Study On The Non-flowback Fracturing Fluid And Its Imbibition Mechanism In Tight Oil Reservoirs

The tight reservoir is considered to be an important successor for its great potential in exploration and exploitation.It is mainly explored by large-scale fracturing in horizon wells.To eliminate the formation damage caused by the residues of the injected fracturing fluid during the stimulation,the fracturing fluid is flown back as much as possible after fracturing immediately.Furthermore,due to the tight matrix and the presence of natural/artificial cracks in the tight reservoirs,the injected water could easily channel along the cracks.As a result,most of the tight reservoirs are developed under natural depletion.However,compared to the tight reservoirs in North America are mainly marine sediments with high-pressure factors,the tight reservoirs in China are mainly in continental sediment with much lower formation pressure.The energy deficiency after the flowback of the huge-volume fracturing fluid enhances the oil production decrease significantly in these low-pressure tight reservoirs during natural depletion.How to supply the formation energy and improve oil recovery factor is one of the critical challenges in the exploration of tight oil reservoirs in China.Imbibition is an important way of oil extraction for the thin pore/throat and numerous cracks containing tight reservoirs.At the same time,a large amount of the fracturing fluids used in large-scale fracturing is required.Based on the above considerations,taking advantage of the injected fracturing fluid to displace oil instead of flowback is proposed in this thesis.Firstly,a low-damage efficient-imbibition nanoparticle enhanced supramolecular fracturing fluid（NESF）was produced via chemical synthesis,chemical modification and formula optimization.The properties of NESF were then evaluated systematically.The mechanism of low damage and effective imbibition were analyzed from the aspects of gel breaking,enhancing hydrophilicity and reducing the oil-water interfacial tension.Secondly,the micro characteristics of NESF imbibition under the reservoir condition after fracturing was studied through the combination of nuclear magnetic resonance（NMR）and computer tomography scanning technology（CT）.Finally,the feasibility of fracturing without flowback was analyzed and the optimization of the shut-in time was worked out.The test results of Fourier transform infrared（FT-IR）,Proton-1 nuclear magnetic resonance（¹H NMR）,Carbon-13 nuclear magnetic resonance(¹³C NMR),high-resolution mass spectra（MS）,and thermogravimetric analysis（TGA）showed that the designed cationic Gemini surfactant HGS containing the groups of amide on the tails and hydroxyl on the C3 spacer,as well as the hydrophobically modified hydroxypropyl guar HMHPG were successfully produced.The optimized formula of NESF was 1.0 wt%KCl as the clay stabilizer,1.0 wt%HGS,0.3 wt%Na Sal as the provider of counterions for VES,1.4 wt%HMHPG and 0.05 wt%hydrophobically fumed nanosilica（HNS）.The viscosity of NESF under 80℃and 170 s^-1 was as high as 143.2m Pa·s;The elastic modulus was found to be higher than the viscous modulus during the whole test.The total setting rate of the proppant was 0.53 g·min^-1 during the dynamic proppant transportation measurement.NESF could be broken in the presence of the hydrocarbon and water in the formation and the viscosity of the broken NESF could be lowered to 1.8 m Pa·s at60℃.The hydrophilic property of the core could be enhanced by the detaching of the oil layers in the presence of HNS and the formation of double adsorption layers of HGS molecules on the sandstone surface.The HGS molecules could also lower the O/W interfacial tension through the formation of an adsorption layer at the O/W interface.Based on the above properties,the NESF met the requirements of low formation damage and high imbibition effects.The loss rate of matrix permeability caused by the NESF filtrate for the water and oil phase was found to be9.4%.The retainment of conductivity of the simulated crack was kept as high as 90.4%after the contamination of broken NESF.The imbibition efficiency of NESF（19.43%）was much higher than that of formation water（11.48%）and the fracturing fluid of crosslinked guar（8.07%）.The results of the imbibition under simulated in-site conditions（IUSIC）showed that the imbibition rate of NESF was lower than formation water at the initial and higher than that at the later stage.In the end,an imbibition recovery factor of 20.7%was obtained with NESF,which was 4.0%higher than that of formation water,especially in the micropores（the imbibition recovery factor with NESF in micro-pores was 5.5%higher than that of formation water）.IUSIC was dominated by the macropores at the initial,and then shifted to mesopores and micro-pores dominated imbibition.A sufficient imbibition time was required for the effective extraction of oil from the micropores.During the imbibition process,the flowing oil changed from continuous phases into dispersed droplets during IUSIC and the sizes of the dispersed oil droplet were in proportion with the O/W interfacial tension.Based on the geometric model of the imbibition in the matrix of HL Section in Ordos Basin,the conversion relationship between the dimensionless time and the practical time was built through the Back-Propagation neural network learning.The length of shut-in time was optimized by the trained BP network using the parameters of permeability,porosity,water contact angle,matrix width and the pressure after fracturing.