Application of magnesium yield measurement from neutron spectroscopy tool in formation evaluation of northern Kuwait fields. 9th Middle East Geosciences Conference, GEO 2010.

Abstract

Evaluation of porosity and lithology has always been done through a combination of density, photoelectric factor (PEF), neutron, gamma-ray, and sonic measurements. None of these directly gives porosity or lithology. Therefore, common practice includes building petrophysical models to extract these reservoir properties. Geoscientists involved in petrophysical analysis using multi-mineral solvers are aware of the difficulty and the uncertainty of the process; for example, changing a fluid property in the model will change the lithology as well as the porosity. The logs themselves are also known to have their own measurement uncertainties. The density log, for example, is affected by bad hole, lithology, barite, and light hydrocarbons. The neutron log is affected by lithology, fluid hydrogen index, and the borehole properties (temperature, pressure, hole size, stand-off, mud cake, mud weight, etc.). The interpretation is also complicated by the fact that different neutron tools from different logging companies have different sensitivities to lithology. Sonic log data is also used for interpretation even though it is affected by fractures, vuggy porosity, anisotropy, etc. The PEF curve is commonly used as an additional tool to solve for the lithology. However, if the mud contains barite the measurement becomes unusable. Dolomite and solid bitumen quantifications have been the challenging issues in carbonate evaluation. The dolomite diagenesis involves the recrystallization which makes the dolomite less susceptible to porosity reduction caused by overburden pressure. This unique characteristic of the crystallized dolomite makes it an important reservoir rock especially in deep carbonate reservoirs. On the other hand, the presence of solid bitumen is always associated with poor reservoir quality. Also, the physical properties of the solid bitumen cause it to appear as hydrocarbon. If not corrected, the formation evaluation result will give incorrect porosity and water saturation computation. New development in neutron capture spectroscopy tool provides significant data to quantify the mineralogy in carbonate, especially the dolomite content through magnesium yield measurement. Combination of the spectroscopy data and magnetic resonance data can be used to identify and correct the solid bitumen effects. Real examples from deep carbonate reservoir in northern Kuwait fields and the validation against core data will be presented.