Practical considerations for diagnostic fracture injection test (DFIT) analysis

Abstract

The diagnostic fracture injection test (DFIT) provides a way to estimate parameters critical to hydraulic fracture design. Existing published models for before-closure and after-closure behaviors rely on assumptions, and failure to honor important realities leads to parameter estimation errors that could degrade hydraulic fracturing design. This paper is to illustrate an accurate way to estimate parameters from the DFIT data, and then discuss how these parameters should be used in the hydraulic fracture design, and at last offer an alternative DFIT design approach.Two fundamental concepts underlying the proposed methodology are the material balance before fracture closure and the diffusive flow in the formation after closure. Before closure analysis must correctly address abnormal leakoff behaviors and near well friction losses. Likewise, accurate permeability and pore pressure estimation requires correct accounting for the leakoff rate into the formation.Strict attention to material balance yields estimations for instantaneous shut-in pressure (ISIP) and perforation and tortuosity friction losses that may, in turn, avoid apparent very high net pressure and stress shadowing. This work also underscores the importance of understanding surface and downhole volumes and rates both in the DFIT design and in the analysis. Further, for the DFIT case with pressure dependent leakoff (PDL), the fracture design for the subsequent hydraulic fracturing treatment should use the apparent leakoff rate associated with the first closure trend instead of the leakoff rate associated with the last closure trend derived from a normal leakoff model. Two successive DFITs in the same perforation, the first treated with low injection rate and small volume and then the second with high rate and large volume, are proved to be a good practice to obtain reliable estimation of formation properties.This paper demonstrates that DFIT parameters have often been misused. With the proposed methodology, the interpretation will be more accurate and reliable for formation evaluation and hydraulic fracture design.