Study On Wellbore Instability Mechanism In Southwest Tarim And Technical Countermeasures

There are rich oil and gas resources and great exploration potential in southwest Tarim.In recent years,exploration efforts in southwest Tarim have been continuously improved,but due to the complex formation,complex well conditions occur frequently in drilling,which restricts the efficient development.Wellbore instability and complex well conditions are the difficulties in the exploration and development of southwest Tarim,and the problem of wellbore instability needs to be solved.The characteristics of complex strata drilled in southwest Tarim are analyzed.The mineral components,physical and chemical properties of the complex strata are tested.The results show that the strong formation hydration is an important cause of wellbore instability.There is some clay in Permian,Carboniferous,Neogene and Paleogene,with an average content of 4.5%-45.5%.There are a lot of illite-smectite mixed layers in some strata,up to 92%.The expansion rate of Bashibulak,Kalatar and Acha formation is about 20%,and the water recovery rate is lower than 13%,showing strong hydration dispersion and expansion ability.In addition,triaxial compression experiments of the field cores and outcrops before and after immersion are carried out,and the results show that the rock strength and cementation are low.The formation fragmentation is another important cause of wellbore instability.The compressive strength of limestone in Ordovician is only53 MPa,and the compressive strength and cohesion of some limestone,dolomite and sand-mudstone are low,which are easy to be broken.Drilling fluid immersion has great damage to rock strength,cohesion is only retained to 14.25%-79.71% of the original rock,and the rock cementation is weakened.At the same time,the formation contains micro-scale and nano-scale pores,and the formation fractures and microfractures development.Formation fragmentation,low rock strength,strong hydration and other factors are the main mechanisms leading to wellbore instability in southwest Tarim.Aiming at the mechanism of wellbore instability,the drilling fluid technical countermeasures are further put forward to ensure wellbore stability.The results show that the wellbore stability measures of drilling fluid are as follows:1.Synergistic synergism of polyalcohol and KCl is introduced to enhance the inhibitory properties of drilling fluid.2.Introduce polyamine to further enhance inhibition.3.Nano paraffin emulsion is introduced to strengthen broad-spectrum nano plugging.Polyalcohol,polyamine and nano paraffin emulsion are introduced into KCl-polysulfonic drilling fluid to form two optimized drilling fluid formulations.After optimization,the plugging and inhibition performance of drilling fluid is improved,filtration loss is reduced,mud cake is thin and compact,and the compressive strength and cohesion of rock are effectively improved.The cohesion is retained to 91.86% of the original rock,ensuring the stability of the formation.These strategies are successfully applied to two new wells in the southwest Tarim,ensuring the performance of drilling fluid and improving wellbore stability,providing support for efficient drilling.In addition,in order to solve the problem of difficulty in taking cores in southwest Tarim,a new method of evaluating wellbore stability by digital core is explored.The variation trend of rock mechanics parameters obtained by digital core is basically consistent with the triaxial compression test results,which proves that it is feasible to evaluate wellbore stability and drilling fluid properties by digital core technology.