Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (1): 241-250.doi: 10.11743/ogg20220120

• Methods and Technologies • Previous Articles    

Mini-frac test interpretation for shale reservoirs based on impulse injection theory and its application

Yanyan Wang1,2(), Weihong Wang1,2, Xiaohu Hu1,2, Hua Liu1,2, Xiaorong Jiang3, Cheng Dai1,2, Sidong Fang1,2   

  1. 1.State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China
    2.Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China
    3.Chengdu University of Technology, Chengdu, Sichuan 610059, China
  • Received:2020-08-06 Revised:2021-11-22 Online:2022-02-01 Published:2022-01-28

Abstract:

Mini-frac tests can be used to evaluate the physical property of shale reservoirs prior to large-scale fracturing, and provide key parameters for resource quantity calculation and evaluation of sweet spots both in geology and engineering. As of now, the well testing interpretation based on conventional shut-in pressure recovery has a very low accuracy in dealing with special issues like mini-frac test, featuring “short-term injection and long-term shut-in”, often accompanied by difficulties in flow pattern identification and parameter diagnosis. In view of the particularity of mini-frac test, the study proposes a well testing interpretation method based on pulse injection theory, where the process of injection and shut-in pressure drop is regarded as a whole, the shut-in pressure response is assumed to be the result of both injection and shut-in periods as considered at the inner boundary of mathematical diffusion-filtration model, and then a mini-frac test interpretation model is established. A pressure dynamic curve is calculated, and the influence of the parameters on the test curve is determined through sensitivity analysis. Pressure of key sections of the test curve is obtained through asymptotic solution, and 3 combined diagnostic curves are introduced to obtain the flow regime information. Finally, an interpretation method and process by mini-frac tests is established with parameter interpretation made for different flow regimes, including dominant flow regime identification and special straight line analysis. This method is then applied to analyze two wells located at different structural positions of Pingqiao anticline in Nanchuan block, Fuling shale gas field, through which its practicality is verified. The interpretation results serve to provide basis for understanding the gas-bearing potential of shale in the area, and predict the regional differences of fracturing effect successfully, which is of guiding value to future delineation of sweet spot zone and development zone of priority. The subsequent production tests also verify the reliability of the results obtained.

Key words: flow regime diagnosis, special straight line analysis, pre-frac parameter interpretation, mathematical diffusion-filtration model, mini-frac test, Fuling shale gas field

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