Optimization of Acid Fracturing to Improve Heavy Oil Production in Naturally Fractured Carbonates

Abstract

Abstract This paper presents detail experimental and theoretical study and the results of successful applications of acid fracturing to enhance heavy oil recovery in Ordovician formation in LN region of TARIM, China. The Ordovician formation in LN field is extreme heterogeneous carbonate with naturally fractures. The reservoir temperature is 140 °C, and the formation fluid has API gravity of 8. To optimize the acid system for this complicated formation condition, we first studied the reaction characteristic of gelled acid compared with straight acids at temperature ranging from 60°C to 140°C. Acid conductivity experiments were conducted with a unique conductivity instrument using limestone samples designed to simulate alternating stages of pad acid fracs and closed acidizing process. The original way to determine acid/rock reaction rate at high temperature is to extrapolate from low temperature test data. Our test has shown that the actual reaction rate at high temperature is much less than that obtained from the low-temperature data. Also, results show that gelled acid is 2.5 times more retarded than straight acid and the delayed nature of gelled acids is useful forgenerating longer etched fractures. Gelled acids also have higher viscosity than straight acid, which reduces fluid loss and friction pressure in acid fracturing applications. Based on the experimental result, some novel approaches were used to evaluate stimulation and improve heavy oil production. FMI logging technique was used pre-and post-stimulation, which shows that fracture numbers and shapes were much different from theoretical reorganization. Three kinds of fractures were observed, including vertical fractures, horizontal fractures and inclined fractures. A theoretical model considering wormholing was used to evaluate pressure decline after acid fracturing. In order to minimize post-stimulation production decline rate in field, a new technique, mixing light oil with heavy oil at bottomhole condition, was introduced into this area to significantly improved oil mobility. Field cases of applying the new approach of acid fracturing in the Ordovician formation in TARIM over five years are presented. The field cases show the improvements in production of heavy oil resulting from the comprehensive approach.