Water is very often associated to the oil production, for geological reasons, but also because it is the most frequent mean of secondary recovery. However all fields are not comparable in their behaviour. In the best cases water is effectively contributing to the oil sweeping and the bulk of the oil reserves can be produced at low water cuts. In other cases, it can be said that water is inevitable to the oil and huge volumes of high water liquids must be lifted from an early stage to produce the oil.
In some cases, mechanical or chemical water shut off techniques can help to reduce the water production however, depending on the specific conditions, they are not always cost effective, their implementation can be tricky, and their efficiency may be limited in time.
Therefore the operator is often left with the standard solution of upgrading its field and process facilities to cope with the produced water constraints. However, due to the increase difficulties resulting from the drastic new environmental regulations, the operators tend to focus more than before on the produced water associated cost.
A typical offshore field production history and capex and opex breakdown was analysed to highlight the impact of the water on a field economy.
The new interest linked to the emerging technology of downhole separation and re-injection has motivated the testing of a DOWS unit on the well LA-90 in the Lacq Superieur field in France and results of this operation are presented.
Considering the shortcomings in the existing static cyclone technology which is implemented in the down hole separation systems, TFE has undertaken since three years a R&D program based on an innovative concept of rotary cyclone. The base of the theory and its implementation are presented along with the promising preliminary results.
In many places, water is inevitable for the oil. Although initial oil production from a field is often dry, the water often invites itself in the well at some stage, sometimes much earlier than initialy anticipated.
Some reservoirs are connected to large aquifers providing a strong pressure support to the oil production. Depending on the geology of the structure and on the reservoir characteristics, different schemes can account for the water production process.
In the bottom drive reservoirs, where the water is directly underlaying the oil, the water coning, resulting from the pressure drawdown applied to the formation, is governing the water production. In this type of configuration, the critical rate per well is generally too low to be economical. In fact, there are little reservoirs where an efficient gravity drainage can be implemented.
For the edge drive reservoirs, production wells are drilled much far away from the oil and water contact, but water tends to channel faster through high permeability drains and reaches the producers sometimes very early in the life of the field.
When the oil layer is only connected to a small aquifer, there is not a sufficient pressure support to compensate for the oil production. Then the reservoir pressure is decreasing with time, which is often very detrimental to the ultimate oil recovery. Hence, a pressure maintenance scheme is required and water (or gaz) has to be injected into the reservoir to balance the oil offtake.
Depending again on the reservoir characteristics and on the geology, the water injection wells can be located at the periphery of the oil layer, away from the producers, or on the contrary they must be drilled between the oil wells. Obviously, the same comments as before, concerning the risk of early water production, can apply to these schemes.
In all cases, an early water breaktrough results in a reduce sweeping efficiency which has a negative impact on the oil recovery. This is even aggravated by the unfavourable mobility ratio between the oil and the water, since the viscosity is generally higher for the oil than for the water. Therefore, a longer production period is required in order to make up for the delayed oil and huge cumulative volumes of water are produced ultimately.