An Experimental Study of a New VES Acid System: Considering the Impact of CO2 Solubility

Abstract

Abstract Viscoelastic surfactant (VES) based self-diverting acid system has been developed for better matrix treatment of carbonate formations. Literature survey indicates that the highly viscous fluid acts as a temporary barrier to reduce further fluid loss into the wormholes and allows complete stimulation of all treating zones. After acid treatment, the viscous fluid is broken by either formation hydrocarbons or pre-flush fluids. However, recent lab work confirmed that a significant amount of surfactant was retained inside the core even when mutual solvent was used. The present study was conducted to better understand these acid systems and determine factors that impact their viscosity build-up and performance inside the carbonate formations. A series of coreflood tests were conducted using low and high permeability carbonate cores at different injection rates. Propagation of the acid, surfactant, and reaction products inside the cores was examined in detail. Samples of the core effluent were collected and the concentrations of calcium, surfactant, and acid were measured. Permeability enhancement and location of any precipitation was detected using CT scanner to the core before and after the acid injection. Material balance was conducted to determine the amount of surfactant that retained in the core. Experimental results show that VES acid was only able to build pressure drop across the core at injection rates less than 1 cm3/min when it was injected inside 80 md permeability cores. However, at injection rates of 1 cm3/min and higher, VES was not able to build any pressure drop across the core when it was injected into 4 md cores. Acid pore volume to breakthrough and amount of VES retained in the core were reduced when low permeability cores were used. Calcium propagated faster than HCl, while surfactant propagated the same rate as HCl. Also, pore volume needed to breakthrough the calcium and the maximum calcium concentrations were independent of the acid injection rate. CT scan confirmed that wormhole branches were observed at the second half of the core. At ambient conditions: viscosity of live VES acid was higher than that of partially neutralized (pH 4.5). At temperatures greater than 140oF, the viscosity of partially neutralized acid was higher than the live acid. G’ of the live VES acid was dominant at ambient temperature, while when temperature increased to 85oF, G"became the dominant characteristics of the live VES acid.