Graded filter-separator and method for use

A downhole toolstring for collecting debris within a wellbore, the downhole toolstring is adapted to connect to a surface equipment via an elongated flexible member, wherein the downhole toolstring comprises:

• a collecting device comprising:
• a leading portion, an opposite trailing portion, a wall connecting the leading portion to the trailing portion, a collecting chamber on an inside of the wall, an inlet at the leading portion, an outlet at the trailing portion, and an elongated filter;
• the collecting device forms a device longitudinal central axis between the leading portion and the trailing portion;
• the elongated filter extends along the device longitudinal central axis and forms a first end portion and an opposite second end portion, wherein the elongated filter at the first end portion is formed with a first permeability and the elongated filter at the second end portion is formed with a second permeability;
• the second end portion is positioned closest to the inlet;
• the elongated filter forms a retentate side and a permeate side;
• and
• the permeate side of the elongated filter is fluidly connected with the outlet.

A method for collecting debris within a wellbore, the method comprises the steps of:

• providing a downhole toolstring comprising a collecting device, the collecting device comprising:
• a leading portion, an opposite trailing portion, a wall connecting the leading portion to the trailing portion, a collecting chamber on an inside of the wall, an inlet at the leading portion, an outlet at the trailing portion, and an elongated filter;
• the collecting device forms a device longitudinal central axis between the leading portion and the trailing portion;
• the elongated filter extends along the device longitudinal central axis and forms a first end portion and an opposite second end portion, wherein the elongated filter at the first end portion is formed with a first permeability and the elongated filter at the second end portion is formed with a second permeability;
• the second end portion is positioned closest to the inlet;
• the elongated filter forms a retentate side and a permeate side;
• and the permeate side of the elongated filter is fluidly connected with the outlet;
• displacing the downhole toolstring into the wellbore such that the leading portion is near or adjacent the debris to be collected;
• collecting the debris from the wellbore within the collecting chamber;
• bringing the collecting device out of the wellbore;
• and
• emptying the collecting device of collected debris outside the wellbore.