Low Damage Surfactant Compoud For Shale Reservoirs And Its Working Mechanism Research

The shale gas reservoir with horizontal well fracturing volume is needed to get the economic capacity, but in the process of shale gas fracturing simulation, low flowback rate of fracturing fluid, liquid phase retantion and other problems which is generally exist in fracturing fulid, It causes damage to shale gas reservoirs. The capillary resistance which is the main factor of liquid phase trapping damage. Based on the surfactant combination mechanism, it is necessary to research one type of low-damage surfactant, which can decrease liquid surface tension and capillary resistance, thus improve liquid flowback rate.Taking Longmaxi shale formation at Weiyuan field as the studying object, the fracturing fluid flowback rate is low. X-ray diffraction, gas permeability method, SEM and other experiment methods were used to analyze mineral constituent, basic properties and microstructure in this field. Results indicate that at Weiyuan field, Longmaxi formation shale gas reservoir, the average permeability is 0.0032×10-3?m2, the average porosity is 2.91%(belong to low porosity and ultra-low permeability reservoir); the clay mineral content is 37.3%, and mainly are illite and illte/ smectite mixed layer, which causes great water sensitivity damage; shale reservoir structure is tight, micro pores and micro fractures develop, and pore diameter is small; outside working fluid invasion is prone to be affected by pillar resistance, causing liquid phase trap and liquid retantion, and decreasing reservoir permeability and gas well productivity.According to shale reservoir characteristics and potential damage factors, and based on surfactant combination mechanism, and taking gemini surfactant BTN and nonionic surfactant OP-10 as the major, the surface tension is the minimum at the mass concentration ratio of 3:1, and then the low-damage surfactant formula was determined. Infrared spectrometry, low temperature nitrogen adsorption experiment, surface tension and contact angle measurement were used to discuss the influence on shale surface chemical structure, physical structure and surface chemical properties. Research indicates that low-damage surfactant adds hydrophobic group onto shale surface, decreases damage of liquid on shale particle pore structures, and decreases influence of time on surface tension and wettability effectively, which protects reservoir surface characteristics effectively.Experimental investigation shows that the low-damage surfactant fracturing fluid system has better compatibility, friction reducing ability, swelling inhibition, and shear resistance. Spontaneous capillary imbibition experiment and fracturing fluid flowback rate recovery experiments were performed to analyze the contrast experiment before and after low-damage surfactant added into fracturing fluid system. Results indicate that low-damage surfactant decreases system surface tension effectively, decreases shale contact angle variation, and decreases capillary resistance during fracturing fluid flowback, which helps fracturing fluid flowback and protects shale reservoirs.