Exploration Wells

A shallow stratigraphic test is a relatively shallow well drilled to provide information on the local geology, i.e. structure, facies, geochemistry, etc. Normally its immediate purpose is not exploration for oil or gas accumulations. 

A deep stratigraphic test is drilled to obtain information about a specific geological condition that might lead to the discovery of an accumulation of hydrocarbons. Such wells are customarily drilled without the intention of being completed for hydrocarbon production. This classification also includes tests identified as core tests and all types of expendable holes related to hydrocarbon exploration. 

A new-field wildcat is a well located on a structural feature or other type of trap which has not previously produced oil or gas. In regions where local geological conditions have little or no control over accumulations, these wells are generally at least two miles from the nearest productive area. Distance, however, is not the determining factor. Of greater importance is the degree of risk assumed by the operator, and his intention to test a structure or stratigraphic condition not previously proved productive. 

New-pool test, deeper-pool test, shallower-pool test Q 

A new-pool test (new-pool wildcat) is a well located to explore for a new pool on a structural feature or other type of trap already producing oil or gas, but outside the known limits of the presently producing area. In some regions where local geological conditions exert an almost negligible control, explor- 

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Once an exploration well has encountered hydrocarbons, considerable effort will still be required to accurately assess the potential of the find. The amount of data acquired so far does not yet provide a precise picture of the size, shape and producibility of the accumulation. 

Even if all of the elements described so far have been present within a sedimentary basin an accumulation will not necessarily be encountered. One of the crucial questions in prospect evaluation is about the timing of events. The deformation of strata into a suitable trap has to precede the maturation and migration of petroleum. The reservoir seal must have been intact throughout geologic time. If a leak occurred sometime in the past, the exploration well will only encounter small amounts of residual hydrocarbons. Conversely, a seal such as a fault may have developed early on in the field s history and prevented the migration of hydrocarbons into the structure. 

Eventually, only the drilling of an exploration well will prove the validity of the concept. A wildcat ls drilled in a region with no prior well control. Wells may either result in discoveries of oil and gas, or they find the objective zone water bearing in which case they are termed dry . 

The technique of mudlogging is covered in this section because it is one of the first direct evaluation methods available during the drilling of an exploration well. As such, the mudlog remains an important and often under-used source of original information. 

The survey requirements will depend on rig type and the extent of the planned development e. single exploration well or drilling jacket installation. Atypical survey area is some 4 km by 4 km centred on the planned location. Surveys may include... 

In exploration wells which show hydrocarbon indications, it is often required to test the productivity of the well, and to capture a fluid sample. This can be used as proof of whether further exploration and appraisal is justified. If the well is unlikely to be used as... 

Classification by objective Exploration Exploration well OOOO Appraisal well OO Exploration well OOOO Geological research wells - Reference well OOO (Wildcat) - Preliminary explo- O ration well (Wildcat) - Structural well O (Prospection well) - Detailed OOO exploration well Exploration well OOOO... 

Development Development well OO Development well OO Exploration well O Auxiliary well O Production well O - oil - gas Injection well O - water - gas - in-situ combustion Appraisal well OO Development well O... 

In offshore operations, exploration wells are almost always plugged and abandoned even when they strike petroleum. Their sole function is to find oil or gas and to delineate the reservoir. The operator uses this information to pick a location for a permanent production platform from which development wells will be drilled to recover as much petroleum as economically possible. In onshore operations, however, successful exploration wells also become producers. 

The study of oxygen diffusion in oxides with mixed-valence 3d-ions presents great interest both in theoretical and practical terms. Such systems with Jahn-Teller (JT) 3d-cations are suitable model objects for analysis of the diffusion process in degenerate or pseudo-degenerate condensed systems. The mechanism of multi-well potential formation has been explored well for JT ions [1,2] and it is possible to give a simple microscopic description of the inter-center interactions and different properties of these systems. The practical interest paid to diffusion properties of the... 

Because of the complexity of the catalytic reactions occurring on polycrystalline samples, many authors have attempted to explore well-defined model oxide surfaces (pure or alkali doped) (518) and to compare the results with those representing polycrystalfine samples. 

Significant oil spills have also occurred from offshore drilling or production platforms as the result of mechanical or operational failure, hi 1979 the IXTOC-I exploration well had an uncontrolled blowout that spilled more than 551,000 tons (500,000 metric tons) of petroleum into the Gulf of Mexico. Smaller spills include one that occurred in 1969 off Santa Barbara in southern California, when about 11,000 tons (10,000 metric tons) were discharged, and the Ekojisk blowout in 1977 in the North Sea off Norway, which totaled 33,000 tons (30,000 metric tons) of crude oU. 

Once a successful exploration well has been drilled, the oil and/or gas flow are pumped to the surface of the earth through the well. At the surface, the petroleum either moves through a pipeline or is stored in a tank or on a ship until it can be sold. 

Petroleum exploration specialists are using a type of geophysical data known as three-dimensional seismic data to study the structures and rock types below the surface of the earth in order to determine where exploration wells might successfully produce petroleum. Geochemists are assessing the results of studies of the chemistry of the surface of the earth and whether or not these results can improve the predictions of scientists prior to drilling expensive exploratory wells. 

Table 1. Arsenic (mg/kg) in hot springs and in production or exploration well brines at various geothermal fields.

Fig. 2. GEA database and location of successful and unsuccessful pre-Cretaceous exploration wells. Successful wells are mostly located near the margins of the deep graben area. Fields are shown by a single well symbol.

It is reasonable to assume that ineffective seals are the cause of exploration failure where porous water-wet reservoirs occur in clearly defined closures, and where the reservoirs are also in direct contact with mature, organic-rich source rocks. We refer to these as proven seal failures. Accordingly, seal failure can be proven in only seven cases, all of which are associated with young inversions. One reason for this is that most exploration wells have not been drilled deep enough to prove or disprove the presence of sealed reservoirs underlying the thick Upper Jurassic claystones that occur in many inversion structures. 

Under Subtitle C of the Resource Conservation and Recovery Act (RCRA), certain wastes containing designated TPH compounds and petroleum-related industrial wastes are listed as hazardous. However, RCRA excludes some TPH-related wastes from regulations (e.g., certain exploration, well development, and productions wastes). The RCRA-listed wastes are also controlled under the Comprehensive Environmental, Response, Compensation, and Liability Act (CERCLA) for accidental releases to the environment. 

The Jurassic layers are mainly carbonates in the north and they become sandy toward the south. In the north, carbonate rocks of Jurassic age form the aquifer of Mount Hermon, which is the main source of the Jordan River through the Dan and Banias springs. Saline water has been found in deeply buried blocks which were reached by oil exploration wells (Bentor, 1969), in northern Israel. 

The numerous outcrops, the tunnels and the extensively cored petroleum, water and geothermal exploration wells make the Swiss Molasse basin an ideal candidate for such a study by providing the opportunity to develop the necessary regional sedi-mentological framework and allowing detailed sampling in the wells and tunnels to study diagenesis in the subsurface. 

Calibration of the thermal model is possible using present-day bottom-hole temperatures from exploration wells (Deming Chapman 1989) and by the comparison of modelled and measured vitrinite reflectance (see Section 5.7.3), an example of which is shown in Fig. 5.50. There are other measurements that can provide time-temperature constraints, such as fission-track analysis (Naeser 1993 Gleadow Brown 1999), homogenization temperatures of fluid inclusions (Roedder 1984) and clay transformations (Hoffman Hower 1979). 

Figure 5. A schematic diagram of a reservoir and its exploring well.

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