Porosity Calculation From Core in the Montney Formation: A New Approach

Abstract

Determining the original gas/oil in-place (OGIP) in any reservoir as accurately as possible is key for resource estimation. Two key parameters for OGIP calculations are porosity (ø) and water saturation (Sw), which are determined either through laboratory analyses or calculations of geophysical log data. In unconventional formations, accuracy of these parameters is paramount as small errors in measuring pore volume and saturations are compounded in total gas-in-place calculations. In this study, we focus on the assessment of core porosity and present a new, innovative and more accurate analytical method for determining ø in the laboratory, which results in improved hydrocarbon saturation determinations and total gas-in-place evaluations. We use samples from the unconventional Montney play of the Western Canadian Sedimentary Basin. The Montney Formation (siltstones and shales) in Canada is well known as a world-class unconventional resource with an OGIP exceeding 450 TCF. Calculating the OGIP of the Montney is challenging, which is typical for such low porosity unconventional reservoirs. Conventionally, most laboratories measure core porosity using a core plug up to 1.5 inches in diameter after cleaning in a Soxhlet extractor or Dean Stark apparatus. After cleaning and drying are complete, the core plug is ready for porosity measurement. Conventionally, it is inserted into a helium pyncnometer (Boyle's Law apparatus), which determines the grain or skeletal density of the sample. The plug is then weighed and the bulk volume measured by mercury immersion (Archimedes' principle) to provide bulk density. This talk presents the results of porosity data from Montney cores determined by 3 commercial laboratories. Each laboratory used a similar approach to porosity measurement as described above. The main difference between the laboratories was sample preparation and time taken to both clean and dry the plugs. Comparisons were made using whole plugs and disaggregated plugs, per the Gas Research Institute (GRI) method (Luffel et al 1993). Identified key differences lead to a new work flow which routinely yielded both higher grain densities and thus higher porosities. The core porosity data is used to calibrate wireline density porosity data to establish a new porosity curve in each well; the resulting porosity map is used in the Montney OGIP calculation. Comparisons of OGIP determined using the new and demonstrate the value of this new technique.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016