Simulation Optimization on Multi-Well Fracturing Sequence of Chang 7 Formation

In order to study the engineering problems of shale oil horizontal well platform fracturing mode, a three-dimensional geological model and a three-dimensional geomechanical model were constructed according to the characteristics of Chang 7 section in Ordos Basin. The fracture propagation under different well spacing, cluster spacing, and fracturing sequence was simulated. The PEBI grid was used to refine the local grid of the fractures and predict the performance of the multi-well pad. The finite element model was used to analyze the change of induced stress field. The numerical simulation analysis shows that: 1) Comparing the simulation results of sequence fracturing and zipper fracturing under the condition of 300 m well spacing, the induced stress generated by zipper fracturing is more conducive to the formation of complex fractures, improving the effect of transformation and higher production after fracturing. 2) Under the condition of 200 meters or 300 meters well spacing, zipper fracturing is carried out, and the cluster spacing is reduced from 5 meters to 3 meters. The stress shadow between wells and sections leads to adverse effects, and the production will decrease after fracturing. Therefore, 5 meters is recommended as the cluster spacing.


Introduction
The Ordos Basin is the second-largest oil and gas basin in China.The shale oil in this basin is mainly found in the Chang 7, Yanchang Formation.These shale oil deposits were formed in a fine-grained sedimentary environment in the semi-deep or deep lake phase.The Chang 71 and Chang 72 are lithological combinations of mud shale interbedded with thin layers of pink and fine sandstone.These layers have a large thickness and wide distribution of hydrocarbon source rock, making them the primary focus of current exploration and development.The development process involves using horizontal well pads for commercial flow, and the sequence fracturing and zipper fracturing methods are commonly used in multi-well pads.Determining the optimal well spacing and fracturing sequences is a key factor in maximizing the recovery rates of unconventional reservoirs.This study aims to determine the effects of different fracturing sequences on the propagation of hydraulic fractures and the evolution of in-situ stresses, leading to the identification of the optimal fracturing sequences and corresponding well spacing.using the arithmetic average method and then spatial differences are obtained using the Gaussian Random function algorithm to create a non-homogeneous physical property model.Based on rock mechanical interpretation data from each well, a three-dimensional geomechanical model is constructed under the constraints of the petrographic model.Overlying, lateral, and subducting rock layers are added to maintain a constant far-field stress state at the boundaries of the geomechanical model.

