RU2759247C1 - Method for conducting multi-stage hydraulic fracturing in conditions of thin bridges - Google Patents

Abstract

FIELD: petroleum industry.

SUBSTANCE: invention relates to the field of the petroleum and gas industry and can be applied for stimulation of boreholes by hydraulic fracturing. Proposed is a method for hydraulic fracturing, including the following stages: a liner with external packers and equal-pass hydraulic fracturing sleeves is lowered, then a wrench is lowered on the tubing/coiled tubing string used to open the sleeves, wherein the tubing/coiled tubing string is placed above the lower sleeve, the external interval is washed with the fitting open on the surface and without back pressure in the annular space. The pumps are stopped simultaneously and the pressure drop curve is recorded. At the second stage of processing, cleaning is performed, wherein the hydraulic fracturing fluid is pumped in order to remove the residual drilling fluid and mud cake, wherein the stage of maintaining the material balance is executed, and the output of the fluid on the surface is monitored by evaluating the content of solid substancing therein, while the fitting is not applied, the pumps are stopped, and the pressure drop curve is recorded. At the third stage, the hydraulic fracturing pressure gradient is estimated by performing a mini hydraulic fracturing with a step test at different flow rates, wherein the fitting is closed to determine the hydraulic fracturing gradient, the pumps are stopped, and the pressure drop curve is recorded. At the fourth stage, the formation is subjected to acidising by placing a volume of acid, equal to the volume of the open wellbore annulus, along the annulus, with the fitting closed on the surface, wherein the entire volume of acid is pumped into the formation at a pressure below the hydraulic fracturing pressure. At the fifth stage, hydraulic fracturing is performed, the interval of the borehole is washed from the acidising reaction products by circulation of 2 to 3 volumes of the treated interval until the reaction products are output, the fitting is closed on the surface, and a cushion is pumped, developing a fracture along the wellbore, the fitting is closed on the surface, and pumping of the hydraulic fracturing fluid is started, developing a fracture along the wellbore. In case of low leakage of the working fluid into the reservoir, with low permeability of the reservoir and an increase in the bottomhole pressure creating a risk of developing the fracture in height, part of the pressure is drained through the fitting on the surface. When the proppant is supplied to the flow, the fitting is closed completely until the end of the work, the pumps are stopped, and the pressure drop curve is recorded, the isolation packer is lifted and the transition to the next processing interval in the borehole is executed, then the operations are repeated in the same sequence until complete coverage of the entire length of the borehole.

EFFECT: limited vertical growth of the hydraulic fracture for the purpose of ensuring uniform development of productive formations and, accordingly, increased petroleum recovery of the reservoir with simultaneously minimised risk of breaching into the over- and underlying formations.

1 cl, 8 dwg

Description

The method of carrying out multi-stage hydraulic fracturing with the growth of fractures from a linear source along a horizontal wellbore belongs to the field of the oil and gas industry and can be used to stimulate wells by hydraulic fracturing in target reservoirs bounded by barriers with stress that slightly differ from the target interval.

Many attempts have been made to develop methods to avoid the propagation of a hydraulic fracture into aquifers and gas-bearing horizons, which can be roughly divided into three main categories. A brief description of these general categories of methods is provided below, and an overall assessment of their effectiveness in achieving the stated objectives is given.

• Hybrid fracturing: Use of low-viscosity / low-viscosity fracturing fluids at reduced injection rates; this approach is widely used in the development of oil reservoirs. This method reduces the overall cost, but is probably limited by a fracture length / height ratio of about 2 [RU 2523316].

• Phase Permeability Modifiers (PMMs): The use of PMMs has been tried many times, but the results appear to be unsatisfactory and inconclusive. This method is expensive and, as a rule, also leads to a decrease in the overall productivity index after hydraulic fracturing (due to severe formation pollution) [RU 2506298].

• Barrier hydraulic fracturing: Placement of an additional hydraulic fracture in the water part of the reservoir to create an additional stress barrier to remove the hydraulic fracture. This is an expensive and time-consuming method, but it may provide a length-to-height ratio of the crack at a level of 2 to 3 (a similar technique is described in the patent [RU 2496977]).

