FIELD APPRAISAL

Keywords reducing uncertainty, cost-effective information, ranking sources of uncertainty, re-processing seismic, interference tests, aquifer behaviour, % uncertainty, decision tree analysis, value of information, fiscal regime, suspended wells, phased development. 

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. 

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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. 

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. 

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. 

During the design phase, facilities (the hardware items of equipment) are designed for operating conditions which are anticipated based upon the information gathered during field appraisal, and upon the outcome of studies such as the reservoir simulation. The design parameters will typically be based upon assessments of... 

U.S. Fish and Wildlife Service (USFWS). 1968. Field Appraisal of Tests to Control Salt Marsh Mosquito with Dursban Applied as a Larvicide And Adulticide in Florida. U.S. Fish Wildl. Serv. Spec. Rep., Pesticide Field Appraisal Program, Atlanta, Georgia. Mimeo. 25 pp. 

Owing to cooling 1n the subsea flowline, subsea wells can be expected to pose more production chemistry problems than would be expected with platform wells. The laboratory and field tests described here provided an effective and cheap means of identifying potential problems during the field appraisal. As a result, provision to overcome the problems could be built into the facilities design at an early stage. 

Nederlof, P. j., Ghsen, M. A. Doyle, M. A. 1994. Application of reservoir geochemistry to field appraisal. In Al-Husseini, M. 1. (ed.) The Middle East Petroleum Geosciences 1994, vol. 2. Gulf-Petrolink, Bahrain, 709-722. 

Smalley, P. C. England, W. A. 1992. Assessing reservoir compartmentalization during field appraisal how geochemistry can help. Society of Petroleum Engineers, Paper No. 25005. 

Fig. 18. Depth plot for a Magnolia Field appraisal well showing gas composition and 6 Cc, obtained from mud gas (IsoTubes) samples against those of associated gases of ten MDT samples obtained.

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. 

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. 

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). 

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. 

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. 

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 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. 

Analytical models using classical reservoir engineering techniques such as material balance, aquifer modelling and displacement calculations can be used in combination with field and laboratory data to estimate recovery factors for specific situations. These methods are most applicable when there is limited data, time and resources, and would be sufficient for most exploration and early appraisal decisions. However, when the development planning stage is reached, it is becoming common practice to build a reservoir simulation model, which allows more sensitivities to be considered in a shorter time frame. The typical sorts of questions addressed by reservoir simulations are listed in Section 8.5. 

There will be some uncertainty as to the well initials, since the exploration and appraisal wells may not have been completed optimally, and their locations may not be representative of the whole of the field. A range of well initials should therefore be used to generate a range of number of wells required. The individual well performance will depend upon the fluid flow near the wellbore, the type of well (vertical, deviated or horizontal), the completion type and any artificial lift techniques used. These factors will be considered in this section. 

Petroleum economics provides the tools with which to quantify and assess the financial risks involved in field exploration, appraisal and development, and allows a consistent approach with which alternative investments can be compared. The techniques are applied to advise management on the attractiveness of such investment opportunities, to assist in selecting the best options, and to determine how to maximise the value of existing assets. 

PSCs are agreed with a schedule for exploration, appraisal and development, and production periods. During these times the terms of the PSC are usually fixed, thus reducing some of the uncertainties associated with tax and royalty systems where the royalty and tax rates may vary over the field lifetime. 

Petroleum economics is used at exploration, appraisal and development stages of the field life, to help to make the following typical decisions ... 

At the development planning stage, a reservoir mode/will have been constructed and used to determine the optimum method of recovering the hydrocarbons from the reservoir. The criteria for the optimum solution will most likely have been based on profitability and safety. The model Is Initially based upon a limited data set (perhaps a seismic survey, and say five exploration and appraisal wells) and will therefore be an approximation of the true description of the field. As development drilling and production commence, further data is collected and used to update both the geological model (the description of the structure, environment of deposition, diagenesis and fluid distribution) and the reservoir model (the description of the reservoir under dynamic conditions). 

There have been earlier reviews of enamine chemistry, but the field has continued to expand rapidly. The need for a new appraisal of the present state of enamine chemistry thus seemed apparent. i... 

The plant engineer should strive to make sure that, whenever possible, his purchase orders specify compliance with the correct standard. Many manufacturers can tmthfully claim that their specifications exceed the minimum requirements of a material standard, in which case they will have no objection to its inclusion in the order. Those who protest too much that its requirements are superfluous may deserve further scmtiny. Choosing the correct specification is not always easy, and requires expertise in the relevant field. This expertise is available from consulting engineers or from an inspection authority, who will also appraise the vendor s designs and, if required, will witness critical stages of constmction and test. 

Service tests will be used (1) where the operating conditions cannot be successfully reproduced in laboratory tests, (2) where the environment does not occur naturally, (3) where real components, as opposed to test specimens, need appraisal, and (4) to confirm laboratory and/or field tests. 

An appraisal of artificial weathering methods was given in a report by Hoey and Hipwood who described the effectiveness of various weatherometer tests such as are described in BS 3900 Part F3 and ASTM 42 1964 (now ASTM G23 1989). Although these tests simulate atmospheric exposure it is not possible to obtain a direct correlation owing to variation in outdoor exposure conditions from place to place, but they serve a very useful purpose in providing a preliminary sorting of paints that can then be tested in the field. 

Classification by objective Exploration Exploration OOOO well Appraisal well OO Delineation well Q New-structure test Q New-pool test Q Deeper-pool test Q Shallower-pool test Q New-licence block QQ appraisal test Fault block O extension test Appraisal/outpost 0 test (delineation) Stratigraphic test O0 New-field wildcat 0 New-pool wildcat 0 Deeper-pool test 0 Shallower-pool test 0 Outpost or 00 extension test Key well 00 - Group one - Group two Parametric well O0 Core well 00 Prospecting well 00 Exploratory well 00 Exploration O000 well Appraisal well 0O... 

Matthiessen, P. and Gibbs, PE. (1998). Critical Appraisal of the Evidence for TBT-Mediated Endocrine Disruption in Molluscs—A concise review of effects of TBT on molluscs. Scheuhammer, A.M and Sandheimich, M.B (Eds.) (2008). Special issue of the journal Ecotoxicology devoted to effects of methyl mercury on wildlife, which gives recent results of field studies conducted in North America. 

What if instrument manufacturers would adopt specific RMs Until now, there has been a general tendency for instrument manufacturers to avoid admitting the need for RMs. If instrument manufacturers could be convinced to make a serious appraisal of needs in the field, they might provide some resources to help meet those needs (Rasberry 1998). After all, the extra cost of providing suitable RMs, as a part of an annual service contract costing US 20 000 on an instmment that is worth US 2 million is quite insignificant. 

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