a) Porosity b)Geomechanical model
Figure 1.Three-dimensional geological modeling and geomechanical modeling.For the purpose of studying the effects of fracturing sequence, well spacing, and cluster spacing, three wells named H1, H2, and H3 were selected.H1 and H2 were spaced 200 m apart while H1 and H3 were spaced 300 m apart.Each well had a lateral length of approximately 500 m and was divided into 17 stages.The proppant loading used in the pumping procedure was 4 t/m and the fluid loading used was 35 m³/m.The cluster spacing between was either 3 m or 5 m and the perforations of two wells were staggered with each other.
The stress field plays a crucial role in the propagation of complex fractures.For the specific area of interest, the difference between the maximum and minimum horizontal principal stress is approximately 6.13 MPa.The stress shadows formed during the fracturing treatment are influenced by factors such as mechanical properties, fracture height, cluster spacing, and stage intervals.One of the primary differences between sequential and zipper fracturing is the time interval between stages.For practical reasons, it is recommended to have 240 minutes for sequence fracturing and 60 minutes for zipper fracturing.The time interval between adjacent stages in a single well is 450 minutes.Consequently, during sequence fracturing, stress shadows are stronger between the stages within one well, and weaker between the wells.Conversely, the zipper fracturing process has stronger inter-well stress shadows and weaker inter-stage stress shadows within single wells.To simulate the zipper fracturing fracture propagation, the UFM model is used, and the results of the simulation are presented in Figure 2  By reducing the two-way horizontal principal stress difference, the direction of fracture extension will be changed, so stress shadowing can have a positive impact on fracture complexity and increase stimulated volume.To calculate the changes of in-situ stress during stimulation treatment, a finite element model was used, and the results are presented in Table 1 and Figure 3.The well spacing in cases 1 to 4 is 300 m.Whether sequence fracturing or zipper fracturing is used, the cluster spacing is reduced from 5 m to 3 m, leading to more pronounced changes in the minimum horizontal principal stress magnitude and direction.Stress reversal occurs in the stimulated area, with the largest changes in the in-situ stress field observed in the case of 3 m cluster spacing and zipper fracturing.The maximum change amplitude is 12.82 MPa.
Table 1.From the distribution of the pressure field after 10 years of production, it can be seen that zipper fracturing has a higher degree of inter-well stimulation, and the drainage pressure realizes full coverage of the inter-well between the two wells.The scheme with zipper fracturing with a 5 m cluster spacing achieved the highest production.The post-frac performance of zipper fracturing was higher than that of sequence fracturing for both 3 m and 5 m cluster spacing conditions.
In the case of zipper fracturing with 300 m well spacing, the cumulative production did not increase when the cluster spacing was reduced from 5 m to 3 m.On the contrary, the cumulative oil production decreased by 10.99% after 10 years of production, reflecting that the smaller the cluster spacing, the stronger the superimposed effect of inter-well and inter-stage stress shadowing.However, the highest average daily production in the first month using 3 m cluster spacing indicates the highest degree of stimulated matrix and larger fracture surface area in the near-well zone.As the bottom flow pressure is a descent process from high to low pressure in the initial production stage, the daily production of horizontal wells rises rapidly during this period, peaks at about 50 d, and then decreases rapidly, with a rate of decrease of more than 70% over the two years of production.

Zipper fracturing and cluster spacing with 200 m well spacing
Results show that when the distance between wells was reduced from 300 m to 200 m, two wells with 5 m cluster spacing resulted in higher cumulative production compared to those with 3 m spacing.The total oil production over 10 years was also higher by 4,820 t, representing a 9.4% increase.However, unlike the case with 300 m spacing, reducing cluster spacing to 3 m resulted in lower daily production rates.     of unconventional reservoirs.The model also simulates the geomechanics and seepage mechanism involved in shale fracturing.The study investigates the impact of fracturing sequence on the production of horizontal wells.From the investigation, the following conclusions can be drawn: (1) The non-homogeneity of a reservoir can cause variable stress shadows during fracturing.To limit the stress shadows between wells and decrease the anisotropy of the horizontal principal stresses, it is recommended to reduce the well spacing.
(2) When comparing the results of sequence fracturing and zipper fracturing with 300 m well spacing, it was found that zipper fracturing induced higher stress changes, which led to complex fracture networks.This, in turn, improved the stimulation effect and the post-frac performance.
(3) It is not recommended to use the cluster spacing of 3 m during zipper fracturing with 200 m or 300 m well spacing.This is because the stress shadowing between wells and stages can have unfavorable effects that decrease post-frac production.Therefore, it is recommended to use a cluster spacing of 5 m.
Figure 1.Three-dimensional geological modeling and geomechanical modeling.For the purpose of studying the effects of fracturing sequence, well spacing, and cluster spacing, three wells named H1, H2, and H3 were selected.H1 and H2 were spaced 200 m apart while H1 and H3 were spaced 300 m apart.Each well had a lateral length of approximately 500 m and was divided into 17 stages.The proppant loading used in the pumping procedure was 4 t/m and the fluid loading used was 35 m³/m.The cluster spacing between was either 3 m or 5 m and the perforations of two wells were staggered with each other.The stress field plays a crucial role in the propagation of complex fractures.For the specific area of interest, the difference between the maximum and minimum horizontal principal stress is approximately 6.13 MPa.The stress shadows formed during the fracturing treatment are influenced by factors such as mechanical properties, fracture height, cluster spacing, and stage intervals.One of the primary differences between sequential and zipper fracturing is the time interval between stages.For practical reasons, it is recommended to have 240 minutes for sequence fracturing and 60 minutes for zipper fracturing.The time interval between adjacent stages in a single well is 450 minutes.Consequently, during sequence fracturing, stress shadows are stronger between the stages within one well, and weaker between the wells.Conversely, the zipper fracturing process has stronger inter-well stress shadows and weaker inter-stage stress shadows within single wells.To simulate the zipper fracturing fracture propagation, the UFM model is used, and the results of the simulation are presented in Figure2.

