Mixed Horizontal Well Pattern and Fracturing Parameter Optimization of Low Permeability Reservoir

Horizontal well fracturing development is significant in improving the development effect of low permeability reservoirs and is widely applied in many low permeability reservoirs of China. In this study, the reservoir numerical simulation is used to establish different mixed horizontal well networks of the target reservoir. The result shows that the best waterflood development effect is achieved when horizontal segment orientation and crack orientation are at an angle of 45°. The horizontal segment length and fracturing parameters of mixed horizontal well networks are optimized. It is considered that the rational horizontal segment length of the horizontal well is 400 m, the rational number of cracks is 3, and the optimal crack conductivity is between 180-200 mD.m. Through the study of mixed horizontal well network and fracturing parameter optimization, the low permeability reservoir efficient horizontal well fracturing development is well guided.


Introduction
Horizontal well fracturing development is significant in improving the development effect of low permeability reservoirs and is widely applied in many low permeability reservoirs of China [1][2][3] .However, due to the difference in reservoir features and crack distribution in different low permeability reservoirs, there are significant differences in horizontal well network and fracturing parameters.In the process of horizontal well network deployment, the effect of natural cracks and hydraulic cracks on the well network should be fully considered.According to domestic well network experience in low permeability reservoirs, the orientation of water injection well alignment should be deployed parallel to the orientation of maximum horizontal principle stress as far as possible to achieve better water flood development effect [4][5][6] .Due to the well location difference, maximum principle stress orientation, and crack orientation, the influence of hydraulic fracturing on development performance is also different.Therefore, it is essential to optimize the relation between the horizontal well network and crack orientation.At the same time, the horizontal segment length and fracturing parameter of the horizontal well are optimized to achieve a rational match between the crack system and the horizontal well network [7][8][9][10] .Based on the matching relation between the horizontal well

Reservoir survey
The target reservoir's main oil-bearing formation is the K1bt1 III oil layer.The reservoir lithology is mainly conglomerate and pebbly sandstone.The reservoir's average porosity is 10.2%, and the average permeability is 15.1 mD.The target reservoir is a medium-low porosity and low permeability reservoir.The mixed well network of vertical well and horizontal well is adopted in the main well area, and the well spacing is 200 meters.There are 17 wells that obtain formation dip logging data.The maximum principle stress orientation is basically the same.The maximum horizontal principle stress orientation is approximately 240-270 ° in an east-west orientation.Since the target reservoir was put into operation in the year of 2010, it has gone through the production trial stage, production capacity construction stage, and production decline stage.In the early stage, from the year of 2011 to 2012, there was a long water-free production period.In the year 2012, the annual oil production was 2.64×10 4 t, and the water cut was 22.4%.In the year 2013, the annual oil production was 3.6×10 4 t, and the water cut rose to more than 60% rapidly.In the year of 2014, the annual oil production decreased to 1.23×10 4 t, and the water cut was close to 80%.At present, the reservoir has entered the stage of low liquid production and high water cut, with daily oil production of 0.8 t per well and water cut of over 85%.The horizontal well development is generally poor, and the water cut increases rapidly after fracturing.The vertical wells between horizontal wells open up with high water content.After water injection of some vertical wells, the corresponding horizontal wells increase liquid production without increasing oil production.

Design principle of horizontal well network
The horizontal well network is much more complex than the vertical well network.Currently, there are various kinds of pure horizontal well networks and vertical and horizontal well combination networks.It is essential to optimize the well network based on reservoir features.Due to the deposition process and stress action of sandstone low permeability reservoir, the reservoir has obvious directional features, such as principal permeability orientation, principle stress orientation, crack orientation, sand body extension orientation, fault trend, and structural lean angle.The main considerations of horizontal well network design are well alignment orientation, water flood orientation, horizontal well extension orientation, and crack extension orientation.
The rational deployment of a horizontal well network is the organic combination of inherent reservoir orientation and artificially controllable orientation.The horizontal extension orientation of the horizontal well is the main consideration orientation.There are some principles in horizontal well network deployment.Firstly, the horizontal well extension orientation is vertical to the main permeability orientation and cuts across the sand body extension orientation.Secondly, the horizontal well extension orientation is 45 ° from the principle stress orientation and crack orientation.Thirdly, when the main permeability orientation is inconsistent with the crack orientation, the main permeability orientation is dominant.Finally, the water flood orientation should not be consistent with the crack orientation of the fracturing horizontal well.

Optimization of mixed horizontal well network
Considering of the target reservoir features, the mixed horizontal well network is designed with a constant injection-production well ratio according to the horizontal well network deployment principle.A total of 10 schemes are designed in this study, including a vertical well network, a horizontal well network, and a mixed vertical and horizontal well network.
Scheme 1: The production well (horizontal well) is parallel to crack orientation, and the injection well alignment (vertical well) is vertical to crack orientation.Scheme 6: The production well is a horizontal well, and the injection well is a vertical well.The angle between the horizontal well segment and crack orientation is equal to 45°, and the production wells are distributed in parallel.
Scheme 7: The production well is a horizontal well, and the injection well is a vertical well.The angle between the horizontal well segment and crack orientation is equal to 45°, and the production wells are distributed radially.
Scheme 8: The production well is a vertical well and horizontal well.The injection well is a vertical well.The injection well alignment is parallel to crack orientation.
Scheme 9: The production well is a vertical well and horizontal well.The injection well is a vertical well.The injection well alignment is vertical to crack orientation.
Scheme 10: The production well is a vertical well and horizontal well.The injection well is a vertical well.The angle between the horizontal well segment and crack orientation is equal to 45°, and the production wells are distributed radially.
Based on the 200 m well spacing of the basic well network, the well network with the same well number is adopted for well network layout.The producing capacity of 1 horizontal well is equal to the producing capacity of 2~3 vertical wells.The reservoir numerical simulation is used to calculate different scheme development for 20 years.The optimal horizontal well network is selected, as shown in Table 1 and Figure 1.According to the numerical simulation results, the difference between various mixed horizontal well network recovery degrees is within 5%, as shown in Figures 2 and 3. Scheme A10 has the highest recovery degree of 17.3%, followed by scheme A6 has a recovery degree of 16.5%, and Scheme A5 has the lowest recovery degree of 14.2%.Scheme A10 has the lowest water content of 95.1%, followed by scheme A9 has a water content of 96.1%, and Scheme A3 has the highest water content of 98.1%.In conditions of the same well spacing and injection-production intensity, Schemes A6, A7, and A10 have higher recovery degrees and lower water content.The best water flood development can be obtained when the horizontal segment orientation is 45°f rom the fracturing orientation.Therefore, Scheme A10 is selected as the optimal scheme from the mixed horizontal well network Schemes A6, A7, and A10.

