Wellbore perforation is a stage of oil well completion; it provides a direct link between

the wellbore and the oil formation through perforation tunnels and the surrounding cement sheath. The quality and quantity of the tunnels have a direct influence on well productivity. Therefore it is crucial that completion engineers should ensure that wellbore perforations are located in the right places, oriented correctly, deep, and clean. Effective perforation design and execution process facilitate natural flow and easy pumping of oil, exclude water from the overlying or underlying units, and keep formation rock particles out of the well.

Process

In order for a wellbore to be productive, the casing and cement are perforated to allow the oil and gas to enter the wellstream. In case of cased-hole completions, casing is required to be run into the reservoir. Perforating equipment are transported to and operated at the site by a specialized crew. The tools are then assembled upon arrival to the site. A wireline, slickline, or coiled tubing runs a reservoir-locating device and a perforation gun into the wellbore. Upon reaching the perforation reservoir level, the gun shoots holes to the sides of the well to allow oil and gas to enter the wellstream. Perforations are created by either firing bullets into the sides of the casing or discharging jets/shaped charges into the casing.

Locations of perforations are determined previously using drilling logs, although intervals cannot be easily located through casing and cement. To solve this problem, a gamma ray collar – correlation log is implemented to correlate with the initial log run on the well and define the locations where perforation is needed.

Potential Hazard

Perforation almost always involves use of explosives, and surface detonation poses a great risk to the lives of drillers. Before starting the perforation, keep all non-essential personnel out of the site and post warning signs. Prohibit the use of communication gadgets or navigational systems, and shut down non-essential electrical systems during gun-arming operations. Operations involving explosives should only be performed under the direct supervision of the special services supervisor. Remnants of explosives should be reported to the aforementioned supervisor.

References:

Behrmann, Larry, and Chee Kin Kong. “The Search for Perfect Perforations.” Schlumberger. Schlumberger Limited, 2014. Web. 19 Dec. 2014. <https://www.slb.com/~/media/Files/resources/mearr/…/perforations.pdf>.

“How Does Well Completion Work?” Rigzone. Dice Holdings, Inc., 1999. Web. 19 Dec. 2014. <https://www.rigzone.com/training/insight.asp?i_id=326>.

“Oil and Gas Well Drilling and Servicing ETool.” Occupational Safety & Health Administration. U.S. Department of Labor. Web. 22 Dec. 2014. <https://www.osha.gov/SLTC/etools/oilandgas/servicing/special_services.html>.

Cementing is another stage in well completion that prepares a wellbore for further stages. The operation mainly involves development and pumping of cement into place to protect and seal the wellbore. This way, non-hydrocarbons, especially water, can be prevented from penetrating into the well.

Cementing is used for other purposes, to seal the annulus after a casing string has been run into a well, to seal an area where flow has been reduced or absent (also known as lost circulation zones), to plug an existing well to control its flow, and to plug an abandoned wellbore.

To displace existing drilling fluids and to fill in the space between the casing and the sides of the drilled well, cement slurry is pumped into the well. The slurry is composed of a special mixture of additives and cement, and is left to harden to seal the well from non-hydrocarbons that might try to enter the wellstream. This is also to ensure that the casing is positioned permanently into place.

Cement Preparation

To determine the amount of cement needed, the diameter and the depth of the borehole are measured using a caliper log. Multifinger caliper logs measure the diameter at various locations using mechanical and sonic means to accommodate irregularities and to determine the volume of the openhole.

The required physical properties and proper set of cement are also put into consideration before starting the cementing and pumping operations. Special mixers are used to combine dry cement with water like hydraulic jet mixers, batch mixers, or re-circulating mixers. The result is wet cement known as slurry. Portland cement is used in this process and is calibrated with additives to form one of the eight different classes of cement which is set by the American Petroleum Institute (API)

There are various types additives mixed into the slurry: accelerators can shorten the setting time for the cement, while retarders can prolong its setting time. On the other hand, lightweight and heavyweight additives can decrease or increase the density of the slurry. Other additives can transform the compressive strength of cement, its flow properties, and its dehydration rates. Bridging materials are added to plug lost circulation zones and antifoam additives are included to prevent foaming within the well. Cost reduction efforts call for the use of extenders.

“How Does Cementing Work?” Rigzone. Dice Holdingd, Inc., 1999. Web. 19 Dec. 2014. <https://www.rigzone.com/training/insight.asp?insight_id=317&c_id=1>.

Types of Well Logs

Various types of well logs have emerged over time due to technological improvements. Different tools are used to measure and determine subsurface characteristics.

Resistivity Logs measure electrical flow through rocks and sediments. This log determines the types of fluid present. Formation waters are salty and are good electrical conductors, whereas oil and fresh water are poor electrical conductors.

Induction Logs assess the interaction of magnetic force and electric flow to determine resistivity. They are used for oil-based drilling or air drilling wells, which are non-electrically conductive, unlike mud or water wells.

Spontaneous Potential (SP) Logs calculate electrical currents generated between drilling fluids and formation water held in the pore spaces to show the permeability of rocks in a prospective oil well. SP logs are referred to many times to determine whether a rock formation is a shale or a sandstone.

Well Logging Methods

Mud Logs refer to the drilling of fluid or mud to provide buoyancy to the drill and to remove cuttings from the well. Together with the driller’s log, evaluation log, cuttings log, and logs of nearby wells, information from mud logs is used to determine the commercial viability of an oil well.

To retrieve data before, well logging tools were used to be lowered into the well for regular intervals. Directional drilling have pushed scientists to develop non-vertical methods of well logging. With Logging While Drilling and Measurement While Drilling (MWD), logging tools can now be placed at the end of the drilling column so drillers can determine the prospect of an oil and gas well.

