Research and application of large-scale fracturing imitating horizontal well technology in vertical well

. Hailaer Basin is rich in geological reserves, but the proportion of undeveloped reserves is high, accounting for 30% of the proved reserves. In order to improve the production degree of reserves and single well production, and realize the purpose of high and stable production of the oilfield. Taking the ultra-low permeability reservoir of oilfield B in Hailaer Basin as an example, this paper puts forward the large-scale fracturing of vertical wells and water injection development technology of imitating horizontal wells. By forming directional ultra long fracturing fractures in the oil layer, this technology can effectively increase the reservoir conductivity, increase the oil well drainage area and water well control area, reduce the well pattern density and improve the production of single well. The research shows that the undeveloped reserves in Hailaer oilfield have the characteristics of low thickness, low permeability and low reservoir quality index. There are 55 large-scale mold fracturing wells in Hailaer oilfield, with an initial oil increase of 2.2 times. The sand amount of large-scale fracturing is 2.5 times that of ordinary fracturing, and the fluid amount and sand intensity are 4.5 times that of conventional fracturing. The vertical well large-scale fracturing imitating horizontal well technology is adopted in Oilfield B. The designed vertical well fracturing fracture is 600m long and 120m wide, and the production effect is good after fracturing. The research results are of great significance to the high and stable production of undeveloped reserves block in Hailaer oilfield and other similar oilfields.


Introduction
With the continuous development of oil fields, the oil production decreases year by year, and the national demand for energy is increasing.In the face of this phenomenon, each oil field gradually strengthens the research on the development technology of proved undeveloped reserves.Xiaowa oilfield adopts the technology of viscosity reduction, plugging removal and rush pumping and forced drainage to realize effective gas injection development and improve the degree of reserve production.Wa 79 well area of Liaohe Oilfield adopts the combination of horizontal well and gas injection, and uses multi-point gas injection to improve heat utilization efficiency and realize the effective development of difficult to recover reserves.Facing the gas injection sources and high investment cost of horizontal wells, these methods have certain limitations in Hailaer oilfield.Therefore, in order to realize the economic and effective development purpose of low investment and high production, this paper puts forward the water injection development mode of large-scale vertical well fracturing imitating horizontal well, so as to give full play to the role of well pattern, realize economic and effective development and improve the production degree of undeveloped reserves.

Characteristics of reserve parameters and
pore structure parameters Statistical comparison of the reserve parameters and pore structure parameters of developed and undeveloped fault blocks in Hailaer oilfield shows that the average effective thickness of undeveloped fault blocks is 6.1m, the permeability is 7.0mD and the reserve abundance is 35.4 × 10 4 t / km 2 , with the characteristics of small fault block, low thickness, low permeability, low reserve abundance and low mobility.

Effect of starting pressure gradient on production
Experts and scholars at home and abroad have conducted a large number of experimental studies on ultra-low permeability reservoirs and found that there are non Darcy flow characteristics in ultra-low permeability reservoirs, that is, there is a phenomenon of starting pressure gradient.When the permeability is higher, the starting pressure gradient is lower and the single well production is higher; The smaller the permeability, the higher the starting pressure gradient and the lower the single well production [1,2] .

Effect of starting pressure gradient on effective driving distance
According to the experimental data of starting pressure gradient in Hailaer oilfield, the relationship curve between starting pressure gradient and permeability is drawn, and through curve regression can be obtained the relationship between the starting pressure gradient λ and permeability K is as follows: � � ���2�� ������ Remarks: λ is the starting pressure gradient, MPa / m; K is permeability, mD.In the hydrodynamic field of steady radial flow with equal output source and sink, the seepage velocity on the mainstream line is the largest, while on the same streamline, the seepage velocity at the same distance from the source and sink is the smallest.The starting pressure gradient here is: Remarks: P h is the bottom flow pressure of water injection well, MPa; P w is the bottom hole flow pressure of production well, MPa; R is the limit injection-production well spacing, m; R w is the wellbore radius, m.According to the actual situation of the oilfield, given the injection production differential pressure and starting pressure gradient parameters, the limit injectionproduction well spacing under the condition of low production pressure can be calculated by the above formula is about 150m.
Because the undeveloped reserves in Hailaer Basin have the above characteristics, the conventional fracturing method can not meet the economic and effective development of the oilfield.Therefore, large-scale fracturing is needed to improve the reservoir conductivity, increase the oil drainage area and improve the production of single wells.

Fracture shape analysis
According to the statistics of the relationship between the fracturing scale of large-scale branch fracture fracturing wells in the periphery of Daqing placanticline and the length width ratio of fractures, it can be seen that under the fracturing scale of a certain amount of sand and liquid, the average length width ratio of artificial fractures is 3.0.When the sand liquid ratio is increased, the fracture length width ratio is further increased, basically reaching more than 3.5.

