Shell and Petroleum Development Oman (PDO) have two main priorities in the management of produced water in Oman: to minimize the volumes produced and to re-use the produced water in a beneficial and cost-effective manner. Before being put to use, most produced

A typical deepwater disposal installation with two pumps can inject up to 141,525 b/d of liquids. (All graphics courtesy of Shell and PDO)
water requires treatment to remove traces of oil, heavy metals, boron, corrosive fluids and other solids. The treatment and disposal of produced water is a significant operating expense for oil and gas companies, and PDO is no exception. Such costs include capital and operating expenses; utilities; and chemicals for lifting, separating, de-oiling, filtering, pumping and injection. In an attempt to minimize the health, safety and environment risks associated with injecting produced water into shallow aquifers and thus save a potential source of water for Oman, Shell and PDO are pursuing a range of technologies to phase out shallowwater disposal (SWD).

Oman’s aquifers
Water flow in the Oman hydrogeological basin is gravity-induced and topographically driven. The flow of meteoric water into the formation is through highland recharge, and the discharge is at relatively low relief areas. There are two mountainous areas in Oman, one in the north of the country (Oman Mountains) and the other in the south (Dhofar Mountains). Recharge takes place over the mountains into the Tertiary rocks. Recharge over the Dhofar Mountains (south) is the most important at present in that it is estimated that as much as 11.8 in. of misty precipitation takes place during the monsoon period (July-September). Precipitation over the Oman Mountains (north) is estimated at 5.9 in.

The hydrostratigraphic subdivision of rocks in the Oman basin is predominantly based on salinity data. Two main groups of aquifers have been recognized:
• The shallowest and currently most active aquifers are the Tertiary carbonates, approximately 1,640 ft (500 m) thick. The Tertiary interval contains two aquifers: the Fars/Dammam and the Umm Er Radhuma (UeR), underlain by the Shammar Shale aquitard. The thickness of this aquitard rarely exceeds 98 ft (30 m), and its continuity is questionable.
• The second main aquifer group comprises the fluvio-glacial and essentially continental deposits belonging to the Haushi and Haima group.

SWD phase out
A review of the salinity of the produced water (the chloride ion being the most mobile contaminant) and the receiving shallow water aquifers (Fars and UeR) for south Oman shows significant variation. The Omani Drinking Water Standard specifies water quality of total dissolved solids (TDS) of 500 mg/l up to a maximum of 1,500 mg/l.

SWD in the Marmul area seems to pose potential environmental risk compared to the Rima and Nimr areas. Hence, PDO was committed to phase out SWD into aquifers with salinity (TDS) lower than seawater (<35,000 ppm) by the end of 2001 for the Marmul area, while being allowed to postpone the SWD phase-out for Nimr and Rima until the end of 2004, in order to mature a potentially attractive alternative to deepwater disposal (DWD) in the form of reed-bed technology (see below) and to further investigate long-term injectivity problems and fracturing into deep disposal aquifers.

Deepwater disposal
To begin the process of phasing out SWD, DWD projects were commenced and completed in the Rima, Nimr, and Marmul areas. Rima DWD consists of one site that contains two phases. In Nimr, there are four deepwater disposal sites where site 1 (Phase 1) consists of five wells, including two observation wells, site 2 (Phase 2) has four wells, site 3 (Phase 3 and 4) five wells, and site 4 (Phase 5 and 6) six wells. Marmul DWD, on the other hand, consists of one site that contains one phase. The Marmul asset is also injecting some of the produced water for hydrocarbon recovery and pressure maintenance.

The surface set-up or configuration of a typical DWD site with two or more variable speed drive (VSD) or fixed speed drive (FSD) pumps can deliver a total injection rate of 22,500 cu m/d at 145 Bar each.

Extensive efforts have been made to design, monitor and optimize the performance of DWD wells. The injection into DWD wells is taking place under fracturing conditions, which is the only way to get high volumes of produced water deep underground. Close monitoring of fracture behavior and fracture growth that might grow vertically out of zone to breach the caprock is pursued. Subsurface studies and fracture simulations have been performed in an attempt to understand and positively manage the fracture behavior during injection. For example, the PWRI-FRAC well simulator, a Shell proprietary fracturing tool that can predict fracture dimensions and growth over time, has been used to estimate fracture dimensions (length and height) and assess the impact of fracture growth to caprock integrity. Coupling the PWRI-FRAC tool with the reservoir simulator has enabled accurate prediction of fracture growth and dimensions and the study of the effect of injection to pressure build-up in the reservoir. Surveillance and data gathering exercises have been the key to understanding fracture behavior and management of DWD projects. Injectivity tests, production logging (PLT), fall-off tests, downhole microseismicity and tiltmeters are well-test methods and fracture monitoring technologies that have been deployed in key DWD projects such as in Nimr. These studying and monitoring efforts have enabled the areas of concern — Marmul, Nimr and Rima — to optimize their well and site locations, improve well injectivity targets, and optimize the entire DWD performance to phase out SWD.

Another approach includes minimizing the volume of produced water through:
• re-using water through biosaline agriculture (reed beds);
• reducing water through downhole treatments (downhole oil and water separation and shut-off); and
• recycling water through PWRI for water injection and waterflood projects.
The company explored alternative technologies to get rid of produced water.
Water re-use
In their Greening the Desert initiative, Shell and PDO are exploring technologies to enable the re-use of produced water in areas around desert oil fields. Where water does not serve any purpose for reservoir management, re-use is seen as an enabler for new value creation by stimulating opportunities such as agriculture or forestry.

