EOR Potential of the Michigan Silurian Reefs Using CO2

Abstract

Abstract The Guelph Formation, historically known as the Brown Niagaran, is a Silurian age formation in the Michigan Basin containing hundreds of pinnacle reefs. These reefs, discovered primarily during the 1970s, have produced nearly half a billion barrels of primary oil. Over 700 reefs make up the northern trend and more than 300 reefs have been located in the southern portion of the basin, many of which have produced more than 5 MM bbls of oil. The EOR potential of these fields is believed to be significant. Few of these fields have been waterflooded and only five have experienced CO2 injection. An ongoing US Department of Energy project is studying the use of CO2 in enhanced oil recovery operations at the Charlton 30/31 reef, which is located in Michigan's Otsego County. This field was discovered in 1974 by Shell and produced 2.6 million bbls of oil during its primary production phase from a reservoir that may be typical of the other reefs in these trends. The reservoir is composed of a limestone matrix with low porosity and low permeability that contains irregular dolomitized intervals. These dolomitized zones, with higher porosity and permeability, control the flow of fluids through these reservoirs. This project utilized 4D seismic, reservoir simulation and a new well drilled into the reef to provide greater understanding of the CO2 EOR potential for this and all of the Silurian reefs in Michigan. Introduction Recently the price oil broke $100 per barrel for the first time. The increase in oil price seen in the last few years has refocused attention on oil productive reservoirs. This is particularly true within the US where the transportation costs associated with delivering the product to the point-of-sale is significantly less than imported products. Due to the decrease in exploration activity brought about by the low prices of the preceding decades, few new oil fields have been located within the US. Many existing fields are older and these have significant potential for enhanced oil recovery. One producing area that could benefit considerably from EOR techniques is the Silurian reef trends of the Michigan Basin, Figure 1. These reefs occur in the Guelph Formation, which is a stratigraphic unit that has historically been referred to as the Brown Niagaran. The first large commercial scale Niagaran reef field was the Boyd Field in St. Clair County. Discovered in 1952, the Boyd has produced over 2 MM bbls of oil and over 21 BCF of gas. From the mid 1940's through the 1960's, a number of publications addressed the regional stratigraphy and paleogeography of the Silurian in the Michigan and Illinois Basins. An early lithofacies analysis of the area's Silurian was conducted by Melhorn (1958). The paleontology, petrography and geometry of northeast Illinois Silurian reefs were described by Ingels (1963). Joudry (1969) published research on potential dolomitization mechanisms in the Southern Michigan Basin Reef Trend. In 1969, the first field in the Northern Silurian Reef Trend was discovered, leading to additional investigations of these reefs. These included works by Mesolello (1974), Shaver (1974) and (1977), Huh (1976), and Nurmi (1977). One of the more prolific workers on these structures during this time was Gill (1973, 1975, 1977 and 1979). In 1987 Cercone and Lohmann discussed diagenesis in these reefs.