There is a known method [RU 2656054] for hydraulic fracturing, including the injection of a process fluid to create, develop and consolidate a fracture, characterized in that the injection is carried out with short stops, the moment and duration of which is determined based on the analysis of the dynamics of fracture closure, and the working process fluid is presented alternating packs with small instant leaks and high viscosity with low-viscosity packs, but with sufficient bearing capacity of the proppant and the last-closing pack of working fluid containing the maximum concentration of the disruptor (breaker). It is understood that these alternating packs of working fluids are slightly mixed during the injection process.

The disadvantage of this technical solution is that the hydraulic fracturing of the formation is performed at one interval (perforation or hydraulic fracturing sleeve), respectively, in the zone of contact between the well and the formation there will be a maximum fracture height and in case of an increase in pore pressure, the fracture will grow along its height. Even stopping the injection for a short time will not be accompanied by a decrease in reservoir pressure in the bottomhole formation zone. It turns out that for hydraulic fracturing in conditions of creation of long fractures and low height, but with a large length along the length, it is not possible to maintain a ratio of 1 to 50 (height to length).

The technical problem, the solution of which is provided in the implementation of the invention, is the development of a method under conditions of low stress contrast between the layers of a multilayer reservoir and the absence of contrast barriers separating the overlying gas-bearing formations and the underlying water, limiting the growth of hydraulic fractures vertically to ensure uniform production of productive formations, increase oil recovery ...

Fracture height limitation can be achieved with a specific well completion approach. The development of any fracture during well completion with an open horizontal wellbore oriented in the direction of maximum stress in such a way that hydraulic fractures are longitudinal. In this case, the crack propagation will occur from a linear source.

The stress contrast along a horizontal wellbore drilled in the azimuth of the plane of maximum rock stress will be less than the stress contrast in the formation. In this case, a lower pressure differential will be observed along the wellbore, and the fracture will grow predominantly along the wellbore. This can be achieved by using two separate hydraulic fracturing sleeves at the top and bottom of an isolated section of an open horizontal wellbore. After circulation of fluid to clean the interval prior to fracturing, the surface choke is closed to initiate a fracture; as soon as the hydraulic fracture has grown over the entire length of the uncased part, the pressure begins to increase and the hydraulic fracture grows in height with the initiation of the fracture as a linear source

The completion of the horizontal well will be done in a segmented manner; after drilling the horizontal section, a liner with a set of annular packers will be run. The distance between the annular packers will be selected for each segment of the open hole; the wellbore must be segmented. Fracturing sleeves will run as part of the liner and the distance between them will be determined at the design stage. Fracturing sleeves should be of the type that runs in a closed form, but they can be opened and closed by running a special tool (wrench) on the tubing / coiled tubing string.

Brief Description of Drawings

1, Schematic diagram / display of a completion method;

Fig. 2, Stage 1 (Circulation stage);

Fig. 3, Stage 2 (Stage of the pack of a gelled liquid for flushing in a turbulent mode);

Fig. 4, Stage 3 (SRT / SDT, estimation of fracture pressure gradient and near-wellbore friction during a step test);

Fig. 5, Stage 4 (acidizing the formation);

Fig. 6, Stage 5 (hydraulic fracturing);

Fig. 7, Radial fracture geometry;

Figure 8, The geometry of an elliptical crack.

