Crosslinked High-Strength Hydrochloric Acid Gel System Rock Mountain Case Histories (1982)

Summary A recently developed high-strength crosslinked acid system involving hydrochloric acid (HCl) concentrations of 0.25 to 28% has been used in acidizing a variety of limestone and dolomite formations throughout the Rocky Mountain region. The specialized chemistry of the crosslinked acid polymer in HCl solution as well as the resultant clay stabilizing properties after the acid reacts are noted.Case histories of the well treatments using high-strength crosslinked HCl are presented along with production history before and after the cross-linked acid production history before and after the cross-linked acid jobs. Data on more than 45 crosslinked acid jobs are presented. Some older wells treated with the crosslinked presented. Some older wells treated with the crosslinked acid averaged a two- to sixfold oil production increase, whereas the potential of new wells was much greater than expected after the treatment was compared with neighboring wells treated conventionally, and production usually has been more sustained. production usually has been more sustained. Introduction The technique of acidizing oil and gas wells is a well-known technology. The process of dissolving soluble carbonate or dolomitic rock has been employed since 1938. The technique of gelling HCl to control fluid loss and to prevent it from reacting during pumping is not new; however, the degradation of the gel by the acid has caused the system to be less efficient than anticipated. The crosslinked acid system overcomes the disadvantages of the gelled acid and allows HCl to be pumped, live and strong, farther into the formation than any previously designed system. previously designed system. The effectiveness of the acid treatment is determined by the degree of formation conductivity achieved. The conductivity between the wellbore and the reservoir depends primarily on the depth of penetration of live acid. The acid can be kept live away from the wellbore by retardation of the acid reaction on the formation. Various techniques and compositions are known for retardation: chemical retardation, acid/oil emulsion, foaming, hybrid acids, gelling, the use of alternate stages of pad and acid, etc. All these techniques provide retardation, but by the time acid reaches the provide retardation, but by the time acid reaches the tip of penetration, the concentration of the acid is reduced considerably or the acid is completely spent. However, the unbroken crosslinked HCl acid maintains nearly the same acid concentration at the tip of the penetration as at the wellbore. penetration as at the wellbore. The shear stability and high viscosity of the crosslinked acid are advantageous to fracture-acidizing operations because the acid produces wider and longer fractures in low-permeability limestone and dolomite formations.Laboratory tests indicate the effective retardation for wells having a bottomhole temperature (BHT) up to 350F. Since the stability of the system decreases with the increase in temperature and acid concentration, it is suggested that for high-BHT wells, a suitable preflush to cool the formation be incorporated in the treatment design. By selection of the appropriate polymer concentration and crosslinker quantity according to the BHT and the strength of the HCl acid to be used for the job, the desired viscosities may be obtained. The crosslinked acid system breaks down with time and temperature, as a practically residue-free live acid of less than 7- to 10-cp practically residue-free live acid of less than 7- to 10-cp viscosity to react with the formation. The system is compatible with various additives used in the acid, such as corrosion inhibitors, surfactants, sequestering agents, and suspending agents. However, some of these additives are not compatible and they should be used only after their compatibility is determined in a prejob laboratory test.The experimental data on the crosslinked acid stability, inhibiting clay swelling, sand suspension tests, reaction rate data, rheology data, fluid loss data, etc. have been published in recent literature. JPT p. 2080