Abstract

Laboratory experiments on samples from a consolidated sandstone reservoir are presented that demonstrate rate type compaction behaviour similar to that observed on unconsolidated sands and soils. Such rate type behaviour can have large consequences for reservoir compaction, surface subsidence and induced seismicity resulting from oil and gas production. During traditional single loading rate laboratory experiments, the effect of loading rate on uniaxial compressibility can be ignored to first approximation. In contrast, strongly non-linear compaction occurs when stepwise increases or decreases in loading rate are applied within a loading cycle, during loading after creep or during loading after partial unloading. A modified version of the RTCM rate type compaction model, built on Dieterich’s rate and state friction equation describes the experimental results well. The new isotach version solves a number of limitations of the original model at the cost of an additional parameter that specifies the split between direct (elastic) and secular (creep) strain. The good fits between the updated model and the experimental data suggest that rate and state friction - e.g. on sliding contact surfaces between assemblies of cemented grains or on sliding micro-cracks - causes the observed non-linear compaction. The same rate and state friction phenomena have been linked to the non-linear rate-dependent earthquake generation in volcanism-induced seismicity, suggesting similar rate dependence for the induced seismicity in depleting oil and gas fields.

1. INTRODUCTION

Production of gas from the large Groningen field in the Netherlands causes surface subsidence as predicted prior to the start of production [1]. Above the centre of the field the subsidence is now some 35 cm, important for a country like the Netherlands which would largely flood without the presence of dykes and active pumping. As for a number of other oil and gas fields [2, 3] subsidence in Groningen was initially much lower than expected based on measurements carried out on samples taken from the reservoir. As for other fields the subsidence later accelerated, coming closer to values expected and leading to the notion of "delayed subsidence" [2]. During the stage of accelerated subsidence, shallow earthquakes started to occur in Groningen that have been increasing in numbers and strength over time (Fig. 1.). An example is the strongest tremor so far with a magnitude of 3.6 that occurred near the village of Huizinge in 2012, causing damage to thousands of homes and raising anxiety among those living on or near the field [4]. The mechanism behind the earthquakes is understood to be (differential) compaction at reservoir level reactivating larger faults with substantial offset [5]. Better understanding of the physics of these processes is important to predict the further evolution of the Groningen subsidence and to find out if it is possible to influence the level of seismicity by production changes. Rate and state friction has been proposed as an explanation for the observed non-linear compaction behaviour of the Groningen field [6, 7]. The same rate and state friction phenomena have been linked to rate-dependent earthquake generation [8, 9]. Hence a number of laboratory experiments, previously carried out to study the effects of changes in loading rate on the compaction behaviour of Groningen reservoir rock, have been re-analysed using a modified (isotach) formulation of the rate type compaction model [10] with the hope to derive new insights into the physics of reservoir compaction and induced earthquake generation.

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