CN114622905A - System and method for testing conductivity of low-concentration proppant - Google Patents
System and method for testing conductivity of low-concentration proppant Download PDFInfo
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- E—FIXED CONSTRUCTIONS
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- E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
- E21B43/25—Methods for stimulating production
- E21B43/26—Methods for stimulating production by forming crevices or fractures
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Abstract
The invention discloses a system and a method for testing the conductivity of a low-concentration proppant, and belongs to the technical field of proppant conductivity testing. The technical scheme is as follows: the method comprises placing proppant into upper and lower cores with arched end faces, and sealing with cushion block with filter screen and thermal shrinkage sleeve to simulate supporting crack; preferably selecting a high-pressure rock core holder, a high-pressure confining pump with a worm and gear labor-saving transmission system and a differential pressure sensor with a proper measuring range, and assembling a test flow; injecting a test fluid into the supporting fracture model, obtaining fluid flow pressure difference and flow parameters under set closing pressure, and calculating the flow conductivity of the supporting fracture; and calculating the propping fracture opening based on the equivalent volume. The method not only makes up the test limitation of the existing proppant flow conductivity test system and method, but also widens the test limit, reduces the configuration difficulty of the test device, and more importantly greatly reduces the test cost.
Description
Technical Field
The invention relates to the technical field of proppant conductivity testing, in particular to a system and a method for testing the conductivity of a low-concentration proppant.
Background
The proppant is a key material for fracturing construction, is used for supporting the fracture, preventing the fracture from being closed and keeping the high flow conductivity of the fracture. The proppant conductivity test is a main way to evaluate the performance of the proppant, optimize the application scheme of the proppant and know and understand the change rule of the proppant conductivity under the oil reservoir condition. With the continuous development of the proppant technology, the proppant can not only effectively support the main fracture of artificial fracturing, but also can enter secondary fractures and natural microcracks to generate support. The primary fracture differs from other fracture proppant placement states by: the proppant in the main fracture has large grain size and high laying concentration; the proppant in the secondary cracks and the micro cracks has small grain diameter and low laying concentration. In order to better know and understand the conductivity of the propping agent in the secondary fractures and the microcracks, the research on the method for testing the conductivity of the low-concentration propping agent is significant, wherein the concentration range of the low-concentration propping agent is the sand laying concentration corresponding to the filling thickness of less than or equal to 2mm, namely<0.2ρvg/cm2，ρvIs volume density, g/cm3。
In the present situation, there are mainly 2 methods for testing the flow conductivity, one is a test method using a recommended standard flow guiding chamber as a support fracture simulation subject: the standard flow guide chamber is an experimental device for simulating the support crack recommended by SY/T6302-2019 industry standard, and consists ofUpper and lower pistons, experiment table, metal filter screen, etc., the laying area of the flow guide cavity proppant formed by the upper and lower pistons and the experiment table is 64.5cm2(ii) a And secondly, a test method taking the artificial fracture core as a supporting fracture simulation main body comprises the following steps: selecting a reservoir rock sample or an outcrop rock sample, preparing the rock sample with a rough fracture surface by using an artificial fracture-making method, simulating an underground supported fracture by filling a proppant or a sliding combination mode, and obtaining the flow conductivity of the supported fracture by using a supported fracture model fluid flow experiment. Although the methods are feasible, the application limit and the application limitation exist, and based on a testing method established by a standard diversion room, the standard diversion room is easy to block a pressure measuring end, an inlet end and an outlet end or fluid circumfluence in the experimental process under the condition of low-concentration proppant, so that the diversion capacity of the proppant cannot be effectively obtained; the test method established based on the artificial fracture core method has the following problems: firstly, the uncontrollable roughness of the wall surface of the crack causes large difference and instability of test results; secondly, the low pressure bearing capacity of the conventional rock core holder and the confining pressure pump leads to a narrow closing pressure test range, and although the problem of low closing pressure can be solved by introducing a large-scale loading system, the experimental device is difficult to assemble and inconvenient; thirdly, application limitations exist, such as the opening degree of the supporting fracture cannot be represented.
At present, more researches on a proppant conductivity test method are carried out: the invention patent with application publication number CN110608037A and patent number CN107806339B establishes a proppant conductivity evaluation method considering the influence of fracture wall morphology and proppant distribution condition on the proppant conductivity based on a standard flow guide chamber; the invention patent with application publication numbers of CN104295281A and CN104295281 combines a metal plate, a metal patch or a rock plate and a propping agent into a fracture network model, and places the fracture network model into a standard diversion chamber to establish a fracture diversion capability test method under the condition of complex fracture network; the invention patent with the patent number of CN110593842B establishes a self-supporting fracture conductivity test method by placing an artificial rock plate into a standard flow guide chamber through a self-supporting fracture model formed by shearing and sliding, but in the test process, particularly when the opening of a supporting fracture is narrow, the problems that an inlet and an outlet are easy to occur in the standard flow guide chamber, a pressure measuring port is blocked, fluid flows around along the gap between the rock plate and the inner wall of the flow guide chamber and the like cannot be effectively tested for the conductivity of a low-concentration proppant. The invention with application publication number CN113029898A establishes a self-supporting fracture conductivity test method based on the designed square core holder, and has the problems that the closed pressure test range is narrow, and the fracture opening degree cannot be represented; the invention patent with application publication number CN111103222A utilizes a triaxial testing machine to establish a self-supporting fracture conductivity test based on a cylindrical core sample, the invention patent of CN113295537A utilizes an MTS rock mechanics test system to establish an unconventional reservoir fracturing fracture permeability evaluation method considering the water and rock action based on the cylindrical core sample, and due to the introduction of a large-scale loading system, the problem of narrow closed pressure test range is solved, but the problem that the opening of the supporting fracture cannot be represented still exists, and the application popularity of the large-scale loading system is limited.
