reserves of a single development layer system means that an independent development layer system needs to have a certain amount of reserves to ensure a certain oil production rate and a certain stable production time during the development process, and to achieve better economic indicators; interlayers are used to ensure that there is no crosstalk and interference between layers during water injection development; crude oil properties mainly consider density, viscosity, freezing point and solubility.

[0046] (1) Division of oil-bearing layer groups and thickness of strata and sand bodies in the target area

[0047] The average thickness of the main target layer of the target oil group is 145m, with shallow burial depth of 550m. 1 , 3 2 , 3 3 Three sub-oil groups, of which the main oil-bearing layer is 3 3 The average thickness of the stratum is 79.7m. 3 It is divided into three small layers, namely 3 3 1 , 3 3 2 , 3 3 3 The average thickness of the strata are 27.5m, 26.1m and 26.1m respectively. The thickness of the sand layers are 11.3m, 12m and 7m respectively (as shown in Table 1).

[0048] Table 1. Division of sublayers in the three oil-bearing layers and thickness of strata and sand bodies

[0049]

[0050] (2) Sedimentary characteristics and sand body distribution characteristics

[0051] The three reservoir groups in the target area belong to the braided river delta sedimentary system, and the subfacies is the braided river delta front sedimentary system. The main microfacies types are underwater distributary channels, estuary bars, and distributary bays. The favorable sedimentary microfacies are mainly underwater distributary channels and estuary bars (as shown in Table 2).

[0052] Table 2 Sedimentary phase classification

[0053]

[0054] The sand bodies in the target area are distributed in the northeast direction. 3 1 , 3 3 2 , 3 33 Three small layers. 3 3 3 The underwater distributary channel is in the development stage. 3 2 The underwater distributary channel is at its peak. 3 1 The underwater distributary channel is in the energy return period. 3 1 , 3 3 2 During this period, the water energy is the strongest and the sand body is best developed.

[0055] ①3 3 3 Sand body distribution

[0056] The sand bodies of this sand layer group are mainly provided by the source in the southeast and northeast of the study area. Two sand body branches and an independent sand body can be seen in the area. The developed branch sand bodies extend for a long distance and are distributed in strips, but they do not run through the entire study area. The thickness center of the sand body is mainly distributed in the main branch sand body, and the thickness is uneven (such as Figure 1 ). The width of the sand body is about 3.1 to 5.2 m, the thickness of the sand body is 1.8 to 18.9 m, and the average thickness of the sand body is 8.6 m. The effective sand body thickness is 0.3 to 8.9 m, with an average thickness of 3.8 m. During this period, two small-scale braided river delta front underwater distributary channels developed in the block from the southeast and northeast to the north, extending to the middle of the study area in a strip-like shape, and transitioned to the flank of the channel in the middle.

[0057] ②3 3 2 Sand body distribution

[0058] The sand bodies of this sand layer group are mainly provided by the southeast and eastern sources of the study area. It can be seen that there are 4 sand body branches developed in the area, extending from the southeast and east to the north. The developed branch sand bodies extend for a long distance and run through the entire study area. The overall thickness of the sand body is 3 3 3 The thickness of the sand body is mainly distributed in the middle (such as Figure 2 The western provenance is relatively isolated, the sand body is relatively thin, and generally thins toward the northwest, is continuous in large areas, and has uneven thickness. The sedimentary facies of the block during this period generally inherited 3 3 3 Development characteristics: underwater distributary channels of the braided river delta front develop from southeast to northwest and converge in the middle, while two other underwater distributary channels of a certain scale develop in the east and transition to sand bodies on the flanks of the channel towards the middle.

[0059] ③3 3 1 Sand body distribution

[0060] The sand bodies of this sand layer group are mainly provided by the southeast and northeastern sources of the study area. There are 4 sand body branches in the area, extending to the north and northeast respectively. The 4 main branch sand bodies fork and intersect at the front end. The single sand body is distributed in a strip shape and tends to converge to the central and northern parts of the study area. 3 2 The section becomes thinner and is thickest at the eastern source and the middle of the exploration area. It presents three high-value areas represented by wells 15, 46, and 55 (e.g. Figure 3 During this period, the sedimentary facies did not change significantly. The block was still dominated by underwater distributary channels at the front of the braided delta. The sand bodies in the southeast and northeast converged in the middle and transitioned to sand bodies on the flanks of the channel.

