Field identification method of the solubility of carbonate geothermal reservoirs

Abstract

The present invention provides a method for field identification of the solubility of carbonate geothermal reservoirs, comprising the following steps: the first step: field outcrop exploration and selection; the second step: field outcrop dissolution observation; the third step: field outcrop rock type identification; the fourth step: field outcrop and stratum macroscopic information collection; the fifth step: stratum burial history restoration; the sixth step: carbonate rock solubility model construction and geothermal reservoir solubility identification. The present invention constructs a carbonate rock solubility model based on a large amount of literature, experimental data and rock properties, conducts a comprehensive analysis of the "field outcrop dissolution condition-rock type-macroscopic information" system, and uses the "inversion backstripping method" to restore the burial history of the stratum, thereby identifying the solubility of carbonate geothermal reservoirs. It has the advantages of being simple, fast and easy to implement, and at the same time, it also has a certain degree of accuracy.

Description

Technical Field

[0001] The invention provides a method for field identification of the solubility of a carbonate geothermal reservoir, belonging to the technical field of geological exploration. Background Art

[0002] Geothermal energy is a special mineral resource. It is a heat energy resource inside the earth that can be developed and utilized by the human economy in the foreseeable future. It is the cleanest green, low-carbon, and recyclable renewable energy on earth. Medium and high temperature geothermal energy resources highlight their unique advantages. Not only is the mining stable and uniform, but it is also truly green, clean, and zero-carbon. Among them, the secondary pores and caves formed by carbonate rock dissolution are one of the most important storage spaces for geothermal energy.

[0003] Since the 1970s, the research on carbonate rock dissolution has developed rapidly, and the experimental understanding of carbonate rock dissolution has been continuously deepened. Early carbonate rock dissolution experiments simulating surface environments have made people realize that carbonate rocks of different compositions, structures and origins have different dissolution rates. Later carbonate rock dissolution experiments simulating burial environments have made people realize that factors such as fluid properties, fluid flow rate, temperature, and pressure play an important role in the dissolution of carbonate rocks. At present, field observation and dissolution experiments are two important methods for judging the solubility of carbonate rocks in a region. However, relying solely on a single field observation is not enough to judge the solubility of carbonate rocks. It is often necessary to combine simple dissolution experiments to obtain specific solubility or dissolution rate for preliminary judgment. For deep burial conditions, high temperature and high pressure dissolution simulation experiments are needed to judge the solubility of carbonate rocks under deep burial. For the purpose of preliminary field judgment, relying solely on field observation is undoubtedly rough and inaccurate. If dissolution-related experiments are carried out, it will be time-consuming, labor-intensive, and uneconomical. Summary of the invention

[0004] This method provides a means for identifying the solubility of carbonate geothermal reservoirs in the field. This method is based on a constructed carbonate rock solubility model, adopts a comprehensive analysis system of "field outcrop dissolution conditions-rock type-macro information", combines satellite images, uses drone photography, compasses, rangefinders and other equipment, and uses the "inversion backstripping method" to restore the burial history of the strata, and then makes a preliminary identification of the solubility of the geothermal reservoir. It solves the current problems of lack of objectivity and accuracy in field observations, as well as the problems of cumbersome, time-consuming and uneconomical dissolution experiments for the purpose of preliminary field identification.

[0005] The specific implementation method of this plan includes the following steps:

[0006] Step 1: Field outcrop exploration and selection;

[0007] Step 2: Observation of dissolution of outcrops in the field;

[0008] Step 3: Identification of rock types in field outcrops;

[0009] Step 4: Collect macroscopic information of field outcrops and their stratigraphic structures;

[0010] Step 5: Restoration of burial history of strata;

[0011] Step 6: Construction of carbonate rock solubility model and identification of geothermal reservoir solubility.

[0012] The specific contents are:

[0013] Step 1: Field outcrop exploration and selection:

[0014] Combined with various geological data, field surveys are carried out and appropriate outcrops for observation are selected. The principles for selecting field outcrops are:

[0015] 1. The exposed area should be as large as possible: A larger outcrop area can better observe the dissolution phenomenon and better reflect the solubility of the area.

