Evaluation of Acid-Gelling Agents for Use in Well Stimulation

Abstract

Abstract Various polymers and other thickening agents have been evaluated as gelling agents for hydrochloric acid (HCl). These materials were compared on the basis of thickening efficiency, acid stability, and residue formation upon spending of the acid. Thickeners tested include xanthan polymers (XP), guar gum (GG), hydroxyethyl cellulose (HEC), carboxymethyl-hydroxyethyl cellulose (CMHEC), polyacrylamide (PAM), polyvinyl alcohol (PVA), polyvinyl-pyrrolidone (P V P), acrylamide/sodium -2-acrylamido-2-methylpropane sulfonate copolymer (PAM/AMPS), and a cationic surfactant that thickens acid when used at high concentrations.Of the thickening agents tested, XP showed the greatest overall potential for use as an acid gelling agent. This polymer is an efficient viscosity builder and has good stability in 15 % HCl at temperatures up to 150 degrees F. Computer model studies, which take into consideration cool-down and gel degradation rate at various stages of acid spending, indicate viscosity is maintained when treating wells that have bottomhole temperatures (BHT's) up to 220 degrees F. Since the gelled acid retains its viscosity during spending, both reaction rate and fluid leakoff are retarded. Laboratory studies simulating downhole spending conditions yield a 3.6-fold increase in spending time for gelled 15 % HCl (22-cp) compared with ungelled acid. Well test results are presented that demonstrate the effectiveness of acid fracturing treatments using HCl with XP. Introduction Gelled acids have been used for many years to improve the effectiveness of fracture acidizing treatments. Gelling the acid allows greater fracture penetration by increasing fracture width, slowing reaction rate, and reducing fluid loss. Unfortunately, most commonly used gelling agents have limited acid stability and are effective only at low-temperature applications. Newer, more acid-stable polymers show promise for use at higher temperatures.When evaluating acid gelling agents, a number of important factors must be taken into consideration. These include the following. Acid Stability. How long is viscosity maintained in the treating acid at various temperatures? Efficiency. What concentration of gelling agent is required to achieve a desired viscosity? Condition of Spent Acid. Is the viscosity of the spent acid low enough to ensure cleanup and are insoluble reaction products formed upon spending of the acid? Acid Stability For an acid gelling agent to be effective, it must be sufficiently stable to retain its viscosity in acid under downhole spending conditions. Tests were performed in which gelling agents were mixed with preheated,]5 % HCl to compare the stability of various acid thickeners. After 1 minute, the initial viscosity was measured with a Fann 35 viscometer at a shear rate of 511 seconds . While maintaining the solution at the test temperature, additional viscosity measurements were made over a period of 1 hour. Table 1 describes the gelling agents that were evaluated, while Figs. 1 and 2 contain the test results.Tests performed at 100 degrees F demonstrate that gelling agents such as GG, hydroxypropyl guar gum (HPG), HEC, and CMHEC have limited stability in 15 % HCl. Consequently, their use at temperatures above 125 degrees F is not recommended. SPEJ P. 415^