Schlumberger

Capture Technology for Complex Carbonates

Date: 08/11/2009

An Elemental Solution to a Weighty Matter

Characterization of reservoir quality with logging tools can be complicated by high mud weights. In northern Kuwait, oil-base mud is often weighted with barite to promote safer drilling in fields known for high concentrations of hydrogen sulfide and high reservoir pressures. This combination of mud components can affect logging measurements and the formation evaluation process, so additional data are required to reduce the assessment's uncertainty.

Reservoir quality is primarily defined by the rock's porosity and permeability. Analyses of cores from carbonate reservoirs show strong correlations between reservoir quality and dolomitization, which increases the values for these two properties. The quantification of dolomite content thus becomes important in classifying rock quality. However, as Djisan Kho, senior petrophysicist for Schlumberger, told a recent gathering of well log analysts, the estimation of dolomite content from conventional measurements can be hindered by a variety of factors. These include barite mud effects, invasion of oil-base mud filtrate, complex lithologies, and sensitivity of measurements to dolomite, as well as differences in each tool's vertical resolution and depth of investigation.

"Porosity and lithology are normally evaluated using a combination of density, photoelectric factor, neutron, gamma ray, and sonic measurements," Kho said. "Unfortunately, none of these tools measures porosity or lithology independently."

This limitation introduces uncertainty into petrophysical evaluations and, where possible, core data is used to validate log readings used in models. "If core data is not available, identifying and correcting anomalous logs can become more complicated and create further uncertainties," Kho told his audience. Additional data may provide answers to help reduce this uncertainty.

The most important variable to be solved for is formation mineralogy, which can be derived from neutron capture spectroscopy data. Recent developments in the ECS elemental capture spectroscopy tool have improved yield measurements of magnesium—a key component of dolomite—to help petrophysicists quantify the amount of dolomite and other minerals in the reservoir rock. ECS spectroscopy data also provides relative yields of elements such as iron, silicon, calcium, sulfur, barium, hydrogen, and chlorine.

Because its performance is unaffected by barite muds, the tool is particularly appropriate for wells such as those in northern Kuwait that require heavy drilling and completion fluids. And since the spectroscopy data expands the ability to solve for more minerals in complex reservoirs, the end result is less uncertainty in lithology, porosity, and water saturation computations.

Reference

Kho D, Al-Awadi M, and Acharya M: "Application of Magnesium Yield Measurement from Elemental Capture Spectroscopy Tool in Formation Evaluation of Northern Kuwait Fields," Transactions of the SPWLA 50th Annual Logging Symposium, The Woodlands, Texas, USA.

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Capture Technology for Complex Carbonates
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