CPG Formation Evaluation & Petrophysics Flashcards

What is the primary reason shaly sands require modified water saturation equations beyond standard Archie's equation?

Answer: Clay minerals conduct electricity independently, causing Archie's equation to overestimate water saturation

Clay minerals provide an additional conductive pathway (cation exchange capacity) that makes the formation appear more conductive than it really is, causing Archie's equation to overestimate Sw.

In NMR logging, what does the T2 distribution primarily represent?

Answer: Pore size distribution, with larger pores having longer T2 relaxation times

The NMR T2 transverse relaxation time is inversely proportional to the surface-area-to-volume ratio of pores, making larger pores correspond to longer T2 times.

What does capillary pressure data from core analysis reveal about the reservoir that routine porosity-permeability data cannot?

Answer: Pore-throat size distribution and fluid saturation height function above the free water level

Capillary pressure curves define the height-saturation relationship above the free water level, which is essential for building accurate static reservoir models.

Which petrophysical parameter is most critical for assessing the economic viability of a tight gas or shale formation?

Answer: Total organic carbon (TOC) combined with effective porosity and gas saturation

In unconventional reservoirs, TOC indicates generation potential and adsorbed gas content, while effective porosity and Sg quantify the free gas in pores available for production.

What is the purpose of applying a 'borehole correction' to wireline log measurements?

Answer: To remove the influence of borehole size, mud type, and standoff on log readings to obtain accurate formation values

Borehole corrections use manufacturer-supplied charts or software to subtract the effect of borehole fluid, diameter, and tool position from raw log readings.

In the context of US regulatory reporting, which federal agency requires petrophysical data submission for offshore formations evaluated with wireline logs?

Answer: Bureau of Safety and Environmental Enforcement (BSEE)

BSEE (formerly MMS) regulates offshore oil and gas operations on the US Outer Continental Shelf and requires well log and formation evaluation data submissions.