Apparatus and methods for drilling

Drilling apparatus for accessing a sub-surface formation containing fluid at a predetermined pressure, the apparatus comprising:

• a drill bit mounted on a tubular drill string for extending through a bore and drilling through a formation containing fluid at a predetermined pressure;
• means for circulating drilling fluid down through the drill string to exit the string at or adjacent the bit and enter an annulus between the string and bore wall, and then continuously upwards through an the annulus between the string and bore wall;
• and
• means for adding energy to the drilling fluid in the annulus above the formation such that the pressure of the drilling fluid above said means is higher than the pressure of the drilling fluid below said means and there is a predetermined differential between the pressure of the formation fluid and the pressure of the drilling fluid in communication with the formation.

A wellbore system for decreasing a circulating pressure of fluid in the wellbore, the system comprising:

• a pore pressure that generally increases as the depth of the wellbore increases;
• a wellbore fluid pressure that is greater than the pore pressure and generally increases as the depth of the wellbore increases;
• an effective circulating density pressure that is greater than the wellbore fluid pressure and generally increases as the depth of the wellbore increases, the difference between the circulating density and the fluid pressure defining a friction head;
• a fracture pressure that is greater than the circulating density pressure and generally increases as the depth of the wellbore increases;
• and
• a pressure decreasing device in a tubular string, a spaced distance from the bottom of the wellbore, the device located at a position proximate the wellbore where the effective circulating density approaches the fracture pressure and wherein the device substantially reduces the friction head and thereby increase the difference between the circulating density pressure and the fracture pressure.

A method of reducing the pressure of fluid in a wellbore, the method comprising:

• placing a tubular string in the wellbore, thereby creating an annulus between the string and walls of the wellbore;
• circulating a fluid down the string and upwards in the annulus;
• utilizing the fluid in the string to operate a fluid driven, downhole pump disposed in the string, the pump having an impeller on an outer surface thereof, the impeller in communication with the fluid in the annulus;
• whereby
• the impeller provides a lifting energy to the fluid in the annulus and reduces the pressure of fluid in the wellbore therebelow.

Drilling apparatus for accessing a sub-surface formation containing fluid at a predetermined pressure, the apparatus comprising:

• a drill bit mounted on a tubular drill string for extending through a bore and drilling through a formation containing fluid at a predetermined pressure;
• means for circulating drilling fluid down through the drill string to exit the string at or adjacent the bit, and then upwards through an annulus between the string and bore wall;
• means for adding energy to the drilling fluid in the annulus above the formation such that the pressure of the drilling fluid above said means is higher than the pressure of the drilling fluid below said means and there is a predetermined differential between the pressure of the formation fluid and the pressure of the drilling fluid in communication with the formation;
• and
• means for agitating cuttings in the annulus, wherein the agitating means is mounted on a body which is rotatable relative to the drill string and is driven to rotate by the flow of drilling fluid through the drill string.

A drilling method in which a drill bit is mounted on a tubular drill string extending through a bore, the method comprising:

• drilling the bore extending through a formation containing fluid at a predetermined pressure;
• circulating drilling fluid down through the drill string to exit the string at the drill bit, wherein the drilling fluid is circulated continuously up an annulus defined by the bore and the drill string after exiting the drill bit;
• and
• adding energy to the drilling fluid in the annulus at a location in the bore such that the pressure of the drilling fluid above said location is higher than the pressure of the drilling fluid below said location and there is a predetermined differential between the pressure of the formation fluid and the pressure of the drilling fluid in communication with the formation.

A drilling method in which a drill bit is mounted on a tubular drill string extending through a bore, the method comprising:

• drilling the bore extending through a formation containing fluid at a predetermined pressure;
• circulating drilling fluid down through the drill string to exit the string at a lower end thereof, wherein the drilling fluid is circulated up an annulus defined by the bore and the drill string;
• and
• adding energy to the drilling fluid in the annulus at a location in the bore where a circulating density pressure approaches, but is below a fracture pressure proximate the location.

A drilling method in which a drill bit is mounted on a tubular drill string extending through a bore, the method comprising:

• drilling the bore extending through a formation containing fluid at a predetermined pressure;
• circulating drilling fluid down through the drill string to exit the string at the drill bit, wherein the drilling fluid is circulated continuously up a flow path defined by the bore and the drill string after exiting the drill bit;
• and
• adding energy to the drilling fluid at a location in the flow path such that the pressure of the drilling fluid above said location is higher than the pressure of the drilling fluid below said location and there is a predetermined differential between the pressure of the formation fluid and the pressure of the drilling fluid in communication with the formation.