[0061] (3) Reservoir characteristics

[0062] ① Petrological characteristics

[0063] The three groups in the target area mainly develop lithic feldspar sandstone and feldspar-quartz sandstone, and the rock mineral composition is mainly quartz, followed by feldspar.

[0064] The total content of interstitial materials in the three groups of reservoirs is 12.4%, mainly carbonate, calcite and authigenic clay. Clay minerals are mainly illite and illite-montmorillonite, followed by kaolinite and a small amount of chlorite.

[0065] ②Physical properties

[0066] The average porosity of the three reservoir groups is 12.2%, and the average permeability is 0.76 mD.

[0067] ③Pore structure characteristics

[0068] The reservoir type is porous reservoir, mainly composed of intergranular solution pores.

[0069] Mercury injection data show that the pore throats are uneven, the pore throat radius distribution range is large, the peak value is not obvious, the average maximum pore throat radius is 0.31μm, the average pore throat median pressure is 5.97MPa, and the average median radius is 0.23μm, indicating that the pore throats are fine and the pore structure is complex.

[0070] Through the analysis of the casting thin section image, it can be seen that the average pore radius in this area is a low value for medium pores, and the average throat width is a fine throat. Overall, it belongs to the medium pore-fine to ultra-fine throat type. The pore shape factor is small and the coordination number is low, indicating that the pore structure is complex and the connectivity between pores is poor (as shown in Table 3).

[0071] Table 3 Image analysis 3 3 Pore ​​characteristics and throat width statistics

[0072]

[0073] (4) Temperature and pressure

[0074] The three groups of oil reservoirs are buried at a depth of 350-650m, with an original formation pressure of 2.06MPa (pressure coefficient 0.45-0.6), a saturation pressure of 0.65MPa, and a geothermal gradient of 2.7℃ / 100m.

[0075] (5) Fluid properties

[0076] The density of the crude oil in the 3rd formation is 0.8102 g / cm 3 The formation crude oil has a viscosity of 6.64 mPa·s, a volume coefficient of 1.0295, a freezing point of 25°C, an initial distillation point of 78.5°C to 89°C, and belongs to medium-quality Class I crude oil. The total mineralization of the formation water is 33447.89 to 49692.04 mg / L, the chloride content is 18540.8 mg / L, and the water type is CaCl 2 type.

[0077] After discussing the static data of the block, the present invention is further explained using two well groups (as shown in Table 4).

[0078] Table 4 Correspondence between oil and water wells in well group

[0079]

[0080] (1) Sand body thickness and connectivity

[0081] Two well groups, 9 wells, 3 3 1 , 3 3 2 The sand bodies of the layers are of certain thickness. Overall, the reservoir of the 2-28-5 well group is thicker than that of the 2-28-8 well group. 3 2 The thickness of the sand body is 3 3 1 Layer thickness, 3 3 1 The maximum sand body thickness of the layer is about 11.4m, the minimum is 1m, and the average is 5.57m. 3 2 The maximum sand body thickness of the layer is about 16m, the minimum is 2.6m, and the average is 8.07m (as shown in Table 5). 3 1 Sand body, 3 3 2 Sand body, from the analysis of sand body connectivity, the sand body is well connected (such as Figure 4 , Figure 5 ).

[0082] Table 5 Well group sand thickness statistics

[0083]

[0084] (2) Oil layer distribution characteristics

[0085] The well group has a high overall oil content, with oil spots accounting for more than 65% (as shown in Table 6). The oil layer is thick. Due to the influence of the river channel morphology and the degree of superposition, the development of the small oil layers in each well is different:

[0086] Table 6 Mud logging display table

[0087] hashtag Fluorescence(m) Oil stain(m) Oil spot(m) Oil spot ratio (%) 2-28-1 7.0 100 2-28-2 25.0 100 2-28-7 0.9 18.4 95.4 2-8-3 1.2 4.2 3.5 39.3 2-28-3 14.9 100 2-28-4 0.5 11.0 95.6 2-28-6 6.7 12.6 65.3

[0088] 3 3 1 The thickness of the oil layer is relatively thin as a whole. The oil layer is mainly concentrated in the 2-28-3 well area and the west of the 2-28-7 well. The maximum effective thickness is about 10.5m and the minimum is 0.5m (such as Figure 6 and Figure 7 ).

