A New Analysis of Pressure Decline for Acid Fracturing

Abstract

Summary Conventional pressure-decline analysis has been used successfully to determine the in-situ fluid-loss coefficients and fracture geometries of minifracture tests or full-scale hydraulic fracturing. However, it fails to interpret the pressure-decline data of acid fracturing because it has a different fluid-loss mechanism and a short testing time. This paper presents a new pressure-decline analysis for full-scale acid fracturing or acid minifracturing to estimate fluid-loss coefficients and evaluate fracture geometries for the Perkins-Kern-Nordgren (PKN) model.1,2 The models established after shut-in have comprehensively taken into consideration the influences of compressibility and thermal expansion of fluids (waste acid and CO2 gas produced by reaction) and the rock volume etched away by acid. The effects of formation temperature and reaction heat are also taken into account in this analysis. The analytical solution derived by Nordgren2 for large fluid-loss is used to calculate the fracture-width distribution along a fracture. The short-test-time data for acid fracturing can be interpreted as a simulation technique used in analyzing. The comparison of the results of conventional analysis and this new analysis for field test data shows that the conventional analysis causes a −56.2% error in the fluid-loss coefficient and underestimates the fluid-loss. The relative errors of the conventional analysis for estimating fluid efficiency and hydraulic-fracture length and width occur 186.2%, 220.8%, and 16.5% respectively.