Summary
This paper details the application of passive seismic monitoring to image reservoir fracturing and deformation from the stage of an initial wellcompletion 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, an umber 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 micro seismic 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-wasterepositories,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.
THE BEHAVIOR CHARACTERISTICS OF A RESERVOIR LEVEE SUBJECTED TO INCREASING WATER LEVELS