[0089] 3 3 2 The oil layer has a large distribution range on the plane, which is 3 3 The oil layer is mainly concentrated around the 2-28-7 well area, with a maximum effective thickness of 11m and a minimum of 3m (such as Figure 8 and Fig. 9 ).

[0090] (3) Well group reserves calculation

[0091] By identifying and characterizing the effective reservoirs of each small oil layer in the target area layer by layer, the oil-bearing area of ​​9 wells in two well groups is calculated to be 0.71km 2 , geological reserves of 22.29×10 4 t, and the average single-well controlled reserves are 2.47×10 4 t (as shown in Table 7).

[0092] Table 7 Well area layer asynchronous injection and production well group reserves calculation table

[0093]

[0094] (II) Apply production data and test data, adopt reservoir engineering methods to conduct dynamic analysis of oil and water wells, and dynamically divide the strata developed by asynchronous water injection. Production data include the production of fluid, oil, water, oil pressure, and casing pressure of oil wells, as well as the injection volume and pressure of water wells, and the water content change law of a single layer. Test data include the production profile and water absorption profile.

[0095] (1) Well group production

[0096] There are 7 oil wells in the well group. According to the fracturing method and production situation of the oil wells (as shown in Table 8), the surrounding oil wells 2-28-1 and 2-8-3 were mechanically divided when they were put into production, and 4 wells including 2-28-2, 2-28-3, 2-28-4 and 2-28-6 were repeatedly fractured, which can ensure 3 3 1 , 3 3 2 Both layers were opened by pressure. So far, the daily fluid production of 7 oil wells is 0.7-4m 3 , daily oil production is 0.5-1.8t, cumulative liquid production is 13677m 3 , with a cumulative oil production of 7868.8t. From the 2-28-6 well, the 3rd hole was filled on April 6, 2020. 3 1(2) , the initial oil production after the perforation repair was 1.3t, an increase of 1t compared with the day before the measure; 2-28-3 well was repaired on March 22, 2020 3 2(1) After the repair, the initial daily oil production was 2.2 tons, which was 1.8 tons higher than before the measure. This shows that the well group 3 3 1 , 3 3 2 Each layer has a certain liquid discharge capacity and a certain production capacity.

[0097] Table 8 Oil well production in well group

[0098]

[0099] There are two water injection wells in the well group. According to the analysis of injection conditions (as shown in Table 9), the cumulative water injection volume of the two wells is relatively high. The total water injection volume of well 2-28-5 is 10280.4m 3 The total water injection volume of Well 2-28-8 is 12367.6m 3 , 3 of the two wells 3 1 , 3 3 2 The layers have a certain water absorption capacity. The 2-28-5 well has a certain water absorption capacity. Fig.10 ), the water absorption layer of well 2-28-5 is 3 3 1(3) , 3 3 2 Layer, 3 3 1(2) No water absorption, according to the analysis of the well injection situation in August 2014, the casing layer 3 3 1(2) It also has a certain water absorption capacity, with a total water injection of 2438m 3 , indicating that the 3 3 1 , 3 32 All layers can absorb water; the 2-28-8 well has an initial water absorption profile analysis (such as Fig.11 ), mainly 3 3 1 layer; after the injection, the oil pipe layer also absorbs water. From the analysis of the water injection situation after the reinjection, 3 3 2 Layer Ratio 3 3 1 The layer absorbs water well, which generally shows that the 3 3 1 , 3 3 2 All layers absorb water.

