A method and system for evaluating heavy oil water flooded layers by neutron gamma identification

Abstract

The application discloses a kind of evaluation method and system for identifying heavy oil water flooded zone by neutron gamma, and the method takes neutron gamma curve and resistivity value as the basis for judging water flooded zone, wherein neutron gamma curve is the preliminary judgment standard, and resistivity value is further, more detailed basis for judgment.The evaluation method finds the change rule of neutron gamma curve after heavy oil reservoir water flooding, combines with heavy oil reservoir structure, studies the reason of neutron gamma change from nuclear physics logging principle, to form the interpretation method of neutron gamma logging for judging heavy oil water flooding.As neutron gamma logging can be implemented in cased well, therefore, neutron gamma curve is measured in new well after cementing, neutron gamma curve of production well is measured again after a certain production cycle, and by using the change of neutron gamma curve at different times, heavy oil water flooding condition can be judged, and remaining oil saturation monitoring of heavy oil block can be realized.

Description

Technical Field

[0001] This invention belongs to the field of petroleum development technology, specifically relating to an evaluation method and system for identifying water-flooded layers in heavy oil using neutron gamma spectroscopy. Background Technology

[0002] As heavy oil thermal recovery technology matures, the scale of heavy oil reservoir development has increased year by year. After years of development, the water flooding degree in heavy oil blocks has increased, affecting the recovery rate of heavy oil fields. Due to the limited displacement radius of heavy oil steam huff and puff development methods, sidetracking horizontal wells and infill wells are generally adopted to improve the recovery rate. Monitoring the water flooding status and remaining oil saturation of heavy oil reservoirs can better study the changes in reservoir parameters in thermally recovered reservoirs, understand the distribution of remaining oil in the middle and late stages of heavy oil development, and provide relevant scientific basis for the deployment of infill wells and the formulation of new well production development plans, as well as for improving the recovery rate of heavy oil reservoirs.

[0003] Currently used techniques for determining water flooding in heavy oil reservoirs mainly include the following logging methods:

[0004] (1) Pulsed neutron logging methods, including carbon-oxygen ratio logging, PNN logging and neutron lifetime, have the following drawbacks: First, the price of neutron tubes is expensive, resulting in high instrument costs and high logging prices; second, the measurement speed is slow and the efficiency is low; third, the detection depth is shallow and is greatly affected by the wellbore environment; and fourth, it can only be measured independently and is difficult to measure in conjunction with other instruments.

[0005] (2) Wide-range logging technology utilizes neutron gamma spectroscopy to calculate parameters such as saturation and formation relative density by analyzing porosity, chloride function, and elemental analysis, thereby monitoring reservoir changes. However, this technology has several drawbacks: firstly, the instruments are expensive, costing around 5-7 million RMB per instrument; secondly, it can only measure independently, although it can currently be connected to domestically produced surface instruments, it cannot perform simultaneous measurements with other instruments.

[0006] (3) Through-casing resistivity logging technology. This technology adopts a point measurement method and is the least efficient of all saturation logging technologies. Due to the complex instrument structure, it has a high failure rate downhole and is greatly affected by the well environment, and is highly sensitive to the test environment.

[0007] Therefore, the above technologies all have different limitations, which restrict the large-scale promotion and application of residual oil saturation monitoring technology in terms of service price, construction efficiency and accuracy. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide an evaluation method and system for identifying heavy oil water-flooded layers using neutron gamma spectroscopy, so as to solve the problems of slow measurement speed, high cost and low efficiency of existing methods for identifying heavy oil water-flooded layers.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A method for evaluating heavy oil water-flooded layers using neutron gamma spectroscopy includes the following steps:

[0011] Step 1: Subtract the standard deviation of the neutron gamma curve of the original well from the standard deviation of the neutron gamma curve of the corresponding side-drilled well to obtain the difference value. If the difference value is less than 40% of the standard deviation of the neutron gamma curve of the side-drilled well, then proceed to Step 2; otherwise, the side-drilled well does not meet the mining requirements.

[0012] Step 2: Compare the deep lateral resistivity of the original well with the apparent formation resistivity of the corresponding side-drilled well. If the difference between the two is less than 12Ω, the side-drilled well is considered to meet the mining requirements; otherwise, the side-drilled well does not meet the mining requirements.

