ReO Forecast™ Software Technology Overview
On any day in the life of an oil or gas field, the production rate achieved is determined by the performance of the total system comprising reservoir, wellbores and surface equipment in combination with contractual or regulatory production limits. The production rate varies over field life as the properties of the individual elements of the total system change.
Many of the business planning decisions made by the operator of an oil or gas field over field life require an evaluation of the production rates (and therefore revenue) that will be obtained from the field given a certain investment in wells and/or production infrastructure. This process is applied over the entire range of investment sizes, from initial field development, the drilling of an individual infill well, or the recompletion of an existing well. The output from an evaluation is used to verify that the investment will meet corporate criteria for economic return, and also to verify well and equipment design.
The forecasts of production rates that are generated to support these planning decisions must consider the combined performance of the various elements of the total system, so that the impact of the investment upon future production and also the viability of various investment options can be predicted as accurately as possible.
ReO Forecast software has been developed to address these needs by integrating the engineering components to deliver accurate forecasts of the various scenarios under consideration. A key strength of the approach in ReO Forecast software is the use of optimization at each time step to ensure optimal planning of the timing of investments – this creates considerable value through minimizing or delaying capital expenditure until it is needed.
Figure 1 Example production profile output
There are several aspects to an oil or gas field planning decision which require the achievable production from the field at future times to be predicted. In general, the questions that need to be addressed are the following:
- Engineering Design: are the planned wells and surface facilities sufficient to deliver the required production rates over (remaining) field life? If not, then what will be required?
- Commercial Evaluation: will the revenue resulting from the planned wells and surface facilities yield a satisfactory economic return on the investment?
Oil companies and engineering contractors address the two questions in a variety of ways, and this means that several different calculation workflows exist. These are:
- Production profile generation
- Flow assurance
- System deliverability calculation
ReO Forecast software has been developed to enable these workflows to be performed in an efficient way including all key engineering and economic constraints so that the output result is the optimal achievable performance.
Many methods are available to forecast field production rates, ranging from simple exponential decline curves to finite difference reservoir simulation models. One of the limitations of most of the methods currently in use is that they do not adequately consider the detailed interactions of the reservoir, wellbore and surface equipment, instead putting the bulk of the modeling effort into only one or two of these areas. Although this may lead to a very thorough understanding of the area(s) where the focus has been applied, the output production forecasts may be unsuitable for complex planning decisions because the total system performance has not been adequately considered.
EPS has developed ReO Forecast software to support engineers and managers making business planning decisions for oil or gas fields. ReO Forecast software is able to forecast production rates while considering the combined performance of the reservoir, wellbore and surface equipment.
One way in which ReO Forecast software achieves this is by integrating the functions of existing EPS software products MatBal®, WellFlo® and ReO® software which are reservoir, well and surface network modeling packages respectively (see Figure 2). ReO software provides the optimization technology.
Figure 2 – Architecture of ReO Forecast software showing key elements of the application
Alternatively, ReO Forecast software allows the reservoir performance to be captured from other applications through the use of reservoir performance tables. In this way ReO Forecast software can use reservoir simulation results, decline curves etc to provide the required information about the performance of the reservoir.
ReO Forecast software can be used for all of the following types of reservoir:
- Dry/wet gas reservoirs, depletion or aquifer influx
- Gas condensate reservoirs depletion or aquifer influx
- Gas condensate reservoirs, gas recycling
- Oil reservoirs (including volatile oil), depletion or aquifer influx, naturally flowing or artificially lifted wells
- Oil reservoirs, secondary recovery, naturally flowing or artificially lifted wells
All these reservoir types are available both in MatBal software and when using tabulated reservoir performance.
In standalone mode,MatBal software can be used to provide a match to historical production and reservoir pressure data using linear or non-linear regression facilities prior to these reservoir models being used in a ReO Forecast software run. Pseudo-relative permeability curves can be entered for wells to predict water cut and producing GOR development as a function of average phase saturations. Again, regression facilities exist within MatBal software to tune the reservoir model, on a well by well basis, to match the historical water cut and GOR evolution.
In difference to the classical material balance approach MatBal software allows the user to build sub-surface models containing several aquifers and reservoirs with complex connectivity as well as wells completed in several reservoir layers, making the product applicable to a wider range of subsurface modeling needs.
Individual well constraints such as maximum water cut, maximum draw down or maximum rate can also be specified. MatBal software can be used in standalone mode to provide forecast for both a single reservoir and multiple connected reservoirs supported by one or more aquifers. Production controls can be imposed at the wellhead. For more information on MatBal software, refer to the dedicated MatBal software technical insert.
If no MatBal software models are available tabulations of cumulative off take, reservoir pressure and evolving GOR and water cut versus time can be used instead to capture reservoir performance. These tabulations can be provided on a reservoir or a well basis as required.
Well models can be provided using the EPS WellFlo application. WellFlo software is a state-of-the-art well modeling package suitable for all well types. It allows the user to build an accurate view of the subsurface production system, providing a wide choice of engineering models for all relevant pieces of subsurface equipment. For more information on WellFlo software, refer to the dedicated WellFlo software technical insert.
Alternatively, well modeling can be provided by MatBal software. The well model in MatBal software is based on parameterized inflow performance relationships (IPRs), and tubing performance characterized by vertical flow performance (VFP) tables, extended to contain wellbore temperature data as well as pressure drop. The VFP tables can model gas-lifted wells, so that ReO Forecast software can optimally allocate lift gas to wells over a field profile.
