Showing papers by "Jack Dvorkin published in 2003"

Rock physics of a gas hydrate reservoir

Abstract: Gas hydrates are solids composed of a hydrogen-bonded water lattice with entrapped guest molecules of gas. There are convincing arguments that vast amounts of methane gas hydrate are present in sediments under the world's oceans as well as in onshore sediments in the Arctic. This hydrate is possibly the largest carbon and methane pool on earth. As such, methane hydrate may be the principal factor in global climate balancing. One may also treat this methane pool as a potential energy source. These considerations ignite the scientific and business community's interest in quantifying the amount of methane hydrate in the subsurface. Gas hydrate reservoir characterization is, in principle, no different from the traditional hydrocarbon reservoir characterization. Similar and well-developed remote sensing techniques can be used, seismic reflection profiling being the dominant among them. Seismic response of the subsurface is determined by the spatial distribution of the elastic properties. By mapping the elastic contrast, the geophysicist can illuminate tectonic features and geobodies, hydrocarbon reservoirs included. To accurately translate elastic-property images into images of lithology, porosity, and the pore-filling phase, quantitative knowledge is needed that relates the rock's elastic properties to its bulk properties and conditions. Specifically, to quantitatively characterize a natural gas hydrate reservoir, we must be able to relate the elastic properties of the sediment to the volume of gas hydrate present and, if at all possible, the permeability. One way of achieving this goal is through rock physics effective-medium modeling. Several attempts to construct a relation between hydrate concentration and the compressional velocity in sediments have followed the path of modifying the popular Wyllie's time average equation which states that total traveltime through rock is the volume-weighted sum of traveltimes through the solid phase and the fluid phase considered independently of each other; i.e., V P −1 = (1-ϕ) V …

Rock Property Determination Using Digital Rock Physics

Abstract: The Digital Rock Physics (DRP) technology is based on a rigorous numerical simulation of physical experiments in a realistic pore space, at the pore-scale level. The output is usually a macroscopic property conventionally measured in the lab. For example, a single-phase viscous fluid flow simulation through a digitized pore space provides absolute permeability. A simulation of electrical current provides conductivity, and a simulation of the stress field provides the elastic moduli and strength. DRP complements lab measurements and, at the same time, enormously enhances the geoscientist’s capabilities because digital experiments can be conducted in real time and on small fragments of rock, such as drill cuttings. We report a feasibility study on DRP technology applied to drill cutting samples to obtain porosity, permeability, and the P- and S-wave velocity.

Effect of glauconite on the elastic properties, porosity, and permeability of reservoir rocks

Abstract: Glauconite is an iron rich variety of clay that can be found as individual pellets, composite grains, and intergranular cement. Its density ranges between 2.4 g/cm3 and 2.95 g/cm3, averaging 2.67 g/cm3. It has a Moh's scale hardness of 2. Authigenic glauconite is formed under a limited range of geologic and geochemical conditions; it typically develops on the outer margins of continental shelves, in areas of low sediment input (Odin, 1980), and its presence is valuable as an indicator of transgressive sequences. Identifying glauconite in the subsurface is important for depositional environment interpretation, stratigraphic correlation, dating, tracing of unconformities, and geochemical exploration for source and reservoir rocks (Srivastava, 1986). A number of commercial hydrocarbon reservoirs are glauconitic sandstones—for example in Colombia, Ecuador, Peru, Venezuela, Australia, Eastern China, North Sea, United States, Canada, Saudi Arabia, and Ireland. Although glauconite tends to exist as grains and as such is part of the rock framework, under moderate overburden pressure, these grains are easily compacted (Figure 1) and may form a pseudomatrix that occludes the original primary porosity. This behavior is in contrast to that observed in clay minerals. This problem, and the fact that there are no published studies about the elastic properties of glauconite and glauconitic sandstones, motivated this research to understand their rock physics properties. We present analyses of data from five lithologies containing varying amounts of glauconite and identify the best seismic attributes to evaluate its presence and the reservoir quality. Figure 1. Optical image of a glauconitic sandstone (made at 20X magnification) showing formation of a pseudomatrix that occludes the original primary porosity. Glauconite=green, Quartz=white. The samples in this study come from Caballos Formation in Putumayo and Upper Magdalena Basins, Colombia (Figure 2), which is described as a marine transgressive blanket sandstone deposited in a shallow …

Model-based prediction of porosity and reservoir quality from P- and S-wave data

Abstract: [1] We present a deterministic methodology for mapping the lithology, pore fluid, and porosity from seismic data. The input is the P- and S-wave data volumes that may come, e.g., from acoustic and elastic inversion or cross-well measurements. The output is the pore fluid type (hydrocarbon versus water), total porosity, and clay content. The key element of this methodology is a site-specific rock physics model that provides the needed transforms from the elastic rock properties to the reservoir properties. This model is established by comparing model-based predictions, such as impedance versus porosity, to the relations present in well log data. Once selected, the model is used to identify the presence of hydrocarbons from a combination of the P-wave impedance and Poisson's ratio. Then the P-wave impedance is used to map porosity and clay content assuming that a deterministic relation exists between the latter two properties. All deterministic equations are calibrated at a well and then are applied to upscaled well log data to confirm their validity at the seismic scale. This methodology is applied to log data from a fluvial environment. The results indicate that the relative error in porosity determination is 14%, which is acceptable for reservoir characterization purposes.

Rock Physics of Gas Hydrate Reservoir

Abstract: C-14 ROCK PHYSICS OF GAS HYDRATE RESERVOIR 1 Summary. Enormous amounts of methane gas hydrate are present in sediments under the world's oceans as well as in on-shore sediments in the Arctic. These hydrates are a potential future energy resource. The most well-developed geophysical tool for exploring large volumes of the subsurface where gas hydrate is found is seismic reflection profiling. To characterize a natural gas hydrate reservoir with seismic data we must be able to relate the elastic properties of the sediment to the volume of gas hydrate present. One way of achieving this goal is through rock physics

Seismic attenuation for reservoir characterization

Abstract: In this report we will show the fundamental concepts of two different methods to compute seismic energy absorption. The first methods gives and absolute value of Q and is based on computation with minimum phase operators. The second method gives a relative energy loss compared to a background trend. This method is a rapid, qualitative indicator of anomalous absorption and can be combined with other attributes such as band limited acoustic impedance to indicate areas of likely gas saturation.

Rock Physics Diagnostic In a Sand/Shale Sequence

Abstract: In this paper we will review the use of Rock Physics Diagnostics applied to log data that illustrates a relational model between porosity, clay and saturation. We use these relations to estimate porosity from elastic impedance attributes. Using statistical fits may work locally around the property values experienced by a well for example but away from the well you need to employ some systematic approach to improve the confidence and reduce the risk associated with such estimations. Such a systematic approach is Rock Physics Diagnostics and we believe that this methodology is essential for extracting rock properties from seismic data.