Optimization of Acid Fracturing to Improve Heavy Oil Production in Naturally FracturedCarbonates

摘要

This paper presents detail experimental and theoretical studyand the results of successful applications of acid fracturing toenhance heavy oil recovery in Ordovician formation in LNregion of TARIM, China. The Ordovician formation in LNfield is extreme heterogeneous carbonate with naturallyfractures. The reservoir temperature is 140 °C , and theformation fluid has API gravity of 8.To optimize the acid system for this complicated formationcondition, we first studied the reaction characteristic of gelledacid compared with straight acids at temperature ranging from60°C to 140°C.Acid conductivity experiments were conductedwith a unique conductivity instrument using limestonesamples designed to simulate alternating stages of pad acidfracs and closed acidizing process. The original way todetermine acid/rock reaction rate at high temperature is toextrapolate from low temperature test data. Our test has shownthat the actual reaction rate at high temperature is much lessthan that obtained from the low-temperature data. Also, resultsshow that gelled acid is 2.5 times more retarded than straightacid and the delayed nature of gelled acids is usefulforgenerating longer etched fractures. Gelled acids also havehigher viscosity than straight acid, which reduces fluid lossand friction pressure in acid fracturing applications.Based on the experimental result, some novel approaches wereused to evaluate stimulation and improve heavy oilproduction. FMI logging technique was used pre- and poststimulation,which shows that fracture numbers and shapeswere much different from theoretical reorganization. Threekinds of fractures were observed, including vertical fractures,horizontal fractures and inclined fractures. A theoretical modelconsidering wormholing was used to evaluate pressure decline after acid fracturing. In order to minimize post-stimulationproduction decline rate in field, a new technique, mixing lightoil with heavy oil at bottomhole condition, was introducedinto this area to significantly improved oil mobility.Field cases of applying the new approach of acid fracturing inthe Ordovician formation in TARIM over five years arepresented. The field cases show the improvements inproduction of heavy oil resulting from the comprehensiveapproach.