[0013] A further improvement of the present invention is that:

[0014] Preferably, the formula for calculating the apparent formation resistivity Rta of the side-drilled well is:

[0015]

[0016] In the formula, ΔD is the calculated value, and its calculation formula is:

[0017] ΔD = 1.67 × (NGRN - NGRBJN) 3 +2.14×(NGRN-NGRBJN) 2 -0.388×(NGRN-NGRBJN)+0.018 (6)

[0018] Wherein, NGRN and NGRBJN are the normalized values ​​of the neutron gamma curves of the original well and the sidetracked well, respectively, and are known values; RtBJ is the deep lateral resistivity of the sidetracked well as a function of time, and is a known value; Rtmin and Rtmax are the normalized minimum and maximum values ​​of the deep lateral resistivity of the original well, respectively.

[0019] The preferred standardized formula for the neutron gamma curves of the original well and the sidetracked well is:

[0020]

[0021]

[0022] Wherein, NGR is the neutron gamma value of the original wellbore over time, and NGRBJ is the neutron gamma value of the sidetracked well over time; NGR min The minimum value among the time-varying neutron gamma values ​​of the original well, NGR max The maximum value of the neutron gamma value of the original well over time, NGRBJ minThe minimum neutron gamma value of the side-drilled well over time, NGRBJ max This represents the minimum neutron gamma value of the side-drilled well over time.

[0023] Preferably, the formula for calculating the standardized value of lateral resistivity RtBJN at the side-drilled depth is as follows:

[0024]

[0025] Where RtBJ is the deep lateral resistivity of the side-drilled well as a function of time; Rt min Rt represents the minimum value of the deep lateral resistivity of the original well over time. max This represents the maximum value of the deep lateral resistivity of the original well over time.

[0026] Preferably, the original well is a well from which exhaust gas has been extracted, and the side-drilled well is a well next to the original well that is being re-extracted.

[0027] Preferably, the side-drilled well corresponding to the original well is treated with high-temperature steam.

[0028] Preferably, during 9 steam injection cycles, the oil saturation is less than 40.0% within a 30m radius of the side-drilled well.

[0029] Preferably, in step 1, if the difference between the standard deviation of the neutron gamma curve of the original well and the standard deviation of the neutron gamma curve of the side-drilled well is less than 30%, then step 2 is executed.

[0030] Preferably, in step 2, when the difference between the deep lateral resistivity of the original well and the apparent formation resistivity of the corresponding side-drilled well is 1.5 to 3 Ω, the side-drilled well is of exploitable value.

[0031] A neutron gamma identification system for heavy oil water-flooded layers includes:

[0032] The standard deviation judgment unit is used to compare the standard deviation of the neutron gamma curve of the original well with the standard deviation of the neutron gamma curve of the corresponding side-drilled well. When the difference between the standard deviation of the neutron gamma curve of the original well and the standard deviation of the neutron gamma curve of the side-drilled well is less than 40%, the resistivity judgment unit is used for judgment; otherwise, the side-drilled well does not meet the mining requirements.

[0033] The resistivity judgment unit is used to compare the deep lateral resistivity of the original well with the apparent formation resistivity of the corresponding side-drilled well. When the difference between the two is less than 12Ω, the side-drilled well is considered to meet the mining requirements; otherwise, the side-drilled well does not meet the mining requirements.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] This invention discloses a method for evaluating heavy oil water-flooded layers using neutron gamma logging. This method uses neutron gamma curves and resistivity values ​​as the basis for judging water-flooded layers, with the neutron gamma curve serving as a preliminary criterion and the resistivity value as a further, more refined criterion. This evaluation method identifies the changing patterns of the neutron gamma curve after water flooding in heavy oil reservoirs, combines this with the heavy oil reservoir structure, and studies the causes of these changes from the perspective of nuclear physics logging principles, thus forming an interpretation method for judging heavy oil water flooding using neutron gamma logging. Since neutron gamma logging can be implemented in casing wells, the neutron gamma curve can be measured in newly cemented wells, and then measured again in production wells after a certain production cycle. By utilizing the changes in the neutron gamma curve over different times, the water flooding status of heavy oil can be determined, enabling the monitoring of the remaining oil saturation in heavy oil blocks. This evaluation method is a low-cost, fast-speed, deep-penetrating method that can be combined with other logging instruments to measure and interpret residual oil saturation. Through theoretical research and field test data analysis, it is the first time that the neutron gamma logging time-lapse method is proposed to solve the problem of monitoring water flooding in heavy oil reservoirs behind casing.

