Acid Treatment Optimization Based on Digital Core Analysis
Authors: Ivanov Evgeny , Demianov Alexander , Sidorenkov Alexander , Beletskaya Anna , Dovgilovich Leonid , Abdrazakov Dmitriy , Stukan Mikhail , Dinariev Oleg 
Published: September 2020

Abstract
Acid treatment is a common well stimulation technique widely used for both oil and gas wells. However, there is a challenge, that for any given acid solution, rock lithology and permeability, and reservoir conditions, there are optimum values of acid injection volume and rate. Deviation from these optimal values during stimulation treatments reduces the acid job efficiency. We developed a robust and efficient method of identification of the optimum parameters based on digital core approach.
The developed workflow includes: a) construction of a digital avatar of a core sample using 3D microCT tomography; b) pore-scale direct reactive flow modeling using a combination of the chemical kinetics/thermodynamics (in assumption of partial local equilibrium) with the method of density functional theory in hydrodynamics (an efficient tool for pore-scale modeling of multiphase flow able to handle different complex physical phenomena); c) core scale simulations in the framework of the Darcy based approach using upscaling from the results of the direct pore-scale simulations; d) input of the obtained parameters into the acidizing simulator to determine optimum acid type, rates, and volumes.
We illustrate the developed workflow on example of an optimum injection rate determination in the case of Silurian dolomite dissolution by hydrochloric acid. The pore-scale simulations were performed using 3D microCT models with 2.5 μm/voxel resolution. These simulations allowed to determine the dependence of dolomite dissolution rate on the fluid injection rate and predict the transport properties of damaged rock. The correlations obtained from high resolution simulations were then applied in core-scale modeling of dissolution process using continuous Darcy based model (with 100 μm/voxel resolution). The transport properties of a core were populated using the results of pore scale simulations.
Then several core scale simulations of dolomite dissolution with different acid injection rates were performed to obtain numerically the dependence of its influence on the number of pore volumes injected until the breakthrough (PVBT). PVBT dependence on the injection rate in a form of characteristic curve was incorporated into the advanced acidizing simulator. Being calibrated this way, the simulator was then used to model acidizing treatment in a dolomite reservoir with the similar properties as digitally acidized core. Modeling showed that post stimulation skin values are lower and expected wormhole length is bigger when digitally calibrated pore volume to breakthrough (PVBT) curve is used, if compared with modelling of the same treatment with non calibrated acid-rock interaction curves. Consequently, outcomes of this well scale modeling suggest that the use of digitally calibrated PVBT curves results, for this case, in optimization of required acid volume and associated operational footprint.
The suggested approach improves the process of obtaining PVBT characteristic curve by application of digital core analysis technique. It allows to test numerous "what if’ scenarios and to evaluate the effect of different factors on mineral dissolution rate at pore scale. This paves the way for improvements in acidizing job design by increasing the consistency between the models used for reactive flow modelling and pore scale heterogeneity of real rocks.