Since 1999, PDO has piloted the treatment of produced water by reed beds at an experimental site of six hectares in the Nimr area. Although reed-bed technology is an
The Greening the Desert initiative makes good use of produced water.
established method for treating industrial wastewater, its application for the treatment of produced water is still novel. Reeds are halophytes that grow well in saline environments. The salinity of reed-bed effluent water in Nimr is moderate i.e., (between 8,000 and 10,000 mg/l). Operations have demonstrated that plant growth can be sustained even in desert environments and that natural processes in the reed-bed degrade residual oil and cleanse the water of heavy metals.

Part of the reed-bed effluent water is fed directly to test plots for irrigation of biosaline agriculture with selected salt-tolerant crops and trees. The remaining volume of treated water is collected in two evaporation ponds and a spray-evaporation pond. The performance of the reed beds has been monitored and evaluated on the basis of key performance parameters.

• Effluent quality. The inlet oil-in-water (OIW) concentration is about 200 to 300 ppm (injection water quality) with a salinity of 6,000 to 8,000 ppm; the outlet concentration for oil is in the range of 0 to 3 ppm with a salinity around 10,000 ppm. Heavy metals are removed to concentrations that meet irrigation standards.
• Oil treatment capacity. Based on chemical analyses over a 6-month period in 2003, a capacity of oil treatment was calculated using standard hydrology methods. The treatment capacity of a reed bed is calculated to be 17.1 ml oil/sq m/d. The trial has shown that occasional spikes of OIW would not be detrimental to the reed beds.
• Lifecycle cost. The unit technical cost for water cleanup by reed-bed systems is estimated in the range of US $0.03 to 0.08/cu m.

Solar-driven desalination techniques have been investigated (Solar Dew) since there is scope for the extraction of a large fraction of fresh water, given moderate salinity.
After treatment by reed beds, two limiting factors for crop selection are the salinity of the water and boron. The salinity of the water after biological treatment could range between 6,000 and 11,000 ppm, boron would have a value between 4 and 7 ppm, and OIW content would be less than 3 ppm. Boron may be a limiting growth factor for certain agricultural options, although species that are salt-tolerant can generally also tolerate high boron concentrations. Boron removal at the present time is not economically viable although technically feasible.

A PDO imperative is that produced water not enter the food chain, thus crops for human consumption or fodder crops are excluded from the list of water-use options. Instead the effluent of the reed beds may be used for agriculture and forestry, e.g., to produce fibers for construction material. A forestry system should be adjusted to local conditions such as soil depth. Eucalyptus trees are salt-tolerant but have been restricted in Oman due to their high water demand. However, since a plantation would not use water from readily available aquifers, an exemption for a desert Eucalyptus forest may be made.

A fairly constant supply of water may be obtained from the reed bed as feed for a forestry system. The water supply should match the variable demand by trees as a result of seasonal variations. The requirement for water storage is minimized, noting that trees may grow with less water than under optimal watering conditions during the hotter months, while during cooler months a larger area can be irrigated at the capacity of the tree uptake.

The relative economics for treatment and re-use of produced water in upscale bioremediation treatment was compared to deepwater disposal. A large-scale reed bed system may be sized using the design data presented here, i.e., remediation potential of 17.1 ml oil per day per square meter of reed bed, allowing an outflow up to 5 ppm OIW. In order to treat 45,000 cu m/day by reed beds, with an average OIW content of 250 ppm in the feed, approximately 65 hectares of reed beds would be required. Calculations show the treatment costs of about $0.05/cu m.

In the case study, the reed-bed effluent would be used to irrigate a eucalyptus forest; the proceeds would be about $0.01/cu m of produced water. Proceeds from forestry to recover costs will come longer term, since it takes time for trees to grow (discounting assumes 12 years).

Water shut-off
PDO’s current water production of 4.5 million b/d is expected to increase to more than 7
Reed beds at the Nimr site process 2,830 b/d of produced water and cut oil-in- water content from 300 ppm to 0 to 3 ppm.
million b/d in the next 10 years. Water shut-off and profile control technology is aimed at reducing water production and, as a consequence, reducing capital and operational expenditure. An effective reduction in the volume of produced water reduces water treatment needs and the environmental impact considerably. At the same time, successful water shut-off can improve sweep efficiency in natural water drive and water-flooded reservoirs, thereby increasing oil production and ultimate recovery.

Mechanical profile control using expandable zonal inflow profiler (EZIP) technology was first applied in PDO in 2004. An EZIP consists of long seals created by swelling of elastomers around standard API pipe in formation water in the pay zone. The length of the seals is chosen to correspond with formation and formation fluid properties, such that flow diversion is achieved. Following water breakthrough, it is possible to shut the water-producing section off, either permanently or temporarily. The most optimal way to go about shutting off sections is yet to be determined as part of an operating philosophy upgrade for PDO’s main water-producing assets.

Conclusions
• There is not a unique solution to the produced water problem; the optimal solution depends on actual field conditions such as the need for pressure maintenance, the quality of the water stream and local opportunities for re-use.
• Full-field deployment of downhole reduction methods like downhole oil and water separation and water shut off technologies could potentially achieve both reservoir pressure maintenance and debottlenecking
of surface facilities.
• Reservoir management opportunities to re-use produced water are particularly limited when water salinity is moderate.