Oil/Gas/Sensors used in E&P
QUESTION: Where are pressure sensors (aka gauges) used in exploration and production phases and what governs which pressure sensing technology is used for the various applications?
This is somewhat vague.
There are pressure gauges all around a drilling rig. Air pressure gauges on air compressor tanks, fluid pressure gauges on the mud pumps, casing and drill pipe pressure gauges on the well control choke panel, hydraulic pressure on the top drive, fuel and oil pressure on the engines, water pressure and I am sure I have left out some but I will assume that you are wondering about measuring the pressure of the formation that is being drilled.
There are two ways this can be determined. One is direct reading from a transmitter that is near the drill bit, the other by weighing the drilling fluid and making calculations.
The newest generation of both land and offshore rigs will have an instrument panel that shows a continuous read out of the status of the rigs equipment including the pressures listed above. In addition to this will be gauges installed on the piece of equipment that can be monitored the old fashioned way; by a person looking at it. There are still many older rigs that are quite capable of drilling and/or working over (well service)less complex or shallower wells that still rely on someone looking at the gauges rather than a computerize panel.
Measuring the formation pressure is a little different. The newer generation of rigs will have a section of the drilling pipe called a drill collar that is installed near the drill bit that can measure the direction of the hole, log the formation and record the pressure at the bottom of the hole. Every liquid has weight and just as a diver encounters increasing pressure (a form of weight) with increasing depth so does a drill fluid exert more pressure against a formation as the hole is deepend. The pressure sub detects the pressure at the bit and since the weight per gallon (or liter) is known, the pressure of the formation can be seen at the computerized control panel.
The older rigs that are not completely instrumented cannot do this is real time. The weight of the drilling fluid per gallon is known and it is weighted as it returns to the surface. Gas that is released from the formation will be entrained in the drilling fluid and cause a reduction in its weight per gallon. The fluid is weighted at the surface when it is pumped into the hole and weighted as it returns and if it weights less then gas is present. This doesn't give the formation pressure but it can be determined with calculations.
During well control operations when the Blow Out Preventer (BOP) is closed, a direct reading of the pressures inside the drill pipe and inside the casing at ground level can be seen and the difference between the two can be used to determine the pressure of the formation at the bottom of the hole.
Regardless of how automated the rig, the person at the scene, Driller, Toolpusher or Company Man must know how to manually read the various surface gauges, know the formulas to use and make the calculations to find the formation pressure.
I hope this has helped. Because I work on these rigs and they are covered in gauges it did seem a little vague. Keep in mind that there really is little difference between a drilling rig and a well service rig, mostly one of scale or size and both can and do use the same methods. Once a well is drilled and is producing is different. The gauges are used for a different purpose, the gauges are on the well head and used to regulate the amount of gas/oil that is sent down the pipe line. If this is your interest please send a follow up.
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QUESTION: Hi Carl,
I had purposely asked the "where are pressure sensors used" question broadly and indeed your response was helpful. Now can we concentrate more fully on where pressure sensors are used downhole in the drilling phase and in the production phase. I believe one finds a pressure sensor at times in the drilling tool and i understand that after drilling there is often a "permanent gauge" placed in the well to measure reservoir pressure. Can you elaborate on drill tool and permanent gauges and discuss what sensing technologies are used in which applications and why?
All of the companies have a wealth of information on the web. This is only a few from Google using the search terms measure pressure while drilling or measure down hole pressure. You will probably gain more from this than from me as the results are from the companies that make them. Watch the Schlumberger animation on how these things work and it also shows the three accepted technologies used on the gauges. Hostile environment in oilfield terms means the presence of corrosive gases such as hydrogen Sulfide or Carbon Dioxide or ultra high temperature.
Also, these gauges are not solid, they are part of a tube, either drilling pipe or production pipe. Drilling fluid or production fluid moves through the inner part of the tubulars and the gauge measures the pressure on the outside of it.
I hope the copy and paste hyperlinks work. Thank you for taking the time to do the rating and again, feel free to follow up.
Used while drilling (exploration phase) the down hole gauges will give real time data of the pressures of the different formations encountered. Optimally, the Hydrostatic pressure exerted by the drilling fluid would equal the pressure of the formation. it would be static. A pressure reading greater that that of the Hydrostatic pressure of the drilling fluid would be a first indication of what is referred to as Under balance. Being under balanced does not necessarily mean the well is going to blow out but it usually means that there will be an influx of gas/oil into the well bore and will have to be handled at the surface by removing it or chemically treating it.
Used during production, it shows the optimum rate to flow the well. The goal being to have maximum production with minimum Draw down of the formation pressure. This is achieved by using different Chokes.