In conclusion, the existing proppant conductivity test method based on the standard flow guide chamber or the artificial core sample has some defects in the low-concentration proppant conductivity test, and the optimization of the proppant application scheme in the secondary fractures and the microcracks is difficult to carry out conveniently, stably and effectively.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the system and the method for testing the flow conductivity of the low-concentration proppant can effectively solve the problems that a pressure measuring end, an inlet end and an outlet end of a standard flow guide chamber are blocked or fluid flows around, can also effectively solve the problems that the flow conductivity of the proppant is large and the opening degree of the fracture cannot be represented due to uncontrollable roughness of the wall surface of a core of an artificial fracture, and effectively improve the understanding and comprehension of the flow conductivity of the proppant in deep part of the fracture, secondary fracture and microcrack.
In one aspect, the invention provides a system for testing the flow conductivity of a low-concentration proppant, which comprises a core holder, wherein a core is contained in the core holder; the inlet of the core holder is connected with a constant flow pump through a pipeline, the outlet of the core holder is communicated with a fluid container through a pipeline, and the fluid container is arranged on a precision balance; the side wall of the rock core holder is connected with a confining pressure pump through a pipeline; a differential pressure sensor is also connected between the inlet and the outlet of the rock core holder; the rock core comprises an upper rock core and a lower rock core which are symmetrically arranged, and a propping agent is paved between the upper rock core and the lower rock core; the contact surfaces of the upper core and the lower core and the propping agent are rectangular, and two end surfaces of the upper core and the lower core, which are vertical to the contact surfaces, are arched; cushion blocks are arranged at two ends of the rock core respectively, a through hole is formed in the center of each cushion block, and a filter screen is arranged at the through hole on one side face, close to the rock core, of each cushion block. The filter screen is used for ensuring that the fluid can enter and simultaneously blocking the propping agent between the upper core and the lower core, so that the propping agent cannot flow into the fluid channel along the flowing direction of the fluid.
Preferably, the core holder further comprises an intermediate container, wherein an inlet of the intermediate container is connected with an outlet of the constant flow pump through a pipeline, and an outlet of the intermediate container is connected with an inlet of the core holder through a pipeline; valves are respectively arranged on a pipeline between the constant flow pump and the core holder, a pipeline between the constant flow pump and the intermediate container, a pipeline between the intermediate container and the core holder, and a pipeline between an outlet of the core holder and the fluid container.
Preferably, a screw rod in a working barrel of the confining pressure pump is connected with the handle through a worm wheel and a worm.
Preferably, the span of the differential pressure sensor is selected by a differential pressure estimation formula, wherein the differential pressure estimation formula is:
wherein Δ p is the estimated pressure difference, pa; μ is the fluid viscosity, pa · s; q is the fluid flow, m3S; tau is the tortuosity of a seepage channel of the proppant layer, and the value is carried out according to the particle size of the proppant, the large value of the particle size is lower, otherwise, the value is higher, and the value range is 1.5-5.5; lfLength of the propped fracture model, m; w is afWidth of the propped fracture model, m; h isf estimateTo estimate the crack opening, m;
wherein h isf estimateCan be calculated from the change in porosity,the calculation formula is as follows:
in the formula, phi is the porosity and decimal of the proppant under different closing pressures; p is closing pressure, MPa; alpha is a constant, and the value is taken according to the compressive strength of the proppant, the compressive strength is high, and is low, otherwise, the value is high, and the value range is 0.010-0.027; csLaying the proppant in g/cm2；ρsIs apparent density of proppant, g/cm3；ρvIs proppant bulk density, g/cm3。
In another aspect, the present invention provides a method for testing the conductivity of a low concentration proppant by using the above system, comprising the steps of:
s1: laying a propping agent between an upper rock core and a lower rock core, assembling cushion blocks at two ends of the rock core to form a propping fracture model, and putting the propping fracture model into a rock core holder; loading confining pressure to a closed pressure value, and acquiring fluid flow pressure difference and fluid flow parameters under the closed pressure value;
s2: and calculating the flow conductivity of the low-concentration proppant by utilizing a Darcy formula in combination with the geometric size of the supporting fracture model, wherein the Darcy formula is as follows:
in the formula, khfIs the flow conductivity of the proppant, um2Cm; q is the fluid flow, cm3/min；wfIs the width of the propping fracture model, cm; h isfOpening degree of the supporting crack model is cm; Δ p is the flow pressure differential, kpa; μ is the fluid viscosity, mpa · s; lfLength in cm for the propped fracture model.
Preferably, the method further comprises a characterization step S31 of the opening degree of the propping fracture: based on the capillary seepage model, calculating the equivalent capillary radius and the corresponding pore volume of the proppant layer by using a Poiseup formula:
in the formula, rpEquivalent capillary radius, cm; μ is viscosity, pa · s; l. thefLength of the propped fracture model, cm; q is the fluid flow, cm3S; Δ p is the test differential pressure, pa; vpIs the pore volume of the proppant layer, mL;
s32: obtaining the volume density and the apparent density of the proppant by using the precision balance and a fluid container, and then calculating the volume of the propping fracture skeleton:
in the formula, CsLaying the proppant in g/cm2；ρvIs proppant volume density, g/cm3；VsIs the volume, mL, of the proppant skeleton in the proppant layer; w is afIs the width of the propping fracture model, cm; lfLength of the propped fracture model, cm; rhosIs apparent density of proppant, g/cm3；
S33: calculating the opening of the proppant propped fracture under different closing pressures:
in the formula, hfCm for supporting crack opening; vpPore volume of the proppant layer, mL; vsIs the volume of the proppant skeleton in the proppant layer, mL; w is afIs the width of the propping fracture model, cm; lfLength in cm for the propped fracture model.
Compared with the prior art, the invention has the following beneficial effects:
1. the arc column rock core and cushion block simulation supporting crack testing system designed by the invention can effectively solve the problems that the pressure measuring end, the inlet end and the outlet end of a standard flow guide chamber are blocked or fluid flows around, and simultaneously can effectively solve the problem that the difference of the flow guide capacity of the propping agent is large due to uncontrollable roughness of the rock core wall surface of an artificial fracture, and effectively improves the understanding and comprehension of the flow guide capacity of the propping agent in deep cracks, secondary cracks and microcracks.
