Field development

Introduction and Commercial Application This section provides an overview of the activities carried out at the various stages of field development. Each activity is driven by a business need related to that particular phase. The later sections of this manual will focus in some more detail on individual elements of the field life cycle. 

The purpose of development appraisal is therefore to reduce the uncertainties, in particular those related to the producible volumes contained within the structure. Consequently, the purpose of appraisal in the context of field development is not to find additional volumes of oil or gas A more detailed description of field appraisal is provided in Section 6.0. 

Based on the results of the feasibility study, and assuming that at least one option is economically viable, a field development plan can now be formulated and subsequently executed. The plan is a key document used for achieving proper communication, discussion and agreement on the activities required for the development of a new field, or extension to an existing development. 

The field development plan s prime purpose is to serve as a conceptual project specification for subsurface and surface facilities, and the operational and maintenance philosophy required to support a proposal for the required investments. It should give management and shareholders confidence that all aspects of the project have been... 

Once the field development plan (FDP) is approved, there follows a seguence of activities prior to the first production from the field ... 

It is expected that seismic iwill become even more important in determining field development strategies throughout the total field life. Indeed, many mature fields have several vintages of seismic, both 2D and 3D. 

Many techniques have been developed for management of the safety and environmental impact of operations, and much science is applied to these areas. The objective of this section is to demonstrate how the practising engineer can have a significant impact on these aspects of a field development, and that safety and the environment should be the concerns of all employees. 

To a large extent the reservoir geology controls the producibility of a formation, i.e. to what degree transmissibility to fluid flow and pressure communication exists. Knowledge of the reservoir geological processes has to be based on extrapolation of the very limited data available to the geologist, yet the geological model s the base on which the field development plan will be built. 

In the following section we will examine the relevance of depositional environments, structures and diagenesis for field development purposes. 

Carbonate rocks are more frequently fractured than sandstones. In many cases open fractures in carbonate reservoirs provide high porosity / high permeability path ways for hydrocarbon production. The fractures will be continuously re-charged from the tight (low permeable) rock matrix. During field development, wells need to be planned to intersect as many natural fractures as possible, e.g. by drilling horizontal wells. 

The diagenetic processes relevant to field development are compaction, cementation, dissolution and replacement. 

The non-hydrocarbon components of crude oil may be small in volume percent, typically less than 1 %, but their influence on the product quality and the processing requirements can be considerable. It is therefore important to identify the presence of these components as early as possible, and certainly before the field development planning stage, to enable the appropriate choice of processing facilities and materials of construction to be made. 

Figure 5.21 helps to explain how the phase diagrams of the main types of reservoir fluid are used to predict fluid behaviour during production and how this influences field development planning. It should be noted that there are no values on the axes, since in fact the scales will vary for each fluid type. Figure 5.21 shows the relative positions of the phase envelopes for each fluid type. 

A common objective of a data gathering programme is the acquisition of fluid samples. The detailed composition of oil, gas and water is to some degree reguired by almost every discipline involved in field development and production. 

To make the correlation results applicable for the field development process it may be desirable to display the correlated units in their true structural position. For instance if water injection is planned for the field, water should enter the structure at or below the owe and move upwards. Hence the correlation panel should visually show the sand development in the same direction. For this, all markers on the panel are displayed and connected at their TVSS position (Fig. 5.43). This is called a structural correlation. 

The maps most frequently consulted in field development are structural maps and reservoir quality maps. Commonly a set of maps will be constructed for each drainage unit. 

Having defined some of the statistical rules, we can refer back to our example of estimating ultimate recovery (UR) for an oil field development. Recall that... 

It is worth noting that if field development using horizontal wells is under consideration, then horizontal appraisal wells will help to gather representative data and determine the benefits of this technique, which is further discussed in Section 9.3. 

Introduction and Commercial Application The reservoir and well behaviour under dynamic conditions are key parameters in determining what fraction of the hydrocarbons initially in place will be produced to surface over the lifetime of the field, at what rates they will be produced, and which unwanted fluids such as water are also produced. This behaviour will therefore dictate the revenue stream which the development will generate through sales of the hydrocarbons. The reservoir and well performance are linked to the surface development plan, and cannot be considered in isolation different subsurface development plans will demand different surface facilities. The prediction of reservoir and well behaviour are therefore crucial components of field development planning, as well as playing a major role in reservoir management during production. 

