Experiences in Permeability Calculations in the Obayied Field (Egypt) Using Magnetic Resonance Imaging

Abstract Permeability is a key factor in well completion in many reservoirs all over the world. The dependency of permeability on the accurate determination of reservoir total and effective porosity, irreducible and movable fluids and the correct correlation between the pore throat and the pore body made it a real challenge. The capability of the Nuclear Magnetic Resonance to provide effective and total porosity, capillary bound fluids and movable fluids provides means for better estimation of rock permeability. Magnetic Resonance Imaging Log (MRIL) was run in two cored wells in Obaiyed field in Egypt to determine the variation in rock permeability to better design well completion program and select best perforation intervals. Obaiyed field is a gas sandstone reservoir with low porosity ranging between 5 to 10 porosity units. Obtaining a reliable permeability in this reservoir faced two challenges; the first due to the reservoir heterogeneity while the second due to the drilling mud. The reservoir rock is heterogeneous and varies from very fine, fine to coarse-grained sandstone. The coarse grained sands represent the high permeability zones, which are imbedded within the fine-grained sand. The key issue in this reservoir is to determine the locations of these high permeability zones within the fine-grained sand beds for well stimulation planning. The determination of the T2 cut-off for both the fine and the coarse grained sands, which vary with depth due to the reservoir heterogeneity, generated a great challenge in determining the formation permeability. A function depending on an average of the T2 distribution is developed to provide the segregation. The wells in Obaiyed field are drilled with oil base mud with very long relaxation time T1 that even exceeds the gas relaxation time. This property of the oil base mud, if it is not taken into account in logging and interpretation, makes it difficult to obtain a reliable porosity since the NMR will under-polarize the invading OBM as well as the residual formation gas. The time domain analysis technique (TDA) was used to overcome this problem. In this paper, our experiences in such complex situation is discussed, core analysis is provided, core to log calibration in two wells is presented and recommendations for future applications in Obayied and other similar fields are suggested.