Research On Reservoir Acidification For Nantun Group In Hailar Oilfield

Hailar Basin is the biggest petroliferous basin which is outside Songliao Basin and belongs to Daqing region. With the proceeding of oil exploration and development, it has become one of the most important suppliers for Daqing Oilfield's sustainable 40 million tons output. Nantun Group is a main pay horizon in this oilfield, such as for Su131 Block in Surenur Oilfield and Bei301 Block in Huhenuren Oilfield. These oilfields generally have features of different geological condition and uneven clay mineral distribution, and have problems of rapid water injection pressure increase and great water adsorbing capacity decrease. The former acidification got unsatisfactory stimulation result of low efficiency, short valid period due to poor targeted formula and unreasonable operational process. In this paper, geological characteristics, physical characteristics and formation sensitivity of this oilfield are analyzed, and clear understanding for formation characteristics has been got. Nantun Group has great physical characteristics difference, and it is strongly sensitive due to its high content of clay, in which, illite and kaolinite are the major components. Based on the above analysis, research on acid formula and performance evaluation are conducted, and the new acid system has features of high corrosion rate, low crush ratio, quick demulsification, easy flow back and prohibiting secondary precipitation, etc, which will improve matrix permeability significantly. Additionally, more appropriate operational process is determined according to the reservoir geological characteristics. Up to now, field test has been performed on 12 wells, and satisfactory stimulation results are achieved. This technology ensures waterflooding proceeding smoothly in Nantun Group.Combined with previous acidification situation conducted in Nantun Group, and analysis on formation geological feature, physical feature and sensitivity, targeted acid formula and operational process are researched. Following results are achieved.1. Research by X-Ray diffraction, polarizing microscope and scanning electron microscopy (SEM) indicates, clay mineral of Su131 Block are mainly kaolinite and illite, and followed by montmorillonite-illite mixed layer, and montmorillonite- chlorite mixed layer. It is medium sensitive to water, with water sensitivity index 0.3~0.7. But two target intervals, layer No.16 above and No.18 below, have different characteristic on acid sensitivity, the former is strongly sensitive to chlorhydric acid, and the latter is strongly sensitive to mud acid.2. According to sensitivity characteristic of Su131 Block, two acid systems are determined, organic acid system for the upper layer can improve the permeability of natural core by 147%, and compound acid and organic acid system for the lower layer can improve the permeability of natural core by 148%. For operational process, multi-stage diverting technique and reasonable operation scheme are adopted to realize separate layer diverting acidification by one treatment, which resolves the problem of no packer operation on water injection wells with two different layers.3. Research by X-Ray diffraction, polarizing microscope and scanning electron microscopy (SEM) indicates, clay mineral of Bei301 Block has great difference in different wells, which is mainly strongly water sensitive mineral montmorillonite, and velocity sensitive mineral, such as kaolinite and illite; some wells has acid sensitive mineral chlorite. Basically, this formation is strongly sensitive to water and medium sensitive to velocity.4. According to strongly water sensitivity of Su131 Block, alcohol-based retarded acid system is determined, which has features of high dissolution rate (14.7%), low crush ratio (2.4%) ,low corrosion rate, and no precipitation under pH 7.1. For operational process, operation parameters are determined according to acid software calculation. Additionally, over-displacing and after-protection technique is applied to guarantee the treatment results.Up to now, field test has been performed on 12 wells with effective rate of 100% and average 12 months valid period, at most 18 months. Injection pressure is decreased by 2.8MPa on average at the initial stage, and water injection volume for single well is increased by 7.3m3.With the proceeding of Hailar Oilfield waterflood development, more and more injection wells with difficulty of water injection and low yield production wells is emerging. This technology provides a set of well stimulation method, which not only ensures the development of Hailar Oilfield, but also instructs acidification operation on similar strongly sensitive formations.