2. The preferred core holder and the high-pressure confining pump with the worm and gear labor-saving transmission system solve the problems of narrow test range and complicated and inconvenient loading system caused by low pressure range of the conventional core holder and the loading system. By adopting the method, the dependence degree of a large loading system such as a pressure testing machine or a three-axis testing machine can be effectively reduced, the convenience of the testing process of the diversion capacity of the proppant is improved, and the testing cost of the diversion capacity of the proppant is reduced.
3. The characterization method for the opening degree of the propping fractures can effectively obtain the opening degree of the propping fractures in the test process, effectively describe the change rule of the propping fractures of the propping agent, and solve the problem that the opening degree of the propping fractures cannot be monitored in the test process of totally-enclosed samples.
Drawings
FIG. 1 is a schematic diagram of the system of the present invention.
Fig. 2 is a schematic structural view of a core of the present disclosure.
Fig. 3 is a front view of the spacer of the present invention.
In the figure, 1-a core holder, 2-a constant flow pump, 3-a fluid container, 4-a precision balance, 5-a confining pressure pump, 6-a differential pressure sensor, 7-an upper core, 8-a lower core, 9-a propping agent, 10-a cushion block, 11-a through hole, 12-a middle container, 13-a valve and 14-a groove.
Detailed Description
The flow conductivity of the proppant is the product of the permeability of a proppant filling layer and the opening of a propping fracture, and is used for describing the flow capacity of a gas seepage channel in a hydrocarbon reservoir. The proppant conductivity under different conditions obtained through experimental tests has the following significance: the method is used for optimizing the design of a proppant application scheme in the fracturing construction design; secondly, the method is used for optimizing the production system of the reservoir after hydraulic fracturing of the reservoir; and thirdly, evaluating the performance of the proppant.
The opening degree of the propping fractures refers to the relative distance between two fracture wall surfaces in the direction perpendicular to the fracture wall surfaces, and is an important parameter for quantitatively describing the fracturing fractures. The opening degree of the supporting crack obtained through experimental tests has the following significance: firstly, on the basis of the flow conductivity of the proppant, the permeability of the proppant filling layer is obtained; and secondly, the method is used for quantitatively describing the change process and rule of the fracturing fracture under the oil reservoir condition.
The invention discloses a system for testing the flow conductivity of a low-concentration proppant, wherein the low-concentration proppant refers to a sand laying concentration with the concentration lower than that of the proppant with the filling thickness of less than or equal to 2mm, namely<0.2ρvg/cm2，ρvIs volume density, g/cm3. Referring to fig. 1, the system comprises a core holder 1, wherein a core is contained in the core holder 1; the inlet of the core holder 1 is connected with a constant flow pump 2 through a pipeline, the outlet is communicated with a fluid container 3 (such as a measuring cylinder) through a pipeline, and the fluid container 3 is arranged on a precision balance 4; the side wall of the core holder 1 is connected with a confining pressure pump 5 through a pipeline; a differential pressure sensor 6 is also connected between the inlet and the outlet of the core holder 1; the rock core comprises an upper rock core 7 and a lower rock core 8 which are symmetrically arranged, and a propping agent 9 is paved between the upper rock core 7 and the lower rock core 8; the contact surfaces of the upper core 7 and the lower core 8 with the propping agent 9 are rectangular, and two end surfaces of the upper core 7 and the lower core 8, which are perpendicular to the contact surfaces, are arched; cushion blocks 10 are respectively arranged at two ends of the rock core, a through hole 11 is formed in the center of each cushion block 10, and a filter screen is arranged on one side face, close to the rock core, of each cushion block 10.
According to the testing closing pressure, a proper core holder 1 and a high-range confining pressure pump 5 with a worm and gear labor-saving transmission system can be selected, and a lead screw in a working barrel of the confining pressure pump 5 is connected with a handle through a worm and a gear. When the electric hydraulic pump is used for driving the confining pump, the electric hydraulic pump system is too large, the occupied area is large, and the cost is high; when the manual confining pressure pump is used, the manual pressure can not reach the high pressure of 69MPa, so the confining pressure pump of the invention makes up the defects of the two modes, abandons an electric hydraulic pump system and can use the manual pressure to reach the closing pressure of 69MPa on the premise of saving labor.
The reduction ratio i of the worm and gear transmission system is calculated by using a reduction ratio calculation formula:
in the formula, i is a reduction ratio and is dimensionless; p is a radical ofpIs confining pressure, pa; dpThe diameter of the confining pressure pump plunger is m; l is lead screw lead, mm; f is the applied force, N; r is the radius of the hand wheel, mm; eta is transmission efficiency, decimal.
And selecting a constant flow pump with the flow range of 1-10mL/min, estimating the pressure difference according to parameters such as the volume density of the proppant, the apparent density, the laying concentration of the proppant, the test closing pressure and the like, and preferably selecting the measuring range of the sensor. The pressure difference sensor selected by experience can cause the flow pressure difference to exceed the measuring range during the experiment, or the measuring range is too large, so that the flow pressure difference can not be accurately obtained. Therefore, the differential pressure estimation method of the invention makes up the defect of selecting a mode by experience, provides the selection basis of the measuring range of the differential pressure sensor, and can ensure the accuracy of the flow differential pressure test within the precision range of the differential pressure sensor.
Wherein, the pressure difference Δ p is estimated by using a pressure difference estimation formula:
wherein Δ p is the estimated pressure difference, pa; μ is viscosity, pa · s; q is the fluid flow, m3S; tau is the tortuosity of a seepage channel of the proppant layer, and the value is carried out according to the particle size of the proppant, the large value of the particle size is lower, otherwise, the value is higher, and the value range is 1.5-5.5; lfLength of the propped fracture model, m; w is afM is the width of the propped fracture model; h isf estimateTo estimate the crack opening, m.
