A New Methodology to Determine the Reasonable Buildup Test Time for Tight Oil Wells: A Case Study from the Ordos Basin

摘要

In this paper, a new methodology to determine the reasonable buildup test time is derived. The methodology has the ability to determine the starting time of quasi-radial flow (STQF) with considering wellbore storage coefficient, permeability, skin factor, reservoir thickness, and oil viscosity. The curves of pressure derivative flatten out but don’t really enter into radial flow stage at the STQF. The STQF is determined with the use of the constraint for the slope of the pressure derivative curve. Using sensitivity analysis, we build the relationship between the dimensionless factors and the dimensionless STQF. Then the STQF is identified as a reasonable buildup time. We finally verify the methodology with the field pressure test data of typical tight oil wells from the Ordos Basin. The results show that our methodology is more practical and allows us to accurately estimate the STQF, without any influence on the interpretation results. The parameters all have a significant impact on the STQF including the wellbore storage coefficient, permeability, skin factor, reservoir thickness, oil viscosity, and the wellbore storage coefficient is what affects the STQF the most. In order to reduce the number of those key factors, we use the approach of non-dimensional treatment and obtain the relationship between the dimensionless factors and the dimensionless STQF. The results show that the relationship between the dimensionless STQF and skin factor is power function. With the increase of the dimensionless wellbore storage coefficient, STQF performs a second order polynomial function. Furthermore, a formula to determine the STQF is obtained and the two coefficients of the equation respectively represent the peak of the pressure derivative curve and the occurrence time of 0.5 straight line referred to pseudo-radial flow. We analyze plenty of pressure test data of typical tight oil wells from the Ordos Basin, and find that the two coefficients are fixed on 75.5071 and 0.2048, respectively. The results are very meaningful to determine the STQF of typical tight oil wells.