A method for determining the reasonable range of backpressure difference after fracturing in high water cut tight sandstone gas reservoirs

Abstract

A method for determining the reasonable backflow pressure difference range after fracturing in a high-water-cut tight sandstone gas reservoir. Through indoor evaluation experiments, the critical backflow pressure differences for inducing reservoir particle migration damage during rapid backflow after fracturing and for the large-scale conversion of formation connate water into movable water production are determined respectively. The smaller value of the two is selected as the upper limit of the reasonable backflow pressure difference range. The backflow pressure difference corresponding to the point where the slopes of the backflow rate and permeability damage rate curves tend to 0 in the relationship curve of backflow pressure difference, backflow rate, and permeability damage rate is used as the lower limit of the reasonable backflow pressure difference range after fracturing in a high-water-cut tight sandstone gas reservoir to determine the numerical range of the reasonable backflow pressure difference. The beneficial effect of the present invention is that it can weaken the particle migration damage induced by large-displacement rapid backflow after fracturing while ensuring a high backflow rate of fracturing fluid and a high recovery rate of gas-phase permeability, avoid the large-scale conversion of formation connate water into movable water production, delay the water breakthrough time of gas wells, realize efficient water control and gas production in the gas reservoir, and ultimately increase the EUR of gas wells.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and more specifically to a method for determining a reasonable backflow pressure difference range after fracturing in a high water - cut tight sandstone gas reservoir. Background Art

[0002] As a typical unconventional natural gas resource, tight sandstone gas is widely distributed in the Ordos Basin, Sichuan Basin and Tarim Basin in China, and has great development value. Among them, the tight sandstone gas reservoirs in the Ordos Basin mostly have the characteristics of "low porosity, low permeability, high water saturation and high formation pressure". How to achieve effective exploitation, large - scale production increase and continuous stable production of such gas reservoirs is even known as a "world - class problem". Tight sandstone gas reservoirs have "poor congenital geological conditions" and basically do not have natural productivity. The large - scale hydraulic fracturing technology is the key and way to develop such gas reservoirs. The formulation of the gas well backflow system after hydraulic fracturing directly affects the effect of fracturing transformation. After hydraulic fracturing, the large - pressure - difference rapid backflow method is usually adopted in tight sandstone gas reservoirs to improve the backflow rate of fracturing fluid, avoid the long - term water - rock interaction between the fracturing fluid and the reservoir, and control the long - term percolation of the fracturing fluid along the tight matrix blocks. However, for high water - cut tight sandstone gas reservoirs, when using a high backflow pressure difference to discharge the fracturing fluid after fracturing, in addition to inducing secondary reservoir damages such as particle migration, gas well sand production and salt precipitation, it will also cause a large amount of conversion of formation connate water into mobile water, resulting in gas well water production. The secondary damages and gas well water production problems induced by high - pressure - difference backflow after fracturing will significantly weaken the reservoir transformation effect, shorten the stable production time of gas wells, and restrict the cumulative gas production of gas wells.

[0003] At present, the formulation of the backflow pressure difference after fracturing in tight sandstone gas reservoirs mostly depends on field experience or blindly pursues large - pressure - difference blowout and rapid liquid drainage, and has not considered reasonably formulating the backflow pressure difference from the perspectives of weakening the secondary reservoir damages induced by large - displacement rapid backflow after fracturing and avoiding the large - amount production of formation connate water into mobile water. Summary of the Invention

[0004] The present invention overcomes the deficiencies in the prior art and provides a method for determining a reasonable backflow pressure difference range after fracturing in a high water - cut tight sandstone gas reservoir.

[0005] The object of the present invention is achieved by the following technical solutions.

