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The Corrected d-exponent, also known as cd-exponent or more correctly dc-exponent (dc-exponent) as used in mud logging and formation pore pressure analysis in the oil industry, is an extrapolation of certain drilling parameters to estimate a pressure gradient for pore pressure evaluation while drilling. Normally this is done in over-pressured zones, but most mud logging contracts require it to be done at all times. It is regarded as one of the best tools for pore pressure evaluation. See mud log for an example of the corrected d-exponent plotted on a mud log. The parameter is an extension ("correction", hence the "c" notation) to the d-exponent method previously used for estimating formation pore pressures. The extension consists of a correction for the mud weight in use, compared to "standard" mud for the region.
Mud logging is the creation of a detailed record of a borehole by examining the cuttings of rock brought to the surface by the circulating drilling medium. Mud logging is usually performed by a third-party mud logging company. This provides well owners and producers with information about the lithology and fluid content of the borehole while drilling. Historically it is the earliest type of well log. Under some circumstances compressed air is employed as a circulating fluid, rather than mud. Although most commonly used in petroleum exploration, mud logging is also sometimes used when drilling water wells and in other mineral exploration, where drilling fluid is the circulating medium used to lift cuttings out of the hole. In hydrocarbon exploration, hydrocarbon surface gas detectors record the level of natural gas brought up in the mud. A mobile laboratory is situated by the mud logging company near the drilling rig or on deck of an offshore drilling rig, or on a drill ship.
In atmospheric science, the pressure gradient is a physical quantity that describes in which direction and at what rate the pressure increases the most rapidly around a particular location. The pressure gradient is a dimensional quantity expressed in units of pascals per metre (Pa/m). Mathematically, it is obtained by applying the del operator to a pressure function of position. The negative gradient of pressure is known as the force density.
The parameters used to calculate dc-exponent values are: drilling rate (ROP), rotary speed, weight on bit, bit diameter and mud weight; it is plotted against drilled depth.
In the drilling industry, the rate of penetration (ROP), also known as penetration rate or drill rate, is the speed at which a drill bit breaks the rock under it to deepen the borehole. It is normally measured in feet per minute or meters per hour, but sometimes it is expressed in minutes per foot.
As a drill bit bores into rock, it will gradually encounter denser formations and therefore slower rates of penetration . (Though there are exceptions such as sands that normally drill faster, or faulted and uplifted formations). The general trend is normally a gradually slowing rate of penetration.
Sands may have above them an impermeable layer of formation, normally shale, that may be hundreds of metres thick. When gas or fluids migrate up through the sand and reach this impermeable layer, pressure may build up in the sand and push up against the impermeable layer of shale. Over time the pressure becomes so great that it begins to fracture the shale, making it weaker and easier to penetrate by a drill bit. When a hole is drilled down towards this sand, it will gradually begin to experience faster rates of penetration as it drills through this shale gets closer to the high-pressure sand. It is this trend that the dc-exponent exposes. An examination of the fractured shale that is being drilled will reveal increasingly larger concave pieces. This is where the term pressure shale comes from.
Shale is a fine-grained, clastic sedimentary rock, composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.
Pressure shale is shale that has been exposed to high pressures that causes it to fracture, usually into large concave pieces. In mud logging, watching for this type of shale is one method of monitoring for the possibility of drilling into high pressures that might cause a blowout.
The basic drillability exponent was published in 1966 by Jorden & Shirley relating the action of tricone bit teeth to an inherent characteristic of the rock, the drillability, or 'd' :
d = log10(R/60N)/log10(12W/106D)
where : R=ROP (ft/hr) N=RPM (rev/min) W=WOB (lbs) D=bit size (ins)
In 1971, Rehm and McClendon (1971) defined the corrected d exponent to account for changes in mud weight where dc-exponent is defined as
dc-exponent = MW1/MW2 * d
and where :
dc = modified d exponent ; MW1 = normal pressure gradient ; MW2 = mud weight (preferably ECD) ECD, Equivalent circulating density is the hydrodynamic pressure experienced at the cutting face of the bit due to the combination of mud density, fluid viscosity, borehole wall friction and cuttings load act to increase the pressure. This can be estimated by calculations, but it has become common in recent years to use a direct-measuring annulus probe in the MWD tools string (if there is one).
