Overbalanced Drilling
Overbalanced drilling is conventional way of drilling, where the pressure (force per unit area) in the wellbore while drilling is maintained above the formation pressure. Difference between the hydrostatic pressure in the well and the fluid pressure in the formation being drilled is called 'Overbalance'. Overbalance is considered the primary barrier for well control and is maintained to disallow flow of formation fluid into the wellbore. Different companies have different policies for maintaining minimum level of overbalance depending on type of well and expected reservoir fluid.
Excessive overbalance can significantly slow the drilling progress and limit removal of drilled cuttings below the bit. High overbalance coupled with poor mud properties across porous formation can also result in thick mud cake risking differential sticking of drill string. It also risks formation damage across reservoir rock.
Wellbore Tortuosity
Wellbore Tortuosity is a ratio of actual distance between two point in a wellbore and the straight line distance between the same two points. Inclination and azimuth measurements define the profile of the well whereas tortuosity is a measure of crookedness and indicates the well quality. High wellbore tortuosity not only affect the drilling process, drilling efficiency and casing running but also impact effectiveness of completions and production equipment in achieving reservoir return rates.
Dog leg severity (DLS) with measurements every 95 ft provide some insight into wellbore tortuosity but is insufficient to fully define it. Information acquired through tortuosity log help define wellbore shape and 3D visualization, facilitating in making critical decisions on well development and placement for artificial lift and other equipment in areas of low tortuosity. Collecting data at 1-ft intervals through tortuosity logs reveals areas of high side forces and high friction that won't be visible with standard MWD measurements carried out at every connection.
Stabbing Guide
Stabbing guide is a handling tools used while running in drill pipe, casing or tubing. It help align male-to-female connection of two separate pipes. Stabbing guides have two sections, a thru-bore (top section) and a counterbore (bottom section). Both thru-bore and counterbore are made of a strong, durable, yet somewhat elastic material. The top section guides the pin end to align with the box end of the other pipe . The bottom end of the stabbing guide helps the pin end slide over and sit in the box end of the pipe.
Making up misaligned ends could cross thread connections leaving a leak path. Cross treaded connection could have thread damage requiring replacement of pipe, which will result in additional time and cost. Using the right stabbing guide prevents thread damage and minimizes potential connection failure. A good stabbing guide will help in ensuring precisely aligned, securely connected pipe sections.
Slip Crushing
When drill pipe, casing, or tubing are run in or pulled out of the hole, they are set on slips at rotary bushing for connecting or disconnecting pipes. When the pipe is sitting on slips, in addition to tensile load due to the buoyed weight of the string, the pipe also experiences radial load or hoop stress exerted by slip segments. The damage to the pipe that can be caused due to this additional stress by slip segments is called ‘Slip Crushing Effect.’ Calculations and analyzing tools help in determining the maximum load that can be placed on the slips without damaging the tubular subjected to a tensile load. Slip crushing is affected by the pipe diameter, slip angle, slip length, and the coefficient of friction between the slip and the master bushing.
Anchor Latch Seal Assembly
Anchor Latch Seal Assembly is used for completions where tubing movement is not desirable. The seal assembly is run in to the polished seal bore and anchor latch is snapped in to the profile at the top of the seal bore with set down weight. Assembly is released when required by upward pull and right hand turn. Standard anchor latch assemblies are manufactured from materials that comply with NACE standards for sulfide stress cracking-resistant metallic materials. Viton, Aflas, and Chevron seals are used with material suitable for pressure, temperature and corrosive environment specific to the application.
PBR
PBR (Polished Bore Receptacle) also known as Seal Bore is designed to accept the seal assembly. PBRs have varied applications in liner top, permanent packer or completion string for landing a production tubing seal assembly or tie back string. This design provides the benefit of achieving containment of flow within the tubing without any leak and allowing tubing movement due to thermal expansion.
