Abstract
Tight gas is one important unconventional hydrocarbon resource that is stored in tight sandstone, whose mechanical property greatly influences the tight gas production process and is commonly believed to be simply elastic when designing the stimulation plan. However, the experimental evidence provided in this work surprisingly shows that tight sandstone can deform in a viscoelastic way. Such an unexpected observation poses a challenge in accurately modelling the deformation process. We solve this problem by adopting the fractional Maxwell model to successfully derive the constitutive equation of tight sandstone, based on which not only all the experimental data can be interpreted quantitatively, but also reasonable and consistent predictions as to tight sandstone’s long-term deformation behaviour can be made. We then investigate the typicality of our results in China’s Changqing oilfield, which is one major centre of tight gas production and where the rock samples for experiments are obtained. It is estimated that a non-negligible portion of 18% tight sandstone samples in this area will probably display viscoelasticity. Finally, our work implies that the mechanical properties of other materials may also need further scrutiny to possibly uncover any unexpected behaviour, overlooking which may result in misleading predictions.
Highlights
Tight gas is one important unconventional hydrocarbon resource that is stored in tight sandstone, whose mechanical property greatly influences the tight gas production process and is commonly believed to be elastic when designing the stimulation plan
We perform a series of creep experiments for tight sandstone samples, which are obtained from the Changqing oilfield, one of the major tight gas development centers in China
Our result surprisingly shows that an unexpected viscoelastic deformation can be observed for some of our rock samples, indicating the failure of the elastic assumption
Summary
Tight gas is one important unconventional hydrocarbon resource that is stored in tight sandstone, whose mechanical property greatly influences the tight gas production process and is commonly believed to be elastic when designing the stimulation plan. The exact underlying physical mechanism is still elusive to date (due to the notoriously complex nature of tight sandstone as a kind of natural composite materials usually with the presence of inhomogeneity and anisotropy), we believe the phenomenological fractional Maxwell model serves well as a way to understand the mechanical response of tight sandstone We use this model to predict the strain rate and the stress relaxation of rock samples in the long run, which are of great importance in assessing to what extent the fracture closure will be, and it turns out that such predictions are fully consistent with previous results. More testing experiments performed to other kinds of reservoir rock are suggested to possibly uncover some counter-intuitive property just like tight sandstone’s viscoelasticity, which can be pivotal for an accurate long-term prediction of oil recovery, but has long been, and will otherwise continuously be overlooked by both academia and industry
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