Inclination
Inclination is the angle of deflection of bit from vertical while drilling operations. The inclination is measured in a vertical plane. There are various methods of measuring wellbore inclination. The simplest one is through an inclinometer.
Heavy Weight Drill Pipe
As the name suggests, the Heavy Weight Drill Pipe (HWDP) is tubular with drill pipe dimensions but has a heavier wall. It is used as the intermediate member of the drill string between drill collars and drill pipes. HWDP usually has longer tool joints than drill pipe. Its weight is less than the drill collar but more than the drill pipe. Having HWDP in the drill string has multiple advantages. It reduces the required number of drill collars in the string while maintaining the drill string's ability to apply the required weight on the bit for drilling. It has less wall contact than drill collar which reduces torque and the chances of differential sticking. HWDP provides smooth weight transition from drill collars to drill pipes and can be run through the hole angle much easily due to less rigidity.
Grid North
Maps are used for various surveys. Since maps are flat surface, the latitude and longitude of spherical earth are converted using mathematical formula called 'Map Projection'. This results in a grid of horizontal and vertical lines on the map. The north on this grid of map is called 'Grid North'. At central meridian, Grid North equals True North.
Geodetic System
Maps used in directional drilling are flat, but the earth is an oblate spheroid, close to an ellipsoid. A point on earth is defined by latitude and longitude whereas, on a flat map, a point is defined by easting and northing. A geodetic system is a means of converting coordinates on earth to map coordinates by using map projections.
Float Sub
A Float Sub is a sub with a float valve installed in its bore. The flapper in the float valve allows circulation but isolates the drill string from the well bore pressure. The float sub has pin and box connections matching with the Bottom Hole Assembly and it is usually placed above the bit or the mud motor. The decision to run a float sub in the drilling string depends on the company policy.
Error Ratio
"Error Ratio" also known as "Separation Factor" is used for assessing the collision risk of two wellbores. It is calculated as a ratio of center-to-center distance and the sum of semi-major axis of error ellipse of the two wellbores. An error Ratio of 1 indicates that the error ellipses of the two well bores touch each other. Error Ratio of less than 1 means that error ellipses overlap and Error Ratio of more than 1 means that the ellipses don't overlap.
Drill Collar
Drill Collars are heavy wall rigid pipes that are used as part of the bottom hole assembly above the drill bit. Drill Collars are stiff tubular, which are added in the drilling assembly so that more weight on the bit can be applied at the bottom for drilling without the risk of buckling the drill string. Drill collars can be either plain or spiral. Spiral drill collars have spiral grooves machined on their outer surface. This reduces the contact area of the drill collar with the hole, reducing the chances of differential sticking.
Drill Bit
A drill bit is a rock-cutting tool used for drilling a hole in the ground. A drill bit could either have fixed cutting blades or rollers. They use different types of cutters made of steel, tungsten carbide, polycrystalline diamond compact (PDC) material, or diamonds for cutting rocks. The design and type of cutting teeth are selected based on the type of rocks to be drilled. Drill bits are broadly classified as roller cone or fixed cutter bits. They are further classified based on the material used for cutting teeth, bearing type, suitability for cutting different compressive strength rocks etc.
A drill bit is connected to the drill string and is run in the well. The drill string is rotated at the surface, which turns the bit at the bottom of the hole to cut the rock into small pieces called drilled cuttings. Downhole drive mechanisms like Positive Displacement Motors (PDM) or Turbines are also used for generating bit rotation and torque downhole.
Directional Survey
Directional Surveys are carried out in a well to measure the angle and direction of a well at a certain depth. Surveys are essential part of directional drilling to establish actual well path and compare it with the planned trajectory.
Directional Drilling
Directional Drilling is the process of drilling a deviated well to reach the Bottom Hole Location (BHL) that is different from the Surface location. Directional drilling is carried out for several reasons. It could be either for accessing specific BHL where the surface location is obstructed or inaccessible or for reaching several BHLs from one surface location or for enhancing reservoir production from a well. Directional Drilling is a specialized service. It needs many special tools and skills for designing & achieving the intended direction of a well.
Connection Gas
Connection Gas is the term used for gas observed on bottoms up after making a pipe connection. It indicates the amount of gas that enters the well bore during connection. Pipe connection causes loss of annular pressure losses effect as the pumps are turned off. The bottom hole pressure in this static condition is equivalent to only the hydrostatic pressure of the mud column. Gas feeds into the well due to the loss of hydrostatic pressure caused by the loss of annular friction pressure. Connection gas is an indication that formation pressure is more than the hydrostatic pressure of the column of mud in a hole in static condition. An increasing trend of connection gas would indicate the need for increasing mud weight to balance the formation pressure.
Casing
A casing is a steel pipe that is run in the well after drilling the hole to a certain depth. The casing is run to prevent the collapse of the drilling hole. In most cases, the casing is cemented to provide effective isolation to avoid communication between the zones behind the casing and the new formations to be drilled below the casing pipe in the next hole section. Various sizes of casing pipes are used in the industry. The grades and connections also vary depending on the application, design loads, and availability.
Bottom Hole Assembly
Bottom Hole Assembly, commonly known as BHA refers to a set of tools between the drill bit and drill pipe. One of the main objectives of BHA is to have enough weight to transfer to the bit to enhance the bit's ability to cut hard rocks. A conventional BHA for drilling vertical wells is designed as a tapered string with a larger diameter and heavier drill collars above the bit. A well-designed BHA produces a smooth hole, minimizes vibration, maintains the intended profile of the well, and improves bit performance.
Bit Sub
Most drill bits have a pin connection on top whereas drill collars have a pin connection at bottom. Bit sub is a BOX x BOX sub with connections matching with pin connections on the bit and drill collar. It is installed right above bit if near bit stabilizer is not planned to be run above bit. Bit subs are also bored out to take a float valve.
Bent Sub
Bent Sub or AKO(Adjustable Kick Off) sub is a small segment of the drill string that is tilted at an angle rather than being straight. It is run above the bit and below the power section. Running a bent sub in the string makes it possible to steer the well as needed through sliding. The tilt of the sub depends on tool size, hole size, and DLS requirement.
Background Gas
Background gas expresses the average gas reading that is observed while drilling. It is also sometimes called "Drill Gas". It is an indication that the formation being drilled contains gas but doesn't necessarily an indication of higher formation pressure. The gas enters the wellbore only when a new formation is drilled. It will seep in from the already drilled part of the well only if the formation has enough permeability and pressure differential. Background gas doesn't indicate an underbalanced condition but may lead to it if the total volume of gas in the system lightens the hydrostatic column enough.
Anti-Collision
Anti-collision is a term used for analysis or report that evaluates the collision risk between two or more wells. In an area where many directional and vertical wells have been drilled, there is always a risk of drilling and penetrating an existing well bore. Carrying out an anti-collision analysis is an essential part of directional well planning in a development drilling.