[0036] This method has excellent application prospects. Firstly, heavy oil reserves are vast, and water flooding is severe, indicating a large market potential for saturation monitoring. Secondly, compared to existing saturation monitoring technologies, this method offers advantages such as lower instrument costs, higher construction efficiency, and deeper detection depth. Due to its low cost and high efficiency, compared to other logging methods such as carbon-oxygen ratio logging, this method is cheaper and more readily accepted by oilfields. Thirdly, it can be used in conjunction with current sonic density measurement, allowing for simultaneous measurement of cementing quality changes during a single well run, providing value-added services. Compared to previous residual oil saturation monitoring technologies, this method can meet the needs of heavy oil field water-flooded layer identification, enabling the large-scale application of residual oil saturation monitoring technology.

[0037] Furthermore, this invention targets sidetracking wells after high-temperature and high-pressure steam huff and puff tests, because the high-temperature and high-pressure steam huff and puff development method significantly modifies the lithological and physical properties of heavy oil reservoirs, and the neutron gamma logging curve can well reflect the changes in lithological and physical properties of heavy oil after water flooding. Attached Figure Description

[0038] Figure 1 A scanning electron microscope image of rocks after steam treatment;

[0039] Figure 2 The figure shows the change in oil saturation in a side-drilled well (13m away from the original well). Figure (a) shows the curves of permeability and oil saturation; Figure (b) shows the curve of oil saturation change.

[0040] Figure 3 The images show a comparison between the original well and the sidetracked well, where (a) is for q40-2-20 and (b) is for q40-24-38.

[0041] Figure 4 A schematic diagram showing that the acoustic transit time is greater than 500 u / m at a well depth of 967-968.5m;

[0042] Figure 5 This is a graph showing the trend of neutron gamma curves as the mud content changes.

[0043] Figure 6 This is a comparison chart of the original well and the well logging data;

[0044] Figure 7 The standardized difference between NGR and RT between the original well and the sidetracked well. Detailed Implementation

[0045] The present invention will now be described in further detail with reference to the accompanying drawings:

[0046] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, unless otherwise explicitly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection or a detachable connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two elements. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0047] This invention discloses a method for evaluating water-flooded layers using neutron gamma identification, which mainly consists of two parts:

[0048] The first part involves determining the evaluation parameters before identifying the flooded layer. This process includes the following steps:

[0049] (i) Determine the variation law of lithological and physical properties of heavy oil reservoirs under steam huff and puff conditions;

[0050] 1. Confirm the significant loss of clay minerals and changes in pore structure.

[0051] Sidetracked wells were drilled near the original well, and high-temperature steam treatment was applied to these wells to determine the changes in lithology and physical properties of heavy oil reservoirs under high-temperature steam extraction conditions. It was found that under high-temperature thermal extraction conditions, a large amount of clay minerals were lost from the reservoir, and the mud content decreased significantly (from 10.1% to 7.7%). The sorting coefficient increased (from 2.07 to 2.4). The steam action dissolved and destroyed particles and mud-filled materials in the reservoir, leading to the development of intergranular pores and dissolution pores. Particles, mud, and heavy oil were dragged out of the reservoir along with the high-temperature liquid, resulting in significant changes to the reservoir structure and a marked improvement in the pore structure of the heavy oil reservoir.

[0052] Table 1. Core sampling data from side-drilled wells and existing wells (13m apart)

[0053] Harai 10.1 2.07 Side-drilling 7.7 2.4

[0054] See Figure 1 This is a scanning electron microscope image of rocks after steam treatment, reflecting the microscopic morphology of the rocks after steam treatment. The image shows that the grains and interstitial material were dissolved into a broken spider web-like structure, with the development of extremely large intergranular pores, dissolution pores, and even casting pores.

[0055] 2. Confirm the changes in the physical properties of the rock.

[0056] Numerical simulations showed that during nine steam injection cycles, the oil saturation within a 30m radius of the sidetracked well was less than 40.0%. The remaining oil saturation data from both the sidetracked and replacement wells effectively validated the simulated reservoir utilization. The injection simulation experiments indicated that increased porosity and permeability led to a decrease in oil saturation. With multiple rounds of steam injection, reservoir condensate gradually increased, resulting in a significant decrease in oil saturation, and a corresponding reduction in recovery rate in the near-wellbore area of ​​severely water-flooded formations.