[0006] A method for determining a reasonable backflow pressure difference range after fracturing in a high water - cut tight sandstone gas reservoir includes the following steps:

[0007] Step 1: Select tight sandstone cores, and through indoor evaluation experiments, obtain the values of the water saturation in the cores under different displacement pressure differences. According to the cumulative difference of the irreducible water saturation before and after, determine the critical backflow pressure difference ΔP at which a large amount of formation connate water is converted into mobile water and produced w ;

[0008] Step 2: Select a tight sandstone core and calculate the critical maximum backflow pressure difference ΔP at which damage occurs through indoor evaluation experiments. v ;

[0009] Step 3: Select the smaller value among the critical backflow pressure differences obtained in Step 1 and Step 2 as the upper limit ΔP of the reasonable backflow pressure difference range after fracturing in a high-water-cut tight sandstone gas reservoir. 上 ;

[0010] Step 4: Use experimental or simulation methods to obtain a curve graph of the relationship between the backflow pressure difference, the backflow rate, and the permeability damage rate or permeability recovery. Select the backflow pressure difference corresponding to the point where the slope of the curve of the relationship between the backflow rate and the damage rate or permeability recovery tends to 0 as the lower limit ΔP of the reasonable backflow pressure difference range after fracturing in a high-water-cut tight sandstone gas reservoir. 下 ;

[0011] Step 5: Combine the upper and lower limit values of the backflow pressure difference range obtained in Step 3 and Step 4 to determine the numerical range of the backflow pressure difference after fracturing in a high-water-cut tight sandstone gas reservoir.

[0012] Step 1 specifically includes:

[0013] (1) Select a tight sandstone core and establish the irreducible water saturation before core fracturing according to the maximum pressure difference provided by the gas reservoir.

[0014] (2) After hydraulically fracturing the sandstone core, with the maximum available production pressure difference as the boundary, gradually increase the pressure difference to displace the core in the irreducible water saturation state, obtain the water saturation data of the core corresponding to different displacement pressure differences, and judge the critical backflow pressure difference ΔP at which a large amount of formation connate water is converted into mobile water, resulting in water production from the gas well. w The numerical value of ΔP w The judgment formula is as follows:

[0015] S w0 -S w1 ≥8%

[0016] In the formula: S w0 is the irreducible water saturation value measured at the maximum pressure difference provided by the gas reservoir before fracturing; S w1 is the different irreducible water saturation values obtained by simulating the displacement of the core in the irreducible water saturation state after increasing the pressure difference; ΔP w is the critical backflow pressure difference at which a large amount of formation connate water is converted into mobile water, resulting in water production from the gas well.

[0017] Step 2 specifically includes:

[0018] (1) Select a tight sandstone core with a relatively high illite relative content and prepare a thin section of the tight sandstone core.

[0019] (2) Place the core thin section into the holder, place filter paper behind the outlet end, and collect the produced minerals.

[0020] (3) Use high-purity nitrogen for displacement, gradually change the displacement pressure difference, collect the filter paper in sequence, and conduct X-ray diffraction analysis.

[0021] (4) Obtain the data of mineral content and soak fluid turbidity under the displacement pressure gradient.

[0022] (5) Select the backflow pressure gradient when illite or kaolinite minerals are contained in the minerals collected at the outlet end and the soak fluid turbidity is greater than 10 as the critical pressure gradient τ for particle migration damage. i .

[0023] (6) Based on the critical pressure gradient and reservoir thickness obtained from the experiment, calculate the critical maximum backflow pressure difference ΔP for damage under reservoir conditions. v ,

[0024] ΔP v =τ i ×r e

[0025] ΔP v The critical backflow pressure difference for reservoir damage, τ i is the critical pressure gradient for particle migration damage; r e is the reservoir thickness.

[0026] The beneficial effects of the present invention are as follows: A method for determining the reasonable backflow pressure difference range after fracturing in a high-water-cut tight sandstone gas reservoir is provided. This method can, while ensuring the most efficient utilization of formation energy to achieve a high backflow rate of fracturing fluid and a high recovery rate of gas-phase permeability, alleviate to a certain extent the particle migration damage induced by backflow after fracturing, avoid a large amount of formation connate water from converting into movable water for production, delay the water breakthrough time of gas wells, realize reasonable gas production allocation for gas wells, and improve the EUR of gas wells. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the flow chart of the invention;

[0028] Figure 2 is the water saturation change curve under different backflow pressure differences;

[0029] Figure 3 is the relationship curve between backflow pressure difference and backflow rate and permeability damage rate. DETAILED DESCRIPTION OF THE INVENTION

[0030] The technical solutions of the present invention will be further described below through specific embodiments.

