Deformation Experiments on Bowland and Posidonia Shale\u2014Part II: Creep Behavior at In Situ pc\u2013T Conditions

J Herrmann,G Dresen,E Rybacki,H Sone

doi:10.1007/s00603-019-01941-2

Abstract

To unravel their long-term creep properties at simulated reservoir conditions, we conducted constant stress deformation experiments at elevated confining pressures, pc = 50–115 MPa, and temperatures, T = 75–150 °C, on Posidonia (GER) and Bowland (UK) shale, which exhibit varying petrophysical and mechanical properties. Depending on applied pc–T conditions and sample composition, recorded creep curves exhibit either only a primary (decelerating) or additionally a secondary (quasi-steady state) and a tertiary (accelerating) creep phase during deformation. At high temperature and axial differential stress and low confining pressure, creep strain is enhanced and a transition from primary towards secondary and tertiary creep behavior is observable. Creep strain of Posidonia shale, which is rich in weak constituents (clay, mica, and organic content), is enhanced when compared to creep strain recorded during deformation of either carbonate- or quartz-rich Bowland shale. Electron microscopy observations revealed that creep strain is mainly accommodated by the deformation of weak minerals and local pore space reduction. In addition, microcrack growth occurred during secondary creep. An empirical correlation between creep strain and time based on a power law was used to describe the decelerating creep phase, also accounting for the influence of confining pressure, temperature, and axial differential stress. The results suggest that the primary creep strain can be correlated with mechanical properties determined from short-term constant strain rate experiments such as static Young’s modulus and triaxial compressive strength.

Highlights

Energy consumption depends largely on conventional and unconventional resources, e.g., coal, oil, or gas (Kuchler 2017)
During 42 experiments, we fixed two of the three parameters pc, T, and σ and changed the remaining third parameter within a given range to investigate the influence of this particular parameter on the resulting creep strain behavior (Table 2)
Constant stress experiments performed on Posidonia (HAR) and Bowland (OC) shale reveal time-dependent creep behavior characteristic of semibrittle deformation, influenced by confining pressure, temperature, and applied stress

Summary

Introduction

Energy consumption depends largely on conventional and unconventional resources, e.g., coal, oil, or gas (Kuchler 2017). As the permeability of shales is very low (e.g., Naumann et al 2007; Holt et al 2012; Villamor Lora et al 2016), reservoir stimulation using hydraulic fracturing is needed to increase well productivity and allow economical exploitation (Li et al 2015). Recorded production curves of fractured wells typically exhibit a rapid decline within the first few years (Hughes 2013; Wang 2016; Al-Rbeawi 2018) as a result of reservoir depletion and fracture closure due to time-dependent proppant embedment (Sone and Zoback 2014; Wang 2016; Cerasi et al 2017). Fracture closure is influenced by temperature (Johnston 1987; Brantut et al 2013; Masri et al 2014), confining pressure (Niandou et al 1997; Petley 1999; Naumann et al 2007; Kuila et al 2011; Islam and Skalle 2013; Brantut et al 2013), stress conditions (Swan et al 1989; Chong and Boresi 1990; Ibanez and Kronenberg 1993; Kwon and Kronenberg 1994; Brantut et al 2013; Sone and Zoback 2013a; Rybacki et al 2015, 2017), and mechanical and petrophysical properties (Brantut et al 2013; Rybacki et al 2016; Cerasi et al 2017; Morley et al 2017; Teixeira et al 2017)

Methods

Results

Discussion

Conclusion