Optimization of drilling fluid for Chang7 shale reservoir in Wuqi Oilfield
Large scale borehole wall falling is easy to occur during the drilling of Chang7 shale oil reservoir in Wuqi Oilfield,which is the main reason affecting the drilling efficiency improvement and comprehensive logging effect of Chang7 Shale oil horizontal wells in Wuqi Block.To solve the problem of shale instability,experimental equipment such as core observation,scanning electron microscopy,and X-ray diffraction analysis were used to analyze the microscopic and physicochemical characteristics of the Chang7 shale.The research shows that the scanning electron microscope observation of Chang7 shaleshows that micro cracks and a high proportion of clay mineral,the relative proportion of Kaolinite is 37.56％,which is easy to be muddy and broken when encountering water;The illite and illite/smectite formation are close to 50％,the rock stratum expansion pressure is high,and the borehole wall is unstable,so pay attention to reservoir protection during drilling;The technical countermeasures of strong plugging and strong inhibition polymer drilling fluid for Chang7 shale in Wuqi area are proposed.The drilling fluid formula is 4％bentonite+appropriate caustic soda+appropriate soda+0.3％(HV-CMC)+2％(NH4-HPAN)+2％(WNP-2)+5％PTGE+1％ultra-fine calcium carbonate+3％nano calcium carbonate+buckwheat bran+Baryte;The evaluation shows that after aging,the viscosity and shear force of the system are low,and the plastic viscosity is 16 mPa·s with a dynamic plastic ratio of 0.44;The filtration loss and lubricity meet the on-site requirements,with an API water loss of 3.8 mL and a friction coefficient of 0.614;The recovery rate of rock cuttings is high,reaching 98.3％,and the permeability recovery rate is high.The performance is stable before and after liquid contamination.This research result has certain reference value for the implementation of horizontal wells in the Wuqi Block.
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