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Keywords:

  • Heterogeneous;
  • Homogeneous;
  • Carbonates

ABSTRACT

Carbonate rocks are heterogeneous at various levels from deposition to diagenesis. Any existing depositional heterogeneity becomes more complex when carbonate rocks are in contact with polar fluids. Our experiments on carbonate rocks show that change in textural heterogeneity leads to heterogeneity in the distribution of storage and flow properties that may govern changes in saturation patterns. This would be akin to any carbonate reservoir with a mix of heterogeneous and homogeneous facies within a formation and their control on saturation distribution in response to a standard imbibition process. Associated with the saturation pattern heterogeneities, the resultant elastic property distributions also change. We quantify this heterogeneity and its effects on flow and seismic properties based on a few textural extremes of fabric heterogeneity in samples that can exist in any typical carbonate reservoir system. Our measurements show that textural heterogeneity can lead to anisotropy in permeability and in acoustic velocities. Permeability anisotropy measurements varied between 40% and 100% while acoustic velocity anisotropy measurements varied between 8% and 30% with lower values for homogeneous samples respectively. Under similar conditions of the saturation experiment (spontaneous imbibition at the benchtop and undrained pressure imbibition at 1000 psi), the imbibing brine replaced 97% of the pore volume in a homogeneous sample (porosity 20% and permeability 2.6 mD) compared to 80% pore volume in a heterogeneous sample (porosity 29% and permeability 23.4 mD). Furthermore, after pressure saturation, a change of +79% in the bulk modulus and -11% in the shear modulus is observed for homogeneous samples in comparison to +34% in the bulk modulus and −1% in the shear modulus for heterogeneous samples, with respect to the dry state moduli values of the samples. We also examined the uncertainties associated with Gassmann models of elastic properties due to variations in fluid saturations.

Our results provide significant information on the saturation and, with it, modulus variations that are often ignored during fluid substitution modelling in time-lapse seismic studies in carbonate reservoirs. We show that the bulk modulus could vary by 45% and the shear modulus by 10% between homogeneous and heterogeneous patches of a reservoir after a water flooding sequence for secondary recovery. Our findings demonstrate the need to incorporate and couple such fabric-controlled saturation heterogeneities in reservoir simulation and in seismic fluid substitution models.