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Why is it so difficult to drill deep wells?
Why is it so difficult to drill deep wells?

Answered Originally: Why is it so difficult to drill deep wells?

There are some big problems. While some of these are obvious, the most important limits are not as you might expect.

The drilling process becomes more difficult at higher depths because the temperatures rise. The heat can also make metallurgy more expensive. Drilling fluids could also be affected by heat and may lose their viscosity properties. We aren't even close to that limit.

The drilling pipe's hanging weight is another important issue. A "mud motor" is used to drill deep wells. It is lowered down a segmented long pipe. The drill mud is pumped down the tube, pushing through the motor to turn the bit against the rock face. Finally, the mud flows up the hole outside of the drillpipe to remove the rock cuttings. To withstand the required pumping pressures, motor torques, this process requires a very thick and heavy pipe. The average pipe weight for drilling very deep wells is 40-50 lb per ft. The pipe becomes longer and heavier, which can easily reach a million pounds (500 tons) for deep wells. This weight must be lifted and lowered by the drilling rig's derrick lifting equipment. The real problem is that the pipe must hold its own weight.

We can make the pipe stronger by making the walls thicker. However, this will make it heavier. Actually, increasing the wall thickness along the entire length of the pipe will not increase its strength. The weight increase is offset by any increase in strength. This is mathematically true for all suspended tension members of constant cross-section. The same problem applies to space elevators. Ultra-deep wells need elaborate tapered string designs. These include light pipe at the bottom and medium pipe in the middle. Heavy pipe is at the surface. This requires a lot of engineering and can extend the depths that we are able to reach. There is a limit to the extent you can take this idea.

It is more efficient to make the drillpipe material harder than to make the pipes walls thicker to go deeper. Special high-strength steels are used in our work. They have yield strengths of 150 ksi or more, which is 3-4x greater than standard structural steel. This really limits the capabilities of steel. The more you make the steel stronger, the harder it will become.

The more vulnerable it is to chemical attack, the better. Subterranean temperatures can be extremely corrosive. The most dangerous underground chemical is the deadly-toxic hydrogen sulfuride gas. It is similar to swallowing cyanide when inhaled. It's also extremely difficult for high-strength materials. Hydrogen embrittlement is where H2S gas releases one its hydrogen atoms at metal surface. This leaves acidic HS on the metal face, which is bad. It also allows a highly reactive H+ ion to enter metal. The hydrogen that is not used in the alloy actually diffuses within the steel. This process creates methane molecules within the drillpipe's metal walls. Methane, a much larger molecule than elemental carbon or elemental hydrogen is created by these new gas molecules. They create nano-bubbles of high pressure and significant stresses in the grain structure within the steel alloy. Extreme embrittlement is the result. These high-strength alloy steels are already very brittle, so hydrogen sulfide attacks renders most advanced steel alloys unusable. They crack, pit and eventually fail. We have to return to soft metals, which are generally less brittle than they were to begin with. These metals should yield strength of 80 ksi or lower. This is how hydrogen embrittlement does not become a fatality.

The depth limit for drilling to 150 ksi yield strength pipe can be halved by using 80 ksi. This is a huge deal. The oil industry is very active in metallurgy and research to solve this problem. We have developed 110 ksi materials which can handle H2S in recent years. 125 ksi is currently being tested. Advanced nickel-based alloys like Inconel can also resist hydrogen attack at higher strengths. However, drillpipe made from that stuff is expensive.

Casing design is another major obstacle that can pose a technical challenge for ultra-deep wells. Casing design is a complex topic that requires some explanation.

While drilling wells, they come across various high-pressure areas and sometimes non-economical hydrocarbon zones along the way. These fluid-bearing areas are more pressure sensitive the deeper you go. The fluids are pushed down by the weight of the rock. Drilling mud has the major function of providing sufficient hydrostatic pressure to "overbalance", or prevent, all pressurized fluids flowing to the surface. If the mud hydrostatic does not contain pressure, it can cause a serious "kick" or even a blow-out. That's game-over.

