Abstract
AbstractProduction of oil and gas from sandstone reservoirs leads to small elastic and inelastic strains in the reservoir, which may induce surface subsidence and seismicity. While the elastic component is easily described, the inelastic component, and any rate‐sensitivity thereof remain poorly understood in the relevant small strain range (≤1.0%). To address this, we performed a sequence of five stress/strain‐cycling plus strain‐marker‐imaging experiments on a single split‐cylinder sample (porosity 20.4%) of Slochteren sandstone from the seismogenic Groningen gas field. The tests were performed under in situ conditions of effective confining pressure (40 MPa) and temperature (100 °C), exploring increasingly large differential stresses (up to 75 MPa) and/or axial strains (up to 4.8%) in consecutive runs. At the small strains relevant to producing reservoirs (≤1.0%), inelastic deformation was largely accommodated by deformation of clay‐filled grain contacts. High axial strains (>1.4%) led to pervasive intragranular cracking plus intergranular slip within localized, conjugate bands. Using a simplified sandstone model, we show that the magnitude of inelastic deformation produced in our experiments at small strains (≤1.0%) and stresses relevant to the Groningen reservoir can indeed be roughly accounted for by clay film deformation. Thus, inelastic compaction of the Groningen reservoir is expected to be largely governed by clay film deformation. Compaction by this mechanism is shown to be rate insensitive on production timescales and is anticipated to halt when gas production stops. However, creep by other processes cannot be eliminated. Similar, clay‐bearing sandstone reservoirs occur widespread globally, implying a wide relevance of our results.
Highlights
Extraction of oil and gas from sandstone reservoirs frequently leads to surface subsidence and induced seismicity (Suckale, 2009; Davies et al, 2013)
Using a simplified sandstone model, we show that the magnitude of inelastic deformation produced in our experiments at small strains (≤1.0%) and stresses relevant to the Groningen reservoir can be roughly accounted for by clay film deformation
We present a first attempt to quantify the inelastic strains expected to develop in clay‐bearing sandstone, deforming by consolidation of and slip on intergranular clay, for the stress changes applied in our Experiment 2 on Z1, and for the increase in effective stress representing that in the Groningen gas field
Summary
Extraction of oil and gas from sandstone reservoirs frequently leads to surface subsidence and induced seismicity (Suckale, 2009; Davies et al, 2013). Reservoir compaction is generally assumed to be poroelastic (Bourne et al, 2014; Dempsey & Suckale, 2017; Zbinden et al, 2017), reversible, rate insensitive, and quantified (Wang, 2000) Both field data (Santarelli et al, 1998) and experimental studies have shown that, even at the small reservoirs strains pertaining to hydrocarbon production, a significant part of the deformation behavior of sandstone reservoirs is inelastic (Bernabe et al, 1994; Schutjens et al, 1995; Hol et al, 2015, 2018; Yale & Swami, 2017; Pijnenburg et al, 2018, 2019).
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