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.

The invention relates to a method for recovery of hydrocarbons present in an underground formation by injection of a saline aqueous solution comprising at least one surfactant, by means of a numerical flow simulator including a model of the evolution of the interfacial tension between the saline aqueous solution and the hydrocarbons as a function at least of salinity, wherein the interfacial tension evolution model is calibrated as follows: i) carrying out interfacial tension measurements for a plurality of emulsions having distinct salinity values corresponding at least to the optimum salinity, to two salinities bounding the optimum salinity in a 5-10% limit, to the zero salinity and to the solubility limit of the salts; ii) determining the constants of the interfacial tension evolution model by minimizing a difference between the model and the interfacial tension measurements.

The invention relates to a method for recovery of hydrocarbons present in an underground formation by injection of a saline aqueous solution comprising at least one surfactant, by means of a numerical flow simulator including a model of the evolution of the interfacial tension between the saline aqueous solution and the hydrocarbons as a function at least of salinity, wherein the interfacial tension evolution model is calibrated as follows: i) carrying out interfacial tension measurements for a plurality of emulsions having distinct salinity values corresponding at least to the optimum salinity, to two salinities bounding the optimum salinity in a 5-10% limit, to the zero salinity and to the solubility limit of the salts; ii) determining the constants of the interfacial tension evolution model by minimizing a difference between the model and the interfacial tension measurements.

A method for determining properties of different cation exchange sites in a rock core sample at a non-preserved state may include displacing all native components out of the rock core sample before subjecting the rock core sample to coreflooding steps to determine a total amount of exchangeable cations adsorbed onto the cation exchange sites; injecting formation brine and then a reservoir crude oil into the rock core sample such that the rock core sample includes indigenous exchangeable cations adsorbed onto the cation exchange sites, cation exchange sites occupied by a crude oil, and one or more fluids; subjecting the rock core sample to coreflooding steps to displace the indigenous exchangeable cations, the crude oil, and the one or more fluids; determining an amount of indigenous exchangeable cations adsorbed onto the cation exchange sites; and determining at least one property of different cation exchange sites.

A method for determining properties of different cation exchange sites in a rock core sample at a non-preserved state may include displacing all native components out of the rock core sample before subjecting the rock core sample to coreflooding steps to determine a total amount of exchangeable cations adsorbed onto the cation exchange sites; injecting formation brine and then a reservoir crude oil into the rock core sample such that the rock core sample includes indigenous exchangeable cations adsorbed onto the cation exchange sites, cation exchange sites occupied by a crude oil, and one or more fluids; subjecting the rock core sample to coreflooding steps to displace the indigenous exchangeable cations, the crude oil, and the one or more fluids; determining an amount of indigenous exchangeable cations adsorbed onto the cation exchange sites; and determining at least one property of different cation exchange sites.

A process for recovering oil from an oil-bearing formation is disclosed comprising providing a first oil recovery formulation comprising a first surfactant able to create low interfacial tension with reservoir crude oil; injecting the first oil recovery formulation into the oil-bearing formation via an injection well (201); providing a second oil recovery formulation having a second surfactant with a higher solubility in water than the first formulation; injecting the second oil recovery formulation into the oil-bearing formation via the injection well; and producing oil to a production well (203).

A process for recovering oil from an oil-bearing formation is disclosed comprising providing a first oil recovery formulation comprising a first surfactant able to create low interfacial tension with reservoir crude oil; injecting the first oil recovery formulation into the oil-bearing formation via an injection well (201); providing a second oil recovery formulation having a second surfactant with a higher solubility in water than the first formulation; injecting the second oil recovery formulation into the oil-bearing formation via the injection well; and producing oil to a production well (203).

A method and apparatus for controlling the flow of fluid in an injection well includes a main valve assembly having a valve and a retrievable nozzle selective lock assembly (RNSLA). The RNSLA is operable when positioned within the valve body to open the valve when fluid is pumped into the well and closes the valve when fluid flow is terminated. The RNSLA includes a replaceable orifice nozzle so that orifices of different dimensions may be used in conjunction with the valve assembly. In an alternate embodiment, the RNSLA includes a variable output nozzle assembly to maintain the valve in a protected open position without chattering over a wide range of flow rates.

A method and apparatus for controlling the flow of fluid in an injection well includes a main valve assembly having a valve and a retrievable nozzle selective lock assembly (RNSLA). The RNSLA is operable when positioned within the valve body to open the valve when fluid is pumped into the well and closes the valve when fluid flow is terminated. The RNSLA includes a replaceable orifice nozzle so that orifices of different dimensions may be used in conjunction with the valve assembly. In an alternate embodiment, the RNSLA includes a variable output nozzle assembly to maintain the valve in a protected open position without chattering over a wide range of flow rates.

A chelating agent can be used to enhance secondary hydrocarbon-fluid recovery during waterflooding operations. A composition can include a fluid and a chelating agent. The chelating agent can increase the viscosity of the fluid, which can enhance the efficacy of the waterflooding operations. The chelating agent can also form complexes with divalent cations in precipitates and solids formed by the divalent cations. The complexes can keep the cations in a soluble form until the composition exits the production well, which can prevent precipitates from forming in the production well and blocking pore throats in the production well.