Method for determining oil saturation of shale bound fluid and shale oil reservoir

Abstract

According to the method for determining the oil saturation of the shale bound fluid and the shale oil reservoir, the method for determining the shale bound fluid comprises the steps that a rock core obtained from a shale oil reservoir section is processed into a plunger-shaped rock core, and the plunger-shaped rock core is saturated with kerosene; determining the nuclear magnetic porosity of the saturated rock core; carrying out a displacement experiment on the saturated rock core by using a displacement medium; determining the nuclear magnetic porosity of the core after displacement is completed; gradually increasing the displacement pressure, carrying out displacement experiments for multiple times, and obtaining corresponding nuclear magnetic porosity; determining a rock core bound fluid cut-off value according to the variation of the nuclear magnetic porosity corresponding to two adjacent displacement experiments; and determining the shale bound fluid according to the cut-off value of the core bound fluid. After the bound fluid cut-off value is determined through the nuclear magnetic resonance experiment, the bound fluid and the movable fluid in the rock core can be determined, the saturation degree of bound oil and the saturation degree of movable oil can be further calculated, and powerful help is provided for calculation of the oil saturation degree of the shale oil reservoir.

Description

Technical Field

[0001] The invention relates to oil exploration and well logging technology, and in particular to a method for determining shale bound fluid and oil saturation of shale oil reservoirs. Background Art

[0002] Reservoirs in which the hydrocarbons in shale are natural gas are called shale gas reservoirs, and reservoirs in which the hydrocarbons are oil are called shale oil reservoirs. Compared with conventional oil and gas reservoirs, shale oil reservoirs have finer grains, smaller pore throats, and more complex pore structures. At the same time, because the enrichment process of hydrocarbons in shale oil reservoirs spans the entire process of hydrocarbon generation, expulsion, and retention of organic matter, its pore formation and preservation mechanism, hydrocarbon occurrence, and flow mechanism are different from those of conventional reservoirs. The above characteristics of shale oil reservoirs make the fluid distribution characteristics in the pores different from those of conventional reservoirs.

[0003] In conventional reservoirs, the bound fluid cutoff value of the core can be determined by measuring the saturated state and centrifugal state of the plunger core by nuclear magnetic resonance, thereby determining the bound fluid in the core. However, due to the characteristics of shale oil reservoirs such as the development of micro-nano pores, integrated source and reservoir, and rich organic matter, it is difficult to displace the fluid in the core through centrifugal experiments, and it is difficult to obtain a cutoff value that accurately reflects the fluid situation in the rock, and it is impossible to effectively distinguish between bound fluid and movable fluid. Summary of the invention

[0004] In view of the technical defects and technical drawbacks in the prior art, an embodiment of the present invention provides a method for determining shale bound fluid and oil saturation of a shale oil reservoir, which overcomes the above problems or at least partially solves the above problems.

[0005] An embodiment of the present invention provides a method for determining shale bound fluid, comprising:

[0006] Processing cores obtained from shale oil reservoir sections into plug-shaped cores and saturating them with kerosene;

[0007] Determine the nuclear magnetic porosity of the core after saturation;

[0008] Use displacement media to conduct displacement experiments on the core after saturation;

[0009] Determine the NMR porosity of the core after flooding is completed;

[0010] gradually increasing the displacement pressure, performing the displacement experiment multiple times, and obtaining the corresponding nuclear magnetic porosity;

[0011] The core bound fluid cutoff value is determined based on the change in NMR porosity corresponding to two adjacent displacement experiments;

[0012] The shale bound fluid is determined according to the core bound fluid cutoff value.

[0013] Another aspect of the present invention provides a method for determining the oil saturation of a shale oil reservoir, comprising:

[0014] Determine the shale bound fluid and shale movable fluid of the shale oil reservoir based on the above method;

[0015] The ratio of the envelope area of ​​the nuclear magnetic resonance T2 spectrum less than the core bound fluid cutoff value to the envelope area of ​​the complete nuclear magnetic resonance T2 spectrum is taken as the bound fluid saturation;

[0016] The ratio of the envelope area of ​​the nuclear magnetic resonance T2 spectrum greater than the core bound fluid cutoff value to the envelope area of ​​the complete nuclear magnetic resonance T2 spectrum is taken as the movable fluid saturation.

