Multiphysics Investigation of Diagnostic Fracture Injection Tests in Unconventional Reservoirs

Abstract

Diagnostic fracture injection tests (DFIT) are commonly used to characterize stress and reservoir properties in unconventional reservoirs. Although simple in concept, interpreting DFIT results can be difficult because several factors can cause results to deviate from ideal DFIT behavior. Some examples of nonideal DFIT behavior include steep pressure declines after shut-in, absence of pseudolinear and pseudoradial flow, and excessive storage indications. Such deviations from ideal DFIT behavior challenge our ability to estimate formation properties reliably. Potential drivers of nonideal behavior include heterogeneous rock properties, complex rock/fluid interaction, thermal effects, phase entry, and natural fractures. The objective of this study is to investigate how these factors can impact DFIT results and interpretations. A comprehensive approach was taken using a combination of pressure transient analysis, frac modeling, analytical leakoff modeling, and detailed numerical simulation of DFIT behavior. The application was for a horizontal well, completed in a shale gas reservoir, which included an actual field DFIT. Detailed modeling included full wellbore transients and storage, hydraulic behavior through induced fractures, as well as complex interactions between rock, fluids, and natural fractures. It was determined that the actual DFIT showed indications of a complex network created by the pump-in. Closure pressure estimates were found to be reliable, between the simulation cases and DFIT analysis. However, a consistency check on initial reservoir pressure had to be used to obtain reasonable estimates compared to the simulation input. Finally, estimates of reservoir conductivity were highly uncertain compared to the actual simulation inputs. Furthermore, the actual DFIT estimates of reservoir pressure and conductivity were found to be overoptimistic. Estimated ultimate recovery using DFIT-based estimates of reservoir quality was shown to be more than twice the estimated ulterimate recovery using reservoir quality estimates made with integrated core, log, and a pressure build-up test.

Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis

“…The clear closure signals of intersected natural fractures on the G-function plots have been observed in numerous other field studies and numerical models (Wallace et al 2014;Jung et al 2016;Nadimi et al 2020;Kamali and Ghassemi 2019;Wang and Sharma 2019). The interaction of the propagating hydraulic fracture with differently oriented natural fractures will cause changes in the pressure transient, as the closure of reactivated natural fractures will change the system stiffness/compliance and, if hydraulic connectivity is lost, the leak-off surface area.…”

Re-examining interpretations of non-ideal behavior during diagnostic fracture injection tests

“…This behavior is an example of the "multiple closures" that have been observed in field DFITs (Mohamed et al, 2011;Wallace et al, 2014).…”

“…These interpretations of non-ideal transients were developed based on published work by Barree and Mukherjee (1996) and Barree et al (2007) and are widely used in DFIT interpretation (Soliman and Kabir, 2012;Soliman and Gamadi, 2012;Cramer and Nguyen, 2013;Padmakar, 2013;Wallace et al, 2014;Meng et al, 2014;Barree et al, 2014).…”

Hydraulic Fracture Propagation in a Poro-Elastic Medium with Time-Dependent Injection Schedule Using the Time-Stepped Linear Superposition Method (TLSM)

“…In unconventional exploration, diagnostic fracture injection tests (DFIT) have become a primary method to determine key reservoir parameters-for later use in hydraulic fracture treatment operations-such as the fluid efficiency, minimum in situ stress, pore pressure, leak-off coefficient, and fracture gradient. DFITs are typically completed in short time-frames spanning a mere couple of days, and commonly less than two weeks, which is significantly shorter as compared to the traditional pressure build-up tests, which could last several weeks due to the requirement of stable-flowing conditions, before the start of the shut-in period [28]. DFITs involve the injection of a relatively small fluid volume, typically at a first stage in the toe of a newly drilled horizontal well, with a constant injection rate to obtain information about the hydraulic fracture treatment parameters as well as the reservoir.…”