CN109946147B - Preparation method of coal-series composite reservoir fracturing physical simulation test piece - Google Patents

Abstract

The invention discloses a method for preparing a physical simulation specimen of coal measure composite reservoir fracturing, which is especially suitable for use in the fields of coal measure gas development geology and rock mechanics. First, collect large fresh coal samples and small rock samples of different lithologies, conduct laser scanning on the cross-section of the large fresh coal samples and calculate the fractal dimension value of the surface topography, carry out X-diffraction analysis and rock According to the test results, different formulations are simulated by using sand, cement and clay to determine the rock combination type and thickness of different lithologic rock layers in coal-measure composite reservoirs. On the same side of the cube specimen, boreholes were drilled at the center of coal samples and self-made rock samples with different lithologies, and steel liquid injection pipes were consolidated in them to simulate boreholes. The externally poured composite cubes of natural rock were prepared into standard-sized coal-measure composite reservoir physical simulation fracturing specimens. The steps are simple, the use is convenient, and the prepared test piece is closer to reality.

Description

Preparation method of physical simulation specimen for coal measure composite reservoir fracturing

technical field

The invention relates to a method for preparing a physical simulation test piece for coal measure composite reservoir fracturing, and is particularly suitable for a method for preparing a physical simulation test piece for coal measure composite reservoir fracturing used in the fields of coal measure gas development geology and rock mechanics.

Background technique

In the eastern margin of the Ordos Basin, eastern Yunnan-western Guizhou and other multi-coal seam areas in my country, different lithologic reservoirs such as coal rock, tight sandstone and mud shale are often vertically superimposed and developed, and the rock layers are thin and frequently interbed, forming coal measures. The composite reservoir is rich in unconventional natural gas resources and has huge development potential.

Coal-measure composite reservoirs generally have the characteristics of low porosity and low permeability, and commingled mining of multi-type reservoirs is the best choice for economical and efficient development of coal-measure gas resources. It is an effective method to implement fracturing stimulation to form a complex fracture network. However, compared with conventional single lithologic reservoirs, coal-measure composite reservoirs have distinct geological characteristics, and the lithologic combination styles are diverse and complex. The fracture extension ability and expansion method determine the gas recovery rate and ultimate recovery rate of coal-measure gas. Therefore, it is the key to release the coal-measure gas production capacity to the maximum to find out the fracture propagation law of coal-measure composite reservoir combined pressure, which is of great significance for the large-scale development of coal-measure gas.

The coal-measure composite reservoir is deeply buried underground, which makes it impossible to directly observe the geometry and extension of the fracture. The physical simulation of fracturing is an important laboratory method to analyze the geometry and extension process of fracturing fractures reliably and effectively. However, there is no physical simulation method of fracturing for coal-measure composite reservoirs in multi-coal seam areas.

Invention patents for physical simulation of multi-rock combination fracturing include:

1. The invention patent with the application publication number CN107060714A discloses a large-scale true triaxial physical model test method for studying the crack extension law of thin interlayer fractures. The method states that each test element is close to the real geological conditions, and through this method, the extension law of hydraulic fracturing fractures in thin interbedded reservoirs can be mastered, and the thin interbedded reservoir can be guided.

The physical model test method involved in the above-mentioned patent only describes the interbedding of two lithological rock layers, and does not involve coal rock. Similar materials emphasize the use of cements of different hardness and particle size, which is difficult to approximate the real coal-measure composite reservoir.

2. The invention patent with the application publication number CN105334090A discloses a method for preparing a fracturing physical model sample of a coal-bearing production layer group. The invention can prepare layered samples of different lithologic combinations in the coal-bearing production layer group.

Although the above-mentioned patents include coal rock and rocks of different lithology, there are shortcomings: First, a large rock cutting machine is used in the preparation of the physical model sample, so that the surface of the coal and the rock is smooth, and the two are in flat contact with each other. The contact interface between actual coal and rock and the irregular shape of roof and floor is different; commingled mining is emphasized in the development of the two coal-measure composite reservoirs, and the single-layer fracturing mode in this invention is not enough to guide the multi-layer combination of actual coal-measure composite reservoirs The extension and expansion of pressure cracks during pressure reconstruction.

