Enhancing Efficiency of HCl Based Stimulating Fluids by Creating In-Situ Carbonic Acid Using Nickel Nanoparticles
Authors: Raj Navneet , Pal T Varun 
Published: March 2015

Abstract
Generally carbonate acidizing job is carried out using HCl based stimulation fluids because of its effectiveness to dissolve and/or to disperse materials obstructing flow. But this effectiveness also creates a major drawback of its use; that is only surface dissolution and low penetration due to fast rate of reaction with carbonates. One method to overcome this problem is to mix appropriate organic acid with HCl but the reactions between organic acids and carbonates is less understood than those of HCl with carbonate rocks because of the presence of CO2 and the precipitated reaction products; the organic salts of calcium and magnesium. Therefore much testing is needed to know the right organic acid to be mixed with HCl for a particular reservoir. Also they normally do not react to their full acid strength because of the release of CO2 from carbonate dissolution and the cost of organic acid is significantly higher than that of HCl for equivalent mass of rock dissolved.
In this paper we propose a new method to overcome these tribulations. In this method, Nickel nanoparticles mixed with water are injected into the formation before injecting stimulating fluid. Afterwards when the HCl based stimulating fluid is injected into the formation, it reacts with the Carbonates and produce CO2. As sson as CO2 is formed, Nickel Nanoparticles converts gaseous CO2 into carbonic acid (aqueous CO2). What nickel nanoparticles do is that it accelerates the natural conversion of CO2 to carbonic acid. Use of nanoparticles reduces the amount of CO2 by converting it into Carbonic acid (enhancing the stimulation job), reducing the cost and facilitating the reaction between organic acid and carbonate rocks.

Introduction
Carbonate reservoirs present tremendous completion, stimulation and production challenges because they are vertically and laterally heterogeneous, with natural permeability barriers, natural fractures and a vast array of porosity types, from intercrystalline to massive vugular and cavernous porosity. Consequently it is very difficult to target the injection of stimulation fluids to a particular zone of interest. This leads to inefficient use of stimulation fluids. So there is a need for the development of new stimulation technique which would result in more efficient use of stimulation fluids and remove the drilling damage at a reasonable cost.
While permeability is a major factor in the distribution of acid along a completion for many reservoirs, pre-stimulation skin damage, intermixed rock types with different acid-rock worm holing characteristics, distance between zones, and differential reservoir depletion also play important roles in the effective stimulation of the reservoirs
Oil and gas companies are developing carbonate reservoirs of deeper and deeper depths in order to meet the demand of increasing worldwide energy consumption. Hydrochloric acid is the most commonly used acid for carbonate acidizing due to its low cost and high dissolving power but enhancing productivity from these reservoirs poses a challenge in stimulation fluids due to the increase in bottom hole temperature. The rapid reaction rate between HCl and carbonate limits the penetration of HCl into the formation, especially at low pumping rates. The reaction of HCl often needs to be retarded by gelling, emulsifying, or adding viscoelastic surfactants. In addition to the high reaction rate, HCl is very corrosive to well tubulars. Expensive corrosion inhibitors can protect the tubulars at high temperatures only for a short period of time. Other problems have been shown in Fig. 1–4.