Retrievable whipstock system

A whipstock assembly for use within a wellbore to cut a window laterally through a casing for passing a workstring to drill a sidetrack borehole from said wellbore, said whipstock assembly comprising:

• a whipstock anchor having at least one gripping surface for urging to move laterally into an interior surface of said casing to grippingly secure said whipstock anchor within said wellbore;
• an outer whipstock having an uphole end for releasibly securing to said workstring, a first deflection surface which extends laterally towards said casing for urging a first milling tool through said casing, and a downhole end for releasibly securing to said whipstock anchor;
• a whipstock assembly release latch having at least one member which extends between said uphole end of said outer whipstock and said workstring for selectively coupling said workstring to said whipstock assembly;
• an inner whipstock having a second deflection surface which extends laterally towards said casing for urging a second milling tool into said casing, and a first end which is secured to said whipstock anchor;
• and
• a whipstock coupling for releasibly securing said outer whipstock to said inner whipstock, and transferring torque and linear force therebetween.

A whipstock assembly for use within a wellbore to cut a window laterally through a casing for passing a workstring to drill a sidetrack borehole from said wellbore, said whipstock assembly comprising:

• a first milling tool having a plurality of cutting surfaces for cutting said casing to provide said window, and which includes a threaded end for securing said first milling tool to said workstring;
• a whipstock anchor for releasibly securing said whipstock assembly within said casing;
• an inner whipstock having a second deflection surface which extends laterally towards said casing for urging a second milling tool into said casing, a first end which is secured to said whipstock anchor, and at least one slot formed into a surface of said inner whipstock;
• an outer whipstock having an uphole end for releasibly securing to said workstring, a first deflection surface which extends laterally towards said casing for urging said first milling tool into said casing, and a downhole end having at least one lug extending therefrom and into said at least one slot for releasibly securing said outer whipstock to said inner whipstock and said whipstock anchor;
• a whipstock assembly release latch having at least one member which extends between said uphole end said outer whipstock and said first milling tool to secure said whipstock assembly to said workstring until said whipstock assembly release latch is selectively operated to release said workstring from said whipstock assembly;
• at least one release shear pin for releasing said at least one lug to traverse within said at least one slot at a predetermined force threshold for releasing said outer whipstock from said inner whipstock for retrieval of said outer whipstock from said wellbore while said inner whipstock remains secured within said wellbore;
• a cylindrical head secured to an upper portion of said outer whipstock and having a retrieval shoulder for latching a spear thereto for securing said outer whipstock to said workstring for retrieval of said outer whipstock from said wellbore;
• an excluder member having a body which circumferentially extends from an exterior portion of said whipstock assembly and laterally toward said casing to prevent said debris from passing along said exterior portion of said whipstock assembly;
• a thin-walled sleeve secured about a portion of said first deflection surface, and having an exterior surface which extends substantially parallel to an interior surface of said casing for defining an exterior shape of said first deflection surface to prevent said first deflection surface from pressing said debris laterally toward said interior surface of said casing when said outer whipstock is urged to move within said wellbore;
• and
• a millable material disposed within an interior space between said thin-walled sleeve and said first deflection surface, and which is milled by said first milling tool during cutting of said window into said casing to at least in part define said exterior shape of said first deflection surface.

A method for cutting a window laterally through a casing within a wellbore for passing a workstring through said window to drill a sidetrack borehole from said wellbore, said method comprising the steps of:

• securing an inner whipstock to a whipstock anchor;
• securing a downhole end of an outer whipstock to said inner whipstock by a whipstock coupling which will release said outer whipstock from said inner whipstock at a preselected force threshold;
• securing an uphole end of said outer whipstock to a workstring with a release latch which is selectively operable to release said outer whipstock from said workstring;
• lowering said whipstock anchor, said inner whipstock, and said outer whipstock within said wellbore to a selected wellbore depth at which said wellbore is to be sidetracked;
• setting said whipstock anchor at said selected depth within said wellbore to grippingly engage said casing and support said inner whipstock within said wellbore;
• selectively operating said release latch to release said workstring from said outer whipstock;
• starting said window into said casing by rotating a first milling tool and lowering said first milling tool into said outer whipstock, which urges said first milling tool to cut through a wall of said casing;
• operating said whipstock coupling to release said outer whipstock from said inner whipstock and removing said outer whipstock from said wellbore;
• lowering a second milling tool downhole within said wellbore;
• and
• completing said window by rotating and lowering said second milling tool into said inner whipstock, which urges said second milling tool laterally through said casing.