Abstract:
A subsea drilling system comprises a pressurized drilling capsule that is sealed against the subsea environment and houses equipment for manipulating a drillstring that extends through an opening in the drilling capsule to the exterior of the capsule. A drilling fluid handling assembly is contained within the drilling capsule that communicates drilling fluid through the drillstring and returns the drilling fluid to the water&#39;s surface via a drilling fluid conduit. A seafloor unit engages the seafloor and the drilling capsule to position the drilling capsule relative to the seafloor, and includes an opening for passing the drillstring into the seafloor. A pressurized transport unit is movable between the drilling capsule and a support vessel at or near the water&#39;s surface for delivering personnel and supplies to the drilling capsule. A method of drilling using the subsea drilling system is disclosed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to offshore or subsea drilling systems or rigs. More particularly, the present invention relates to submersible systems for deepwater drilling for hydrocarbons or other natural resources. 
   2. Summary of the Prior Art 
   Drilling for hydrocarbons (oil and natural gas) offshore, in some instances hundreds of miles away from the nearest landmass, poses a number of different challenges over drilling onshore. The actual drilling mechanism used to delve into the seafloor often is much the same as can be found on an onshore rig. However, with drilling at sea, the seafloor can sometimes be thousands of feet below water level. Thus, while with onshore drilling the ground provides a very large load carrying capacity platform from which to drill, at sea, an artificial drilling platform with the same load carrying capacity must be constructed to support the offshore drilling equipment. 
   Drilling offshore dates back as early as 1869, when a T. F. Rowland developed an offshore drilling rig design. The rig was designed to operate in very shallow water, but the seafloor-anchored, four-legged tower bears much resemblance to a land drilling rig and modern offshore rigs, with the drilling platform at the water level instead of resting on the ground. It wasn&#39;t until after World War II that the first truly offshore well was drilled in the Gulf of Mexico. Since then, offshore production, particularly in the Gulf of Mexico, has been very successful, with the discovery and development of a great number of large oil and gas deposits. 
   There are two basic types of offshore drilling rigs: those that can be moved from place to place, allowing for drilling in multiple locations, and those that are permanently placed. Movable rigs are often used for exploratory purposes because they are much less expensive to use than permanent platforms. Once large deposits of hydrocarbons have been found, a permanent platform is built to allow their extraction. There are a number of different types of movable offshore rigs. These include drilling barges and ships, jack-up rigs, submersible rigs, submersible rigs, and semi-submersible rigs. 
   Drilling barge rigs are used mostly for inland, shallow water drilling. This typically takes place in lakes, swamps, rivers, and canals. Drilling barges are large, floating platforms, which must be towed by tugboat from location to location. Suitable for still, shallow waters, drilling barges are not able to withstand the water movement experienced in large open water environments. 
   Drillships are exactly as they sound: ships designed to carry out drilling operations. These drillships are specially designed to carry drilling platforms out to deep-sea locations. A typical drillship will have, in addition to all of the equipment normally found on a large ocean ship, a drilling platform and derrick located on the middle of its deck. In addition, drillships contain a hole (or “moonpool”), extending right through the ship down through the hull, which allows for the drillstring to extend through the boat, down into the water. Drillships are often used to drill in very deep water, which can often be quite turbulent. Drillships also use dynamic positioning systems. Drillships are equipped with electric motors with propellers on the underside of the ship&#39;s hull, capable of propelling the ship in any direction. These motors are integrated into the ship&#39;s computer system, which uses satellite positioning technology, in conjunction with sensors located on the drilling template at the sea bottom, to ensure that the ship is directly above the drill site at all times. 
   Jack-up rigs are similar to drilling barges, with one difference. Once a jack-up rig is towed to the drilling site, three or four “legs” are lowered until they rest on the sea bottom. This allows the working platform to rest above the surface of the water, as opposed to simply floating. However, jack-up rigs are suitable only for shallow waters (approximately 450 feet), as extending the legs down too deeply would be impractical. These rigs are typically safer to operate than drilling barges, as their working platform is elevated above the sea water level. 