[0057] Table 2 Comparison of coring properties between side-drilled wells and original wells (13m apart)

[0058] Harai 22.3 2.364 Side-drilling 27.4 2.903

[0059] The throughput simulation experiment showed that the oil saturation of sidetracked wells with a spacing of 10-20m decreased from an average of 61% to 45%; when a replacement well was deployed at a spacing of 20-30m from the original well, the oil saturation decreased from 60.1% to 56.5%.

[0060] See Figure 2 To understand the variation of oil saturation during steam injection in sidetracking, the degree of oil reservoir utilization in sidetracked wells mainly depends on reservoir properties. Reservoirs with better physical properties, higher permeability, and higher porosity have lower remaining oil saturation and higher oil reservoir utilization. Figure 2 ).

[0061] 3. To study the response characteristics of neutron gamma logging curves in heavy oil water-flooded layers;

[0062] (1) Research on qualitative identification technology after wearing a mask

[0063] By comparing the sidetracking data from the original well and other sidetracking wells in the same wellbore, and analyzing the statistical curve variation patterns (such as...) of the sidetracking well after steam injection treatment... Figure 3 a) and b) Selected heavy oil blocks: qXXX block (28 wells, over 300 data sets), JXXX block (45 wells, over 2300 data sets), WXXX block (6 wells, 27 data sets), HXXX block (42 wells), DXXX block (10 wells), and SXXX block (6 wells). The following patterns were observed:

[0064] The resistivity decreases after the coating is applied; the acoustic wave pattern becomes less regular after the coating is applied; and the neutron and gamma ray levels generally increase after the coating is applied.

[0065] See Figure 3 The charts show a comparison of different original wells and their corresponding logging data. In the late stages of steam injection in heavy oil reservoirs, with multiple rounds of development, the steam injected into the target formation condenses into water. Since some of this condensate cannot be extracted and remains in the target formation, a "water retention" phenomenon occurs. Simultaneously, as crude oil is continuously extracted from the development formation, the oil content in the near-wellbore zone naturally decreases, further exacerbating the "water retention" phenomenon and resulting in a significant increase in the well's water cut. Once the relative change in oil and water content in the target formation reaches a certain level, it indicates that the target formation has entered different levels of water flooding.

[0066] The decrease in resistivity RT in the figure is analyzed as follows: When the formation is a primitive heavy oil layer, the pores and throats of the rock are occupied by oil. Oil is non-conductive, and only a layer of water film on the throat walls and in the micropores is conductive, resulting in a high resistivity value. When the formation begins to be flooded, the injected steam water has a very low salinity. According to conventional experience, the resistivity of the bottom layer is similar to that of the oil layer, and the resistivity should not gradually decrease. However, due to the different solubility properties of water and oil, the steam condensate dissolves salts in the formation and exchanges ions with bound water in the oil layer, thus increasing the conductivity of the formation and causing the formation resistivity value and magnitude to decrease.

[0067] Analysis of the small change in AC in the figure: For reservoirs with the same porosity, the sound velocity of the reservoir rock will vary depending on the properties of the fluid contained in the pore space. Research results show that for sandstone with the same porosity, the sound velocity is higher when it is water-bearing than when it is oil-bearing. Moreover, the larger the porosity of the sandstone and the higher the sound velocity of the sandstone skeleton, the more significant the difference in sound velocity between water-bearing and oil-bearing sandstones with the same porosity. However, it should be noted that even for pure sandstone with a porosity of 20%-30%, the sound velocity when completely water-bearing is only 7%-15% higher than that when completely oil-bearing. Furthermore, considering that in reality, the pores of oil-bearing sandstone reservoirs cannot be entirely filled with oil (i.e., the oil saturation cannot be 100%), such as... Figure 4 As shown. Therefore, it is quite difficult to determine whether a sandstone reservoir contains oil or water based solely on differences in sound velocity.

[0068] Water flooding NGR changes: The macroscopic capture section value of natural gas decreases while the skeleton value remains unchanged, resulting in an increase in the NGR value. Heavy oil water flooding is caused by a decrease in the removal of mud and an increase in formation water salinity, which reduces the hydrogen index and increases the NGR value.

[0069] 4. Utilize nuclear physics principles to study the reasons for changes in neutron-gamma logging curves caused by variations in lithological properties;

[0070] Because high-temperature and high-pressure steam injection can cause reservoir stimulation in three ways, this analysis reveals the physical principle that changes in neutron gamma logging curves are significantly affected by the hydrogen content index.

