Onshoring

As decommissioning approaches, enhanced recovery e.g. chemical flooding processes are often considered as a means of recovering a proportion of the hydrocarbons that remain after primary production. The economic viability of such techniques is very sensitive to the oil price, and whilst some are used in onshore developments they can rarely be justified offshore at current oil prices. 

Exploration activities are potentially damaging to the environment. The cutting down of trees in preparation for an onshore seismic survey may result in severe soil erosion in years to come. Offshore, fragile ecological systems such as reefs can be permanently damaged by spills of crude or mud chemicals. Responsible companies will therefore carry out an Environmental Impact Assessment (EIA) prior to activity planning and draw up contingency plans should an accident occur. In Section 4.0 a more detailed description of health, safety and environmental considerations will be provided. 

For onshore operations various types of landrigs are available, ranging from truck mounted light rigs to heavy landrigs weighing several hundred tons. 

Whether onshore or offshore drilling is carried out, the basic drilling system employed in both cases will be the rotary rig (Fig. 3.7) and the following summarises the basic functions and parts of such a unit. Three basic functions are carried out during rotary drilling operations ... 

A site survey will be carried out, from which a number of parameters can be established, e.g. carrying capacity of the soil at the planned location, possible access routes, surface restrictions like built-up areas, lakes, nature reserves, the general topography, possible water supplies. The survey will allow the adequate preparation of the future location. For instance, onshore in a swamp area the soil needs to be covered with support mats. 

Not surprisingly, costs are several times higher than conventional wells. Nevertheless, overall project economics may favour ERD over other development options. For example, BP developed the offshore part of the Wytch Farm Oilfield (which is located under Poole Harbour in Dorset, UK) from an onshore location. The wells targeted the reservoir at a vertical depth of 1,500 meters with a lateral displacement of over 8,000 meters (Fig. 3.20). The alternative was to build a drilling location on an artificial island in Poole Bay. ERD probably saved a considerable amount of money and advanced first oil by several years. 

In a normally pressured reservoir, the pressure is transmitted through a continuous column of water from the surface down to the reservoir. At the datum level at surface the pressure is one atmosphere. The datum level for an offshore location is the mean sea level (msl), and for a onshore location, the ground water level. 

Another type of gravity separator used for small amounts of oily water, the oil interceptor, is widely used both offshore and onshore. These devices work by encouraging oil particles to coalesce on the surface of plates. Once bigger oil droplets are formed they tend to float to the surface of the water faster and can be skimmed off. A corrugated plate interceptor (CPI) is shown below and demonstrates the principle involved. However there are many varieties available. Plate interceptors can typically reduce oil content to 50-150 ppm. 

The most common contaminants in produced gas are carbon dioxide (COj) and hydrogen sulphide (HjS). Both can combine with free water to cause corrosion and H2S is extremely toxic even in very small amounts (less than 0.01% volume can be fatal if inhaled). Because of the equipment required, extraction is performed onshore whenever possible, and providing gas is dehydrated, most pipeline corrosion problems can be avoided. However, if third party pipelines are used it may be necessary to perform some extraction on site prior to evacuation to meet pipeline owner specifications. Extraction of CO2 and H2S is normally performed by absorption in contact towers like those used for dehydration, though other solvents are used instead of glycol. 

Steel jackets are constructed from welded steel pipe. The jacket is fabricated onshore and then floated out horizontally on a barge and set upright on location. Once in position... 

Generally EOR techniques have been most successfully applied in onshore, shallow reservoirs containing viscous crudes, where recoveries under conventional methods are very low. The Society of Petroleum Engineers publishes a regular report on current EOR projects, including both pilot and full commercial schemes (the majority of which are in the USA). In the 1992 report, EOR methods could be divided into three basic types ... 

Whether on land or offshore, the principle of satellite development is the same. A new field is accessed with wells, and an export link is installed to the existing (host) facility. Development is not always easier on land, as environmental restrictions mean that some onshore fields have to be developed using directional drilling techniques (originally associated with offshore developments). A vertical well can be drilled offshore away from the host facility, and the well completed using a subsea wellhead. 

All pipelines will be circulated clean and those that are buried, or on the seabed, left filled with water or cement. Surface piping will normally be cut up and removed. Flexible subsea pipelines may be reeled-in onto a lay barge and disposed of onshore. 

Onshore processing facilities, and modules brought onshore, have to be cleaned of all hazardous compounds and scrapped. Cellars of single wells, drilling pads, access roads and buildings will have to be removed. If reservoir compaction affects the surface area above the abandoned field future land use may be prevented, in particular in coastal or low land environments. 

Dissolved Minerals. The most significant source of minerals for sustainable recovery may be ocean waters which contain nearly all the known elements in some degree of solution. Production of dissolved minerals from seawater is limited to fresh water, magnesium, magnesium compounds (qv), salt, bromine, and heavy water, ie, deuterium oxide. Considerable development of techniques for recovery of copper, gold, and uranium by solution or bacterial methods has been carried out in several countries for appHcation onshore. These methods are expected to be fully transferable to the marine environment (5). The potential for extraction of dissolved materials from naturally enriched sources, such as hydrothermal vents, may be high. 

In an attempt to stimulate onshore production of synthetic quartz and piezoelectric devices in the 1970s, Brazil imposed an embargo on exports and ultimately raised the price several-fold for small quartz crystals used as the starting material for quartz growth. However, sources of suitable pure quartz were located in the United States and Canada, including vein and pegmatic deposits (1). Synthetic processes compatible with the natural U.S. quartz starting material from a variety of sources were developed, and U.S. production became relatively independent of imports (1). 

Equipment Leaks of VOG from Onshore Natural Gas Processing Plants... 

Seawater services on onshore platforms, for drinking, washing and firefighting services on off-shore platforms Seawater lift pumps for cooling gas compressors on oil platforms... 

In the case of older pipelines and offshore pipelines, protection current densities can amount to several mA m" For older onshore pipelines, the protection current densities are determined by a drainage test according to Section 3.4.3. 

Onshore or offshore gas plants are designed for either LNG rejection and gas injection, or LNG rejection and transmission for sale. In the case of offshore plants, onshore facilities further process the natural gas before transmission for sale. In either case, natural gas must be treated and then refrigerated to make rejection of heavy hydrocarbons possible. In plants where natural gas is treated for sale purposes, water and hydrocarbon dew points of the gas must also be controlled. 

In a similar study, Exxon describes an onshore plant where once again gas processing with a turboexpander was chosen over the other alternatives. The results of that report are detailed in Table 3-7. 

One of the most comprehensive economic studies was done in two phases. The first phase addressed whether the location of the treating facilities should be offshore or onshore. The second phase evaluated the process design options. The outcome of the first phase recommended onshore natural gas treating facilities the second phase recommended implementation of the turboexpander process design. The process options evaluated for this project are listed below ... 

MeGlashan, R. S. and Hviding, G., Troll Onshore Development Balaneing Costs, Energy Effieieney, and Emissions, 71st Annual GPA Convention, Anaheim, California, Mareh 1992. 

Dry Face Seal Application, Atmospheric Bearing Housing (Onshore Application)... 

CASE 9 USE OF MAGNETIC BEARINGS BY NORSKE SHELL IN AN ONSHORE APPLICATION... 

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