A carbonate field developed by Petroleum Development Oman is one of the largest fields in the Sultanate of Oman and has been running for more than 4 decades. An integrated wells, reservoir, and facility management approach has been implemented to create more focus and discipline with the aim of achieving an efficiently monitored and controlled asset and highly synchronized multiteam actions.
Introduction
The field is located in the north of the Sultanate of Oman, and it is one of Oman’s largest fields. The reservoir layers dip uniformly at 15° to the northeast (Fig. 1). The field is a highly complex carbonate reservoir of the Natih formation and is subdivided into seven units, Natih A through Natih G, and the Shuabia reservoir. It has been running since 1967 and continues to contribute significantly.
The development strategy began with natural depletion with vertical wells and followed with two mechanisms, gas/oil gravity drainage (GOGD) and waterflood. The GOGD process concentrates more on fractured reservoir rock, while the waterflood process targets the layers with a relatively low degree of fracturing.
The GOGD development consists of five crestal gas-injection wells and rows of down-flank producers that tap oil from a fractured oil rim. The reservoir pressure is managed by gas injection. The gas-injection rates are set to replace voidage and to maintain reservoir pressure. For the matrix wells, the reservoir pressure is managed by waterflood (water injection).
This combination of different drive mechanisms in a highly fractured reservoir that yields to interference and gas/water short-circuiting makes the management of the field very challenging.
To overcome these challenges, an integrated well, reservoir, and facility management strategy has been established in a way that ensures integration with multiple disciplines and uses many tools. As a result, an improvement in production has been seen that reflects successful implementation of the strategy.
Wells and Reservoir Reviews
Wells are reviewed on a yearly basis in order to have a systematic approach for reviewing wells. Approximately 26 sectors (more than 450 wells) from different reservoirs have been reviewed collaboratively. Multidisciplinary involvement from the well and reservoir management (WRM) team, the development and planning (DP) team, and the new oil (NO) team, and alignment to mature identified activities, played a crucial role in effectively sharing ideas and lessons learned and implementing activities.
A well book review involved a detailed discussion of reservoir geology, properties, challenges, performance, and data gathering for each well. The main objectives of the well book review are
- Discuss the challenges and performance of the sectors.
- Identify opportunities, and decide the way forward for each well.
- Sustain no-further-action (NFA) production by taking advantage of opportunities.
- Ensure and maintain integration among cluster teams (WRM, DP, and NO).
Wells Optimization and Restoration
Optimization is defined as production gained following an activity to increase production potential (above NFA production). Restoration is defined as production gained following an activity to bring back a failed or quit well.
Optimization. Because of the presence of faults/fractures in the field, the tendency of gas/water breakthrough is very high; thus, mechanical shutoff is performed frequently to solve this issue.
Restoration. Field A is a small field located north of the main field. Most of the wells were closed in 2000 because of surface and subsurface issues [e.g., beam-pump failures, high gas/oil ratio (GOR)]. The field was reviewed in 2010 to restore the production from this small field. A decision was made to recomplete the wells with electrical submersible pumps (ESPs). Six wells were restored, with approximately 200% oil gain.
Reservoir Management
Managing a reservoir is one of the drivers for sustaining NFA production. Daily monitoring is required of any changes in the reservoir using available tools. Pressure maintenance, oil-rim movement, and interference are examples of the daily reservoir monitoring.
Cycle Stopcock (CSCK) Wells
The CSCK process is specific to GOGD fracture producers, and it involves shutting in high-GOR wells for a certain period to allow the local oil rim to thicken and consequently produce at a lower GOR when the well is opened up. Stopcocking is a well-optimization procedure that aims to reduce gas production by temporary closure of a well to allow replenishment of the fractured oil rim.
A CSCK well is opened and closed manually according to a specified schedule that is prepared according to well performance.
Gas-Breakthrough Control (GBC)
GBC is a new technology applied in the field in late 2007. It uses an automatic pressure-controller valve, and it has become a requirement for all new GOGD wells in the field. This technology basically reduces the need to have wells on the CSCK process.
