Abstract

This paper describes an inverse problem for compaction analysis by implementing a recently developed constitutive model in 1-D simulations. The paper uses this effective constitutive model to account for strain rate dependent reservoir compaction during depletion, re-pressurization and water-flooding. By designing an inverse problem, we present a workflow to assess whether it is possible to estimate reservoir properties based on sparse measured data in off shore hydrocarbon reservoirs. To this purpose, a multi-layered reservoir model from the Dan field with various material properties under overburden stress is simulated as an objective model. The methodology is to solve an inverse problem to optimize the elasto-plastic material properties by means of history matching. The data used for the history matching process includes vertical displacement monitored with GPS survey at the top layer. By minimizing the error between simulation and objective data, the simulator optimizes the mechanical material properties of each layer such as the elastic modulus, pore collapse stress and water weakening effect with an effective procedure. It will be presented how the model matched the observed data from the field.

Introduction

As pressure changes during the production of hydrocarbon from reservoirs, chalk may compact due to change in in situ stresses and saturating fluids. Subsurface deformation is transferred to the sideburden, overburden and earth surface, resulting into wellbore instability and sea-floor subsidence. The mechanical properties of the reservoir rock need to be determined to accurately predict field deformation in 3-D by using geomechanical models and, ultimately, to reduce associated costs and risks. Triaxial tests such as uniaxial/hydrostatic compaction tests are commonly carried out in the laboratory to quantify these mechanical properties (Havmøller & Foged, 1996; Johnson et al., 1989). Triaxial tests use x1 cm-long core plugs and are designed to characterise one single chalk lithology or reservoir rock type. Flaws such as fractures, stylolites and clay seams are commonly avoid when selecting core plugs for testing. However, reservoirs are geologically heterogeneous with vertical and lateral changes in petrophysical and lithological properties that are not investigated in laboratory. Moreover, upscaling technics for e.g. porosity and permeability from the geo-statistical description to reservoir grid are necessary to use laboratory data acquired at the cm-scale for reservoir modeling carried out at the km-scale. Studies on the upscaling methods for geo-mechanical properties are still scarce in literature, and when available, those studies have mainly focused on the elastic properties (Hu et al., 2009, Fokker, 2012 and Settari et al., 2013). With advances in field of coupled reservoir and geo-mechanical modeling, guidelines for the upscaling of other mechanical properties related to plasticity are required to assign parameters at the grid scale of field simulators.

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