oa Application of Magnesium Yield Measurement from Neutron Spectroscopy Tool in Formation Evaluation of Northern Kuwait Fields

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