Crushed-Rock Vs. Full-Diameter Core Samples for Water-Saturation Determination in a Tight-Gas Siltstone Play

Abstract

Summary The efficacy of crushed-rock samples vs. small plugs or full-diameter core samples for measurement of porosity, permeability, and fluid saturation is an important consideration in the evaluation of tight-gas reservoirs and shale-gas reservoirs. Crushed-rock core analysis methods originally developed for shale reservoirs are now, in some cases, being extended to low-quality tight-gas reservoirs. In this study, crushed-rock and full-diameter core measurements from two wells drilled with oil-based mud are compared to evaluate which of the two core-analysis methods is more reliable for water-saturation assessment of a major North American tight-gas siltstone play (Montney Formation, western Canada). Measurements from the studied full-diameter core samples have wide ranges of water saturation (10 to 45%) and bulk volume water (BVW) (0.5 to 2.6%). In contrast, measurements from crushed-rock samples have much narrower ranges of water saturation (10 to 20%) and BVW (0.2 to 0.7%). The lower values and limited range of water-content measurements from crushed-rock samples suggest a significant degree of artificial water loss during sample handling in the laboratory. This conclusion is supported by comparing core-measured BVW with deep-resistivity values from openhole well logs. Full-diameter BVW measurements correlate well with log resistivity, indicating they are generally representative of in-situ reservoir conditions. Crushed-rock BVW values, on the other hand, show no correlation with log resistivity. The results of this study suggest caution is warranted in the use of crushed-rock samples for water-saturation measurements of siltstones or silty shales. Failure to recognize artificial water loss from crushed-rock siltstone samples could lead to an erroneous interpretation of irreducible water saturation at in-situ reservoir conditions with potentially serious implications for resource evaluation and exploitation.