Sandstone acidization using in situ generated hydrofluoric acid (HF) in low-permeability sandstone reservoirs was carried out by exothermic reaction of an acidic precursor (ammonium fluoride NH4F) with an oxidant (sodium bromate NaBrO3).Novel authigenic acid mixtures were prepared to react with pure quartz samples,and reaction products were analyzed by inductively coupled plasma techniques to identify Si ions.Core intrusion experiments were performed using sandstone cores.Scratch tests and NMR scans were performed on the core samples before and after treatment.The results showed that the new autogenous acid mixture could dissolve the quartz sample and reduce its mass by 80 mg.The mass concentration of dissolved Si ions exceeded 90 mg·L-1,demonstrating the ability of the chemical mixture to generate HF.The initial core permeability was measured to be 33 mD at a flow rate of 2 cm3·min-1.After the acid pre-rinse phase,the core permeability decreased to 31 mD.Immediately after the first treatment cycle,the core permeability increased and reached 41 mD,which showed an increase of nearly 40% in the permeability of the sandstone core.ICP analysis of the solution showed a total of 10.5 mg of chelated Si ions.In addition to the high temperatures generated near the wellhead,the nitrogen generated due to the exothermic reaction resulted in an increase in pressure to approximately 4.14 MPa.Scratch tests showed an increase in the uniaxial compressive strength of the samples of approximately 12.4 MPa.The dynamic Young's modulus and Poisson's ratio increased by approximately 9.83 MPa and 0.02,respectively.Through the application of this novel autogenous acid,it was possible to deep acidize sandstone reservoirs,minimize wellbore corrosion,and avoid handling hazardous chemicals such as HF.
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