Figure 2 .
Figure 2. Hydraulic fracturing simulation results with 300 m well spacing.By reducing the two-way horizontal principal stress difference, the direction of fracture extension will be changed, so stress shadowing can have a positive impact on fracture complexity and increase stimulated volume.To calculate the changes of in-situ stress during stimulation treatment, a finite element model was used, and the results are presented in Table1and Figure3.The well spacing in cases 1 to 4 is 300 m.Whether sequence fracturing or zipper fracturing is used, the cluster spacing is reduced from 5 m to 3 m, leading to more pronounced changes in the minimum horizontal principal stress magnitude and direction.Stress reversal occurs in the stimulated area, with the largest changes in the in-situ stress field observed in the case of 3 m cluster spacing and zipper fracturing.The maximum change amplitude is 12.82 MPa.Table1.Minimum horizontal principal stress change of post-frac.

Figure 3 .
Figure 2. Hydraulic fracturing simulation results with 300 m well spacing.By reducing the two-way horizontal principal stress difference, the direction of fracture extension will be changed, so stress shadowing can have a positive impact on fracture complexity and increase stimulated volume.To calculate the changes of in-situ stress during stimulation treatment, a finite element model was used, and the results are presented in Table1and Figure3.The well spacing in cases 1 to 4 is 300 m.Whether sequence fracturing or zipper fracturing is used, the cluster spacing is reduced from 5 m to 3 m, leading to more pronounced changes in the minimum horizontal principal stress magnitude and direction.Stress reversal occurs in the stimulated area, with the largest changes in the in-situ stress field observed in the case of 3 m cluster spacing and zipper fracturing.The maximum change amplitude is 12.82 MPa.Table 1.Minimum horizontal principal stress change of post-frac.Case Fracturing sequence Cluster spacing /m Minimum horizontal principal stress change /MPa 1 sequence fracturing 3 -8.65-9.05 2 sequence fracturing 5 -8.98-8.99 3 zipper fracturing 3 -9.14-12.82 4 zipper fracturing 5 -9.13-8.67 (a) sequence fracturing, 3 m cluster spacing (b) sequence fracturing, 5 m cluster spacing ICMSOA-2023 Journal of Physics: Conference Series 2755 (2024) 012033 IOP Publishing doi:10.1088/1742-6596/2755/1/0120335 (c) zipper fracturing, 3 m cluster spacing (d) zipper fracturing, 5 m cluster spacing

Figure 5 .
Figure 5. Normalized production prediction for the 1500 m lateral length of two Wells.

(a) Case 5 :Figure 6 .
Figure 6.Top view of post-frac minimum horizontal principal stress field.After 10 years of production, a comparison of the pressure field distributions of the two horizontal wells in Figures7 and 8reveals that utilizing the zipper fracturing method and reducing the well spacing from 300 m to 200 m results in a larger pressure drop between the two wells.This larger pressure drop is beneficial for achieving full recovery of the reserves.

Figure 8 .
Figure 8. Normalized production prediction for the 1500 m lateral length of two Wells.
This study is based on the understanding of shale oil exploration and development in the Ordos Basin.It uses a state-of-the-art workflow to construct a 3D model that describes the natural non-homogeneity ICMSOA-2023 Journal of Physics: Conference Series 2755 (2024) 012033 IOP Publishing doi:10.1088/1742-6596/2755/1/0120337

Table 2 .
Minimum horizontal principal stress change of post-frac.