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FIGURE 1. Size of molecules and hydrate structures.

Summary Sorption-induced strain and permeability were measured as a function of pore pressure using subbituminous coal from the Powder River basin of Wyoming, USA, and high-volatile bituminous coal from the Uinta-Piceance basin of Utah, USA. We found that for these coal samples, cleat compressibility was not constant, but variable. Calculated variable cleat-compressibility constants were found to correlate well with previously published data for other coals. Sorption-induced matrix strain (shrinkage/swelling) was measured on unconstrained samples for different gases: carbon dioxide (CO2), methane (CH4), and nitrogen (N2). During permeability tests, sorption-induced matrix shrinkage was demonstrated clearly by higher-permeability values at lower pore pressures while holding overburden pressure constant; this effect was more pronounced when gases with higher adsorption isotherms such as CO2 were used. Measured permeability data were modeled using three different permeability models that take into account sorption-induced matrix strain. We found that when the measured strain data were applied, all three models matched the measured permeability results poorly. However, by applying an experimentally derived expression to the strain data that accounts for the constraining stress of overburden pressure, pore pressure, coal type, and gas type, two of the models were greatly improved. Introduction Coal seams have the capacity to adsorb large amounts of gases because of their typically large internal surface area (30 to 300 m2/g) (Berkowitz 1985). Some gases, such as CO2, have a higher affinity for the coal surfaces than others, such as N2. Knowledge of how the adsorption or desorption of gases affects coal permeability is important not only to operations involving the production of natural gas from coalbeds but also to the design and operation of projects to sequester greenhouse gases in coalbeds (RECOPOL Workshop 2005). As reservoir pressure is lowered, gas molecules are desorbed from the matrix and travel to the cleat (natural-fracture) system, where they are conveyed to producing wells. Fluid movement in coal is controlled by diffusion in the coal matrix and described by Darcy flow in the fracture (cleat) system. Because diffusion of gases through the matrix is a much slower process than Darcy flow through the fracture (cleat) system, coal seams are treated as fractured reservoirs with respect to fluid flow. However, coalbeds are more complex than other fractured reservoirs because of their ability to adsorb (or desorb) large quantities of gas. Adsorption of gases by the internal surfaces of coal causes the coal matrix to swell, and desorption of gases causes the coal matrix to shrink. The swelling or shrinkage of coal as gas is adsorbed or desorbed is referred to as sorption-induced strain. Sorption-induced strain of the coal matrix causes a change in the width of the cleats or fractures that must be accounted for when modeling permeability changes in the system. A number of permeability-change models (Gray 1987; Sawyer et al. 1990; Seidle and Huitt 1995; Palmer and Mansoori 1998; Pekot and Reeves 2003; Shi and Durucan 2003) for coal have been proposed that attempt to account for the effect of sorption-induced strain. Accurate measurement of sorption-induced strain becomes important when modeling the effect of gas sorption on coal permeability. For this work, laboratory measurements of sorption-induced strain were made for two different coals and three gases. Permeability measurements also were made using the same coals and gases under different pressure and stress regimes. The objective of this current work is to present these data and to model the laboratory-generated permeability data using a number of permeability-change models that have been described by other researchers. This work should be of value to those who model coalbed-methane fields with reservoir simulators because these results could be incorporated into those reservoir models to improve their accuracy.

