Oil and Gas: Site Selection and Preparation
CHOOSING THE SITE
The operating company decides where to drill by considering several factors. The most important is that the company knows or believes that hydrocarbons exist in the rocks beneath the site. In some cases, the operator drills a well in an existing field to increase production from it. In other cases, the operator drills a well on a site where no one has found oil or gas before. The company often hires geologists to find promising sites where no production exists. Geologists explore areas to try to determine where hydrocarbons may exist. Major companies sometimes have a staff of geologists; independents often hire consulting geologists or buy information from a company that specializes in geological data.
Legal and economic factors are also important in the selection of a drilling site. For example, the company must obtain the legal right to drill for and produce oil and gas on a particular piece of land. Further, the company must have money to purchase or lease the right to drill and produce. What's more, it must have money to pay for the costs of drilling. The costs of obtaining a lease and drilling for oil or gas on the lease vary considerably. Costs depend on such factors as the size of the reservoir, its depth, and its location (offshore and remote sites cost more to drill and produce than readily accessible land sites). A company can easily commit several million dollars to find, drill for, and produce oil and gas. The rewards, of course, can be great, but so can the expenses.
The operating company takes several steps before telling the drilling contractor exactly where to place the rig and start, or spud, the hole. The company carefully reviews and analyzes seismic records. Legal experts thoroughly examine lease terms and agreements. They ensure that the operating company has clear title and right-of-way to the site. Surveyors establish and verify exact boundaries and locations. The company also confirms that it has budgeted the necessary drilling funds and that the funds are available.
On land, operating personnel usually try to choose a spot directly over the reservoir. With luck, the surface will be accessible and reasonably level. They also try to pick a location that will not suffer too much damage when the contractor moves in the rig. In an area that is especially sensitive, the operator and contractor take extra steps to ensure that as little harm as possible occurs. Offshore, the operator hopes that the weather is reasonably good, and, if using a bottom-supported rig, picks a spot where the ocean bottom (the mud line) can adequately hold any rig supports in contact with it.
PREPARING THE SITE
On land sites, the operator hires a site-preparation contractor to prepare the location to accommodate the rig. If required, bulldozers clear and level the area. This contractor also builds an access road and, if necessary, a turnaround. Offshore, the operator simply marks the spot with a buoy. On all jobs, contractors and operators make every effort to keep damage to a minimum because no one wishes to harm the environment. Further, if harm does occur, the contractor and operator have to pay to correct or mitigate the damage, which can be expensive.
The contractor uses various materials to prepare the surface and roads around a land location. Near the coast, oyster shells are popular. In other locations, gravel may be the choice. A contractor may lay boards to allow access in rainy weather. In the far north, permafrost presents a special problem because the heat generated under and near the rig may melt the permafrost. Thus, the rig may settle into the thawed soil. In permafrost, therefore, the contractor spreads a thick layer of gravel to insulate the area. If gravel is scarce, polyurethane foam may be used.
At a land site, the site-preparation contractor may dig a reserve pit. A reserve pit is an open pit that is bulldozed from the land next to the rig. Reserve pits vary in size, depending on how much room is available at the site. Usually, reserve pits are relatively shallow, maybe no more than 10 feet (3 metres) deep and are open on top. In the early days of drilling, the reserve pit was mainly a place to store a reserve supply of drilling mud. Today, however, drilling mud used in actively drilling the hole is seldom stored in the reserve pit,although, in an emergency, it can be.
Modern reserve pits mainly hold rig wastes temporarily. For example, cuttings carried up the hole by the drilling mud fall into the reserve pit. After finishing the well, the drilling contractor or operator removes any harmful material that may be in the pit and properly disposes of it. A bulldozer then covers it with dirt and levels it. If necessary, the contractor lines a reserve pit with plastic to prevent soil and groundwater pollution. In especially sensitive areas, such as in a migratory bird flyway or in a wildlife refuge, contractors cover the pit with netting to prevent birds from landing in it. In addition, they may put up a fence to keep cattle or wildlife out.
The operator may make additional preparations before moving in the rig. The terrain, the well's depth, the underground pressures expected, and the operator's and contractor's preferences determine how they start the well. At land sites where the operator has ordered a deep, high-pressure well, for example, a work crew, using dirt moving equipment, may dig a rectangular pit, or cellar. Sizes vary, but a typical cellar is about 10 feet (3 metres) on a side and perhaps io feet (3 metres) deep. The exact size and depth depend on the characteristics of the well and the rig's configuration.