[0017] The embodiments of the present invention achieve at least the following technical effects: after determining the bound fluid cutoff value by nuclear magnetic resonance experiment, the bound fluid and movable fluid in the core can be determined, so that the bound oil saturation and movable oil saturation can be further calculated, providing powerful assistance for the calculation of oil saturation of shale oil reservoirs.

[0018] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structure described in the written description, claims, and drawings.

[0019] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0021] Figure 1 A flowchart for determining shale bound fluid according to an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the nuclear magnetic resonance T2 spectrum of a well core in a kerosene-saturated state and a carbon dioxide displacement process according to an embodiment of the present invention;

[0023] Figure 3 This is a schematic diagram of the change of nuclear magnetic porosity in the kerosene-saturated state of a well core and the carbon dioxide displacement process according to an embodiment of the present invention;

[0024] Figure 4 It is a schematic diagram of determining the cutoff value of the saturated kerosene state and the bound fluid state of a core of a certain well according to an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0026] Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. When the following description refers to the drawings, the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Instead, they are merely examples of devices and methods consistent with some aspects of the present invention as detailed in the appended claims.

[0027] The measurement object of nuclear magnetic resonance technology is hydrogen atoms. The fluid in the pores inside the rock is oil or water, both of which contain hydrogen atoms. The nuclear magnetic resonance T2 spectrum obtained by carrying out nuclear magnetic resonance measurement on the fluid in the rock can reflect the fluid information and pore information of the core. The nuclear magnetic resonance cutoff value refers to an exact value found through nuclear magnetic resonance experimental research, and the nuclear magnetic resonance T2 spectrum is divided by this value. The fluid in the rock pores corresponding to the nuclear magnetic resonance T2 spectrum less than the cutoff value is all bound fluid, and the fluid in the rock pores corresponding to the nuclear magnetic resonance T2 spectrum greater than the cutoff value is all movable fluid. Based on the current situation that it is difficult to displace the fluid in the core of the shale oil reservoir by technical means such as centrifugation, the present invention uses carbon dioxide as a displacement medium, continuously increases the displacement pressure of carbon dioxide, and gradually displaces the movable oil in the shale oil core, so that the remaining fluid in the core is all bound fluid, and the bound fluid cutoff value is finally determined by using the nuclear magnetic resonance T2 spectrum of the core saturated oil state and the nuclear magnetic resonance T2 spectrum of the last displacement state.

[0028] like Figure 1 FIG. 1 is a flow chart of a method for determining shale bound fluid according to an embodiment of the present invention, comprising:

[0029] Step 101: coring a shale oil reservoir section, processing the core into a plunger-shaped core, and saturating the plunger-shaped core with kerosene.

[0030] This embodiment is mainly aimed at shale oil reservoirs, which have the characteristics of developed micro-nano pores, integrated source and reservoir, and rich in organic matter.

[0031] Step 102: Determine the nuclear magnetic porosity of the core after saturation.

[0032] Specifically, after saturation is completed, the core can be loaded into a core holder to perform nuclear magnetic resonance experimental measurement to obtain the nuclear magnetic resonance T2 spectrum of the saturated kerosene core. The nuclear magnetic resonance T2 spectrum initially obtained in the experiment reflects the dimensionless signal amplitude, and the porosity of water is 100%. A certain amount of water is used as the nuclear magnetic porosity calibration standard in the laboratory, and the nuclear magnetic resonance information of the water is used to calibrate the nuclear magnetic resonance information of the core, thereby converting the core nuclear magnetic resonance T2 spectrum from signal amplitude to porosity information, and the envelope area of ​​the entire nuclear magnetic resonance T2 spectrum is the core nuclear magnetic porosity.

[0033] Nuclear magnetic resonance as a physical phenomenon was independently discovered by Purcell of Harvard University and Bloch of Stanford University in 1946. Nuclear magnetic resonance technology identifies the fluid and its content in rock pores by collecting hydrogen nuclear magnetic resonance signals. In the 1950s, nuclear magnetic resonance technology was gradually applied to the petroleum field. It has now become an indispensable technology for reservoir fluid identification and pore structure research in well logging interpretation and evaluation.

[0034] Step 103: Perform a displacement experiment on the saturated core using a displacement medium, and determine the nuclear magnetic porosity of the core after the displacement is completed.

[0035] Specifically, carbon dioxide can be used as a displacement medium to carry out a displacement experiment on the core in the holder. The first displacement pressure is set to 15 MPa. After the displacement is completed, a nuclear magnetic resonance experiment is measured to obtain the nuclear magnetic resonance T2 spectrum of the saturated kerosene core and determine the nuclear magnetic porosity in this state.

