Appraisal

The WSA process is ideal for making 98 mass% H2S04(f) acid from low SO2 ( 6.5 volume%) strength, H20(g)-bearing, feed gases. This is because it uses a unique condensation system that condenses H2S04(f) while absorbing very little H2O in the acid. 

The process recovers 99- -% of its feed sulfur to acid. Higher recoveries than this require scrubbing of the process offgas. A double condenser design, which improves S capture and permits higher SO2 concentration feed gas, has been proposed and sold but is not yet in operation (Rosenberg, 2009a,b). 

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This Hydrocarbon Exploration and Production is going to take you through all of the major stages In the life of an oil or gas field from exploration, through appraisal, development planning, production, and finally to decommissioning. 

Keywords exploration, appraisal, feasibility, development planning, production profile, production, abandonment, project economics, cash flow... 

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. 

Imagine for a moment that the exploration activities carried out in the previous section have resulted in a successful discovery well. Some time will have passed before the results of the exploration campaign have been evaluated and documented. The next step will be the appraisal of the accumulation, and therefore at some stage a number of additional appraisal wells will be required. The following section will focus on these drilling activities, and will also investigate the interactions between the drilling team and the other E P functions. 

Review involves a systematic appraisal by a government agency or independent review panel. 

It is important to realise that knowledge of depositional processes and features in a given reservoir will be vital for the correct siting of the optimum number of appraisal and development wells, the sizing of facilities and the definition of a reservoir management policy. 

Data gathering in the water column should not be overlooked at the appraisal stage of the field life. Assessing the size and flow properties of the aquifer are essential in predicting the pressure support which may be provided. Sampling of the formation water is necessary to assess the salinity of the water for use in the determination of hydrocarbon saturations. 

The range of uncertainty in the UR may be too large to commit to a particular development plan, and field appraisal may be required to reduce the uncertainty and allow a more suitable development plan to be formed. Unless the range of uncertainty is quantified using statistical techniques and representations, the need for appraisal cannot be determined. Statistical methods are used to express ranges of values of STOMP, GIIP, UR, and reserves. 

Each of the input parameters has an uncertainty associated with it. This uncertainty arises from the inaccuracy in the measured data, plus the uncertainty as to what the values are for the parts of the field for which there are no measurements. Take for example a field with five appraisal wells, with the following values of average porosity for a particular sand ... 

The slope of the expectation curve indicates the range of uncertainty In the parameter presented a broad expectation curve represents a large range of uncertainty, and a steep expectation curve represents a field with little uncertainty (typical of fields which have much appraisal data, or production history). 

The purpose of this exercise is to identify what parameters need to be further investigated if the current range of uncertainty in reserves is too great to commit to a development. In this example, the engineer may recommend more appraisal wells or better definition seismic to reduce the uncertainty in the reservoir area and the net-to-gross ratio, plus a more detailed study of the development mechanism to refine the understanding of the recovery factor. Afluid properties study to reduce uncertainty in (linked to the shrinkage... 

Introduction and Commercial Application JUe objective of performing appraisal activities on discovered accumulations is to reduce the uncertainty in the description of the hydrocarbon reservoir, and to provide information with which to make a decision on the next action. The next action may be, for example, to undertake more appraisal, to commence development, to stop activities, or to sell the prospect. In any case, the appraisal activity should lead to a decision which yields a greater value than the outcome of a decision made in the absence of the information from the appraisal. The improvement in the value of the action, given the appraisal information, should be greater than the cost of the appraisal activities, otherwise the appraisal effort is not worthwhile. 

Appraisal activity should be prioritised in terms of the amount of reduction of uncertainty it provides, and its impact on the value derived from the subsequent action. 

The objective of appraisal activity is not necessarily to prove more hydrocarbons. For example, appraisal activity which determines that a discovery is non-commercial should be considered as worthwhile, since it saves a financial loss which would have been incurred if development had taken place without appraisal. 

This section will consider the role of appraisal in the field life cycle, the main sources uncertainty in the description of the reservoir, and the appraisal techniques used to reduce this uncertainty. The value of the appraisal activity will be compared with its cost to determine whether such activity is justified. 

Appraisal activity, if performed, is the step in the field life cycle between the discovery of a hydrocarbon accumulation and its development. The role of appraisal is to provide cost-effective information with which the subsequent decision can be made. Cost effective means that the value of the decision with the appraisal information is greater than the value of the decision without the information. If the appraisal activity does not add more value than its cost, then it is not worth doing. This can be represented by a simple flow diagram, in which the cost of appraisal is A, the profit (net present value) of the development with the appraisal information is (D2-A), and the profit of the development without the appraisal information is D1. 

Figure 7.1 Net present value with and without appraisal...

The appraisal activity is only worthwhile if the value of the outcome with the appraisal information is greater than the value of the outcome without the information... 

In other words, the cost of the appraisal must be less than the improvement in the value of the development which it provides. It is often necessary to assume outcomes of the appraisal In order to estimate the value of the development with these outcomes. 

Field appraisal Is most commonly targeted at reducing the range of uncertainty in the volumes of hydrocarbons in place, where the hydrocarbons are, and the prediction of the performance of the reservoir during production. 

In determining an estimate of reserves for an accumulation, all of the above parameters will be used. When constructing an expectation curve for STOIIP, GIIP, or ultimate recovery, a range of values for each input parameter should be used, as discussed in Section 6.2. In determining an appraisal plan, it is necessary to determine which of the parameters contributes most to the uncertainty in STOIIP, GIIP, or UR. 

The ranking process is an important part of deciding an appraisal programme, since the activities should aim to reduce the uncertainty in those parameters which have the most impact on the range of uncertainty in HCIIP or UR. 

Seismic surveys are traditionally an exploration and appraisal tool. However, 3-D seismic is now being used more widely as a development tool, i.e. applied for assisting in selecting well locations, and even in identifying remaining oil in a mature field. This was discussed in Section 2.0. Seismic data acquired at the appraisal stage of the field life is therefore likely to find further use during the development period. 

Appraisal activity should be based upon the information required. The first step is therefore to determine what uncertainties appraisal is trying to reduce, and then what information is required to tie down those uncertainties. For example, if fluid contacts are a major source of uncertainty, drilling wells to penetrate the contacts is an appropriate tool seismic data or well testing may not be. Other examples of appraisal tools are ... 

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. 

The stated objective of appraisal activity is to reduce uncertainty. The impact of appraisal on uncertainty can be shown on an expectation curve, if an outcome is assumed from the appraisal. The following illustrates this process. 

Figure 7.3 Impact of appraisal well A on expectation curve...

If well A is oil bearing, then the low case must increase, though the high case may not be affected. If well A is water bearing (dry), then the medium and high cases must reduce, though the low case may remain the same. For both outcomes, the post-appraisal expectation curve becomes steeper, and the range of uncertainty is reduced. 

Note that it is not the objective of the appraisal well to find more oil, but to reduce the range of uncertainty in the estimate of STOMP. Well A being dry does not imply that it is an unsuccessful appraisal well. 

The choice of the location for well A should be made on the basis of the position which reduces the range of uncertainty by the most. It may be for example, that a location to the north of the existing wells would actually be more effective in reducing uncertainty. Testing the appraisal well proposal using this method will help to identify where the major source of uncertainty lies. 

As discussed at the beginning of this section, the value of information from appraisal is the difference between the outcome of the decision with the information and the outcome of the decision without the information. 

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