- Title : Completion Technology for Unconsolidated Formations
- Publish : N/A
- Type Document : pdf
- Release : Juny 1995
- Total Page : 256 Page
- Size : 5.33 Mb
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Decrypted Contents
Nature of Sand Production
The conditions which can cause sand production and the probable condition of the formation outside of the casing after sand is produced can be determined by the factors that affect the beginning of sand production. These factors must describe both the nature of the formation material and also the forces that cause the formation structure fail. The strength of a sandstone is controlled by:
The conditions which can cause sand production and the probable condition of the formation outside of the casing after sand is produced can be determined by the factors that affect the beginning of sand production. These factors must describe both the nature of the formation material and also the forces that cause the formation structure fail. The strength of a sandstone is controlled by:
- The amount and type of cementing material holding the individual grains together
- The frictional forces between grains
- Fluid pressure within the pores of the rock
- Capillary pressure forces
The type of failure that is likely to occur in sandstone has been investigated by several researchers. Work at Exxon1 indicates that the nature of a failed perforation tunnel is indicative of a shear failure that will occur when the compressive strength of the rock is exceeded. In addition, the Exxon work indicates that in weakly consolidated sandstones, a void is created behind the casing. Exxon concluded that the rock’s compressive strength should be a good indicator of sand production potential, and that sand production will probably cause a void behind the casing that can be filled with gravel pack sand during a gravel packing operation. The details of the research work performed by Exxon may be found in Reference 1
In general, the compressive strength of a rock is primarily controlled by the intergranular frictional forces, therefore, the strength of the rock will increase as the confining stress on the rock increases. In the situation of failure of the rock matrix surrounding a perforation tunnel, the rock will be in an unconfined state of stress, so sand production should be related to the unconfined compressive strength of the rock. The degree of consolidation (intergranular cementation) will be more important than intergranular frictional forces. The stresses that cause the rock to fail in this situation include the mechanical stress resulting from the overburden material, and the drag forces associated with the flow of viscous fluids through the rock matrix. The overburden stress is partially supported by the pore pressure within the rock; so the stress actually working to cause failure of the rock (i.e., the effective stress) is the difference between the overburden stress and the pore pressure.
The mechanical failure of unconsolidated rock surrounding a perforation is analogous to the failure of a loose material surrounding a tunnel in soft earth. The mechanism for load transfer surrounding a tunnel in such a situation was described by Terzaghi2 in 1943. As the earth material over the tunnel yields, the stress originally held in the yielded material is relieved and transferred to the more rigid material surrounding the tunnel. However, a portion of the original stresses is supported by intergranular friction above the tunnel. In tunneling operations, if there is no intent to provide internal support to the tunnel, then the common practice is to excavate a tunnel height approximately twice the tunnel width to create a stable arch so that the material above the tunnel will not collapse (see Figure 2.1). The arch is made more stable through the presence of cohesive forces as well as from surface tension stresses if the granular material is wet.
An altered state of stress exists in the material above a tunnel. This altered state of stress extends to a height above the tunnel approximately five times the width of the tunnel. The material in the area that is more than five times the width of the tunnel base above the tunnel does not feel any of the effects of the excavation, and remains in its original stress state.
The mechanical failure of unconsolidated rock surrounding a perforation is analogous to the failure of a loose material surrounding a tunnel in soft earth. The mechanism for load transfer surrounding a tunnel in such a situation was described by Terzaghi2 in 1943. As the earth material over the tunnel yields, the stress originally held in the yielded material is relieved and transferred to the more rigid material surrounding the tunnel. However, a portion of the original stresses is supported by intergranular friction above the tunnel. In tunneling operations, if there is no intent to provide internal support to the tunnel, then the common practice is to excavate a tunnel height approximately twice the tunnel width to create a stable arch so that the material above the tunnel will not collapse (see Figure 2.1). The arch is made more stable through the presence of cohesive forces as well as from surface tension stresses if the granular material is wet.
An altered state of stress exists in the material above a tunnel. This altered state of stress extends to a height above the tunnel approximately five times the width of the tunnel. The material in the area that is more than five times the width of the tunnel base above the tunnel does not feel any of the effects of the excavation, and remains in its original stress state.
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