   Submersible rigs, also suitable for shallow water, are like jack-up rigs in that they come in contact with the ocean or lake floor. These rigs typically consist of platforms with two hulls positioned on top of one another. The upper hull contains the living quarters for the crew, as well as the actual drilling platform. The lower hull works much like the outer hull in a submarine—when the platform is being moved from one place to another, the lower hull is filled with air—making the entire rig buoyant. When the rig is positioned over the drill site, the air is let out of the lower hull, and the rig&#39;s lower hull submerses to the sea or lake floor. This type of rig has the advantage of mobility in water, however, once again, its use is limited to shallow water areas. 
   Semi-submersible rigs are the most common type of deep water offshore drilling rigs, combining the advantages of submersible rigs with the ability to drill in deep water. Semi-submersible rigs work on the same principle as submersible rigs; through the “inflating” and “deflating” of its lower hull. The main difference with a semi-submersible rig, however, is that when the air is let out of the lower hull, the rig&#39;s lower hull does not submerge to the seafloor. Instead, the rig is partially submerged, but still floats above the drill site. When drilling, the lower hull, filled with water, provides stability to the rig. Semi-submersible rigs are held in place by huge anchors, each weighing upwards of ten tons. These anchors, combined with the submerged portion of the rig, ensure that the platform is stable and safe enough to be used in turbulent offshore waters. Semi-submersible rigs can be used to drill in much deeper water than the rigs mentioned above. Semi-submersible rigs can be used for drilling in very deep water by replacing the anchors with multi-computer controlled propellers which are commonly known as a “dynamic positioning system.” 
   The oil and gas industry in its search for additional hydrocarbon reserves in deep water currently utilizes drillships or semi-submersible drilling units. These are generally called “floating” drilling units and they have a riser (a large-diameter pipe) connecting the floating drilling unit to the BOP stack and template at the sea bottom or floor. The water depth limitation of the current deep water drilling method is approximately 10,000 feet for several reasons:
         1. The effect of the long column of drilling fluid (mud) in the riser on the subsea shallow unconsolidated formation integrity.   2. The maximum riser weight the floating drilling unit can support.   3. The ability to maintain the floating drilling unit within a very limited radius above the subsea well location, especially under high current and waves.
 
The latest generations of the floating drilling units are several thousand tons variable load capacity and cost up to $500,000 per day rental and these floating drilling units are currently in a very short supply.
       

   Therefore, a need exists for a submersible drilling rig or system adapted for deepwater drilling that eliminates the need for a long riser and the associated difficulties presented in position-keeping, as well as avoiding the skyrocketing cost and limited availability of deep water floating drilling units. 
   SUMMARY OF THE INVENTION 
   It is a general object of the present invention to provide an improved offshore drilling assembly of the submersible type. This and other objects of the present invention are achieved by providing a subsea drilling system comprising a pressurized drilling capsule that is sealed against the subsea environment and houses equipment for manipulating a drillstring that extends through an opening in the drilling capsule to the exterior of the capsule. A drilling fluid handling assembly is contained within the drilling capsule which communicates drilling fluid through the drillstring and returns the drilling fluid to the water&#39;s surface via a drilling fluid conduit. A seafloor unit engages the seafloor and the drilling capsule to position the drilling capsule relative to the seafloor, and includes an opening for passing the drillstring into the seafloor. A pressurized transport unit is movable between the drilling capsule and a support vessel at or near the water&#39;s surface for delivering personnel and supplies to the drilling capsule. 
   According to the preferred embodiment of the present invention, the drilling capsule is an elongate pressure vessel having an opening at one end for connection to the equipment and personnel transport units, and an opening at an opposing end through which the drillstring passes. 
   According to the preferred embodiment of the present invention, the equipment for manipulating a drillstring includes a top drive mounted to a mast by a rack-and-pinion. 
   According to the preferred embodiment of the present invention, the seafloor unit comprises a generally flat guided legs template having the opening and having wire guiding lines extending from the template to the water&#39;s surface. A plurality of leveling legs extends upward from the template to engage a lower surface of the drilling capsule. A tubular member extends from the opening in the guided legs template, the opening and tubular member cooperate to receive the drillstring and guide it into the formation to be drilled. 
   According to the preferred embodiment of the present invention, a plurality of buoyancy capsules are secured to the drilling capsule. The buoyancy capsules have adjustable buoyancy so that the drilling capsule can selectively float, sink, or have neutral buoyancy. 
   According to the preferred embodiment of the present invention, a pressurized emergency personnel capsule is associated with the drilling capsule to permit human personnel to exit the drilling capsule in an emergency. 
