CN115961930A - Method for testing diversion capacity of branch cracks of low-sand-laying-concentration shale - Google Patents

Abstract

The improved method for testing the diversion capability of the shale branch fracture with low sand laying concentration provided by the invention avoids the problems that when a core holder is used for confining pressure, a propping agent in the fracture is extruded and embedded into a rubber cylinder due to creeping of the propping agent in the fracture when a propping agent filling fracture prepared by standard cylindrical shale core is processed, and the displacement differential pressure measurement is too large due to unreasonable pressure monitoring point position, and can more accurately determine the diversion capability of the shale fracture with low sand laying concentration.

Description

A method for testing the conductivity of branched fractures in shale with low sand concentration

technical field

The invention relates to the technical field of shale oil development, in particular to a method for testing the conductivity of branch fractures of shale with low sand concentration.

Background technique

Shale oil is an important unconventional oil and gas resource, which occurs in nanoscale pore throats and fracture systems in shale reservoirs, and is mostly distributed along sheet-like bedding planes or micro-fractures parallel to it, in the form of adsorption and free Exist, the oil is lighter and the viscosity is lower. my country has abundant shale oil reserves, and the amount of technically recoverable resources ranks third in the world, but it is still in the early stage of exploration and development, starting in 2010, about 30 to 40 years later than the United States. Different from the marine deposits of shale oil reservoirs in the United States, the organic-rich shale in my country is mainly continental lacustrine deposits. These shales have relatively large clay mineral content and are less brittle. They are suitable for horizontal well construction and staged fracturing. Difficulty is caused, and the well wall is prone to collapse. my country's shale oil resources are mainly distributed in the Mesozoic and Cenozoic basins. The basins were formed late, and the degree of thermal evolution is relatively low. The shale oil quality is relatively thick and the fluidity is poor, which also poses difficulties for development. Therefore, the shale oil fracturing technology in the United States is not fully applicable to my country. my country needs to strengthen the research on the occurrence characteristics of shale oil, and develop shale oil fracturing development technology suitable for my country's geological characteristics.

Evaluation of the conductivity of shale fractures is an important content in the design of shale fracturing schemes. At present, the conventional method of measuring the conductivity of shale fractures is to use API diversion instrument, but it is only suitable for high sand concentration. In the case of low sand concentration, the side of the fracture is easy to close, and the actual action on the fracture cannot be measured. The problem of displacement pressure difference. The difficulty in measuring the conductivity of fractures filled with low sand concentration by using standard cylindrical shale cores lies in the preparation of fractures filled with low sand concentration. Due to the small size of the standard cylindrical shale core, when the circular rubber cylinder of the core holder is used to exert confining pressure on the core, the proppant particles filled between the two half-moon core pieces are prone to creep and be squeezed out of the fracture Embedding the rubber tube, especially when the sand concentration is high and the confining pressure is high, the fracture width will be significantly reduced after the proppant is squeezed out of the fracture, and the accurate and stable fracture conductivity cannot be measured; in addition, when measuring the core When there is a displacement pressure difference at both ends, the pressure monitoring points are generally arranged at both ends of the core holder, and the inner diameter of the conventional pipeline is only 1mm, resulting in the measured displacement pressure difference being greater than the actual pressure difference acting on the two ends of the core, especially when laying When the sand concentration is high, the proportion of the flow pressure difference generated in the non-core section between the two pressure monitoring points increases, resulting in a small fracture conductivity, which seriously affects the accuracy of fracture conductivity measurement.

Contents of the invention

In view of the above problems, the present invention is proposed to provide a method for testing the conductivity of branched fractures of shale with low sand concentration, which overcomes the above problems or at least partially solves the above problems.

According to one aspect of the present invention, a method for testing the conductivity of branched fractures of shale with low sand concentration is provided, the testing method comprising:

Process cylindrical shale cores broken along the central axis into proppant-filled fractures;

The fracture conductivity test of the proppant-filled shale was carried out using a core holder and a displacement device.

Optionally, processing the cylindrical shale core disconnected along the central axis into proppant-filled fractures specifically includes:

Measure the fracture wall area of the disconnected half-moon shale core, and weigh the proppant according to the sand concentration;

Adhere a layer of double-sided adhesive to the fracture wall of the half-moon shale core, and pre-lay a layer of proppant on it to prevent the proppant from slipping near the fracture wall, then lay the remaining proppant on the fracture surface, and place the two Two half-moon-shaped shale cores are merged and aligned; a circular metal filter with the same diameter as the core end face is placed at each end of the shale core, and two strips of thin aluminum with a thickness of 0.5-1 mm are covered along the cracks on the side of the core. sheet to prevent the proppant from being squeezed out of the fracture after the core is subjected to confining pressure;

