Monitoring the Oil Field With Live-Well Microseismics

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 19693, "Monitoring the Oil Field of the Present Using Live-Well Microseismic Technology," by Stephen A. Wilson, Gilles Le Floch, and Rob Jones, Schlumberger, prepared for the 2008 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed. Microseismics provides direct 4D information about stress changes in the reservoir that are caused by injection and production operations. These stress changes emit packets of seismic energy and information about all the small slip events triggered by oilfield operations. Until recently, microseismic monitoring required deploying seismic sensors in inactive monitor wells. It is now possible to deploy seismic monitoring in live wells. Such monitoring can take place during active injection or production and in offshore environments where monitor wells are rarely available. Introduction It is possible to detect tiny slip magnitudes corresponding to only a few microns of movement. Such movements may correspond to slip on a fracture, movement within the cement, slip on bedding, the opening of a fracture, or even precursory slip in the completion. Mapping these changes in both space and time provides unique insight into stress change in the reservoir, the overburden, and the well. Although this activity is triggered by oilfield activity, the emitted energy derives from strain energy stored within the rock mass. In the oil industry, microseismic monitoring has been limited to short-term monitoring of hydraulic-fracturing operations. Microseismic monitoring has been used to detect and, subsequently, limit integrity problems; map fault reactivation, thereby identifying drilling hazards; improve well longevity and identify compartments; map casing failure; and position wells better to improve production and injection efficiency. Successful microseismic monitoring requires the use of sensors that are acoustically well-coupled to the rock and acoustically decoupled from manmade-noise sources. Such sensors must form part of a system that is designed to record low-energy signals. Microseismic Basics Microseismic monitoring involves listening to a rock mass to detect and locate the release of seismic energy from any discrete naturally occurring seismic source (Fig. 1). In this context, "naturally occurring" could include movement of any manmade downhole structure but would exclude any manmade seismic source. In contrast, conventional reflection-seismic monitoring uses artificial seismic sources to image either subsurface layering or to infer changes in subsurface properties. Both techniques measure vibrations in the ground. However, the sources of the data are different, and each technique processes the vibration data in very different ways to derive value (Fig. 2). An individual microseismic event provides information about its source and the rock mass through which it passes in travelling from source to receiver. It is necessary to detect and accurately locate a population of microseismic events to provide useful and meaningful reservoir information. The detection rates from actively producing fields range from a single event per week to more than 100 events per day.