SUMMARY OF THE INVENTION

Aiming at the shortcomings of the above technologies, a method for preparing a physical simulation specimen for coal-measure composite reservoir fracturing is provided. The method uses natural fresh bulk coal samples and similar materials to quantitatively characterize coal-rock surface fractal dimension values. The irregularity of the rock contact interface truly reflects the differences in the mechanical properties and interlayer interface properties of different types of rock formations, and improves the reliability of the fracturing physical simulation results.

In order to achieve the above technical goals, the method for preparing a physical simulation specimen for coal measure composite reservoir fracturing of the present invention includes the following steps:

Step 1. Collect large fresh coal samples and small rock samples of different lithologies in the mine, mark the sampling orientation and horizon, the size of large coal samples is more than 5cm×30cm×30cm, and the size of coal is about 30cm×30cm The surface of the sample is a natural section, to ensure that the interface between the coal and the rock is an irregular contact surface, and the size of the small rock sample should be greater than 8cm×8cm×8cm;

Step 2, use a laser scanner to scan the natural section of the large fresh coal sample obtained in step 1, calculate the fractal dimension value of the surface topography of the section, and quantitatively characterize the irregularity of the surface of the coal sample according to the value of the fractal dimension. The larger the value, the more irregular the surface of the coal sample;

Step 3: Carry out X-diffraction analysis and rock mechanics testing on small rock samples of different lithologies to obtain the mineral composition and content and rock mechanics parameters of rocks with different lithologies, and use sand, cement and clay to prepare simulations according to the testing results. The formulas of various rock samples with different lithologies are obtained, and the rock samples prepared by each formula are correspondingly marked as C1, C2, ...;

Step 4, based on the observation of the core of the coal-measure composite reservoir, determine the rock combination type and the thickness of different lithologic rock layers of the coal-measure composite reservoir;

Step 5, use the cubic steel mold to prepare the coal-bearing multi-lithologic rock combination cube specimen: first, referring to the rock combination type of the coal-measure composite reservoir determined in step 4, design the position of the large fresh coal sample in the mold, the fresh coal sample The two sides of 30cm × 30cm are used as the top and bottom surfaces, and the rock sample formula corresponding to the actual rock type on the top surface of the fresh coal sample is selected to simulate the rock formation on the outside thereof, and silica gel is applied to the top surface of the fresh coal sample. During the period, pour the mold into the mold until the thickness reaches the design requirements and wait for solidification. After solidification, a layer of simulated rock sample rock layer on the top of the fresh coal sample is formed; select the rock sample formula corresponding to the actual rock type on the bottom surface of the fresh coal sample to simulate the rock layer on the outside thereof. Apply silica gel on the bottom surface of the coal sample, and pour it into the mold before solidification according to the rock sample formula until the thickness reaches the design requirements and wait for solidification. After solidification, a layer of simulated rock sample rock layer at the bottom of the fresh coal sample is formed; Continue to pour different rock sample formula liquids on the outside of the top and bottom of the rock sample layer until the entire mold is completed, in which a layer of silica gel is applied for each layer of simulated rock sample rock layer poured, waiting for the last layer of simulated rock sample rock layer to solidify; the thickness of different rock sample rock layers According to step 4, the actual observed rock layer thickness value in the composite reservoir core is scaled by 1:100 to complete the preparation of a coal-bearing multi-lithologic rock combination cube specimen;

In step 6, on the same side of the cube specimen prepared in step 5, boreholes were drilled in the central part of the coal sample and the self-made rock samples with different lithologies, and the steel liquid injection pipe was consolidated in the hole by resin glue. The simulated wellbore is 16 cm long and 15 mm in diameter; the steel liquid injection pipe used is 20 cm long, 12 mm in outer diameter and 6 mm in inner diameter, and the steel pipe is cemented to the simulated wellbore with resin glue at a depth of 14 cm;

Step 7, externally pouring the coal-bearing multi-lithologic rock composite cube by using the sealing material to prepare a standard size coal-measure composite reservoir physical simulation fracturing specimen.

The different lithologic rock layers in step 1 include sandstone layers, argillaceous sandstone layers, sandy mudstone layers and mudstone layers.

When choosing the best formula in step 3, first put the sand, cement and clay into the mixer for stirring, and the stirring time is at least 1 hour. After it is uniform, add water and stir again. During this process, about 50mL of retarder can be added appropriately. The standing time after solidification is at least 2 weeks; the small rock sample is a plunger sample with a diameter of 2.5cm and a height of 5cm; the optimal formula is determined based on the Young's modulus, Poisson's ratio, Poisson's ratio, Comparison of mechanical parameters such as cohesion, internal friction angle, and tensile strength.

