Study and Application of a New Self-diverting Acid for Use in Sandstone Fracturing

The main reservoirs in the Bohai oilfield are characteristic of big permeability differences between different production layers, and high heterogeneity. This has resulted in the acid solution flowing mainly into the high permeability layers, further aggravating the permeability differences, making it difficult to effectively improve the conductivity of the layers with medium and low permeability, and resulting in poor acidizing job quality. A new self-diverting acid was developed to deal with these problems. The self-diverting acid was viscosified with ZX-1, an amphoteric surfactant. Laboratory experiments have been conducted on the rheology, compatibility, gel-breaking and the diverting acid stimulation performance of the self-diverting acid. It was shown that the simulated fresh acid had viscosity of 6 mPa·s, which was easy to inject into reservoir formations. The viscosified simulated self-diverting acid had viscosity of 60 mPa·s, highly resistant to shearing. And the simulated residue acid had viscosity of only 2 mPa·s, easy to be flowed back. The self-diverting acid was well compatible with corrosion inhibitors, iron ion stabilizers, swelling inhibitors and cleanup additives, and no precipitation and residue were observed in the experiments. Gel breaking of the self-diverting acid was easy to realize with isopropyl alcohol, demulsifiers, ethylene glycol mono-butyl ether and acid solutions. Viscosity of the self-diverting acid after gel breaking was less than 10 mPa·s. Part of the gel within the viscosified acid may not be broken at the time of application, but with time, it will automatically become broken, rendering no permanent damage to the potential of reservoir. The self-diverting acid had good diverting property, with the permeability difference ratio increased from the initial value of 2.00 to the final value of 10.70. After acid stimulation, the permeability of the low-permeability layers was enhanced remarkably, and the permeability differences between the high-and low-permeability layers were reduced. When the permeability difference reached 10.7, diverting acid stimulation can still be achieved. Based on the results of the field application, the self-diverting acid will find wide application in the future.