By mid-2012, more than 126 GBC devices were installed in the field. The devices have shown outstanding performance in reducing the gas rates. This has resulted in
- Fewer CSCK wells
- High-gas wells producing continuously with lower GORs
- More-stable well performances
Surveillance Strategy
The surveillance plan consists of routine and nonroutine activities. Routine activities consist of static, gradiometric, and flowing surveys. Nonroutine activities consist of pressure-transient tests, memory production logging, reservoir-saturation logging, chemical-based tracer surveys, CO2 tracer surveys, pressure-buildup and -falloff tests, and step-rate and interference tests.
Routine-Activities Example—Gradiometric Survey. A gradiometric survey is performed to identify the fracture gas/oil content and oil/water contact in the GOGD system. It is a useful tool to see the changes in oil-rim thickness and movements of contacts, which can be used to predict the performance of oil producers, leading to better management of the wells.
Nonroutine-Activities Example—Memory Production Logging. Production logging is performed frequently to monitor a well’s production or injection profile. Memory production logging is also used to identify gas- and water-entry points in producers in order to plan for water- or gas-shutoff interventions efficiently.
Exception-Based Surveillance
With a large number of wells and daily issues, there was a need to monitor those wells. Each well has an operating envelope where all targets are defined. Any deviation from these targets will trigger an alarm.
New Surveillance Technology
Distributed Pressure Sensor. Pressure and temperature gauges and fluid-level sensors are to be installed to observe the position and thickness of the oil rim in real time, to allow faster operational and developmental decisions.
Sonic Gauge. A sonic gauge is a downhole device that is designed to attach to a well’s completion. Sensors convert downhole pressure and temperature information and transmit it to the surface along the production tubular. This provides real-time temperature and pressure measurements of the reservoir.
Gas-Balance Management
Gas balance is tracked and managed on a daily basis through a specially designed interface.
Maintaining the reservoir pressure is critical to optimize the oil rim. The balance between injection and production should be established to meet that.
From the subsurface side, the wells are managed to produce at optimum oil rate with low gas production. This is achieved by
- Effective gas management
- Daily gas balance for oil-rim management
- GBC installation in new and existing wells
- Operating high-gas wells with CSCK method
- Downhole gas-shutoff techniques
From the operation side, compressor availability and efficiency also affect the gas-handling capacity.
Water-Balance Management
Water balance is also tracked and managed on a daily basis. The water-injection strategy is to maximize injection under the matrix conditions, to avoid hydraulic fracturing, and for water-injection volumes to replace all the volumes produced from the waterflooded reservoirs.
The water produced from the gathering stations is routed to all the water injectors and disposal wells. Water is also consumed by the rig and hoist. In general, the oil deferment because of water-capacity limitation can reach 1.6% of total production.
The team tried to reduce the oil deferment through
- Additional perforation in water injectors
- Installation of horizontal ESPs to inject at higher rates
- Drilling new disposal wells
- Downhole water-shutoff techniques
Collaborative Work Environment
The Field Collaboration Centre is an enabling platform that integrates people, processes, and tools within a specialized “virtual” facility to facilitate improved communications, cross collaboration, and high-quality and time-efficient decisions.
Realized benefits include
- Creating physical or virtual proximity between key roles and functions
- Improving operational site-to-office and office-to-site communications
- Accelerating decision cycle times and high-quality collaborative decisions
- Increasing mutual appreciation and understanding of optimization opportunities
- Providing direct feedback during activities
- Improving knowledge sharing and transfer
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 16523, “Oman’s Large-Carbonate-Field Production Improvement Through Integrated Well, Reservoir, and Facility Management,” by S.M. Al-Khadhuri, M.M. Al-Harthi, and A. Alkalbani, Petroleum Development Oman, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2013 International Petroleum Technology Conference. Reproduced by permission.