1. Field Development Planning Process
For the exploitation of a hydrocarbon field the process of identifying the concepts technically feasible and associated to the best economical performance is called field development planning process. Oil and gas exploration and exploitation require a large amount of economical resources mainly in offshore environments thus, field development planning has the main objective of maximizing the revenue for a given investment, this is maximizing the utility index (UI) defined as UI = NPV/NPI, where NPV is the net present value and NPI is the net present investment value. Scenarios with the greatest median (P50) NPV and lowest spread between P10 and P90 NPV will be selected [1]. Economical evaluation becomes complicated since for example date of initial production and price of hydrocarbons vary randomly.
It is convenient to identify all the feasible concepts to exploit a field, especially for undeveloped fields, to assure that any possible concepts that provide value is not discarded. This process is usually performed in a workshop where personnel representing the technical specialties involved participate defining the information available, the objectives of the project and the strategy to reach the objectives, as a result of this workshop a field development concepts matrix is obtained. This matrix usually has a decision variable as heading in each column for example, hydrocarbon to be exploited, hub concept, well type, transport option, etc. an example of this matrix is shown in Table 1. The number of feasible field development scenarios is the result of all possible combinations for each decision variable, for Table 1 the number of scenarios is 2 × 5 × 4 × 2 = 80.
It is recommended to validate the technical feasibility of each of the concepts since a decision variable can be feasible on its own but when combined with others the outcome might not be feasible, from Table 1 for example, oil & gas exploitation transported by tanker would not be feasible since gas cannot be transported in a tanker.
After the technical screening, NPI for each option is estimated by using commercial data bases and operators experience; it is important to estimate costs during the
Table 1. Example of field development concepts matrix.
full service life of the field from planning studies up to abandonment. Well costs are the major expenditure thus, well type selection is usually done following the same approach presented in this work and is performed simultaneously to the field development planning activities.
On the other hand, the production profile associated to each development option has to be calculated to estimate the income due to hydrocarbons sale. Production profiles can be calculated from simple models like exponential declination or using more complex ones based on energy balance where reservoir, wells and pipe systems are coupled in a model to estimate the production versus time, this later process can be cumbersome and usually consumes several hours depending on the model complexity and computer process speed.
The annual income associated to hydrocarbons sale is estimated from the production profile assuming economical premises such as oil price, gas price and interest rate. NPV for each field development option is estimated from the annual income due to hydrocarbons sale and the annual expenditure associated to capital expenditure (Capex), drilling expenditure (Drillex), operational expenditure (Opex) and abandonment expenditure (Abex).
Since NPV for each field development option involves a high level of uncertainty, probability distributions are assigned to the most relevant variables such as volume of reserves, oil & gas sale price, Drillex and Capex, etc.; this leads to perform stochastic analysis varying the relevant variables within their upper and lower limits by the Montecarlo method and analyze the outcomes in a probabilistic manner. A flow chart of the process described previously is shown in Figure 1, where it is depicted that the coupling of the reservoir, well and pipes models integrates the asset model from which the production profiles are determined and provide the income due to hydrocarbons sale. Stochastic analysis is due to the random nature of the variables involved in these models.
As mentioned, well costs are the major expenditure of the total costs to develop a field thus, it is recommended to find the optimum number of wells for a given development option, a practical approach for finding the optimum number is by plotting NPV versus UI (NPV/NPI) where UI is the investment efficiency ratio. The objective is finding the number of wells that maximizes NPV and UI simultaneously for a given development option, this leads to find the number of wells that generates the maximum economical value with the best investment efficiency since production versus number of wells is governed by reservoir characteristics.
2. Concept Selection Study Case
Figure 2 shows an example of three field developments concepts for an offshore gas field where a tie back to