How an Oil Well Works: From Spud to First Oil (Infographic)

An oil well is a pressure problem before it is an oil problem. Every step below, from the first seismic survey to the last cement plug, exists to make a hole through rock that would rather not hold one, keep the pressure inside it where it belongs, and let oil and gas out on purpose rather than by accident. This infographic draws the eight steps in order; the notes underneath link every term to its full definition.

Key takeaways

• A well is drilled and cased in stages. Each casing string is set because the rock above cannot stand the mud weight the rock below needs, so the finished well is three or four steel pipes nested inside each other, each cemented to the rock.
• The primary barrier against a blowout is the weight of the drilling mud, not the blowout preventer. The BOP is the backup, closed only when the mud loses the fight and a kick comes in.
• Drilling makes the hole; completion makes it produce. Logging, perforating, tubing, a packer and a Christmas tree turn a cased hole into a well.
• Reservoir pressure does the lifting at first. When it fades, artificial lift takes over, most visibly the pump jack.
• Every well ends the same way: cement plugs from bottom to top and the wellhead cut off below ground in plug and abandonment.

What a well actually is

Picture a straw pushed into wet sand. Left alone the sand collapses into it, water seeps in from the sides, and if there is gas trapped below it comes up whether you want it to or not. A well is the engineering that stops all of that: steel pipe to hold the hole open, cement to seal the pipe to the rock, a column of heavy fluid to hold back whatever pressure the rock contains, and valves at the top so that nothing moves unless a person opens something. Once you see the well as a pressure envelope rather than a pipe, each of the eight steps below has an obvious reason to exist.

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The eight steps

1. Find the prospect

Nobody drills on a hunch anymore. A seismic survey sends sound into the ground and listens for the echoes from each rock layer, building a picture of the folds and faults below. Geologists look for a trap: a shape where oil that migrated up from a source rock has been caught under a seal. A promising area is a play; a specific drillable target inside it is a prospect. The survey truck on the infographic is the onshore version; offshore, ships tow the sources and receivers. How the oil came to be there at all is its own story: How Oil and Gas Form.

2. Spud the well

Before the rig arrives, a short, very wide pipe called the conductor casing is driven or drilled into the loose ground at surface so the top of the hole cannot wash out. Then the rig is raised, the top drive or rotary table is ready to turn pipe, and the bit makes hole for the first time. That moment is the spud, and the spud date is the reference point for every report about the well from then on.

3. Drill with mud

The bit sits at the bottom of the drill string, hundreds or thousands of feet of hollow steel pipe. The mud pump forces drilling mud down the inside of that pipe, out through nozzles in the bit, and back up the gap between the pipe and the rock, the annulus. On the way it cools the bit, carries the broken rock, the cuttings, up to surface, and, because it is heavier than water, holds back the pressure in the formations. At surface the shale shakers screen the cuttings out, the mud drops into the mud tanks, and the pump sends it round again; the whole loop is mud circulation. A mud logger reads the cuttings and the gas in the returning mud to tell the geologists what the bit is drilling through. Most modern wells are steered with directional drilling, and shale wells turn to run horizontally through the reservoir for a mile or more.

4. Case and cement

A hole cannot be drilled to the reservoir in one pass. Shallow rock is weak and would fracture under the mud weight that deeper, higher-pressure rock needs, and fresh-water zones near surface must be sealed off before anything else is done. So the well is built in stages: drill a section, run casing to the bottom of it, pump cement down the pipe and up the outside to bond it to the rock (cementing), then drill a smaller hole out of the bottom of that string and repeat. Surface casing protects the water and anchors the blowout preventer; intermediate strings seal off troublesome zones; production casing is set across the reservoir. Deep wells hang a liner from the previous string instead of running pipe all the way to surface.

5. Keep it controlled

Once surface casing is cemented, the blowout preventer is bolted onto the wellhead. It is a stack of large hydraulic valves that can close around the drill pipe, shear through it, or seal an open hole, and on a normal day it never closes. The mud is the primary barrier. If the pressure in a formation exceeds the pressure of the mud column, formation fluid pushes into the well and the pit level rises: a kick. Well control is the discipline of spotting that early, shutting the BOP, and circulating the kick out under controlled pressure while heavier mud is pumped in. A kick that is not controlled becomes a blowout.

6. Complete the well

Reaching the reservoir does not make a well produce; completion does. Logging tools run on wireline measure the rock behind the casing and find the pay zone. Perforating guns fire shaped charges through the casing and cement into the reservoir so fluid can enter. A narrower pipe, the production tubing, is run inside the casing with a packer sealing the gap, so produced fluids travel up the tubing rather than wearing the casing. On top, the Christmas tree, a stack of valves and gauges, controls flow. In tight rock the well is also stimulated by hydraulic fracturing: fluid is pumped at pressures high enough to crack the reservoir, proppant sand holds the cracks open, and the well is cleaned up in a period called flowback.