[0071] (1) Cavities appear in heavy oil reservoirs

[0072] Heavy oil reservoirs are typically shallow, with layers composed mainly of muddy and asphalt cement. During steam injection, the asphalt undergoes pyrolysis due to the high-temperature steam, dissolving some of the other cementing materials and particles, leading to severe sand production. Steam injection also alters the reservoir's pore structure, increasing pore space and even creating cavities. These cavities cause a decrease in the overall hydrogen content of the reservoir.

[0073] (2) The phenomenon of reduced pore space in heavy oil reservoirs.

[0074] Montmorillonite is the most abundant clay mineral in heavy oil reservoirs (relative content 76.2%, in the early stages of water flooding in heavy oil reservoirs). With increasing steam injection cycles, porosity tends to decrease. This is because some clay cement is dispersed on the particle surface or coated within the particles, detaching from its original position and entering the pore space. Montmorillonite expands several to ten times, or even 30 times, its volume upon contact with water, forming partially pore-filling cement, thus reducing porosity.

[0075] (3) The decrease in clay content leads to a decrease in reservoir density and a looser structure.

[0076] As the number of steam injection cycles increases, the high temperature and pressure cause the lithology to become loose, and the clay is continuously carried away with the extracted oil and water. Since the density of clay is several times higher than that of oil and water, the reduction of clay minerals will inevitably lead to a decrease in reservoir density.

[0077] In summary, regarding the above three phenomena, see Figure 5 When the reservoir has partial "cavities", clay minerals (mud) are lost, and the pore space is reduced, the overall hydrogen content of the reservoir decreases, the ability to slow down thermal neutrons deteriorates, and neutrons continue to collide with formation elements for a long time to emit gamma rays, so the neutron gamma curve generally rises.

[0078] In addition, the following two factors also contribute to the general increase in neutron gamma curves:

[0079] a) The density of clay minerals is generally 2.5–2.8 g / cm³. 3 Between these two points, the loss of clay minerals has the greatest impact on the hydrogen content index.

[0080] b) When the source distance is 35 cm, the neutron gamma value is only affected by the mineralization degree. When the source distance is greater than 55 cm, it is mainly affected by the hydrogen content index.

[0081] Based on the above determination process, this invention combines the time-shifting method to determine that neutron gamma and resistivity can be used as indicators to judge the degree of water flooding in heavy oil. By determining the measured values ​​of neutron gamma and resistivity over time, the qualitative analysis of the water-flooded heavy oil layer is achieved.

[0082] Part Two: Evaluation Methods

[0083] A qualitative interpretation method for water-flooded heavy oil formations is achieved by comparing the changes in neutron gamma curves obtained at different times in production wells using the time-lapse method. This method includes the following steps:

[0084] 4.1 Research on Qualitative Judgment Technology for Heavy Oil Water-Flooded Layers

[0085] Utilizing the general increase in neutron gamma ray levels in water-flooded zones, the neutron gamma ray of the original well was compared with that of a sidetracked well adjacent to the original well. The qualitative determination of the water-flooded zone was achieved using the neutron gamma ray curve over time, referencing changes in other curves. The original well was an abandoned well that had already been fully exploited; the neutron gamma ray curve of the sidetracked well was larger than that of the original well.

[0086] Specifically, if the neutron gamma curve of the sidetracked well differs significantly from that of the original well, it indicates a high water content and a high water-flooded layer. If the two neutron gamma curves differ only slightly, it indicates a low water content and potential for exploitation. More specifically, if the difference between the standard deviations of the neutron gamma curves of the sidetracked well and those of the original well exceeds 40% of the standard deviation of the sidetracked well's neutron gamma curve, it indicates a high degree of water flooding and high water content, making it unprofitable. If the difference is between 30% and 40% of the standard deviation of the neutron gamma curve of the sidetracked well, it indicates that the exploitability needs to be investigated based on local conditions. If the difference is less than 30% of the standard deviation of the neutron gamma curve of the sidetracked well, it indicates a low degree of water flooding and the potential for further exploitation.

