Abstract Obtaining accurate knowledge of variations in the magnitude of the least horizontal principal stress, S hmin , with depth is of great practical importance in the oil and gas industry. We demonstrate that accurate knowledge of the magnitude of S hmin with depth can be obtained utilizing two fundamental concepts. First, frictional faulting constraints place bounds on principal stress magnitudes in relatively stiff (high Young’s modulus) rocks. These bounds depend on depth, pore pressure and whether a site is characterized by a normal, strike-slip or reverse faulting stress state. Second, in more ductile clay-rich formations such as those typical of most US unconventional plays, varying degrees of viscoplastic stress relaxation (VSR) reduces differences between principal stress magnitudes. We focus in this paper on the importance of how layer-to-layer variations of lithology control the magnitude of S hmin and how the two concepts above can be used to accurately predict variations of the magnitude of S hmin with depth. It is well established that the effectiveness of multistage hydraulic fracturing is critical to stimulate production in unconventional oil and gas reservoirs. Using several case studies, we illustrate how predictable variations of stress magnitude with depth affect hydraulic fracture propagation. We also demonstrate that widely used elastic loading models do not accurately predict variations with depth. In the context of VSR, essentially complete stress relaxation also helps explain occasional observations of the least principal stress being nearly equal to the overburden stress in some clay-rich formations. In such cases, sub-horizontal hydraulic fracture propagation is expected due to the low tensile strength of bedding planes.
Parenterally administered drugs are secreted directly into the gastric juice. The concentration ratio (concentration of drug in gastric juice divided by concentration in plasma) depends on the dissociation constant of the drug. Thus strong acids appear in gastric juice in negligible concentration, weak acids and weak bases in measurable amount and stronger bases in highest concentration. The stronger bases appear in a limiting concentration ratio of about 40 to 1 when gastric juice flow is maximal, the limitation apparently being due to complete clearance by the gastric mucosa. The pattern of drug secretion can be explained by the concept that the membrane separating plasma from gastric juice has the characteristics of a lipoid membrane that allows the passage of drugs in their undissociated form while restricting passage of dissociated form.
Inositol 1,4,5-trisphosphate is an intracellular second messenger, produced upon stimulation of the phosphoinositide system, capable of mobilizing calcium from intracellular stores. We have recently identified high levels of specific binding sites for inositol 1,4,5-trisphosphate in brain membranes (Worley, P. F., Baraban, J. M., Colvin, J. S., and Snyder, S. H. (1987) Nature 325, 159-161) and have now further characterized these sites. In cerebellar membranes, inositol 1,4,5-trisphosphate binding sites are abundant (20 pmol/mg protein) and display high affinity and selectivity for inositol 1,4,5-trisphosphate (KD approximately equal to 40 nM), whereas other inositol phosphates such as inositol 1,3,4,5-tetrakisphosphate (Ki approximately equal to 10 microM) and inositol 1,4-bisphosphate (Ki approximately equal to 10 microM) exhibit much lower affinity for this site. Submicromolar concentrations of calcium strongly inhibit inositol 1,4,5-trisphosphate binding (IC50 approximately equal to 300 nM). A sharp increase in binding occurs at slightly alkaline pH. These results suggest that actions of inositol 1,4,5-trisphosphate are regulated by physiological alterations in intracellular pH and calcium concentrations.
Negatively charged liposomes are endocytosed by the coated vesicle system and accumulate in acidic intracellular vesicles. Liposomes that become unstable at acidic pH improve cytoplasmic delivery of membraneimpermeant macromolecules such as calcein (CAL) and FITC dextran (18 or 40 kDa). Oleic acid (OA): phosphatidylethanolamine (PE) (3:7 mole ratio) liposomes become permeable to CAL at pH < 7.0. Control liposomes of phosphatidylserine : PE or OA : phosphatidylcholine are stable at pH 4–8. OA:PE liposomes promote cytoplasmic delivery of encapsulated CAL to CV‐1 cells, as evidenced by the emergence of diffuse, cytoplasmic CAL fluorescence. Delivery requires metabolic energy and is partially inhibited by chloroquine or monensin, which raise the pH of intracellular vesicles.
Based on chemical analyses of some 2 000 rainwater samples, the relation between the pH and the total amount of acid or of base respectively is studied. Considering the carbon dioxide-water system the theoretical relation between pH and total amount of acid or bicarbonate is calculated. A regular deviation is observed between the empirically found and theoretically calculated relation. Starting from chemical analyses of the most important compounds in atmospheric precipitation, a quantitative hypothesis is formulated of how these compounds originally were associated with acids or bases. It is further assumed that these have netralized each other and that the system has come to an equilibrium with the carbon dioxide pressure of the atmosphere. Good agreement is obtained between the predicted amount of acid or base and the measured one, and this is taken as a clear indication of the validity of the model. This model on the stoichiometric relation between acids and bases turns out to be a useful tool both for an interpretation of the amount of acid found in precipitation and in estimating future deposition of acid by precipitation. DOI: 10.1111/j.2153-3490.1972.tb01581.x