Month: July 2009

The phase of petroleum operations that immediately follows successful exploratory drilling. At this stage, appraisal wells are drilled to further define the resources of the structure. During appraisal, delineation wells might be drilled to determine the size of the oil or gas field and how to develop it most efficiently. When an appraisal well is carried out its location is usually sited to test another part of the structure where the operator expects there to be similar hydrocarbon bearing sands, at a location several kilometers away from the first exploration well, and probably closer to the predicted gas-water contact, in order to see if they can pin down where the edges of the reservoir are in order to best estimate the size thereof. In addition, if the appraisal well shows thicker sands, then this is a further positive indication of the quantity of hydrocarbons, if they come in thinner then potentially there could be less gas present then estimated after the exploration well and after the initial logging and flow rate tests. The greater the number of wells placed the higher the accuracy

A license given by the licensing authorities to drill an appraisal well

Drilling undertaken to define hydrocarbon volumes and options for development. See also appraisal well

The aim of the appraisal phase is to provide an accurate estimate of hydrocarbon reserves in order to make the right decision about whether and how to develop the discovery and to understand and narrow the range of uncertainty in reserves estimates. The key tools include: geological and geophysical interpretation, well test data evaluation, reservoir modeling, economic and fiscal terms. This is a period of high economic risks as on the one hand a complete appraisal program needs to be conducted so that sufficient information is available to take the right decision as to whether to drill or abandon and on the other hand it is important to know when to bring this phase to an end.

Usually a well is drilled which may last 30-40 days and electric logs are run immediately down the drill hole a process that takes about 3 days. The electric logs provide an 85% certainty as to whether there is potential for oil and gas and potential for the existence of a reservoir. This phase, however, needs to be followed by a fuller examination of the data, including especially of the fluid in the reservoir. Indeed, the electric logs do not suffice on their own as they do not testify as to whether the source rock is permeable enough to enable for the free flow of hydrocarbons. Whether the drilling will be a commercial discovery can only be known after all the production testing has been completed. This is carried out by a drill string test in the open hole to take a mud sampling out from the contents of the reservoir to the surface and examine the sample for the presence of oil and/or gas. This process takes another 7-15 days to complete. The drill stem test would be carried out by a reservoir engineer and the electric logs would be carried out by a petro-physicist