Significance This work reports direct measurements of methane emissions at 190 onshore natural gas sites in the United States. The measurements indicate that well completion emissions are lower than previously estimated; the data also show emissions from pneumatic controllers and equipment leaks are higher than Environmental Protection Agency (EPA) national emission projections. Estimates of total emissions are similar to the most recent EPA national inventory of methane emissions from natural gas production. These measurements will help inform policymakers, researchers, and industry, providing information about some of the sources of methane emissions from the production of natural gas, and will better inform and advance national and international scientific and policy discussions with respect to natural gas development and use.

Emissions from 377 gas actuated (pneumatic) controllers were measured at natural gas production sites and a small number of oil production sites, throughout the United States. A small subset of the devices (19%), with whole gas emission rates in excess of 6 standard cubic feet per hour (scf/h), accounted for 95% of emissions. More than half of the controllers recorded emissions of 0.001 scf/h or less during 15 min of measurement. Pneumatic controllers in level control applications on separators and in compressor applications had higher emission rates than controllers in other types of applications. Regional differences in emissions were observed, with the lowest emissions measured in the Rocky Mountains and the highest emissions in the Gulf Coast. Average methane emissions per controller reported in this work are 17% higher than the average emissions per controller in the 2012 EPA greenhouse gas national emission inventory (2012 GHG NEI, released in 2014); the average of 2.7 controllers per well observed in this work is higher than the 1.0 controllers per well reported in the 2012 GHG NEI.

Methane emissions from liquid unloadings were measured at 107 wells in natural gas production regions throughout the United States. Liquid unloadings clear wells of accumulated liquids to increase production, employing a variety of liquid lifting mechanisms. In this work, wells with and without plunger lifts were sampled. Most wells without plunger lifts unload less than 10 times per year with emissions averaging 21,000-35,000 scf methane (0.4-0.7 Mg) per event (95% confidence limits of 10,000-50,000 scf/event). For wells with plunger lifts, emissions averaged 1000-10,000 scf methane (0.02-0.2 Mg) per event (95% confidence limits of 500-12,000 scf/event). Some wells with plunger lifts are automatically triggered and unload thousands of times per year and these wells account for the majority of the emissions from all wells with liquid unloadings. If the data collected in this work are assumed to be representative of national populations, the data suggest that the central estimate of emissions from unloadings (270 Gg/yr, 95% confidence range of 190-400 Gg) are within a few percent of the emissions estimated in the EPA 2012 Greenhouse Gas National Emission Inventory (released in 2014), with emissions dominated by wells with high frequencies of unloadings.

Our main result is the discovery of an optimum acid injection rate to obtain acid breakthrough in linear corefloods of carbonates using a minimum total acid volume. Low rates result in acid spending at the core surface while high rates result in the formation of multiple, highly ramified wormholes. At the optimum intermediate rate, a single, small wormhole penetrates the core. The optimum acid rate is found to be a function of the rock composition and reaction temperature as well as the pore size distribution of the virgin formation rock. All of these factors are included in the theory developed here. This theory provides a quantitative prediction of the optimum rate. The practical ramifications of the results are also considered. Introduction Williams et al. recommend that carbonate acidizing treatments be carried out at the highest rate possible without fracturing the reservoir rock. This strategy is in complete accord with the findings reported by Paccaloni and Tambini based on their extensive field studies. Daccord et al. have, on the other hand, proposed a design procedure that requires low acid injection rates to achieve optimum stimulation with a given acid volume. Experiments reported to date do not, in fact, resolve this conflict. Hoefner and Fogler conducted acidizing experiments using both Indiana limestone and dolomite cores. For dolomite at room temperature, higher injection rates resulted in more total acid being required to achieve a given acid penetration in agreement with the concepts of Daccord et al. With Indiana limestone, smaller quantities of acid were found to be required to achieve a given penetration as the acid flux was increased. Thus, the reported laboratory experiments are ambiguous and do not support either one of the conflicting recommendations. One purpose of this paper is to resolve this issue by showing experimentally the existence of an optimum acid injection rate. Thus, whenever the injection rate exceeds the optimum, a reduction in rate will improve performance. Similarly, rates that are less than the optimum must be increased. Furthermore, the optimum will be shown to be a complex function of the reservoir composition, temperature, and pore structure of the virgin rock, so that there can be no simple rules as to whether slow or fast rates are best. P. 675^

The efficiency of the matrix acidizing process in carbonates depends strongly on the wormholing phenomenon - if worm holes are formed, the effects of near wellbore damage can be overcome with relatively small volumes of acid. Numerous previous studies have shown that worm hole patterns can be placed in these general categories:compact dissolution in which most of the acid is spent near the rock face;the wormholing pattern; anduniform dissolution in which many pores are enlarged, as typically occurs in sandstone acidizing We have developed a theory of the wormholing process which predicts when the wormholing pattern is most efficiently created as a function of the acid flux and other treatment variables. By testing this theory with several independent sets of laboratory data, we can now demonstrate the important roles that surface reaction rate and fluid loss play in the wormholing process. This theory accurately predicts the optimal flux (that which leads to dominant wormholes with a minimum of branching and hence a minimum acid volume) for experiments with HCl in limestone and dolomite at several temperatures and with acetic acid in limestone. Surface reaction rate differs by several orders of magnitude in these experiments and is the only process variable that differs greatly among them. Paradoxically, though worm holes are formed because the overall reaction rate is controlled by mass transfer in the wormholes, diffusion rates play only a minor role in the wormholing process. Fluid loss through the walls of the wormholes ultimately limits the distance to which worm holes can propagate. Because of this effect, laboratory linear core floods will give optimistic predictions of worm hole penetration distances. We developed a cylindrical flow model to represent the flow field around a worm hole propagating from a wellbore which illustrates how to translate laboratory results to field conditions. We have used these theories to predict optimal acid formulations and injection rates for field conditions. In general, the lower the reaction rate (such as at low temperatures in dolomites or with weak acids in limestones), the lower the injection rate required, making it easier to propagate dominant wormholes under matrix treating conditions in the field. Introduction Numerous studies of the wormholing process in carbonate acidizing have shown that the dissolution pattern created can be characterized as being one of three types:compact dissolution in which most of the acid is spent near the rock face;the wormholing pattern; anduniform dissolution in which many pores are enlarged, as typically occurs in sandstone acidizing. These studies have also shown that the acidizing process is most efficient (defined as the process that will enhance near-wellbore permeability to the greatest depth with the smallest volume of acid) when the wormholing pattern develops. A third observation common to these studies is that the pattern created depends on acid flux, with the compact pattern created at relatively low acid flux, the worm hole pattern developing at intermediate flux, and the uniform pattern at high flux. Of course, there is not an abrupt transition from one pattern to another. As acid flux is increased, the compact pattern will change to one in which large diameter worm holes are created; further increases in flux yield narrower wormholes which propagate farther for a given volume of acid injection; and finally, as acid flux is increased more, the worm holes become more and more branched until ultimately the uniform pattern is observed. P. 775^