Gas reservoirs are produced by expansion of the gas contained in the reservoir. The high compressibility of the gas relative to the water in the reservoir (either connate water or underlying aquifer) make the gas expansion the dominant drive mechanism. Relative to oil reservoirs, the material balance calculation for gas reservoirs is rather simple. A major challenge in gas field development is to ensure a long sustainable plateau (typically 10 years) to attain a good sales price for the gas the customer usually requires a reliable supply of gas at an agreed rate over many years. The recovery factor for gas reservoirs depends upon how low the abandonment pressure can be reduced, which is why compression facilities are often provided on surface. Typical recovery factors are In the range 50 to 80 percent. 

The main differences between oil and gas field development are associated with ... 

In a gas field development, producers are typically positioned at the crest of the reservoir, in order to place the perforations as far away from the rising gas water contact as possible. 

The above descriptions may suggest that rather few wells, placed in the crest of the field are required to develop a gas field. There are various reasons why gas field development requires additional wells ... 

Typical recovery factors for gas field development are in the range 50 to 80 percent, depending on the continuity and quality of the reservoir, and the amount of compression installed (i.e. how low an abandonment pressure can be achieved). 

The amount of detail input, and the type of simulation model depend upon the issues to be investigated, and the amount of data available. At the exploration and appraisal stage it would be unusual to create a simulation model, since the lack of data make simpler methods cheaper and as reliable. Simulation models are typically constructed at the field development planning stage of a field life, and are continually updated and increased in detail as more information becomes available. 

At the field development planning stage, reservoir simulation may be used to look at questions such as ... 

The production profile for oil or gas is the only source ofrevenueior most projects, and making a production forecast is of key importance for the economic analysis of a proposal (e.g. field development plan, incremental project). Typical shapes of production profile for the main drive mechanisms were discussed in Section 8.2, but this section will provide some guidelines on how to derive the rate of build-up, the magnitude and duration of the plateau, the rate of decline, and the abandonment rate. 

At the stage of field development planning, reservoir simulation would normally be used to generate production profiles and well requirements for a number of subsurface development options, for each of which different surface development options would be evaluated and costs estimated. 

In gas field development, the recovery factor is largely determined by how low a reservoir pressure can be achieved before finally reaching the abandonment pressure. As the reservoir pressure declines, it is therefore common to install compression facilities at the surface to pump the gas from the wellhead through the surface facilities to the delivery point. This compression may be installed in stages through the field lifetime. 

The quality and quantity of fluids produced at the wellhead is determined by hydrocarbon composition, reservoir character and the field development scheme. Whilst the first two are dictated by nature the latter can be manipulated within technological and market constraints. 

When an oil or gas field has just been discovered, the quality of the information available about the well stream may be sparse, and the amount of detail put into the process design should reflect this. However, early models of the process along with broad cost estimates are needed to progress, and both design detail and cost ranges narrow as projects develop through the feasibility study and field development planning phases (see Section 12.0 for a description of project phases). 

Figure 10.34 Typical Subsea Field Development Options...

Typically, a Subsea Field Development or Subsea Satellite Development would consist of a cluster of special subsea trees positioned on the seabed with produced fluids piped to the host facility. Water injection, as well as lift gas, can be provided from the host facility. Control of subsea facilities is maintained from the host facility via control umbilicals and subsea control modules. 

In 1986 when the oil price crashed to 10 a barrel, operators began to look very hard at the requirements for offshore developments and novel slimline, reduced facilities platforms began to be considered. The reduced capital outlay and early production start up capability, coupled with the added flexibility, ensured that all companies now consider subsea systems as an important field development technique. Although the interest and investment in subsea systems increased dramatically, subsea systems still had to compete with the new generation of platforms, which were becoming lighter and cheaper. 

Only one set of pipelines and umbilicals (as with the template) are required from the manifold back to the host facility, saving unnecessary expense. Underwater manifolds are becoming very popular as they offer a great deal of flexibility in field development and can be very cost effective. 

The operations group will develop general operating and maintenance objectives for the facilities which will address product quality, costs, safety and environmental issues. At a more detailed level, the mode of operations and maintenance tor a particular project will be specified in the field development plan. Both specifications will be discussed in this section, which will focus on the input of the production operations and maintenance departments to a field development plan. The management of the field during the producing period is discussed in Section 14.0. 

---