Experimental study of the coupled cord downhole seismic source

Abstract

INTRODUCTION To transmit sufficient energy within a reasonably short time interval without causing damage to the well (formations, cement, casing) is the most challenging requirement a practical downhole seismic source must fulfil. Actually introducing a powerful source in a well is not sufficient because the acoustic impedance contrast between the fluids in the well and the surrounding formations or the casing is very high (70-90%). The acoustic energy tends to remain in the well as tube waves bouncing between the top and the bottom of the fluid column. In fact, whether explosives or an airgun are used, the well bottom is very frequently the most energetic radiation point regardless of the actual source depth. Because it must fulfil conflicting requirements, designing an efficient and versatile downhole seismic source is no easy task. Chemical explosives were certainly the most obvious possible solution and were actually used very early for limited experiments. Studies of the use of explosive charges as downhole seismic sources are still going on (Chen and Eriksen, 1989) but have not led yet to commercial services. This paper describes a Coupled Cord Source and presents the results of experiments carried out in the reverse VSP and configurations. REQUIREMENTS TO BE FULFILLED BY A DOWWHOLE SEISMIC SOURCE The above remarks lead to the conclusion that a practical downhole seismic source must necessarily have an elongated near field radiation pattern in order to reconcile sufficient instantaneous transmission, low linear power density and sufficient coupling with the borehole wall. Downhole seismic sources must fulfil the following requirements if they are to be adopted for routine well seismic surveys: One solution tested in well-to-well and in wellto-surface configurations is the Coupled Cord downhole seismic source which may be briefly described as follows. 1 Sufficient bandwidth, from 10 Hz to about 1000 Hz, the high frequencies being required for well-to-well acquisition in order to take advantage of the capability of transmitting high frequencies over long distances in a well-towell configuration. Low density RDX cord strands are spiraled in grooves around a steel frame. The steel frame and the grooves bring the low density cord close to the wall, so that even at high pressures the shock wave reaches the wall and considerably increases energy transmission through the wall by comparison with an acoustic situation. 2 Sufficient energy: it should be possible to produce a sufficient S/N within an affordable time interval, probably not exceeding lo-20 min per source station. 3 No damage should be caused to the well (formations, cement, casing). Cementation is easily damaged and in some cases it may still be advisable to operate before cementing. Connections controlled by diodes allow 14 strands to be fixed separately when using a standard 7-conductor wireline. Field experiments were carried out in well-tosurface and well-to-well configurations. In the former case, the upper limit of the useful bandwidth was 300 Hz, and in the latter case, 650 Hz. This comparison was carried out in poorly consolidated sediments and under the water table for the well-to-well set up. 4 For reasons of cost-efficiency, a sufficient number of source stations should be completed before the device needs to be removed from the well. Considering that a wireline trip takes about 30 to 90 minutes depending on depth, it seems that 5 levels per trip might be considered as a minimum. 5 The diameter of the device should be adequate for operations in most wells. BRIEF REVIEW OF TBE DIFFERENT TYPES OF DOWEEOLE SEISMIC SOURCES A thorough check of the casing condition before and after the experiments showed that no damage A broad spectrum of different sources has been was caused. presented in various meetings and publications. They belong to the following categories: The explosive caps and the RDX cord are of the type commonly available on oil fields where they are used on perforating guns. 1 Chemical explosrves (exogen i.e. RDX) Compact charges Explosrve cord 2 Airguns (connected to the surface through a hose)