Effects of Faults and Fracture Corridors on Gas Productivity

By

 Laure Moen-Maurel¹, Dominique Etchegoyen¹, Sylvie Delisle¹

(1) Totalfinaelf, Pau, France

 Gas production from tight (3% porosity) carbonate reservoirs is possible only with the presence of a pervasive fracture network. Production simulation requires a modeling in which the fracture network is implicitly integrated. Mass transfer and depletion can be modeled, using lateral transmissivity variations where necessary. Barriers with decreased permeability would thus be zones of ineffective fracturing.

Even in case of weak aquifer activity, well production may rapidly be altered by water breakthrough. The case study shows erratic water production at depths that defy the gravity law of GWC uprise, and at pressures and times which lead to the certainty that vast reserves will remain inaccessible from the water-invaded wells.

In order to recover the gas, the challenge consists in localizing the faults and fracture corridors which are responsible for the water drainage into the wells, and to avoid triggering them (with side-tracks...). Results show that these water-prone corridors occur at various scales from a fault with evident throw to a localized fracture corridor made of closely spaced open joints. The physical connections to the aquifer must thus be evaluated, as wells located away may be protected.

Field examples of comparable tectonic history provide guidelines for the representation of the various scales of the fracture network, as well as rheologic laws (i.e. the relationship between the density of the brittle fabrics, the competency and the thickness of the reservoir).

Ahead of simulation the recognition of fabrics that may be responsible for water breakthrough is thus encouraged in a pluridisciplinary approach in order to optimize development operations.