Multiscale Characterization and Simulation of Acidizing-Induced Pore Structure Evolution in Low Permeability Carbonates

Abstract

Acid stimulation is widely used for carbonate reservoirs to remove near wellbore formation damage. Reliable prediction of pore structure attributes changes as a function of time, space, and length scale of heterogeneity in multiscale continuum model is necessary for designing successful matrix acidizing. The objective is to investigate pore structure attributes of low permeability carbonates and use these attributes in acidizing model.

Samples characterized before injection for petrography, pore structure and distribution using thin sections, XRD, SEM, NMR and Micro-CT. Three different sets of coreflood experiments with different injection rates were performed. Hydrochloric acid was injected at different Damkohler numbers by changing injection rate. Pore volume to breakthrough (PVTB) for each injection rate was determined based on the pH of the effluent fluid. Multiscale continuum model was developed to match the experimental data and find the best relationship between the porosity and permeability for better matching.

Multiscale characterization used to derive the statistical distributions of porosity, permeability, pore size, and surface area. These distributions were then used as inputs to the numerical model for effective representation of the actual rock properties instead of using random distributions of these attributes. The coreflooding experiments with very low concentration of HCl shows that the PVTB increases with increasing the injection rate. The parameters of power law constitutive relations that relate the pore structure attributes with porosity were derived by matching the model with the experimental data. Change of permeability during dissolution is related to the change of tortuosity and the effective hydraulic radius. For homogeneous dissolution, the increase of permeability is basically due to the decrease of tortuosity, whereas for heterogeneous dissolution it’s due to a combination of decrease of tortuosity and increase of hydraulic radius. For intermediate dissolution regime, the permeability increases significantly for a small change in porosity that results from the large increase of the effective hydraulic radius. The relationships between pore structure parameters and porosity used for developing multiscale continuum mathematical model that incorporates the heterogeneity of carbonates and the constitutive relationships.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018