Global Model for Fracture Falloff Analysis

Abstract

Hydraulic fracturing is undeniably the crucial technology involved in the development of tight and/or unconventional gas reservoirs. The fracture geometry and pumping execution, as well as the well architecture, can be designed to maximize the well productivity, provided the reservoir permeability is known. Because it is difficult to estimate permeability from conventional pressure transient tests, various authors have shown how fracture-injection/falloff tests designed for final fracture design calibration (i.e. estimation of closure stress, leak-off coefficient and fracture fluid efficiency) can be used to estimate reservoir permeability as well.This paper presents a new comprehensive model for the analysis of fracture falloff, following a step-rate or constant rate injection test. The derivative of the falloff pressure change with respect to the logarithm of superposition time shows a progression of straight trends that include a newly introduced elastic closure-dominated flow enabling determination of the closure stress (3/2 slope), post-closure formation-linear flow (1/2 slope) and infinite-acting pseudo-radial (0 slope). This model provides a robust complete assessment tool that allows quantification of all fracture parameters (closure stress, fracture extent, leak-off coefficient and fracture fluid efficiency) as well as reservoir permeability, provided that enough time is allowed for the falloff to reach pseudo-radial flow regime. Both oil and gas reservoirs can be effectively evaluated.While the approach is leveraging well-accepted pressure transient analysis methods, the inclusion of elastic closuredominated flow is new. This model can be used to design fracture-injection/falloff tests that would allow determination of all the involved parameters, including reservoir permeability. Field data validate the model and demonstrate the value of this approach.

Effect of natural fractures on determining closure pressure

“…G-function, log-log (diagnostic) and Bourdet plots with derivatives taken with respect to shut-in time, superposition time and time-integrated pressure are methods that can be used to interpret the pressure transient behavior and flow type (Barree et al 2009(Barree et al , 2013Bourdet et al 1989;Marongiu-Porcu et al 2011). Plots of pressure versus the square root of time and G-function are two major methods for diagnosing fracture-closure pressure (Nolte 1997;Nolte and Smith 1981).…”

Permeability Estimation from Fracture Calibration Test Analysis in Shale and Tight Gas

“…(Figure 2. 1) In tight gas or shale gas formation the short and low rate injection-fall off test using slick water as injection fluid is favored because slick water is the injection fluid for the main fracture treatment and because the closure time increases with increased injection volume (Marongiu-Porcu et al 2011). Sometimes, a step rate test is performed for a prior estimation of the closure pressure, p c , which can be used to ensure that shut-in period following injection is monitored longer than the complete time of fracture closure.…”

“…featuring a specific flow regime to analyze various elements of the falloff behavior. The exception is the approach byMarongiu-Porcu et al (2011) that uses the same log-log diagnostic plot used for standard pressure transient analysis. The next chapter will show how the same plot reveals the behavior described in this chapter and displays the important trends in a global context that avoids erroneous parameter estimations based on apparent straight lines on specialized plots.…”

Fracturing unconventional formations to enhance productivity

“…Mohamed et al (2011) recently developed a third approach. Marongiu-Porcu et al (2011) expanded on Mohamed's approach.…”