Downhole apparatus

The apparatus of any of the preceding claims wherein the positive displacement motor operates using the Moineau principle and includes a lobed rotor which rotates within a lobed stator, the stator having one more rotor than the rotor.

The apparatus of any of the preceding claims, in combination with a drill bit connected to the housing.

The apparatus of any of the preceding claims, wherein the valve includes first and second valve members each defining a respective axial flow opening and which openings are aligned to collectively define an open axial drilling fluid flow port through the valve, the first member being rotatable about a longitudinal axis of the housing to vary the alignment of the openings and thus vary the open area of said port to, in use, provide a varying flow therethrough and variation of the fluid pressure.

A flow pulsing drilling method comprising the steps;

• providing a valve in a drill string bore including first and second valve members each defining a respective axial flow opening and which openings collectively define an open axial flow port through the valve;
• and
• rotating the first member about a longitudinal axis to vary the alignment of the openings such that the open area of said axial flow port varies with said rotation to provide variable flow therethrough and thus produce varying fluid pressure in the drilling fluid.