Wherein h isf estimateCan be based on porosityThe change is calculated by the following formula:
in the formula, phi is the porosity and decimal of the proppant under different closing pressures; p is the closure pressure, MPa; alpha is a constant, and the value is taken according to the compressive strength of the proppant, the compressive strength is high, and is low, otherwise, the value is high, and the value range is 0.010-0.027; csLaying the proppant in g/cm2；ρsIs apparent density of proppant, g/cm3；ρvIs proppant volume density, g/cm3。
Referring to fig. 2, a standard cylinder core sample with phi of 25 x 50-80mm is prepared by using 316 stainless steel or a natural outcrop core, and is cut along an axial section to form an arc cylinder core with completely symmetrical upper and lower parts, the chord length L of an end face arch is 23.9-24.8mm, and the height H of the arch arc is 8.9-11.0 mm.
Referring to fig. 3, two phi 25 x 10-20mm encapsulation pads 10 on which stainless steel screens can be placed are prepared from 316 stainless steel for supporting the encapsulation of the fracture model end face. A groove 14 is formed in the surface, close to the core, of the cushion block 10, and a filter screen is placed in the groove 14.
Preparing a low-concentration proppant supporting crack model by using an aluminum foil adhesive tape and a heat-shrinkable film:
(1) aligning the lower rock core 7 and two cushion blocks 10 provided with stainless steel filter screens by using an aluminum foil adhesive tape to form a proppant cavity;
(2) and adding a propping agent 9 into the propping agent cavity, covering the core 7, and then sleeving a heat-shrinkable film for packaging, thereby preparing the low-concentration propping agent propping fracture model.
Finally, referring to fig. 1, the test system of the present invention is assembled to construct a low concentration proppant conductivity test procedure.
The system also comprises an intermediate container 12, wherein the inlet of the intermediate container 12 is connected with the outlet of the constant flow pump 2 through a pipeline, and the outlet of the intermediate container 12 is connected with the inlet of the core holder 1 through a pipeline; valves 13 are respectively arranged on a pipeline between the constant flow pump 2 and the core holder 1, a pipeline between the constant flow pump 2 and the intermediate container 12, a pipeline between the intermediate container 12 and the core holder 1, and a pipeline between an outlet of the core holder 1 and the fluid container 3.
The intermediate container 12 is used for pumping special liquid (such as fracturing fluid gel breaking liquid and the like) which cannot be pumped by the constant flow pump 2 directly into the core holder 1. If the fluid is distilled water, the distilled water can be directly pumped into the core holder 1 along a pipeline by the constant flow pump 2 without passing through the intermediate container 12; when the fluid is a special liquid, the valves at both ends of the intermediate container 12 need to be opened, the valves on the distilled water pipeline need to be closed, and distilled water is pumped into the lower end space of the intermediate container 12 through the constant flow pump 2, so that the special liquid at the upper end is pushed into the core holder 1.
On the basis of the work, flow parameters under test liquid laminar seepage are obtained by using a fluid flow experiment of a low-concentration proppant propping fracture model, and the flow conductivity of the low-concentration proppant is calculated by using a Darcy formula, and the specific operation is as follows:
s1: placing a low-concentration proppant propping fracture model into a rock core holder shown in figure 1, and loading a fixed sample with confining pressure of 1 MPa; starting a constant-flow pump to inject test liquid, and discharging air of the differential pressure sensor and the test system; loading confining pressure to a closed pressure value, and acquiring parameters such as fluid flow pressure difference and flow of the lower support fracture model under the closed pressure;
s2: and calculating the flow conductivity of the low-concentration proppant by using a Darcy formula in combination with the geometric dimension of the low-concentration proppant propping fracture model, wherein the Darcy formula is as follows:
in the formula, khfIs the flow conductivity of the proppant, um2Cm; q is the fluid flow, cm3/min；wfIs the width of the propping fracture model, cm; h isfOpening degree of the supporting crack model is cm; Δ p is the flow pressure differential, kpa; μ is the fluid viscosity, mpa · s; lfFor supporting fracture mouldLength of the form, cm.
And a characterization step S31 of the opening of the propping fracture: based on the capillary seepage model, calculating the equivalent capillary radius and the corresponding pore volume of the proppant layer by using a Poiseup formula:
in the formula, rpEquivalent capillary radius, cm; μ is viscosity, pa · s; lfLength of the propped fracture model, cm; q is the fluid flow, cm3S; Δ p is the test differential pressure, pa; vpPore volume of the proppant layer, mL;
s32: obtaining the volume density and the apparent density of the proppant by using the precision balance and a fluid container, and then calculating the volume of the propping fracture skeleton:
in the formula, CsLaying the proppant in g/cm2；ρvIs proppant volume density, g/cm3；VsIs the volume, mL, of the proppant skeleton in the proppant layer; w is afIs the width of the propping fracture model, cm; lfLength of the propped fracture model, cm; rhosIs apparent density of proppant, g/cm3；
S33: calculating the opening of the proppant propped fracture under different closing pressures:
in the formula, hfCm for supporting crack opening; vpIs the pore volume of the proppant layer, mL; vsIs the volume, mL, of the proppant skeleton in the proppant layer; w is afIs the width of the propping fracture model, cm; lfLength in cm for the propped fracture model.
Example 1
At a spreading concentration as low as 0.16g/cm2The proppant conductivity test of 40/70-mesh quartz sand is taken as an example to further illustrate the low-concentration proppant conductivity test method, and specifically comprises the following steps:
1. optimizing equipment and constructing a flow test process for the conductivity of the low-concentration proppant
(1) When the maximum value of the closing pressure is 69MPa, a rock core holder capable of resisting the pressure of 80MPa and a high-range confining pressure pump with a worm and gear labor-saving transmission system are selected, and parameters such as the diameter (25mm), the lead (10mm), the radius (10cm) of a hand wheel, the transmission efficiency (0.6) of the worm and gear, the applied force (100 plus 200N) and the like of a plunger of the confining pressure pump are known according to a formula
The reduction ratio i of the worm and gear transmission system can be calculated to be 5-10;
(2) selecting a constant flow pump with the flow range of 1-10mL/min, wherein the known volume density of the proppant is 1.632g/cm3And an apparent density of 2.646g/cm3The laying concentration is 0.16g/cm2The maximum closing pressure 69MPa is measured, the differential pressure at the maximum flow rate is estimated by using a differential pressure estimation formula, and a differential pressure sensor (μ ═ 1MPa · s) with the measurement range of 200kPa is preferably selected. The results of the evaluation are shown in the following table:
(3) and (3) assembling the optimized equipment according to the flow shown in the figure 1 to construct a flow for testing the flow conductivity of the low-concentration proppant.