[0100] Table 9 Injection situation of water injection wells in well group

[0101]

[0102] (2) Injection-production connectivity

[0103] 2-28-5 Well Group: Analysis from the injection-production connectivity diagram (e.g. Fig.12 ), 3 3 1 , 3 3 2 The sand bodies in the layers are well connected. Analyzing from the perforation sections of the oil and water wells, the injection-production correspondence rate is high, and stratified asynchronous injection and production are possible. In addition, the corresponding oil wells 2-28-3, 2-28-4, and 2-28-6 have been repeatedly fractured, which can meet the requirement of a certain liquid production in each layer during stratified production.

[0104] 2-28-8 Well Group: Analysis from the injection-production connectivity diagram (e.g. Fig.13 ), 3 3 1 , 3 3 2 The thickness of the sand body is large. From the analysis of the perforation section of the oil and water wells, the injection-production correspondence rate is high. 3 1(1) There is no corresponding water injection well in the layer, and all of them can be injected and produced asynchronously in layers. When the surrounding oil wells 2-28-1 and 2-8-3 were put into production, they were mechanically pressurized. 2-28-2 has been repeatedly fractured, and the repeated fracturing plan for well 2-28-7 has been released. Well 2-28-7 has also been tested in layers, which can meet the requirements of layered production, and each layer has a certain liquid production.

[0105] (3) Effect characteristics

[0106] According to the production curves of oil and water wells and the connection between injection and production, a dynamic analysis was conducted on the 7 oil wells and 2 water injection wells in the well group, combined with the water absorption profile, reservoir physical properties and sand body superposition thickness. The oil wells were all affected to varying degrees. The main reason was that the well group was located in an area with good physical properties and relatively large effective sand body thickness (such as Fig.10 , Fig.11 ), good injection-production connectivity (such as Fig.12 , Fig.13 ), the corresponding cumulative water injection volume of the wells was relatively high, and the chloride ion decreased significantly. In the later period, with the increase of cumulative water injection volume, during the production process, the water content of some wells increased in steps. After state adjustment, the water content decreased, and the overall effect was relatively good. The main effective direction of the single well was (as shown in Table 10).

[0107] Table 10 Statistics of effective directions of oil wells

[0108]

[0109] Step 2: Relocate and sample the cores of the coring wells in the well area, conduct indoor experiments, and test the multiphase seepage characteristics, imbibition characteristics, and starting pressure gradient characteristics. According to the study of the seepage mechanism of the cores of different layers in the coring wells in the well area, the multiphase seepage characteristics of the cores of different single layers are normalized, and the oil displacement mechanism of the layer is clarified by combining the results of the imbibition characteristics and the starting pressure gradient experiment, that is, whether the oil displacement mechanism is mainly displacement, the combined effect of displacement and imbibition, or the main effect of imbibition.

[0110] (1) Characteristics of relative permeability curves of different types of reservoirs

[0111] 3 The main reservoirs are divided into two categories: main channel reservoirs and secondary channel reservoirs. The two types of reservoirs have certain differences in physical properties, petrological characteristics, pore throat structure characteristics, etc., and their phase permeability curves also show different characteristics (as shown in Table 11).

[0112] Table 11 Statistical table of relative permeability curves of oil-water relative permeability experiments in three different types of reservoirs

[0113]

[0114] Main channel reservoir: The average porosity of the samples is 13.16%, the average permeability is 1.01mD, the average bound water saturation is 35.6%, the water saturation at the isotonic point is 54.7%, the oil-water phase permeability at the isotonic point is 0.11, the residual oil saturation is 32.4%, the water phase permeability at the residual oil is 0.568, the average oil recovery efficiency in the water-free period is 18.9%, and the average oil recovery efficiency in the final period is 49.7%. The physical properties of the main channel reservoir samples are relatively good, with the largest average porosity and permeability; the bound water saturation is relatively low, the water relative permeability is the largest when there is residual oil, the two-phase co-seepage zone is the widest, and the oil recovery efficiency is also the highest. It can be seen that the phase permeability curve of the main channel reservoir is a reservoir type with good seepage capacity in the study area 3. This type of reservoir has strong seepage capacity, which is conducive to the flow of oil and water (such as Fig.14 ).

