Applications of Pre-Frac Injection/Falloff Tests in fissured Reservoirs - Field Examples

Abstract

Abstract A method has been developed, based on the pressure versus 6-function curve and its derivatives, to characterize the pressure dependent behavior of formations from injection fall off test data. Characteristic shapes of the derivative curves differentiate between various closure mechanisms including pressure dependent leakoff associated with fissure opening. The dominant closure mechanism has been found to influence the treating behavior of the formation and help define the optimum fracture treatment design for the interval. In this paper, field examples of each major type of closure mechanism are presented along with the fracture treating response observed. The impact of natural fractures on production and injection will also be discussed. Introduction This paper is an extension of work originally presented at the SPE Annual Conference and Exhibition in 1996. The previous paper described the expected pressure falloff response of the 6-function and its derivatives, for pressure dependent leakoff and other factors affecting rate of fracture closure. The derivative response was predicted using the GOHFER fracture design simulator. This work adds to our understanding of pressure falloff behavior through the acquisition and analysis of extensive field data examples. In the pioneering work by Nolte in 1979, fracture pressure decline analysis was proposed as a key to successful fracture stimulation treatment design. The method proposed was to inject a small fluid volume at constant rate and analyze the pressure decline to determine the fluid efficiency and pad volume requirement for achieving the desired fracture length. This technology appeared to offer a key to improved stimulation design. By 1985 it was realized that this method, while very successful in many instances, did not generally identity wells which would be very difficult to hydraulically fracture. Many screenouts occurred which were not predicted by the existing pressure falloff analysis techniques. One possible cause of the noted failures was suspected to be pressure dependent leakoft, or fluid loss which increases with treating pressure.