A Fracture Injection Test Design and Interpretation for the Determination of Reservoir Permeability

Abstract

Permeability is an existential property in oil and gas reservoir production. Its value becomes even more important for the design of hydraulic fractures, in very wide use in the industry. Optimization of fracture geometry depends greatly on the permeability value. Methodologies such as the Unified Fracture Design cannot be applied properly in the absence of a reliable value of permeability. Massive amounts of income ride on the optimum deployment of fractures. While core and log-derived permeability values are often employed, they are usually unreliable and irrelevant to the forecast of well performance. Well-test-derived permeability is far superior but it suffers because of two reasons: 1) the well may not be flowing at all because of very small permeability and near-well damage and, 2) the test in an e.g., 0.1 md reservoir may require a week of duration, at least, just to reach infinite acting radial flow, whose appearance is necessary, for interpretation. The recent activity in shale formations has lowered the permeability values by orders of magnitude, towards nanodarcy (0.000001 md). This makes well tests even more unattainable. We are proposing here an alternative approach, the use of a fracture injection test (Minifrac) whose pressure falloff interpretation can lead to the traditional Nolte analysis providing the leak-off coefficient and closure pressure. However, because the overwhelming majority of DFIT fluids in low permeability formations is water with little if any polymer, the viscosity controlled and filter cake controlled components of the leak-off coefficient are non-existent. This suggests that only the compressibility controlled leak-off component dominates. This leak-off coefficient is directly proportional to the square root of reservoir permeability, whose calculation becomes readily available. In some ways what the energy of the minifrac does is to shock the formation, forcing back a rapid response, one that a well test cannot. Application of the technique to dozens of wells has led to very realistic permeability values which subsequently were corroborated by production data analysis. The technology and approach presented here can fill a major gap in proper fracture design and reservoir exploitation.