Low Viscosity Acid Platform: Benchmark Study Reveals Superior Reaction Kinetics at Reservoir Conditions
Authors: Sayed Mohammed , Cairns Amy J. , Sahu Qasim 
Published: January 2020

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.