Experimental investigation on the fracture propagation of three-stage acid fracturing of tight sandstone gas reservoir

The three-stage acid fracturing technology is an effective technology for tight sandstone reservoirs with undeveloped natural fractures. Its filed test has achieved good results in the tight sandstone field test in the Daning block of China. The core of this technology is to complete the pump injection procedure of "conventional fracturing, acid fracturing and conventional fracturing" in sequence. However, fracture propagation behaviors vary greatly under different acidizing fracturing conditions. In this paper, the influence of different factors on the uneven degree of hydraulic fracture etching and fracture propagation was studied by triaxial hydraulic fracturing physical simulation experiment. The effects of different acid concentration, acid dosage and acid reaction time on fracture propagation behaviors were analyzed. The results show that the fractures formed by the fracturing with displacement of 160 mL/min in the laboratory did not penetrate the overburden and underburden, and longer fractures and small branch fractures were formed in the reservoir. Hydraulic fractures showed a trend of continuous expansion in the sandstone reservoir, indicating that the fracturing effect was favorable. The experimental displacement is converted to the field displacement of 8 m(3)/min. The influence degree of acid concentration, acid dosage, acid reaction time on fracture propagation of three-stage acidizing fracture decreases in turn. Acid concentration and acid dosage are positively correlated with the area of fracture propagation. The influence of acid injection reaction time on fracture propagation is the lowest, and the influence of acid injection reaction time on fracture propagation area is significant only in the case of high acid dosage or high acid concentration. The influence of acid immersion time on fracture permeability is far less than that of acid concentration on fracture permeability. However, the greater the acid concentration, the greater the impact of acid immersion time on fracture permeability. The experimental results can provide empirical reference for the onsite construction of the three-stage acid fracturing in tight sandstone reservoirs.