Low Viscosity Retarded Acid System: A Novel Alternative to Emulsified Acids
Authors: Aldakkan Bashayer , Gomaa Ahmed M. , Cairns Amy J. , Sayed Mohammed , Alnoaimi Khalid 
Published: October 2018

Abstract
Acid systems are widely recognized by the oil and gas industry as an attractive class of fluids for efficient stimulation of carbonate reservoirs. One of the major challenges in carbonate acidizing treatments is adjusting the convective transport of acid deep into the reservoir while achieving a minimum rock face dissolution. Conventional emulsified acids are hindered by several limitations; low stability at high temperatures, a high viscosity that limits pumping rate due to frictional losses, the potential of formation damage, and the difficulty to achieve homogenous field-scale mixing. The objective of this paper is to introduce an engineered low-viscosity retarded acid system without the need for gelation by a polymer or surfactant, or emulsification by diesel.
The proposed acid system combines the use of a strong mineral acid (i.e. hydrochloric acid "HCl") with an organic compound that is highly soluble in the acid. It is based on reducing the free water in the system and, consequently, restricting the ionic separation of the proton from the acid. This results in a retardation behavior that is necessary to maintain a controlled reaction for the system used. The retardation behavior testing includes dissolution experiments, compatibility testing (using X-Ray Diffraction "XRD" and inductively coupled plasma "ICP"), coreflood study (at rates of 2 and 5 cm3/min at 300°F and 3000 psi) and corrosion rate testing (conducted at 300°F). Finally, a reaction kinetics analysis (at disc rotational speeds of 250 rpm, 500 rpm, 700 rpm and 1500 rpm) was performed to evaluate the retarded acid performance. The new acid system showed 4 times reduction in the core weight loss due to dissolution in acid compared to plain 15wt% HCl. XRD and Computed Tomography (CT) scans illustrated the acid compatibility with dolomite by forming no precipitation that is attributed to the organic compound used in the system. The proposed acid can create a dominant wormhole pattern requiring only 0.53 Pore Volume (PV) of acid to achieve breakthrough in the Indiana limestone core. It is important to note that emulsified acids require 0.8 PV of fluid compared to 1.4 PV for 15 wt.% HCl acid package. The proposed system was determined to have negligible corrosion rate of 0.002 lb/ft2 after 4 hours exposure at 300°F. Reaction kinetics study proved the delayed acid reaction with calcite by an order of magnitude compared to 32 wt.% HCl.
Conventional emulsified acids require expensive energization, through nitrogen (N2) or carbon dioxide (CO2), to overcome frictional losses and meet high injection rates requirement for fracturing and stimulation in deep formations. This novel system is characterized by having a low-viscosity and high thermal stability system that can be mixed on the fly. This approach addresses the main challenges of emulsified acid systems and will offer a cost-effective solution to cover a wide range of applications in matrix or fracturing applications, and high-temperature conditions that require a thermally-stable acid system.