[0115] Secondary channel reservoir: The average porosity of the samples is 10.53%, the average permeability is 0.42mD, the average bound water saturation is 46.9%, the water saturation at the isotonic point is 61.1%, the oil-water phase permeability at the isotonic point is 0.063, the residual oil saturation is 30.4%, the water phase permeability at the residual oil is 0.348, the average oil recovery efficiency in the water-free period is 17.9%, and the average oil recovery efficiency in the final period is 43.0%. The physical properties of the secondary channel reservoir samples are relatively poor, and the average porosity and permeability are small; the bound water saturation is relatively high, the water relative permeability is small when there is residual oil, the two-phase co-seepage zone is narrow, and the oil recovery efficiency is relatively low. The phase permeability curve of the secondary channel reservoir is representative of the reservoir type with poor seepage characteristics of the 3 reservoirs in the study area, and the seepage capacity is weaker than that of the main channel reservoir (such as Fig.15 ).

[0116] (2) Reservoir imbibition characteristics

[0117] The water-injection oil displacement experiment lasted about 10 days, and the infiltration gradually reached a balanced state. The weight of the sample after infiltration was higher than the wet weight after saturation with oil, and the water displacement efficiency was calculated according to the formula (Table 12).

[0118] Table 12 Statistics of oil recovery rate from water absorption flooding

[0119]

[0120]

[0121] The imbibition experiment shows that the direct water imbibition recovery rate of matrix cores is distributed between 15.22% and 38.62%, with an average of 23.2%; the direct water imbibition recovery rate of fracture cores is between 26.72% and 39.52%, with an average of 34.8%. The imbibition recovery rate of fracture reservoirs is higher than that of matrix reservoirs, indicating that the presence of fractures greatly improves the imbibition recovery rate.

[0122] Core direct water imbibition, in the first 50h of natural imbibition, with the change of time, the imbibition recovery increases rapidly, at this time the imbibition displaces more non-wetting phase fluid; after 50h, the imbibition rate gradually weakens, spontaneous imbibition tends to be stable, and finally the wetting phase no longer displaces non-wetting phase fluid (such as Fig.16 In the initial stage of imbibition, the imbibition rate is high and the oil recovery efficiency increases rapidly. As the imbibition time increases, the imbibition rate decreases as a whole and the increase in oil recovery efficiency gradually slows down until the imbibition stops (e.g. Fig.17 ).

[0123] (3) Start-up pressure gradient characteristics

[0124] The formation temperature of the 3 oil layer is 34.5℃, the viscosity of the formation crude oil is 3.5mPa.s, and the salinity of the formation water is 35.1. Therefore, the start-up pressure gradient measurement of 30 cores was carried out at room temperature using 34600ppm standard brine and formation crude oil with a viscosity of 3.5mPa.s at 34.5℃. The experimental test results are as follows (see Table 13).

[0125] Table 13 Regional startup pressure gradient statistics

[0126]

[0127]

[0128] The porosity of the rock samples is mainly distributed in the range of 7.35-16.74%, with an average of 10.68%. The gas permeability distribution range is 0.202-2.474mD, with an average of 0.878mD. The starting pressure gradient is mainly distributed in the range of 0.019-0.436MPa / m, with an average starting pressure of 0.087MPa / m.

[0129] The gas permeability of oilfield rock samples has a good power relationship with the starting pressure gradient. The smaller the permeability of the rock sample, the higher the starting pressure (e.g. Fig.18 ); when the permeability K is less than 0.3mD, the start-up pressure is relatively large; when the permeability K is distributed between 0.3mD and 0.7mD, the start-up pressure rises slowly as the permeability decreases; when the permeability K is greater than 0.7mD, the start-up pressure gradient of the reservoir is relatively small; that is, the start-up pressure gradient of the main channel reservoir is smaller than that of the secondary channel reservoir. This phenomenon shows that in the development of low permeability reservoirs, when the actual permeability decreases to a certain value, the start-up pressure gradient will have a huge impact on the development of the reservoir. That is, when the permeability K is less than 0.3mD, the oil displacement mechanism is mainly based on imbibition, when the permeability K is distributed between 0.3mD and 0.7mD, the oil displacement mechanism is the combined effect of displacement and imbibition, and when the permeability K is greater than 0.7mD, the oil displacement mechanism is mainly based on displacement.