The size of the mold in step 5 is 30cm×30cm×30cm, and the inner wall of the mold is coated with lubricant; the coal-bearing multi-lithologic rock assembly includes, for example, sandstone layer-mudstone layer-coal sample-mudstone layer-sandstone layer, sandstone layer-mudstone layer- Coal sample-sandstone layer-mudstone layer, mudstone layer-sandstone layer-coal sample-sandstone layer-mudstone layer, mudstone layer-sandstone layer-coal sample-mudstone layer-sandstone layer, the cementing time is at least more than 10 hours.

In step 7, the sealing material is resin, its thickness is 50mm, and the standard size is 35cm×35cm×35cm; after pouring the coal-bearing lithologic rock composite cube, it is dried under natural conditions for 2-3 weeks.

Beneficial effects:

For coal-measure composite reservoirs, the significant difference between rock mechanical properties and interlayer interface properties is its typical geological feature. The method proposed in the present invention comprehensively utilizes similar materials and fresh large coal samples to quantify the size of the fractal dimension of the coal rock surface. Characterize the irregularity of the coal-rock contact interface, and truly reflect the differences in mechanical properties and interlayer interface properties of different types of rock formations. Combined with the laser scanning of rock surface morphology and the calculation of fractal dimension values, it can approximate the interface state between different layers of composite reservoirs. The real geological conditions can be used to prepare fracturing physical simulation specimens that meet the characteristics of coal-measure composite reservoirs, so as to improve the credibility of fracturing physical simulation results. The method has simple steps, is convenient to use, and has a high degree of experimental simulation.

Description of drawings

Fig. 1 is a technical flow chart of the method for preparing a physical simulation test piece for coal measure composite reservoir fracturing according to the present invention.

Detailed ways

Embodiments of the present invention will be further described below in conjunction with the accompanying drawings:

As shown in Figure 1, the method for preparing a physical simulation specimen for coal measure composite reservoir fracturing of the present invention includes the following steps:

Step 1. Collect large fresh coal samples and small rock samples of different lithologies in the mine, mark the sampling orientation and horizon, the size of large coal samples is more than 5cm×30cm×30cm, and the size of coal is about 30cm×30cm The surface of the sample is a natural section, to ensure that the interface between the coal and the rock is an irregular contact surface, and the size of the small rock sample should be greater than 8cm×8cm×8cm; the different lithologic rock layers include sandstone layers, argillaceous sandstone layers, and sandy mudstone layers. and mudstone formations;

Step 2, use a laser scanner to scan the natural section of the large fresh coal sample obtained in step 1, calculate the fractal dimension value of the surface topography of the section, and quantitatively characterize the irregularity of the surface of the coal sample according to the value of the fractal dimension. The larger the value, the more irregular the surface of the coal sample;

Step 3: Carry out X-diffraction analysis and rock mechanics testing on small rock samples of different lithologies to obtain the mineral composition and content and rock mechanics parameters of rocks with different lithologies, and use sand, cement and clay to prepare simulations according to the testing results. Formulas of various rock samples with different lithologies are obtained, and the rock samples prepared by each formula are marked as C1, C2, ...; when choosing the best formula, sand, cement and clay need to be put into the mixer first for mixing, and the mixing time is at least After 1 hour, add clean water and stir again. During this process, about 50mL of retarder can be appropriately added, and the time after solidification is at least 2 weeks; the small rock sample is a plunger sample, with a diameter of 2.5cm and a height of 5cm; The determination of the optimal formula is based on the comparison of mechanical parameters such as Young's modulus, Poisson's ratio, cohesion, internal friction angle, and tensile strength between the self-made sample and the actual small rock sample;

Step 4, based on the observation of the core of the coal-measure composite reservoir, determine the rock combination type and the thickness of different lithologic rock layers of the coal-measure composite reservoir;

