P.M.T.M. Schutjens, SPE (1, T.L. Blanton, SPE (2, J.W. Martin (3, B.C. Lehr (1 and M.N. Baaijens (4

Abstract

Laboratory experiments were done to investigate compaction of core taken from a 5 km deep, highly overpressured sandstone reservoir as a function of depletion, stress path, and time. Guided by a numerical analysis of the reservoir compaction and associated total stress change, a best-estimate and an unfavourable reservoir stress path were applied. Most samples showed a linear relationship between axial strain and total axial stress up to 65 MPa, which reflects the target depletion level. However, high-porosity samples compacted following the unfavourable reservoir stress path showed linear compaction up to only 52 MPa increase in total axial stress (i.e. up to 80% of target depletion). Non-linear (accelerating) compaction occurred in nearly all high-porosity samples with increasing total axial stress; total strains reached 4 to 23 ms after 65 MPa increase in total axial stress, typically followed by 20 to 50 axial ms creep at time periods of 11 to 3 weeks. Microstructural analysis revealed that the non-linear compaction is due to an increasing activity of grain-scale brittle deformation with increasing stress, which gradually leads to pervasive microfracturing including numerous intraand transgranular microcracks, collapse of weak minerals and grain size reduction. No strain localisation occurred. Analysis of the experimental data suggest that high-porosity layers loaded following the unfavourable stress path could show vertical compactions in the range 50 ms to 100 ms should they be depleted by more than 52 MPa. Monitoring of reservoir compaction and field stress path evolution is therefore advised.

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