Thursday, January 11, 2018

Seismic While Drilling

Seismic while drilling promises elegant solutions to some limitations of conventional borehole seismics. But so far, it has failed to grab a significant slice of the market. Recent advances may create new opportunities for the technique.

 The aim is to turn conventional borehole seismics on its head. Instead of locating seismic sources on the surface and conveying receivers downhole, seismic while drilling uses bit vibrations as a downhole source and surface geophones to measure signals. This inversion promises timely seismic information without interrupting drilling and without deploying any downhole hardware. Further, the service may be employed in environmetally sensitve areas where surface seismic sources are disruptive - for example, in jungle roads built specially to accomodate sources. Despite these advantages, a widely-used seismic-while-drilling service has so far proved elusive.

This article outlines the princlpes of seismic while drilling and its potential uses, looks at developments to date, and examines how recent research initiatives are rekindling interest in the technique.

The principles

Well location is usually selected using surface seismic images. Once drilling is underway, it is useful to know the bit's position relative to the seismic section. However, this information is not easily available because the vertical axis of the seismic section is measured not in distance but in "two-way-time" - the time the seismic waves take to travel through the earth, bounce off a subsurface reflector and return to surface.

To relate the position of the bit to the seismic section, it is necessary to convert the vertical axis from time to depth. This conversion requires knowledge of the velocity of seismic waves through the formation. Velocity varies significantly with rock type and usually has to be measured, rather than modeled, using a combination of sonic logs and borehole seismics of a well after it has been drilled. 

The theory of borehole seismics has been known for many decades. At its simplest, a geophone deployed on wireline records the time that seismic waves take to travel from a surface source to a receiver at known depth in the well. These times are doubled to tie in with two-way time on the surface seismic section. This simple service is known as a "checkshot" survey.

But there are subtleties that add significantly to the usefulness of borehole seismics. Good quality data, sampled finely and in sufficient depth, enable a vertical reflection image or vertical seismic profile (VSP) to be created. In a basic VSP survey, the seismic source is static and the geophone  is moved to different levels in the well. The image may be displayed either in time, to match the surface seismic section, or in depth, to match wireline logs.

Alternatively, the geophone location may be fixed and the surface source moved along a line that "walks away" from the rig. Walkaway VSP produces an image of the subsurface with lateral coverage that is typically between half and a quarter the well depth. In deviated wells, various combinations of VSP and walkaway VSP may be employed to provide the required images.

Today, borehole seismics delivers a range of high-resolution images. However, like all wireline-delivered services, drilling must stop and the drillstring must be removed prior to running the survey. Therefore, borehole seismics is typically carried out during openhole logging, usually just before casing is run. The results certainly offer useful information, but this may be too late.The well may already be in the wrong place - for example, on the wrong side of a fault subsequently revealed by walkaway VSP - and a costly sidetrack may be needed. Furthermore, it may be expensive or impossible to locate sufficient surface sources to create a satifactory walkaway VSP image.

 In seismic while drilling, compressional waves emitted by the active bit radiate both directly to surface and downward from the bit, reflecting off formation boundaries. By using surface geophones to detect this sound, the inverse checkshot, VPS and walkaway VSP surveys may be obtained. 

  These techniques offer several advantages over conventional borehole seismics: drilling need not stop and, because the measurements are made continously, the information allows well trajectory decisions to be made before it is too late. Further, using the bit as a source may make it practical to perform large-scale borehole seismic jobs where surface sources are impractical - for example in towns or environmentally sensitive areas. However, seismic while drilling presents significant technical challenges. The signal emitted by conventional seismic sources is well controlled - either an impulsive explosion or a sweep from a vibrator of known signature - making the time between its emission and detection relatively easy to determine. On the other hand, the bit's signal is essentially continous and uncontrollable. A geophone on surface records continous seismic radiation as it is transmitted through the ground.

In addition, the environment around a drilling rig is very noisy. The comparatively low-level energy of the drillbit seismic signal is often completely submerged in noise. Onshsore, the geophone traces include several noise components. Some noise that correlates with seismic signal is caused by bit vibrations travelling up the drillstring and the fluid-filled annulus and then "rolling" along the air-ground interface to the geophones-this is called correlated ground roll. Uncorrelated ground roll comes from the vibrations of surface equipment like the mud pumps and engines. Random noise is caused by events like a passing truck or train. 

The challenge is to recognize the unknown and variable signature of the bit, to improve the signal-to-noise ratio and to convert a continous emission to one in which discrete seismic events may be recognized.

For seismic while drilling, the filter exploits differences in the moveout of components within the traces. Ground roll approaches the geophones from the side and exhibits moveout across the array. However, the wavefront of the seismic signal approaches the array from below, has zero moveout and is in phase. By using these differences in moveout to distinguish between different parts of the trace, the adaptive filter effectively attenuates the ground roll, while allowing the seismic signal to pass. 

Random noise may be removed by cross-correlating the individual geophone traces with the average of the two traces measured by the accelerometers on the drillstring. This crosscorrelation also gives the time shift between accelerometer and geophone signals- the difference in signal velocity through the drillstring and formation.

 Determining formation velocity also requires knowing the drillstring travel time. As already noted, the many components in the string complicate calculation of this travel time and a number of methods have been proposed - like Elf's double crosscorrelation process.

The continous checkshot system uses a new technique called drillstring imaging, also devised at SCR, to model changes in the acoustic impedance of the drillstring, giving a better understanding of the velocity of the signal through the drillstring. The time shift and the drillstring travel time are then used to compute the formation travel time.

The processing capitalizes on the relative abundance of geophone data and tracks the wavefront as it travels through the formation, potentially estimating the bit signal and the earth's response without using accelerometer data. However, by employing the accelerometer input, data may be compressed, making it feasible to store the massive volume of information collected over three or four days.

Each trace contains a common bit signature, and noise that varies from trace to trace. With time-delay curves, stacking and deconvolution filtering, new signals are created that represent what the traces would have looked like if the source had been a noiseless pulse - the earth impulse response. This converted form is then migrated to create an image.

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