[0016] 2. The stratification should be as good as possible: Detailed geological dissection and geometric measurement are required for the selected field outcrops. If the stratification is poor, it will affect the estimation of the thickness of the stratum, thereby affecting the determination of the solubility of the deeply buried geothermal reservoir.

[0017] 3. The degree of obstruction should be as low as possible: Outcrops with good exposure are conducive to the observation of dissolution and the identification of lithology, and are more conducive to the identification of the solubility of the area.

[0018] Step 2: Observation of dissolution of outcrops in the field:

[0019] Based on on-site field surveys and combined with drone photography, a comprehensive observation of the selected field outcrops was conducted. The specific contents are as follows:

[0020] 1. Overall observation of the dissolution of field outcrops: Combined with drone photography, identify large dissolution cracks and dissolution holes in the field outcrops and surrounding areas, and estimate the area they cover, the number of holes and cracks, etc., to perceive the dissolution of the area as a whole, and record and take pictures.

[0021] 2. Detailed observation of rock dissolution, crystallization, and calcium filling: Through detailed observation and analysis of rock outcrops in the field, small dissolution holes, cracks, crystallization, and calcium filling displayed are recorded and photographed to support the determination of the solubility of the area.

[0022] Step 3: Identification of rock types in field outcrops:

[0023] Based on the observation of the dissolution of the outcrops in the field, combined with various literature and map data, the rock type of the outcrops in the field is identified. The specific contents are as follows:

[0024] 1. Empirical method to identify rock types: Combined with the literature and map data collected in the area, the rock types of field outcrops are preliminarily determined based on the lithological characteristics of limestone, dolomite limestone, dolomite, etc.

[0025] 2. Experimental method to identify rock types: Due to the differences in component content among limestone, dolomite limestone, dolomite, etc., they can be distinguished by dripping dilute hydrochloric acid on the fresh surface of the rock outcrop in the field, thereby further determining the rock type of the outcrop in the field.

[0026] Step 4: Field outcrop and stratigraphic macro information collection:

[0027] Using field equipment such as compasses and laser rangefinders, combined with satellite images, we collected macro information such as the location, geometric scale, dip angle, thickness, etc. of the outcrops in the field. The specific contents are as follows:

[0028] 1. Use satellite images to collect macroscopic information of outcrops: Use satellite images to collect background information such as the location of outcrops in the field, topography and geomorphology, and the number of outcrops in the same area. Use distance measurement, area measurement and other functions to measure the length, width, area and other geometric scale information of outcrops to form a preliminary understanding of outcrops in the field and their outcrop areas.

[0029] 2. Use field equipment to collect macroscopic information of the strata: Use compass, laser rangefinder and other field equipment to measure the dip angle, length, height and other information of the outcrop, and trace the outcrop up and down to estimate the thickness range of the outcrop strata.

[0030] Step 5: Restoration of burial history of strata:

[0031] According to the known well data, geological data, etc., the "inversion backstripping method" is used to restore the burial history of the strata, so as to judge the solubility of the carbonate geothermal reservoir. The specific contents are as follows:

[0032] 1. Formation compaction recovery: According to the porosity, volume and other parameters of the rock, (in, is the porosity at depth Z; is the surface porosity; c is the compaction coefficient, m -1 ; z is depth, m) exponential relationship, and a quantitative relationship between the porosity of different lithologies and depth is established.

[0033] 2. Restoration of stratum erosion: The stratum thickness is corrected by compaction, ancient water depth and ancient sea level. The stratum cross-section comparison method, acoustic time difference method, stratum cross-section method, vitrinite reflectivity method, wave analysis method, method of calculating stratum erosion thickness using density difference between strata above and below the erosion surface, apatite fission track method, seismic layer velocity method and sedimentation rate method are used to restore stratum erosion thickness.

[0034] 3. Formation thickness recovery: Based on formation compaction recovery and formation erosion recovery, when the top and bottom burial depths of the target layer are known, the rock skeleton thickness of each layer is calculated, and the "inversion backstripping method" is used to obtain the formation thickness at different burial depths, and the burial history curve of each formation is established.

