Well drilling is the process of drilling a borehole in the earth to facilitate the extraction of a natural resource such as ground water, brine, natural gas or petroleum, for the injection of a fluid from surface to a subsurface reservoir, or for subsurface formations evaluation or monitoring. These drilling processes are sometimes referred to as borehole drilling, and the resulting borehole is often referred to as a well. Wells can be drilled to extract (produce) hydrocarbons, such as oil and gas, from the earth. The term ‘oil well’ is often used to describe a well designed to find and produce petroleum oil hydrocarbons. In the case of an oil well, some natural gas is typically produced along with oil. Due to these types of wells commonly producing one or both of oil and natural gas, they are often referred to as ‘oil wells’ or ‘oil and gas wells.’ The term ‘gas well’ is often reserved to describe a well designed to find and produce primarily natural gas.
Creating an oil and gas well typically involves several stages, including drilling, completion and production stages. The drilling stage typically involves drilling a hole (a borehole or wellbore) into the ground using a drill bit. The ground includes a geographic formation (reservoir) expected to contain a concentration of oil and gas that can be extracted (produced). As the well is drilled deeper sections of steel pipe (casing) are often used to line the inside diameter of the borehole to provide structural integrity. In some instances concrete is placed between the outside of the casing and the borehole to improve the structural integrity of the borehole. The drilling process is typically facilitated by a drilling rig which sits at the surface and provides for operating the drill bit, hoisting, lowering and turning drill pipe, circulating drilling fluids, and generally controlling down-hole operations.
The completion stage involves making the well ready to produce oil, gas and other hydrocarbons. In a cased-hole completion, small holes (perforations) are typically made in the portion of the casing that is located in the production zone of the well. The perforations provide a path for oil, gas and other hydrocarbons to flow from the reservoir into a production tubing that routes them to the surface. In an open-hole completion (e.g., a well having production zone that does not have casing), sand or gravel can be installed into the uncased section to maintain integrity of the borehole, while allowing oil, gas and other hydrocarbons to flow from the reservoir into the borehole. In some instances, the completion stage includes pumping fluids into the well to fracture, clean or otherwise prepare the reservoir to produce oil, gas and other hydrocarbons.
The production stage involves producing (e.g., extracting and capturing) oil, gas and other hydrocarbons from the reservoir via the well. During production, the drilling rig is typically removed and replaced with a collection of valves, referred to as a ‘Christmas tree’ or ‘production tree’, that regulates pressures, control flows, and allows access to the borehole in case further completion work is needed. A pump jack or other mechanism can be provided to assist in extracting the hydrocarbons, especially in instances where the pressure in the well is low and the hydrocarbons do not flow freely to the surface. The flow from the outlet valve of the production tree can be connected to a distribution network, including pipelines, tanks and transport vehicles to supply the production to refineries, export terminals, and so forth.
Various types of testing and measurements, including logging, can be conducted throughout the various stages of drilling a well. Well logging, also known as borehole logging, is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole. The log can be based either on visual inspection of samples brought to the surface (geological logs) or on physical measurements made by instruments (logging tools) lowered into the hole (geophysical logs). Logging can take several forms, including wireline logging, logging while drilling, coring, and mud logging. Wireline logging involves lowering a ‘logging tool’—or a string of one or more instruments—on the end of a wireline into borehole and recording petrophysical properties using a variety of sensors. Coring involves obtaining an actual sample of a rock formation from the borehole. Mud loggings involves preparing logs based on rock or soil cuttings brought to the surface by mud circulated through the borehole.
Some forms of wireline logging employ sources and detectors to determine characteristics of the geographic formation, such as reservoir saturation which can be used to determine concentrations of oil and water in a reservoir. During use, a source emits a signal into the surrounding geologic formation, and a detector senses the resulting signals. Characteristics of the resulting signals can be processed to determine the various characteristics of the geographic formation. A pulsed neutron (PN) logging tool, for example, is routinely used in reservoir surveillance to log the relative concentration of carbon and oxygen (e.g., a C/O ratio), among other reservoir parameters, that can be used to derive the reservoir saturation. A relatively high C/O indicates an oil bearing formation, whereas a relatively low C/O indicates a water-bearing formation. A traditional PN tool employs a source (e.g., a neutron source), and multiple detectors (e.g., gamma ray detectors). During operation, the PN source emits signals (e.g., neutrons) into the geologic formation surrounding the borehole, and the detectors sense signals (e.g., gamma rays) generated as a result of the emitted signals. Each of the detectors is provided at a given distance (spacing) from the source to optimize the measurements of the formation. The spacing between a given detector and the source is often referred to as the ‘source-to-detector spacing’ for that detector. The location of the source and the detectors on traditional PN logging tools are fixed. That is, the locations of the source and the detectors—and, thus, the distance between the source and the respective detectors, or between the detectors themselves—are set at a given position for use in a given set of logging conditions.