RU2789895C1 - Method for hydraulic fracturing in fractured carbonate formations - Google Patents

Abstract

FIELD: oil industry.

SUBSTANCE: invention relates to the field of oil production and can be used in the planning of hydraulic fracturing in carbonate formations characterized by natural fracturing. The method for hydraulic fracturing in carbonate formations characterized by natural fracturing includes conducting a complex of hydrodynamic studies, while before hydraulic fracturing at the well, hydrodynamic studies are carried out by the method for unsteady sampling, followed by removal of the pressure recovery curve or level, according to which the presence of natural fracturing is assessed. When confirming the presence of natural fracturing in the formation, before hydraulic fracturing, at the first stage, the fracturing fluid is injected at a rate of 1.0 - 3.5 m³/min to involve an additional number of natural cracks located in the well drainage zone. At the second stage, the fracturing fluid is pumped at a rate of 2.0 - 3.5 m³/min, already into the cracks formed at the first stage, to create several main cracks; at the final stage, the fracturing fluid is pumped at a rate of 3.0 - 3.5 m³/min to create a hydrodynamic connection between the cracks formed at the first and second stages with voids located in the remote part of the formation.

EFFECT: increase in the efficiency of hydraulic fracturing in carbonate rocks with natural fracturing, and obtaining a greater technological effect: additional oil production.

1 cl, 10 dwg

Description

The invention relates to the field of oil production and can be used in planning hydraulic fracturing in carbonate formations characterized by natural fracturing.

The method of developing oil and gas deposits using hydraulic fracturing (HF) is known to be widely used both in world and domestic oil production practice (Reference book on oil production / Edited by Sh.K. Gimatudinov. - M .: Nedra , 1974, pp. 451-461).

The disadvantage of this method is the low technical and economic efficiency due to selective hydraulic fracturing in individual wells of the development system. As field practice shows (Zankiev M. Ya. Classification and diagnostics of the effectiveness of hydraulic fracturing technology in the conditions of Slavneft-Megionneftegaz. .), well flow rates after hydraulic fracturing continuously decrease exponentially and after a period of time from several months to several years return to their original state. Often the costs of hydraulic fracturing are not compensated by additional oil produced. Hydraulic fracturing in the known method is used mainly to increase the productivity of wells. There is practically no increase in the oil recovery factor.

A known method of hydraulic fracturing of a carbonate reservoir (patent RU No. 2460875, IPC E21V 43/26, publ. 10.09.2012 in Bull. No. 25), including the descent into the well of a tubing string - tubing with a packer and its subsequent landing, descent into tubing string coiled tubing strings - coiled tubing below the lower end of the tubing, pumping water-insulating cement through the coiled tubing, hydraulic fracturing of the carbonate formation with bottom water, and the lower end of the coiled tubing is lowered to the level of oil-water contact - WOC, sealing the space between the tubing strings and the coiled tubing, pumping water-insulating cement CT is used to isolate the bottom water in the carbonate reservoir with filling the well from the bottomhole to the level of the WOC, after which the space between the tubing and CT strings is depressurized and the CT string is raised so that its lower end is 1-2 m below the roof of the carbonate reservoir, after which determine the total volume of the fracturing fluid according to the formula: V _g =k⋅h _p ; where V _g is the volume of the fracturing fluid, m ³ ; k=1.4-1.6 - conversion factor, m ³ /m; h _p - the thickness of the productive part of the reservoir, m, seal the space between the tubing strings and the GT and produce the injection into the GT of the first portion of the fracturing fluid in the amount of 60-70% of the total volume - V _g under a pressure of not more than 25 MPa and at a speed of not more than 2 m ³ /min, after which the remaining volume of the fracturing fluid is pumped into the GT in 3-5 cycles, alternating with the injection of a proppant, which is used as a 25% inhibited hydrochloric acid, and the acid volume is determined depending on the thickness of the productive part of the carbonate reservoir , based on the volume of 0.2 m ³ of acid per 1 m of formation thickness for each injection cycle, at the end of the last injection cycle, the acid is squeezed with an aqueous solution of a surfactant in the volume of the CT column, followed by holding for 1-2 hours, after which the column is removed GT from the tubing string and put the well into operation.

The disadvantages of this method are the complexity of implementation due to the large number of operations, the equipment involved and the use of various chemicals, the high costs of chemicals, since the filling goes from the bottomhole to the level of water contact, the need for operations in combination with geophysical surveys, which together leads to great material costs.