Step 5, use the cubic steel mold to prepare the coal-bearing multi-lithologic rock combination cube specimen: first, referring to the rock combination type of the coal-measure composite reservoir determined in step 4, design the position of the large fresh coal sample in the mold, the fresh coal sample The two sides of 30cm × 30cm are used as the top and bottom surfaces, and the rock sample formula corresponding to the actual rock type on the top surface of the fresh coal sample is selected to simulate the rock formation on the outside thereof, and silica gel is applied to the top surface of the fresh coal sample. During the period, pour the mold into the mold until the thickness reaches the design requirements and wait for solidification. After solidification, a layer of simulated rock sample rock layer on the top of the fresh coal sample is formed; select the rock sample formula corresponding to the actual rock type on the bottom surface of the fresh coal sample to simulate the rock layer on the outside thereof. Apply silica gel on the bottom surface of the coal sample, and pour it into the mold before solidification according to the rock sample formula until the thickness reaches the design requirements and wait for solidification. After solidification, a layer of simulated rock sample rock layer at the bottom of the fresh coal sample is formed; Continue to pour different rock sample formula liquids on the outside of the top and bottom of the rock sample layer until the entire mold is completed, in which a layer of silica gel is applied for each layer of simulated rock sample rock layer poured, waiting for the last layer of simulated rock sample rock layer to solidify; the thickness of different rock sample rock layers According to step 4, the actual observed rock layer thickness value in the composite reservoir core is scaled by 1:100 to complete the preparation of a coal-bearing multi-lithologic rock composite cube specimen; the size of the mold is 30cm×30cm×30cm, and the inner wall of the mold is painted Lubricants; coal-bearing multi-lithologic rock assemblies include such as sandstone layer-mudstone layer-coal sample-mudstone layer-sandstone layer, sandstone layer-mudstone layer-coal sample-sandstone layer-mudstone layer, mudstone layer-sandstone layer-coal A variety of combinations including sample-sandstone layer-mudstone layer, mudstone layer-sandstone layer-coal sample-mudstone layer-sandstone layer, and the cementing time is at least 10 hours;

In step 6, on the same side of the cube specimen prepared in step 5, boreholes were drilled in the central part of the coal sample and the self-made rock samples with different lithologies, and the steel liquid injection pipe was consolidated in the hole by resin glue. The simulated wellbore is 16 cm long and 15 mm in diameter; the steel liquid injection pipe used is 20 cm long, 12 mm in outer diameter and 6 mm in inner diameter, and the steel pipe is cemented to the simulated wellbore with resin glue at a depth of 14 cm;

Step 7: Use the sealing material to externally pour the coal-bearing multi-lithologic rock combination cube to prepare a standard size coal-measure composite reservoir physical simulation fracturing specimen; the sealing material is resin, and its thickness is 50mm, and the standard size It is 35cm×35cm×35cm; after the coal-bearing multi-lithologic rock combination cube is poured, it is dried under natural conditions for 2 to 3 weeks.

Claims (5)

1. A preparation method of a coal-series composite reservoir fracturing physical simulation test piece is characterized by comprising the following steps:

step 1, collecting a large fresh coal sample and small rock samples with different lithological properties under a mine, marking the sampling direction and horizon, wherein the size of the large coal sample is more than 5cm multiplied by 30cm, the surface of the coal sample with the size of about 30cm multiplied by 30cm is a natural section, the interface of coal and rock is ensured to be an irregular contact surface, and the size of the small rock sample is more than 8cm multiplied by 8 cm;

step 2, scanning the natural section of the large fresh coal sample obtained in the step 1 by using a laser scanner, calculating a fractional dimensional value of the surface appearance of the section, and quantitatively representing the irregularity degree of the surface of the coal sample according to the value, wherein the larger the fractional dimensional value is, the more irregular the surface of the coal sample is;

step 3, carrying out X-diffraction analysis and rock mechanics detection on small rock samples with different lithological properties, obtaining mineral compositions and contents of rocks with different lithological properties and rock mechanics parameters, simulating formulas of the rock samples with different lithological properties by using sand, cement and clay according to detection results, and correspondingly marking the rock samples prepared by the formulas as C1, C2 and … Cn;

step 4, determining the rock combination type and different lithologic rock thicknesses of the coal-series composite reservoir based on the observation of the coal-series composite reservoir rock core;