A drilling rig is used to create a borehole or well in the earth's sub-surface, for example in order to extract natural resources such as gas or oil. During such drilling, data is acquired from the drilling rig sensors for a range of purposes such as: decision-support to monitor and manage the smooth operation of drilling; to make detailed records of the geologic formations penetrated by a borehole; to generate operations statistics and performance benchmarks such that improvements can be identified, and to provide well planners with accurate historical operations-performance data with which to perform statistical risk analysis for future well operations. The terms measurement while drilling (MWD), and logging while drilling (LWD) are not used consistently throughout the industry. Although these terms are related, within the context of this section, the term MWD refers to directional-drilling measurements, e.g., for decision support for the smooth operation of the drilling, while LWD refers to measurements concerning the geological formation made while drilling.
As with all matters relating to pore pressure analysis, the method cannot be applied blindly. In particular, models and constants need to be adjusted to the particular basin being drilled. The method was developed for the delta of the Mississippi/ Missouri river system in the United States, and works reasonably well there. However, basins with different sediment sources cannot be assumed to have the same compaction profiles (because they may have different depositional clay mineralogy). Basins with different pore fluid chemistry will have differing hydrostatic pressure profiles, leading to different dc-exponent profiles. The presence of post-depositional carbonate cements in mudrocks will make formations appear abnormally hard to drill. In particular, the use of PDC-type bits with a shearing cutting action (instead of the chipping action that Jorden & Shirley (1966) assumed in their chip-holddown model) will lead to dc-exponent plots that differ from tricone or bi-cone bits in the same formations. While doing wildcat exploration work in a region, the method can be applied "by the book", but after drilling the first well, one would need to carefully re-evaluate the data collected to try to improve the model for the particular basin in question. While it can be used successfully, one must always validate the information presented by dc-exponent plots by examining multiple other pore pressure indicators.
Reference: Jorden, J.R. and Shirley, O.J.: "Application of Drilling Performance Data to Overpressure Detection", Journal of Petroleum Technology, p1387-1394, Vol.18, No.11, Nov 1966.
Well drilling is the process of drilling a hole in the ground for 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. Drilling for the exploration of the nature of the material underground is best described as borehole drilling.
In petroleum exploration and development, formation evaluation is used to determine the ability of a borehole to produce petroleum. Essentially, it is the process of "recognizing a commercial well when you drill one".
Well logging, also known as borehole logging is the practice of making a detailed record of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface or on physical measurements made by instruments lowered into the hole. Some types of geophysical well logs can be done during any phase of a well's history: drilling, completing, producing, or abandoning. Well logging is performed in boreholes drilled for the oil and gas, groundwater, mineral and geothermal exploration, as well as part of environmental and geotechnical studies.
A drilling rig is an integrated system that drills wells, such as oil or water wells, in the earth's subsurface. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person and such are called augers. Drilling rigs can sample subsurface mineral deposits, test rock, soil and groundwater physical properties, and also can be used to install sub-surface fabrications, such as underground utilities, instrumentation, tunnels or wells. Drilling rigs can be mobile equipment mounted on trucks, tracks or trailers, or more permanent land or marine-based structures. The term "rig" therefore generally refers to the complex equipment that is used to penetrate the surface of the Earth's crust.
A mud engineer works on an oil well or gas well drilling rig, and is responsible ensuring the properties of the drilling fluid, also known as drilling mud, are within designed specifications.
Well control is the technique used in oil and gas operations such as drilling, well workover and well completion for maintaining the hydrostatic pressure and formation pressure to prevent the influx of formation fluids into the wellbore. This technique involves the estimation of formation fluid pressures, the strength of the subsurface formations and the use of casing and mud density to offset those pressures in a predictable fashion. Understanding pressure and pressure relationships is important in well control.