[0031] Example 1

[0032] In this embodiment, a high water cut tight sandstone gas reservoir in a certain domestic block is selected as the research object. The reservoir burial depth is 1300 - 2200m, the average pore pressure is 18.8MPa, and the average temperature is 55.1°C. Taking Well L in this block as an example, the content, characteristics and effects of the present invention will be further described in detail with reference to the attached drawings. The specific implementation method is as Figure 1 shown, and the specific implementation steps are as follows:

[0033] Step 1: Select the tight sandstone core in the study area, refer to the maximum pressure difference provided by the gas reservoir, and establish the irreducible water saturation of the core. The specific calculation results are shown in Table 1.

[0034] Table 1 Variation of water saturation under different displacement pressure differences

[0035]

[0036]

[0037] As Figure 2 shown, the irreducible water saturation value established by the gas drive method is 66.24%. After hydraulic fracturing, taking the maximum available production pressure difference as the boundary, the pressure difference is gradually and slowly increased to displace the core in the irreducible water saturation state. At a pressure difference of 3.9MPa, the cumulative difference in irreducible water saturation before and after is 8.23%. Then this 3.9MPa pressure difference is the critical backflow pressure difference ΔP at which a large amount of formation connate water is converted into movable water, resulting in water production in the gas well w ;

[0038] Step 2: Select the tight sandstone core and conduct a fine particle migration evaluation experiment. Combine with X-ray diffraction to analyze the occurrence of illite minerals under different displacement pressure gradients. If the minerals collected at the outlet end contain illite or kaolinite minerals, and the turbidity of the soaking solution increases by more than 10, it is considered that the backflow pressure gradient at this time is the critical pressure gradient at which fine particle migration damage occurs. The detection values are shown in Table 2. To exclude that the turbidity change is not caused by experimental misoperation or other reasons, combining the illite content and the turbidity of the soaking solution, 0.2MPa / cm is selected as the medium critical maximum backflow pressure gradient, and ΔP v is 20MPa.

[0039] Table 2 X-ray diffraction and turbidity analysis results under different displacement pressure differences

[0040]

[0041] Step 3: Convert the pressure gradient obtained in Step 2 to the reservoir conditions, and compare ΔP v with ΔP w , and select the minimum value of the two as the upper limit ΔP of the reasonable backflow pressure difference range after fracturing of the high water cut tight sandstone gas reservoir上 , it can be seen by comparison that ΔP w has a smaller value. Therefore, the upper limit of the reasonable backflow pressure difference range after fracturing for high-water-cut tight sandstone gas reservoirs is 3.9 MPa;

[0042] Step 4: As Figure 3 shown, use experimental or simulation methods to obtain the relationship curves of backflow pressure difference with backflow rate and permeability damage rate or permeability recovery rate. Select the backflow pressure difference corresponding to the slope of the backflow rate and permeability damage rate curves tending to 0 as the lower limit ΔP 下 , that is, ΔP 下 is 2 MPa;

[0043] Step 5: Comprehensive analysis shows that the reasonable backflow pressure difference range after fracturing for high-water-cut tight sandstone gas reservoirs is between 2 and 3.9 MPa. Within this range of backflow pressure difference, it is possible to ensure a high backflow rate of fracturing fluid and a high recovery rate of gas-phase permeability while avoiding a large amount of formation fluid production and inducing particle migration damage, thus realizing efficient water control and gas production in the gas reservoir.

