METHOD AND TOOL FOR FRACTURING AN UNDERGROUND FORMATION

摘要

1. A method for fracturing an underground formation surrounding a borehole for the production of hydrocarbon fluids, the method comprising: moving into the borehole a fracturing tool which is adapted to exert a pressure which varies in a circumferential direction against the borehole wall; positioning the fracturing tool at a selected downhole location and circumferential orientation in the borehole; expanding the fracturing tool such that the tool exerts a circumferentially varying pressure against the borehole wall during a selected period of time, thereby initiating in the surrounding formation at least one fracture which intersects the borehole wall at a selected orientation; and inserting a proppant into at least one fracture during at least part of said period of time. 2. The method of claim 1, wherein period of time during which the tool exerts a circumferentially varying pressure against the borehole wall is at least 5 seconds. 3. The method of claim 2, wherein the fracturing tool is equipped with a series of formation crushing pins which penetrate into, and are retracted from, the initiated fracture when the tool is in the expanded position thereof, thereby pushing crushed formation debris into each fracture, which debris forms the proppant which keeps each fracture at least partly open after retraction of the fracturing tool. 4. The method of claim 2, wherein the fracturing tool comprises at least two substantially longitudinally cut and complementary pipe segments, which are co-axial to a central axis of the tool and which are, when the tool is expanded, pushed radially from the central axis and against the borehole wall by means of a hydraulic, mechanical, or heat activated memory metal actuator mechanism. 5. The method of any of claims 3 or 4, wherein the fracturing tool is positioned within an expandable slotted tubular in a well inflow zone within a hydrocarbon fluid bearing formation, which tubular is expanded against the formation as a result of the expansion of the fracturing tool and which tubular is perforated by the formation crushing pins when the pins penetrate into the fractures. 6. The method of claim 5, wherein the fracturing tool comprises two complementary pipe halves, which are each at least 5 m long and are radially movable in opposite directions relative to the central axis of the tool and the crushing pins extend through openings between the pipe halves and are expandable in radial directions relative to the central axis of the tool which directions are substantially orthogonal to the directions in which the pipe halves are movable and wherein the fracturing tool is oriented and expanded while the rock crushing pins are actuated to insert crushed formation particles into the opened fracture, and subsequently moved over a length which substantially corresponds to the length of the pipe halves and oriented and expanded while the rock crushing pins are actuated to insert crushed formation particles into the opened fracture, which sequence of steps is repeated until a substantial part of the formation around the well inflow area has been fractured such that elongate fractures are created in the formation over a substantial length of the well inflow zone which fractures intersect the borehole wall at a predetermined orientation. 7. The method of claim 4, wherein the fracturing tool is positioned within an expandable slotted tubular in a well inflow zone within a hydrocarbon fluid bearing formation, which tubular is expanded against the formation as a result of the expansion of the fracturing tool and which tubular is perforated by the formation crushing pins when the pins penetrate into the fractures. 8. A method for enhancing fluid production from a hydrocarbon fluid production well, the method comprising inserting a slotted tubular into the inflow zone of the well and sequentially expanding and perforating adjacent sections of the slotted tubular by moving and expanding a fracturing tool within the slotted tubular in accordance with the method according to claim 6. 9. The method of claim 8, wherein the fracturing tool forms part of a drilling assembly and a drilling fluid comprising drill cuttings is pumped from the drill bit into the fractures surrounding the tool and the tool is equipped with a screen which allows drilling fluid to be pumped back towards the drill bit but which prevents drill cuttings of a size larger than the sieve openings of the screen to re-enter the borehole. 10. A tool for fracturing an underground formation surrounding a borehole for the production of hydrocarbon fluids, the tool comprising: a tool body having a central axis, which tool body is rotatably connected to an orienting sub such that the tool body is rotatable about the central axis relative to the orienting sub; an orienting mechanism for orienting the tool body in a predetermined angular position relative to the central axis; a number of tubular or semi-tubular expansion elements mounted on the tool body such that each expansion element is movable in a radial direction relative to the central axis of the tool body; an expansion mechanism for pressing each expansion element during a selected period of time against the formation in such a manner that in use the expansion elements exert a circumferentially varying pressure against the borehole wall; and means for inserting a proppant into at least one fracture during at least part of said period of time. 11. The tool of claim 10, wherein the tool comprises a pair of semi-tubular expansion elements which are radially movable in opposite directions relative to the central axis of the tool body and the proppant inserting means comprise a series of rock crushing pins which are radially movable relative to the central axis in directions which are substantially orthogonal to said opposite directions. 12. The tool of claim 10, wherein the proppant injection means comprise a proppant slurry injection system. 13. The tool of claim 12, wherein the tool forms part of a drilling assembly and surrounds a section of a drill string which is located at a selected distance from a drill bit such that the expansion elements are expandable and fracture the surrounding formation while drilling operations take place and drill cuttings are injectable as a proppant into the fractured formation.