The application of horizontal drilling has made exploitation of oil from low matrix permeability, naturally fractured reservoirs feasible. However, enhancing recovery from this reservoir type presents a distinctive problem. The low porosity and permeability matrix is virtually unaffected by the fluids injected into the extremely high permeability fractures. These fractures are multi-leveled and range from large scale macro-to small scale micro-fractures.

The effects that the presence of fractures on oil recovery due to water imbibition was studied. Both unadulterated and carbonated water imbibition tests were conducted on Indiana Limestone cores confined by a fluid pressure of 800 psi. Water was enriched with CO2 at a 500 psi carbonation pressure for the carbonated water cases. Magnetic Resonance Imaging (MRI) methods were used to study the effects that micro-fractures have on their surrounding rock matrix during oil production by water imbibition.

The presence of micro-fractures revealed that several factors may assist oil displacement:

  • Capillary forces drive water into the microfractures

  • Increase in the surface area available for water imbibition

  • Oil trapped in fractures can be displaced by this process.

  • An adequate wettability facilitates the process

Inclusion of CO2 improved oil recovery by:

  • Increasing the oil mobility

  • A solution gas drive induced by the evolving CO2 when the system pressure is decreased below the carbonation pressure.

The presence of micro-fractures wide enough to allow appreciable fluid movement, increased oil production when compared to production from homogeneous rock. Imaging studies showed water channels were formed within the fracture system.

The presence of micro-fractures used the beneficial effects of capillary forces within the fracture system to create fluid paths that assist oil production. CO2 dissolved into the water being imbibed by the rock was found to accelerate oil recovery rates and increase ultimate oil recovery when compared to unadulterated water imbibition.

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