Optimization of horizontal segment length
Based on the optimal horizontal well network scheme A10, the horizontal segment length, the crack quantity, and the crack conductivity are optimized.It is generally believed that the horizontal segment is longer, the single well control area is larger, and the production rate is higher.But for some specific reservoirs, when the horizontal segment length is close to a constant value, the production no longer increases or increases very little.The reservoir numerical simulation is used to study the development effect under different horizontal segment lengths, with horizontal segment lengths of 200 m, 250 m, 300 m, 350 m, 400 m, and 450 m, respectively.The development effect of different schemes for production 20 years is calculated by reservoir numerical simulation.The optimal horizontal segment length is selected, as shown in Figure 4.According to the numerical simulation results, as the rise of horizontal segment length increases, the formation pressure loss increases, and the producing capacity increases slowly.In addition, increasing horizontal segment length will significantly raise drilling costs and drilling risk.All adverse factors determine that the horizontal segment length is not necessarily too long, and there is an optimal value for the horizontal segment length.When the horizontal segment length exceeds 400 m, the cumulative oil production tends to approach, so the rational horizontal segment length of 400 m is determined.

Optimization of crack quantity
The quantity of cracks plays a crucial role in the productivity variation of fracturing horizontal wells.When the horizontal segment length is 400 m, the crack quantity in the horizontal well is designed to be 1~5 for sensitivity analysis research.The reservoir numerical simulation is used to calculate the development effect of different schemes for production 20 years.Then the optimal quantity of crack is selected, as shown in Figure 5.As the quantity of crack rises, the cumulative production and recovery degree also rise, but the raised amplitude slows down gradually.When the crack quantity is greater than 3, increasing the crack quantity has little effect on the recovery degree.And the cumulative production of 4 cracks and 5 cracks basically coincided.As the quantity of cracks rises, the oil drainage area of the reservoir gradually rises, and the seepage resistance decreases.And the production capacity of the fracturing horizontal well gradually rises, but the raised amplitude gradually decreases.When the quantity of cracks is small, the production capacity of fracturing horizontal wells rises significantly.When the quantity of cracks is large, the interference between cracks leads to a small rise in production capacity.So, the crack quantity of horizontal wells is not too large.Therefore, it is recommended that the rational quantity of crack is 3, corresponding to a horizontal segment length of 400 m.

Optimization of crack conductivity
The crack conductivity can be controlled artificially in hydraulic fracturing and has a substantial influence on the fracturing effect.The oil well fracturing effect depends on the liquid supply capacity of formation to crack and the liquid supply capacity of crack to the wellbore.Therefore, in order to achieve a good match between design crack conductivity and formation liquid supply capacity, it is essential to optimize the crack conductivity.
The numerical model is established to analyse the impact of crack conductivity on production capacity.The crack conductivity is set to be 50, 100, 150, 180, 200, 220, 250 and 280 mD.m.By using reservoir numerical simulation to calculate different scheme development for 20 years, the optimal crack conductivity is selected, as shown in Figure 6.relatively obvious.However, increasing the crack conductivity also means an increase in fracturing costs and expenses.When the crack conductivity is higher, the heterogeneity of the oil layer is stronger.Therefore, it is not that the higher the crack conductivity is, the better the development effect is.In the actual production, it is not essential to blindly pursue high conductivity but to optimize the overall scheme.According to the research result, the optimal crack conductivity is between 180 ~ 200 mD.m.

Conclusion
(1) The reservoir numerical simulation is used to establish different mixed horizontal well networks of the target reservoir.The result shows that the best water flood development effect is achieved when horizontal segment orientation and crack orientation are at an angle of 45°.
(2) The optimal horizontal segment length of a mixed horizontal well network is studied.The rational horizontal segment length is considered to be 400 m.
(3) The optimal fracturing parameter of mixed horizontal well network is studied, the rational quantity of horizontal well crack is 3, and the optimal crack conductivity is between 180-200 mD.m.

Figure 2 .Figure 3 .
Figure 2. Recovery degree of different horizontal well network scheme

Figure 4 .
Figure 4. Cumulative oil production of different horizontal segment length

6 Figure 5 .
Figure 5. Recovery degree of different crack quantity

Figure 6 .
Figure 6.Cumulative oil production of different crack conductivity Scheme 2: The production well (horizontal well) is vertical to crack orientation, and the injection well alignment (vertical well) is parallel to crack orientation. 3

Table 1 .
Scheme of mixed horizontal well network