Drilling, Completing and Producing a Well
Landowners are contacted and contracted to allow oil companies to conduct seismic surveys which use shock waves sent through the rock layers storing the results for the geologists analyze and interpret what oil and gas lies beneath the surface. If the results of the seismic surveys appear promising, the oil company will lease the minerals from the mineral owner. The mineral owner is usually paid a lease bonus plus a negotiated royalty on the production revenue. The royalty allows the mineral owner to not have to pay any of the expenses. The costs are paid by the working interest owners. The royalty owner is highly advised to get legal help when negotiating a lease with any operator since costs after the sale of the production can make a deep cut into their royalty. Landmen assist the oil and gas attorney to determine who the right royalty owners are; getting them to sign leases and providing run sheets with ownership which allow an attorney to prepare a title opinion which results in a division order for the proper payment of revenues paid to the royalty owners and working interest owners. Contacting royalty owners is getting much more complicated since families grow in size and move to different parts of the world and it is a challenge to find them. A survey crew is hired to mark the precise location where the rig will drill the well. GPS coordinates are used to mark the designated spot for the planned drilling. The operator must obtain permits, lease agreements, titles to drill in the area. Usually a construction contractor must install: 1) a road must be constructed in order to allow access to the rig location; 2) an area cleared for the rig, equipment, mobile homes, multiple service trucks and 3) pits to hold the drilling water and mud. A reserve pit is dug and sometimes lined with plastic so it can hold drilling mud and rock cuttings which will be trucked away from the site. The rig contract is on a daywork, footage or turnkey basis. The daywork is used the most frequently since the other two contracts put a financial risk on the rig contractor. The rig usually runs on diesel fuel and sometimes electricity; however, there will be a need for electricity and water for the drill site. The rig works around the clock on a 24-hour schedule using two shifts of workers. Hopefully there is a near-by natural source of water; otherwise a water well will have to be drilled. Sometimes this is done with the help of a water witch individual to locate water underground by a pull on the stick. A small rig is used to drill a water well. A shallow, starter hole is drilled with a drill truck. The hole has a conductor casing run and cemented in place. It will also drill the mouse and rat holes which are used to hold the pipe during the drilling. A mouse hole holds ready the next drill pipe to be run and a rat hole holds the rig’s kelly. Moving and assembling the rig (rig up) to the new location is a big project which can require sometimes up to 45 semi-truck loads lasting a minimum of three days or more to move in and rig up. Once the rig is safely in place, it must be inspected to make certain it is ready to start drilling the well. To keep the well from blowing out due high pressure a blow out preventer (BOP) is installed on top of the casing. It is tested during numerous phases for safety. The BOP can seal off the well for a blowout if needed. Drill pipe is run into the hole and turned with a Kelly and a turntable. Drill pipe is turned using a hydraulic swivel device on all newer flex rigs. After a single drill pipe has drilled and reached its length, another drill pipe must be screwed into it so the drilling can continue. After the rig drills past the water bearing zone or water table, surface casing is run and cemented in place to prevent the flow of water outside the casing. Surface casing is run and cemented back to the surface to protect shallow fresh water intervals. The drilling continues and the intermediate and smaller diameter casing is run into the surface casing and cemented in place. Centralizers are also used on most straight holes; this keeps the casing centered so the cement can be centrally spaced within the wellbore for a better cement job. After each casing is run and cemented, the well hole must be tested to make sure the cement is not leaking. In highly deviated wells, centralizers are used to keep the completion string centered within the hole to get a good cement job. Drill bits eventually wear out and can drill at least 6,000 feet especially if they are diamond bits. Once a bit is worn out during the drilling, it must be replaced which requires removing the entire string of drill pipe (tripping out) which can take several hours. The cuttings made by the drill bit as it crushes the rock are conveniently pushed by the mud flow and carried up the outside of the drill pipe to the surface and removed by shale shakers, de-sanders, de-silters and de-gassers removing gas from the mud so the mud can be circulated again. The drill mud also helps to keep the bit cool. Also, the mud keeps pressure on the formation so the formation cannot have a well to blowout and uncontrollably produce oil into the air and all over the ground. The drill mud density is adjusted with additives to keep pressure on the formation. he mud is also useful since it forms a cake on the