Sandwich the shale core between two cylindrical metal cores, wrap a layer of aluminum foil paper tape with a shaping effect on the outside of the core, place it in a thermoplastic tube, and use a hot air blower to make the thermoplastic tube shrink and stick to the outside of the core. Prevent cracks from opening and proppant falling off; the metal core has a through hole with an inner diameter of 3mm along the axis, and a mesh diversion groove is processed at the end close to the shale core to facilitate the passage of test fluid and reduce the pressure between the monitoring points at both ends of the core The flow frictional pressure difference acting on the non-shale core section; a pressure guide groove with a diameter of 3 mm is also drilled perpendicular to the direction of the through hole away from the shale core, and connected to the pressure sensor.

Optionally, the fracture conductivity test of the proppant-filled shale using a core holder and a displacement device specifically includes:

Load the shale core wrapped in thermoplastic tube into the core holder;

Inject water into the outer annular space of the rubber cylinder of the core holder to apply a certain confining pressure to the core;

Two pressure sensor probes pass through the outer wall of the core holder and the rubber tube, and are respectively connected to the pressure guide grooves at the ends of the two metal cores before and after the shale core, and are used to measure the displacement pressure acting on the front and back of the shale core. Difference;

A back pressure valve is installed at the rear end of the core holder to control the fluid pressure in the shale core fracture; the test fluid is stably injected into the core fracture at the front end of the core holder, and the flow rate is 0-10ml/min; the experiment process Continuously monitor the pressure at the front and rear ends of the shale core, and stop the experiment when the displacement pressure difference between the front and rear ends of the core is stable;

Assuming that the shale matrix is tight and the permeability is negligible compared with the permeability of proppant-filled fractures, the displacement equation of shale cores containing proppant-filled fractures is:

By transforming the equation, the formula for calculating the conductivity of proppant-filled shale fractures is as follows:

Among them, Q is the test fluid flow rate, ml/s; A is the fracture cross-sectional area of the shale core end surface, cm ² ; ΔP is the displacement pressure difference, 0.1 MPa; μ is the test fluid viscosity, mPa·s; Length, cm; K is shale core fracture permeability, μm ² ; W _f is shale core fracture width, cm; D is shale core diameter, cm; KW _f is fracture conductivity, μm ² ·cm.

Optionally, the test fluid is any one of real crude oil, simulated oil, and brine.

The present invention provides an improved method for testing the conductivity of branched fractures of low-sand concentration shale, which avoids the proppant-filled fractures prepared by processing standard cylindrical shale cores in the fractures when confining pressure is applied with a core holder. The proppant creeps and is extruded and embedded in the rubber cylinder, and the position of the pressure monitoring point is unreasonable, which leads to the large displacement pressure difference measurement, which can more accurately measure the conductivity of shale fractures at low sand concentration.

The above description is only an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention, it can be implemented according to the contents of the description, and in order to make the above and other purposes, features and advantages of the present invention more obvious and understandable , the specific embodiments of the present invention are enumerated below.

Description of drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the accompanying drawings that need to be used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For Those of ordinary skill in the art can also obtain other drawings based on these drawings without making creative efforts.

Fig. 1 is a schematic diagram of a single-layer proppant-laid fracture prepared by core column processing provided by an embodiment of the present invention;

Fig. 2 is a schematic diagram of cracks wrapped with aluminum foil paper prepared by core column processing provided by the embodiment of the present invention;

Fig. 3 is the rock core schematic diagram after the thermoplastic pipe is installed additionally that the embodiment of the present invention provides;

Fig. 4 is a diagram of the composition and structure of branch fractures with low sand concentration prepared by processing standard cylindrical shale cores provided by the embodiment of the present invention;

Fig. 5 is a schematic diagram of the assembly effect of branch fractures with low sand concentration prepared by processing standard cylindrical shale cores provided by the embodiment of the present invention;

Fig. 6 is a schematic diagram of an experimental device for testing the conductivity of branch fractures of low sand concentration shale provided by the embodiment of the present invention.

Detailed ways

Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be embodied in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided for more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

The terms "comprising" and "having" and any variations thereof in the description, embodiments, claims and drawings of the present invention are intended to cover non-exclusive inclusion, for example, including a series of steps or units.

The technical solutions of the present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments.