Sometimes, the workers line the cellar with boards or pour concrete walls to keep it from caving in. The cellar accommodates a tall stack of high-pressure control valves under the rig. The bottom of the stack will sit in the cellar, below ground level. Since the crew installs the stack below ground level,the rig's substructure-the base of the rig does not have to be as tall to allow the rig floor to clear the stack. In short, a cellar provides more working room under the rig.
Some rigs use a special pipe called the "kelly," which is part of the drill string. The kelly is part of the system that rotates the bit. Rigs with kellys require a rathole-a shallow hole drilled off to the side of the main borehole. On land, the operator sometimes hires a special truck-mounted, light-duty unit called a "rathole rig" to drill the rathole. Or, after the rig is set up (rigged up), the drilling crew may drill the rathole with special equipment. Offshore, if the rig needs a rathole, it is a large-diameter length of pipe that extends below the rig floor. In the case of drilled ratholes, the crew extends pipe from the drilled part of the rathole up to the rig floor. The rathole goes through the rig floor and protrudes a few feet, or a half metre or so, above it.
During drilling, the crew uses the rathole to store the kelly temporarily. A kelly can be up to 54 feet, or 17 metres, long. Even the tallest land rig substructures are only about 40 feet (12 metres) high and most are even shorter. The contractor therefore has to drill part of the rathole; otherwise, the rathole would extend too high above the rig floor to be accessible.
The rathole rig or the main rig itself may also drill a mousehole on land sites. A mousehole, like a rathole, is also a shallow hole lined with pipe that extends to the rig floor. The mousehole is a lined hole into which the crew puts a length, or joint, of drill pipe during drilling operations. When crew members are ready to add the joint to the drill string as the hole deepens, they add it from the mousehole. A joint of drill pipe is around 30 feet (9 metres) long. If the regular rig's substructure is appreciably shorter than this height, then the rathole crew also drills a mousehole.
The rathole crew may also drill the first, or top, part of the main borehole. The operator can, in some cases, save time and money by having the rathole rig actually start, or spud, the main hole before moving in the regular rig. The rathole crew backs the rathole rig to the cellar. A special bit starts the main hole in the middle of the cellar. This hole is shallow in depth but large in diameter. Termed conductor hole, it may be 36 inches (91 centimeters) or more in diameter (fig. 66). It may be only tens of feet (or metres) deep or it may be hundreds of feet (or metres) deep, depending on the surface conditions.
The rathole crew lines the conductor hole in the cellar with conductor pipe. Conductor pipe, or casing, keeps the hole from caving in. It also conducts drilling mud back to the surface when regular drilling begins. The crew often secures the conductor pipe in the hole with cement or concrete. With the conductor pipe, rathole, and mousehole prepared, the drilling contractor can move in the rotary rig to drill the rest of the hole.
On drilling locations where the ground is soft, a rathole rig and crew may not be needed. Instead, the contractor can usually move in the regular rig and its crews right away. Once the drilling crew members get the regular rig ready, they rig up a pile driver and drive the conductor casing into the ground, just as Colonel Drake did at Oil Creek. Thus, people in the oil patch sometimes call conductor casing "drive pipe-" After driving the casing, the rig crew begins drilling inside it.
If the ground is too hard for the conductor pipe to be driven, crew members can use the regular rig to drill the conductor hole. What's more, they may also drill the rathole and mousehole, using special equipment on the regular rig.
MOVING EQUIPMENT TO THE SITE
After the operator selects and prepares the drill site, the contractor moves the rig to the site. Crew members move most land rigs by loading the rig components onto trucks. The trucks then carry the components to the site where crew members put the components back together and begin drilling. In remote areas, such as in Jungles and arctic regions, crew members may load rig components onto cargo airplanes or helicopters. Boats often tow offshore rigs from one site to another. On the other hand, some offshore rigs are self propelled-that is, built-in units on the rig provide the means to move it. Sometimes, especially where a rig has to be transported a long distance, a special ship carries the rig.
Moving Land Rigs
Virtually all land drilling rigs are portable. If the rig is small enough to be built on a truck, a person simply drives it from one place to another. Once at the site, the rig stays on the truck and drilling commences. Rigs too big to fit onto one truck are designed differently. Fabricators design medium and large rigs so that a contractor's crew can take it apart, load its components onto several trucks, helicopters, or cargo planes, and move it to the drilling site. At the site, crew members put the rig together, or rig up. After they drill the well, they dismantle the rig, or rig down.
From The Primer of Oilwell Drilling, 6th edition Copyright © 2001 Petroleum Extension Service (PETEX®) of The University of Texas at Austin. All rights reserved