wall of the well bore (hole) and helps to keep the wall from caving in and sticking the drill pipe causing a fishing job to try to save the well and the drill pipe. If the fishing job takes too long or fails, the well will have to be side tracked or re-drilled. It should be noted that a lot of people do not know that the temperatures in the below zones can be very hot. After the well has been drilling a while, a mud logger will arrive, catch and test rock cutting samples from the mud that is being forced back to the surface to determine the presence of oil. He measures the pressure and lowers gas sensors into the hole to determine if the reservoir rock has been reached. The mud logger uses a microscope or ultraviolet light is used to see whether oil is in the cuttings. Once the bit reaches total depth of the targeted zone, the rig is used to circulate and condition the hole so the well can be safely logged without getting stuck. The well is logged to see if the well can economically produce oil and/or gas. When the well is being logged, the logging company drives in a truck that has a portable laboratory. Electric logging tools are selected and lowered to the bottom of the well and the reeled back up slowly to measure and record the characteristics of the well formations. A drill-stem test, which can be dangerous, can be done by lowering a device with valves and pressure gauges into the well to measure the pressures. A core might be run to obtain core samples of the reservoir rock. There are two types of core procedures. A core barrel is attached to the end of the drill pipe and rotated so it cuts a cylindrical core a few inches in diameter. A side wall core is also used where a small charge is fired to force a many (30) small cylinders into the wall of the well bore in order to obtain one-inch diameter by three inches long core samples from the well bore. If the Operator of the well is lucky and skillful and the well is capable of economically producing, the rig crew will run the drill pipe back into the hole again circulate and condition the hole to make sure the hole is still intact. If the well is not economical it will be plugged and abandoned by cementing at different depths to safely seal off the well. Once the rig crew considers the well ready, the drill pipe is removed and the production casing is run through the middle of the larger diameter casings from the top of the well to the bottom of the well or total depth and is cemented up to a designated depth behind the production casing and inside the well bore’s formation. When cementing the production casing, a rubber plug precedes the cement slurry down the casing and seats in the float collar while cement is circulated through a small hole in the plug. The cement slurry then flows out through the opening in a guide shoe and is forced up the annular space between the casing and the hole to fill the annular space. Then a top plug with no hole is released as the last of the cement slurry is forced down the well until it seats the bottom plug in the float collar with salt water or drilling mud leaving the casing clear of cement. This lets the cementer know to stop the cement pumps. Afterwards, there is a period of time which is waiting on cement (WOC) which gives the cement time to harden. The casing is inserted at the surface into a Christmas tree or wellhead assembly. This is the main assembly of valves that controls flow from the well and well interventions. The drilling rig is disassembled (rig down) and moved to the next location to be drilled.
Completing a well procedure:
Once the well is drilled and logged, a work-over unit (which is a much smaller and cheaper rig) is brought in and rigged up. A cement bond log is run to make sure the cement job on the production pipe worked as planned. A cased hole bond log, gamma ray log and collar locator can be run. The crew runs tubing which has even a smaller diameter into the well through the middle of all the other casings run by the drilling rig. The function of the replaceable tubing will be to carry the oil and gas from the bottom of the well to the surface of the well. A perforating gun is then run down to a calculated and choice formation(s). A wire line is used for this purpose since it is very fast and does not have to be assembled like drill pipe must be done. Once the gun is in place, an electrical signal fires the gun shooting holes in the casing and cement into the formation. Afterwards, depending on the rock quality, an acid and frac are usually done in order to clean the formation and to leave sand in formation cracks created by the high-pressure frac which uses multiple trucks to create and deliver high tremendous pressure down hole through the perforations into the zone of interest; and most importantly, it is done to achieve a commercial flow rate. Oil might be unable to flow readily into the well bore because the formation has a very low permeability. Acidizing removes drilling mud and opens pore throats in the rock near the wellbore; it also cleans the formation, thereby improving the flow of reservoir fluids. A strong acid (usually HCl) is used to dissolve rock formations, but this acid does not react with the hydrocarbons. As a result, the hydrocarbons are more accessible. Acid can also be used to clean the wellbore of some scales that form from mineral laden produced water.
