Firefood Microseismic Monitoring: Results And Potential For Process Control
Authors: Nyland E. , Dusseault M. 
Published: March 2010

A pilot fireflood in a 750m deep high permeability unconsolidated channel sand in eastern Alberta was instrumented for microseismic emission to evaluate the technique for in-situ process monitoring. The pilot consists of a central air injection well, eight surrounding production wells on artificial lift, and two internally placed observation wells.

Three surface geophones and a downhole 12-hydrophone string in a single well were deployed. Data were collected for 22 hours. Event detectors and cassette recorders were used for data collection. The recorders are triggered by a preset signal-to-noise ratio, allow pre- and post-event buffer recording, and can record 200 samples per second. The devices are self-triggering so that one large signal automatically causes recording on other channels.

Monitoring during twelve hours of steady air injection at 625m3/s yielded a clear event frequency of about 50 events/h. The events are mostly clear compressional wave arrivals, each consisting of 1 to 2 cycles with a repeat signal, in-phase with the first, occurring shortly afterwards (0.2–0.3 sec later). Stopping air injection caused a gradual drop in event frequency, and the rate was re-established when air injection was continued. The surface geophone yielded no useful data associated with in-situ processes. Spectral frequency analysis and knowledge of material properties indicated that the events are shears of a stick-slip nature associated with the strain weakening behavior of the over-compacted reservoir sands.

The recording accuracy implies a capability to locate events to within 10m in space, and signal analysis indicates that some discrimination between events as a function of processes may be achieved. The system is therefore directly applicable to mapping of firefloods and can thus be used as an aid to process control. Other in-situ recovery technologies may also produce useful signals for process control. In particular, steam flooding, hydraulic fracturing, and high-pressure injection operations likely generate mappable microseismic events which can be applied directly to production/injection strategy.

Introduction

As part of an ongoing program of studies relating to the process of fireflood recovery of heavy oil. Noreen Energy Resources Ltd. and the University of Alberta have monitored a fireflood with a vertical string of Mark Products T27 hydrophones in an observation well very close to the burn zone.

An enhanced oil recovery experiment is being carried out in the Bodo pool at a depth of 750m in east-central Alberta. A fireflood scheme is active using a central air injection well and eight peripheral wells in a square pattern on pump. The air injection rate is 625m3 /h and the pattern contains two observation wells monitored for temperature changes. The project (Fig. 1) was monitored for microseismic activity for a 22-hour period in the spring of 1981 using a string of hydrophones in one observation well and a set of three surface geophones. The purpose of the microseismic experiment was to find out if seismic energy was being emitted as the result of the process, and to assess whether this energy could be used to clarify in-situ processes and ultimately be applied to aid process control by delineating fire flood fronts and velocities.