[0044] The above has described the embodiments of the present invention in detail, but the content described is only the preferred embodiments of the present invention and cannot be considered as limiting the scope of implementation of the present invention. All equivalent changes and improvements made according to the scope of the present invention application should still fall within the scope covered by the patent of the present invention.

Claims

1. A method for determining the reasonable flowing-back pressure difference range after fracturing in a high water cut tight sandstone gas reservoir, characterized in that It includes the following steps: Step 1: Select a tight sandstone core. Through indoor evaluation experiments, obtain the values of the water saturation in the core under different displacement pressure differences. According to the cumulative difference in the irreducible water saturation before and after, determine the critical backflow pressure difference ΔP at which a large amount of the formation's original water is converted into movable water and produced w ; Step 2: Select a tight sandstone core and calculate the critical maximum backflow pressure difference ΔP at which damage occurs through indoor evaluation experiments v ; Step 3: Select the smaller value among the critical backflow pressure differences obtained in Step 1 and Step 2 as the upper limit ΔP of the reasonable backflow pressure difference range after fracturing in high water cut tight sandstone gas reservoirs 上 ; Step 4: Use the method of experiment or simulation to obtain a curve graph of the relationship between the backflow pressure difference, the backflow rate, and the permeability damage rate or permeability recovery. Select the backflow pressure difference corresponding to the curve slope of the relationship between the backflow rate and the damage rate or permeability recovery tending to 0 as the lower limit ΔP of the reasonable backflow pressure difference range after fracturing in a high-water-content tight sandstone gas reservoir 下 ; Step 5: Combine the upper and lower limit values of the backflow pressure difference range obtained in Step 3 and Step 4 to determine the numerical range of the backflow pressure difference after fracturing in the high water cut tight sandstone gas reservoir.

2. The method for determining the reasonable flowing-back pressure difference range after fracturing in a high water cut tight sandstone gas reservoir according to claim 1, wherein Step 1 specifically includes: (1) Select a tight sandstone core and establish the irreducible water saturation before core fracturing according to the maximum pressure difference provided by the gas reservoir. (2) After hydraulic fracturing of the sandstone core, taking the maximum available production pressure difference as the boundary, gradually increase the pressure difference to displace the core in the bound water saturation state, obtain the water saturation data of the corresponding core under different displacement pressure differences, and judge the critical backflow pressure difference ΔP at which a large amount of formation connate water is converted into mobile water, resulting in water production in the gas well. w The value of ΔP w The judgment formula is as follows: S w0 -S w1 ≥8% Where: S w0 is the irreducible water saturation value measured at the maximum differential pressure provided in the gas reservoir before fracturing; S w1 is the different irreducible water saturation values obtained by core displacement of irreducible water saturation state after simulated fracturing with increased differential pressure; ΔP w is the critical backflow differential pressure at which a large amount of formation connate water is converted into mobile water, resulting in water production from the gas well.

3. A method for determining a reasonable flowing-back pressure difference range after fracturing in a high water cut tight sandstone gas reservoir according to claim 1, characterized in that Step 2 specifically includes: (1) Select a tight sandstone core with a relatively high illite content and prepare a thin section of the tight sandstone core. (2) Place the thin section of the core in a holder, place a filter paper at the outlet end, and collect the produced minerals. (3) Use high-purity nitrogen displacement, gradually change the displacement pressure difference, collect the filter papers in sequence, and conduct X-ray diffraction analysis. (4) Obtain the data of the mineral content and the turbidity of the soaking solution under the displacement pressure gradient. (5) Select the backflow pressure gradient when the minerals collected at the outlet end contain illite or kaolinite minerals and the turbidity of the soaking solution is greater than 10 as the critical pressure gradient τ for particle migration damage i； (6) Calculate the critical maximum flowback pressure difference ΔP for damage under reservoir conditions based on the critical pressure gradient and reservoir thickness obtained from experiments. v , ΔP v = τ i × r e ΔP v is the critical backflow pressure difference for reservoir damage, τ i is the critical pressure gradient for particle migration damage; r e is the reservoir thickness.

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