As technology makes more headway at this present time, well logs are now computer generated to interpret gathered information immediately while drilling. These logs are sophisticated that not only they can record measurements, but they can also notify drillers of a potential hazard and can transmit data to offsite servers through satellite.

Potential Hazards

Because logging involves tools that are radioactive, electric, and/or sonic in nature, a driller may be at risk of radiation exposure. At best, drillers should wear appropriate personnel protective equipment (PPE) when using the tools. Authorized and qualified personnel should only be the ones allowed to handle logging tools, while other non-essential workers should be kept away from the rig floor and marked-off sites where radiation hazards may be present. Damage to any radioactive logging tools should be reported immediately.

Other drillers can also get injured by unexpected pressure release. To minimize this occurrence, it is highly advisable to check for the presence of trapped pressure before the tool housing is opened.

References:

How Does Well Logging Work? (1999, January 1). Retrieved December 19, 2014, from https://www.rigzone.com/training/insight.asp?insight_id=298&c_id=1

Oil and Gas Well Drilling and Servicing eTool. (n.d.). Retrieved December 19, 2014, from https://www.osha.gov/SLTC/etools/oilandgas/servicing/special_services.html

Well logging records the events, subsurface formations, and depth measurements of geologic formations during drilling. Each well log is a detailed record which can include visual observations (geological logs) or physical measurements made through instruments (geophysical logs) penetrated through a borehole. Some types of geophysical logs can be recorded during any phase of an oil well’s life, namely drilling, completion, production, and abandonment. The concept is borrowed from ship logging in which events aboard a vessel are recorded. But instead of plotting a timeline, well logging is recorded through depth.

History

In the early 19th century, well loggers scaled oilfield derricks and kept a simple chronicle of events that happened at certain depths, which include problems, types of formations encountered, drilling speed, and oil and gas flows.

In the early part of the following century, Conrad Schlumberger envisioned the use of electrical measurements to scale and map out subsurface formations. He and his brother Marcel eventually performed the world’s first well log in France in 1927 using resistivity, a measurement of how strongly a given material opposes the flow of electric current. century, well loggers scaled oilfield derricks and kept a simple chronicle

Method

A graph and corresponding notes represent drilling records and well depths. Logging tools are inserted into the well to measure the electrical, acoustic, radioactive, and electromagnetic properties of subsurface formations. To collect data, logging tools are incorporated into the drilling tool or drilling tools are lowered into the wells at regular intervals.

Drillers and engineers use well logs to measure formation thickness, formation tops, porosity, water saturation, temperature, types of formations encountered, presence of oil and gas, estimated permeability, reservoir pressures, and formation dips. Well logging does not only record wellbore events, but also ultimately predicts whether a well is commercially viable or not, and whether casing, cementing, and completion should be run on a well.

Reading a Well Log

A well log includes a header that provides specific information about the well like the operating company and the type of log, as well as the main log section and the graph. The vertical part of the chart represents the depth reached, while the horizontal part represents the measurement scale.

Each major section of the log contains inserts which identifies each curve. Curves on the logs – called traces, readings, or measurements, can be represented by solid, long-dashed, short-dashed, or dotted lines to represent different measurements used on the log. To ensure accuracy, the final part of the log includes the tool calibrations used before and after the log was conducted.

References:

Helmenstine, Anne Marie. “What Is Electrical Resistivity?” Electrical Resistivity Definition. About Education, 11 June 2014. Web. 19 Dec. 2014. <http://chemistry.about.com/od/chemistryglossary/g/Electrical-Resistivity-Definition.htm>.

“How Does Well Logging Work?” How Does Well Logging Work? Rigzone, 1 Jan. 1999. Web. 19 Dec. 2014. <https://www.rigzone.com/training/insight.asp?insight_id=298&c_id=1>.

“Oil and Gas Well Drilling and Servicing ETool.” Oil and Gas Well Drilling and Servicing ETool. Occupational Safety & Health Administration. Web. 19 Dec. 2014. <https://www.osha.gov/SLTC/etools/oilandgas/servicing/special_services.html>.

Wireline operations are part of special services in the oil and gas industry. The operations include slicklines and electric lines utilized for perforating, logging, bailing, downhole tool setting, fishing, swabbing, and other workover efforts.

Wirelines

A wireline, also called electric line, is an electric cable used to transmit data regarding the conditions of the wellbore and to lower tools into it. It may consist of single or multi-strand cables that are braided. Aside from gathering data for other workover tasks, wirelines are also used to perform wireline logging.

Slicklines A slickline is a thin non-electric cable inserted into a well to deliver and retrieve wellbore tools like gauges, plugs, and valves. It can also be used to adjust valves and sleeves located downhole, and to repair tubing within wellbore. Wrapped around a drum at the back of a truck, the slickline is raised and lowered into the well by reeling the wire in and out hydraulically.

Wireline Logs

Wireline logs measure properties of well formations using electric lines. They were first developed by the Schlumberger brothers in 1927. Wireline logs are constant downhole measurements sent through wirelines to help drillers, engineers, and geologists make real-time decisions about drilling operations – quite different from MWD and mud logs. Wireline logs assess wellbore dimensions and sonic properties, and can measure conductivity, resistivity, and formation pressure.

The logging tool called a sonde is located at the bottom of the wireline. Measurements are taken by lowering the wireline to the prescribed depth and then raising it out of the well. In an effort to sustain tension on the line, measurements are continuously taken on the way up.

Workover Operations

Workover operations cover remedial work to sustain, restore, and/or enhance production. Although frequent, but not necessarily always, workover operations require production shut-in. a well-servicing unit is used in workover operations to haul things in and out