Because of the change in well design, so that long horizontal wells, many with a number of lateral feeds, known collectively as Maximum Reservoir Contact (MRC) wells, located within the top 5 ft of the reservoir are now used, initial declines in production in the major fields have largely stopped. This is partially explained because as a vertical well through an oil reservoir sees the pool getting smaller, the length of well productively exposed to the oil in the reservoir reduces, and so there is a steady reduction in well performance over the years. Where the oil is, however, being pushed up to the horizontal well by an under-flood of injected water (which sustains the differential pressure between the fluid in the rock and the well bore) then the exposed length, and the driving pressure both remain relatively constant, and production is sustained at closer to a constant level until the water flood reaches the well, when the well dies.
Aramco have changed the design of their MRC wells so that the arrival of water at one location along a lateral is no longer sufficient to kill the well. Installed valves isolate the region of the well where the water enters, and the rest of the well can remain in production. But it will produce at a reduced rate, and is an early warning that the water levels are nearing the well location, and that, before long, the well will no longer be able to sustain the x,000 bd production which has been the characteristic of most Saudi wells for over five decades. This is not to decry the efforts that have been made to recover residual oil left in those fields. As I have noted Aramco have been diligent in seeking to find additional ways to extract the oil that has been left as the waterfloods progressed through their senior fields. But they are also smart enough to know that alternate fields would have to be developed, and brought on line as the limits in the older fields are reached, as they now have been, in some cases for years. There is an interesting feature to the sources of production, back in 1994.
Figure 1 Saudi oil production, by field, in1994 (Matt Simmons, “Twilight in the Desert”)
Figure 2. Oilfields in Saudi Arabia (from Aramco via energy-pedia
The immediately obvious characteristic is that they are, at least to some degree (as with Berri) land-based. (Other less obvious differences are a change in the host rock and the quality of the oil). The most significant of the remaining four is Safaniya which came on line in 1951. But it was not, immediately, produced.
Aramco, in accordance with the terms of its concession, went ahead with the careful development of the field. Between 1951 and 1954, 17 wells were drilled, but they were not produced. . . . . . When it was first put in production in 1957, it flowed 50,000 barrels of crude oil a day from 18 wells. At the beginning of 1962 it possessed the facilities to handle 350,000 barrels a day (almost 128 million barrels a year) from 25 wells.It was found to be the world’s largest offshore oilfield, and Matt Simmons has conjectured (in Twilight in the Desert) that it is connected to Khafji and through that field into Burgan. When Saudi oil production peaked in 1980/81 he notes that it was producing at over 1.5 mbd. Since then production fell to around 600 kbd, but then has increased back to 900 kbd with plans now afoot to bring it back up to full volume of earlier levels of production, which will require additional forms of artificial lift this being the electrical submersible pumps that have already been introduced into Ghawar.
The phase 1 upgrade at Safaniya is anticipated to be completed by next year. The oil is found in sandstone, rather than the carbonates at Ghawar, and in the original development Matt has noted that the weak nature of the sand was causing the wells to collapse as the oil was removed, and that the flow of water into the reservoirs was bypassing a lot of the oil being left in place. As other fields have been developed, they have largely been brought into production fairly quickly. This has not been the case at Safaniya, which as Matt noted:
. . .holds the entire remaining spare daily oil supply of any magnitude . .In the sense that other fields and opportunities take a little time to bring on line this remains true.