A numerical model of hydraulic fracture propagation is introduced for a representative reservoir (Yuanba continental tight sandstone gas reservoir in Northeast Sichuan). Different parameters are considered, i.e., the interlayer stress difference, the fracturing discharge rate and the fracturing fluid viscosity. The results show that these factors affect the gas and water production by influencing the fracture size. The interlayer stress difference can effectively control the fracture height. The greater the stress difference, the smaller the dimensionless reconstruction volume of the reservoir, while the flowback rate and gas production are lower. A large displacement fracturing construction increases the fracture-forming efficiency and expands the fracture size. The larger the displacement of fracturing construction, the larger the dimensionless reconstruction volume of the reservoir, and the higher the fracture-forming efficiency of fracturing fluid, the flowback rate, and the gas production. Low viscosity fracturing fluid is suitable for long fractures, while high viscosity fracturing fluid is suitable for wide fractures. With an increase in the fracturing fluid viscosity, the dimensionless reconstruction volume and flowback rate of the reservoir display a non-monotonic behavior, however, their changes are relatively small.

With the deepening of oil and gas exploration, the development of unconventional oil and gas resources has been attached equal importance to that of conventional oil and gas resources [^{8} t, of which unconventional oil is 4209.4 × 10^{8} t (accounting for 72.2%), and unconventional natural gas is 195.4 × 10^{12} m^{3} (accounting for 27.8%) (

Generally, tight sandstone gas reservoirs have strong reservoir heterogeneity. Hydraulic fractures tend to cross zones during fracturing and excessive vertical propagation of fractures leads to extra energy loss, which will make it hard for fractures to extend horizontally in the target zone effectively (

The gas reservoir A in Yuanba area in Northeast Sichuan Province is a typical deep continental tight sandstone gas reservoir of China. The average completion depth of reservoir A is 5 km, with more than 6 km of maximum burial depth, 1.99% average porosity and 0.069 mD average permeability. Its reservoir porosity and permeability are very low, and it is a typical ultra-low porosity and permeability reservoir [

A numerical model of hydraulic fracture propagation was established based on the Discrete Fracture Network (DFN) model, which combined with fractured reservoir geology enabled hydraulically induced discrete fractures to initiate and extend vertical and horizontal fractures across three principal planes. The discrete fracture network can be solved based on continuity equation, mass conservation equation, constitutive relation and momentum equation [

continuity equation:

DFN momentum equation:

In this article, a numerical model of hydraulic fracturing fracture propagation for gas wells in gas reservoir A of Yuanba area in Northeast Sichuan Province is established [

Stress (MPa) | Fracture length (m) | Fracture width (cm) | Fracture height (m) |
---|---|---|---|

1 | 138.85 | 0.408 | 93.643 |

2 | 135.52 | 0.372 | 61.427 |

4 | 178.95 | 0.445 | 43.009 |

6 | 204.57 | 0.477 | 34.967 |

8 | 213.89 | 0.489 | 32.511 |

10 | 218.86 | 0.496 | 31.337 |

According to literature research, the importance of different parameters on hydraulic fracture propagation is ranked as: inter-layer stress difference > displacement > reservoir thickness > Young’s modulus heterogeneity > fracturing fluid viscosity > permeability heterogeneity, among which inter-layer stress difference, fracturing fluid displacement and reservoir thickness are significantly related to fracture geometry [

Under complex geological conditions, the propagation of hydraulic fracture network not only depends on the characteristics of the target layer, but also is closely related to the mechanical properties of the upper and lower spacers. The stress difference between the target layer and the upper and lower spacers is an important factor which affects the vertical propagation of fractures. When the stress difference between the inter-layer is too small, the fracture is easy to channel the layer in the fracturing process, resulting in vertical fractures and energy loss. As a result, it is difficult to extend the fracture in the horizontal section of the target layer. According to the actual geological conditions in the Northeast Sichuan area, six hydraulic fracturing numerical simulations are carried out with the stress difference of inter-layer being 1, 2, 4, 6, 8 and 10 MPa.

Construction displacement (m^{3}/min) |
Fracture length (m) | Fracture width (cm) | Fracture height (m) |
---|---|---|---|

2 | 177.49 | 0.329 | 35.825 |

4 | 182.09 | 0.329 | 40.243 |

8.5 | 187.19 | 0.338 | 43.009 |

12 | 187.38 | 0.349 | 43.376 |

15 | 187.97 | 0.357 | 43.531 |

20 | 189.04 | 0.365 | 43.545 |

The pumping displacement of fracturing fluid during construction has a significant influence on the expansion form of fracture network. The fractures ^{3}/min.

^{3}/min, the growth rate of fracture size slows down and the fracturing efficiency decreases. Considering the fracturing construction cost and reservoir reconstruction volume, the construction displacement should be roughly set at 8.5 m^{3}/min.

