ABSTRACT:

Here we present the results of a geo-mechanical study combined with a new way of 4D interpretation, applied to a depleting offshore gas condensate field. The 4D interpretation uses the predicted stress changes from the geo-mechanical study, by bringing them to the seismic domain in the form of 'synthetic' time-shifts. It is demonstrated that, by comparing measured time-shifts with these 'synthetic' time-shifts, discrepancies in geomechanics (e.g. in the rock properties and formation stiffness contrasts) and reservoir dynamics modeling (e.g. undepleted pockets and activity of waterdrive) become much better traceable. The rock properties and the pressure distributions were then iteratively calibrated and harmonized with the measured 4D and other field observations (GPS and compaction-log). The applied workflow reduced uncertainties in forward predictions of stress changes from the geo-mechanical model in and around the compacting reservoir, and enabled the design of a new strategy for sand management in the field.

1. INTRODUCTION

In petroleum engineering, a geo-mechanical field evaluation focuses on understanding the subsurface mechanical in-situ conditions and the changes in the subsurface induced by pressure depletion or pressure maintenance. Given the structural complexity of certain fields, numerical modeling techniques are nowadays a preferred tool to predict the induced changes in stresses, strains, properties and failure condition of the rock formations.

  • Top-seal integrity.

  • Subsidence or uplift of the seabed.

  • Fault slip.

  • Borehole instability.

  • Risk of sand or fines production.

  • Completion failure.

  • Undepleted reservoir pockets.

  • Sealing or non-sealing faults.

Depending on the problem(s), anticipated the geo-mechanical forward model predictions are used to address specific aspects that impact the economy of the field at different stages. In the e a geo-mechanical evaluation focuses on e.g.: At the e a geo-mechanical assessment looks at e.g.:However, the use of geo-mechanical modeling also provides a new opportunity [1, 2], by incorporating geo-mechanical results of the whole earth [3] in the time-lapse seismic (4D) monitoring of producing oil and gas fields. It can enhance the current reservoir technologies to better detect e.g.: Here we present the workflow of a geo-mechanical study combined with this new way of 4D interpretation, applied to an offshore gas condensate field [4].

2. GEO-MECHANICS AND 4D

The structural geometry of the field, the initial stress state, the formation rock properties and the pressure depletion (or injection) scenario from the dynamic reservoir simulator, are the primary unknowns in the geo-mechanical field equations for calculating production induced stress changes (¿

s

).

2.1. Compaction and Subsidence

The subsurface is subjected to the total stress (

s

) of the overburden and the horizontal tectonic stresses, which are carried in the rocks by effective stress (

s

eff) on the grain-to-grain contacts and the fluid pressure (p) in the pores:

s

eff=

s

-ap

I

(1)

where, a and

I

, denote the coefficient of Biot and the unit tensor, respectively.

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