[0087] like Figure 6 In the early June 1988 period, before sidetracking well qXX-2-XX, the monthly oil production was 569 tons and water production was 237 cubic meters per second. 3 The water cut was 29.4%; before sidetracking in March 1993, the monthly oil production was 114 tons and the water production was 140 cubic meters per second. 3 The water cut was 53.3%. After sidetracking in April 1993, the monthly oil production was 656 tons and the water production was 290 tons. 3 The moisture content is 30.7%. Figure 6 The leftmost layer (1st layer) without water flooding in 1988 has a neutron-gamma logging characteristic value of 1.12 at 937.5m. The rightmost layer, after sidetracking in 1993, has a neutron-gamma value of 1.47 at the same depth (937.5m). The graph shows that the original well's neutron-gamma values ​​in the heavy oil layers 1-6 were generally between 1.12 and 1.19, while the sidetracked well's values ​​were generally between 1.26 and 1.55. The difference between the two curves is 0.35 API, indicating a general trend of increased neutron-gamma values ​​after water flooding in heavy oil.

[0088] (5) By utilizing the correlation between the neutron gamma logging curve of the casing well and the resistivity logging curve of the open hole well in the new well, a calculation formula for simulating resistivity using neutron gamma logging is established, thereby realizing the quantitative calculation of the remaining oil saturation in the original well using resistivity.

[0089] The apparent resistivity Rta is calculated based on the difference between the normalized NGR value of the sidetracked well and the normalized NGR value of the original well, and the difference between the normalized RT resistivity of the original well and the normalized RT resistivity of the sidetracked well.

[0090] 5.1 Establishment of the Simulated Resistivity Formula

[0091] The deep resistivity RT and neutron gamma values ​​measured in the original well and the sidetracked well were standardized respectively. The difference between the standardized resistivity ΔRT and the difference between the standardized neutron gamma values ​​ΔNGR were calculated between the two wells. The product of the squares of the standardized resistivity difference ΔRT and the standardized neutron gamma difference ΔNGR, and the difference between the standardized neutron gamma values ​​ΔNGR, were used to establish a cubic equation. The resulting simulated resistivity formula is as follows:

[0092] (NGRN-NGRBJN) 2 (RtBJN-RtN)=-1.67×(NGRH-NGRBJN) 3 +2.14×(NGRN-NGRBJN) 2 -0.388×(NGRN-NGRBJN)+0.018 (1)

[0093] The correlation coefficient R = 0.995

[0094] The parameters in the formula are explained as follows:

[0095] NGRN and NGRBJN are the standardized values ​​of the original well neutron gamma curve and the standardized value of the sidetracked neutron gamma curve, respectively; the specific calculation formula is as follows:

[0096]

[0097]

[0098] Wherein, NGR is the neutron gamma value of the original wellbore over time, and NGRBJ is the neutron gamma value of the sidetracked well over time. min The minimum value among the time-varying neutron gamma values ​​of the original wellbore, NGR max The maximum value of the neutron gamma value of the original well over time, NGRBJ min The minimum neutron gamma value of the side-drilled well over time, NGRBJ max This represents the minimum neutron gamma value of the side-drilled well over time.

[0099] RtN and RtBJN are the standardized values ​​of the lateral resistivity at the original well depth and the side-drilled well depth, respectively. The specific calculation formula is as follows:

[0100]

[0101]

[0102] Where Rt is the deep lateral resistivity of the original well over time, and RtBJ is the deep lateral resistivity of the side-drilled well over time. min Rt represents the minimum value of the deep lateral resistivity of the original well over time.max This represents the maximum value of the deep lateral resistivity of the original well over time.

[0103] Set ΔD = -1.67 × (NGRN - NGRBJN) 3 +2.14×(NGRN-NGRBJN) 2 -0.388×(NGRN-NGRBJN)+0.018 (6)

[0104] Then formula (1) is transformed into

[0105] (NGRN-NGRBJN) 2 (RtBJN-RtN)=ΔD (7)

[0106]

[0107]

[0108] Combining formulas (8) and (9), we can see that

[0109]

[0110] In the formula, NGRN, NGRBJN, and RtBJN are all known. Rtmin and Rtmax in RtN are also constants to ensure numerical consistency for the same physical quantity. Therefore, only Rt is unknown. Since Rta = Rt in the calculation, the apparent formation resistivity Rta of the side-drilled well can be calculated. Specifically, formula (10) is transformed into...

