Summary
This paper details the application of passive seismic monitoring to image
reservoir fracturing and deformation from the stage of an initial well
completion to final field production. Instrumented oil fields with seismic
arrays either permanently installed or temporarily deployed on wireline offer
the possibility of imaging production activities in a real-time sense that
complements other seismic-reflection and engineering measurements. During the
well-completion stage of development, real-time microseismic imaging offers the
possibility of monitoring well stimulation. Fracture images may be used to
optimize the fracture design and the net present value (NPV) of well
production, as well as understand fracture complexity and the associated
well-drainage pattern to target future well placement. During production
stages, time-lapse microseismic imaging may be used for image deformation
associated with fracturing or fracture reactivation from pressure or stress
changes, strains in the overburden in fields with casing-deformation problems,
and image fronts associated with secondary recovery. In this paper, several
case studies are used to illustrate various potential applications, along with
discussion of the potential limitations. The reservoir conditions necessary for
the successful application of the technology are presented along with a
potential method to quantify the technical feasibility at a particular
site.
Introduction
With the current industry trend toward instrumented oil fields and
smart-well completions, the permanent deployment of geophones or other acoustic
sensors to complement standard engineering gauges is being promoted as a way to
map reservoir dynamics. The biggest push is from active time-lapse seismic,
although the deployment of permanent seismic instrumentation is also
potentially an ideal route to monitor passive seismicity. Passive monitoring of
acoustic emissions, or small-magnitude microearthquakes (microseismicity)
associated with stress changes in and around the reservoir, can also be used to
image the reservoir dynamics. Passive monitoring has the benefit of more fully
using the seismic sensors to monitor during periods between conventional
seismic surveys, directly imaging fracturing and deformation, and offers
complementary information to both active time-lapse images and engineering
measurements.
Microseismic events, related to either induced movements on pre-existing
structures or the creation of new fractures, capture deformations as the rock
mass reacts to stresses and strains associated with pressure changes in the
reservoir. The microseismicity can be used to localize the fracturing or to
deduce geomechanical details of the deformation. Since the Rangely experiment
in the late 1960s,1 a number of passive seismic experiments have been pursued
in the petroleum industry with varying degrees of success.2–5 Recently, a
number of independent operators have successfully implemented passive seismic
studies to address specific issues. The majority of these studies are under the
umbrella of hydraulic fracturing,2,3 where the microseismicity is used to map
the fracture growth directly during well stimulations. However, a number of
other studies have been used to image deformations associated with primary
production,4 secondary recovery,4 or waste-injection operations.5 In the vast
majority of these cases, an array of seismic sensors is deployed by wireline to
monitor for a specific period. This requires finding a well “close to the
action” to facilitate detection of these small passive signals without
impacting production.
Permanent sensor deployment in an instrumented oil field circumvents the
chronic problem of well availability. In numerous fields, microseismicity is
continually occurring, and if the instrumentation were in place to record the
data properly, additional information on the reservoir performance could be
gained. As an aside, it is worth considering how much of the “noise” recorded
in conventional seismics may be actually valuable microseismic data. The key
will be to design the seismic arrays properly to cover both conventional active
seismics (e.g., reflection and tomography) and specific issues associated with
passive recording. This paper will outline a viewpoint of the potential
applications and technical issues associated with passive seismic monitoring.
Because passive seismics is probably best viewed as being in its infancy in the
petroleum industry, it is worth standing back and considering applications in
other industries in which the technology is more mature. In mining, real-time
microseismic data are used by supervisors to decide if it is safe to send
miners underground.6 Microseismic data are also crucial in a number of other
rock-engineering applications, such as excavation stability in nuclear-waste
repositories,7 geotechnical stability,8 and performance of geothermal
reservoirs.9 Permanent instrumentation in oil fields also should allow the
maturity of the technology to help solve certain geomechanical problems in the
petroleum industry.
This article generally will focus on borehole deployments because passive
monitoring will most likely involve borehole arrays to keep the instrumentation
close to the action and maximize sensitivity. In some special cases, where
induced seismic activity can be detected at surface, permanent surface arrays
could be used in a context similar to the picture painted in this paper.
However, for the most part, the following discussion will focus on borehole
arrays.
© 2005. Society of Petroleum Engineers
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