Date of Award

2016

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Geophysics (MS)

Administrative Home Department

Department of Geological and Mining Engineering and Sciences

Advisor 1

Wayne D. Pennington

Committee Member 1

Gregory P. Waite

Committee Member 2

Mir Sadri

Abstract

The goal of this study was to evaluate permeability and study the controls on permeability in a gas saturated formation. Conventional well logs, mineral identification crossplots and empirical models were applied to analyze different lithologic and diagenetic features and to examine the effect that these features may have on the reservoir. An unusual feature was observed, and required detailed examination: there existed (in two wells) five zones of lower resistivity (higher water saturation) above the gas-water contact. This is unexpected, as above that contact, the water is usually at irreducible water saturation. I conclude that the lower resistivity (the higher water saturation) is due to unusual mineralogy containing small grain size, ineffective microporosity and secondary porosity within specific grains, and support this conclusion with a variety of indicators.

Two wells in New Zealand’s Taranaki Basin were used for this study. First, various routinely applied methods were used to assign the boundaries of the gas-saturated zones of the Mangaa C-1 sandstone and to identify the mineralogy. From Archie and non-Archie (Simandoux, Schlumberger, Indonesia, and Dual-Water) models, four subzones in Karewa-1 well and two subzones in Kahawai-1 well were recognized as high water saturation intervals within the Mangaa C-1 gas saturated formation.

The analysis of saturation can be used to identify grain size (and pore size) distribution, which turned out to be critical in understanding the high water saturation zones. Bulk volume water analysis was used together with created lithology logs and with core descriptions that had been made available to recognize the detrimental diagenetic zones in the gas formation. Dissolution of minerals, grain size distributions, and different pore type characteristics increase bulk volume water in high water saturation zones while keeping the formation at irreducible water saturation, but at levels that are elevated in comparison with higher-quality (and lower irreducible water saturation) zones both above and below.

The various irreducible water saturation zones were then used to predict the absolute permeability of those zones using several empirical models. Then, different flow unit characterization methods were applied to better understand the different quality rocks within the formation. One approach represents a new attempt to compare results for pore size classifications. My results showed that diagenesis is more detrimental to reservoir quality than grain size within the Mangaa C-1 gas sandstone, and that there is no transition or fully-water saturated zone under the gas reservoir at Karewa-1 well, while the transition zone exists for the same formation at Kahawai-1 well.

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