Fracturing scale analysis
According to the statistics of the construction parameters of conventional fracturing and large-scale fracturing in Hailaer oilfield, it can be seen that the sand adding amount of large-scale fracturing is 2.5 times that of ordinary fracturing, and the liquid adding amount and sand adding intensity are 4.5 times that of conventional fracturing, forming 3-4 branch fractures.There are micro fractures near the well, with an artificial fracture width of 80-100m, a fracture length of 250-390m and a sand adding amount of 100-150m 3 .To sum up, adopting large-scale fracturing technology to exploit Hailaer oilfield has achieved good development results, and this technology is also an effective means to exploit the undeveloped reserves of Hailaer oilfield.
Based on the continuous rise of oil price at present, in order to achieve the purpose of low investment and high production, it is necessary to optimize and adapt the "large-scale fracturing" and "injection production well pattern", reduce the well pattern density and achieve an effective breakthrough in the production technology of undeveloped reserves.

Research on water injection development technology of imitation horizontal well
Based on the idea of water injection development of horizontal wells, the scheme of large-scale fracturing of vertical wells imitating horizontal wells is designed.Through large-scale fracturing of vertical wells, directional ultra long fracturing fractures are formed in the oil layer, which is equivalent to drilling through a horizontal section hundreds of meters long in the oil layer.From the schematic diagram of fracturing half fracture length, oil well drainage area and water well control area, it can be seen that with the increase of fracturing half fracture length, the oil well drainage area and water well control area also gradually increase [3][4][5] .At the same time, considering the influencing factors of in-situ stress direction, combine the fracturing long fracture with the well pattern, optimize the design of injection production well pattern, and timely inject water linearly into the fracturing well, so as to achieve the purpose of water injection, obtain higher recovery, reduce drilling, reduce investment, greatly improve recovery and economic benefits, and realize the economic and effective development of ultra-low permeability reservoir.Due to the high investment cost of large-scale fracturing, in order to reduce investment risks and improve economic benefits, according to the technical requirements and economic boundary requirements of large-scale fracturing of imitation horizontal wells, combined with the analysis of previous large-scale fracturing effects, the applicable conditions of water injection development technology of imitation horizontal wells are formulated: ① in order to realize the directional extension of the length of main fractures, the ratio of the maximum horizontal stress difference coefficient to the minimum horizontal stress difference coefficient should be ≥ 1.5 on the plane; ② Permeability ≤ 20 mD, small physical properties, large increase of oil drainage radius after pressure; ③ Since the overall fracturing requires a certain oil-bearing area, the oil-bearing area needs to be ≥ 0.5 km 2 ; ④ In order to ensure that the oil well after fracturing has a certain production capacity, the thickness of oil layer shall be ≥ 5m; ⑤ In order to prevent pressure channeling reservoir, oil-water distribution requires oil layer or dry layer; ⑥ In order to avoid conflicts between layers, the perforated well section shall be ≤ 50m.The thickness of the oil layer is small.The average porosity is 16.3% and the average permeability is 1.59mD.It is a medium porosity and ultra-low permeability reservoir.The average permeability variation coefficient of oil group II of oil layer X is 1.34, the breakthrough coefficient is 4.82, the permeability grade difference is 48.14, and the heterogeneity is serious.

Effect analysis
According to the prediction results of seismic and geological reservoir sand body in fault block B, aiming at maximizing the effective fracture swept volume, the integration method of vertical well directional long fracture fracturing imitating horizontal well and well pattern optimization design is adopted to realize the purpose of increasing fracture controlled reserves, expanding well spacing and reducing well pattern density.In the first year, the average daily water injection is 57m 3 and the water injection pressure is 18.5MPa.

Figure 1 .
Figure 1.Comparison of parameters between developed and undeveloped areas of Hailaer Oilfield

Figure 2 . 2 . 3
Figure 2. Scatter diagram of the relationship between porosity and reservoir quality index in developed and undeveloped blocks

Figure 3 .
Figure 3. Relation curve between starting pressure gradient and permeability

Figure 5 .
Figure 5.Control area of oil and water wells with different fracture lengths

Figure 6 .
Figure 6.Comparison between vertical well and imitation horizontal well

5. 1
Block overview Fault block B is located in the west of sudelte structural belt.The elevation of the high point of fault block structure is about -1025m.The stratum is high in the northwest and low in the southeast.It is a slope zone cut by NE trending clockwise fault, and the stratum dip angle is about 11 °.The maximum principal stress azimuth is 70 ° ~ 115 °, with an average of 96.6 °.The average effective thickness of the single layer of the main small layer of oil group I of oil layer X is 1.0 ~ 2.0m, and the average effective thickness of the single layer of the main small layer of oil group II is 1.0 ~ 2.2m.

Figure 7 .
Figure 7. Production curve of production well in B block

Table 1 .
Statistical results of fracturing scale and artificial fracture length width ratio

Table 2 .
Comparison of construction parameters of conventional fracturing and large-scale fracturing

Table 3 .
Production increase multiples of Hailaer Oilfield Block