[0035] Step 6: Construction of carbonate rock solubility model and identification of geothermal reservoir solubility:

[0036] Based on literature and experimental data, according to the composition, structure and other properties of various carbonate rocks, a carbonate rock solubility model is constructed. Combined with the information collection, comprehensive analysis and burial history restoration of field outcrops and strata from the first to fifth steps, the solubility of carbonate geothermal reservoirs is judged. The specific contents are as follows:

[0037] 1. Constructing a carbonate rock solubility model: Based on a large amount of literature and actual experimental data, combined with the composition, structure and other rock properties of various carbonate rocks, a carbonate rock solubility model is constructed with rock type as the main factor, carbonate fluid medium as the representative, dissolution rate as the horizontal axis, and burial depth as the vertical axis.

[0038] 2. Determination of the solubility of geothermal reservoirs: Based on the macroscopic information of field outcrops and their strata and the burial history of the strata, combined with the rock types identified in the field and the observed dissolution conditions, according to the carbonate rock solubility model, the solubility of shallow-medium buried and deep buried geothermal reservoirs is determined.

[0039] The present invention provides a certain technical method for discriminating the solubility of carbonate geothermal reservoirs through outcrops in the field. Based on a large amount of literature, experimental data and rock properties, a carbonate rock solubility model is constructed, and a comprehensive analysis of the "field outcrop dissolution situation-rock type-macro information" system is carried out. The "inversion backstripping method" is used to restore the burial history of the stratum, so as to discriminate the solubility of the geothermal reservoir. This method solves the problems of subjectivity and accuracy in field observations, as well as the time-consuming, labor-intensive and uneconomical problems of dissolution experiments for the purpose of preliminary field judgment. In terms of field discrimination of the solubility of carbonate geothermal reservoirs, it has the advantages of being simple, fast and easy to implement, and at the same time, it also has a certain degree of accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic diagram of the process of the present invention;

[0041] Figure 2 It is the carbonate rock solubility model under carbonic acid conditions of the present invention. DETAILED DESCRIPTION

[0042] The specific technical solution of the present invention is described in conjunction with the accompanying drawings.

[0043] The embodiment of the present invention provides a specific implementation method of a field determination method for the solubility of carbonate geothermal reservoirs, see Figure 1 This method was first applied in the project of "Selection of favorable areas for geothermal energy distribution and evaluation of resource potential in a county in Dali Prefecture, Yunnan Province", and achieved good results. The specific contents include the following:

[0044] Step 1: Field outcrop survey and selection.

[0045] In a certain county, the crustal activity is strong, the geological structure is complex, and the magma activity is frequent, forming a geothermal resource belt rich in medium and high temperature geothermal resources. At the same time, there are many carbonate reservoirs in the area, with rich dissolution phenomena, including large and small dissolution holes, cracks, crystals, etc. According to the principle of field outcrop selection, 10 representative field outcrop points were selected for observation and recording.

[0046] Step 2: Observe the dissolution of outcrops in the field, identify rock types, and collect macroscopic information.

[0047] Based on the preliminary investigation, detailed investigations were carried out on the 10 selected outcrop sites, including but not limited to comprehensive observation of the dissolution conditions of the outcrops and their surrounding areas, definition of the rock types of the outcrops, and collection of macroscopic information on the outcrops and their strata.

[0048] Step 3: Restoration of burial history of the strata.

[0049] Burial history refers to the change in burial depth of a sedimentary unit or a series of units (sequence or stratum) in a basin from the beginning of deposition to the present or a certain geological period, including geological events such as sedimentary interruptions, erosion and faults. Burial history is the basis for basin analysis and simulation. Studies on geothermal history, hydrocarbon generation history, hydrocarbon transport history, and accumulation history all await the resolution of this issue. Usually, the restoration of burial history of strata includes stratum compaction restoration, stratum erosion restoration, stratum thickness restoration, and paleo-water depth correction.

