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W H AT DOE S I N TEQ M A N U FACTUR E ? – 5 MWD-System. The plumb line measures the inclination of the borehole. The drilling engineers have agreed that a vertical hole has the inclination 0 degrees and a horizontal one the inclination 90 degrees etc.). Whenever a new run of drillpipes is screwed on to the drill string, the MWD-System uses the short moment of stillness in the borehole to measure the inclination and direction particularly precisely. The technical expert talks about performing a “Survey”. Our directional driller on the rig calculates the precise path of the borehole to date from the survey data. Apart from the inclination and the direction (“azimuth”) of the borehole, the MWD-System also has the task of establishing the direction in which the kick-off sub of the downhole motor, the “Toolface“ is pointing at that precise moment because our directional driller needs this information to control the drilling operation. Inclination and azimuth therefore provide information on what the hole drilled to date looks like, whilst the toolface determines the direction in which drilling will continue. To realign the kick-off sub on the motor (the „toolface“), the directional driller simply turns the drill string (and therefore also the downhole motor) cautiously a little to the left or the right. One further question arises: the MWD is at the bottom of the hole, maybe several thousand meters deep whilst the directional driller is standing at the top on the rig. How do the measured values actually go from the MWD to the directional driller? As data transmission by cable in the screwed together drill string or via radio through the rock has proved problematical, it was possible to implement a different variation: the transmission of the measured values in the form of targeted pressure fluctuations in the drilling fluid. As you know, this is pumped constantly through the drill string to the bit to pick up the cuttings there and deliver them to the surface through the annular gap between the drill string and the borehole wall. Now, there is a valve in the MWD which is operated (i.e. opened and closed) by a computer, also housed in the MWD. If the valve is “closed” (one part of the flow cross-section still remains free …), the pumps at the top on the rig must generate more pressure to keep the fluid in motion than is the case with the valve open. Thus the valve can generate fluctuations in pressure in the drill string by opening and closing. The expert prefers to talk about “pressure pulses“, therefore, the valve in the MWD is called a “pulser valve“. But … a valve that can transmit measured data? How is that meant to work? First of all, the measured values are converted into a “binary code” by a computer in the MWD. This is the computer language which, instead of consisting of 0 5 10 15 46 47 20
W H AT DOE S I N TEQ M A N U FACTUR E ? – 5 W h at Do W e K now A bout The Rock Dr illed Out ? many different letters or numbers, only consists of zeros and ones. It is a similar situation to Morse code which of course only has dots and dashes but yet any desired words and sentences can be transmitted with it. So the computer begins to open and close the valve in a specific time sequence. For example, the open valve stands for a zero, the closed one for a one. Whenever the valve closes in the MWD, the pressure rises in the entire drill string. When the valve opens, the pressure drops back to its original value. Right at the top of the drill string (about where our directional driller is waiting for his data) there is a pressure sensor which measures the pressure fluctuations in the drill string and passes them on to a computer connected to the system. This evaluates the measurement by first of all decoding the binary code (much pressure = “1”, little pressure = “0”) and then converts it into actual measured values for azimuth, inclination and toolface. The “MWD-operator”, an expert who is sent together with the evaluation equipment to operate it on site, ensures that the entire data transfer runs smoothly and checks whether the data transferred are complete and correct. The crews on the rig do not notice much of the whole procedure. They only look at the digital display on the rig and, together with our directional driller, are delighted with the measured values shown. Our most recent MWD-System is called “OnTrak”. In addition to the sensors already described for procuring the directional drilling information (inclination, azimuth and toolface) it has further “sense organs”. The measurement of the pressures in the drill string and in the borehole gives information about the stability of the borehole and tells us whether the discharge of the debris from the hole is working in the best possible way. The measurement of the natural radioactivity and conductivity of the rock enables information to be gained on exactly which rock is being drilled at present and whether gas, water, or oil is to be found in the pores (this will be described in greater detail in the following chapter). Finally, a vibration module indicates whether the drill string with the sensitive evaluation systems is rotating evenly deep down in the hole or is vibrating dangerously. Well, so much technology is hidden in some of the “tubes” on the shelves you see on our company site. Could you have imagined that? OK, we know now where we are drilling to. But the drilling path originally planned is not always the best one. For example, the reservoir could in fact be a little higher or lower than the geologist thought. If in this case we stick stubbornly to the original path planned for the well, in the worst case we might drill past the reservoir altogether! Or it could be that by chance, at the particular point in the reservoir in which we want to place the well, the pores in the rock are particularly thin and we would therefore only be able to deliver a very small amount of oil, whilst a few meters to the left or right, the really good big pores remain unnoticed. Is there perhaps some way of establishing when drilling the current path of the well is advantageous or whether it would be better to modify it a little? I’m sure you already suspect it: yes, we can find out! Modern drilling assemblies have a wide range of well thought out evaluation systems on board which enable us to obtain valuable information about the nature of the rock in the hole. If the path of the borehole is constantly optimized during drilling on the basis of the results of the evaluation, the technical expert calls this „Geosteering“. In the chapters which follow, you will learn about the most impor- 0 5 10 15 48 49 20
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