Limestone or dolomite reservoir rock results are better with acid pumped down the well and into the formation perforations so that the acid can dissolve channels allowing the oil to flow into the wellbore.
Sandstone reservoir rock results are better using a frac with lots of water and proppants, which can contain aluminum pellets, sand, walnut shells, pumped down the well under heavy pressure into the formation perforations resulting in fractures and propping the fractures so the remain open to allow the oil and gas to flow. Dense reservoir rock results are better using a frac. Proppants include small diameter sand or ceramic beads pumped into the formation.
Producing a well procedure:
The well is then produced and monitored by a crew. The Christmas tree allows them to control the flow of oil from the well by using a choke. Hopefully the well will produce which is caused by the down hole pressure. Gas expands as it gets closer to the surface since there is no longer any formation pressure. After the well will no longer produce without assistance, artificial lift will be required. A pumping unit (pump jack) is moved in and set on a cement pad. A tubing anchor prevents the tubing from moving while the well is pumping. A bottom hole pump is run on the end of rods that fit inside the tubing. The sucker rods are attached to a polish rod at the surface which is attached to the pump jack. The production packer isolates the annulus between the tubing and the inner casing and the foot of the well. Usually, the production packer is placed close to the foot of the tubing, shortly above the production zone. The pumping unit pulls up and down on the rods and the rods pull the oil through a flow, no-flow valve pump due to suction that draws oil into the pump. Blast joints may be used on the tubing within the region of the perforations. These are thick, walled subs that can withstand the fluid abrasion from the producing zone. Gravel packers many times are needed to control lots of produced formation sand. The tubing hanger sits on top of the wellhead and holds the production tubing. An electric submersible pump can be used for artificial lift instead of a pumping unit to help provide energy to drive the oil and water to surface if reservoir pressure is insufficient. The pumping unit receives its power either from an engine or a motor which drive the gear box that moves a lever which pushes and pulls the polish rod up and down.
A tank battery is constructed with storage tanks with stairwells (usually two 400 barrel tanks for oil and a 210-barrel tank for salt water disposal, heater treater, separator, water tank, flow lines and a meter loop for production. A contract with the gather for the purchase of oil and gas is signed if not already in place. Oil buyers pick up the oil from the holding take at periodically; gauge it and leave a ticket in the well’s mail box. Oil and salt water are separated at the surface which means there will be salt water disposal (SWD) costs incurred since tank trucks are required to haul the water to an authorized salt water disposal well. Fluid level tests are important since they keep the well rods from pounding fluid and causing damage. Dynamotor tests are important to keep the pumping jack well balanced with the rod string versus the pump jack weights.
Wells that flow oil into the wellbore slowly might be put on a timer so they only turn the pump jack on periodically. Wells eventually will not have enough formation pressure to move the oil since only a portion of the oil and gas is recoverable by primary production methods. Once this happens, secondary recovery methods will become necessary. This is done by injecting a substance, e.g., water, into the producing formation to increase the pressure in order to drive the oil to the well bore. Sometimes abandoned wells can be reworked and used as injection wells for this purpose. A water supply well might have to be drilled to provide an adequate supply of water. Water flooding is the most common and efficient process which can last up to ten years.
The well is produced from the lowest zone which can be plugged off when the oil production is small enough and becomes economically unfeasible to set a plug to seal off that zone and perforate the behind casing zone that is higher up the hole.