1) Describe the following types of wells according to objective: Exploration, Appraisal, Development and Production.
Exploration wells, these types of wells are drilled after an exploration phase (Seismic Interpretation, geology studies, etc) had been done. The purpose of drilling exploration wells is to know whether or not there is hydrocarbon in the area of interest. Also, they are called wildcat, which is the well that the company drills to determine whether oil or gas exist in a subsurface rock formation.
Appraisal wells, Theses types of wells are drilled after the exploration wells have shown the presence of hydrocarbons. The purpose of drilling appraisal wells is to make an estimation of the size of the possible reservoir, such area and reserves. Also, they are called confirmation wells, because they try to verify whether the wildcat well tapped a rock layer with enough hydrocarbons for the company to develop it.
Development and production wells, these wells are drilled after the appraisal wells have shown that the reservoir is big enough to be produced and, with enough hydrocarbons reserves to exploit and have profits. The purposes of these wells are to develop and produce the hydrocarbon in situ to sell it in the world market.
2. List the reasons and applications of directional, Horizontal and extended reach wells.
For many reasons it may not be possible or desirable to drill a vertical well. There may be constrains because of the surface location. In the subsurface, multiple targets, the shape of the structure, faults, etc. may preclude a vertical well. So directional wells are the option to avoid this constrains.
Main applications of directional wells
1. Sidetracking.
Sidetracking was the original directional drilling technique. Initially sidetracks were “blind”. The objective was simple to get past a fish. Oriented sidetracks are most common. They are performed when, for example, there are unexpected changes in the geological configurations
2. Inaccessible locations.
Target located beneath a city, a river or in environmentally sensitive areas makes it necessary to locate the drilling rig some distance away. A directional well is drilled to reach the target
3. Salt Dome Drilling.
Salt domes have been found to be natural traps of oil accumulating in strata beneath the overhanging hard cap. There are several drilling problems associated with drilling a well through salt formations. These can be somewhat alleviated by using a salt saturated mud. Another solution is to drill a directional well to reach the reservoir, thus avoiding the problem of drilling through the salt.
4. Fault Controlling.
Crooked hole are common when drilling nominally vertical. This is often due to faulted sub-surface formations. It is often easier to drill a directional well into such formation without crossing the fault lines.
5. Multiple exploration wells from a single well bore.
A single well bore can be plugged back a certain depth and deviated to make a new well. A single well bore is sometimes used as point of departure to drill others. It allows explorations of structural locations without drilling other complete wells.
6. Onshore Drilling.
Reservoirs located below large bodies of water, which are within drilling reach of land, are being tapped by locating the wellheads on land and drilling directionally underneath the water. This saves money and land rigs are much cheaper.
7. Offshore Multi-Wells Drilling.
Directional drilling from a multi-well offshore platform is the most economic way to develop offshore oil fields. Onshore a similar method is used where there is space restrictions, e.g. jungle, swamp. Here the rig is skidded on a pad and the wells are drilled in “clusters”.
8. Multiple sands for a single well bore.
In this application a well is drilled directionally to intersect several incline oil reservoirs. This allows completion of the well using a multiple completion system. The well may have to enter at specific angle to ensure maximum penetration of the reservoir.
9. Relief well.
The objective of a directional relief is to intersect the borehole of a well, which is blowing and allowed to be killed. The borehole causing the problem is the size of the target. To locate and intersect the blowing well at a certain depth, a carefully planned directional well must be drilled with great precision.
3. List the types and designs of directional wells. Horizontal, J type, S type.
1. Horizontal wells.
This type of well consists of a vertical section, a build section, a tangent section, a second build section, and a horizontal section to reach the target.
Given the lateral distribution of reservoir rock or reservoir fluids, a horizontal well may provide the optimum trajectory. The build up rate of angle is the main distinction from a drilling point of view. Mediums radius wells are preferred since they can be drilled, logged and completed with fairly standard equipment. The horizontal drilling target can be controlled within a vertical window of about 2m. To target the horizontal section with the require accuracy a pilot hole may be drilled first for depth control. Accurate directional surveys are critical for the optimum positioning of the well.
