Middle Marrat resrervoir of Jurrasic age is a tight carbonate reservoir with vertical and horizontal heterogeneous properities. The well placement over deep elonogted anticline with steep dips, geosteering and lowering of production liner are challenging, therefore the field is being developed using horizontal wells cutting across multiple reservoir layers to maximize reservoir contact and driange
The low clay content in Marrat reservoirs gives low gamma ray counts, which makes the identification of reservoir layers identification difficult. In addition, the high-resistivity responses from hydrocarbon-bearing pay-zones and from the tight layers make the identification of the reservoir sweet spots difficult as well. Slim-hole magnetic resonance (NMR) logging was deployed in wash-down mode for identifying reservoir sweet-spots as a lithology-independent porosity and formation fluid characterization tool.
Magnetic resonance was acquired with dual wait time enabled T2 polarization to differentiate between moveable water and hydrocarbons. After acquisition, the standard deliverables were porosity, the effective porosity ratio, and the permeability index to evaluate the rock qualities. Porosity was divided into clay-bound water (CBW), bulk-volume irreducible (BVI) and bulk-volume moveable (BVM). Rock quality was interpreted and classified based on efftective porosity and permeability index ratios. The ratio where a steeper gradient was interpreted as high flow zones, a gentle gradient as low flow zones, and a flat gradient was considered as tight baffle zones.
Based on the MR flow units and fluid types (CBW, BVI and BVM), the drilled interval was classified into six compartments of high flow, one compartment of low flow, and five tight baffle zones. Accordingly, the perforation plan was optimized to fit the high flow units only. Comparing to the conventional log analysis, NMR excluded approximately 1000 ft of non-productive reservoirs, leading to a significant cost savings (250,000 USD) in perforationand production optimization.
The well was drilled with a distance-to-bed boundary tool; however, enormous potential exists to use this slim MR tool as a non-radioactive sourceless solution for geosteering through reservoir sweet spots while delivering wells that are more productive safely.