As shown in Figure 1, a method for testing the conductivity of branched fractures of shale with low sand concentration, the test method includes:

Cylindrical shale cores broken along the central axis are processed into proppant-filled fractures:

Measure the fracture wall area of the disconnected half-moon shale core, and weigh the proppant according to the sand concentration;

Adhere a layer of double-sided adhesive to the fracture wall of the half-moon shale core, and pre-lay a layer of proppant on it to prevent the proppant from slipping near the fracture wall, then lay the remaining proppant on the fracture surface, and place the two Two half-moon-shaped shale cores are merged and aligned; a circular metal filter with the same diameter as the core end face is placed at each end of the shale core, and two strips of thin aluminum with a thickness of 0.5-1 mm are covered along the cracks on the side of the core. sheet to prevent the proppant from being squeezed out of the fracture after the core is subjected to confining pressure;

Sandwich the shale core between two cylindrical metal cores, wrap a layer of aluminum foil paper tape with a shaping effect on the outside of the core, place it in a thermoplastic tube, and use a hot air blower to make the thermoplastic tube shrink and stick to the outside of the core. Prevent cracks from opening and proppant falling off; the metal core has a through hole with an inner diameter of 3mm along the axis, and a mesh diversion groove is processed at the end close to the shale core to facilitate the passage of test fluid and reduce the pressure between the monitoring points at both ends of the core The flow frictional pressure difference acting on the non-shale core section; a pressure guide groove with a diameter of 3 mm is also drilled perpendicular to the direction of the through hole away from the shale core, and connected to the pressure sensor.

The fracture conductivity test of the proppant-filled shale is carried out using a core holder and a displacement device:

Load the shale core wrapped in thermoplastic tube into the core holder;

Inject water into the outer annular space of the rubber cylinder of the core holder to apply a certain confining pressure to the core;

Two pressure sensor probes pass through the outer wall of the core holder and the rubber tube, and are respectively connected to the pressure guide grooves at the ends of the two metal cores before and after the shale core, and are used to measure the displacement pressure acting on the front and back of the shale core. Difference;

A back pressure valve is installed at the rear end of the core holder to control the fluid pressure in the shale core fracture; the test fluid is stably injected into the core fracture at the front end of the core holder, and the flow rate is 0-10ml/min; the experiment process Continuously monitor the pressure at the front and rear ends of the shale core, and terminate the experiment when the displacement pressure difference between the front and rear ends of the core is stable; the test fluid is any one of real crude oil, simulated oil, and brine.

Assuming that the shale matrix is tight and the permeability is negligible compared with the permeability of proppant-filled fractures, the displacement equation of shale cores containing proppant-filled fractures is:

By transforming the equation, the formula for calculating the conductivity of proppant-filled shale fractures is as follows:

Among them, Q is the test fluid flow rate, ml/s; A is the fracture cross-sectional area of the shale core end surface, cm ² ; ΔP is the displacement pressure difference, 0.1 MPa; μ is the test fluid viscosity, mPa·s; Length, cm; K is shale core fracture permeability, μm ² ; W _f is shale core fracture width, cm; D is shale core diameter, cm; KW _f is fracture conductivity, μm ² ·cm.

In the shale branch fracture conductivity experiment, the temperature condition is 80°C, the confining pressure is 25MPa, the core fracture type is flat fracture, the initial fracture width is 1.73mm, the proppant type is ceramsite, and the mesh number is 40-70 The sanding concentration is 3kg/m ² , the sanding area is 6.05568cm ² , and the proppant mass is 1.816704g. The branch fractures prepared by core column processing are shown in Figure 1-5. The front and rear ends are obtained by reducing the back pressure. The pressure difference between them can be used to obtain the permeability of the fracture, and then the conductivity of the fracture. During the experiment, the change of back pressure is 20-15-10-5-0MPa, and the effective closing pressure is 5-10-15-20-25MPa. Light oil is used as the test fluid, the viscosity of crude oil is 5.1mPa s, and the flow rate is 0.1 ml/min, carry out the displacement experiment, the experimental flow chart of the core displacement device is shown in Figure 6, the pressure monitoring data record is exported by the monitoring software, the stable permeability is obtained through the laboratory experiment, and the conductivity is calculated by the formula as shown in Table 1 shown.

Table 1 Calculation of conductivity capacity based on indoor experimental results

Beneficial effects: The method for testing the conductivity of branched fractures of shale with low sand concentration provided by the present invention proposes a new method for processing and preparing branched shale fractures, which can use cylindrical shale cores of standard sizes to process and prepare proppant fillings Fractures, so that the proppant filled in the shale fractures will not be squeezed out of the fractures and embedded in the rubber tube due to the confining pressure, which will lead to unreasonable narrowing of the fracture width and decrease in conductivity; at the same time, the core clamping is optimized The position and structure of the pressure difference monitoring point of the fracture device, the large through-hole of the metal core, the pressure guide groove, and the rubber tube make the displacement pressure difference at both ends of the shale fractures monitored more accurate. It is suitable for measuring the conductivity of fractures with a sand concentration of 0-5kg/ ^m2 within a large variation range, especially for the case of high sand concentration and high confining pressure, which can ensure the stability of fracture filling and monitor the pressure difference of fractures The accuracy of the measurement results is more accurate than the conventional method. The invention is ingeniously designed and conveniently implemented, and can provide basis and support for testing the conductivity of proppant-filled shale fractures.