[0036] Carbon dioxide is the most commonly used oil displacement medium. Compared with centrifugal and other displacement media, it can better penetrate into micro-nano pores and displace the movable fluid in the micro-nano pores, leaving only the remaining bound fluid in the core.

[0037] Step 104, gradually increasing the displacement pressure, performing the displacement experiment multiple times, and obtaining corresponding nuclear magnetic porosity.

[0038] Specifically, multiple carbon dioxide displacement experiments were subsequently carried out, and the displacement pressure was gradually increased by 5 MPa each time. Nuclear magnetic resonance experimental measurements were carried out after each displacement was completed to obtain the nuclear magnetic resonance T2 spectrum of the core in this state and calculate the nuclear magnetic porosity.

[0039] Step 105: Determine the core bound fluid cutoff value according to the change in nuclear magnetic porosity corresponding to two adjacent displacement experiments.

[0040] Specifically, when the change in nuclear magnetic porosity corresponding to two adjacent displacement experiments does not exceed a predetermined threshold, a nuclear magnetic resonance cutoff value is obtained, and the core bound fluid cutoff value is determined according to the nuclear magnetic resonance cutoff value.

[0041] The NMR cutoff value refers to an exact value found through rock physics experimental research, so that the fluid in all pores smaller than this value in the reservoir rock is bound fluid, and the fluid in all pores larger than this value is movable fluid. Therefore, the key to determining the bound fluid in the core is to use NMR experiments to determine the bound fluid cutoff value.

[0042] The cutoff value is a value that divides the nuclear magnetic resonance T2 spectrum into two parts. The nuclear magnetic resonance cutoff value can be calculated from the nuclear magnetic resonance T2 spectrum data of the core saturation state and any displacement state; the bound fluid cutoff value means that the part of the nuclear magnetic resonance T2 spectrum that is less than this value corresponds to the bound fluid information, and the part that is greater than this value corresponds to the movable fluid information. Not every nuclear magnetic resonance cutoff value can be used as the bound fluid cutoff value. Only when there is no movable fluid in the core that is displaced out, the nuclear magnetic resonance cutoff value determined by this state can be considered as the bound fluid cutoff value. The present invention uses the change in the nuclear magnetic resonance porosity corresponding to two adjacent displacement experiments as an identifier for identifying whether the movable fluid is completely displaced out of the core. When the change in the nuclear magnetic resonance porosity corresponding to two adjacent displacement experiments does not exceed a predetermined threshold, it indicates that only bound fluid remains in the core pores, and the movable fluid has been displaced out of the core. Therefore, the nuclear magnetic resonance cutoff value obtained by using the nuclear magnetic resonance T2 spectrum data of the core saturation state and the last displacement state is considered to be the core bound fluid cutoff value.

[0043] Step 106: determine the shale bound fluid according to the core bound fluid cutoff value.

[0044] Specifically, the object of measurement of nuclear magnetic resonance technology is hydrogen atoms. The fluid in the pores of rocks is oil or water, both of which contain hydrogen atoms. The nuclear magnetic resonance T2 spectrum obtained by nuclear magnetic resonance measurement of the fluid in the rock can reflect the fluid information and pore information of the core. The nuclear magnetic resonance cutoff value refers to an exact value found through nuclear magnetic resonance experimental research, and the nuclear magnetic resonance T2 spectrum is divided by this value. The fluid in the rock pores corresponding to the nuclear magnetic resonance T2 spectrum less than the cutoff value is bound fluid, and the fluid in the rock pores corresponding to the nuclear magnetic resonance T2 spectrum greater than the cutoff value is movable fluid.

[0045] The portion of the NMR T2 spectrum that is smaller than the cutoff value corresponds to the bound fluid in the core, thus realizing the method for determining the bound fluid in shale.

[0046] This embodiment provides a method for determining the oil saturation of a shale oil reservoir. First, based on the above method for determining the shale bound fluid, the shale bound fluid and the shale movable fluid of the shale oil reservoir are determined.

[0047] Then, the nuclear magnetic resonance T2 spectrum reflects the information of two types of fluids in different occurrence states in the core, including bound fluid and movable fluid. The nuclear magnetic resonance T2 spectrum less than the bound fluid cutoff value of the core reflects the bound fluid information in the core. Therefore, the ratio of the envelope area of ​​this part of the nuclear magnetic resonance T2 spectrum to the envelope area of ​​the complete nuclear magnetic resonance T2 spectrum is used as the bound fluid saturation; the nuclear magnetic resonance T2 spectrum greater than the bound fluid cutoff value of the core reflects the movable fluid information in the core. Therefore, the ratio of the envelope area of ​​this part of the nuclear magnetic resonance T2 spectrum to the envelope area of ​​the complete nuclear magnetic resonance T2 spectrum is used as the movable fluid saturation.