2. Preparation of arc column core and cushion block
(1) Preparing a standard cylinder sample with the diameter of 25 multiplied by 50mm by using 316 stainless steel, cutting the standard cylinder sample into an upper core and a lower core of a circular arc cylinder which are completely symmetrical up and down along an axial section, wherein the chord length L of an end face arch is 24.5mm, and the height H of an arc is 10.0 mm;
(2) two cushion blocks with the diameter of 25 multiplied by 20mm and capable of placing stainless steel filter screens are prepared by 316 stainless steel, and the stainless steel filter screens with the meshes of 200 are placed for supporting the end face encapsulation of the crack model.
3. Low-concentration proppant supporting crack model prepared by using aluminum foil adhesive tape and thermal shrinkage film
(1) Aligning and combining the lower core and two cushion blocks filled into the stainless steel filter screen by using an aluminum foil adhesive tape to form a propping agent cavity;
(2) uniformly adding 1.96g of quartz sand proppant of 40/70 meshes into the proppant cavity, covering a core, and then sheathing a thermal shrinkage film for packaging to prepare a low-concentration quartz sand support fracture model.
4. Obtaining flow parameters under the flow seepage of a distilled water layer by using a fluid flow experiment of a low-concentration quartz sand support fracture model, and calculating the flow conductivity of a low-concentration proppant by using a Darcy formula
(1) Placing the low-concentration quartz sand support fracture model into a rock core holder in a testing process, loading a fixed sample with confining pressure of 1MPa, starting a constant-flow pump to inject distilled water, and discharging air of a differential pressure sensor and a testing system; loading confining pressure to different set closing pressure values, bearing pressure for a certain time, and acquiring parameters such as fluid flow pressure difference and flow under the closing pressure;
(2) combining the geometric dimension of the low-concentration quartz sand supported fracture model, calculating the flow conductivity of the proppant by using a Darcy formula, wherein the calculation result is shown in the following table:
5. calculating the equivalent capillary radius, pore volume and proppant skeleton volume of the quartz sand proppant layer, and further calculating the opening of the propped fracture
(1) Based on a capillary seepage model, calculating the equivalent capillary radius and the corresponding pore volume of the quartz sand proppant layer by using a Poiseul formula, wherein the calculation result is shown in the following table:
(2) the volume density of the 40/70-mesh quartz sand obtained by using a precision balance (precision 0.001g) and a 100mL density bottle is 1.632g/cm3And an apparent density of 2.646g/cm3Calculating the initial opening of the supporting crack to be 0.098cm and the framework volume of the quartz sand to be 0.741mL, wherein the known mass of the quartz sand is 1.96 g;
(3) the opening degree of the 40/70-mesh quartz sand supported fracture under different closing pressures is calculated, and the calculation result is shown in the following table:
6. stability analysis of low concentration proppant conductivity test method
Aiming at the data of three times of experiments of 1.96g of 40/70-mesh quartz sand proppant, the standard deviation of the test fluctuation is analyzed, the standard deviation of the three-time flow conductivity is 0.01-1.03, and the standard deviation of the three-time supported fracture opening is 0.0001-0.0003, which are all relatively stable. The following table shows the standard deviation of the conductivity and the opening of the support fracture obtained by three tests:
example 2
At a laying concentration as low as 0.2g/cm2Proppant conductivity test of 70/140 mesh silica sand as an example to further illustrate the low concentration of the present inventionThe proppant conductivity testing method specifically comprises the following steps:
1. optimizing equipment and constructing a flow test process for the conductivity of the low-concentration proppant
(1) The calculation method of the worm gear transmission ratio is the same as that of embodiment 1;
(2) selecting a constant flow pump with the flow range of 1-10mL/min and the proppant volume density of 1.550g/cm3And an apparent density of 2.639g/cm3The laying concentration is 0.2g/cm2The maximum closing pressure 69MPa is measured, the differential pressure at the maximum flow rate is estimated by using a differential pressure estimation formula, a differential pressure sensor (mu is 1MPa · s) with the measurement range of 300kPa is preferably selected, and the estimation result is shown in the following table:
(3) and (3) assembling the optimized equipment according to the flow shown in the figure 1 to construct a flow for testing the flow conductivity of the low-concentration proppant.
2. Preparation of arc column core and cushion block
(1) Preparing a standard cylinder sample phi of 25 multiplied by 80mm by using 316 stainless steel, cutting the standard cylinder sample into an upper core and a lower core of a circular arc cylinder which are completely symmetrical up and down along an axial section, wherein the chord length L of an end face arch is 24.8mm, and the height H of an arc is 11.0 mm;
(2) two cushion blocks with the diameter of 25 multiplied by 10mm and capable of placing stainless steel filter screens are prepared by 316 stainless steel, and the stainless steel filter screens with the meshes of 200 are placed for supporting the end face encapsulation of the crack model.
3. Low-concentration proppant supporting crack model prepared by using aluminum foil adhesive tape and thermal shrinkage film
(1) Aligning the lower core with two cushion blocks filled in the stainless steel filter screen by using an aluminum foil adhesive tape to form a proppant cavity;
(2) 3.968g of quartz sand proppant with the size of 70/140 meshes is uniformly added into the proppant cavity, the core is covered, and then a thermal shrinkage film is sleeved for packaging to prepare a low-concentration quartz sand support fracture model.
