So how do we find oil and gas ? First, lets stick to conventional sandstone (coarse grained) sediments and dry gas. These are perhaps the simplest after water wells.
Oil and gas drilling used the technology available from water bore drilling and improved on this for both. A rock roller bit ( invented by Howard Hughes – an older version of Elon Musk) is attached to the end of a drill pipe and rotated to chip away at rock. Many different bit types are now available, depending on the rock being drilled. A derrick (or truck) holds the pipe in place and a diesel motor supplies the energy.
Air, water or mud are pumped down the hole to wash the cuttings up to the surface. As the well is drilled the driller (or geologist) describes and records the cuttings, looking for reservoir quality sands. Once these are reached the well is “completed”. There are many ways to do this and the most appropriate is selected. The reservoir may be sealed by cement and then perforated or it may be gravel packed and let to flow freely. Some of this decision is based on the pressure the water is under. Artesian water will be under enough pressure to flow to surface by itself. Otherwise a pump is required (eg a windmill).
The well is also likely to be cased with poly or steel pipe and cemented into place. These days it is requirement that any different pressure sands are separated with cement plugs. It is also a requirement that no toxic chemicals are used in any of the drilling or completion processes.
Water wells often do not flow very well due to a variety of reasons (tight sands, limestone (calcification) or iron growths etc. In these circumstances the well is stimulated to get a better flow. Hydraulic Fracturing is one method used. To quote AGE Developments “Hydraulic Fracturing (hydro-fracking) is a method of stimulating and improving the yield of a water bore through the injection of high pressure water into the formation. This causes fractures in the rock to open up, which allows a greater flow of water to pass through the surrounding zone into the bore.”
According to the Australian Standards for water bores, a number of chemicals can and are being used to stimulate these bores. A number of chemicals are listed including calcium hypochlorite ( disinfectant), Copper sulphate, Potassium permanganate, hydrochloric, sulphuric and phosphoric acid, Sodium hexametaphosphate, and unknown proprietary products such as wetting agents and corrosion inhibitors.
Another method of stimulation of water bores is to blow up the formation with dynamite. One water drilling company even states that we used to do this but don’t any more. Anecdotally, this was common practice amongst land owners and may still be. Just look up “dynamite water bore” on Google.
How is the water located?
Groundwater is generally found in wide spread aquifers with sweet spots of high porosity (space between rock grains) and permeability ( connectedness of these spaces). A well drilled anywhere that the sand is present should produce water. “Dirty” water is often intersected if a well encounters coals or organic shales within the aquifer. Aquifers maybe connected or substantially separated (with different pressures, maybe confined (top and bottom aquitard well defined) and consolidated or unconsolidated. When they are unconsolidated, pumping water from them will cause subsidence.
Aquifers, particularly within limestones (carbonates) can also be like an underground river. This is the model preferred by water diviners. They suggest that a force exists which draws down their divining rod if a body of water is present below. Many landowners still prefer to use a diviner in preference to a hydrogeologist. Diviners are seen by some as more connected to the land. Some see the use of geophysical methods to detect underground water as justification and that diviners merely do the same thing in their bodies. Sadly, no diviner has yet been able to scientifically demonstrate their ability to divine water, even with massive rewards offered. We will go into geophysical methods in another blog.
Oil and Gas Drilling
This is much the same process as water drilling but with many more health and safety controls and a much larger technical budget.
Seismic surveys are first conducted (in Australia at least) and these data are used to map the subsurface geology. Again another blog. Hydrocarbons are lighter (more buoyant) than water and will move to the top of reservoirs. Geophysicists map the subsurface, looking for these anticlinal traps (ie buried hills). Some areas will be prone to finding gas ( eg continental setting with significant coal) and other areas will be prone to oil (eg black shale source rocks). Exactly what will be found in a particular well is only known after drilling. Prior to that it is only conjecture.
Exploration wells are first drilled and, if successful these are followed by appraisal and development wells. Exploration wells tend to be simple vertical holes ( as are water bores), are drilled with mud ( the weight helps control fluid movement from the well) and are cased with steel casing (often up to three concentric rings). Wells have complex Blow Out Preventers (BOPs) to prevent any unwanted gas escape and are cemented around the casing.
If unsuccessful the well is Plugged and Abandoned ( P&A). In this case cement plugs will separate all aquifers. Wells are logged ( using many different methods) and geologists can very accurately define porous and non porous rocks. If successful, the well is first completely cased ( steel pipe and cement). Any potential productive zones are then perforated to allow the gas ( or oil or gas liquids) to flow to surface. Gas will always be under pressure but oil may not be.
The only time that explosives are used during drilling and completion are in this perforation process or in the gathering of side wall cores. In both cases the explosives are similar to shot gun charges and are intended to blow a small hole in the casing or in the borehole to collect a sample the size of a match box.
Stimulation ( Fraccing)
In the same way a water well may require stimulation, a gas well may require a similar process. In a conventional gas well (ie gas within a sandstone reservoir) this is usually done because the pore space is clogged or the sandstone is tight. The exact nature of the flow problem will determine the process and any chemicals used in the stimulation ( frac) process.
Stimulation ( or fraccing) is similar to that in water wells. In Australia it is better controlled, with all products used stated on Company websites ( they are much the same products with no toxic chemicals allowed). In the case of gas wells being hydraulically fractured, water is pumped into the formation and a proppant used to keep the pore space open once the water is let out again. The proppant is almost always sand grains although some synthetic products have also been used.
In the South Australian and Queensland Cooper Basin fraccing of conventional reservoirs has been used to increase gas flow rates since the early seventies with over 700 fracs to date. More recent wells will require fraccing on a more regular basis ( the good ones were addressed first). Without fraccing there would likely be no gas business in the Cooper Basin.
Unconventional gas and Coal Seam Gas (CSG)
Stimulation in these gas fields is a different proposition. I will look at these in a future blog. In Australia we have a handful of unconventional shale gas wells and even less frac jobs within them to date. One is on commercial production in the Cooper Basin as far as I am aware.
CSG wells do not require fraccing to the same degree and the process may be different ( eg cavitation). We will go into that in a different blog. Only about 8 percent of Australian CSG wells are fracced. Santos, for example has stated that they won’t frac any of their CSG wells in the Gunnedah Basin near Narrabri.