[0048] The embodiment of the present invention determines the cutoff value of rock bound fluid in shale oil reservoir based on core nuclear magnetic resonance experiment and carbon dioxide displacement experiment. The results of carbon dioxide displacement of kerosene-saturated core are as follows: Figures 2 to 4 As shown, under displacement pressures of 30Mpa and 35Mpa, the change in core porosity is only 2.7%. Within the error range of nuclear magnetic porosity measurement, the movable oil in the core is completely displaced in this state, and only bound fluid remains in the core. The determined bound fluid cutoff value is 3.5 milliseconds, the bound oil saturation is 36.5%, and the movable oil saturation is 63.5%. It can be seen that the invention is an effective method for determining bound fluid in shale.

[0049] The specific values ​​of the core saturated with kerosene in a certain well and the porosity and relative error, cutoff value, bound oil saturation, movable oil saturation during carbon dioxide displacement are shown in Table 1 below:

[0050] Table 1

[0051]

[0052] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage and optical storage, etc.) containing computer-usable program code.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining shale bound fluid, characterized in that: The method comprises: Processing cores obtained from shale oil reservoir sections into plug-shaped cores and saturating them with kerosene; Determine the nuclear magnetic porosity of the core after saturation; Use displacement media to conduct displacement experiments on the core after saturation; Determine the NMR porosity of the core after flooding is completed; gradually increasing the displacement pressure, performing the displacement experiment multiple times, and obtaining the corresponding nuclear magnetic porosity; The core bound fluid cutoff value is determined based on the change in NMR porosity corresponding to two adjacent displacement experiments; The shale bound fluid is determined according to the core bound fluid cutoff value.

2. The method of claim 1, wherein the process of determining the nuclear magnetic porosity comprises: A nuclear magnetic resonance experiment is performed on the core to measure and obtain a nuclear magnetic resonance T2 spectrum, and the nuclear magnetic porosity is calculated based on the T2 spectrum.

3. The method according to claim 2, characterized in that: The NMR experiment has an echo interval of 0.1 milliseconds and a signal-to-noise ratio of at least 15.

4. The method according to claim 1, characterized in that: The displacement medium is carbon dioxide.

5. The method according to claim 1, characterized in that: The change is calculated according to the following formula:

6. The method according to any one of claims 1 to 5, characterized in that: The displacement pressure of the first displacement experiment was 15 MPa, and the displacement experiment was performed multiple times with the displacement pressure increased by 5 MPa each time.

7. The method according to claim 1, characterized in that: The process of determining the core bound fluid cutoff value includes: when the change in nuclear magnetic resonance porosity corresponding to two adjacent displacement experiments does not exceed a predetermined threshold, obtaining the nuclear magnetic resonance cutoff value from the nuclear magnetic resonance T2 spectrum data of the last displacement state as the core bound fluid cutoff value.

8. The method according to claim 6, characterized in that: The predetermined threshold is 5%.

9. The method according to claim 1, characterized in that: Determining the shale bound fluid according to the core bound fluid cutoff value includes: The fluid in the rock pores corresponding to the nuclear magnetic resonance T2 spectrum less than the core bound fluid cutoff value is determined as the shale bound fluid.

10. The method according to claim 9, characterized in that Also includes: The fluid in the core whose nuclear magnetic resonance T2 spectrum value is greater than or equal to the core bound fluid cutoff value is determined as shale movable fluid.

11. A method for determining the oil saturation of a shale oil reservoir, characterized in that: include: Determining the shale bound fluid and shale movable fluid of the shale oil reservoir based on the method of claim 10; The ratio of the envelope area of ​​the nuclear magnetic resonance T2 spectrum less than the core bound fluid cutoff value to the envelope area of ​​the complete nuclear magnetic resonance T2 spectrum is taken as the bound fluid saturation; The ratio of the envelope area of ​​the nuclear magnetic resonance T2 spectrum greater than the core bound fluid cutoff value to the envelope area of ​​the complete nuclear magnetic resonance T2 spectrum is taken as the movable fluid saturation.

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