How Downhole Conditions Impact Acidizing Performance in Carbonate Reservoirs?

Abstract

Abstract Carbonate reservoirs consist mainly of calcite or dolomite or a mixture of both minerals. Hydrochloric acid (HCl) is frequently injected to stimulate the productivity of such formations. At temperatures above 200°F the reaction rate between HCl and carbonate becomes so fast that the treatment can result in shallow penetration and face dissolution. The HCl-carbonate reaction kinetics is complicated by the released carbon dioxide (CO2) phase behavior. Typical experiments on carbonate acidizing use 1,000 psi system back pressure as a mean to control the release of CO2 and keep it in the solution. The CO2 can be partially in the gaseous phase, which creates forced convection and therefore increase in apparent reaction rate. Acidizing treatments are usually performed at higher pressures, and temperatures, to push the acid to either create wormhole or etched fracture surfaces. The overestimated reaction rate due to the unrealistic experimental parameters can lead to underestimation of acid penetration and over-design of acid volume and injection rate requirements. The insight of pressure effect on acid-carbonate reaction rate has been published before. This paper is aimed at studying the combined effect of downhole condition of pressure and temperature on the acid and rock reaction rates and the overall performance (mainly wormhole propagation and structure) of the acid solution in more depth. Core flooding experiments are performed to cover a wider range of parameters of permeability, injection rate, and more importantly pressures and downhole temperatures to simulate more realistic reservoir conditions. Core samples were scanned using a micro-CT scanner to evaluate the wormhole propagation inside the core sample. In addition, the kinetics of CO2 phase transition, the solubility of CO2 in water are also investigated as functions of pressure and temperature to help better explain the core flooding results. It is found that the CO2 solubility in water at 300°F increases 4 times when pressure increases from 1500 psi to 8,000 psi. At 250°F and under similar conditions of injection rate and acid concentration, increasing the system pressure from 1,500 to 7,000 psi caused a 50% reduction in the acid volume needed to achieve breakthrough indicating an improvement in the acid penetration rate into the rock. At injection rate of 5 ml/min and under a pressure of 5,000 psi, 15 wt.% HCl breaks through core even at temperature as high as 350°F without causing any face dissolution. At high pressure, the HCl acid penetrates deeper than the preconceived notion because it is naturally more retarded than previously believed due to its full supercritical phase and higher CO2 solubility. It is very critical that the reaction rate measurements are performed under realistic downhole conditions to better simulate and design the acidizing treatment.