Abstract

Conventional mudlogging utilizes a total gas (TG) detector along with a gas chromatograph’s C1–C5 gas measurements, which are used for the hydrocarbon-bearing formation evaluation. Differential total gas (DTG) detection system provides a direct measurement of light and heavy gas components calibrated to discriminate formation fluids (water, oil, condensate, wet and dry gas, and coal-bed methane). Combination of TG and DTG measurements leads to better understanding of the reservoir properties such as porosity, permeability, fluid types, and hydrocarbon saturation. It also allows calculating of hydrocarbons in place. Using the real time log, the production capability of the reservoir zone at a fixed pressure drop can be calculated.

This work covers principles of joint interpretation of the TG and DTG curves and examples from a real-time mudlogging conducted on vertical and horizontal wells drilled for both conventional and unconventional reservoirs. They include identification of the water-bearing fault in a horizontal well; oil-water contact; gas-condensate contact; gas-condensate-oil zone; oil-water zone; and tight zone within the reservoir. The case study of the DTG application for shale oil demonstrates the definition of oil production capable intervals and lesser production capable intervals, which is important for an effective fracking design.

Presented examples demonstrate an additional value of including DTG data into petrophysical formation evaluation workflow.