Evaluation experiment of low-damage complex acid system

The sandstone oil reservoirs in Huabei Oilfield are characterized by low porosity, low permeability, high content of sensitive minerals and high reservoir temperature. And the conventional acid system is suffered from the problems of fast reaction with reservoirs, short effective penetration distance and obvious secondary precipitation. To solve these problems, the molecular structure and performance of a new complex acid system were analyzed and evaluated by means of experimental researches, including NMR carbon spectrum and NMR hydrogen spectrum of complex acid, retard performance, chelation capacity to metallic ion, corrosion inhibition performance and acidizing effect characterization from two aspects of core flow test and wettability measurement. It is indicated that acid-rock reaction speed of the new complex acid system is lower than 40% of that of conventional earth acid system. The chelation value of metallic ion Ca²⁺, Fe³⁺ and Mg²⁺ is 272 mg/g, 455 mg/g and 167 mg/g, respectively. The steel corrosion rate is 2.873 g/(m²·h), which is much lower than the primary industrial standard (10-20 g/(m²·h)). In the acid flow test on the formation core, the permeability is increased by over 200%. The core after the acidizing is water wettability prone. This new complex acid system with the advantages of retard, scale inhibition and deep penetration is of great significance to the low-permeability reservoir acidizing stimulation and hydrocarbon recovery improvement of sandstone oil accumulations in Huabei Oilfield.