A method of clearing well casing (3) or tubing from a target region of an oil/gas well (1) borehole is provided. The clearance of the oil/gas well bore hole being achieved by employing chemical agents (9) that consume, weaken or melt the well casing/tubing. In some aspects of the method the well casing is cleared to expose the rock formation within which the well borehole is formed so that the rock formation can be accessed from within the well casing/tubing. In other aspects of the method the removal of inner tubing structures is used to facilitate the unimpaired deployment of repair tools down the well borehole.

A method of clearing well casing (3) or tubing from a target region of an oil/gas well (1) borehole is provided. The clearance of the oil/gas well bore hole being achieved by employing chemical agents (9) that consume, weaken or melt the well casing/tubing. In some aspects of the method the well casing is cleared to expose the rock formation within which the well borehole is formed so that the rock formation can be accessed from within the well casing/tubing. In other aspects of the method the removal of inner tubing structures is used to facilitate the unimpaired deployment of repair tools down the well borehole.

A method for reducing water production from a hydrocarbon bearing subterranean formation includes identifying a high permeability zone in the formation and injecting a dense CO.sub.2 composition from a production well into the high permeability zone. The dense CO.sub.2 composition includes dense CO.sub.2 and a thickener soluble in the dense CO.sub.2. The thickener includes a copolymer that is the polymerized reaction product of monomers that include at least one alkenyl ether or dialkenyl ether monomer, at least one acrylate or methacrylate monomer, at least one structural monomer, and at least one allyl ester monomer. After injecting the dense CO.sub.2 composition into the high permeability zone, the method includes withdrawing hydrocarbons from the hydrocarbon bearing subterranean formation through the production well. The dense CO.sub.2 composition blocks pores in the high permeability zone to reduce or prevent flow of water from the high permeability zone into the production well.

A method for reducing water production from a hydrocarbon bearing subterranean formation includes identifying a high permeability zone in the formation and injecting a dense CO.sub.2 composition from a production well into the high permeability zone. The dense CO.sub.2 composition includes dense CO.sub.2 and a thickener soluble in the dense CO.sub.2. The thickener includes a copolymer that is the polymerized reaction product of monomers that include at least one alkenyl ether or dialkenyl ether monomer, at least one acrylate or methacrylate monomer, at least one structural monomer, and at least one allyl ester monomer. After injecting the dense CO.sub.2 composition into the high permeability zone, the method includes withdrawing hydrocarbons from the hydrocarbon bearing subterranean formation through the production well. The dense CO.sub.2 composition blocks pores in the high permeability zone to reduce or prevent flow of water from the high permeability zone into the production well.

A method of forming a barrier to overcome lost circulation in a subterranean formation. The method includes injecting a polymer-sand nanocomposite into one or more lost circulation zones in the subterranean formation where the polymer-sand nanocomposite is formed from sand mixed with a polymer hydrogel. Further, the polymer hydrogel includes a hydrogel polymer, an organic cross-linker, and a salt. The sand additionally comprises a surface modification. The associated method of preparing a polymer-sand nanocomposite lost circulation material for utilization in forming the barrier is provided.

A while drilling mud loss treatment method includes providing a drilling tool main body with a through bore connected to an above arranged wired drill pipe string with a communication line to a topsides monitoring and control system, the drilling tool main body connected to a below arranged one or more drill collar sections with a lower of said drill collar sections connected to a drill bit, and drilling in a well. The main body is provided with an annular tank with a swellable sealant and the annular tank has a valve to an outlet to the through bore. A control system in the main body receives MWD sensor signals from an MWD sensor system and controls the valve having a valve actuator. The control system is, during drilling, running a monitoring and control algorithm using the signals as input for detecting an undesired mud loss state during drilling, and, if a mud loss state is detected, to command said valve actuator to open said valve upon detecting an undesired mud loss state, so as for ejecting said swellable sealant to said through bore.

A while drilling mud loss treatment method includes providing a drilling tool main body with a through bore connected to an above arranged wired drill pipe string with a communication line to a topsides monitoring and control system, the drilling tool main body connected to a below arranged one or more drill collar sections with a lower of said drill collar sections connected to a drill bit, and drilling in a well. The main body is provided with an annular tank with a swellable sealant and the annular tank has a valve to an outlet to the through bore. A control system in the main body receives MWD sensor signals from an MWD sensor system and controls the valve having a valve actuator. The control system is, during drilling, running a monitoring and control algorithm using the signals as input for detecting an undesired mud loss state during drilling, and, if a mud loss state is detected, to command said valve actuator to open said valve upon detecting an undesired mud loss state, so as for ejecting said swellable sealant to said through bore.

A method of cementing may include: measuring a feeding rate of water and a feeding rate of cement blend into a cement blender; calculating a cement retarder feeding rate and/or an accelerator feeding rate using a thickening time model, wherein the thickening time model uses at least a thickening time requirement, the feeding rate of water, and the feeding rate of cement blend, to calculate the cement retarder feeding rate and/or the accelerator feeding rate; introducing a cement retarder at the cement retarder feeding rate and/or an accelerator at the accelerator feeding rate into the cement blender; mixing at least the water, cement blend, and at least one of the cement retarder and/or the accelerator in the cement blender to provide a cement slurry; and placing the cement slurry in a wellbore.

A method of cementing may include: measuring a feeding rate of water and a feeding rate of cement blend into a cement blender; calculating a cement retarder feeding rate and/or an accelerator feeding rate using a thickening time model, wherein the thickening time model uses at least a thickening time requirement, the feeding rate of water, and the feeding rate of cement blend, to calculate the cement retarder feeding rate and/or the accelerator feeding rate; introducing a cement retarder at the cement retarder feeding rate and/or an accelerator at the accelerator feeding rate into the cement blender; mixing at least the water, cement blend, and at least one of the cement retarder and/or the accelerator in the cement blender to provide a cement slurry; and placing the cement slurry in a wellbore.

A downhole sealant injection system includes a first casing configured to be positioned in a wellbore and a second casing configured to be positioned in the wellbore within the first casing. Cement at least partially fills an annulus between the interior of the first casing and the exterior of the second casing. A first sealant injection tool is configured to be attached to the exterior of the second casing, and is positioned at a downhole location and within an annulus between the interior of the first casing and the exterior of the second casing. The sealant injection tool includes a plurality of nozzles configured to inject sealant into voids within the cement in the annulus between the interior of the first casing and the exterior of the second casing.