4. Obtaining flow parameters under the flow seepage of a distilled water layer by using a fluid flow experiment of a low-concentration quartz sand support fracture model, and calculating the flow conductivity of the low-concentration proppant by using a Darcy formula
(1) Placing a low-concentration quartz sand support fracture model into a rock core holder in a test process, loading a fixed sample with confining pressure of 1MPa, starting a constant flow pump to inject distilled water, and discharging air of a differential pressure sensor and a test system; loading confining pressure to different set closing pressure values, bearing pressure for a certain time, and acquiring parameters such as fluid flow pressure difference and flow under the closing pressure;
(2) combining the geometric dimension of the low-concentration quartz sand supported fracture model, calculating the flow conductivity of the proppant by using a Darcy formula, wherein the calculation result is shown in the following table:
5. calculating the equivalent capillary radius, pore volume and proppant skeleton volume of the quartz sand proppant layer, and further calculating the opening of the propped fracture
(1) Based on the capillary seepage model, the equivalent capillary radius and the corresponding pore volume of the quartz sand proppant layer are calculated by using a Poiseup formula, and the calculation result is shown in the following table:
(2) the volume density of the 70/140-mesh quartz sand obtained by using a precision balance (precision 0.001g) and a 100mL density bottle is 1.550g/cm3And an apparent density of 2.639g/cm3The mass of the known quartz sand is 3.968g, the initial opening of the supporting fracture is calculated to be 0.129cm, and the framework volume of the quartz sand is 1.504 mL;
(3) the opening degree of the 70/140-mesh quartz sand supported fracture under different closing pressures is calculated, and the calculation result is shown in the following table:
6. stability analysis of low concentration proppant conductivity test method
(1) Aiming at the three experimental data of 3.968g of 70/140-mesh quartz sand proppant, the standard deviation is used for analyzing the test volatility, the standard deviation of the three-time flow conductivity is 0.02-0.13, and the standard deviation of the three-time support fracture opening is 0.0002-0.0007, which are all relatively stable. The following table shows the standard deviation of the conductivity and the opening of the propped fracture obtained by three tests:
example 3:
at a laying concentration as low as 0.1g/cm2The proppant conductivity test taking the 30/50-mesh ceramsite fluid as the supernatant (viscosity mu is 3mpa · s) of the fracturing fluid gel breaking liquid as an example further illustrates the conductivity test method of the low-concentration proppant, which specifically comprises the following steps:
1. preferably selecting equipment, constructing a flow test process of the conductivity of the low-concentration proppant:
(1) the calculation method of the worm gear transmission ratio is the same as that of embodiment 1;
(2) selecting a constant flow pump with the flow range of 1-10mL/min and the proppant volume density of 1.660g/cm3An apparent density of 3.059g/cm3The laying concentration is 0.1g/cm2The maximum closing pressure 69MPa is measured, the differential pressure at the maximum flow rate is estimated by using a differential pressure estimation formula, and a differential pressure sensor (μ ═ 3MPa · s) with the measurement range of 50kPa is preferably selected. The results of the evaluation are shown in the following table:
(3) and (3) assembling the optimized equipment according to the flow shown in the figure 1 to construct a flow for testing the flow conductivity of the low-concentration proppant.
2. Preparation of arc column core and cushion block
(1) Preparing a standard cylinder sample with the diameter of 25 multiplied by 80mm by using 316 stainless steel, cutting the standard cylinder sample into an upper core and a lower core of an arc cylinder which are completely symmetrical up and down along an axial section, wherein the chord length L of an end face arch is 24.8mm, and the height H of an arc is 11.0 mm;
(2) two cushion blocks with the diameter of 25 multiplied by 10mm and capable of placing stainless steel filter screens are prepared by 316 stainless steel, and the stainless steel filter screens with the meshes of 200 are placed for supporting the end face encapsulation of the crack model.
3. Low-concentration proppant supporting crack model prepared by using aluminum foil adhesive tape and thermal shrinkage film
(1) Aligning the lower core with two cushion blocks filled in the stainless steel filter screen by using an aluminum foil adhesive tape to form a proppant cavity;
(2) uniformly adding 1.984g of 30/50-mesh ceramsite proppant into the proppant cavity, covering a core, and then sheathing a thermal shrinkage film for packaging to prepare a low-concentration ceramsite supported crack model.
4. Obtaining flow parameters of a fracturing fluid gel breaking liquid supernatant (viscosity mu is 3mpa · s) under laminar seepage by using a fluid flow experiment of a low-concentration ceramsite supported fracture model, and calculating the flow conductivity of a low-concentration proppant by using a Darcy formula
(1) Placing a low-concentration ceramsite supporting crack model into a rock core holder in a testing process, adding a supernatant of fracturing fluid gel breaking liquid into an intermediate container, opening an intermediate container channel valve, closing an intermediate container bypass valve, loading a confining pressure 1MPa fixed sample, starting a constant flow pump, injecting liquid, and discharging air of a differential pressure sensor and a testing system; loading confining pressure to different set closing pressure values, bearing pressure for a certain time, and acquiring parameters such as fluid flow pressure difference and flow under the closing pressure;
(2) and calculating the flow conductivity of the low-concentration ceramsite by using a Darcy formula in combination with the geometric dimension of the support crack model, wherein the calculation result is shown in the following table:
5. calculating the equivalent capillary radius, pore volume and proppant skeleton volume of the ceramsite proppant layer, and further calculating the opening of the propped crack
(1) Based on a capillary seepage model, calculating the equivalent capillary radius and the corresponding pore volume of the ceramsite proppant layer by using a Poisea formula, wherein the calculation result is shown in the following table:
(2) the volume density of the 30/50-mesh ceramsite is 1.660g/cm by using a precision balance (the precision is 0.001g) and a 100mL density bottle3And an apparent density of 3.059g/cm3The mass of the known ceramsite is 1.984g, the initial opening of the supporting crack is calculated to be 0.060cm, and the framework volume of the ceramsite is 0.649 mL;
(3) the opening degree of the 30/50-mesh ceramsite supported crack under different closing pressures is calculated, and the calculation result is shown in the following table:
comparative example 1
Aiming at 20/40-mesh ceramsite, the laying concentration is as low as 0.1g/cm2The artificial fracture core method is compared with the proppant conductivity test method under the condition, so that the low-concentration proppant conductivity test method is further explained, and the method specifically comprises the following steps:
1. optimizing equipment and constructing a flow test process for the conductivity of the low-concentration proppant
In the method of the invention, the volume density of the ceramsite is 20/40 meshes (1.54 g/cm)3) Apparent density (2.68 g/cm)3) The proppant spreading concentration (0.1 g/cm)2) The method is used for testing parameters such as closing pressure (69MPa), plunger diameter (25mm), lead (10mm), transmission system efficiency (0.6), hand wheel radius (10cm), applied force (100-200N) and the like, can preferably select a labor-saving high-range confining pressure pump (80MPa, reduction ratio of 5-10) and a pressure difference sensor with the range of 10KPa, and realizes accurate measurement of the flow conductivity of the supporting agent in a high-closing pressure range.
