Study On The Damage Mechanism Of The K₁bs Gas Layer In The Keshen A Well Block Of The Kelasu Gas Reservoir

The resource of tight sandstone gas in China is abundant,which is about 26.81×1012 m3,accounting for 42.5%of the total gas geological resources.Tight gas production proportion of total gas production increased year by year,the proportion reached 39%in 2014.The tight gas has become China's natural gas industry development important resources.Keshen A wellblock K1bs gas reservoir of Kelasu gas filed is a typical tight sandstone gas reservoir and the buried depth is over 8000 m,the formation temperature is about 195 ?,the formation pressure is about 130 MPa,which belongs to the ultra depth high temperature and high pressure gas reservoir.Keshen A-1 gas well is the deepest well and first one to receive commercial hydrocarbon flow in our country land,which received a output of 30×104m3 nature gas per day at the target stratum(about 8038 m)during formation testing.Based on the deepen understanding of geologic feature in Keshen A wellblock and the deficiencies of evaluation methods to tight sandstone reservoir form industrial standard and cited references,the evaluation methods are improved and using them to overall evaluation of the formation damage mechanism.There are the main research achievement of the paper:(1)Through comprehensive analysis of core,the geological characteristics of reservoir are clear and definite.The main rock type is lithic arkose sandstone.The absolute content of clay minerals is about 4.44%,and illite occupied the highest proportion.The reservoir has characters of poor physical property.The matrix permeability ranges from 0.001 mD to 0.02 mD and porosity range from 2%to 6%.The pore types include dissolved pore,primary intergranular pore.The formation develops high angle fracture,which are filled fully or partly by Calcite.dolomite and anhydrite.Some fracture was filled with nothing.In a word,reservoir characteristics include high formation water salinity,low to ultralow pore permeability,rich in clay mineral,high capillary pressure,low original water saturation.high irreducible water saturation.exist nature fracture and so on.which can induce formation damage in the process of production and development.(2)In view of the tight sandstone gas reservoir,evaluation methods of sensitivity test were established and perfected.The result showed that reservoir has a character of medium to strong water sensitivity,low to medium salt sensitivity and alkali sensitivity,and acid sensitivity is not exit.Study on stress sensitivity to the reservoir showed that the matrix permeability stress sensitivity is extremely strong and pore and fracture permeability stress sensitivity is strong using industry standard method and the stress sensitive coefficient method synthetically.(3)Evaluation methods of aqueous phase trapping damage test were established and perfected.The result showed that matrix formation exit a strong aqueous phase trapping damage,but the existence of nature fracture system improved filtration capacity,and naturally fractured formation have a relatively low aqueous phase trapping damage compare to matrix formation.The influences of aqueous phase trapping damage,in order by degree of strength,are matrix permeability,water saturation,nature fracture,drawdown pressure,invading fluid salinity and so on.Rmoving of aqueous phase trapping experiments were launched,the result is 1%SCDX-1 have a good effect to improve aqueous phase trapping damage to matrix formation.Using 1%SCDX-1 can decrease irreducible water saturation about 20 percentage.(4)The result of evaluation of the damage mechanism and degree of drilling fluid showed that drilling fluid damage to matrix formation is not large.The main damage is caused by drilling fluid filtrate invasion and invasion depth is less than 3 cm.The damage degree of porosity and fracture cores is closely related to the fracture opening degree.When the opening degree is less than 15 ?m,the damage to formation degree is weak.Three types of drilling fluid system used in the field are evaluated.The result showed that the median particle size of drilling fluid is well matched with pore radius,full filled fracture aperture,half filled fracture aperture,while is ill matched with unfilled fracture aperture.The sealing performance of drilling fluid to full filled fracture and half filled fracture is better than unfilled fracture.