After an area has been licensed, a combination of pre-existing and newly acquired seismic data, nearby and regional well data and regional geological analyses are used to define where to drill exploration and subsequently appraisal and development wells with the lowest risk of failure. When a well location has been selected a geological programme is written defining both the direct objectives and the information that must be collected, whilst the well is being drilled, to achieve those objectives. The type and extent of the planned geological programme controls the total geoscience evaluation budget. The Well-site Geologist is responsible for all aspects of evaluation related to this geological programme, including supervising the mud loggers (in association with the Well-site Drilling Engineer), quality control and communications, and is the custodian of all the data collected.
There are many ways in which the geology of the section being drilled is evaluated both during drilling and subsequently. The geological data derived from a well whilst it is drilling fall into three categories dependent on which Service Company is primarily responsible for collection:
- Mud logging used to monitor and analyse changes in the drilling parameters and mud content;
- Coring used for retrieval of full-hole cores to better define the section geology;
- And/or pipe-conveyed logging for acquisition of electric logs, other logs and samples with tools deployed on a cable on a pipe.
These are all used to better define the composition and fabric of the drilled section. Although LWD, deployed on a tool behind the bit, is increasingly used for geoscientific analysis its role originated as a bit steering method so costs are here ascribed to the engineering programme although evaluation of geological data from this form of log remains within the geoscience sector.
Samples, cores and wireline and other logs from exploration and appraisal wells make it possible to predict the size, shape and lateral extent of any reservoir rock present, and to predict reservoir parameters such as porosity, permeability, fluid saturation, gas-oil ratio, and the depth of the oil-water or gas-oil contact.
With this information a model can be constructed that will determine the expected reservoir performance, the most likely range of total recoverable reserves and the most cost-effective development or continued development programme. Some geological inferences can also be made from the results of drillstem tests (DSTs) and other production tests which are usually run at the end of successful exploration and appraisal wells.
During drilling a daily geological report is prepared by the Well-site Geologist and transmitted to the operating company's head office where it is evaluated by the Exploration Manager and his or her staff, who were initially responsible for preparing the geological programme. At each cost-sensitive decision point the Well-site Geologist will make drilling recommendations but the Exploration Manager, in consultation with partners in the well, will make most of the decisions. At the end of a well the Well-site Geologist prepares a geological report, which is a summary document containing and analysing the geological well data. All key sample, log and test data are compiled into a single log called the composite log. A copy of this and other logs form part of the geological report, which is submitted to the responsible govemment department and all partners. After exploration and appraisal wells these data, incorporated with existing data, lead to decisions as to whether to proceed with further operations.
The cost of geoscience services, not including shared costs such as communications, safety, inspection and transport, depend very much on well objectives. Exploration wells in brand new areas cost more than exploration wells in mature areas or appraisal wells and they cost substantially more than simple development wells.
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