Shallow Gas Pockets Research Articles

Isotopically light methane in natural gas: bacterial imprint or diffusive fractionation?

The evaluation of the economic importance of bacterial methane is still a matter of discussion, linked to the evolution of the geochemical understanding. Methane occurrence or accumulations with light isotopic ratios, both for carbon and hydrogen, were thought in earlier days to be primarily due to migration. Then, from several works presented in the eighties, it was claimed that (a) bacterial methane had the same light isotopic signatures as those found in some gas accumulations, mainly at shallow depths, where bacterial activity is possible, and that (b) migration could not induce measurable isotopic fractionation. This gave a non ambiguous origin (bacterial occurrence) for any gas having 12C-enriched methane. Looking back at the experimental and physical evidence of this last assessment, and taking into account the consequences of some new series of gas migration experiments, we now consider that both explanations for isotopically light methane (bacterial generation or migration fractionation) are realistic. Using a simple diagram (ethane over methane versus the carbon isotopic ratio of methane), and modelling the two discussed processes (mixing of bacterial and thermal gas, or diffusion of thermal gases), it is possible in several cases to decipher the origin of the gas series. It would appear for example that Italian gases correspond grossly to a simple mixture between bacterial and thermogenic gases, whereas other gases of various origins (head space gases, gases from German coals, shallow gas pockets in recent sediments, Mexican deep overmature gases) show a trend that we interpret as a result of diffusive fractionation.

Italy--Adriatic Sea--Barbara--A Giant Gas Field Marked by Seismic Velocity Anomaly--A Subtle Trap: ABSTRACT

Barbara gas field, discovered in 1971, is located in the northern sector of the Adriatic offshore. The field is a gentle anticline involving Quaternary clastic sediments and shaped by carbonate Mesozoic morphology. The presence of shallow gas pockets at the crest of the structure distort the seismic signal to such an extent that structural reconstruction using seismic data is not possible. Moreover, time delays and ray-path anomalies do not allow the use of staking velocities for the depth conversion. Seismic attribute analysis, instead of velocities, and time delays on the isochrone maps are providing a key to the understanding of seismic anomalies and are an indirect tool for reconstructing the real structural configuration of the field. The appraisal story of the field illustrates how the previously mentioned complications influenced its delineation and how an understanding of these complications helped in upgrading the reserves from an initial value of 10 billion ECM of gas to 40 billion ECM. Additional data acquired with the development wells tend to increase the estimate. Therefore, Barbara field is the most important Italian gas field of the decade. The producing formation is composed of very thin-bedded sandstone and shale intercalations, representing the peculiarity of this reservoir.more » Development of the field is being achieved with six production platforms and 72 wells.« less

Seismic evidence for an extensive gas-bearing layer at shallow depth, offshore from Prudhoe Bay, Alaska

High-resolution seismic reflection data, recorded offshore from Prudhoe Bay, Alaska, were processed digitally to determine the reflectivity structure of the uppermost layers of the seafloor. A prominent reflector, found at 27 m below the mud line (water depths 7–9 m), has a negative reflection coefficient greater than 0.5. The large acoustic impedance contrast, coupled with a report of gas encountered at a corresponding depth in a nearby drillhole, shows that the reflector is the upper boundary of a zone containing gas. The gas exists in sandy gravel capped by stiff, silty clay. Analysis of unprocessed conventional high-resolution records from the region indicates that the gas-bearing layer may extend over an area of at least 50 km 2 at a depth of 20–35 m below the mud line. Similar-appearing reflectors (Reimnitz, 1972), previously unexplained, occur in patches over wide regions of the shelf where offshore oil development is beginning at a rapid pace. This suggests the exercise of caution with respect to possible hazards from shallow gas pockets.

Curved Well Conductors and Offshore Platform Hydrocarbon Development

This paper discusses drilling offshore directional wells with conventional equipment through curved conductor pipes. Curves and graphs are presented to compare the lateral-reach characteristics of directional wells drilled through curved and vertical pipes. Well trajectory, directional drilling data, and tubular information illustrate the success achieved by drilling wells through curved conductors. Introduction The 1970 Federal Offshore Louisiana Lease Sale required the development of shallow-depth hydrocarbons from sites in 150 to 400 ft of water. Because the lateral reach of a conventional vertical conductor was limited in depths ranging from 3,500 to 6,000 ft, several platforms would be required to handle these requirements. The potential for extended lateral reach prompted a review of potential for extended lateral reach prompted a review of unconventional methods to improve platform well reach. (The term, conductor, used in this paper refers to the part of the well tubulars often called drive pipe.) Fig. 1 illustrates the reach improvement obtained when the rate of building drift angle increased from 2 degrees to 4 degrees per 100 ft (55 degrees maximum). If an angle builds faster below the per 100 ft (55 degrees maximum). If an angle builds faster below the conductor, the coverage of vertical conductors is improved noticeably, as evidenced by the area factor values in Fig. 1. Initiating well deflection above the water instead of below the conductor also improves the reach significantly. In discussing the problem with drilling engineers, we found that a well could be drilled around a curve of 6 degrees per 100 ft if the curvature was smooth and drillstring weight below the curvature minimized. This rapid increase in drift angle through the 150- to 400-ft water zone of an offshore platform can improve the mud-line drift angle 9 degrees to 24 degrees, respectively. When an angle improvement above the mud line is combined with that below the mud line, the curved conductor becomes a very useful tool. This method was evaluated with a successful field-test well in 1970. Well Planning Considerations Generally, curved conductors are most advantageous at water depths of 150 ft when developing hydrocarbon reservoirs shallower than a true vertical depth of 7,000 ft. These conductors have applications not only in conventional rig development drilling, but also in exploratory drilling from platforms with conditions that normally could require the more expensive mobile rigs. The improved reach characteristics of curved conductors has a significant impact on the choice of platform sites. Locations may be selected at some distance from the center of hydrocarbon accumulation to avoid surface and near-surface obstructions such as salt plugs, shallow faults, sea-bed reef deposits, gas seeps, shallow gas pockets, mud slides, debris, and shipping fairways. Final pockets, mud slides, debris, and shipping fairways. Final choice of platform location also is influential when considering the most efficient reservoir development and optimization of the drilling and completion techniques. The design of an individual well trajectory is influenced by reservoir size, required maximum lateral well displacement, and drilling or completion considerations. Drilling and completion efficiency generally is reduced with an increasing wellbore angle and by tong sections with varying hole angle. The curve conductor can minimize these problems by reducing the length of wellbore with varying angle and the maximum drift angle required to reach a target. JPT P. 440