Characterization and Modelling of Increasing Permeability While Producing a Gas Fractured Reservoir
Authors: Rolando J.-P. , Massonnat G.J. , Grasso J-R. , Odonne F. , Meftahi R. 
Published: December 2007

The 40-year period of production of the huge gas field of Lacq Profond (France) has resulted in a depletion of approximately 600 bars, a considerable figure among the highest in the hydrocarbon production world history.

Surprisingly, together with this depletion, a significant increase in productivity was recorded at the wells over the same period (20 times more). This phenomenon identified by interpreting the data obtained from well testing can be explained by an increase in permeability during the field production phase.

This effect is surprising insofar as the decrease in fluid pressure in the pore volume causes the actual stress to be increased, and this should compact the pore network and thereby reduce permeability.

This phenomenon is explained by the poro-elastic deformation suffered by the Lacq reservoir since production was initiated and evidenced by taking into account the microseismic activity within the field.

This paper gives an original method to characterize and model the fracture network, explain the increase in permeability with field production and the way this phenomenon is used for flow simulation and matching the 40-year production history.

Introduction

Usually, in the absence of pressure maintenance (active aquifer or injection), production of hydrocarbons causes pore pressure to decrease within the reservoir, and thereby actual stress to increase. This entails a reduction of pore threshold and absolute permeability. Surprisingly, in the huge gas field of Lacq Profond (France), a sharp increase in productivity has been recorded from well testing on most wells over a 40-year period of production.

The reasons for this phenomenon have been discussed and the increase in absolute permeability with depletion is undoubtedly selected and correlated with the earthquakes recorded within and in the vicinity of the reservoir.

Conventional approaches for characterizing and modelling a fractured reservoir always come up against the two following difficulties : identifying the fractures at the wells, and extrapolating the well data to a 3D model.

Generally, the indirect data from processes such as mud-logging, production or injection logging and well testing are more appropriate than the methods consisting in identifying the fractures at the wells (from cores and wireline logs) for accurately describing the fracture system reservoir properties. In the gas-bearing field of Lacq Profond a lot of data of this kind are available from the wells and used to develop a new method providing different fracture indexes. These indexes allow the required dual porosity parameters to be quantified and cross-correlated.

Extrapolating well data to a 3D model is generally supported by the conceptual structural model, seismic interpretation and production history analysis. In the case of the Lacq field, the monophasic production of gas related to the depletion does not make it possible to define guidelines for building a 3D reservoir model. Nevertheless, the 20-year surface seismic monitoring of the Lacq gas field gives a unique opportunity for analysing the impact of induced earthquakes (related to poroelastic deformation) on reservoir behavior and building a 4D reservoir model.

Setting

The gas field of Lacq Profond has produced more than 220. 10 9 Sm3 of gas since 1957, with a resulting depletion of approximately 600 bars.

The field is a carbonate reservoir of the Portlandian (Uppermost Jurassic) to Barremian (Lower Cretaceous) stages.

The field is a large west-east-oriented anticline (Fig. 1) patterned by the Pyrenean stage. The northern portion is a uniform-dip monocline, while the southern part of the field is extensively patterned with west-east-oriented compressive bands.

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