Evaluating seepage radius of tight oil reservoir using digital core modeling approach

摘要

The main development method of tight oil was depletion development. It is determined that the seepage radius of tight reservoir is crucial to the layout and prediction development of tight oil development. Since the pore throat level of tight reservoirs was frequently nanoscale, the reservoir pressure propagation velocity was even faster than conventional medium high permeability. The fracture system in the tight oil reservoir, which is the main flow channel of oil flow to the direction of wellbore, is more sensitive to the sharp change in reservoir pressure. Hence, the stress-sensitive absolute permeability of the reservoir near the wellbore will fluctuate significantly during the depletion development phase, rather than a constant value. At present, the numerical simulation and graph method, which are commonly utilized in the regression and prediction of the depletion-developing seepage radius. Both do not consider the stress sensitivity phenomenon which caused by nanoscale pores of tight oil and the change of the permeability in the model. As a result, the prediction is not accurate and the model needs to be modified. Meanwhile, the percolation parameters at the scale of the tight oil permeability cannot be accurately obtained in using conventional experimental methods owing to the limitations of the accuracy of the experimental instruments. In this paper, the test of fluid permeability under overburden was carried out to verify the stress sensitivity of the tight core. The microscopic mechanism of nanoscale pore-throat stress sensitivity in tight reservoirs was analyzed from the perspective of tight reservoir mineral composition and nanopore characteristics by using Quantitative Evaluation of Minerals by Scanning Electron Microscopy (Qemscan) and MAPS micro-image splicing technology. Nanoscale pore network model was established with nano-CT scanning and the digital core seepage simulation of tight core was figured out. The accuracy of the digital core seepage simulation method was verified in comparison with the indoor over pressure liquid permeability test. The conventional reservoir model was modified by using the digital core seepage simulation result, and the new model has a high degree of confidence in predicting the tight oil seepage radius.