Downhole apparatus and associated methods

1. A downhole indexing apparatus for cyclically varying a configuration or operational mode of a downhole tool according to a predetermined sequence, the apparatus comprising:

a tubular body including a throughbore;

an indexing member selectively moveable relative to the tubular body in response to a signal between a first axial position and a second axial position, wherein movement of the indexing member relative to the tubular body is defined by an indexing pin and slot coupling arrangement;

an indexing fluid chamber uphole of the indexing member, wherein the indexing fluid chamber is in fluid communication with the throughbore via an indexing chamber port; and

wherein the signal for moving the indexing member comprises change in a differential fluid pressure acting across the indexing member;

the apparatus further comprising a balance piston for supporting the indexing member at at least one of the axial positions;

wherein the indexing member is selectively movable between the first axial position and the second axial position in response to the signal, the second position comprising the supported position, and wherein the balance piston is configured to move from a passive position to an active position to support the indexing member at the second position;

wherein the indexing member is biased towards the supported position by a biasing force, and the balance piston is configured to at least partially counteract the biasing force when the indexing member is in the supported position;

wherein the balance piston is configured to engage or contact the indexing member at the second position, thereby reducing load on the indexing member and consequently the indexing pin.

2. The apparatus of claim 1 , wherein the biasing force provides a preload at the supported position, maintaining the indexing member in the supported position, and wherein the balance piston is configured to reduce the preload at the supported position.

3. The apparatus of claim 1 , wherein the biasing force comprises a fluid pressure force component, and the balance piston is configured to at least partially counteract the biasing force's fluid pressure force component.

4. The apparatus of claim 1 , wherein the apparatus is configured to expose the balance piston to a similar fluid pressure as the biasing force's fluid pressure, and wherein the apparatus is configured to propel the balance piston towards the active position and the indexing member from the unsupported or first position towards the supported or second position with fluid from a same source.

5. The apparatus of claim 1 , wherein the indexing fluid chamber is intermediate the biasing fluid chamber and the balance piston fluid chamber.

6. The apparatus of claim 1 , further comprising a coupling arrangement for controlling movement of the indexing member, wherein the coupling arrangement comprises a protrusion and a slot, wherein the protrusion engages the slot.

7. The apparatus of claim 6 , wherein the slot is configured to allow the balance piston to support at least at a portion of the biasing force and/or indexing force and/or counterforce at the first and/or second and/or third and/or fourth axial positions.