A known method of hydraulic fracturing in a well (patent RU No. 2473798, IPC E21B 43/26, publ. 27.01.2013, bull. No. 3), including perforation of the walls of the casing string of the well in the formation interval with channels with a depth not less than the length of the stress concentration zone in the rocks from the wellbore, running a pipe string with a packer, setting the packer above the roof of the perforated productive formation, pumping gelled fracturing fluid into the underpacker zone, creating hydraulic fracturing pressure in the underpacker zone and forcing gelled fracturing fluid with proppant into the formed fracture. Before hydraulic fracturing, the pipe string is filled with process fluid and the total volume of gelled fracturing fluid V _r is determined by calculation, which is divided into two parts, of which 2/3 V _r is the volume of the crosslinked gel, and 1/3 V _r is the linear gel. The hydraulic fracturing process starts with injection into the well through the pipe string of a gelled fracturing fluid - a cross-linked gel with a dynamic viscosity of 150-200 cPa until a fracture is formed in the formation. After creating a fracture in the formation, the volume of cross-linked gel remaining from 2/3 V _r is pumped in equal portions in 3-5 cycles with the addition of proppant fraction 12-18 mesh at a flow rate of 1.5-2 m ³ /min. Moreover, the proppant is introduced into the cross-linked gel in steps with increasing concentrations from 200 to 1000 kg/m. Further, without stopping the hydraulic fracturing process, in the well through the pipe string, increasing the flow rate to 2.5-3 m ³ /min, the fracturing fluid is pumped in equal portions in 3-5 cycles - a linear gel with a dynamic viscosity of 30-50 cPa with the addition of proppant fraction 20 -40 mesh with step increase in concentration from 200 kg/m ³ to 1000 kg/m ³ . After injection of the last portion of the linear gel with proppant into the well pipe string, they are forced into the formation with the process fluid. At the same time, during the squeezing process, the flow rate of the process fluid is reduced to 0.5-1 m ³ /min for 1-3 min and the injection is resumed again at a flow rate of 2.5-3 m ³ /min until the linear gel with proppant is completely squeezed into the formation. After that, exposure is performed for the time required for the injection pressure to drop by 70-80%. The packer is unpacked and removed with the pipe string from the well.

The disadvantages of this method are:

- firstly, the complexity of the technology for implementing the method, associated with an increase in the proppant concentration during the fixation of the fracture, with a decrease in the flow rate of the process fluid to 0.5-1 m ³ / min for 1-3 min and subsequent resumption of injection at a flow rate of 2.5- 3 m ³ /min until the proppant is completely squeezed into the formation;

- secondly, the low efficiency of hydraulic fracturing, due to the fact that first a fracture is formed, and then it is cyclically fixed with proppant, while the end sections of the fractures have time to close by the time they are filled with proppant, as a result of which there is an uneven distribution of proppant in the formation fracture, which reduces the flow capacity of fractures and limits the flow of formation fluid into the wellbore;

- thirdly, low coverage of the formation by hydraulic fractures, since the fracture is opened and fixed with proppant in only one direction, i.e. in the direction of tension.

- fourthly, the low conductivity of the fracture, since the spent gelled fracture fluids are filtered into the formation, clogging its pores.

The closest method of the same purpose to the claimed invention in terms of a combination of features is a method of hydraulic fracturing by pumping fluid under high pressure into the well, which provides opening in the reservoir, in particular the productive formation, of existing fractures or the creation of new fractures, which significantly improve the hydrodynamic connection between the reservoir and the well. At the same time, a crack binder is introduced into the fracturing fluid - a proppant (for example, quartz sand or walnut shells, or glass beads), which penetrate into the cracks, remain in them when the well is put into operation and keep the cracks open (Prince Usachev P .M., Hydraulic fracturing. M.. Nedra. 1986. p.164). This method is taken as a prototype.

The disadvantages of this method are:

- firstly, the high labor intensity and high cost of implementing the method, due to the fact that hydraulic fracturing requires a large number of pumping units of sand mixing machines, tankers, and pumping units are designed to pump liquid media under pressure up to 70 MPa, and sand mixing machines are designed for transportation of a crack fixer - a proppant, preparation of a sand-liquid mixture and its supply to the intake of pumping units. Tank trucks are used to transport liquids and supply them to sand mixing and pumping plants, while piping the mouth requires the use of special high-strength fittings designed for high pressures up to 70 MPa. To protect the production string from high pressure during hydraulic fracturing according to the method described above, it is mandatory to use high-strength packers with anchor devices;

- secondly, when high pressures are reached, not only the productive formation is ruptured, but also the overlying and/or underlying screening bridge formations. This leads to intensive watering of the extracted products and, in general, to a decrease in the efficiency of work, in particular, work to intensify oil production. In addition, a full-scale hydraulic fracturing leads to the formation of a large-scale fracture, usually a single one, with a long-range strike far beyond the boundaries of the colmatation zone. The intensified formation does not drain the production in this case through the entire thickness of the formation, but leads to catastrophic losses of the working agent both at the stage of the actual hydraulic fracturing and at subsequent stages of stimulation.