   According to the preferred embodiment of the present invention, the drilling capsule is of sufficient length to handle at least one section of conventional, 30-foot drill pipe. 
   According to the preferred embodiment of the present invention, the drilling capsule is a pressure vessel of double-wall construction having an inner and an outer wall. The space between the inner and outer walls is selectively pressurized. 
   In another aspect of the present invention, an improved method of drilling a formation under the water is achieved by pressurizing a drilling capsule assembly and maintaining the drilling capsule assembly in a pressurized state suitable for human occupancy. The drilling capsule assembly is submerged by altering its buoyancy. The drilling capsule assembly is secured to the formation by drilling a pilot hole in the formation from within the drilling capsule assembly and by inserting a portion of the drilling capsule assembly into the pilot hole, specifically the surface casing. A drillstring is delivered to the drilling capsule assembly, the drillstring including at least one length of 30-foot drillpipe. A bottomhole assembly is made up into the drillstring and rotated from within the drilling capsule to bore a hole in the formation. The drillstring is selectively removed from the hole and the drilling capsule assembly, wherein the drillstring remains assembled and includes at least one length of 30-foot drill pipe, and preferably several lengths. 
   Other objects, features, and advantages of the present invention will become apparent with reference to the figures and the detailed description of the invention, which follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows the drilling capsule assembly in transport between drilling locations accompanied by a support vessel. 
       FIG. 1B  illustrates the drilling capsule assembly changing from horizontal to vertical position for a controlled descent to the seafloor. 
       FIG. 1C  shows the drilling capsule assembly securely positioned on the seafloor with the surface casing in the formation. 
       FIG. 2  is a longitudinal section view of the drilling capsule assembly at neutral buoyancy with the sealed end of the surface casing just penetrating the soft seafloor. 
       FIG. 3  is a cross-section view of the drilling capsule assembly showing the interior arrangement of the drilling capsule in an operating compartment. 
       FIG. 4  illustrates the drilling capsule assembly spudding or boring into the seafloor to insert the surface casing tubular member. 
       FIG. 5  depicts the drillstring being removed in a continuous length from the top opening of the drilling capsule. 
       FIG. 6  illustrates the drillstring having exited the drilling capsule after drilling the hole section of the well. 
       FIG. 7  illustrates the drillstring stored at the seafloor with the transport capsule in neutral buoyancy. A second string of casing is fed through the top opening of the drilling capsule to be seated and cemented in the well. 
   

   DETAILED DESCRIPTION 
   Referring now to the Figures and particularly to  FIGS. 1A through 1C ,  2  and  3 , an Underwater Seafloor Drilling Rig (USDR) or system  11  according to the present invention is depicted in a submarine environment. This environment, which is referred to as “undersea” or “sub-sea,” can be any underwater environment in which there is a floor or seafloor  3  and a water surface  1 . Such subterranean environments include freshwater seas and lakes, as well as offshore oceanic environments. 
   USDR  11  comprises a drilling capsule  101  which is a steel pressure vessel of multi-wall constructions that is designed to withstand pressures encountered in the undersea environment. Drilling capsule  101  is filled with a continuous supply of air or appropriate human-breathable gas at equivalent atmospheric pressure to provide a safe environment for the operating crew of the USDR  11  and to help withstand the hydrostatic pressure exerted on the outer walls of drilling capsule  101 . Preferably, drilling capsule  101  has an outer diameter of 19.5 feet and a length of 89 feet. It is of steel (AISI 4340 heat-treated alloy with a yield strength of 140 ksi), with double-wall construction. A two-inch “dead space” between the inner and outer hulls is pressurized with an inert gas to assist in equalizing the pressure between the atmospheric interior pressure and the primarily hydrostatic pressure on the exterior of the capsule. 
   As best seen in  FIGS. 2 and 3 , a plurality or pair of buoyancy capsules  103  is secured to the exterior of drilling capsule  101 . Buoyancy capsules  103  can be selectively filled with air (or other gases) or water (or other liquids) via pumps  103 B to alter the buoyancy of the assembled buoyancy capsules  103  and drilling capsule  101  so that the assembly may have positive, negative, or neutral buoyancy, as the situation requires. Buoyancy capsules are provided at their lower end with thrusters (propellers or jets)  103 A to provide steerability of the drilling capsule assembly  101 ,  103 . Drilling capsule  101  and buoyancy capsules  103  together comprise a drilling capable assembly. 
