ABSTRACT: Rebound hardness (RHN) has recently gained considerable interest as a rock mechanical parameter in the petroleum industry. However, few published studies target a comprehensive integration among RHN and reservoir parameters, which can be highly valuable in reservoir characterization and production. This study focuses on the integration among RHN and facies, mineralogy, natural fractures, reservoir quality, and rock mechanical properties of the unconventional “Mississippian Limestone” play in north-central Oklahoma, USA. In 2415 feet (736 m) of core, RHN correlates with porosity, permeability, and critical rock mechanical properties, suggesting its potential value as a quick and inexpensive tool in reservoir characterization and production design. RHN exhibits varying patterns in relation to porosity and permeability in different play areas, likely related to different depositional settings and sampling bias. Therefore, the prediction of reservoir quality from RHN should be tailored among different play areas with a well-defined sampling protocol. Results also indicate that RHN correlates well with mineralogy but exhibits limited variability among many fractured and non-fractured zones. This suggests that the present-day rock mechanics are likely a combination of earlier “unaltered” and later “altered” characteristics, indicating that the temporal evolution of rock mechanical properties should be considered in reservoir characterization and production design.
The “Mississippian Limestone” Play, located primarily in Oklahoma and southern Kansas (Figure 1), has been developed using conventional vertical drilling techniques for over half a century and has recently become one of the most active unconventional resource plays in North America. The associated strata - the “Mississippian Limestone” (MISS) - is an informal stratigraphic nomenclature which includes the Mississippian (Early Carboniferous)-aged strata present across the U.S. Southern Mid-Continent, including parts of Kansas, Missouri, Arkansas, and Oklahoma (Figure 1). As opposed to the historic “Mississippian Limestone” play, there are several recently discovered play areas nearby that target the Mississippian section, such as the “STACK” play southwest of the “Mississippian Limestone” play area (Figure 1).
Norbisrath, Jan Henrik (Statoil) | Grammer, G. Michael (Oklahoma State University) | Vanden Berg, Beth (BP America) | Tenaglia, Max (University of Miami) | Eberli, Gregor P (University of Miami) | Weger, Ralf J (University of Miami)
Nanopore geometry and mineralogy are key parameters for effective hydrocarbon exploration and production in unconventional reservoirs. This study describes an approach to evaluate relationships between low-frequency complex resistivity spectra (CRS), nanopore geometry, and mineralogy to use CRS to provide estimates of reservoir parameters concerning hydrocarbon saturation, storage, and producibility. For this purpose, the frequency dispersion of CRS was analyzed in 56 mudrock core plugs from the Vaca Muerta Formation (VMF) (Jurassic/Cretaceous) in Argentina, along with cementation factors (m), carbonate content (CO3), and total organic carbon (TOC). To quantify the nanoporosity, a subset of 23 samples was milled with broad ion beam (BIB) and imaged with scanning electron microscopy (SEM); the image grids of these samples were stitched together into high-resolution BIB-SEM mosaics and analyzed with digital image analysis (DIA) techniques. Results show that porosity is the dominant control on electrical properties in the mudrocks analyzed as part of this study. There is no conclusive evidence that pore geometry influences the electrical properties in the analyzed mudrocks. Pore-geometry parameters [dominant pore size (DOMsize) and perimeter over area (PoA)] do not correlate with electrical properties. Instead, mineralogy shows a first-order correlation with electrical properties, where cementation exponents are higher in rocks with high TOC and low CO3 content. CRS can be used to estimate porosity and cementation factors with high correlation coefficients of R2=0.71 and R2=0.95, respectively. Estimates of the 2D interfacial surface area (ISA2D), which is a function of both pore geometry and porosity, achieve an R2=0.59. The results of this study suggest that low-frequency dielectric rock properties, if measured downhole, could be useful to identify primary producing intervals in unconventional reservoirs, and to accurately determine cementation factors independent of formation fluids and porosity.
The Mid-Continent Mississippian Limestone is an unconventional carbonate reservoir with a complex depositional and diagenetic history. Oil and gas have been produced from vertical wells for over 50 years, but recent horizontal activity in low porosity, low permeability zones have made it crucial to understand the petrophysical characteristics to better target producing intervals. Because of the wide variability and complexity of pore systems in carbonate reservoirs, simple porosity/permeability transforms developed for siliciclastic reservoirs often provide erroneous results in carbonates. However, laboratory measured sonic velocity response in carbonates, when combined with an analysis of pore system architecture, has been shown to provide a proxy for permeability in carbonates with pore systems at the macro- and micro-scale. This project is focused on testing this relationship for micro-to nanoscale pore systems.
The sonic velocity response (compressional and shear wave) for a sub-set of samples from Mid-Continent Mississippian limestones varies from 6500 to 5000m/sec (Vp) and 4500-2500m/sec (Vs). Overall trends of the data confirm observations from previous studies regarding the expected range of sonic velocity for low porosity, low permeability carbonates. Porosity values in the horizontal direction ranges from 0.5-7%, although locally porosity values may be as high as 20%. Pore diameter ranges in size from the mesopore (4mm-62.5 μm) to nanopore (1μm-1nm) size, with the majority of the pores in the micro- to nanopore size range. Pores viewed with SEM show the largest pores are mostly oblong to oval shaped, intercrystalline to vuggy mesopores with a diameter of 25- 100μm while the smallest are circular shaped intercrystalline to vuggy nanopores with a diameter of 5-10μm with 50-100nm pore throats.
Because pore size and shape have significant influence on the permeability in these reservoirs, sonic velocity data coupled with characterization of macro- to nanoscale pore architecture shows promise of predicting both key reservoir facies and key producing intervals within these unconventional carbonate reservoirs, in particular when tied to primary depositional facies and incorporated within a high-resolution sequence stratigraphic framework.
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.