Main application
Reduced production in a field may be due to many factors, including gas and water conning or formations with good but vertical permeability. Engineers can then plan and drill a horizontal drain hole in order to increase production. Others application are in developing geothermal field and in mining.
2. Extended Reach Drilling (ERD),
An extended reach well is loosely defined as having a horizontal displacement of at least twice the vertical depth. Extended reach drilling (ERD) will be considered,
· Where surface restrictions exist
· Where marginal accumulations are located several miles from existing platforms/clusters
· Where ERD allows a reduction in the number of platform required
3. J type wells.
In this type of the well consists of a vertical section, a build section, a tangent section where the acquired angle is held constant till it reaches the target.
4. S type wells.
In this type of well, it consists of a vertical section, a build section, a tangent section and a drop section to reach the target.
4. Explain what well planning is.
Well plan is an organized process used to put together data that will be used to design a successful well. A well plan is a very important stage prior to the commencement of the drilling operations to ensure that all aspects are tailored to meet the specific objectives of that well.
The data provided by the client are used by the well planner to develop a geometric profile for the safest and easiest path from the surface location to the target.
5. Name and describe the following basic components of a well plan:
a. Location.
A place where the well is going to be drilled, usually includes the name of the well and field. The first thing to do is to define a local coordinate system, originating at the structure reference point. In many land wells, this will be the surface location. The target location is then converted to this local coordinate system, if necessary
b. Target Location and Size.
Target location is a point defined in space by geographical coordinates at given true vertical depth. A well profile could have multiples target.
c. Selection of Well profile;
Smoothest way to reach the target is selected with less dog leg and more easily reachable way is selected.
d. Anti-collision considerations.
The well planner uses three techniques to analyze the proposed trajectory of the wellbore. The techniques are: anti-collision, traveling cylinder analysis and ellipses of uncertainty. The goal of these techniques is to eliminate the possibility of a collision with a nearby well.
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e. Well Profile features:
i. Kick off point and start of drop
A point in the wellbore at a given vertical depth below the surface location where the well is to be deviated away from vertical. It is deviated in a given direction up to a given inclination and at a given build rate. The selection of the kickoff point is made by considering the geometrical well path and the geological characteristics of the formation.
. Start of Drop is the location where the wellbore starts dropping inclination (applicable for S-shape well).
ii. End of build and end of drop
End of build, it is the point where the angle stop building.
End of Drop, it is the point where the angle stop dropping or decreasing.
1) Surface Location: Surface location is a point from where the well is started.
2) Azimuth: Azimuth is the clockwise angle of departure from a reference direction i.e. north, measured in a horizontal plane. It is also referred as “Direction”
3) Target Location: A defined area at a prescribed vertical depth and location, which will be intersected by the wellbore.
4) Vertical Section: The vertical section is the length of the projection of the horizontal displacement onto the vertical plane of projection.
5) End of Build-up: The inclination of the well bore is not in increased.
6) Kickoff Point: A point in the wellbore at a given vertical depth below the surface location where the well is to be deviated away from vertical.
7) Quadrant: The angle in the horizontal plane measured from either a N or S reference direction towards E or W, defining the direction of the wellbore.
8) Build-up Rate: It is the rate at which the angle of inclination of the well bore is increased.
9) Polar Co-ordinates: Distance projected in a horizontal plane from a fixed reference point and the angle of this line from a reference direction, usually True North, e.g. 500 feet @150°
10) Drop off rate: It is the rate at which the inclination drops in well bore.
11) Turn Rate: It is the rate at which the azimuth of the well bore is changed.
12) Rectangular Coordinates: Defined as the projected distance along two reference axes, which are at right angles, the distance being measured from the intersection of the axes along the axes. These axes are in the horizontal plane and typically are NS and EW.
13) Hold Angle: Drilling continues without changing the angle and holding The same angle.
14) Start of Drop: The point along the wellbore trajectory at which we start to drop inclination.
15) End of Drop: The point along the wellbore trajectory at which we end to drop inclination.
16) TVD (True Vertical Depth): The vertical depth measured from surface to the end of the wellbore profile.
Tangent Section: Occurs after the build up section and is a section where the inclination is kept constant for a certain distance.
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