Decipher Productivity Secret to Optimise Well Stimulation for Keshen Tight Gas Reservoir

Abstract Keshen Gas Field is part of Kuqa Foreland thrust belt, located in Southern foothill of Tianshan Mountain, northern margin of the Tarim basin, West China. The main pay zone, Cretaceous Bashijiqike (K1bs), has extreme reservoir conditions of ultra-deep (6500-8000 m), high temperature (170-190°C), high pressure (110-120 MPa)(HTHP). Thick salt and gypsum layers with high dip angle above the pay zone also increase safety concerns during drilling and completion. Because of its low porosity (2-7%), low matrix permeability (0.001-0.5 md) and high heterogeneity of nature fractures (NF), well production rates vary largely, ranging from zero to 300,000 m3/d. Stimulation has become the only way to enhance single well performance and gain economic production for the long run. Finding controlling factors for well production and effective workflow is the key to realize efficient field development for this tight gas reservoir. In this study, an integrated workflow covering comprehensive reservoir characterization (evaluations of geology, geophysics, geomechanics and reservoir engineering), engineering design and execution (stimulation design and operation) was formulated and conducted to discover main controlling factors which influence production. Firstly, wells were divided in three groups based on their current production rates. Wells in best group have good production in DST tests and higher production after stimulation. Wells in better group have no production in DST tests but obtaining good production after stimulation. Wells in bad group have no production in DST tests, yet very low production after stimulation. Then correlate well production with structure, reservoir property, geomechanics and natural fracture properties to reveal the key controlling factors. Further study was focused on combining multiple parameters to decipher productivity secret based on multi-domain evaluation results. Lastly it was found out that the well performance in this field is controlled by both Stress and NF, i.e. the angle (θ) between NF and the maximum horizontal stress. When θ is small, well productivity is good. Otherwise well productivity is poor. According to this new finding, different stimulation methods were chosen, designed and executed in the field. Through integrated study on different levels, from core analysis to reservoir evaluation, the main factors controlling the tight gas field productivity were deciphered. Together with the understanding of the reservoir, the selection criteria for well stimulation were formulated with the intersection angleθ. After application of study results in stimulation design, gas production shows significant improvement: the average daily production rate increases 50% per well.