Integrity issues and water production have been the main challenge in high viscosity fields in the South of Oman salt basin. The complex heavy oil fields started production in 1985 and to date, produce heavy oil (21 API) from more than 1,000 penetrate sandstone reservoirs of which 80% are horizontal. The historical performance of the fields were analysed from 2015 to characterize rock behaviour. This showed that water cut progression was naturally occurring in fractures, which are limited in length and not vertically extensive, creating unexpectedly high initial water production. From first production until 2004, the water cut of the field gradually increased from zero to 30%, averaging 20% in the 10-year period. After 2004, the average initial base sediment and water (BSW) of new wells jumped to 85%. Even wells with initially low BSW developed high water cut within two to three months.
During early stages of production, when the initial water-cut shows matrix-like behaviour, the bottom water is not in contact with the fracture network. However, after years of production and rise of the water table, the fracture behaviour became dominant as water came into contact with the small vertical fractures. Several multi-disciplinary studies were conducted by a major operator in Oman to identify causes and recommend mitigations. Several options, including precise swellable packers placement and smart well completion technologies, were investigated to stop water encroachment into the wells and to improve productivity.
Autonomous advantages
Independent global completions service company Tendeka has worked with the operator for nearly 20 years, which currently has around 209 producing oil fields and more than 8,000 active wells.
In 2018, to control the flow of the fluids and reduce unwanted water production in the South of Oman salt basin, Tendeka developed and deployed the Autonomous Inflow Control Device (AICD) to be piloted in two newly drilled horizontal wells in two fields located in the basin. The reservoir contains viscous oil (400-600 cp) and exhibits high inflow potential due to favourable rock properties and strong bottom water aquifer. Each well was completed with AICDs along a completed lateral length of 500m.Horizontal wells were used to maximize reservoir oil contact and improve recovery from the reservoir. The wells, in general, produce oil with very high water cut (>95%).
AICD technology is comprised of mechanical devices installed with the sand face completion (Figure 1).As an active flow resistance element distributed along the length of the horizontal wellbore section, the device can delay and reduce the proportions of water breakthrough. Due to the real-time reaction to the local fluid dynamics within the tool it works by imposing a relatively strong resistance for low viscous fluids over high viscous oil. Water inflow into the wellbore is thereby restricted and oil production increased. It is important to note that AICDs are not downhole separators and cannot change water to oil.
To improve oil production, there must be a variation in the oil saturation or oil cut along the length of the wellbore. In other words, what comes in must go out. Therefore, if the whole length of the wellbore is at 90% water cut, the AICD cannot make an improvement. AICDs react to the properties of the fluids passing through them and can act as an ‘insurance policy’ on well performance uncertainty.
It operates without the need for human or surface interventions and electric or hydraulic power (Figure 2). Once installed and the completion is in the ground, no interpretations, actions or decisions are required.
The AICD valve is assembled as part of the sand screen joint (Figure 3). The flow path from the reservoir is marked by arrows. The reservoir fluids enter the completion through the sand screen filter and flows into the inflow control housing where the AICD is mounted. The fluids then flow through the AICD and into the production stream and flows to surface together with the production from the rest of the screens.
New wells – AICD on trial
The typical well performance of the first field (field-A) was mostly water with 600 m3/d of gross. It had suffered from high water cut since day one, as the average of BSW for the field is 96 to 97%. Normally, this field would be completed with 4.5’’ wired wrapped screen (WWS) with swellable packers to isolate the major fracture.
The first AICD trial was performed in one of five wells (A-111) drilled in this field. It showed excellent performance where the water cut remained below 5% with 50 m3/d oil gain. The well was completed with seven compartments with 4.5’’ blank, 14 AICDs and seven swellable packers. This was in comparison to 98% of the offset wells.
The second well is B-130 in field-B. The typical performance of this new well is 10-20 m3/d oil with BSW of 75 to 90%. The completion of the well is normally 7’’ WWS with or without packers, dependent on the condition of the well. This was completed with five compartments with 7” blank, 13 AICDs and five swellable packers. The well test results showed 90 m3/d net oil and <5% water cut.
AICDs in existing wells
The typical well performance of the first field (field-A) was mostly water with 600 m3/d of grIn order to control the water production for existing wells, the normal practice is to isolate some compartments from the horizontal section mechanically or in some cases, by chemical water shut-off (WSO). To conduct well intervention, a hoist was required for both procedures:
• The chemical method was used to restrict or plug the high permeable zone that has high water influx
• The mechanical WSO prevents the production from the selected zone by either using cement or specific device.
A good interpreter of the fractures in horizontal wells can greatly help select the expected high water sections. However, this method has many uncertainties. Mechanical WSO installed in different ways, for example, depends on the orientation of the well and the crossed lithology.
The most common mechanical WSO is by making segmentations with swellable packers and WWS. After a period, the horizontal section can either shut-off from the heal, mid or toe sections.
As fields A and B have a high water cut of more than 98%, different WSO methods were applied with a 50% success factor as outlined below:
• 120 m3/d to 58 m3/d
• 100% net oil gained (17 m3/d to 29 m3/d)
• 300% reduction in water production (92 m3/d to 29 m3/d).
The trials on new and existing wells in the South Oman fields have shown that the AICD application is an economical solution to restrict water production and gain oil for viscous oil reservoir. It is also a very effective solution for water shut-off compared to conventional completions with mechanical or chemical water shut-off.
Essentially, the higher the number of compartments, the more efficiently AICDs perform to restrict water production and gain oil. This also results in a clearer contrast in viscosity between the fluid we want to produce (Oil) and the fluid we want to control (water or gas).
