This example shows how a modeling tool can predict changes reservoir behavior during water injection. This enables engineers to optimally develop a field. (Images courtesy of Shell)

Successfully managing water is one of the key challenges for the oil industry. With three barrels of water produced on average for every barrel of oil, water disposal costs the industry more than US $125 million a day.

Water plays a key role in oil production by helping to maintain reservoir pressure and by sweeping oil toward producing wells. Injecting water into a reservoir is a complex process that, if not managed properly, can result in bypass of oil and early water breakthrough into producing wells. Injection pressure that is too high can cause fracturing of the reservoir, which can create a short circuit between the injector and producers. In addition, horizontal permeability differences in the reservoir can create poor areal sweep or fingering effects. And vertical permeability differences can result in cusping or coning.

Injection-induced fractures

Understanding production-induced changes in the subsurface is prerequisite to reservoir management strategies, including waterflooding.

Shell has developed a unique set of tools that allow it to optimize its injection strategy by improving sweep and minimizing early water breakthrough. An integrated fracture and reservoir modeling tool enables simulation of the impact of fractured injection and provides a definite advantage over the standard industry solution, which uses non-integrated modeling packages.

Shell’s reservoir fracture simulator (FRAC-IT) captures the impact of complex geology and pressure effects due to production and injection. The simulator package uses geological and rock mechanical information along with data on pressure and the rate and quality of the injected water. The results are more accurate simulations that facilitate waterflooding optimization and improve field development plans. During field development, the model is regularly updated with production data to calibrate the model and improve the quality of the predictions.

The predictive power of the FRAC-IT software was validated in the Haima West field in Oman, where a field trial demonstrated a close relationship between the injection rate and fracture length. The ability to manage induced fractures led to a production increase of 50% to 100% in three out of four pilot producers. Using this result and fracture predictions, Shell adopted a controlled field-wide injection strategy that contributed to the arrest of field decline. Haima West is once again showing an increasing production trend.

Treating source water

Waterflooding requires large volumes of water that can come from various sources. In offshore operations, water comes from the sea. In any case, water has to be evaluated prior to injection because the combined effect of seawater, formation water, and hydrocarbons can cause a lethal reaction called “souring.” Souring occurs in the injection zone where different fluids blend together and sulfate-reducing bacteria thrive. Under these circumstances, the bacteria react with sulfates to produce highly poisonous and corrosive hydrogen sulfide. Obviously, water quality is of critical importance, and oxygen content, bacteria, and solid matter need to be maintained at low levels.

The best approach is to model, test, and mitigate souring to prevent hazards to workers and to reduce material damage and downtime of installations.

Shell demonstrated the efficiency of this method at the Bonga field offshore Nigeria. This project, entirely dependent on waterflooding with seawater, requires that 300,000 bbl of water per day be injected into the reservoir. Prior to the waterflooding, the anticipated degree of souring was modeled, and early nitrate injection was identified as the preferred mitigation method. The critical combination of water treatment and nitrate injection will enable Bonga to sustain oil production at 225,000 b/d.

Minimizing water production

Shell has developed a unique type of elastomer that swells when it comes in contact with water. This effective low-cost solution to the water management challenge was reached by selecting the appropriate elastomers and vulcanizing them onto well-completion pipes. The resulting product, Zonal Inflow Profiling (ZIP), is an intelligent-completion technology that reduces water inflow by up to two-thirds, separating and isolating different producing zones from one another.

ZIP technology can be combined with standard pipes (E-ZIP) and expandable pipes (X-ZIP) to stop inflow. This technology has been effective in the field. By the end of 2006, it had increased production by more than 1.5 million bbl of oil in more than 150 wells.

Profound understanding of fluid flows is critical to managing reservoir performance and optimizing production inflow. That is why intelligent completion technology is part of the equation. “Smart well technology” allows the operator to monitor and optimize production using remote control valves.

Smart wells monitor reservoir temperature and pressure, detect seismic activity, and measure the produced volume as well as the composition of well fluids. They also have a control and telemetry link to the surface that allows production to be optimized simply by changing valve settings. Water management benefits greatly from smart-well technology because it permits water inflow control by selectively closing or opening valves.

The added value of smart-well completions has been illustrated in projects like Saih Rawl in Oman, where smart wells have reduced water production considerably in multilateral producers. In this setup, a remote control site manipulates each individual multilateral branch and shuts it off from the system without affecting any of the others. Selective lateral production enables better reservoir management, maximizes oil production throughout the life of the well, and ensures more effective reservoir drainage.

Downhole dehydration

Downhole separation is becoming increasingly important as mature fields challenge water-management technologies.

In the early ’90s, Shell pioneered a new concept in oil/water separation called downhole dehydration. This patented technology consists of a static oil/water separation chamber installed inside the well bore. The separation chamber has a single inlet for receiving well fluids and two outlets — one that discharges a water-enriched component into a discharge well section, and one that produces an oil-enriched component to the surface.

This gravity-based separation method was piloted in the Amal field in Oman in 1994/95 and was followed by hydrocyclone-based methods in the late ’90s. Today, another means of limiting water production is by improving sweep efficiencies and reducing water inflow into the wells. Of course, there are still large volumes of water that need to be disposed. More than 50% of Shell’s produced water is re-injected for pressure support or in a waterflood. The rest of the produced water is either injected for disposal or treated and discharged in deepwater.