Numerical simulation: Diverting study and analysis on nanoparticle-viscoelastic-surfactant acid in high-temperature carbonate reservoir

Abstract

Uniform acidizing has been an urgent challenge in strongly heterogeneous and high-temperature carbonate reservoirs. Nanoparticles have been shown to enhance the viscosity of viscoelastic-surfactant (VES) fracturing fluids, thereby improving the proppant carrying capacity of the fluid. However, there is almost no understanding of the nanoparticles on wormhole propagation and diversion in VES acid at low pH values and high salinity. Here, the rheological and adsorption model of nanoparticles in VES acid are established based on experimental results, and the multi-field and multi-scale coupling model of nanoparticles-VES acid is established by introducing the temperature field and two-scale continuum model. Based on the model, the propagation pattern and diverting performance of the wormholes were analyzed. The effects of permeability contrast, temperature, and adsorption on the propagation pattern and diverting performance are systematically investigated. The results show that the synergistic effect of nanoparticles and adsorption divided the invaded zone of spent acid into high-viscosity and low-viscosity zones, and the propagation capacity of the wormholes in the low-permeability reservoir was increased by 17%. The propagation and diverting process of wormholes can be divided into three stages. In stage II, when the additional resistance increased by spent acid is enough to resist the permeability contrast and the wormholes, more acid starts to flow into the low-permeability reservoir. The higher flow rate in the high-permeability reservoir makes the propagation path more complicated, and more branching wormholes are formed. At a reservoir temperature of 393 K, the nanoparticle-VES acid has the poor diverting ability for reservoirs with permeability contrast greater than ten. When the maximum adsorption capacity is 0.008 mol/kg, the diverting performance of nanoparticle-VES acid and VES acid were close. Reducing the adsorption is a very effective way to enhance the diverting performance. When the adsorption capacity is larger, the influence distance of the high viscosity zone formed by the dominant wormholes is limited, and the number of main wormholes in the reservoir becomes significantly large. This study clarifies the propagation law and diverting characteristics of wormholes using nanoparticle-VES acid, fills the technical gap of nanoparticles in acidizing, and provides guidance and a basis for the stimulation of uniform acidizing in high-temperature carbonate.