The precise depth at which a target formation will be penetrated by the bit is never known with certainty until the well is actually drilled. Yet maximising recoverable reserves depends on optimal wellbore placement, which in turn depends on reacting to the newly confirmed knowledge of a formation boundary’s whereabouts the moment it is discovered.
A new technology, RockSense, provides at-bit bed boundary identification while Coiled Tubing Drilling and gives engineers and directional drillers as new tool in the Geosteering toolkit.
Richard Stevens, Coiled Tubing Applications Engineer at AnTech Ltd.
MED OIL & GAS SUMMER/AUTUMN MAGAZINE 2018: ANTECH'S ROCKSENSE TECHNOLOGY
12th November 2018
Mechanical Specific Energy was first proposed by Teale in 1965. MSE provides a way to characterise the energy required to drill a length of hole in a formation. It follows that, as the drill-ability of the formation changes, so does the energy required to drill it. It turns out that a plot of MSE vs Depth for a section of hole is a very good proxy for porosity, and the fact that it is an at-bit measurement makes it an extremely useful tool to have when navigating vertically limited formations.
The calculation of MSE requires real time measurement of Weight-on-Bit (WOB) and Torque. Historically, downhole Weight-on-Bit and Torque were derived from surface measurements, with corrections applied for the effects of buoyancy and friction. The error inherent in these empirical corrections was usually greater than the signal in the subtle changes in MSE due to differing formation characteristics. In other words, the signal from the formation was lost in the noise of the corrections. Latterly, technology advances have permitted downhole measurement of WOB and Torque, but mud pulse bandwidth limitations have imposed severe constraints on the definition that has been possible.
The latest generation of Coiled Tubing Drilling BHAs, featuring integrated downhole sensors and high speed wired telemetry, provide a technology platform that finally makes high definition MSE measurements possible, opening a new window on the downhole environment as drilling progresses.
The technology has sound, intuitive footings. Picture yourself a passenger in a moving car. Even with your eyes closed, you know the type of road you’re travelling on (freeway, city street, dirt road) by the road noise you can hear. Anyone who has ever drilled a hole in a masonry wall to hang a picture, or fit shelves will have experienced the principle at first hand. Without even thinking about it, the person holding the drill is aware when he is drilling the plaster, when he is drilling brickwork and when / if he hits a void in the brickwork.
Consider now a motor turning a drill bit which drills a hole in a sample of rock. We can measure the power input to the motor as the hole is drilled to gain an understanding of the type of rock we’re drilling. If it was an electric motor we would simply measure the voltage applied and the current flowing during drilling and multiply them to get the instantaneous power. For a positive displacement mud motor it’s slightly more complicated, but it can be done. If differential pressure and flow rate can be measured then, given knowledge of principal operating constants for the motor, an expression for power in terms of pressure and flow rate can be written.
If we were then to integrate this power as the hole progresses we would have a value of Energy expended per foot of hole drilled and would therefore have a relative indicator of the changes in formation being drilled.
In this fashion, by continually monitoring torque, WOB, pressure and ROP, AnTech’s RockSense provides information about the formation being drilled, as drilling proceeds. The high data rate offered by wired telemetry, permits multiple measurements to be made per foot of hole drilled, yielding inch level resolution in the processed signal.
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