   At the lower end of drilling capsule  101 , there is an aperture or opening provided with a watertight hatch  105  that permits tubular members, such as drill pipes or casings, to exit the lower end of drilling capsule  101 . A series or stack of conventional spherical and ram blowout preventers (BOP)  107  are secured to the lower end of hatch  105  to provide a water- and pressure-resistant seal while allowing drilling equipment to pass through them. 
   Also at the lower end of drilling capsule  101 , but contained within the atmospheric pressure of drilling capsule  101 , a closed, continuous drilling fluid (mud) circulation system  111  is provided. Drilling fluid system  111  includes mud pumps, filter screens and other conventional mechanisms for mixing, filtering, and delivering drilling fluid through the drill pipe to the formation being drilled. The drilling fluid returns through the annulus surrounding the drillstring. The drilling fluid is necessary to cool and lubricate the drill bit, provides a means of removing the formation cuttings, and provides a fluid stabilizing column over the bottom and walls of the borehole being drilled to prevent or diminish the effects of a blowout. 
   A mud standpipe  113  and flexible conduit or hose  115  conduct drilling fluid from the drilling fluid system  111  to a top drive  117 . Top drive  117  may be hydraulically or electrically driven and serves to lift and rotate the drillstring. Top drive  117  is coupled to a mast  119  of a rack-and-pinion design. The use of a top drive and rack-and-pinion system allows use of conventional rotary drilling techniques in the confined area and limited weight required for a minimum size of the drilling capsule  101 . The mast  119  of the rack-and-pinion design is driven by multiple electric motors to reduce the power requirement of the drilling operation and increase the load handling of the rig racks. 
   According to the preferred embodiment of the present invention mast,  119  is approximately 78 feet tall and, accordingly, is dimensioned to handle two sections of conventional (30 foot) drill pipe  121 . An umbilical flex hose bundle  123  is coupled to drilling capsule  101  and supplies breathable gas, electrical power, drilling fluid, and fresh water to drilling capsule  101  from surface  1 . An umbilical flex hose bundle  123 A returns the circulating mud, cuttings, and pressurized gas/breathing air to surface vessel  5 . The air will be injected in the return mud line  123 A to reduce the pumping horsepower required. 
   At the upper end of drilling capsule  101 , a transport capsule  201  is connected to an aperture in drilling capsule  101  by a hatch assembly  203 . Below hatch assembly  203  and contained within drilling capsule  101  is an upper BOP  205 . Another BOP  207  is above hatch assembly  203  and is associated with transport capsule  201 . As will be discussed in greater detail below, transport capsule  201  is a vessel pressurized with human-breathable gas and serves to transport human operators and equipment between drilling capsule  101  and the water&#39;s surface  1 . Accordingly, pressurized transport capsule  201  typically has positive buoyancy, but the buoyancy of the transport capsule  201  can be adjusted to negative or neutral buoyancy by pump  201 A. 
   At the lower end of drilling capsule  101 , a guided legs template  301  is provided. Guided legs template  301  is a flat steel reinforced template with apertures or holes drilled appropriate for the equipment secured to template  301 . This equipment includes a length of casing  303  (a relatively large diameter tubular member usually used to line or “case” a borehole after drilling but before production) and a plurality or four wire guideline posts  305  to which wire guidelines are attached that, optionally, can extend to the water&#39;s surface. Casing  303  may be up to 500 feet in length and extends from the lower central portion of template  301 . As will be described later, casing  303  terminates at its lower end in a cementing shoe. A plurality of leveling legs  307  extend from the upper surface of template  301  to engage the lower surfaces of drilling capsule  101  to selectively level and maintain upright drilling capsule  101 . Casing  303  is connected to drilling capsule by BOP stack  109  and hatch coupling  105 . 
   With reference to  FIG. 2 , a transparent insulating cylinder  601  is installed around the BOP stack  109 . Cylinder  601  will maintain a higher temperature around the BOP stack  109  due to the higher temperature of the returning circulating mud to prevent freezing of its pipes and quick couplings. The transparency of the cylinder  601  will allow an ROV (Remotely Operated Vehicle) to routinely examine the BOP stack  109 , as required by regulation and common practice in deep water drilling. Cylinder  601  is attached to drilling capsule  101  in a manner to allow upward and downward sliding. The position of cylinder  601  with its sliding capability permits the ROV or divers to perform minor adjustments or repairs on the BOP  109 . The capability to inject hot water/liquid into cylinder  601  inner diameter space when needed may be available. 