In the artificial fracture core method, the confining pressure pump for simulating the closing pressure can only be loaded to 40MPa due to the lack of a speed reduction system, and the flow conductivity test under the condition of high closing pressure (>40MPa) cannot be tested; and because of lacking the corresponding differential pressure estimation, the differential pressure sensor with the proper measuring range can not be effectively selected, the problem that the differential pressure test is inaccurate due to the fact that the differential pressure exceeds the testing measuring range or the measuring range is too large in the testing process can exist, and the accuracy of the testing data is limited.
2. Preparation of low-concentration proppant propping fracture model
In the method, the proppant fracture model is provided with the simulation fracture wall surface and the end surface cushion block with fixed and uniform roughness, the proppant can be repeatedly and uniformly paved, the migration of the proppant to the end surface under high closing pressure can be prevented, and the stability and the repeatability of the proppant fracture model are ensured.
In the artificial fracture core method, the roughness of the fracture wall surface is uncontrollable, no packaging cushion block is provided, the spreading state of the propping agent is unrepeatable and uncontrollable, the propping agent cannot be prevented from moving to the end surface under high closing pressure, and the limitations of instability and unrepeatability of a propping agent fracture model exist.
3. Calculation of conductivity of low concentration proppant
In the method, under the condition that the closing pressure is 69MPa, the standard deviation of the three test results of the diversion capacity of the low-concentration proppant is 0.03, the stability is good, the accurate diversion capacity of the proppant fracture can be obtained, and the analysis of the variation rule of the diversion capacity of the proppant can be well carried out.
In the artificial fracture core method, the closing pressure can only be loaded to 40MPa difficultly and cannot be increased any more, the standard deviation of the three-time test result of the diversion capacity of the low-concentration proppant is larger and is 5.67, and the stability is poor. The method cannot obtain the flow conductivity of the proppant under the condition of high closing pressure, and cannot effectively analyze the flow conductivity rule of the proppant.
4. Opening of low concentration proppant propped fracture
According to the method, the calculation method of the opening degree of the propping fractures is provided, the opening degrees of the propping fractures under different closing pressures can be obtained, the stability is good, the change rule of the propping fractures can be described quantitatively, and the permeability of the propping fractures can also be obtained.
However, in the artificial fracture core method, there is no corresponding characterization method, the opening parameter of the supporting fracture cannot be obtained, the change rule of the fracturing fracture cannot be described quantitatively, and the permeability of the supporting fracture cannot be obtained.
Comparative example 2
Aiming at 40/70-mesh ceramsite, the laying concentration is as low as 0.1g/cm2The method for testing the diversion capability of the low-concentration proppant further comprises the following steps:
1. core sizes of different arc columns
| Core model type of arc column | End face size (mm) | Length (mm) |
| Definition model of the invention | The chord length L is 23.9mm, and the arc height H is 8.9mm | 50 |
| Other models | The chord length L is 23.3mm, and the arc height H is 8.0mm | 50 |
According to the volume density of 40/70 meshes of ceramsite (1.60 g/cm)3) Apparent density (3.18 g/cm)3) The proppant spreading concentration (0.1 g/cm)2) Testing parameters such as closing pressure (69MPa), plunger diameter (25mm), lead (10mm), transmission system efficiency (0.6), hand wheel radius (10cm), applied force (100-:
| device | Measuring range | Reduction ratio |
| Labor-saving high-range confining pressure pump | 80MPa | 5-10 |
| Differential pressure sensor | 20KPa | / |
2. Preparation of low-concentration proppant propping fracture model
Weighing 1.195g of 40/70-mesh ceramsite and the model of the invention to prepare a low-concentration proppant propping fracture model; 1.165g of 40/70-mesh ceramsite and other models are weighed to prepare a low-concentration proppant propping fracture model.
3. Proppant conductivity calculation for supported fracture models prepared from different arc column cores
From the comparison of the data, the arc column core model of the invention and the low-concentration proppant propping fracture model prepared from 1.195g of 40/70-mesh ceramsite are as follows: the flowing pressure difference is relatively stable in three times of tests under the same closing pressure and flow, the pressure difference change trend in each test process is relatively stable, the pressure difference is increased along with the increase of the closing pressure, and the pressure difference is consistent with a theoretical rule, so that the arc column rock core model can obtain relatively stable flow conductivity of the propping agent.
Other arc column core models and a low-concentration proppant propping fracture model prepared from 1.165g of 40/70-mesh ceramsite: under the same closing pressure and flow, the fluctuation of the flowing differential pressure in three tests is large (except for 6.9 MPa), the fluctuation of the differential pressure change trend in each test process is large, the differential pressure sometimes rises and falls along with the increase of the closing pressure, and the theoretical law is not met, so that the arc column core model cannot obtain the accurate proppant flow conductivity.
4. Opening calculation of proppant propped fractures of different core models
The radius, pore volume and opening degree of equivalent capillary tubes of the arc column rock core model and the low-concentration proppant supporting fracture model prepared from 1.195g 40/70-mesh ceramsite are reduced along with the increase of closing pressure, and the curve is consistent with the theoretical rule, which shows that the arc column rock core model can obtain accurate supporting fracture opening degree.
