Abstract
In recent years, petroleum exploration and production from the Bakken Formation in Williston Basin have gained great momentum and pushed the oil production to over one million barrels per day. Estimation of original oil and gas in place and recoverable hydrocarbons of the Bakken Formation requires an accurate estimation of porosity. Unfortunately, uncertainty in porosity measurements in tight rocks such as Bakken could be as high as +/-1.0 porosity unit if a single method is applied. This is significant considering that most Bakken sample porosity is less than 8%. In this study, we measured the porosity of tight rocks utilizing four different methods: gas expansion method, radius-of-investigation method, pressure buildup method, and oscillating pulse method. In this way, porosity of over 200 Bakken core samples have been measured in this study. The uncertainty in the porosity measurement is reduced by applying proper experiment procedures and cross-checking by different approaches. The comparability of results from different methods increased. Experience and lessons learned from this study can be a good guideline to the accurate measurement of other tight rock porosity. Therefore, a more accurate evaluation of hydrocarbons in place is possible and the primary production design and enhanced oil recovery method selection can be more appropriate. Furthermore, difference among the results is useful for indicating the heterogeneity and/or microcracks of the rock, which are crucial in drilling and completing unconventional oil and gas wells.
1. INTRODUCTION
During the last decade, petroleum exploration and production from the Bakken Formation in Williston Basin have gained great momentum and pushed the oil production to over one million barrels per day. However, producing hydrocarbons from the Bakken Formation is challenging due to its low porosity and permeability. The high uncertainty in the estimation of original oil and gas in place and recoverable hydrocarbons of the Bakken Formation is reflected by the wide range estimated by many researchers. In addition, porosity is one of the key parameters in modeling fluids flow in reservoir matrices. Unfortunately, uncertainty in porosity measurements in tight rocks such as Bakken could be as high as +/- 1.0 porosity unit if a single method is applied. This is significant considering that most Bakken sample porosity is less than 8%. An accurate estimation of Bakken rock porosity is important to successfully produce from Bakken reservoir.
