glossary

MWD signals are transmitted in real time (see real time data) either through the fluid in the borehole and casing (mud pulses), or through the earth formations (electromagnetically). The type of drilling fluid (compressible or incompressible) and the conductivity of geological formations may dictate the appropriateness of one telemetry type or another. The type of telemetry affects data rate, the depth at which an MWD system can transmit in real time back to the surface, and various operational procedures.

The force per unit cross-sectional area required to pull a substance apart.

Pertaining to any reservoir or wellbore treatment performed through the tubing string. Through-tubing treatments are generally associated with live-well operations, thereby causing minimal interruption to production (see oil production) and eliminating the need to kill the well.

Gas produced from a relatively impermeable (see permeability) reservoir rock. Hydrocarbon production from tight reservoirs can be difficult without stimulation operations. Stimulation of tight formations can result in increased production from formations that previously might have been abandoned or been produced uneconomically. The term is generally used for reservoirs other than shales.

A device that turns the drillstring. It consists of one or more motors connected with appropriate gearing to a short section of pipe that in turn may be screwed into a saver sub or the drillstring itself. The topdrive is suspended from the hook, so the rotary mechanism is free to travel up and down the derrick. This is radically different from the more conventional rotary table and kelly method of turning the drillstring because it enables drilling to be done with three joint stands instead of single joints of pipe. It also enables the driller to quickly engage the pumps or the rotary while tripping pipe, which cannot be done easily with the kelly system. Modern topdrives are a major improvement to drilling rig technology and are a large contributor to the ability to drill more difficult extended-reach wellbores. In addition, the topdrive enables drillers to minimize both frequency and cost per incident of stuck pipe.

The planned end of the well, measured by the length of pipe required to reach the bottom. The total depth is also the bottom of a particular hole section, where drilling is stopped, logs are run and casing is cemented before starting the next, smaller diameter hole section. For most operations, the driller is interested in the length of the hole or how much pipe will fit into the hole and therefore wants to know total depth. This must not be confused with the true vertical depth.

The rate at which data are transmitted to the surface in real time (see real time data), usually quantified in bits per second. Effective data rate, however, is a function of transmission rate, data resolution, parity checking and data compression.

A configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation (see permeability) through which hydrocarbons will not migrate (see migration).

A common drill bit with three rotating cones on the bottom.

The process of moving drill pipe down (or up) a previously drilled wellbore section without circulating mud or rotating the drill pipe.

The vertical distance from a point in the well (usually the current or final depth) to a point at the surface, usually the elevation of the rotary kelly bushing. This is one of two primary depth measurements used by the drillers, the other being measured depth (MD). TVD is important in determining bottomhole pressures, which are caused in part by the hydrostatic head of fluid in the wellbore. For this calculation, measured depth is irrelevant and TVD must be used. For most other operations, the driller is interested in the length of the hole or how much pipe will fit into the hole. For those measurements, measured depth, not TVD, is used.

Turn-around time refers to the time period, which is required for MWD-Systems after their removal to be installed again. Tasks are the emptying of the storage card and the exchange of batteries. Furthermore, their functions are checked in course of a systems check.

A system that allows a downhole tool to transmit and receive information from the operator to either query the MWD tool or change some operating parameters such as data sampling times, transmitted data sequence and quality control checks.