The variation of the principal stress of formations with the working and geo-mechanical conditions can trigger wellbore instabilities and adversely affect the well completion. A finite element model, based on the theory of poro-elasticity and the Mohr-Coulomb rock damage criterion, is used here to analyze such a risk. The changes in wellbore stability before and after reservoir acidification are simulated for different pressure differences. The results indicate that the risk of wellbore instability grows with an increase in the production-pressure difference regardless of whether acidification is completed or not; the same is true for the instability area. After acidizing, the changes in the main geomechanical parameters (i.e., elastic modulus, Poisson’s ratio, and rock strength) cause the maximum wellbore instability coefficient to increase.

At present, well-completion methods mainly include bare-hole completion and shot-hole completion. The bare-hole completion method suits carbonate or sandstone reservoirs without sand production, which are hard and dense. The shot-hole completion method is suitable for sandstone and fractured carbonate reservoirs, which must be sand-free. Therefore, it is necessary to choose the completion method based on the wellbore stability for different types of reservoirs.

For unstable formations, the completion method with a propped arrangement will be adopted to avoid wellbore collapse during the production process. There are three aspects must be considered as the wellbore stability estimation, including rock mechanics parameters, in-situ stress field, and wellbore stability mechanics. Combining the three aspects, the wellbore stability evaluation model can obtain a rational drilling fluid density range to manage wellbore stability. Westergard [

From the above studies, it is clear that the current research on wellbore stability mainly about mechanical and chemical aspects, and there is a lack of research on the effect of the production process on wellbore stability. To this end, this paper establishes a finite element model based on the theory of poroelasticity and considers the influence of different working conditions and geomechanical conditions. The Mohr–Coulomb rock damage criterion is then used to determine the rock damage state. From this, the factor of wellbore instability and wellbore stability are calculated to determine whether the wellbore is stable. Finally, a simulation study is carried out with field data to provide a model basis for studying wellbore stability during production.

There are three aspects of wellhole stability mechanics research; the surrounding rock mechanical characteristics is basis, the ground stress is fundamental cause, and the wellbore stability mechanics model effectively solves wellbore stability [

The formation rocks are stable through the interaction of overburden pressure, horizontal

As shown in

In a homogeneous, isotropic and linearly elastic formation, the redistributed wellbore envelope stress expression is obtained by using the Fairhurst slant well equation [

The stress component at the wellbore (

where

In the vertical well scenario, the polar axis direction is the same as the maximum principal stress direction (i.e.,

where

In porous media, the formation pore medium around the wellbore, as shown in

where

The wellbore stability evaluation mainly includes three steps:

Stress analysis on wellbore surrounding rock, to get three ground stresses.

Consideration of the rock failure criterion to assess the stability of the wellbore.

For instability in a wellbore, regulation of the drilling fluid density to stabilize the wellbore.

Currently, the most used strength criteria are the Mohr–Coulomb (M–C) criterion and the Drucker–Prager (D–P) criterion [

where

In

Firstly, based on the theory of poroelasticity, a three-dimensional mechanical model of the wellbore is established through the finite element analysis method. And then, the three-way principal stresses at each point around the well perimeter are taken, and the rock’s state of damage is judged by the Mohr–Coulomb rock damage criterion. Finally, the coefficients of wellbore instability and wellbore stability are calculated.

The key point of the study on the wellbore stability mechanical mechanism is the analysis of the stress state and stress distribution in the wellbore. Considering the effect of pore pressure changes during production on the effective stress of wellbore rocks, the mechanical analysis model obeys the following assumptions:

Rock initialization around the wellbore is assessed by using a pore elastic method with effective stress control for rock skeleton deformation and damage.

The numerical model obeys the elastic-plastic mechanics of porous media. When the damage occurs in the formation, the Mohr–Coulomb criterion is used for the strength criterion.

The percolation of the drilling fluid conforms to Darcy’s law. The effects of wellbore temperature variation and the geochemical reaction between drilling fluid and formation are ignored.