The facilities from the wellhead to the export point are modeled using ReO software. Based on Advanced Sequential Linear Programming, ReO software simultaneously simulates and optimizes the network and facilities model based on actual product prices, costs and physical constraints. ReO software allows both Black Oil and Fully Compositional fluid descriptions and performs comprehensive hydraulic and thermodynamic flow modeling. A key feature of ReO software is the ability to model surface networks and facilities of any complexity and detail. Facility models available within ReO software include :-
- Wells (Injection, Production - Natural & Artificial lift)
- Chokes (Fixed and Variable diameter, Pressure Reducers)
- Pumps (Centrifugal and Reciprocating)
- Heat Exchanges (Simple and Detailed)
- Simple Compressors
- Detailed Compressors (Multi-stages, Turbines and Gearboxes)
- Separators (Three and Two Phase)
- And many other models
Following a ReO Forecast software run the details of the facility model solutions at each time step are available as separate scenarios or snapshots within the ReO software model. For more information on ReO software please refer to the dedicated ReO software technical insert.
Applications of ReO Forecast software
Production rates from a field are invariably limited by different constraints which come into and out of play as recovery of the hydrocarbons proceeds. Production optimization must be applied to maximize revenue from the field subject to the constraints being honored. Examples of typical production optimization challenges are determining the optimal allocation of lift gas to individual wells, maximizing revenue subject to produced fluid handling limits and making best use of available compression capacity.
ReO Forecast software is able to incorporate production optimization into the profiles it generates, so that they provide the production rates which maximize revenue at each point in the field’s life given the prevailing constraints. ReO Forecast software can do this because ReO software has a general and very powerful optimization capability. ReO models can be set up with objectives and constraints so that there is more than one feasible solution to the network, allowing the optimizer to choose automatically the one with the highest revenue. The optimizer is scalable to very large and complex networks, including those with hundreds or thousands of wells, and loops and branches.
The production optimization that ReO software performs essentially maximizes instantaneous revenue, and this is appropriate for many situations. However in other situations, there may be longer term reservoir management issues which must be considered either instead of, or in addition to, production optimization. ReO Forecast software allows such reservoir management strategies to be implemented in the forecast generation.
An example of this situation would be where there is a need to maintain uniform depletion across multiple reservoirs which contribute to a single sales contract. If there is excess well deliverability and the production from every reservoir has equal value and cost per barrel, the instantaneous production optimization problem is degenerate, and reservoir management considerations apply instead. In this example, the reservoir management strategy would be represented within ReO Forecast software through the use of group producing rules. The group producing rules define the relative contributions of wells and groups of wells to higher level production targets/constraints when there is excess capacity available. A range of producing rules is available within ReO Forecast software to:
- Choke Back of wells based on High or Low WCT, WGR, GOR, CGR or deliverability
- Scale Back in proportion to maximum well deliverability or in accordance with guide rates
- Produce to target rates and deviations
In most cases the wells and surface equipment required to produce a field, change over time in response to changing conditions in the reservoir. For example, additional infill wells may be drilled, additional fluid handling capacity installed, secondary recovery initiated or compressors re-wheeled. One of the most dramatic demonstrations of this is in the Brent field in the North Sea, where the production facilities have been converted from primarily oil to primarily gas production so that the reservoir can be depressurized.
ReO Forecast software is able to model field configuration changes over field life, which includes bringing wells and items of equipment on-stream or off-stream, and also modifying production constraints. The changes can be made at times specified in a pre-defined schedule, and/or can also be made automatically in response to producing rules. For example, rules may be defined to abandon wells if they exceed a maximum water cut, or to drill new wells to maintain required field production rates.
For Field Development studies ReO Forecast software can...
- drill new wells to maintain production rates (in accordance with a pre-defined drilling schedule)
- abandon wells at pre-determined criteria such as maximum water cut or GOR
- bring on or power up compression when needed to meet performance targets
...as well as following pre-defined schedules for any facility or well changes.
Flow Assurance - Well and Facilities Availability
No wells or surface equipment can be relied upon to be available for production 100% of the time. There will inevitably be downtime as a result of shut-downs for scheduled maintenance, and for unscheduled failures or process trips.
One of the design aspects examined when considering a field development or incremental investment is the trade-off between capital expenditure, reliability and operating cost. In general, equipment that is more expensive to purchase has greater reliability and lower operating cost when in service. For example, an offshore field development with subsea wells is cheaper than a platform; however, the failure of a subsea wellhead will be more difficult to rectify, incurring both additional costs and additional shut-in time waiting for suitable vessels and equipment to be mobilized. ReO Forecast software allows production profiles to be generated which incorporate well and facilities availability.
If the facilities and well availabilities are less than 100%, in order to produce at a certain production rate on average, the facilities must be capable of producing at a higher rate to counteract the periods when the field is shut down, and the total well deliverability must be higher still. For example, if the facilities availability is 95%, and the required production is 100,000 STB/D on average, the facilities must be designed for a throughput of 100,000/0.95=105,300 STB/D. If the well failures are independent of facilities failures, and the well availability is 90%, the total well deliverability needs to be 105,300/0.90 =117,000 STB/D.
ReO Forecast software determines the required well and facilities production capacities on the basis of the higher rates, so that the required production rates can be delivered on average.