Study On The Wellbore Instability Mechanism And The Anti-collapse Drilling Fluid Technology For Deepwater Shallow Formation

With the continuous development of offshore oil drilling technology,the deep and ultra-deep water zones have already been the new research focus for the marine oil and gas exploration and development.Compared with onshore or the shallow water drilling,the environment of deepwater drilling is worse,bringing more technical problems.The wellbore instability in deepwater shallow formation is one of the main tasks that need to be solved.Considering the characteristics of unconsolidated sediments in deepwater shallow formation,the experimental method for evaluating the wellbore stabilization property of drilling fluid for deepwater shallow formation was established.The optimized sample preparation conditions to improve the SSI were derived.The sand(60~80 mesh)and calcium based bentonite with the ratio of 7∶3 were mixed while adding the seawater.The weight ratio of the mixed sand/bentonite and the sea water was 85∶15.After that the mixtures will be pressed under the pressure of 15 MPa for 2 h.Drilling fluid inhibition of hydration dispersing was evaluated by this improved SSI experiment method.In addition,a laboratory evaluation experiment method for the chemical cementation stability of unconsolidated formation was established.The chemical cementation and consolidation property of drilling fluid for deepwater shallow formation was evaluated by the needle penetration,permeability and compressive strength tests of the sand mudstone samples(the ratio of sand(40~60 mesh)and calcium based bentonite was 7∶3)immersed in drilling fluids.Based on the analysis of deepwater shallow formation mineral compositions and structures,physic-chemical characteristics,rock-soil properties,the influencing factors of wellbore instability,and the chemical cementation and consolidation evaluation method,the wellbore instability mechanisms in deepwater shallow formation were revealed.The results showed that,due to the replacement by seawater on the upper of shallow formation,the deepwater shallow formation was under the low compaction and poor consolidation conditions.The minerals in shallow formation were mainly quartz and clay with the low recovery of cuttings(1.33%),demonstrating the strong hydration dispersion.The drilling fluid filtrate easily invaded into the formation due to the development of micro pores or cracks,leading to the clay hydration.In addition,the safe density window of drilling fluid was narrow,which can add more chances for the wellbore collapse and lost circulation issues.For the wellbore instability characteristics in deepwater shallow formation,the technical countermeasures of multiple collaborative anti-collapse drilling fluids were proposed: Strengthening physical plugging and chemical cementation and consolidation of the wellbore,inhibiting clay hydration dispersion,reasonably controlling the drilling fluid density and considering the drilling fluid rheology at low temperature and hydrate inhibition.Through the improved SSI experiment method,the inhibitive anti-collapse agents were optimized.The results showed that the mixed using of sodium silicate,polyamine SDJA and inhibitor KCl had good effect on stabilizing the sample.The SSI of the sample after immersed was 95.2.Based on the chemical cementation and consolidation evaluation method,the anti-collapse agent with chemical consolidation was optimized.The results showed that solid sodium silicate and aluminium-based anti-collapse agent had good chemical cementation and sealing effect on the unconsolidated core samples.In addition,for the deepwater shallow flow,the resin glue formula was optimized.The compressive strength of the consolidated sample could reach 10.84 MPa after resin glue treatment for 24 h.Using the optimized chemical cementation agents and inhibitive anti-collapse agents,and considering the low temperature rheology and the hydrate inhibition of deepwater drilling fluid,two sets of anti-collapse drilling fluid formulas were optimized.Evaluation experiments showed that,two formulas of the anti-collapse drilling fluid for deepwater shallow formation had an good effect on wellbore stability.In addition,they showed good low temperature rheology and hydrates inhibition.