The technical result achieved by the invention is to increase the efficiency of hydraulic fracturing in carbonate rocks by pumping the fracturing fluid at low speed to involve an additional number of natural fractures located in the well drainage zone, as well as at a distance from it and, as a result, obtaining a greater technological effect (additional oil production).

The specified technical result in the implementation of the invention is achieved by the fact that in the known method of hydraulic fracturing in carbonate reservoirs, characterized by natural fracturing, including, before hydraulic fracturing in the well, conducting hydrodynamic studies by the method of transient sampling, followed by taking a pressure or level recovery curve (PBU / KVU), which assess the presence of natural fracturing; when confirming the presence of natural fracturing in the reservoir before hydraulic fracturing, at the first stage, the fracturing fluid is injected at a rate of 1.0 - 3.5 m ³ / min to involve an additional number of natural fractures located in the well drainage zone; at the second stage, the fracturing fluid is pumped at a rate of 2.0 - 3.5 m ³ /min, already into the fractures formed at the first stage, in order to create several main fractures; at the final stage, the fracturing fluid is injected at a rate of 3.0 - 3.5 m ³ /min to create a hydrodynamic connection between the fractures formed at the first and second stages with voids located in a remote part of the formation.

Hydrodynamic studies of wells by the method of pressure (level) restoration allow obtaining a large amount of information about the presence of a system of natural fractures in the drainage zone of the studied well. The use of the KAPPA Workstation software product (Saphir module) or analogue programs in which such algorithms are implemented when processing the pressure recovery curve (level) will allow:

• to diagnose the presence of a system of natural fractures in the formation drainage zone;

• to diagnose the presence of cracks in the drainage zone of the formation, formed during the hydraulic fracturing;

• Qualitatively and quantitatively assess the parameters of natural fracturing of the reservoir and rock fractures.

It has been experimentally established that the hydraulic fracturing operation in three stages at the fluid injection rate declared at each stage provides the most efficient hydraulic fracturing with the highest technological efficiency.

When the injection rate exceeds 3.5 m ³ /min, a hydraulic fracture occurs, which prevents the creation of an extended network of fractures and, as a result, incomplete involvement of the formation volume in the process of reserves development; as well as rapid deformation (collapse) of the hydraulic fracture and failure to achieve the technological efficiency of the measure.

When the fluid injection rate at the first stage is less than 1.0 m ³ /min, it is not possible to pump the fracturing fluid into the formation and create a network of fractures in the well drainage zone.

If the liquid injection rate at the second stage is less than 2.0 m ³ /min, it is impossible to achieve the expected technological effect, since at the first stage a system of fractures was not created in the well drainage zone and, accordingly, the main hydraulic fractures are not formed.

When the fluid injection rate at the final stage is less than 3.0 m ³ /min, it is impossible to achieve the expected technological effect, since the main fractures were not created at the first and second stages, which at the third stage would create a hydrodynamic connection between the fractures formed at the first and second stages, with voids located in the remote part of the formation.

In the presence of natural fracturing in the formation, before hydraulic fracturing, the fracturing fluid is pumped at a rate of 4.5–6 m ³ /min to create a hydraulic fracture. At a lower injection rate in a carbonate reservoir, characterized by the absence of natural fracturing, it is impossible to create a hydraulic fracture.

The proposed method is illustrated in the drawings shown in Fig. 1-10.

Figure 1 - View of the graph of the pressure recovery curve before hydraulic fracturing in logarithmic coordinates, characterizing the presence of natural fracturing.

Figure 2 - View of the graph of the pressure recovery curve before hydraulic fracturing, processed by the graphical analysis method of Warren-Root, confirming the presence of natural fracturing.

Figure 3 - View of the graph of the pressure recovery curve after hydraulic fracturing in logarithmic coordinates, characterizing the presence of a network of fractures.

Figure 4 - View of a complex network of hydraulic fractures formed in carbonate reservoirs with natural fractures.

Figure 5 - The results of microseismic monitoring.

Figure 6 - View of the graph of the pressure recovery curve before hydraulic fracturing in logarithmic coordinates, characterizing the absence of natural fracturing.

Figure 7 - View of the graph of the pressure recovery curve before hydraulic fracturing, processed by the graphical analysis method of Warren-Root, confirming the absence of natural fracturing.

Figure 8 - View of the graph of the pressure recovery curve after hydraulic fracturing in logarithmic coordinates, characterizing the formation of a hydraulic fracture, in which the filtration flow in the reservoir is considered bilinear.

