GEOL 115: The Earth’s Energy Resources

Hydrocarbon exploration still boils down to drilling. The primary drilling device used in the oil industry is the rotary drilling rig. This piece of equipment is so essential to the energy industry that the number of rotary rigs in operation is monitored continuously and the results are followed with great interest. By considering where rigs are working and how many rigs are working, one can quickly assess the state of the oil and gas industry.

Offshore exploration wells are not drilled from platforms but rather from Jack-up rigs, ship-shaped drilling vessels or semisubmersibles.

The best vessels in operation are semisubmersibles and drillships that can drill in 7,000-8,500 feet of water and support a crew of 200 people for months without significant resupply. They are dynamically positioned and in some cases can drill two wells at once.

One of the most important pieces of equipment on a drilling rig is the BOP stack (Blow out preventer). The BOP stack helps the drilling crew keep control of the well. There are three sets of rams in the BOP stack. 1)The pipe rams clamp down on the pipe and keep it in the well. As long as you control the drill pipe you have a good chance of getting control of the well. 2)The blind rams close when there is no drill pipe in the hole and the 3)Shear rams cut through drill pipe that is in the well. Use of the blind rams or shear rams indicate that you’re having a bad day.

Blow-outs occur when fluid in the formation flows to the surface under circumstances that are not controlled. These can be quite spectacular and dangerous. Blowouts start when formation fluid enters the well bore. This is called “taking a kick.”

If the formation fluid includes gas, as the gas flows toward the surface the gas bubble will expand rapidly. The expanding gas will displace the drilling mud from the drill pipe. When the drilling mud is lost, the formation pressure will exceed the pressure exerted by the fluid remaining in the well bore, the well flows to the surface, and excitement ensues. Take a look at the flowchart that drillers must go through when they realize that they have taken a kick. As you can imagine, the text doesn’t quite convey the excitement inherent in those steps that are highlighted in pink. This link has a several videos of blowouts and their results (I am not responsible for the content at that link).

Among the hazards associated with blowouts is fire. Once a fire occurs the rig typically collapses and you have to move a bunch of scrap away from the hole before you can put the fire out. If you want to see the basic steps in putting out an oil well fire (circa 1968) check out the first 15 minutes or so of the old John Wayne movie titled “The Hellfighters.” For a more modern and less dramatic view you can check out this link provided by one of the successors to Red Adair, Boots and Coots and other pioneers.

You could probably trace the beginning of the environmental movement to the Santa Barbara Channel oil spill. On January 28, 1969 a drilling crew on Platform A in the Dos Cuadras oil field took a kick while tripping out of the hole.  Because of the shallow reservoirs at Dos Cuadras, Union Oil had obtained a waiver allowing them to set only 238 feet of casing below the seafloor.  Casing was set to a drilled depth of 514′ but the rig floor was 88′ above sea level and the water was 188′ deep (88′+188′+238″=514′).  There is nothing odd about such a variance.  It does represent one of the small misfortunes that led to the catastrophic nature of that blow-out, however.

This graph (go to the course documents section of Blackboard and download “Pressure Graph to go with Blog post for Lec 20-22″ if the graph isn’t visible to your right) illustrates the pressure conditions relevant to the A21 well. The blue line indicates the hydrostatic pressure gradient (0.44 psi/ft) whereas the buff line indicates the lithostatic pressure gradient (1 psi/ft).  The hydrostatic pressure gradient is the pressure that would be exerted by a column of salt water whereas the lithostatic pressure gradient is the pressure exerted by a column of rock.  Pressures can’t exceed the lithostatic gradient because they would lift the overlying rock up.  The red line designates the typical pressure gradient in the trend of oil fields of which the Dos Cuadras field was a part.  The pressure at the bottom of the A21 well was only a little in excess of the hydrostatic pressure.  The problem was that the drilling crew had difficulty controlling the kick.  A series of mishaps ensued when they attempted to control the well.  The crew finally closed the blind rams, but only after all the drilling mud had been ejected from the well and a natural gas mist had begun flowing from the well bore.  Although this controlled the well bore,  the well bore was now filled with gas rather than mud and the bottom hole pressure was transmitted to the formation at the bottom of the casing.  This is where the waiver regarding the casing program was important.  The seafloor was doomed to fail if the well bore filled with gas from a depth of 3000′.  The hydrostatic pressures at 3000′ are well in excess of the lithostatic pressures at 514′.  The well bore did fill with gas, the pressures exceeded the lithostatic pressures and the seafloor failed in the area of the platform.  Oil began to flow into the Santa Barbara Channel.

At least 11.5 days passed before the blow-out was controlled. There are many natural seeps in the Santa Barbara Channel and environmental regulations at the time did not require that seeps on leases be mapped before exploration or production activities commenced.  We will, therefore, never know to what extent the well was finally controlled.

Prior to that blowout offshore oil development generally proceeded with little organized opposition. There has been considerable opposition to offshore petroleum development in areas other than the Gulf of Mexico ever since.  It matters little whether the exploration is within sight of land.  Significant progress has been made in the science of well control in the ensuing 40 years.  Blowouts remain rare, but potentially catastrophic events.

We also discussed subsurface blowouts in class.  Although these lack the spectacular fires and other visual effects that accompany surface blowouts, subsurface blowouts are serious problems.   Subsurface blowouts occur when formation fluids leave their current “home” enter a well bore and then flow into a formation that is at lower pressure.  These fluids can eventually reach the surface.  This is what apparently has happened with the Lusi Mud Volcano.