The above specific implementation manners have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific implementation modes of the present invention, and are not used to limit the protection scope of the present invention. Within the spirit and principles of the present invention, any modifications, equivalent replacements, improvements, etc., shall be included in the protection scope of the present invention.

Claims (4)

1. A method for testing the diversion capacity of a branch fracture of a shale with low sand laying concentration is characterized by comprising the following steps:

processing the cylindrical shale core disconnected along the central axis into a proppant filled fracture;

and testing the flow conductivity of the fracture filled with the proppant by adopting a rock core holder and a displacement device.

2. The method for testing the diversion capacity of the shale branch fracture with the low sanding concentration as claimed in claim 1, wherein the processing of the cylindrical shale core broken along the central axis into the proppant filled fracture specifically comprises:

measuring the fracture wall surface area of the disconnected half-moon-shaped shale core, and weighing a propping agent according to the sand laying concentration;

adhering a layer of double faced adhesive tape to the fracture wall surface of the half-moon-shaped shale core, pre-laying a layer of propping agent on the fracture wall surface to prevent the propping agent near the fracture wall surface from slipping, laying the rest propping agent on the fracture surface, and combining and aligning the two half-moon-shaped shale cores; two ends of the shale core are respectively provided with a circular metal filter screen with the same diameter as the end face of the core, and the side face of the core is covered with two strip-shaped aluminum sheets with the thickness of 0.5-1mm along the crack to prevent the propping agent from being extruded out of the crack after the core is subjected to confining pressure;

the shale rock core is clamped between two cylindrical metal rock cores, a layer of aluminum foil adhesive tape with a shaping effect is wrapped on the outer side of the rock core and is placed in a thermoplastic pipe, and the thermoplastic pipe is contracted and tightly attached to the outer side of the rock core by using a hot air blower, so that the fracture opening propping agent is prevented from falling off; the metal core is provided with a through hole with the inner diameter of 3mm along the axis, and one end close to the shale core is provided with a grid type flow guide groove so as to facilitate the passing of test fluid and reduce the flow friction pressure difference acting on a non-shale core section between pressure monitoring points at two ends of the core; and a pressure guide groove with the diameter of 3mm is drilled at one end far away from the shale core and is perpendicular to the direction of the through hole, and the pressure guide groove is connected with a pressure sensor.

3. The method for testing the diversion capability of the branch fracture of the shale with the low sanding concentration as claimed in claim 1, wherein the testing the diversion capability of the fracture filled with the proppant by using the core holder and the displacement device specifically comprises:

filling a shale core wrapped by a thermoplastic pipe into a core holder;

injecting water into the outer annular space of the rubber cylinder of the core holder, and applying certain confining pressure to the core;

two pressure sensor probes penetrate through the outer wall of the rock core holder and the rubber cylinder, are respectively connected with pressure guide grooves at the end parts of the front metal rock core and the rear metal rock core of the shale rock core and are used for measuring the displacement differential pressure acting on the front and the rear metal rock cores of the shale rock core;

a back pressure valve is arranged at the rear end of the core holder to control the fluid pressure in the shale core fracture; stably injecting a test fluid into the core fracture at the front end of the core holder, wherein the flow rate is 0-10ml/min; continuously monitoring the pressure of the front end and the rear end of the shale rock core in the experiment process, and stopping the experiment when the displacement differential pressure of the front end and the rear end of the rock core is stable;

assuming the shale matrix is dense and has negligible permeability compared to the permeability of the proppant-filled fracture, the displacement equation for a shale core containing proppant-filled fractures is:

and transforming the equation to obtain a calculation formula of the conductivity of the proppant filled shale fracture as follows:

wherein Q is the flow rate of the test fluid, ml/s; a is the fracture sectional area of the end face of the shale core, cm ² (ii) a Delta P is the displacement pressure difference, 0.1MPa; μ is the test fluid viscosity, mPa · s; delta L is the shale core length, cm; k is the permeability of shale core crack in mum ² ；W _f The width of the shale core crack is cm; d is the diameter of the shale core in cm; KW _f Mu m for fracture conductivity ² ·cm。

4. The method for testing the branch fracture conductivity of the shale with the low sanding concentration as claimed in claim 1, wherein the test fluid is any one of real crude oil, simulated oil and brine.

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