Study On Lost Circulation Mechanism And Real-time Diagnosis Method In Halfaya Oilfield

Halfaya oilfield is an important overseas cooperative development block of China.Due to the complex and changeable formation lithology longitudinally in this block,lost circulation occurred frequently while drilling,which poses a threat to safe drilling.The traditional research mainly focus on leakage pressure model,plugging formula evaluation and plugging technology countermeasures,etc.However,when drilling in a block with unclear thief zone,it is often faced with challenges.It is difficult to timely predict well leakage and formulate effective engineering countermeasures,which can induce malignant mud loss underground and further increase non-production time.In view of the above challenges,the characteristics and mechanism of lost circulation in the complex strata of Halfaya oilfield are systematically analyzed.Based on the wellbore big data,the diagnosis model of lost circulation while drilling is established to diagnose the risk of lost circulation and guide safe drilling.The specific research work of this paper is as follows:(1)Study on the status and characteristics of lost circulation in complex formationsThe field data of lost circulation are analyzed and the distribution rules and characteristics of complex formation leakage are obtained.The results showed that 137of the 213 wells encountered lost circulation,and the mud loss amount of a single well ranged from 5 m~3 to 4595.6 m~3,among which 29 wells encountered severe leakage with mud loss amount above 381.6m~3.Mishrif and Nahr Umr are formations of most severe leakage,with lost circulation in both drilling and cementing process.According to different characteristics of leakage,leakage can be divided into overpressure leakage,fracturing leakage and induced leakage.(2)Study on leakage mechanism and pressure model of complex formationsCombined with mud logging and logging curves,the corresponding geological characteristics and mechanical mechanism of different lost circulation types are analyzed,and the corresponding leakage pressure model is given,and the fracture leakage is calculated and analyzed.The results show that the formation with fractures and dissolution pores is prone to overpressure leakage and induced leakage during drilling,and the formation with closed fractures or weak planes is prone to fracturing leakage during cementing.The calculation results of fracture leakage show that the rate of loss has a power exponential relationship with bottom hole pressure difference,fracture width,drilling fluid consistency coefficient and flow index,and increases with bottom hole pressure difference and fracture width,but decreases with consistency coefficient and flow index.(3)Diagnosis method of lost circulation while drilling based on random forestThe mud logging data and lost circulation information of typical drilled wells are collected,the data of different drilling depths are marked to distinguish the well leakage points from non-well leakage points,and the data are preprocessed to form a training sample matrix with output response.The random forest algorithm is employed to study the training data,and various feature parameters are used to classify the well leakage points and non-well leakage points,and the corresponding relationship between feature parameters and lost circulation is established to obtain the diagnosis model while drilling.