Fracture monitoring

A method of downhole monitoring of hydraulic fracturing comprising the steps of:

• interrogating an optic fibre arranged down the path of a well bore to provide a distributed acoustic sensor;
• sampling data from a plurality of longitudinal portions of the fibre;
• and
• processing said data to provide an indication of at least one fracturing characteristic, wherein the indication of at least one fracturing characteristic comprises an indication of proppant wash-out.

A method of downhole monitoring of hydraulic fracturing comprising the steps of:

• interrogating an optic fibre arranged down the path of a well bore to provide distributed acoustic sensing;
• sampling data from a plurality of longitudinal portions of the fibre;
• and
• processing said data to provide an indication of at least one fracturing characteristic comprising the step of performing transient detection to determine at least one of a direction to, a range to or a location of the origin of the transient.

A method of hydraulic fracturing of a well bore comprising the steps of:

• flowing a fracture fluid down the well bore under pressure,
• performing the method of monitoring as claimed in any preceding claim;
• and
• adjusting flow parameters of the fracture fluid in response to said indication of at least one fracturing characteristic.

A system for downhole monitoring, said system comprising:

• a fibre optic interrogator adapted to provide distributed acoustic sensing on an optic fibre arranged along the path of a well bore;
• a sampler arranged to sample a plurality of channels output from said interrogator to provide acoustic data from a plurality of portions of said fibre at each of a plurality of times;
• and a data analyser adapted to process said sampled data to detect fracture characteristics and provide indications of said fracture characteristics wherein the indication of at least one fracturing characteristic comprises an indication of at least one of:
• (a) the intensity levels;
• (b) the frequencies, and (c) the spread of the frequencies of acoustic disturbances in at least one longitudinal sensing portion of the fibre in the vicinity of a fracture site, and wherein said indication(s) are used to provide an indication of the flow of proppant and fluid into a fracture.