Charge pump for electric submersible pump (ESP) assembly

A method of lifting liquid in a wellbore, comprising:

• running an electric submersible pump (ESP) assembly into a wellbore, wherein the ESP assembly comprises
• an electric motor having a first drive shaft;
• a seal section coupled at a lower end to an upper end of the electric motor having a second drive shaft coupled to the first drive shaft;
• a charge pump assembly disposed downstream of the seal section, wherein the charge pump assembly comprises
• a third drive shaft coupled to the second drive shaft,
• a first fluid mover mechanically coupled to the third drive shaft and having a fluid inlet and a fluid outlet,
• a fluid reservoir concentrically disposed around the third drive shaft and located downstream of the first fluid mover, wherein an inside surface of the fluid reservoir and an outside surface of the third drive shaft define a first annulus that is fluidically coupled to the fluid outlet of the first fluid mover, and
• a second fluid mover mechanically coupled to the third drive shaft and having a fluid inlet and a fluid outlet, wherein the second fluid mover is located downstream of the fluid reservoir, and wherein the fluid inlet of the second fluid mover is fluidically coupled to the first annulus
• wherein the charge pump assembly is configured to flow substantially all of a fluid received by the first fluid mover out an outlet disposed at a downstream end of the charge pump assembly;
• a gas separator assembly coupled at an upstream end to the outlet of the charge pump assembly, having a fourth drive shaft coupled directly or indirectly to the third drive shaft and having an inlet in fluid communication with an outlet of the charge pump assembly, having a gas flow path and liquid flow path separator having a gas phase discharge port open to an exterior of the gas separator assembly and a liquid phase discharge port;
• and
• a production pump assembly coupled at an upstream end to a downstream end of the gas separator assembly and having an inlet in fluid communication with the liquid phase discharge port of the gas flow path and liquid flow path separator;
• turning the third drive shaft of the charge pump assembly by the electric motor of the ESP assembly;
• drawing reservoir fluid from the wellbore into the charge pump assembly by the first fluid mover of the charge pump assembly;
• moving the reservoir fluid downstream by the first fluid mover within the charge pump assembly;
• filling the fluid reservoir of the charge pump assembly with the reservoir fluid;
• flowing the reservoir fluid from fluid reservoir of the charge pump assembly to the fluid inlet of the gas separator assembly;
• discharging a first portion of the reservoir fluid via the gas phase discharge port of the gas flow path and liquid flow path separator to an exterior of the gas separator assembly;
• discharging a second portion of the reservoir fluid via the liquid phase discharge port of the gas flow path and liquid flow path separator to the inlet of the production pump assembly;
• pumping the second portion of the reservoir fluid by the production pump assembly;
• and
• flowing the second portion of the reservoir fluid out of a discharge of the production pump assembly and via a production tubing to a surface location.

A method of assembling an electric submersible pump (ESP) assembly at a wellbore location, where the ESP assembly comprises

• an electric motor having a first drive shaft;
• a seal section coupled at a lower end to an upper end of the electric motor having a second drive shaft coupled to the first drive shaft;
• a charge pump assembly disposed downstream of the seal section, wherein the charge pump assembly comprises
• a third drive shaft coupled to the second drive shaft,
• a first fluid mover mechanically coupled to the third drive shaft and having a fluid inlet and a fluid outlet,
• a fluid reservoir concentrically disposed around the third drive shaft and located downstream of the first fluid mover, wherein an inside surface of the fluid reservoir and an outside surface of the third drive shaft define a first annulus that is fluidically coupled to the fluid outlet of the first fluid mover, and
• a second fluid mover mechanically coupled to the third drive shaft and having a fluid inlet and a fluid outlet, wherein the second fluid mover is located downstream of the fluid reservoir, and wherein the fluid inlet of the second fluid mover is fluidically coupled to the first annulus
• wherein the charge pump assembly is configured to flow substantially all of a fluid received by the first fluid mover out an outlet disposed at a downstream end of the charge pump assembly;
• a gas separator assembly coupled at an upstream end to the outlet of the charge pump assembly, having a fourth drive shaft coupled directly or indirectly to the third drive shaft and having an inlet in fluid communication with an outlet of the charge pump assembly, having a gas flow path and liquid flow path separator having a gas phase discharge port open to an exterior of the gas separator assembly and a liquid phase discharge port;
• and
• a production pump assembly coupled at an upstream end to a downstream end of the gas separator assembly and having an inlet in fluid communication with the liquid phase discharge port of the gas flow path and liquid flow path separator,
• the method, comprising:
• coupling a downstream end of the electric motor to an upstream end of the seal section, including coupling the first drive shaft to the second drive shaft;
• lowering the electric motor, and seal section partially into the wellbore;
• coupling a downstream end of the seal section directly or indirectly to an upstream end to an upstream end of the charge pump assembly, including coupling the second drive shaft third drive shaft
• lowering the electric motor, seal section, and charge pump assembly partially into the wellbore;
• coupling the gas separator assembly to the ESP assembly so an inlet of the gas separator assembly is in fluid communication with the outlet of the charge pump assembly;
• lowering the electric motor, seal section, fluid intake, charge pump assembly, and gas separator assembly partially into the wellbore;
• coupling a downstream end of the gas separator assembly to an upstream end of the production pump assembly;
• and
• lowering the electric motor, seal section, charge pump assembly, gas separator assembly, and production pump assembly partially into the wellbore.