The optimization of methods for the quantitative evaluation of risks in drilling engineering is an effective means to ensure safety in situations where high temperature and high pressure blocks are considered. In such a context, this study analyzes the complexity of the drilled wells in such blocks. It is shown that phenomena such as well kick, loss, circulation, and sticking, are related to the imbalance of wellbore pressure. A method for risk quantitative evaluation is proposed accordingly. The method is used to evaluate the risk for 9 drilled wells. By comparing the predictions of the method with actual historical data related to these wells, it is found that the coincidence rate is about 95%.

Offshore has become the main battlefield of oil and gas exploration. Most of the blocks are developed in high-temperature and overpressure formations, with huge natural gas reserves and overall exploration and development value. However, this region’s drilling and completion construction is faced with problems such as high temperature and high pressure, narrow safety density windows, and strong uncertainty of formation pressure information. In construction, situations such as well kick, well circulation, borehole collapse, and pipe sticking frequently occur, seriously restricting the exploration and development process [

The above literature research found that most existing methods apply the classical risk analysis method in the field of drilling projects, and results are often qualitative and semi-quantitative. Feedback from the field shows that this often cannot meet the safety requirements of offshore high-risk drilling construction. To verify the advantages and disadvantages of the above evaluation methods, this study takes a block in offshore as an example to analyze the applicability of quantitative risk assessment.

The buried depth of the reservoir in this block is 3800∼4100 m. The geothermal gradient of the basin increases gradually from north to south and from west to east. The formation pressure has the characteristics of more pressure steps and faster pressure uplift. The measured equivalent density of deep formation has reached about 2.30 g/cm^{3} [

The risk is mostly concentrated in 3900∼4200 m. The drilling formations are mostly deep and complex, there are mainly sandy mudstone, especially mudstone formations with large sections of shale in the formation. There are sticking accidents in higher formations, which needs reverse reaming and reaming operation, and lost circulation accidents in its Lower Formation. The reason is that a narrow pressure window requires high-density drilling fluids to balance formation pressure. However, The use of high-density drilling fluid will cause the pressure of the liquid column in the wellbore to be greater than the fracture pressure, resulting in well loss in fracturing formation [

Formation pore pressure, formation fracture pressure and collapse pressure profile are important basis for wellbore structure design. Due to complex formation conditions, incomplete interpretation data, and difficulty in selecting model parameters, the single-value formation pressure predicted by the conventional method is difficult to reflect real formation pressure, so the prediction of formation pressure has uncertainty. Guan et al. [

Quantitative risk analysis method based on pressure balance can be summarized as the following steps:

Calculation of formation pore pressure with credibility: According to the velocity of the seismic layer, the single-value pressure is calculated combined with Fillippone;

By combining Fillippone and Eaton methods, Eaton index is back-analysised to determine the probability distribution of whole Eaton index, and Eaton index of the obtained probability distribution is substituted into Eaton model. The distribution of pore pressure with confidence will be obtained, with different confidence profiles shown in

Through the statistical analysis of logging data, core laboratory experimental data and LOT data, the structural stress coefficients of each well are obtained, and then these coefficients are counted to obtain range and distribution of their values;

Calculated rock mechanical parameters and tectonic stress coefficient are substituted into the calculation formula of formation collapse and fracture pressure [

Finally, according to practical drilling fluid density and formation pressure measured data of adjacent wells, four formation pressure profiles with credibility are corrected by translation or interpolation;

Based on constraint conditions of safe drilling fluid density design, four types of risks including well kick, lost circulation, wellbore collapse, and differential pressure pipe-stuck, are mainly considered. Combining formation pressure profile with the above three constructed credibility, four drilling engineering risk evaluation models are defined, as shown in

Risk type | Risk model |
---|---|

Well kick | |

Lost circulation | |

Borehole collapse | |

Pipe sticking |

In the formula, ^{3}.

Combined with the above risk assessment method, the authors predicted the risk of 9 drilled wells in this block, and compared them with the actual situation. Taking Well A as an example for specific analysis, the drilling depth is 4368 m, and the risk of the well mainly occurs in Formation 6 with depth range of 4061∼4330 m. There are four times of lost circulation, two times of well kick, and simultaneous occurrence of lost circulation and well kick, which is due to the inaccurate grasp of formation pressure [

In order to evaluate quantitative evaluation method from the whole block, the authors used this method to evaluate the risk of 8 remaining wells, and compared them with the actual situations.

In fact, there is no risk in Well D, but

In order to systematically evaluate this method, the authors performed a statistical analysis of actual risk of the block and used a quantitative evaluation method. The near risk occurrence points in the statistics are recorded as one, such as accidents at 4100 and 4080 m in Well G, and evaluation method can predict two points in the depth section, which is consistent with actual situation. The maximum probability profile greater than 0.5 is defined as high probability, and lower than 0.5 is defined as a low probability.

It can be concluded from

Actual (count) | High probability (count) | Low probability (count) | Coincidence rate | |
---|---|---|---|---|

Well kick | 4 | 4 | 2 | 100% |

Well collapse | 0 | 0 | 1 | 100% |

Lost circulation | 5 | 6 | 0 | 80% |

Sticking drill | 3 | 1 | 4 | 33% |

Secondary statistics of sticking | 1 | 1 | 4 | 100% |

Average coincidence rate | 95% |

The quantitative evaluation method of underground engineering risk obtains an underground complex risk probability profile with geological engineering information, and quantified probability at the corresponding depth. Compared with the complex downhole situation in actual drilling progress, it has a good consistency.

Through pre-drilling risk assessment, the wellbore structure and mud design of any depth can be optimized, which brings good construction significance to the pre-drilling risk assessment for developing wells.

The deficiency of this method is that the source data requires high accuracy and richness. The model parameters should be as accurate as possible to establish more accurate pressure profiles, and to obtain accurate risk assessment results.

The authors received no specific funding for this study.

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