[0050] In the early stage, a large number of documents were consulted to sort out the data on the burial history of a certain county. Using the known well data and geological data, the stratum compaction restoration, stratum erosion restoration and stratum thickness restoration were carried out.

[0051] Step 4: Construction of carbonate rock solubility model.

[0052] Based on a large amount of literature and actual experimental data, combined with the composition, structure and other rock properties of various carbonate rocks, a carbonate rock solubility model is constructed with rock type as the main factor, carbonate fluid medium as the representative, dissolution rate as the horizontal axis and burial depth as the vertical axis, such as Figure 2 shown.

[0053] Step 5: Determination of the solubility of geothermal reservoirs.

[0054] Secondary pores and caves formed by carbonate rock dissolution are one of the most important storage spaces for geothermal energy. Therefore, understanding the solubility of geothermal reservoirs is of great significance for selecting favorable areas for geothermal energy distribution and evaluating resource potential.

[0055] Combined with the information collection, comprehensive analysis and burial history restoration of field outcrops and their strata, based on the carbonate rock solubility model ( Figure 2 ) to conduct a comprehensive field identification of the solubility of the thermal reservoir units in a certain county.

[0056] In step 1, the specific steps are as follows:

[0057] ① Conduct research in a certain county in advance and plan field observation routes and locations based on satellite images.

[0058] ② Carry out field surveys and search for field outcrops that meet the selection principles along the planned route.

[0059] ③ After comprehensive comparison of the age, lithology, advantages and disadvantages of the field outcrops, a total of 10 field outcrops were finally selected.

[0060] In step 2, the specific steps implemented for each selected field outcrop are as follows:

[0061] ① Investigate and organize information on outcrop points in advance, including geographical location, stratigraphic age, stratigraphic thickness, lithology, dissolution phenomenon, etc.; at the same time, collect background information such as the development location, topography and geomorphology of field outcrops, and the number of outcrops in the same area on satellite images, and use distance measurement, area measurement and other functions to measure the length, width, area and other geometric scale information of the outcrop development, so as to form a preliminary understanding of the field outcrops and their outcrop areas.

[0062] ② Carry out overall observation of the outcrop points in the field, and use drones to comprehensively observe the dissolution conditions of the outcrops and surrounding areas, and record the estimated coverage area, number of holes and cracks, etc.; carry out detailed observations simultaneously, and use two or more people to describe the dissolution phenomena of the outcrop points, and comprehensively record the objective description of each outcrop point, including longitude and latitude, stratigraphic age, lithology, dissolution phenomena, etc., and retain the corresponding photos.

[0063] ③ Observe the outcropping rocks in the field in detail, combine the survey data and the lithological characteristics of limestone, dolomite limestone, dolomite, etc. to preliminarily determine the rock type, and then further determine the rock type by dripping dilute hydrochloric acid on the fresh surface of the rock.

[0064] ④ Use a compass to measure the dip angle of the field outcrop, use a laser rangefinder to measure the length, height and other information of the field outcrop, and trace the outcrop up and down to estimate the thickness range of the outcrop stratum.

[0065] ⑤ According to the dissolution conditions, rock types and macroscopic information of the field outcrops, 10 field outcrop sites in a certain county were divided into 4 heat storage units.

[0066] In step 4, the specific steps are as follows:

[0067] ① Investigate the mechanism and main controlling factors of the differences in the solubility of different types of carbonate rocks, and preliminarily form a ranking of the solubility of different types of carbonate rocks.

[0068] ②Using high temperature and high pressure dissolution dynamics simulation equipment, we carried out simulation experiments on the dissolution of different types of carbonate rocks (including limestone, dolomite limestone, argillaceous dolomite limestone, argillaceous limestone, lime dolomite, argillaceous dolomite, dolomite, argillaceous dolomite, etc.) from the surface to the deep burial environment. Based on the experimental data, we established a carbonate rock solubility model represented by carbonate fluid, such as Figure 2 shown.

