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Well S12Seria and Nearby Fields

The Seria field (Figures 5.30, 5.31, 5.32, 5.33 and 5.34) was discovered in 1929 when the exploration well Seria-1 (S-1), situated on an area of very strong surface oil and gas seeps and on a gravity anomaly first defined in 1926, began flowing oil and gas while drilling at 288 metres (see the story of the discovery of the Seria field).

Development of the shallow, heavy (20° API, 940 kg/m3) oil proceeded rapidly and by 1936, production had reached 1,590 m3/day m from 77 wells. With the subsequent development of the deeper, light (38° API, 835 kg/m3) oil, production rose to 2,700 m3/d. By the outbreak of World War Il, 158 wells had been drilled.

After the war, the predominantly light oil reservoirs were intensively developed and in 1956 production peaked at 18,780 m3/d. Offshore development began in 1955 from multi-well marine platforms situated near the shoreline. By 1958, 500 wells had been drilled in the field.

In the late 1950's production began to decline rapidly. Several secondary recovery projects in the 1960's were unable to moderate this decline. Development activity was reduced and by 1977 production had fallen below 4,770 m3/d. Because of increased oil prices in the 1970's further primary development (for example in the Tali area) and further secondary recovery projects became attractive. Production increased to 5,550 m3/d by 1985. The interest in secondary recovery projects waned with the decrease in oil prices in the mid 1980's. Water injection into some heavy oil reservoirs is now employed, albeit on a limited scale. A pilot project for Enhanced Oil Recovery (EOR) in the Panaga Main block, involving surfactant, proved unattractive. Several infill drilling campaigns of varying extent have been carried out during the last 20 years: the most recent targeted the Tali TB block in 1993/94. By 1991, the Seria field had produced a total of a billion barrels of oil (159 million m3).

A total of 774 wells have now been drilled in the field, of which 307 are currently producing (1/1/1996). The average daily production of the Seria field over 1995 was 2,400 m3/d, almost equally divided between heavy and light oil.

The Seria field is a giant field by world standards, and cumulative oil production has already exceeded 162 million m3. The ultimate recoverable oil is estimated at 175 million m3 (recovery of 38%) and 46 109 m3 of gas (recovery of 90%).

The Rasau and Enggang fields constitute smaller, fault and dip bound structures lying along the same structural trend (Figures 5.30, 5.31). The Tali accumulation is considered as part of the Seria field. Cumulative production from the Seria (including Tali), Rasau and Enggang accumulations has now reached 166 million m3 of oil.

The Seria area is covered by 3D exploration seismic, acquired from 1989 to 1992.

A detailed history of the technical and social aspects of the development of the Seria field was published in 1975 by the Muzium Brunei (Harper, 1975).

The internal complexities and high degree of compartmentalisation of the Seria field has required a large number of development wells. Within the field, over 500 different fault and dip closed accumulations have been found to date, each containing several of the more than 70 known vertically stacked hydrocarbon-bearing reservoirs (Figures 5.32, 5.33 and 5.34).

Two distinctly different types of oil occur: lighter (30 - 45°API, 876 - 802 kg/m3) waxy crudes are found at deep levels from 1,200m to 3,000m. Above 1,200m, bacterial biodegradation has resulted in relatively heavy (17 -25°API, 953 - 904 kg/m3) non-waxy oils with a higher sulphur content. Some 80% of the proven light oil is located from 1,200 to 2,100 metres depth in the westerly part of the field; mainly within two large relatively unfaulted blocks on the southwest flank: the Panaga Main and Mumong Main blocks (Figure 5.31). These blocks together contain 35% of the total field STOIIP. The heavy oil is more evenly distributed over the field, and much of it is contained within relatively small fault blocks in the intensely faulted crestal areas of the field. The presence of accumulations on both flanks of the anticline suggests that charge has been available from both the Inner Shelf syncline to the north, and the Badas syncline to the south. Maturity maps of the area support this, and show extensive present day oil and gas potential from both synclines.

There is evidence that continuous secondary migration has occurred within the field itself, partly driven by the arrival of newly generated hydrocarbons. The breakdown of partially sealing faults due to continuing structural uplift, and pressure changes resulting from reservoir depletion, have also played a role in the re-distribution of hydrocarbons.

Although pressures in the objective reservoirs of the Seria field are generally close to hydrostatic, overpressures approaching lithostatic pressure have been encountered at deeper levels (Figure 5.35). In the Tali and Enggang area (Figure 5.31), inflation overpressures result in sudden large pressure increases across specific shale breaks.

Oil recovery mechanisms and recovery factors vary over the Seria field, depending on the density of the oil and the degree of fault compartmentalisation and aquifer support. Light to medium oil reservoirs, contained in relatively large blocks situated on flanks exposed to aquifer support, have generally higher water drive and consequently higher recovery factors. Such blocks (e.g., Mumong Main and Tali) have recovery factors in some reservoirs as high as 50%. Even so, in the Mumong Main block, the effects of varying permeability between reservoirs, and a strong directional component of aquifer support, mean that the degree of water drive is highly variable, both from reservoir to reservoir, and laterally across the block. In this block, areas and reservoirs with low relative permeability to water (in comparison to oil) experience relatively little water drive, and may therefore contain by-passed oil. The main direction of aquifer support appears to be from the southeast, and the western area of the block has relatively low water drive. In such areas, where water drive is weak, gravity drainage (with gas/oil segregation) becomes significant.

These effects have resulted in the current situation in the Mumong Main block, in which there are secondary gas caps in some reservoirs and water-invaded zones in others.

In areas of greater fault intensity such as the relatively isolated crestal blocks, aquifer influx is minimal, and gravity drainage and solution gas drive are the major recovery mechanisms for the light oil. The faulting may allow cross-flow to occur between the reservoirs; an effect which makes the volumes of individual reservoirs difficult to define from production history.

Recovery from the heavy oil reservoirs, which are at relatively shallow depths, is from a combination of depletion drive, solution gas drive and water drive. Water drive is small to moderate, and as for the light oil reservoirs, aquifer support is limited in areas of high fault intensity and compartmentalisation. Production from the heavy oil reservoirs is often at high water cut owing to an unfavourably high mobility ratio of water in comparison to oil, and relatively shorter column lengths. In some areas the recovery factor is as low as 10%.

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