Overpressure

About: Overpressure is a research topic. Over the lifetime, 3236 publications have been published within this topic receiving 34648 citations.

Magnitude, mechanisms, and prediction of abnormal pore pressure using well data in the Krishna–Godavari Basin, east coast of India

Abstract: Abnormal pressures are encountered during exploration drilling in different parts of Krishna–Godavari Basin on the east coast of India. The nature and stratigraphic occurrences of the overpressure zones vary across the basin. Three different clusters of wells, covering a large part of the basin encompassing both onshore- and offshore-drilled wells, are analyzed to capture this variation. A wide range of pore-pressure gradients from normal to as high as 18 MPa/km was observed in the present data set. The tops of overpressure zones demonstrate a large range from 2200 to 3000 m (6562 to 9842 ft). These depths generally correspond to either a Miocene deltaic sequence or Cretaceous synrift and postrift sequences. Available well data reveal two main reasons for the development of overpressure. Considerably high pore-pressure regimes in the Cretaceous sequence in the eastern corner of the basin are found to be mainly caused by gas generation, whereas disequilibrium compaction is proposed as the main cause for overpressure in the other parts of the basin. The outcome of this analysis provides a fair idea of the nature, magnitude, and distribution of the overpressure, and this will also help to strategize further exploration activities in the basin.

Flame acceleration of premixed methane/air explosion in parallel pipes

Abstract: The explosion propagation characteristics in parallel pipes have been studied for the first time in two different types of pipe. It was found that flame speed and overpressure in two branches of equal length were close when the ignition was at the pipe head. The explosion violence was strengthened after the flame and blast wave were superimposed. When the ignition was acted in the corner of one branch, the peak overpressure near the meeting point was higher than that at the two adjacent points, while the flame speed showed a downtrend. When the parallel pipe was not full of gas, the peak overpressure in the two branches followed similar trends and showed an obvious downtrend. However, when the blast wave reached the meeting point, the peak overpressure also showed an obvious upward trend until the end of the pipe. The flame accelerated in the two different branches of unequal length, but it slowed down when traveling to the meeting point, and the peak overpressure evolution was contrary to the flame speed. The results suggested that the violence of the underground gas explosion near the meeting point was more serious, such that the equipment and people in this area should be paid more attention.

Porosity trends in the Skagerrak Formation, Central Graben, United Kingdom Continental Shelf: The role of compaction and pore pressure history

Abstract: This paper describes reservoir properties in the Triassic Skagerrak Formation in the Central North Sea. This prolific sandstone reservoir often possesses anomalously high porosity for its depth of burial. Simple statistical analysis of wire-line-log-derived porosity data is used to derive empirical trends as a function of both depth and vertical effective stress that show variations between neighboring hydrocarbon fields and between different parts of the basin. Porosity data from the Josephine (J) Ridge (Quadrant 30 of the United Kingdom Continental Shelf [UKCS]) show a marked degradation with depth, but the porosities are significantly higher than in similarly deeply buried areas such as the Puffin high to the west (Quadrant 29) or the Forties–Montrose high to the north (Quadrant 22). To understand the porosity patterns better the data have been analyzed by plotting against vertical effective stress. This allows a better comparison to be made between fields and wells within the high-pressure–high-temperature (HPHT) realm. High pressure here refers to fluid pressures above 10,000 psi (), whereas high temperatures are above 300°F (149°C). Results show that porosity and fractional effective reservoir (the proportion of net sandstone with a porosity greater than a predetermined cutoff) decrease systematically with increasing vertical effective stress. Data from the different J Ridge fields fall on a common compaction trend even though they are derived from structures with marked variations in present-day depth of burial and static formation overpressure. Trends from the other areas of the Central Graben (the Puffin and Forties–Montrose highs) indicate more indurate reservoir states. The observed porosity trends are independent of fluid type within the reservoir and the absolute magnitude of overpressure. The main observed hydrocarbon effect is the result of buoyancy forces. The analysis supports the contention that, after accounting for facies-related grain-size variations, compaction controls average reservoir properties. Differences in compaction state between areas are postulated to relate primarily to structurally controlled timing of overpressure development relative to burial, and how these affect the resultant vertical effective stress history. Both the Puffin and Forties–Montrose highs are directly attached to the basin margins across stepped faults. These marginal terraces were open to lateral fluid flow for longer probably because across-fault seals were only established late in the burial history when higher temperatures promoted cementation and the destruction of permeability within fault cores. As a result, they developed overpressures in the last 5–10 m.y. or so and are largely normally compacted. The J Ridge horst block is hydrologically more isolated within the basin center by across-fault juxtaposition seals. Here, overpressure development appears to have started earlier, possibly between 50 and 60 Ma, retarding compaction and allowing preservation of higher porosities. Compaction continues to present day driven by the large static vertical effective stress gradients in these deeply buried reservoirs. The observed empirical trends offer a means of predicting average reservoir properties in deep untested exploration targets.