Figure 9 - View of the hydraulic fracture, in which the filtration flow in the reservoir is considered bilinear.

In Fig.10 - The results of microseismic monitoring.

An example of the implementation of this method.

The implementation of the method is considered on the example of one well that exploits a carbonate deposit. At the well, before hydraulic fracturing, hydrodynamic studies were carried out using the pressure recovery method, which were processed in the KAPPA Workstation program (Saphir module). Figure 1 shows a graph of the pressure recovery curve before hydraulic fracturing in logarithmic coordinates, characterizing the presence of natural fracturing. Figure 2 is a graph of the pressure recovery curve before hydraulic fracturing, processed by the graphical analysis method of Warren-Root, confirming the presence of natural fracturing. The fact of the presence of natural fracturing indicates the need for a hydraulic fracturing operation in three stages in order to obtain the maximum technological effect. At the first stage, the fracturing fluid was injected at a low rate (1.0 - 3.5 m ³ /min) to involve an additional number of natural fractures located in the well drainage zone; at the second stage, the fracturing fluid is pumped at a rate (2.0 - 3.5 m ³ /min) into the fractures already formed at the first stage in order to create several main fractures; at the final stage, the fracturing fluid is injected at a rate of 3.0 - 3.5 m ³ /min to create a hydrodynamic connection between the fractures formed at the first and second stages with voids located in a remote part of the formation. Figure 3 shows a graph of the pressure recovery curve after hydraulic fracturing in logarithmic coordinates, characterizing the presence of a network of natural fracturing. A view of the complex network of hydraulic fractures formed in naturally fractured carbonate reservoirs is shown in Fig. 4. The hydraulic fracturing procedure at the well was accompanied by microseismic monitoring. According to the results of microseismic monitoring, it is noted that a system of cracks of submeridial orientation (azimuths from 0° to 45°) was formed within a radius of 200-250 m (figure 5). Carrying out the activity in accordance with the proposed method made it possible to increase additional oil production by an average of 15-20% and the duration of the effect of the activity by 30-35% compared to previously carried out activities in similar geological and physical conditions.

At the well, before hydraulic fracturing, hydrodynamic studies were carried out using the pressure recovery method, which were processed in the KAPPA Workstation program (Saphir module). Figure 6 shows a graph of the pressure recovery curve before hydraulic fracturing in logarithmic coordinates, characterizing the absence of natural fracturing. Figure 7 is a graph of the pressure recovery curve before hydraulic fracturing, processed by the Warren-Root graphical analysis method, confirming the absence of natural fracturing. The fact that there is no natural fracturing indicates the need for a one-stage hydraulic fracturing operation at a rate (4.5–6 m ³ /min) to create a hydraulic fracture. Figure 8 shows a graph of the pressure recovery curve after hydraulic fracturing in logarithmic coordinates, indicating that a hydraulic fracture has formed in the reservoir, in which the filtration flow in the reservoir is considered bilinear. The type of hydraulic fracture, in which the filtration flow in the reservoir is considered bilinear, is shown in Fig.9. The hydraulic fracturing procedure at the well was accompanied by microseismic monitoring. According to the results of microseismic monitoring, it is noted that one main fracture was formed along the entire length of 300 m (Fig. 10).

The application of the proposed method allows to increase the technological efficiency of hydraulic fracturing in carbonate reservoirs.

Conditions for implementation: Before hydraulic fracturing of the well, hydrodynamic studies should be carried out by the method of pressure restoration (PBU) or level (LLR).

Implementation tools: To calculate the presence of a system of natural fractures, the use of the KAPPA Workstation program (Saphir module) is necessary. In case of its absence, it is possible to use the Warren-Root graphic-analytical method, which also allows you to visually assess the presence of fracturing, as well as quantify the size of the opening of natural fractures.

Claims (1)

A method for performing hydraulic fracturing in carbonate reservoirs characterized by natural fracturing, including conducting a complex of hydrodynamic studies, characterized in that before hydraulic fracturing, hydrodynamic studies are carried out in the well by the method of transient selections, followed by the removal of a pressure or level recovery curve, which is used to assess the presence natural fracturing; when confirming the presence of natural fracturing in the reservoir before hydraulic fracturing, at the first stage, the fracturing fluid is injected at a rate of 1.0 - 3.5 m ³ / min to involve an additional number of natural fractures located in the well drainage zone; at the second stage, the fracturing fluid is pumped at a rate of 2.0 - 3.5 m ³ /min, already into the fractures formed at the first stage, to create several main fractures; at the final stage, fracturing fluid is injected at a rate of 3.0 - 3.5 m ³ /min to create a hydrodynamic connection between the fractures formed at the first and second stages with voids located in a remote part of the formation.

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