Reservoir characterisation: microseismics now a key tool

Paul Boughton

William Wills looks at the increasing use of passive seismic and microseismic monitoring within the oil and gas industry

Passive seismic monitoring has been used on a global scale for many years to predict potential earthquake or volcanic eruption activities. Today, passive seismic and microseismic (MS) applications are now being widely used within the oil and gas industry. With a current industry trend towards more long term deployments of geophones to complement traditional surface source driven seismic techniques to map reservoir dynamics, there is an increasing demand for acquisition hardware to be developed parallel with the rapidly improving microseismic processing speeds, visualisation, and analytical techniques.

Changing reservoir dynamics

Microseismic events can be described as small magnitude microearthquakes (microseismicity) associated with local stress changes in and around a reservoir.

These stress changes can occur when removing fluids from existing geological structure (micro-subsidence) or be induced by pumping fluids back into reservoirs. These changing reservoir dynamics will generate the microseismic events which can then be employed to pinpoint where and to what extent reservoir geology changes are taking place, and to map any fracture pattern or other rock property changes.

As in all seismic survey applications the most accurate results are achieved by measuring higher frequencies and having a high signal to noise ratio. Hence the evolution of geophone arrays being placed closer to the source of micro-seismic events in much quieter ambient noise locations, ie deep underground. Here an added advantage comes from being able to more readily identify and utilise the shear wave energy generated by the micro earthquakes.

Hydro-fracturing techniques

Arrays of three component seismic receivers locked into nearby monitoring wells are now in common use for both passive and induced MS monitoring.

MS induced events are epitomised by the monitoring of oil or gas bearing rocks being treated with hydro-fracturing techniques used to enhance hydrocarbon recovery, particularly from 'tight' zones or reservoirs where production has significantly slowed. Known as EOR (Enhanced Oil Recovery), 'fraccing' is just one of several methods utilised, others examples are, among many, steam and water flood.

The primary principle is to use the most appropriate methods to stimulate production. Fluids are pumped into the reservoirs, often at high pressure. The fluid used for such recovery depends on the process needed to make the hydrocarbons flow from the host rock. Such applications include heat for low viscosity oil and pressure to persuade migration of hydrocarbon fluids towards where the producer needs them, ultimately the surface.

Additionally, these fluids can be employed to fracture rocks to produce less restrictive travel paths and contain a proppant for propping open existing and induced rock cracks and fractures.

Hedging against future shortages

There is a modern trend to store natural gas back underground to create reserves, taking advantage of periods where there is a commercial surplus or when hedging against future supply shortages.

Quite often carbon gas is used for stimulating EOR. This has naturally a commercial benefit as well as capturing undesirable carbon. It is anticipated that future carbon sequestration will involve pumping waste CO2 into suitable subsurface structures and chambers. One such formation intended for sequestration is located above the Sleipner field, one of the larger natural gas producers in the North Sea.

The methodology for monitoring such procedures therefore already exists but can always be improved and refined.

Low noise, high sensitivity

As previously emphasised the need for downhole tools with low noise and high sensitivity is crucial to identify the low amplitude, high frequency seismics. Microseismic tools are now developing which embody the combined components of very low electronic noise and high gain digitisers with multiple omni directional geophones arranged within a sensor pack in order to achieve high sensitivity.

Such downhole systems are evolving from dedicated VSP arrays into genuine multi-purpose systems with a large bandwidth of ~1600Hz, very low electronic noise levels and extremely high sensitivity, which is ideal for hydraulic fracture surveys.

In order to keep survey/drilling costs down, even slimmer tools are required, these will need to withstand increasingly higher temperatures and pressures for longer periods of time. In certain large long duration projects, deployment of permanent sensor instrumentation in monitoring wells may potentially be a more optimum solution.

Whatever commercial or environmental methods are used involving pumping fluids deep into the subsurface 'traps' for enhanced hydrocarbon recovery or carbon capture and storage (CCS) purposes, monitoring and control is required.

Currently, arrays of underground multi-component, receivers strategically placed in nearby monitoring wells are used to capture microseismic events or geophysical changes taking place in the zones of interest.

These 'cross-well' configuration procedures require careful planning, highly sensitive and accurate recording and expert data manipulation and interpretation.

Pre and post seismic studies using a non-destructive high energy, high frequency, down-hole seismic source is also expected to play a major role in all these types of operations (Fig 1).

Confidence is high

There is a considerable amount of money and effort currently being invested into all procedural phases of the enhanced oil recovery and carbon capture and storage and confidence is high that such operations will benefit from successful, accurate and safe techniques ensuring, where appropriate, commercial and/or environmental benefits.

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William Wills is Geotechnical Support Engineer with Avalon Sciences Ltd, Somerton, Somerset, UK.

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