Summary. The minimum miscibility pressure (MMP) for a gas/oil pair can be measured within 1 hour with the rising-bubble apparatus (RBA). Development of miscibility between a gas bubble and an oil can be observed visually. The measurements of the MMP with the RBA compare favorably with those based on slim-tube experiments and predictions from phase-behavior studies. Introduction For a gas flood in an oil reservoir, MMP is the lowest possible operating pressure at which the gas can miscibly displace oil. The MMP of a gas/oil pair is traditionally determined by flooding an oil-saturated slim tube with a gas at four or five different pressures; the MMP is found from the pressure dependence of oil recovery. Stalkup describes slim tubes in detail. We designed and built the RBA as a reliable, fast alternative to a slim tube for measuring MMP. With the RBA, direct visual observations of miscibility development can be made. In contrast to the slim tube, pressure dependence of oil recovery is not used to indicate MMP with the RBA. This is not a great loss, however, because we do not believe that oil recovery and its dependence on pressure as measured in a slim tube correspond to what might occur pressure as measured in a slim tube correspond to what might occur in an oil reservoir. Coreflooding, combined with simulation and PVT studies, is probably the best way to determine the sensitivity PVT studies, is probably the best way to determine the sensitivity of field-scale oil recovery to flooding pressure. RBA Design and Operation. The heart of the RBA is a flat glass tube mounted vertically in a high-pressure sight gauge in a temperature-controlled bath. The rectangular internal cross section of the glass tube is 0.04 × 0.20 in. [1 × 5 mm]. The visible portion of the tube is about 8 in. [20 cm] long. The sight gauge is backlighted for visual observation and photography of rising bubbles in the oil. With the flat side of the tube perpendicular to the direction of the incident light, gas bubbles are visible even in opaque crudes. A hollow needle for injecting gas bubbles into the glass tube is mounted at the bottom of the sight gauge. For further details, see Ref. 2. The RBA as now designed can operate at up to 300 deg. F [420 K]. For pressures up to 5,000 psi [34 MPa], a single-window sight gauge is pressures up to 5,000 psi [34 MPa], a single-window sight gauge is used; for pressures up to 10,000 psi [69 MPa], a multiple-window sight gauge is used. In preparation for an experiment, the sight gauge and glass tube are filled with distilled water. Enough oil is then injected into the glass tube to displace all but a short column of water in the tube's lower end (Fig. 1). Next, a small bubble of gas of the desired composition is launched into the water. The buoyant force on the bubble causes it to rise through the column of water. then through the water/oil interface. As the bubble rises through the oil, its shape and motion are observed and photographed with a motor-driven 35-mm camera. Between 5 and 30 seconds are needed for the bubble to rise the length of the oil column. After two or three bubbles have risen through the column of oil, the "used" oil is replaced with fresh oil. For a gas/oil pair, rising-bubble experiments are repeated over a range of pressures. From the pressure dependence of the behavior of the rising bubbles, MMP is inferred. Multiple-Contact Miscibility Process. We believe that the mass-transfer process that occurs as the gas bubble rises through the oil in the glass tube is similar to the multiple-contact process described for gas displacements of oil in a slim tube. This multiple-contact miscibility process is frequently shown in a pseudoternary diagram (Fig. 2). Of course, the phase behavior is pressure-dependent. With increasing pressure, the two-phase region shrinks in size and the critical tie-line shifts. As a rising bubble contacts fresh oil in the glass tube, an overall Composition alpha forms in the two-phase region, with equilibrium phases of Compositions g1 and l1. Because the bubble of Composition g1 is buoyant, it rises to contact fresh oil. An overall Composition alpha 2 with Equilibrium Compositions g2 and 2, results. As this process proceeds, the composition of the bubble creeps around the two-phase region until it becomes miscible with the oil. If the compositions of the gas bubble and oil are on the same side of the critical tie-line, miscibility cannot be generated. But if the gas and oil compositions lie on opposite sides of the critical tie-line (as in Fig. 2), multiple-contact miscibility is possible. At the MMP, the critical tie-line extends through the crude-oil composition. For pressures above the MMP with some gas/oil pairs, it is possible for the two-phase region to be so small that the gas and oil are first-contact miscible. Interpretation of RBA Experiments Direct visual observations and photographs obtained over a range of pressures from RBA experiments are used to determine the MMP of a gas/oil pair at a constant temperature. MMP is inferred from the pressure dependence of the behavior of the rising bubbles. Bubble behavior varies significantly over a range of pressures and can be divided into three distinct patterns. 1. Far below MMP, a bubble retains its initial near-spherical shape as it rises through the column of oil, although the size of the bubble decreases as gas transfers to the oil phase. As the pressure approaches MMP, a bubble still remains nearly spherical on top, but the bottom interface of the bubble changes from spherical to flat or "wavy." 2. At or slightly above MMP, tail-like features quickly develop on the bottom of a rising bubble, which remains spherical on top. Then, starting at the bottom of the bubble, the gas/oil interface vanishes, and the contents of the bubble rapidly disperse in the oil. This type of behavior suggests a multiple-contact miscibility process, not a first-contact process, because the bubble did not process, not a first-contact process, because the bubble did not immediately disperse when it first contacted the oil at the water/oil interface. During this multiple-contact process, the volume of the bubble is almost constant (a 10 to 20 % shrinkage is common) until the interface starts to deteriorate. 3. At pressures far above MMP, a bubble will disperse more rapidly than at pressures just above MMP; with some oils, CO2 bubbles disperse immediately after reaching the water/oil contact in the glass tube (first-contact miscibility). SPERE P. 522

Children, 4-6 years old, 153 Caucasian and 50 African-American, from a pre-school and kindergarten programme in a low income industrial area, who participated in a voluntary oral health examination, were questioned and examined for signs and symptoms of craniomandibular disorders (CMD) and of oral parafunctions. Most of the CMD signs and symptoms were mild. Eight per cent had recurrent (at least 1-2 times per week) TMJ pain, and 5% had recurrent neck pain, African-American children more often than Caucasian children (P < 0.05). Seventeen per cent had recurrent headache. Three per cent had recurrent earache. Pain or tiredness in the jaws during chewing was reported by 25% of the children, more often by African-American than by Caucasian children (P < 0.001) and more often by girls than by boys (P < 0.05). Pain at jaw opening occurred in 10% of the children, more often in the African-American than in the Caucasian group (P < 0.001). Thirteen per cent of the children had problems in opening the mouth. Deviation during opening was observed in 17% and reduced opening in 2%. Reduced lateral movements, locking or luxation were not observed in any child. Palpation pain was found in the lateral TMJ area in 16%, in the posterior TMJ area in 25%, in the temporalis and masseter areas in 10%, and pain for all regions was found more often in the African-American than in the Caucasian children (P < 0.01). Thirty-four per cent of the African-American, and 15% of the Caucasian children admitted to having ear noises (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)