A method for obtaining in-situ stress measurements in a well is accomplished by installing a membrane packer on a drill string. The packer membrane is attached near the drilling tool and is capable of being radially expanded by fluid pressure to abut against the borehole. When a measurement of the in-situ stress is desired, a 3 way valve is actuated to divert drill string fluid into the packer until the membrane contacts the borehole. Further increments of fluid addition at this point are accomplished only by large increases in inflation fluid pressure. Pressure is increased (with little increase in volume) at a ramp rate until it reaches a value equal to the least principal stress, at which point the formation microfractures. The microfracture(s) result in a decreased pressure and/or volume ramp rate, which indicate their presence. Further pressurization would generate similar indications of fractures resulting from intermediate and maximum principle stresses. After fracturing, the pressure is reduced, and the 3 way valve is actuated to return the fluid to the drill bit.
The invention is a method to determine stress within a formation, the method comprising the steps of: providing a closed reference pressure volume within the formation; providing a flexible diaphragm which can be exposed on one side to formation, and on the other side to the closed reference pressure volume; providing a switch wherein the switch generates a signal based on the diaphragm being in a position indicative of the diaphragm being flexed by pressure on one side of the diaphragm being greater than pressure on the other side of the diaphragm; cycling a pressure within the reference pressure volume to between a pressure at which a signal is generated and a pressure at which a signal is not generated; and determining the formation stress as the pressure at which the signal changes. Another aspect of the invention is the apparatus useful in this method. The switch is preferably an electrical contact that is activated by movement of the diaphragm.
A borehole technique for in-situ determination of principal stresses operating in a plane normal to the borehole includes using a downhole jack to independently initiate three spaced apart fractures in a subterranean formation, measuring the breakdown pressure required to initiate the fractures and then using the measured breakdown pressures in two-dimensional axial transformation equations to compute the maximum and minimum stresses that are active in the normal plane. The technique is useful while drilling the borehole by lowering a jack having three platens that can be independently activated to bear against the borehole wall along three radii which are offset from each other about the borehole axis. In use each platen is extended in turn to bear against the borehole wall until a fracture is initiated.
A downhole tool facilitates in-situ borehole measurement of stress required to initiate a fracture in a desired direction in subterrean formation. The tool is constructed by longitudinally splitting a cylinder in half, and in use the tool includes an inflatable packer disposed within the split cylinder to force each half of the split cylinder against the borehole wall when the packer is inflated. Each half of the split cylinder, which serves as a borehole platen, is made to better conform to an irregular surface of a borehole wall by dividing the split cylinder into multiple segments that are loosely joined so as to allow limited independent movement of each segment with respect to adjacent segments.
A drill stem has a testing tool and a drill bit. To drill, the drill stem is rotated and weight is applied to the bit from the surface. To test a formation, the drill stem is left in the borehole. Compressed gas purges mud from the drill stem. The testing tool has upper and lower collars. The lower collar has a valve seat therein. A valve member is dropped from the surface down inside of the drill stem to seat in the valve seat. When the valve member seats, the lower collar is unlatched from the upper collar, and a piston drives fluid into an inflatable packer to set the packer. With the well shut in, the drill stem test can be initiated. The valve member can be lowered on a wireline and can be selectively removed or placed in the valve seat, wherein alternating shut in and flow periods for the drill stem test can be conducted. In addition, the testing tool can be used in combination with a circulating sub, which circulating sub has a bypass valve that is actuated by a dead man. Together, the testing tool and the circulating sub can be used to control blow outs and thief zones.
A hydraulic strain sensor for use with a downhole tool includes a housing having two chambers with a pressure differential between the two chambers. A mandrel is disposed in the housing. The mandrel is adapted to be coupled to the tool such that the weight of the tool is supported by the pressure differential between the two chambers. A pressure-responsive sensor in communication with the one of the chambers is provided to sense pressure changes in the chamber as the tool is accelerated or decelerated and to generate signals representative of the pressure changes.