step 5, preparing a coal-containing multi-lithologic rock combined cubic test piece by using a cubic steel mold: firstly, according to the coal-series composite reservoir rock combination type determined in the step 4, designing the position of a large fresh coal sample in a mold, taking two surfaces of the fresh coal sample, 30cm multiplied by 30cm, as a top surface and a bottom surface, selecting a rock sample formula corresponding to the actual rock type of the top surface of the fresh coal sample to simulate a rock stratum on the outer side of the fresh coal sample, coating silica gel on the top surface of the fresh coal sample, pouring the fresh coal sample into the mold during the non-solidification period after blending according to the rock sample formula until the thickness reaches the design requirement, waiting for solidification, and forming a layer of simulated rock sample rock stratum on the top of the fresh coal sample after; selecting a rock sample formula corresponding to the actual rock type of the bottom surface of the fresh coal sample to simulate the rock stratum on the outer side of the fresh coal sample, coating silica gel on the bottom surface of the fresh coal sample, mixing according to the rock sample formula, pouring into a mould while the fresh coal sample is not solidified until the thickness meets the design requirement, waiting for solidification, and forming a layer of simulated rock sample rock stratum at the bottom of the fresh coal sample after solidification; continuously casting different rock sample formula liquids on the outer sides of the top surface and the bottom surface of the simulated rock sample rock stratum in sequence according to the same method until the whole mould is completed, wherein each time one simulated rock sample rock stratum is poured, one layer of silica gel is coated, and the last simulated rock sample rock stratum is waited to be solidified; and (4) according to the thickness of different rock sample rock stratums and the actually observed rock stratum thickness value in the composite reservoir rock core in the step 4, the thickness of the rock sample rock stratums is determined according to the following formula that: scaling by 100 to finish the preparation of a coal-containing multi-lithologic rock combined cubic test piece;

step 6, respectively drilling holes in the central parts of the coal sample and the self-made rock samples with different lithologies on the same side surface of the cubic test piece prepared in the step 5, and solidifying a steel injection pipe in the holes by using resin glue to simulate a borehole, wherein the simulated borehole is 16cm in length and 15mm in diameter; the length of the used steel injection pipe is 20cm, the outer diameter is 12mm, the inner diameter is 6mm, the steel injection pipe is fixedly bonded to the simulated borehole by resin glue, and the length of the steel injection pipe in the simulated borehole is 14 cm;

and 7, performing external pouring on the coal-containing multi-lithologic rock combined cube by using a sealing material to prepare a coal-series composite reservoir physical simulation fracturing test piece with a standard size.

2. The preparation method of the coal-series composite reservoir fracturing physical simulation test piece according to claim 1, characterized by comprising the following steps: the rock layers of the different lithological rock samples recorded in the step 1 comprise sandstone layers, argillaceous sandstone layers, sandy argillaceous rock layers and argillaceous rock layers.

3. The preparation method of the coal-series composite reservoir fracturing physical simulation test piece according to claim 1, characterized by comprising the following steps: when the optimal formula is selected in the step 3, firstly, sand, cement and clay are placed into a stirrer to be stirred for at least 1 hour, clear water is added into the mixture to be stirred again after the mixture is uniform, about 50mL of retarder is added in the process, and the mixture is placed for at least 2 weeks after solidification; drilling a plunger sample in a small rock sample with the size of more than 8cm multiplied by 8cm to carry out rock mechanical property detection, wherein the size of the plunger sample is 2.5cm in diameter and 5cm in height; the determination of the optimal formulation is based on the comparison of the young's modulus, poisson's ratio, cohesion, internal friction angle and tensile strength of the home-made sample with the actual small rock sample.

4. The preparation method of the coal-series composite reservoir fracturing physical simulation test piece according to claim 1, characterized by comprising the following steps: the size of the die in the step 5 is 30cm multiplied by 30cm, and the inner wall of the die is coated with a lubricant; the coal-containing multi-lithology rock combination comprises a plurality of combination modes including a sandstone layer-a shale layer-a coal sample-a mudstone layer-a sandstone layer, a sandstone layer-a mudstone layer-a coal sample-a sandstone layer-a mudstone layer, a mudstone layer-a sandstone layer-a coal sample-a sandstone layer-a mudstone layer, and a mudstone layer-a sandstone layer-a coal sample-a mudstone layer-a sandstone layer; the cementing time of the homemade rock sample exceeds 10 hours.

5. The preparation method of the coal-series composite reservoir fracturing physical simulation test piece according to claim 1, characterized by comprising the following steps: in the step 7, the sealing material is resin, the thickness of the sealing material is 50mm, and the standard size is 35cm multiplied by 35 cm; and after the coal-containing multi-lithologic rock combined cube is poured, drying for 2-3 weeks under a natural condition.

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