Study On The Characteristics Of Imbibition Induced Cracks And Its Influence In Shale Gas Reservoir

Shale gas reservoirs have low porosity and permeability.Horizontal well associated with a large scale of hydraulic fracturing play an important role in developing shale gas.More than ten thousand cubic meter liquid is injected into the shale formation during a large scale of hydraulic fracturing.The flowback rate usually is very low.For example,the flowback rate is 3% in the first period of Fuling shale gas field.However,the gas production rate is high in some shale gas formations from field data.Its mechanism and controlling factors are unclear.Some experts think the new crack produced by the interaction between shale and fracturing fluid is the reason for this phenomenon.On the whole,the mechanism,controlling factors and appraising method need further research.The typical marine shale was selected as the research samples.The characteristics of shale reservoirs were compared.The characteristic of imbibition induced cracks was investigated.The principle of imbibition influenced by induced cracks was investigated.The fluid saturation change with imbibition time was studied at micro scale.The permeability variation with saturation was researched by using the pulse-decay permeability method,especially for the induced cracks condition.At last,the models of saturation and permeability change influenced by induced cracks were established.The results are listed as follows:(1)the interaction between shale and fracturing fluid can cause induced cracks in marine shale reservoir of southern China.However,there is difference between different areas and formations.Induced cracks capacity has positive relationship with organic pores,middle size pores and initial permeability;(2)the induced cracks enhance the liquid imbibition capacity.At the same time,it improves the imbibition rate and diffusion rate.This is the one of mechanisms for low flowback rate.A new method was established based on repeat imbibition.This method can characterize induced cracks imbibition.It is beneficial to distinguish the induced cracks capacity among different shale reservoirs;(3)the permeability increase with increasing imbibition water saturation in some shale rocks with abound of induced cracks.The induced cracks have more contribution to permeability than the permeability decrease caused by aqueous phase trapping.This is the reason for less aqueous phase damage in some shale reservoir;(4)micro scale experiment indicates that liquid move from cracks and large pores to deep matrix by capillary force during diffusion period.The water saturation decreasing in cracks and large pores is beneficial to gas production from micro pores.At the same time,it is good for deep matrix gas to flow into the well bottom by micro cracks and large pores;(5)the reservoir water saturation characteristic model under the influence of induced cracks indicates that the imbibition liquid saturation increase with increasing induced cracks density.The gas permeability increase with increasing the ratio between induced oganic cracks and total induced cracks in our established model.Our models are conducive to the reservoir saturation and permeability prediction after hydraulic fracturing.