[0111]

[0112] In the formula:

[0113] ΔD = -1.67 × (NGRN - NGRBJN) 3 +2.14×(NGRN-NGRBJN) 2 -0.388×(NGRN-NGRBJN)+0.018 (7)

[0114] The Rta of the sidetracked well calculated by formula (6) is compared with the measured Rt of the original well. The smaller the difference between the two, the lower the water cut and the lower the water flooding degree of the sidetracked well. If the difference is large, the higher the water flooding degree. When the difference is greater than 12Ω, the sidetracked well is not worth mining. When the difference is less than 12Ω, it can be further judged whether it is worth mining based on the actual situation. Preferably, when the difference is between 1.5 and 3Ω, the sidetracked well is worth mining.

[0115] The present invention also discloses an embodiment in which the above-mentioned evaluation method is implemented through a system, the evaluation method comprising:

[0116] The standard deviation judgment segment unit is used to compare the standard deviation of the neutron gamma curve of the original well with the standard deviation of the neutron gamma curve of the corresponding side-drilled well. When the difference between the standard deviation of the neutron gamma curve of the original well and the standard deviation of the neutron gamma curve of the side-drilled well is less than 40%, step 2 is executed; otherwise, the side-drilled well does not meet the mining requirements.

[0117] The resistivity judgment unit is used to compare the deep lateral resistivity of the original well with the apparent formation resistivity of the corresponding side-drilled well. When the difference between the two is less than 12Ω, the side-drilled well is considered to meet the mining requirements; otherwise, the side-drilled well does not meet the mining requirements.

[0118] 5.2 Verify the simulated resistivity formula using actual production results.

[0119] See Figure 7 A comparison of measured depth lateral resistivity and calculated resistivity was made using the modeling method of well XXX40-2-20C (e.g.) Figure 7 Before sidetracking, the well produced 569 tons of oil and 237 cubic meters of water per month in early June 1988. 3 The water cut was 29.4%; before sidetracking in March 1993, the monthly oil production was 114 tons and the water production was 140 cubic meters per second. 3 The water cut was 53.3%. After sidetracking in January 1994, the monthly oil production was 23 tons and the water content was 192 m³. 3 The water cut was 89.3%. After sidetracking in February 1994, the monthly oil production was 54 tons and the water production was 1162 m³. 3 With a water cut of 95.6%, after sidetracking in March 1994, the monthly oil production was 66 tons and water production was 311 cubic meters per second. 3 The water cut was 82.5%. After sidetracking began in April 1994, the monthly oil production was 656 tons and the water production was 290 tons. 3 The water cut was 30.7%. After sidetracking in January 2018, the monthly oil production was 4 tons and the water content was 154 m³. 3 The water cut was 97.5%, and production was halted. From the above analysis, it can be seen that the geological conditions are met. The measured deep lateral resistivity from side-drilling in the heavy oil reservoir is basically consistent with the neutron gamma-ray inversion resistivity. Therefore, the calculated resistivity has a certain degree of reliability. The remaining oil saturation can be determined using Archie's formula based on the calculated resistivity.

[17] This method can provide a quantitative interpretation of water flooding in heavy oil. However, it should be noted that this method is not universally applicable; the correlation coefficients of calculation results may be low in some blocks or wells, and achieving fully quantitative calculations remains quite challenging.

[0120] Example

[0121] Sidetracking is a new inclined well drilled after a window is opened in the original wellbore, with formation characteristics similar to the original well. Figure 6 This is a comparison chart of the logging curves from a well completed in 1988 and those from a newly drilled sidetracked well in the same wellbore in 1993. The comparison shows that after five years of high-temperature thermal recovery in the same formation, the lithological and physical properties have changed significantly, reflected in the logging curves. Compared to the original well data, the resistivity in water-flooded zones is significantly lower, while the neutron-gamma ray logging curve is significantly higher. In reflecting the characteristics of water-flooded zones, the casing-mounted neutron-gamma ray logging curves show a high degree of consistency with the open-hole resistivity logging curves. Since neutron-gamma ray logging can be performed inside the casing, this change in the neutron-gamma ray logging curves can be used to monitor residual oil saturation in the original well using the time-lapse method of neutron-gamma ray logging.