   USDR  11 , according to the present invention, preferably includes an emergency personnel capsule  501 , which is coupled through a capsule hatch  503  to a drilling control room  505  within capsule  101 . The emergency personnel capsule  501  is pressurized and available for emergency evacuation of drilling capsule  101  by drilling personnel. Drilling control room  505  contains equipment for controlling and monitoring top drive  117  and drilling fluid system  111 , among other operations of mast  119  and drilling operations in general. Duplicate drilling controls exist on the surface support vessel  5 . 
   In the preferred embodiment, drilling capsule  101  contains an air heater, air dehumidifier, oxygen storage units, emergency power supply sources, and an air compressor to circulate the air from the drilling capsule  101  and inject same in the return mud line  123 A. The injection of air in the return mud line  123 A will reduce the pumping horsepower required to lift the return mud to the surface support vessel  5 . 
   With reference now to all of the Figures, the operation of the USDR  11 , according to the present invention, will be described. After a drilling site is selected, the support vessel  5  and the drilling capsule  101 , with its two buoyancy capsules  103  and at least one closed BOP  109  attached to its bottom hatch  105 , will move from one location to another location at the surface  1  or 10-20 feet below the surface to avoid strong waves, as illustrated in  FIG. 1A . Drilling capsule  101  will move with its propeller and/or may have a self-propelled system or may be relocated with the assistance of another seagoing vessel. 
   When drilling capsule  101  and support vessel  5  arrive at the designated location, the ballast water in the buoyancy capsules  103  of drilling capsule  101  are adjusted to bring the drilling capsule assembly to the surface. The balance of BOP stack  109  and large (20″-30″) surface casing  303 , with a drillstring assembly  121  inside template  301  and pipe and cable bundle  123  and  123   a , are attached to the drilling capsule  101  (as illustrated in  FIG. 1B ). 
   Ballast water or other liquid is pumped into tanks in the lower end of drilling capsule  101 , buoyancy capsules  103 , and casing  303  based on a computer simulation program to bring the assembly to vertical position 20 to 50 feet below the sea surface to avoid wave action, which will generate stress. Continuous regulation of the ballast water generates a controlled descent of drilling capsule  101  to 800 to 900 feet from the seafloor  3 . Drilling capsule  101  stops at this level and the attached side thrusters  103 A are utilized to move the assembly laterally to the desired drilling location as guided by a plurality of three sonar beacons located at the seafloor  3 . 
   During the descent of drilling capsule  101  from the surface  1  to the sea bottom  3 , the fluid pressure between the inner and outer hulls of drilling capsule  101  is continuously regulated and adjusted. Preferably, the inner and outer hulls are subjected to a fluid pressure inversely proportional to their radius. At the bottom location, the fluid pressure between drilling capsule  101  inner and outer hulls will be further regulated to compensate for changes in fluid compressibility and temperature. 
   After positioning drilling capsule  101  at the desired drilling location, drilling capsule  101  will continue a slow descent until the surface casing shoe at the end of casing  303  is penetrated a few feet into the soft sea bottom (as shown in  FIG. 2 ). At this time, the drilling crew descends to drilling capsule  101  by means of a transport capsule  201 . 
   Drillstring  121  will slide down through the continuous drilling fluid circulating system  111  and BOP  109  to drill the casing float plug and 6 to 8 feet of a larger hole than the casing outside diameter using an Enlarge While Drilling (EWD) tool  401  available from Tri-Max Industries, Inc. The ballast water in buoyancy and drilling capsules  101 ,  103  is then adjusted to generate 1000 to 3000 pounds of negative buoyancy in the drilling capsule  101 . This will transfer 1000 to 3000 pounds of downward force to the surface casing  303 . Seawater is continuously circulated around casing  303  and the attached thrusters  103 A can be employed, if necessary, to generate slow clockwise rotation of drilling capsule  101  and casing  303  to facilitate the descent of the surface casing below the seafloor  3  until the depth of casing  303  is reached and template  301  rests on seafloor  3  (as shown in  FIG. 4 ). 
