Openhole Multistage Acid-Fracturing Increases Hydrocarbon Production in Deep Naturally Fractured Carbonate Horizontal Wells in Ta-zhong Gasfield of Tarim Basin, West China

Abstract The naturally fractured carbonate reservoir of Ta-zhong has been developed for over ten years due to new technologies. Since 2008, a program was undertaken to test openhole multistage acid-fracturing technology in the Ta-zhong gasfield to determine if deep fractured carbonate reservoir could be effectively developed by horizontal wells. This paper will introduce the challenges including the segregated completion through optimized design and individual acid-fracturing treatment through simulation, production history of stimulated horizontal wells and the comparisons with offset wells. Ta-zhong carbonate reservoir is buried from 5000m to 7000m TVD with low matrix permeability and porosity, the hydrocarbon is mainly stored in the interconnected natural fractures and vugs, so a novel segregated completion method was gradually formed during the development from the reservoir description. To avoid circulation loss or blowout during drilling horizontal well, the horizontal interval was usually close to the top of reserves with natural fractures and vugs by control while drilling. The reserves and numbers of segments will be distinguished by 3D seismic, well course and stress direction, and then the distances between reserves and horizontal interval was calculated through horizontal well track superimposed on the seismic profile, each section divided by openhole packers need special acid-fracturing technique to get sufficient height or length to communicate the hydrocarbon reserves. Simulation work gives the results which pad and ground cross-linked acid multi-alternative injection fracturing is favorable for longer length, and pad acid-fracturing is favorable for higher height. The performances of frac fluid and ground cross-linked acid are also showed in the paper, the residual viscosity of gel fluid is greater than 150mPa·s in the conditions of 140? and 170s−1 shear, and ground cross-linked acid's is more than 200mPa·s in the conditions of 120? and 170s−1 shear. Production comparisons showed that stimulated horizontal wells extended the production life of development wells and improved development effectiveness. Several years’ production history analysis of TZ62 block highlighted that the average hydrocarbon rate of multistage acid-fracturing horizontal wells was several times the stimulated vertical offset wells.