Penetration rates

Better well control allows at-balance or even underbalanced drilling, resulting in higher penetration rates and reduced potential for formation damage. 

If a situation arises whereby formation fluid or gas enters the bore bole the driller will notice an increase in the total volume of mud. Other indications such as a sudden increase in penetration rate and a decrease in pump pressure may also indicate an influx. Much depends on a quick response of the driller to close in the well before substantial volumes of formation fluid have entered the borehole. Onoe the BOP is closed, the new mud gradient required to restore balance to the system can be calculated. The heavier mud is then circulated in through the kill line and the lighter mud and influx is circulated out through the choke line. Once overbalance is restored, the BOP can be opened again and drilling operations continue. 

Data transmission rate per foot is a function of both pulse frequency and rate of penetration. Sensors acquire and transmit data samples at fixed time intervals and therefore the sampling per foot is a function of rate of penetration. Current tools allow a real time sampling and transmission rate similar to wireline tools as long as the penetration rate does not exceed about 100 ft/h. If drilling progresses faster or if there are significant variations in penetration rate, resampling by depth as opposed to time intervals may be required. 

The Washburn equation has most recently been confirmed for water and cyclohexane in glass capillaries ranging from 0.3 to 400 fim in radii [46]. The contact angle formed by a moving meniscus may differ, however, from the static one [46, 47]. Good and Lin [48] found a difference in penetration rate between an outgassed capillary and one with a vapor adsorbed film, and they propose that the driving force be modified by a film pressure term. 

The chemical and mechanical dispersion of the drilled soHds tends to increase the percentage of smaH-sized soHds in a mud as drilling progresses. The incorporation of a limited amount of drilled soHds (several volume percent) is an economical way of increasing the density of low density muds, but it also reduces penetration rates hence, drilled soHds are usually kept to a minimum. The common clay and formation soHds encountered in normal drilling operations are as foUows ... 

Wood in its untreated form has good resistance or endurance to fire penetration when used in thick sections for walls, doors, floors, ceilings, beams, and roofs. This endurance is due to low thermal conductivity, which reduces the rate at which heat is transmitted to the interior. Typically, when the fire temperature at the surface of softwood is 870—980°C, the inner char 2one temperature is - 290° C, and 6 mm further inward, the temperature is 180°C or less. The penetration rate of this char line is mm/min, depending on the species, moisture content, and density (45,46). Owing to this slow... 

The contact an e or adhesion tension of a binder solution with respecd to a powder can be determined from the slope of the penetration profile. Washburn tests can also be used to investigate the influence of powder preparation on penetration rates. The Bartell cell is related to the Washburn test except that adhesion tension is determined by available gas pressure which opposes penetration. [Bartell Osterhof, Ind. Eng. Chem., 19, 1277 (1927).]... 

Powder Hand. Pmc., 2, 323 (1990).] Increasing penetration rate increased granule size and decreased asymmetry of the granule-size distribution. 

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

To discover the effective potential ranges for electrochemical protection, the dependence of the relevant corrosion quantities on the potential is ascertained in the laboratory. These include not only weight loss, but also the number and depth of pits, the penetration rate in selective corrosion, and service life as well as crack growth rate in mechanically stressed specimens, etc. Section 2.4 contains a summarized survey of the potential ranges for different systems and types of corrosion. Four groups can be distinguished ... 

Corrosion starts rapidly at many points simultaneously. The maximum penetration rate, however, depends on the type of soil and the existence of foreign cathodic structures [16,17]. 

The number of corrosion spots increases with time, but the maximum penetration rate remains roughly constant locally and with time. The penetration rate corresponds to a Gaussian distribution curve [18]. 

The basic requirement for materials intended for fabricating chemical apparatuses is mostly corrosion resistance because this determines the durability of equipment. Often, corrosion data are reported as a weight loss per unit of surface area per unit of time. It is easy to transfer from such data to the penetration rate using the following relation ... 

Also, the higher degree of shear thinning provides for lower bit viscosities. This enables more effective use of hydraulic horsepower and faster penetration rates. In addition, shear thinning promotes more efficient operation of the solids removal equipment. 

It is a well-known fact in drilling practice that clear (fresh) water is the best drilling fluid as far as penetration rate is concerned. Therefore, whenever possible, drilling operators try to use minimum density and minimum solids drilling fluids to achieve the fastest drilling rate. Originally, the low solids-clear (fresh) water muds were used in hard formations, but now they are also applied to other areas. 

In hard rock drilling areas with loss of circulation, the application of preformed (mixed at the surface) stable foam shows four to ten times higher penetration rate than clay-based muds. 

The unavoidable addition of solids comes from the continual influx of drilled cuttings into the active mud system. Undesirable solids increase drilling cost because they reduce penetration rate through their effect on mud specific weight and mud viscosity. 

HP — Hole problems HR — Hours on bit PP — Pump pressure PR — Penetration rate RIG — Rig repairs TD — Total depth/casing depth TQ — Torque TW — Twist off WC — Weather conditions... 

Select a bit that provides the fastest penetration rate when drilling at shallow depths. 

When choosing diamonds for a particular drilling situation, there are basically three things to know. First, the quality of the diamond chosen should depend on the formations being drilled. Second, the size of the diamond and its shape will be determined by the formation and anticipated penetration rate. Third, the number of diamonds used also is determined by formation and the anticipated penetration rate. 

Bit Hydraulic Horsepower. The effective level of hydraulic energy (hydraulic horsepower per square inch) is the key to optimum bit performance. The rule-of-thumb estimate of diamond bit hydraulic horsepower HPj and penetration rates is shown in Table 4-99. The bit hydraulic horsepower is dependent upon the pressure drop across the bit and the flowrate. 

Maximum Drilling Rate. In fast drilling operations (soft formations), the maximum penetration rate is limited by the maximum pressure available at the bit. This is the maximum allowable standpipe pressure minus the total losses in the circulating system. 

The volumetric flowrates required for air and natural gas drilling are basically determined by the penetration rate and the geometry of the borehole and drill string. There must be sufficient compressed air (or gas) circulating through the drill bit to carry the rock cuttings from the bottom of the borehole. 

No loss-of-circulation problem No formation damage Very high penetration rate Low bit costs Low water requirement No mud requirement No ability to counter subsurface pore pressure problems Little ability to carry formation water from hole Hole erosion problems are possible if formations are soft Possible drill string erosion problems Downhole fires are possible if hydrocarbons are encountered (gas only) Specialized equipment necessary... 

Improved penetration rates (relative to mud drilling) Ability to counter high subsurface pore pressure problems ... 

Bottomhole Assemblies. In general, the drill pipe, drill collars and, in particular, bottomhole assemblies for air (or gas) drilling operations are the same as those in mud drilling. However, because the penetration rate is much greater in air (or gas) drilling operations due to the lack of confining pressure on the bit cutting surface, care must be taken to control unwanted deviation of the borehole. Thus, for air (or gas) drilled boreholes, a packed-hole or stiff bottom-hole assembly is recommended. 

Finally, inadequate hole cleaning results in an overloading of the annulus with cuttings, especially in very high penetration rate, poor mud properties, and insufficient annular velocity or circulation time. Inadequate hole cleaning can also be experienced in deviated wells with the formation of cutting beds on the low side migrating in a sand dune fashion. 

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