Near-surface holes are generally drilled with mud densities that are slightly heavier than seawater (SG=1.1). As the well becomes deeper, the mud "weighted up", to 10, 11, and 12 pounds per gallon. Wells that are extremely deep may be twice as dense than fresh water (around 17-18 pounds per gallon). This is the limit of the fluids we can economically and reliably make with the right properties.

The bigger problem is how this density affects rocks that have been previously drilled. The hydrostatic pressure in the well increases when the fluid density is increased to drill deeper. You must stop drilling if you require 12 ppg of mud to control the bottomhole pressure. However, the exposed rock higher up in the well cannot withstand the pressure exerted 11 ppg without fracturing. You can't lift rock cuttings up to the surface and there is no way to maintain enough pressure to fracture the rock. You will likely have an underground blastout, which could cause the well to burst. This is a problem when drilling deep wells. The pressure in deep rocks can literally cause the rocks below to burst. By making a big conduit between these differently-pressured rock formations, we've created a big potential problem.

To isolate weaker rocks, cement a thick steel pipe that runs against the wellbore wall. The steel can withstand the pressure while cement seals off rock faces. It is necessary to casing the well in order to drill deeper than just a few hundred feet. This prevents wellbore failure, isolates aquifers and stops deep pressures from destroying low-lying rock formations. The basic principle of oil well drilling involves drilling a hole as deep and without damaging anything. After that, you need to case it off. Next, add more mud to the hole, then switch to a smaller drill bit and drill as far as possible through the casing without damaging anything. Next, case the new section. Continue until you reach your depth target.

This process has the downside of making the hole smaller as you get deeper. One might begin at the surface with 1,000 feet of 22-inch diameter casing. Next, we drill down to the bottom. We continue drilling until the bottomhole pressure is close to the fracture pressure at 22" casing shoes. Then we set 18" casing in order to isolate weaker rocks. Next, we drill the bottom another few thousand feet and then set 16" casing. Each casing point shrinks your hole as you must fit the drillbit through the casing. The process continues with 13-3/4", 10-3/4", 7-5/8", then 5 1/2", until you run out of sizes your drillpipe can fit through.

The biggest problem with ultra-deep drilling is casing design. This means that you have to get to the bottom before your hole becomes too small. This is even more difficult when regular-depth drilling takes place in geologically unusual areas, such as salt formations. Unexpected pressure can cause drilling rigs to have to place a casing too high in the well. This could make it difficult to reach depth. Casing design presents a significant engineering challenge. It must be safe to reach the desired depth, and hopefully with enough room to provide a contingency size for any unexpected rock pressures. Although there are ways to increase the limit, you can't go wrong with larger casing (eg 36") at the surface. However, this quickly becomes impossible.

It is a nightmare for engineers to push the boundaries of well depth. It's also dangerous -- drilling deeper than necessary can make it difficult to predict the types of rocks and pressures that you will encounter. Unexpectedly high-pressured sour gases pockets can cause a commotion at the drilling site. This could result in the death of all personnel, as well as destroying the rig and causing the casing to erode uncontrollably for several months before a relief well is drilled. It is possible to end up with a huge crater of toxic hellfire and not break the depth record.

There is no reason to drill so deep. The less likely that rock formations contain oil, the hotter they get. When crude oil gets too hot and is too deep, it pyrolitically becomes natural gas. Natural gas is dangerous to extract from deep formations. The low density of natural gas carries the subterranean pressures all to the surface, making it extremely dangerous. Today's surface safety and processing equipment can't withstand the heat and pressure of traditional gas reservoirs that are deeper than 30,000 feet. At 350F, the limit for surface equipment right now is about 25,000 psi. This seriously limits engineering design and metallurgy. Higher pressures mean that the well cannot be safely flown to the surface. There is no economic reason to drill that deep.

The world's drilling depth record is impossible to beat due to high costs, low profit potential, and extreme safety risks. Technology is constantly improving.

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