NMR Logging Improves Wellsite Efficiency, Completion Decisions, and Formation Evaluation in a Freshwater, Shaly Reservoir

Rio Ceibas is a freshwater shaly reservoir in the Honda formation of Colombia. A typical well has 15+ sands, which may contain light oil, medium oil or mobile water. Formation evaluation is difficult. The fresh water and the high volume of smectite clay cause resistivity logs to respond more to the volume of clay than to the difference between oil and water. Nuclear measurements do not indicate clearly the most productive intervals. A focused effort was started in 1998 to increase production using cost-effective modern technology, one element of which was the introduction of nuclear magnetic resonance (NMR) logs. At the wellsite, NMR permeability is used to choose points for wireline pressure tests. The percentage of dry tests has been reduced from 24% to 7%. This saves significant rig time since pressures must be monitored regularly in many sands. As verified on cores, the presence of an NMR signal with T2 > 300ms is a sign of light hydrocarbons. When other data are inconclusive, this indicator helps to decide whether to perforate a sand. NMR free fluid and permeability are then used to decide which intervals in the sands have the best producibility. This method saves costs by reducing perforating intervals without reducing production. For reservoir description, NMR has contributed to improved estimates of effective porosity, and qualitative permeability. Both cores and logs have been analyzed to establish interpretation parameters such as bound- and clay- water cutoffs, and permeability coefficients. In those sands that produce clean oil, the irreducible water from NMR logs is an indicator of water saturation, because of the difficulty of obtaining a reliable value from conventional logs. It is also used to determine resistivity interpretation parameters for calculations in other sands. The new formation evaluation procedure improves the match between predicted and actual production.