Claims

1. A method for determining the solubility of carbonate geothermal reservoirs in the field, characterized in that: The following steps are included: Step 1: Field outcrop exploration and selection; Step 2: Observation of dissolution of outcrops in the field; Step 3: Identification of rock types in field outcrops; Step 4: Collect macroscopic information of field outcrops and their stratigraphic structures; Step 5: Restoration of burial history of strata; Step 6: Construction of carbonate rock solubility model and identification of geothermal reservoir solubility; Specific: The second step is to conduct a comprehensive observation of the selected field outcrops based on on-site field surveys and combined with drone photography; the specific contents are as follows: (1) Overall observation of the dissolution of field outcrops: Combined with drone photography, identify large dissolution cracks and dissolution holes in the field outcrops and surrounding areas, estimate the area they cover and the number of holes and cracks, perceive the dissolution of the area as a whole, and record and take photos; (2) Detailed observation of rock dissolution, crystallization, and calcium filling: Through detailed observation and analysis of the rocks outcropping in the field, the small dissolution holes, cracks, crystallization, and calcium filling conditions displayed are recorded and photographed to support the results of the determination of the solubility of the area; The third step is to identify the rock types of the outcrops in the field based on the observation of the dissolution conditions of the outcrops in the field and in combination with the literature and map data. The specific contents are as follows: (1) Empirical method for identifying rock types: Based on the literature and map data collected in the area and the lithological characteristics of different types of carbonate rocks, the rock types of outcrops in the field are preliminarily determined; (2) Experimental method to identify rock types: by dripping dilute hydrochloric acid on the fresh surface of the rock outcrop in the field to further determine the rock type in the field; The fourth step is to use equipment and satellite images to collect macro information of field outcrops, including the development location, geometric scale, dip angle and thickness of the outcrops. The specific contents are as follows: (1) Use satellite images to collect macroscopic information of outcrops: Use satellite images to collect the development background information of field outcrops, including the development location, topography and geomorphology of the outcrops, and the number of outcrops in the same area. Use distance measurement and area measurement functions to collect geometric scale information of outcrop development, including length, width, and area, to form a preliminary understanding of the field outcrops and their outcrop areas. (2) Use field equipment to collect macroscopic information of the strata: use a compass to measure the dip angle of the field outcrop, use a laser rangefinder to measure its length and height, and trace the outcrop up and down to estimate the thickness range of the outcrop strata; The fifth step is to restore the burial history of the strata based on the known well data and geological data using the "inversion backstripping method" to determine the solubility of the geothermal reservoir. The specific contents are as follows: (1) Formation compaction recovery: Based on the porosity and volume parameters of the rock, Exponential relationship, to establish the quantitative relationship between porosity and depth of different lithologies; among them, is the porosity at depth Z; is the surface porosity; c is the compaction coefficient, m -1 ; z is the depth, m; (2) Stratigraphic erosion restoration: Three corrections are made to the stratum thickness, including compaction, paleo-water depth, and paleo-sea level, to restore the stratum erosion thickness; (3) Formation thickness recovery: Based on formation compaction recovery and formation erosion recovery, the rock skeleton thickness of each layer is calculated when the top and bottom burial depths of the target layer are known. The "inversion backstripping method" is used to obtain the formation thickness at different burial depths, and the burial history curve of each formation is established; The sixth step is to construct a carbonate rock solubility model based on literature and experimental data, according to the composition and structural properties of various carbonate rocks, and combine the information collection, comprehensive analysis and burial history restoration of field outcrops and strata in the first to fifth steps to identify the solubility of geothermal reservoirs; the specific contents are as follows: (1) Constructing a carbonate rock solubility model: Based on literature and actual experimental data, combined with the rock properties of various carbonate rocks, including composition and structure, a carbonate rock solubility model is constructed with rock type as the main factor, carbonate fluid medium as the representative, dissolution rate as the horizontal axis, and burial depth as the vertical axis; (2) Determination of the solubility of geothermal reservoirs: Based on the macroscopic information of field outcrops and strata and the burial history of the strata, combined with the rock types identified in the field and the observed dissolution conditions, the solubility of geothermal reservoirs is determined according to the carbonate rock solubility model.

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