The technological parameters and rheological properties of fracturing fluid have an important influence on the construction effect of hydraulic fracturing. With other conditions invariant, the higher the viscosity of fracturing fluid, the wider the fracture. When the fracture volume is invariant, the fracture length is shorter while the fracturing fluid viscosity is higher. Reasonable selection of fracturing fluid viscosity plays a decisive role in the successful reconstruction of reservoir. Combined with the types and rheological parameters of fracturing fluids used in the actual fracturing process in Yuanba area, five kinds of hydraulic fracturing numerical simulations with levels of 100, 300, 500, 800 and 1000 mPa⋅s of fracturing fluids viscosity are set.

Fluid viscosity (mPa·s) | Fracture length (m) | Fracture width (cm) | Fracture height (m) |
---|---|---|---|

100 | 348.68 | 0.239 | 34.69 |

300 | 271.66 | 0.311 | 36.911 |

500 | 223.71 | 0.369 | 39.79 |

800 | 178.95 | 0.445 | 43.01 |

1000 | 146.7 | 0.527 | 45.77 |

Increasing fracturing fluid flow-back rate can reduce the damage on fracture wall and formation, and reduce fresh water consumption in fracturing. As an important index to evaluate fracturing effect, fracturing fluid flow-back rate is directly related to the production of fractured reservoir [

After historical fitting to ensure the accuracy of the established numerical reservoir model of gas reservoir A in Yuanba area in Northeast Sichuan, the characteristic well H in the target block is selected to carry out the study on the gas and water production law after single well pressure. In this article, the rectangular reservoir with 700 m × 300 m × 400 m I × J × K including well H was intercepted as the study area, the longitudinal grid step size was divided according to the thickness of small beds, and the fractures were preset according to the numerical simulation results of fracture propagation (

Parameter | Value | Parameter | Value | Parameter | Value |
---|---|---|---|---|---|

Reservoir pressure (MPa) | 89.27 | Reservoir temperature (K) | 343.15 | Reservoir size (m × m) | 700 × 300 |

Target layer thickness (m) | 61 | Water saturation | 0.24~0.71 | Young’s modulus |
28~80 |

Matrix porosity | 0.027~0.084 | Matrix permeability |
0.01~0.093 | Poisson’s ratio (dimensionless) | 0.17~0.32 |

After fracturing, the gas and water production dynamics of gas wells are similar, showing the characteristics of rapid rise of production at first, then rapid decline, and finally being stable. Its full life cycle can be divided into three stages: ① rapid production stage, ② production decline stage, and ③ steady production stage (

By comparing the gas and water production dynamics under different inter-layer stress difference conditions (

By comparing the fracture size, dimensionless reconstruction volume and gas water production dynamics under different fracturing fluid viscosities (

1) The inter-layer stress difference can effectively control the joint height and promote the development of joint length. Large displacement fracturing is conducive to improving the efficiency of fracture formation and expanding the fracture size. Low viscosity fracturing fluid is suitable for making long fractures, while high viscosity fracturing fluid is suitable for making wide fractures. The dimensionless reconstruction volume of reservoir has a negative correlation with inter-layer stress difference and a positive correlation with the fracturing displacement. It increases first and then decreases with the increase of fracturing fluid viscosity. The dimensionless reconstruction volume of reservoir has a small difference under different fracturing fluid viscosity.

2) The flow-back rate is mainly determined by the physical properties of the reservoir and the properties of the fracturing fluid. Under different influencing factors, the flow-back rate after pressure is highly correlated with the dimensionless reconstruction volume of the reservoir. The lower the fracturing fluid reservoir intrusion, the higher the fracture formation efficiency, and the larger the dimensionless reconstruction volume, the higher the fracturing fluid flow-back rate. What’s more, rapid flow-back can effectively reduce fracturing fluid reservoir intrusion and improve reservoir flow-back efficiency.

3) Gas well production can be divided into three stages. The production in the first stage is determined by dimensionless reservoir reconstruction volume; the production in the second stage is determined by dimensionless reservoir reconstruction volume and reservoir physical property; and the production in the third stage is determined by reservoir physical property. The smaller the inter-layer stress difference is as well as the larger the fracturing displacement is, the larger the dimensionless reservoir reconstruction volume is and the higher the gas production is. What’s more, the higher the viscosity of the fracturing fluid and the lower the gas production in the case of similar dimensionless reservoir reconstruction volume, the longer the fracture length is, the higher the third-stage production and the final production are.

4) In the case of boundless bottom water or clear water layer, the water production of fractures with different sizes has little difference. The influence of different factors on the water production of gas wells is mainly reflected in the first production stage. The larger the dimensionless reconstruction volume of the reservoir, the larger the water production in the initial production stage, and eventually tends to be consistent with the progress of production time.

None.

The authors received no specific funding for this study.

The authors confirm contribution to the paper as follows: study conception and design: Yan Liu and Tianli Sun; data collection: Bencheng Wang; numerical simulation: Yan Feng; analysis and interpretation of results: Yan Liu, Tianli Sun and Yan Feng; draft manuscript preparation: Bencheng Wang and Yan Feng. All authors reviewed the results and approved the final version of the manuscript.

The authors confirm that the data supporting the findings of this study are available within the article.

The authors declare that they have no conflicts of interest to report regarding the present study.