Low Viscosity Acid Platform: Benchmark Study Reveals Superior Reaction Kinetics at Reservoir Conditions

Abstract

Stimulation of carbonate reservoirs using suitable acid systems is common practice to alleviate damage caused by the drilling and completion operations and to increase permeability by way of acid fracturing operations in tight formations. This process creates conductive channels in the pay zone allowing for oil and gas recovery. Several drawbacks however can be encountered, particularly at elevated temperatures, when using strong mineral acids, i.e., hydrochloric acid (HCl). Under these harsh conditions, propagation of live acid deep into the formation is hindered by unfavorable reaction kinetics between the acid and rock matrix. To address some of these challenges, the industry moved towards emulsifying the mineral acid in a hydrocarbon phase, i.e., diesel. The advantages to this strategy are two-fold, i.e., it provides a temporary barrier between the acid and the formation and offers some degree of corrosion protection for the metal tubulars during the pumping stage. The two main drawbacks are high friction losses and cumbersome mixing requirements that create complexity and impose quality control challenges in the field. To address some of these challenges, we recently reported on the design and characterization of a new acidizing platform consisting of a suite of low-viscosity acid systems (LVAS) having the desired retardation profile for acid stimulation under harsh reservoir conditions. To better design acid stimulation treatments in the field, it is imperative to gain a deeper understanding of the reaction kinetics between the fluid and rock matrix, specifically the reaction rate constant and diffusion coefficient. In this paper, a comprehensive kinetics study was conducted for an LVAS formulation. This previously reported system is comprised of a pre-engineered blend of strong organic and mineral acids. This study was performed at 3000 psi, across a wide temperature range (180-350°F) and disk rotational speeds (250 up to 1000 rpm) using a custom designed rotating disk apparatus. Homogenous calcite disk samples were cut in 1.0" thickness and 1.5" diameter. The mineralogy and homogeneity of the core samples were characterized using powder X-ray diffraction (PXRD). The reaction rate constant for the Newtonian hybrid acid system was measured at 300°F and 500 rpm and determined to be 3.3×10−6 gmole/cm2.s. The performance of this fluid system was benchmarked to a previously reported weak organic acid system (i.e., 20 wt% glutamic Di-acetic acid [GLDA]). Notably, LVAS was able to control the calcite/rock reaction on a magnitude comparable to 20 wt% GLDA. In addition, the diffusion coefficient for LVAS was found to be similar to that of 15 wt% HCl-based emulsified acid systems. The LVAS allows faster batch mixing compared to emulsified acid, achieving low viscosity and manageable corrosion. The measured reaction parameters and the calculated activation energy for LVAS was higher than that of other acid systems indicating the ability of LVAS to control the reaction with calcite rocks. The LVAS presents advantageous performing qualities enhancing conductivity in the rock and porous matrix.

A Calibrated Computational Fluid Dynamics Model for Simulating the Rotating Disk Apparatus

“…Understanding reaction kinetics between the rock and acid is necessary to optimize the acidizing process. The rotating disk apparatus (RDA) has been used to study reaction kinetics between rocks and Newtonian and non-Newtonian fluids (Hansford and Litt 1968;Boomer et al 1972;Lund et al 1973;Roberts and Guin 1975;Prakongpan et al 1976;Anderson 1991;Fredd and Fogler 1998;Alkattan et al 1998;Conway et al 1999;Taylor and Nasr-El-Din 2009;Rabie et al 2014;Li et al 2016;Liao et al 2017;Abdelgawad et al 2018;Kotb et al 2018;Sayed et al 2020). However, no standard exists in the literature for the acid volume used in RDA experiments.…”

“…The theory of an RD was first applied to study the electrochemical processes using an RDE (Daguenet 1968;Ellison et al 1971;Chin and Litt 1972;Nanis and Klein 1972;Vahdat and Newman 1973;Bruckenstein and Miller 1977;Mohr and Newman 1976;Albery and Bruckenstein 1983;Dong et al 2008), and was later adopted to analyze the chemical dissolution reactions using RDA (Boomer et al 1972;Lund et al 1973, Lund et al 1975Fredd and Fogler 1998;Taylor et al 2004;Alkhaldi et al 2010;Rabie et al 2014;Khalid et al 2015;Aldakkan et al 2018;Hall-Thompson et al 2020;Sayed et al 2020;Kotb et al 2021).…”

“…The rotating disk apparatus (RDA) has been typically used for studying reaction kinetics between different rocks and acid systems (Boomer et al 1972;Lund et al 1973a;Roberts and Guin 1975;Fredd and Fogler 1998;Conway et al 1999;Taylor and Nasr-El-Din 2009;Rabie et al 2014;Li et al 2016;Liao et al 2017;Abdelgawad et al 2018;Kotb and Nasr-El-Din 2020;Sayed et al 2020;Ivanshin et al 2021). In the seminal work by Levich (1942), the author studied the rate of mass transfer of H + in Newtonian fluids to a rotating disk in a reactor and developed the following equation to calculate it: is the diffusion coefficient of H + , Cb is the bulk concentration of the reactant, Cs is the reactant concentration on the disk surface, ω is the disk rotational speed, υ is the kinematic viscosity of the fluid, Vy is the acid velocity in the axial direction, and y is the axial distance from the disk to the point of the measurment of Vy.…”

Review of Acid Diffusion Measurement Methods in Porous Media

“…The effect of temperature on the acid rock interaction between a calcite rock and a low-viscous acid system (LVAS-1) (a pre-engineered mixture of HCl and organic acid) was conducted using RDA by Sayed et al 106 At different temperatures of 180, 300, and 350 °F, the acid diffusion coefficients were determined. The results indicated that an increase in the temperature of the system increased the acid diffusion coefficient.…”

Optimization and Uncertainty Quantification Method for Reservoir Stimulation through Carbonate Acidizing

“…Experimental work in the core flooding setup was first conducted to understand the parameters that control the acidizing in carbonate rocks. These experimental studies analyzed the impact of rock properties, injection conditions, pressure, mineralogy, temperature, and reaction kinetics on the created channels. − In those linear core flooding experimental studies, the acidic solution is injected at a different rate in a cylindrical core sample, and the pore volume to the breakthrough was measured along with the other pressure measurements. ,, …”