Study On The Quantitative Prediction Method Of Lost Circulation In Fractured Formation In Pre-exploration Area

Lost circulation was a common drilling complication during oil and gas exploration and production in fractured formations,and could occur at different stages of exploration.In the pre-exploration area,particularly in new deep oil and gas fields,the geology was complex,formation information was unclear,and drilling data was limited,which increased the risk of lost circulation.According to statistical data,the proportion of wells that experiencing severe lost circulation incidents in the Bozhong block of the Bohai Sea was as high as 47.62% of the total number of wells since 2018.One of the ongoing challenges in current research was how to effectively integrate multiple data sources,such as seismic data and well-log data,to improve the accuracy of obtaining characteristic parameters for fractured formations in exploration areas.In addition,there was an urgent need to establish a prediction method for lost circulation diagnosis suitable for different fracture characteristics and rock mechanical properties,so as to overcome the regional limitations of existing methods and improve prediction accuracy.By addressing these issues,a better understanding of the characteristics and lost circulation mechanism of fractured formations could be achieved,and drilling plans and lost circulation response strategies could be optimized to reduce the risk of drilling losses.A technique for predicting characteristic parameters of fractured formations in pre-exploration areas was developed based on the theory of multi-source information fusion,3D geological mechanics modeling method,and seismic attribute recognition method.Firstly,the accuracy of seismic layer velocity was improved using the XGBoost machine learning algorithm,coupled with multiple sources of data including core data,well logging data,seismic data,and drilling summary reports.This facilitated3 D modeling and acquisition of various mechanical parameters.The modified seismic layer velocity improved the prediction accuracy of mechanical parameters in the study area.Secondly,by coupling imaging logging data and original seismic data,the fracture index of the study area was calculated.This technique enabled the high-precision extraction of geological information from points to 1D well profiles and to 3D space.Existing methods for predicting pre-drilling lost formations typically relied on statistical analysis of post-drilling data,rather than fully utilizing seismic data.In this paper,the core concept of geological engineering integration was used,and the Analytic Hierarchy Process was used to couple parameters such as fracture index,minimum horizontal stress,and brittleness index to establish a lost circulation risk index.This index,combine with pre-drilling lost data,was used to establish a quantitative prediction method for lost formations based on geological engineering integration.The three-dimensional lost circulation risk index(LCRI)of the C block in the Bohai Oilfield was established in this paper,where a higher index value indicated a higher risk of formation loss.Based on this index,high,medium,and low-risk lost formation zones were defined for the C block.Application in the C block of the Bohai Oilfield indicated that this method could accurately identify areas with high risk of lost formations in the entire space.The lost circulation pressure was an important reference indicator for wellbore leak prevention and plugging.Currently,there was no mature theory for calculating and predicting lost circulation pressure.Existing models could not accurately predict the lost circulation pressure of complete formations,large fractured formations,and large fractured pore formations simultaneously.By combining field data and introducing the wellbore leak risk index,a classification criterion for lost circulation types and corresponding quantitative calculation models for lost circulation pressure were constructed.A new fine-grained calculation method for lost circulation pressure in fractured formations was established,which achieved automatic and accurate matching of lost circulation pressure calculation models with complex formations.The application in the C block of the Bohai Oilfield showed that this method could precisely characterize the lost circulation pressure in the study area.To investigate the lost circulation mechanism of fractured formations,a 3D finite element model for predicting lost circulation was established on the Abaqus platform.The model integrated multiple processes such as wellbore circulation,fracture propagation,and rock deformation,and provided parameters such as wellbore pressure,drilling fluid lost circulation rate,and fracture morphology with respect to time.The model considered complex mechanisms such as wellbore pressure,rock deformation,and fracture propagation,and comprehensively analyzed the effects of engineering and geological factors on parameters such as fracture aperture,fracture length,drilling fluid lost circulation rate,and bottomhole pressure.Corresponding engineering parameters and fracture leakage diagrams were also provided.The results showed that engineering factors such as drilling fluid density,viscosity,and pump discharge rate could cause severe fluctuations in bottomhole pressure in deep formations,leading to rapid increases in drilling fluid lost circulation rate and fracture propagation.However,these factors had little effect on fracture aperture.Geological factors did not directly affect bottomhole pressure but affected the change patterns of drilling fluid lost circualtion rate and fracture characteristics.In addition,when multiple induced fractures existed,induced fractures with larger initial apertures were more likely to develop into dominant fracture leakage channels.By using the methods provided by the model,drilling engineering design could be optimized to improve the anti-leakage and plugging capabilities of fractured formations.