Abstract
In acid fracturing, while the near wellbore area of the fracture receives fresh, unspent acid, sections of the fracture further down the length receive partially spent acid due to reaction of acid. As a result, there is a distribution of acid concentration along the fracture length. However, almost all experimental studies on acid fracture conductivity have been conducted with fresh acid, which is representative of what occurs near the wellbore.
An experimental study was conducted to investigate the effect of spent acid on the resulting fracture conductivity in a laboratory facility designed to perform acid fracture conductivity characterization. Gelled HCl acid systems with different degrees of acid spending were used to etch fractured cores under identical treatment conditions, which were set to mimic field conditions. Detailed etched surface characterization, fluid analysis, and conductivity measurements were performed on acid etched fracture faces.
Experimental results clearly show that the amount of rock dissolved and actual etching pattern depend highly on acid concentration. While there was less fracture face etching with more spent acid system, etching pattern also changed with acid spending, which sometimes offset the additional etched volume of unspent acid. As a result, the retained conductivity was generally higher for the more spent acid and the decline in conductivity with closure stress was more subtle with the spent acid due to probably less weakening of the fracture face.
While there have been many developments in enhancing the etched length of acid fractured wells, a major reason for the lack of long etched fractures is acid spending along the fracture. In order to understand the etched fracture profile, there is a need to realize how the degree of acid spending limits etching and affects the resulting fracture conductivity. Based on the newly discovered effect of acid spending on conductivity, acid etched length and conductivity can be predicted more accurately and acid fracture treatments better designed.