Run the liner (and completion equipment) and pressurize the entire system. Then a displacement tool is lowered into the well on the tubing / coiled tubing string and with its help the upper and lower hydraulic fracturing sleeves are opened. The tubing / coiled tubing string is placed above the lower hydraulic fracturing collar and an isolation packer is installed there (Fig. 1), where 1 - annular packers; 2 - hydraulic fracturing couplings; 3 - production string of the liner; 4 - string of tubing / coiled tubing used for injection; 5 - open bore; 6 - packer on tubing / coiled tubing string. All stages of hydraulic fracturing and stimulation operations are performed in this area as follows: - Flush the interval as shown in FIG. 2. (At this stage, careful observance of the material balance is necessary: constant monitoring of the values of QP, QR and QL is necessary, where QP = QR + QL (At this stage, the choke on the surface is open, and therefore there is no back pressure in the annulus)); - The pumps are stopped (at the same time) and the pressure drop curve is recorded (preferably using a downhole pressure gauge on the tubing / coiled tubing string); - The second treatment step will be a cleanup step in which a turbulent gel pack is pumped in to remove mud and filter cake as shown in FIG. 3 (the stage of maintaining the material balance is again performed, and control of the release of the drilling fluid to the surface, assessed for the content of solids), the choke is not used; - The pumps are stopped (at the same time) and the pressure drop curve is recorded (preferably using a downhole pressure gauge on the tubing / coiled tubing string); - At the third stage, the hydraulic fracturing pressure gradient is estimated by performing mini fracturing with a step test (see Fig. 4), at different rates (SRT / SDT) (during this stage, the choke is closed to determine the fracturing gradient and SRT / SDT); - The pumps are stopped (at the same time) and the pressure drop curve is recorded (preferably using a downhole pressure gauge on the tubing / coiled tubing string); - In the fourth step, the formation is acidified as shown in FIG. 5 (the volume of acid equivalent to the annulus volume of the open hole of the treatment interval) is placed along the annulus. At this stage, the choke is closed and the entire volume of acid is injected into the formation (below the fracturing pressure); - The pumps are stopped (at the same time) and the pressure drop curve is recorded (preferably using a downhole pressure gauge on the tubing / coiled tubing string); - At the fifth stage, hydraulic fracturing is performed (see Fig. 6). First, the well interval is flushed from the acidizing reaction products by circulating 2-3 volumes of the treated interval until the reaction products come out. After flushing, the choke is closed and the injection of the hydraulic fracturing fluid begins to develop the fracture along the wellbore. In cases of low leaks of working fluid into the formation (due to low reservoir permeability) and an increase in bottomhole pressure, which creates a risk of developing a fracture in height, it is allowed to undercut some of the pressure through the choke. After the start of the proppant supply to the flow, the choke is completely closed until the end of the work. The pumps are stopped (at the same time) and the pressure drop curve is recorded (preferably using a downhole pressure gauge on the tubing string). Then the release packer is lifted. When moving to the next segment in the well, the stimulated zone is isolated (produced by closing the sleeves with a tool / wrench). Then the operations are performed in the same sequence until the full coverage of the entire length of the horizontal wellbore. After performing hydraulic fracturing, a trip (round trip) of the tool / key is performed on all segments to open all hydraulic fracturing sleeves. At the final stage, the ESP unit will be run and the well will be put into operation.

Table 10 presents the observational data obtained at various stages.

For the particular case of the absence of stress contrasts along the depth, let us consider the relationship between the variables that takes place during crack growth; this relationship is determined by reference to the diagram shown in FIG. 7.

Based on the general theory of hydraulic fracturing. We can show how the following equation is applied:

Where: P _viscous = Viscous component of effective fracture pressure (psi)

Q = Pump flow (actually 50% flow) at fracturing (bbl / min)

E '= Modulus of plane deformation E' = E / (1-ν ² ), where ν is the coefficient. Poisson (psi)

H _f = Total Fracture Height (ft)

X _f = Total length of fracture formed (ft)

μ = Viscosity of fracturing fluid (cP)

P _viscous is part of the effective net pressure. For the case where there is no significant vertical stress contrast, when the process develops from a point source, the solution for the minimum energy consumption for crack propagation will always be radial geometry (as shown in Fig. 7). In these cases, the formed crack half-length X _f will develop uniformly, with a simple ratio to the height of the formed crack H _f 2: 1, i.e. radially.

This simplified fracture geometry illustrates the root cause of the problem with fracturing in the underlying water and / or gas cap zone; the formation of the crack half-length is inextricably linked with the formation of the corresponding crack height. As noted above, this is because the fracture is generated from a point source, in which case the result will be a radial fracture. The question arises: are hydraulic fractures always generated as point sources, or is there a way to create fractures from a line source; as we will see, the line source will create a different fracture geometry.

The ratios given in Table 9 show the results of various existing wellbore construction options, depending on the creation of point or line sources. The following cases are considered: cased and cemented or open segmented wellbores, for cases when they are drilled as vertical / deviated wells or horizontal wells oriented in the σ _{h min} direction, or horizontal wells oriented in the σ _{h shaft} direction. We now discuss each of these results in turn:

• Point source: As we have seen, when a fracture is formed from a point source, the result is a radial fracture with simplified geometry. Its distribution in height negatively affects the opening of the zone below the gas-oil (GOC) or above the oil-water (OWC) contact. Therefore, we can exclude all scenarios in which the fracture geometry forms as a point source, since they will not form the required fracture geometry as required in this case.

• Linear source (vertically oriented): In the case of vertical wells, cemented (fully perforated) or open hole, we see that the result of fracturing will be a line source. However, this line source is directed vertically, that is, directly at the gas-oil or oil-water contacts and is limited by the reservoir thickness, so this scenario is obviously not of interest either.

