Changes

Jump to navigation Jump to search
Line 176: Line 176:  
In stress-sensitive reservoirs, fractures may dilate during injection and close during drawdown. These effects are most pronounced in low-permeability, overpressured, and naturally fractured reservoirs.<ref name=Lorenz_1999 /> Pressure depletion as a result of production will change the stress state of a reservoir (e.g., Hillis<ref name=Hillis_2001 />).
 
In stress-sensitive reservoirs, fractures may dilate during injection and close during drawdown. These effects are most pronounced in low-permeability, overpressured, and naturally fractured reservoirs.<ref name=Lorenz_1999 /> Pressure depletion as a result of production will change the stress state of a reservoir (e.g., Hillis<ref name=Hillis_2001 />).
   −
From a mechanical aspect, sandstone reservoirs are porous structures that form a load-bearing framework supporting the weight of the overburden. Reservoir depletion increases the effective stress on the grain framework; this is the difference between the total stress acting on all sides of the rock and the pore fluid pressure. The effective stress is applied at the grain to grain contacts. This leads to elastic deformation of the rock (recoverable on depletion reversal) and, with increasing stress, inelastic deformation. Inelastic deformation mechanisms include microcrack growth and closure, cement breakage, grain rotation, and sliding as well as deformation in clay, mica, and diagenetically altered feldspar grains (Bernabe et al., 1994; Schutjens et al., 1998, 2004; Wong and Baud, 1999). These mechanisms result in the compaction of the rock and a reduction in the porosity. Because the reservoir remains physically connected to the rock surrounding it, the overburden and underburden will also deform in response to reservoir depletion.
+
From a mechanical aspect, sandstone reservoirs are porous structures that form a load-bearing framework supporting the weight of the overburden. Reservoir depletion increases the effective stress on the grain framework; this is the difference between the total stress acting on all sides of the rock and the pore fluid pressure. The effective stress is applied at the grain to grain contacts. This leads to elastic deformation of the rock (recoverable on depletion reversal) and, with increasing stress, inelastic deformation. Inelastic deformation mechanisms include microcrack growth and closure, cement breakage, grain rotation, and sliding as well as deformation in clay, mica, and diagenetically altered feldspar grains.<ref name=Bernabeetal_1994 /> <ref name=Schutjensetal_1998 /> <ref name=Schutjensetal_2004 /> <ref name=Wongandbaud_1999 /> These mechanisms result in the compaction of the rock and a reduction in the porosity. Because the reservoir remains physically connected to the rock surrounding it, the overburden and underburden will also deform in response to reservoir depletion.
 
  −
Compaction can lead to the reactivation of normal faults (Teufel et al., 1991; Goulty, 2003). In the Valhall and Ekofisk fields, offshore Norway, faults that were initially located in the crest of the field's anticlinal structure are thought to have spread out to the flanks as a result of reactivation induced by depletion and compaction of the Chalk reservoir. Casing failures have been attributed to shear along these spreading faults (Zoback and Zinke, 2002). Small earthquakes can be common around some producing oil and gas fields (Segall, 1989).
  −
 
  −
It is common to find that faults that were sealing over geological time in a reservoir start to leak after a few years of production. This may be noticed where a production anomaly occurs, such as newly drilled attic oil wells showing swept zones; a sudden, unexpected rapid rise in water or hydrocarbon production from production wells drilled close to faults; or an inexplicable source of pressure support appearing in the mid life of a producing well. In one example from the Endicott field in Alaska, a major sealing fault within the reservoir was known to act as a pressure barrier from early production data. Later on, it was established that radioactive tracer had crossed the fault from an injection well to a production well, and this indicated that the fault seal had broken down with production (Shaw et al., 1996).
  −
 
  −
Dincau (1998) analyzed fault breakdown with production in the South Marsh Island 66 field, offshore Louisiana. The faults most likely to break down were those with limited predicted shale gouge and where the reservoir unit was fault juxtaposed against itself. Faults with an extensive predicted shale gouge and where they juxtapose one reservoir unit with a different unit were more likely to hold a pressure differential.
  −
 
  −
Examples of fault breakdown are often mentioned as a side issue in technical papers dealing with other aspects of field production. These include the Iagufu-Hedinia area of Papua New Guinea (Eisenberg et al., 1994), the Tia Juana field in Venezuela (Marquez et al., 2001), and the Veslefrikk field, offshore Norway (Pedersen et al., 1994). Fault breakdown is often attributed to the breaching of the capillary seal of the fault rock as a result of large differences in pressure across the fault. It is also possible that in some instances, fault breakdown is the result of fault reactivation induced by differential compaction between adjacent fault compartments, one significantly more depleted than the other. It is possible that the phenomena could be more common in depleting fields than is generally appreciated.
  −
 
  −
 
  −
 
  −
 
  −
 
      +
Compaction can lead to the reactivation of normal faults.<ref name=Teufeletal_1991 /> <ref name=Goulty_2003 /> In the Valhall and Ekofisk fields, offshore Norway, faults that were initially located in the crest of the field's anticlinal structure are thought to have spread out to the flanks as a result of reactivation induced by depletion and compaction of the Chalk reservoir. Casing failures have been attributed to shear along these spreading faults.<ref name=Zobackandzinke_2002 /> Small earthquakes can be common around some producing oil and gas fields.<ref name=Segall_1989 />
    +
It is common to find that faults that were sealing over geological time in a reservoir start to leak after a few years of production. This may be noticed where a production anomaly occurs, such as newly drilled attic oil wells showing swept zones; a sudden, unexpected rapid rise in water or hydrocarbon production from production wells drilled close to faults; or an inexplicable source of pressure support appearing in the mid life of a producing well. In one example from the Endicott field in Alaska, a major sealing fault within the reservoir was known to act as a pressure barrier from early production data. Later on, it was established that radioactive tracer had crossed the fault from an injection well to a production well, and this indicated that the fault seal had broken down with production.<ref name=Shawetal_1996 />
    +
Dincau<ref name=Dincau_1998 /> analyzed fault breakdown with production in the South Marsh Island 66 field, offshore Louisiana. The faults most likely to break down were those with limited predicted shale gouge and where the reservoir unit was fault juxtaposed against itself. Faults with an extensive predicted shale gouge and where they juxtapose one reservoir unit with a different unit were more likely to hold a pressure differential.
    +
Examples of fault breakdown are often mentioned as a side issue in technical papers dealing with other aspects of field production. These include the Iagufu-Hedinia area of Papua New Guinea,<ref name=Eisenbergetal_1994 /> the Tia Juana field in Venezuela,<ref name=Marquezetal_2001 /> and the Veslefrikk field, offshore Norway.<ref name=Pedersenetal_1994 /> Fault breakdown is often attributed to the breaching of the capillary seal of the fault rock as a result of large differences in pressure across the fault. It is also possible that in some instances, fault breakdown is the result of fault reactivation induced by differential compaction between adjacent fault compartments, one significantly more depleted than the other. It is possible that the phenomena could be more common in depleting fields than is generally appreciated.
    
==See also==
 
==See also==

Navigation menu