| Carbonate reservoir,as an important reservoir for oil and gas reservoir development,has always played an important role in the field of oil and gas exploration and development.In China,carbonate reservoir is widely distributed,and the exploration and development of carbonate reservoir is quite active.In conventional drilling engineering,borehole collapse and instability has always been one of the main problems restricting safe and rapid drilling.In the carbonate strata,the occurrence of borehole collapse is rare in general.However multiple Wells in the PZ gas field of western Sichuan,targeting at the carbonate strata of maantang-leikoupo formation,had collapsed and dropped pieces to varying degrees,and leading to stuck drilling and restricting the efficient development of oil and gas resources.Based on the study of macroscopic and microscopic characteristics of carbonate rocks in this area,combining with indoor acoustics and mechanics experiment,the collapse mechanism of carbonate rock was analyzed,and introduced formation integrity coefficient of formation strength to establishing the calculation model of fractured carbonate rock collapse of density,pointed out the mechanism and main factors of borehole collapse in the carbonate strata.The main research contents and results of this paper are as follows:(1)The structural characteristics of carbonate rocks in the PZ gas field were studied.The field data was collected and the geological location information and geological characteristics of the gas field was analyzed;through field exploration,the outcrop of the stratum was observed,and the macroscopic structure of the stratum was understood by combining the core sample taken underground.The microstructure and composition of carbonate rocks in this area were analyzed by XRD and SEM.From the macro and micro to the carbonate rocks in the region have a general understanding.(2)The physical and chemical properties of carbonate rocks were analyzed experimentally.According to the physical and chemical properties of carbonate rocks,the expansion test,pore permeability test and triaxial mechanical test were carried out.The triaxial mechanical test considered the influence of fracture Angle and drilling fluid immersion.It is mainly used to evaluate the influence of hydration property,seepage field and fracture trend on the strength of carbonate rocks.The results show that due to the low clay content,the influence of hydration of carbonate rocks is not obvious,while the influence of fracture trend on rock strength is obvious.(3)The formation integrity of carbonate rocks is studied.Integrity coefficient was introduced to characterize the degree of fractured formation.The feasibility of integrity coefficient was analyzed through laboratory experiments.Then,based on field logging data,the formation integrity of 5 Wells that had been drilled was analyzed.Finally,the empirical formula of formation integrity coefficient and formation strength(cohesion C)of carbonate strata in this area was fitted by laboratory experiments.(4)Collapse density of carbonate rocks is predicted.Based on the above research,this paper analyzes the effective stress field of surrounding rock,established the calculation model of collapse density with considering the integrity of the formation,and analyzed the collapse density with the date of PZ113.The results show that seepage and fragmentation degree of the strata have a great influence on the collapsed density of carbonate,without considering seepage and broken formation,collapsed density is around 1.25 g/cm3;the collapsed density is about 1.4 g/cm3 when the seepage is taken into account but the degree of formation fragmentation is not.The collapsed density is between 1.3-1.5 g/cm3 without considering the seepage but the degree of formation fragmentation is considered;considering both the seepage and the degree of formation fragmentation,the collapsed density is between 1.6-1.9g/cm3.Both the seepage and fragmentation of the strata will cause the increase of the collapsed density,and when the fragmentation degree of strata is obvious,it will increase the influence of the seepage and lead the increase of the collapsed density. |
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