The overall trend of the parameter values such as equivalent capillary radius, pore volume, propping crack opening and the like of other arc column rock core models and low-concentration propping agent propping crack models prepared from 1.165g of 40/70-mesh ceramsite is reduced along with the increase of closing pressure, but the phenomenon of increase appears at certain closing pressure points, such as 6.9MPa to 13.8MPa for the first time, the equivalent capillary radius is increased from 0.025cm to 0.026cm, and the pore volume is increased from 0.01cm3Increased to 0.011cm3The opening of the propping crack is increased from 0.323mm to 0.324 mm; and a second time of 13.8MPa to 27.6MPa, and a third time of 6.9MPa to 13.8 MPa. The results of the test points are contrary to the theoretical law, and the result shows that the arc column core model cannot obtain the opening degree of the supporting crack accurately.
5. Stability analysis of low concentration proppant conductivity test method
The comparison of the data shows that the standard deviation of the proppant flow conductivity test of the arc column core model is 0.04-0.07, the standard deviation of the proppant propping crack opening test is 0.0002-0.0004, and the proppant has good stability and reliability. And the standard deviation of the proppant flow conductivity test of other arc column core models is 0.26-2.21, the standard deviation of the proppant propping fracture opening test is 0.0002-0.0019, and the stability is relatively poor and unreliable.
Although the present invention has been described in detail by referring to the drawings in connection with the preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made on the embodiments of the present invention by those skilled in the art without departing from the spirit and scope of the present invention, and these modifications or substitutions are within the scope of the present invention/any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (6)
1. The system for testing the flow conductivity of the low-concentration proppant comprises a rock core holder (1), wherein a rock core is contained in the rock core holder (1); the inlet of the core holder (1) is connected with a constant flow pump (2) through a pipeline, the outlet is communicated with a fluid container (3) through a pipeline, and the fluid container (3) is arranged on a precision balance (4); the side wall of the core holder (1) is connected with a confining pressure pump (5) through a pipeline; a differential pressure sensor (6) is also connected between the inlet and the outlet of the core holder (1); the rock core is characterized by comprising an upper rock core (7) and a lower rock core (8) which are symmetrically arranged, and a propping agent (9) is paved between the upper rock core (7) and the lower rock core (8); the contact surfaces of the upper core (7) and the lower core (8) and the propping agent (9) are rectangular, and two end surfaces of the upper core (7) and the lower core (8) which are vertical to the contact surfaces are arched; cushion blocks (10) are arranged at two ends of the rock core respectively, a through hole (11) is formed in the center of each cushion block (10), and a filter screen is arranged on one side face, close to the rock core, of each cushion block (10).
2. The system according to claim 1, further comprising an intermediate container (12), wherein an inlet of the intermediate container (12) is connected with an outlet of the constant flow pump (2) through a pipeline, and an outlet of the intermediate container (12) is connected with an inlet of the core holder (1) through a pipeline; valves (13) are respectively arranged on a pipeline between the constant flow pump (2) and the core holder (1), a pipeline between the constant flow pump (2) and the intermediate container (12), a pipeline between the intermediate container (12) and the core holder (1), and a pipeline between an outlet of the core holder (1) and the fluid container (3).
3. The system according to claim 1, characterized in that the lead screw in the working cylinder of the confining pressure pump (5) is connected with the handle through a worm gear and a worm.
4. The system of claim 1, wherein the span of the differential pressure sensor is selected by a differential pressure estimation formula, wherein the differential pressure estimation formula is:
wherein Δ p is the estimated pressure difference, pa; μ is the fluid viscosity, pa · s; q is the fluid flow, m3S; tau is the tortuosity of a seepage channel of the proppant layer and takes a value of 1.5-5.5; l. thefLength of the propped fracture model, m; w is afWidth of the propped fracture model, m; h isf estimateTo estimate the crack opening, m;
wherein h isf estimateCan be calculated according to the porosity change, and the calculation formula is as follows:
wherein phi is different closing pressure of propping agentPorosity, decimal; p is closing pressure, MPa; alpha is constant and takes 0.010-0.027; csLaying the proppant in g/cm2；ρsIs apparent density of proppant, g/cm3；ρvIs proppant volume density, g/cm3。
5. The method for testing the conductivity of low concentration proppant using the system of any one of claims 1-4, comprising the steps of:
s1: laying a propping agent (9) between an upper rock core (7) and a lower rock core (8), assembling cushion blocks (10) at two ends of the rock core to form a propping fracture model, and putting the propping fracture model into a rock core holder (1); loading confining pressure to a closed pressure value, and acquiring fluid flow pressure difference and fluid flow parameters under the closed pressure value;
s2: and calculating the flow conductivity of the low-concentration proppant by utilizing a Darcy formula in combination with the geometric size of the supporting fracture model, wherein the Darcy formula is as follows:
in the formula, khfIs the flow conductivity of the proppant, um2Cm; q is the fluid flow, cm3/min；wfIs the width of the propping fracture model, cm; h isfOpening degree of the supporting crack model is cm; Δ p is the flow pressure differential, kpa; μ is fluid viscosity, mpa · s; lfLength in cm for the propped fracture model.
6. The method of claim 5, further comprising a characterization step S31 of the propping fracture opening: based on the capillary seepage model, calculating the equivalent capillary radius of the proppant layer and the corresponding pore volume by using a Poiseup formula:
in the formula, rpEquivalent capillary radius, cm; μ is viscosity, pa · s; lfLength of the propped fracture model, cm; q is the fluid flow, cm3S; Δ p is the test differential pressure, pa; vpIs the pore volume of the proppant layer, mL;
s32: obtaining the volume density and the apparent density of the proppant by using a precision balance (1) and a fluid container (1), and then calculating the volume of the propping fracture skeleton:
in the formula, CsLaying the proppant in g/cm2；ρvIs proppant volume density, g/cm3；VsIs the volume, mL, of the proppant skeleton in the proppant layer; w is afIs the width of the propping fracture model, cm; lfLength of the propped fracture model, cm; rhosIs apparent density of proppant, g/cm3；
S33: calculating the opening degree of proppant propping fractures under different closing pressures:
in the formula, hfCm for supporting crack opening; vpPore volume of the proppant layer, mL; vsIs the volume, mL, of the proppant skeleton in the proppant layer; w is afIs the width of the propping fracture model, cm; l. thefLength in cm for the propped fracture model.
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