[0130] Step three: Based on the stratum division and oil recovery mechanism research, determine the asynchronous water injection development method and injection parameters for different strata.

[0131] Using reservoir engineering methods, field test methods and numerical simulation methods, the injection and production method of the oil field stratified asynchronous injection and production well group was finally determined as follows: first produce 3 3 2 Layer, reproduce 3 3 1 Layer; the layer change condition is: the daily oil production of the well group is less than 3m 3 (average single well less than 0.6t) when the layer is changed (such as Fig.19 ); The single layer water injection volume during asynchronous injection and production is: 9m 3 / d.

[0132] The achievements of the above implementation cases are: (1) effectively solving the problems of high water injection pressure, failure to inject water and low water injection utilization rate in the process of water injection development of ultra-low permeability oil reservoirs; (2) compared with layer-by-layer up-flow production, the oil production rate increased by 21.06%, the water injection volume was saved by 28.19%, and the predicted final recovery rate increased by 4.7%. Ultra-low permeability oil reservoirs adopt stratified asynchronous water injection according to different oil displacement mechanisms, which effectively improves the economic benefits of ultra-low permeability oil reservoirs and provides reference experience for the economic and efficient development of similar oil reservoirs.

[0133] In other embodiments of the stratified asynchronous water injection method for ultra-low permeability reservoirs: according to actual conditions, data such as multiphase seepage characteristics, imbibition characteristics and start-up pressure gradient characteristics may be readily available without the need for experiments.

[0134] In other embodiments of the stratified asynchronous water injection method for ultra-low permeability reservoirs: reservoir characteristics, static data, test data, production data, etc. may also include more parameters, including more dynamically divided reference data.

[0135] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. The patent protection scope of the present invention shall be based on the claims. All equivalent structural changes made using the contents of the description and drawings of the present invention should also be included in the protection scope of the present invention.

Claims

1. A stratified asynchronous water injection method for ultra-low permeability reservoirs, characterized in that: The following steps are involved: Step 1: statically and dynamically divide the strata developed by asynchronous water injection, apply production data and test data, adopt reservoir engineering methods to perform dynamic analysis of oil and water wells, and dynamically divide the strata developed by asynchronous water injection. The test data includes liquid production profile and water absorption profile. The strata developed by asynchronous water injection are statically divided according to static data. The static data includes differences in oil layer characteristics, reserves of a single development stratum, interlayers and pressure-temperature systems between strata, and crude oil properties. According to the division results, the strata developed by asynchronous water injection are determined. Before carrying out step 2, the cores of the coring wells in the well area are firstly returned, sampled, and indoor experiments are carried out to test the multiphase seepage characteristics, imbibition characteristics, and start-up pressure gradient characteristics. Step 2: According to the study of the seepage mechanism of cores of different layers in the coring wells in the well area, the multiphase seepage characteristics of cores of different single layers are normalized, and the oil displacement mechanism of the layer is clarified in combination with the imbibition characteristics and the results of the start-up pressure gradient experiment, that is, whether the oil displacement mechanism is mainly displacement, displacement and imbibition, or imbibition. When the permeability K<0.3mD, the oil displacement mechanism is mainly imbibition. When the permeability K is distributed between 0.3mD and 0.7mD, the oil displacement mechanism is the joint action of displacement and imbibition. When the permeability K>0.7mD, the oil displacement mechanism is mainly displacement. Step three: Based on the stratum division and oil recovery mechanism research, determine the asynchronous water injection development method and injection parameters for different strata.

2. The method for asynchronous water injection in ultra-low permeability reservoirs according to claim 1, characterized in that: The production data include the produced fluid, oil, water, oil pressure, casing pressure of the oil well, the injection volume and pressure of the water well, and the water content variation law of the single layer.

3. The method for stratified asynchronous water injection in ultra-low permeability reservoirs according to claim 1 or 2, characterized in that: The oil layer characteristics include deposition conditions, deposition environment, reservoir lithology, permeability, geometry and distribution of major oil sand bodies.

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