• Line source (horizontally oriented): Only in the case of a horizontal well oriented in the direction of maximum horizontal stress, a horizontally oriented line source is created. As we will see, such a linear source will stimulate fracture growth along the wellbore between the GOC and OWC in an elliptical manner, which minimizes the growth of the fracture in height; this is exactly the scenario that is most desirable.

As Table 9 shows, only one of these scenarios results in a line source fracture that has the orientation we want.

FIG. 8 shows an example of the schematic development of hydraulic fractures in the form of confocal ellipses. The machining interval is D, and the lengths of the major and minor axes are equal to X _f and H _f / 2, respectively. The condition of constant distance between focuses defines a family of ellipses represented by the equation:

Where: α = Crack length / height ratio (2X _f / H _f ) and

r = Dimensionless crack penetration (H _f / D))

Thus, the fracture geometry can be fully described in terms of fixed size D and variable r. The crack surface area A _{f is} then described by the following equation:

Based on the assumption that the crack has the shape of an ellipsoid, and the width at the center is equal to w _f , the total volume of the crack V _{f is} described by the following equation:

Fracture stiffness (the condition under which the hydraulic fracture begins to develop from a point source), S, depends on the plane deformation modulus E 'of the formation and the lengths of the major and minor axes of the crack. Green and Sneddon, 1950, and Daneshy, 1973, show that applying a uniform overpressure Pnet in a fracture will result in an ellipsoidal deformation with a center width wf.

Where: E (k) = Second order complete elliptic integral, and

k = Length / height ratio function = 1-α ² or = (1 + r ² ) ^-1

The value of k increases from 1 to π / 2 as crack penetration increases, and the geometry of the crack changes from a Kristonovich-Girtsma-De Klerk crack to a radial one. More information on the E (k) function can be found in Abramovitz and Stegun, 1964.

The above equations show that for a horizontal well in open hole conditions, with an effective perforated (open) interval D, determined by the location of the upper and lower hydraulic fracture sleeves, this will create an elliptical fracture with a side ratio equal to 2X _f / H _f .

The technical result of hydraulic fracturing according to the above technology will be to restrict the growth of the hydraulic fracture vertically to ensure uniform production of productive formations, and, accordingly, increase oil recovery and at the same time minimize the risk of breakthrough into the upper and lower formations.

Claims (1)

The method of hydraulic fracturing, which includes the following stages: a liner is run with annular packers and equal-bore hydraulic fracturing couplings, then a key is lowered on the tubing / coiled tubing string, with the help of which the couplings are opened, while the tubing / coiled tubing string is placed over the lower collar, the annular interval is flushed at open choke on the surface and no back pressure in the annulus, stop the pumps at the same time and record the pressure drop curve; at the second stage of treatment, cleaning is carried out, where the hydraulic fracturing fluid is pumped to remove the remnants of the drilling mud and mud cake, while the stage of maintaining the material balance is performed, and the control of the fluid release to the surface, evaluating it for the content of solids, while the choke is not used, is stopped pumps and record the pressure drop curve; at the third stage, the hydraulic fracturing pressure gradient is assessed, performing mini-fracturing with a step test, at different rates, while the choke is closed to determine the fracturing gradient, the pumps are stopped and the pressure drop curve is recorded, at the fourth stage, the formation is acidified by placing along the annulus the volume of acid equal to the volume of the open hole annulus, with the choke closed on the surface, while the entire volume of acid is pumped into the formation at a pressure below the hydraulic fracturing pressure; at the fifth stage, hydraulic fracturing is performed, the well interval is flushed from the reaction products of acid treatment by circulation of 2-3 volumes of the treated interval until the reaction products emerge, the choke is closed on the surface and the cushion is injected, developing a crack along the wellbore, the choke is closed on the surface and the injection is started hydraulic fracturing fluid, developing a fracture along the wellbore, while in the case of low leaks of the working fluid into the formation, with low reservoir permeability and an increase in bottomhole pressure, which creates a risk of the development of a fracture in height, they ensure that part of the pressure is released through the choke on the surface, when proppant the choke flow is completely closed until the end of the work, the pumps are stopped and the pressure drop curve is recorded, the release packer is lifted and the transition to the next treatment interval in the well is carried out, then the operations are repeated in the same sequence until the entire length of the well is completely covered, after Hydraulic fracturing operations at all treatment intervals run the ESP unit and start the well.

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