Casing is large diameter pipe that is assembled and inserted into a recently drilled section of a borehole. Similar to the bones of a spine protecting the spinal cord, casing is set inside the drilled borehole to protect and support the wellstream. The lower portion is typically held in place with cement. Deeper strings usually are not cemented all the way to the surface, so the weight of the pipe must be partially supported by a casing hanger in the wellhead.
In geotechnical engineering, drilling fluid, also called drilling mud, is used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. One of the functions of drilling mud is to carry cuttings out of the hole.
Petrophysics is the study of physical and chemical rock properties and their interactions with fluids.
Underbalanced drilling, or UBD, is a procedure used to drill oil and gas wells where the pressure in the wellbore is kept lower than the static pressure then the formation being drilled. As the well is being drilled, formation fluid flows into the wellbore and up to the surface. This is the opposite of the usual situation, where the wellbore is kept at a pressure above the formation to prevent formation fluid entering the well. In such a conventional "overbalanced" well, the invasion of fluid is considered a kick, and if the well is not shut-in it can lead to a blowout, a dangerous situation. In underbalanced drilling, however, there is a "rotating head" at the surface - essentially a seal that diverts produced fluids to a separator while allowing the drill string to continue rotating.
Drilling fluid invasion is a process that occurs in a well being drilled with higher wellbore pressure than formation pressure. The liquid component of the drilling fluid continues to "invade" the porous and permeable formation until the solids present in the mud, commonly bentonite, clog enough pores to form a mud cake capable of preventing further invasion.
A mud motor is a progressive cavity positive displacement pump (PCPD) placed in the drill string to provide additional power to the bit while drilling. The PCPD pump uses drilling fluid to create eccentric motion in the power section of the motor which is transferred as concentric power to the drill bit. The mud motor uses different rotor and stator configurations to provide optimum performance for the desired drilling operation, typically increasing the number of lobes and length of power assembly for greater horsepower. In certain applications, compressed air, or other gas, can be used for mud motor input power. Normal rotation of the bit while using a mud motor can be from 60 rpm to over 100 rpm.
In the oil industry, mud weight is the density of the drilling fluid and is normally measured in pounds per gallon (lb/gal) (ppg) or pound cubic feet (pcf). In the field it is measured using a mud scale or mud balance. Mud can weigh up to 22 or 23 ppg.
Oilfield terminology refers to the jargon used by those working in fields within and related to the upstream segment of the petroleum industry. It includes words and phrases describing professions, equipment, and procedures specific to the industry. It may also include slang terms used by oilfield workers to describe the same.
In the oil and gas industry, a drill bit is a tool designed to produce a generally cylindrical hole (wellbore) in the earth’s crust by the rotary drilling method for the discovery and extraction of hydrocarbons such as crude oil and natural gas. This type of tool is alternately referred to as a rock bit, or simply a bit. The hole diameter produced by drill bits is quite small, from about 3.5 inches (8.9 cm) to 30 inches (76 cm), compared to the depth of the hole, which can range from 1,000 feet (300 m) to more than 30,000 feet (9,100 m). Subsurface formations are broken apart mechanically by cutting elements of the bit by scraping, grinding or localized compressive fracturing. The cuttings produced by the bit are most typically removed from the wellbore and continuously returned to the surface by the method of direct circulation.
Oil well control is the management of the dangerous effects caused by the unexpected release of formation fluid, such as natural gas and/or crude oil, upon surface equipment of oil or gas drilling rigs and escaping into the atmosphere. Technically, oil well control involves preventing the formation fluid, usually referred to as kick, from entering into the wellbore during drilling.
Geologic overpressure in stratigraphic layers is caused by the inability of connate pore fluids to escape as the surrounding mineral matrix compacts under the lithostatic pressure caused by overlying layers. Fluid escape may be impeded by sealing of the compacting rock by surrounding impermeable layers. Alternatively, the rate of burial of the stratigraphic layer may be so great that the efflux of fluid is not sufficiently rapid to maintain hydrostatic pressure.