BoZi is a normal-temperature and high-pressure gas reservoir located in northwest China. Referencing the BoZi formation properties, the two sets of geomechanical parameters are set to represent formation rock properties before and after acidification. The geomechanical parameters are presented in

BoZi | Original conditions | After acidification |
---|---|---|

Well depth (m) | 7014–7084 | 7014–7084 |

Wellbore size (mm) | 216 | 216 |

Original formation pressure (MPa) | 125.72 | 125.72 |

Rock density (g/cm^{3}) |
2.618 | 2.618 |

Young’s modulus (MPa) | 33560 | 55933.3 |

Poisson’s ratio | 0.207 | 0.1242 |

Angle of internal friction | 24.68 | 14.808 |

Cohesion force (MPa) | 13.69 | 8.214 |

Biot coefficient | 0.808 | 0.808 |

Maximum principal stress (MPa) | 179.823 | 179.823 |

Minimum principal stress (MPa) | 145.107 | 145.107 |

Overlying rock pressure (MPa) | 163.085 | 163.085 |

Reservoir porosity | 0.05 | 0.05 |

Reservoir permeability (mD) | 0.89 | 0.89 |

As shown in

Case | Formula for expression of the distribution of pore pressure |
---|---|

Production pressure difference, 60 MPa | |

Production pressure difference, 40 MPa | |

Production pressure difference, 30 MPa | |

Production pressure difference, 20 MPa | |

Production pressure difference, 10 MPa |

As shown in

As shown in

Based on the stress balance theory, the original formation stress state of the numerical model is restored.

Using the cell deletion method, the cells in the wellbore are removed so that a wellbore can be formed. And the stress state of the wellbore will be refreshed by stress concentration.

The different production pressure is set and the pore pressure variation is simulated to clarify the change characteristics of pore pressure under different working conditions.

As the reservoir pore pressure decreases, there is insufficient fluid filling the pore of rocks, which causes the effective rock stress to increase, wellbore principal stress to change, and the gap between the maximum and minimum principal stress to be obvious. As shown in

In the original condition, the principal stress variation is mainly concentred on the four wellbore points at 0, 90, 180, and 360 degrees. The minimum principal stress extends uniformly outward along the circumference of the wellbore after production. And the maximum principal stress is mainly concentred on two points, at 0 and 180 degrees.

During Bozi reservoir production, the pore pressure shows a logarithmic distribution about the location of the wellbore center. The distribution rules are presented in

The difference between the maximum and minimum principal stress is increased as the changes of principal stress in the wellbore. Comparing the scenario of 60 and 40 MPa production pressure difference (

After the acidizing operation, the formation rock mechanical strength decreases, the elastic modulus increases, and the Poisson’s ratio decreases. These will lead to an increased risk of wellbore destabilization.

The finite element analysis method with the Mohr–Coulomb rock damage criterion is used to establish finite element models for different working conditions and strata based on the theory of poroelasticity. The numerical simulation results reveal that different working and geomechanical conditions will affect the magnitude of the ground stress in the wellbore, increasing the risk of wellbore instability during production.

The model’s practicality is validated through the original formation and post-acidification conditions of the BoZi gas reservoir. With the increase of production pressure difference, the wellbore stability coefficient and the wellbore disability risk are increased in the original formation.

The acidised formation in the same production system has a bigger wellbore instability area and a lower production pressure difference threshold. In the on-site scenario, the numerical simulation results can help engineers choose a better production system for acidized formation production without the occurrence of wellbore instability accident.

None.

This work is financially sponsored by Tarim Oilfield “Study on Adaptability Evaluation and Parameter Optimization of Completion Technology in Bozi Block, Tarim Oilfield” (Item Number: 201021113436).

Study conception and design: Junyan Liu and Ju Liu; data collection: Yan Wang and Shuang Liu; analysis and interpretation of results: Shuang Liu, Qiao Wang, and Yihe Du; draft manuscript preparation: Yihe Du, Junyan Liu and Ju Liu. All authors reviewed the results and approved the final version of the manuscript.

The data belongs to Tarim Oilfield which is very strict to the data security, as a result, for protecting the power requirements of Tarim Oilfield, the supporting data cannot be released.

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