Using Magnetic Resonance Logging to Evaluate Thin-bedded Oil Bearing Formations in the Gulf of Suez: A Case Study
Neil Hewitt1, George Clemenceau1, Gamal Ragab Gaafar2
(1) BP, CAiro, Egypt (2) GUPCO, Cairo, Egypt
In the highly saline Hammam Faraum sands of the Belayim formation, zones with greater than 2 ohmms are regarded as potentially oil productive, yet many of these do not produce at all. Traditional sonic and nuclear porosity measurements do not clearly identify the productive intervals. In an effort to better differentiate productive versus non-productive zones and thus design more cost-effective completions, nuclear magnetic resonance (NMR) technology was introduced.
The formation was deposited as inter-bedded mudstones and arkosic sands in a fan delta complex. Above and below the Hammam Faraum are thick evaporite beds. A typical well has a dozen or more sand bodies, each of which may contain oil, mobile water or no mobile fluids at all. The salt saturated water and the numerous thin clays cause resistivity log baseline of less than an ohmm.
During the reservoir description phase, NMR contributes to improved estimates of effective porosity, free fluid volume and qualitative permeability. With several dozen wells drilled to deeper targets in the field, cores and logs had been taken and analyzed to establish interpretation parameters.
In adding the new measurement, a third fluid is defined; irreducible water. This is equal to porosity minus NMR Free Fluid Index and is used to define the ‘ineffective’ pore space. In the two larger sand bodies, known to produce clean oil, the irreducible water from NMR logs is very low and the new measurement makes only a slight difference. In the thinner, less well defined sands, the new measurement correctly identifies which have high irreducible water. Adding irreducible water in the formation evaluation procedure improves the recognition of pay zones which otherwise would not be perforated.
Other log data has proven inconclusive in the thinner, more marginal intervals. NMR free fluid index helps to decide whether to perforate a sand. This method saves testing and perforating costs without reducing production.