Paper: A New Chemical Retarder For Acid Systems

Carbonate formations have been treated with acid for many years to increase fracture length and conductivity and, thereby, stimulate production. Fracture acidizing of carbonate formations, however, requires consideration of several parameters. One important parameter is the distance acid will penetrate the fracture before completely reacting. This distance is referred to as the acid penetration distance. Other parameters include dynamic fracture geometry and fracture conductivity. Although these three parameters are largely controlled by-formation properties, they can be strongly influenced by the treating fluids used and the techniques employed to place these fluids. Acid penetration distances have been increased to various extents with chemically retarded acids, gelled acids, emulsified acids and acid systems composed of hydrochloric, acetic and/or formic acids. A new chemical retarder has been developed which can be used with emulsified and non-emulsified acids to help increase acid penetration into fractured limestone formations. The unique chemical retarder is chemically and physically adsorbed on the formation where it slows the reaction rate of acid. Besides retarding the acid-limestone reaction effectively, adsorption also appears strong enough to withstand turbulent flow. Although a retarder may greatly increase penetration distance, other parameters, such as fracture conductivity, also have to be considered if the retarder is to be used. For example, during a retarded acid treatment inadequate fracture conductivity may result if fracture cooling causes over retardation of the acid near the wellbore. Placement techniques are in use, however, that could resolve this problem. During treatment, a less retarded acid can be injected before the retarded acid for better conductivity near the wellbore, and the density of preflush can be balanced with the density of partially spent acid for better acid distribution along the fracture face. This paper will discuss the laboratory evaluation of the new retarder's effect on acid penetration distances and reaction times with non-emulsified and emulsified 15% HCl, 20% HCl, 28% HCl and a 7-1/2% HCl-10% formic acid mixture. Also to be discussed, are acid placement techniques used recently in twenty one Mexican wells with the new retarder. Results of these techniques thus far are also listed.