[0122] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for evaluating heavy oil water-flooded layers using neutron gamma spectroscopy, characterized in that, The neutron gamma curve is measured in the original well after cementing. After the production cycle, the neutron gamma curve of the corresponding side-drilled well is measured again. The change of the neutron gamma curve at different times is used to determine the water flooding status of heavy oil and realize the monitoring of the remaining oil saturation of heavy oil blocks. The evaluation method includes the following steps: Step 1: Subtract the standard deviation of the neutron gamma curve of the original well from the standard deviation of the neutron gamma curve of the corresponding sidetracked well to obtain the difference value. If the difference value is less than 40% of the standard deviation of the neutron gamma curve of the sidetracked well, then proceed to Step 2; otherwise, the sidetracked well does not meet the production requirements. The sidetracked well corresponding to the original well is treated with high-temperature steam. Step 2: Compare the deep lateral resistivity of the original well with the apparent formation resistivity of the corresponding side-drilled well. When the difference between the two is less than 12Ω, it is determined that the water flooding degree of the reservoir corresponding to the side-drilled well is low, and the side-drilled well is deemed to meet the production requirements. Otherwise, the side-drilled well does not meet the production requirements.

2. The evaluation method for identifying heavy oil water-flooded layers using neutron gamma spectroscopy according to claim 1, characterized in that, Apparent formation resistivity of the side-drilled well Rta The calculation formula is: In the formula, D is a calculated value, and its calculation formula is: Wherein, NGRN and NGRBJN are the normalized values ​​of the neutron gamma curves of the original well and the sidetracked well, respectively, and are known values; RtBJ is the deep lateral resistivity of the sidetracked well as a function of time, and is a known value; Rtmin and Rtmax are the normalized minimum and maximum values ​​of the deep lateral resistivity of the original well, respectively.

3. The evaluation method for identifying heavy oil water-flooded layers using neutron gamma spectroscopy according to claim 2, characterized in that, The standardized formulas for the neutron gamma curves of the original well and the sidetracked well are as follows: in, NGR The original wellbore represents the neutron gamma value over time, while NGRBJ represents the sidetracked well's neutron gamma value over time. NGR min This represents the minimum value among the time-varying neutron gamma values ​​of the original well. NGR max This represents the maximum value among the time-varying neutron gamma values ​​of the original well. NGRBJ min This represents the minimum neutron gamma value of the side-drilled well over time. NGRBJ max This represents the minimum neutron gamma value of the side-drilled well over time.

4. The evaluation method for identifying heavy oil water-flooded layers using neutron gamma spectroscopy according to claim 2, characterized in that, The formula for calculating the standardized value of lateral resistivity RtBJN at the depth of side-drilled wells is as follows: in, RtBJ The deep lateral resistivity of the side-drilled well varies with time; Rt min This represents the minimum value of the deep lateral resistivity of the original well over time. Rt max This represents the maximum value of the deep lateral resistivity of the original well over time.

5. The evaluation method for identifying heavy oil water-flooded layers using neutron gamma spectroscopy according to claim 1, characterized in that, The original well is a well from which exhaust gas has been extracted, and the side-drilled well is a well next to the original well that is being re-extracted.

6. The evaluation method for identifying heavy oil water-flooded layers using neutron gamma spectroscopy according to claim 1, characterized in that, During 9 cycles of steam injection, the oil saturation is less than 40.0% within a 30m radius of the side-drilled well.

7. The evaluation method for identifying heavy oil water-flooded layers using neutron gamma spectroscopy according to claim 1, characterized in that, In step 1, if the difference between the standard deviation of the neutron gamma curve of the original well and the standard deviation of the neutron gamma curve of the sidetracked well is less than 30%, then step 2 is executed.

8. The evaluation method for identifying heavy oil water-flooded layers using neutron gamma spectroscopy according to any one of claims 1-7, characterized in that, In step 2, when the difference between the deep lateral resistivity of the original well and the apparent formation resistivity of the corresponding side-drilled well is 1.5~3Ω, the side-drilled well is worth mining.

9. An evaluation system for identifying heavy oil water-flooded layers using neutron gamma spectroscopy to implement the evaluation method of claim 1, characterized in that, include: The standard deviation judgment unit is used to compare the standard deviation of the neutron gamma curve of the original well with the standard deviation of the neutron gamma curve of the corresponding side-drilled well. When the difference between the standard deviation of the neutron gamma curve of the original well and the standard deviation of the neutron gamma curve of the side-drilled well is less than 40%, the resistivity judgment unit is used for judgment; otherwise, the side-drilled well does not meet the mining requirements. The resistivity judgment unit is used to compare the deep lateral resistivity of the original well with the apparent formation resistivity of the corresponding side-drilled well. When the difference between the two is less than 12Ω, the side-drilled well is considered to meet the mining requirements; otherwise, the side-drilled well does not meet the mining requirements.

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