7. Produce the oil

At first the reservoir pressure does the work, pushing oil, gas and water up the tubing and through the flowline to a separator, where gas leaves from the top, oil from the side and water from the bottom, each metered and sent its own way. Pressure declines as the reservoir is drained, a curve every engineer watches through decline curve analysis, and eventually the well needs artificial lift: a pump jack nodding at surface, gas lift injected down the annulus, or an electric submersible pump downhole. Over the years a workover rig may return to repair tubing, replace pumps or move the perforations.

8. Plug and abandon

When the well can no longer pay for its own upkeep, it is retired. Plug and abandonment sets cement plugs at intervals from the reservoir to the surface, so no fluid can ever migrate between zones or reach the surface, then removes the wellhead, cuts the casing off below ground level and restores the site. The abandonment cost is booked long before the day arrives, because a well that is not properly plugged is a liability rather than an asset.

A worked example: an onshore shale well

A small rig drives the conductor pipe a week before the drilling rig arrives. The drilling rig moves onto the pad and spuds. It drills the surface hole to below the deepest fresh water, runs surface casing and cements it to surface, then nipples up the blowout preventer and tests it. It drills the intermediate section through any weak or overpressured zones and cases that off too. Then it drills the production hole: vertical to the kick-off point, a curve, and a lateral running horizontally through the shale. Production casing is run to the end of the lateral and cemented, and the rig is released to move to the next slot on the pad. A fracturing crew arrives with pumps, sand and water and treats the lateral in stages from the toe back to the heel. Flowback cleans the well up over days to weeks, the permanent flowline and separator are tied in, and the well is turned to sales. It will produce for decades, with a pump installed somewhere in its first years, before it is finally plugged.

Where it goes wrong

• Kicks. An unexpected high-pressure zone, or mud made too light, lets formation fluid into the well. Caught early it is routine; missed, it is the most dangerous event in drilling.
• Lost circulation. The opposite problem: the mud is heavier than a weak zone can hold and drains away into it, dropping the pressure everywhere else in the well.
• Stuck pipe. The drill string sticks in a caving hole or against a permeable wall. Freeing it, or cutting it off and sidetracking around it, can cost days.
• Bad cement. Cement that channels or fails to bond leaves a path behind the casing. It shows up later as pressure on the annulus and can take a workover to fix.
• Screen-out. During fracturing the sand bridges in the fracture or the perforations, pressure spikes and the stage ends early with sand left in the wellbore.
• Sand and water. Pulling too hard on a new well drags proppant back into the wellbore and wears out chokes, valves and pumps.

Variants

• Vertical, deviated and horizontal. Conventional reservoirs are often drilled vertically. Shale and tight rock need long laterals, which is why the horizontal well became the standard onshore design in the United States.
• Onshore and offshore. Offshore the rig floats or stands on the seabed, the blowout preventer may sit on the seabed with a riser running up to the rig, and the Christmas tree can be a subsea tree tied back to a platform or an FPSO through flowlines and umbilicals.
• Oil, gas and injection wells. The construction is the same. Gas wells need less lift and more attention to pressure; injection wells run the flow the other way, pushing water or gas into the reservoir to maintain pressure.

Frequently asked questions

How deep is an oil well?

Most onshore wells reach somewhere between a few thousand and about fifteen thousand feet of measured depth, and a horizontal well adds a lateral that can run two miles or more. The deepest offshore wells pass thirty thousand feet.

How long does it take to drill one?

An onshore shale well is commonly drilled in one to three weeks from spud to rig release. A deepwater well can take months. Completion and flowback add weeks after the rig has left.

What does spud mean?

To spud a well is to begin drilling it: the first time the bit turns to make hole. The spud date is the date that happened, and it is the starting point for lease terms, regulatory reports and every drilling performance metric.

What is the difference between the wellhead and the Christmas tree?

The wellhead is the structural fitting at the top of the well that the casing strings hang from and that the blowout preventer bolts onto during drilling. The Christmas tree is the stack of valves installed on top of the wellhead once the well is completed, to control production.

Why is casing set in stages?

Because the rock changes with depth. Shallow rock is weak and would fracture under the mud weight needed to hold back pressure deeper down, so each section is drilled with the mud it can stand, then cased and cemented before the next, heavier section begins.

What is a kick?

A kick is an unplanned flow of formation fluid into the well, which happens when the pressure in a rock layer exceeds the pressure of the mud column. It is detected as a rise in the mud pits or a change in flow, and handled by closing the blowout preventer and circulating the intruding fluid out under control.

Does every well need hydraulic fracturing?

No. Conventional reservoirs with good permeability flow without it. Shale and other tight rocks have almost no natural permeability, so hydraulic fracturing is what makes them produce at all.

What happens to a well at the end of its life?

It is plugged and abandoned: cement plugs are set from the reservoir to the surface, the wellhead is removed, the casing is cut off below ground and the site is restored. See plug and abandonment.

Part of the OilfieldTerms.com guides. Every term above links to its definition in the glossary.