   After monitoring the drill pipe pressure and the annulus pressure (the pressure between the inner diameter of casing  303  and outer diameter of the drill pipe), drillstring  121  is pulled into drilling capsule  101  and fed out through upper hatch  203  ( FIG. 5 ). Transport capsule  201  is attached to the top of drillstring  121  and controls its upward movement relative to drilling capsule  101 . Before  121  exits from drilling capsule  101 , transport capsule  201  pumps water out of the bottom section of transport capsule  201  to generate a very small positive buoyancy. When  121  exits drilling capsule  101  completely ( FIG. 6 ), transport capsule  201  pumps water into its lower section to generate a small negative buoyancy and drillstring  121  will rest a safe distance from drilling capsule  101  with its end penetrating a few feet into the soft seafloor ( FIG. 7 ). 
   The EWD, bit and any stabilizers or special over-sized tools  401  are removed from drillstring  121  inside drilling capsule  101  by the drilling crew. The bottom of drillstring  121  is sealed before exiting drilling capsule  101  to assure its positive buoyancy. 
   A cementing head assembly is attached to casing  303 . The casing cementing head has bottom and top cementing plugs inside it. Leveling legs  307  bring drilling capsule  101  to a vertical position. Casing  303  is cemented in place using cement and displacement fluid carried from the surface through conduit  123 . 
   After cementing and waiting for the cement to set, drilling is ready to commence for the next section of the well. Another transport capsule  201  containing or carrying sections of drill pipe and the bit for the next interval, as well as logging and other tools, attaches to hatch  203  of drilling capsule  101 . Once the tools are “offloaded” into drilling capsule  101 , the casing cementing head and the first casing drilling tools are loaded into transport capsule  201  and transported to surface support vessel  5  via transport capsule  201 . 
   The bit and logging tools then are made up into drillstring  121 , transported back to drilling capsule  101  from its seafloor storage position, and rotary drilling is commenced using mast  119  and top drive  117 . Additional sections of drill pipe, additional bits, and other drilling tools are supplied generally as described above by one or more transport capsules  201 . Drilling personnel also can be added or removed from drilling capsule  101  in the same manner. Drilling can be monitored or controlled from the surface through signals duplicating the information in the drilling control room  505  at the surface support vessel  5  through conduit  123 . The drilling personnel crew can rotate shifts every  12  hours to support vessel  5  via transport capsule  201 . 
   When drilling is complete, drilling personnel attach a wellhead connector to casing  303  and drilling capsule  101  is detached from lower BOP stack  109 , leaving template  301 , casing  303 , and all the casings and liners in place. Drilling capsule  101  then can be moved as described above for another drilling operation in another location. Another guided legs template  301  and associated equipment must be attached to drilling capsule  101  prior to commencement of drilling operations elsewhere. This may occur at or near the surface  1 . 
   A significant benefit of the USDR system according to the present invention is that drilling and drilling fluids or mud circulation begin at seafloor  3 , which permits a much larger range for the mud weight while drilling each interval. This is due to no longer having the weight of the column of mud in the riser that conventionally extends from the seafloor to the surface. The fluid column in the riser adds significant hydrostatic pressure on the borehole at the mud line (which is just below the seafloor), where no compaction of rock strength from the overburden exists. With seabed water temperatures as low as 30° F.; the riser also becomes a chilling chamber for drilling fluids. This hydrostatic pressure from the column of mud can be quite substantial. For example, in 8,000 feet of water depth, a 9.9 pound per gallon (ppg) mud will exert an additional 4,100 psi of hydrostatic pressure in the shallow section of a drilled hole. This starting baseline of hydrostatic head pressure results in multiple tubulars (casings or liners) needing to be run over short upper well intervals in order to stay between the pore pressure and overburden gradient. The USDR system allows the hole or well to be drilled in many cases with fewer tubular strings (casing, or liners), leading to substantial cost savings. Also, the range of mud weight which can be run in each interval would be increased (larger window for mud weight). This larger window would allow greater control of the pressures on the formation in the borehole, enabling a safer and more successful ability to stay above the pore pressure (minimum pressure required to prevent an influx of fluids from the formation) and below the overburden gradient (fluid pressure causing fracture of the formation and the beginning of a lost circulation situation which could lead to a well control event). 
   The invention is described with reference to a preferred embodiment(s) thereof. It is thus not limited, but is susceptible to variation and modification without departing from the scope and spirit of the invention.