PBR assembly works as an expansion joint when extreme tubing movements are expected and can also be used as separation tool, facilitating removal of production tubing. Length of PBR depends of application requirement. PBRs allowing even 30 ft stroke are available. PBRs are manufactured of high quality material with tight tolerance and are polished before running seal assembly in the receptacle.
Seal Bore
Seal Bore also known as PBR (Polished Bore Receptacle) is designed to accept the seal assembly. Seal bores have varied applications in liner top, permanent packer or completion string for landing a production tubing seal assembly or tie back string. This design provides the benefit of achieving containment of flow within the tubing without any leak and allowing tubing movement due to thermal expansion.
It works as an expansion joint when extreme tubing movements are expected and can also be used as separation tool, facilitating removal of production tubing. Length of seal bore assembly depends of application requirement. Seal bores allowing even 30 ft stroke are available. Seal bores are manufactured of high quality material with tight tolerance and are polished before running seal assembly in the receptacle.
Permanent Packer
As the name suggests, permanent packers are used for longer term or permanent installations in a well either to isolate the wellbore or tubing to casing annulus. Permanent packers can be removed only through milling. They have higher differential and temperature rating than retrievable packers. Permanent packers are also simpler in design since they do not require retrieval mechanism.
BSW
BS&W is a very commonly used acronym in oil & gas industry. It stands for Basic Sediments and Water. When reservoir fluid flows to surface, the flow of reservoir fluid includes water and suspended particles of impurities like dirt, sand, paraffin, scale, rust etc. BS&W varies greatly from field to field. Water and suspended particles are separate from reservoir fluid before taking measurement during well testing and prior to transporting hydrocarbons to production facilities.
Brine
Brine is water with high salt concentration. It contains salts such as sodium, calcium, or bromides in solution. Brine is commonly produced along with oil from the reservoir. Various brine formulations are also used as completion fluid where salt is used to increase fluid density. It is also used for drilling in sensitive formations where either clay swelling is expected or where solid particles as weighting agent could cause formation damage.
Brines formulations can be prepared to achieve densities ranging from 8.4 to over 20 ppg. Common salts used in the preparation of simple brine systems include sodium chloride, calcium chloride, and potassium chloride. Complex brine systems may contain zinc, bromide, or iodine salts, but these brines are more corrosive and costly.
Retrievable Packer
The retrievable packers are used for achieving temporary isolation for performing intervention activities. They are available in different differential pressure and temperature ratings, but are normally used for less severe well conditions. Retrievable packers are available with different setting mechanism like mechanical set or hydrostatic set options. Similarly, the release options could be either pull to release, cut to release etc. Retrievable packers offer flexibility and cost benefit in cases where removal is anticipated in the short term, or require repeated setting and unsetting or packer. Retrievable packers have more complex design than permanent packers due to setting and retrieving mechanism built inside the packer.
Stimulation
Simulation is the process of enhancing the production from reservoir. Once the well is perforated, it is tested to ascertain the production rate from the reservoir by recording parameters like pressure, temperature, flow rate etc. Depending on the type of reservoir and its properties, appropriate methods is adopted to enhance the flow rate.
Stimulation methods used for enhancing well production could be mechanical, chemical or combination of the two. Fracturing & Acidizing are the two commonly used methods for enhancing productivity. Fracturing is performed above the fracture pressure of reservoir formation, whereas acidizing or matrix treatment is performed below the fracture pressure of the reservoir. Fracturing is aimed at creating new highly conductive flow paths where as matrix treatment aims at restoring natural permeability of the reservoir rock by clearing near wellbore damage.
Well Testing
Well testing is the process of acquiring reservoir data and formation fluid properties after drilling the well to the planned target reservoir. The acquisition of data helps in analyzing reservoir characteristics and skin damage during drilling or other operations carried out in the well. The reservoir modelling carried out based on collected data is used for devising completion, stimulation and field development strategies to optimize reservoir production. It also helps in determining production potential of the well for investment decisions.
There are several ways to test a well. A wireline formation tester can be run in the well for capturing samples and measuring properties downhole. A Drill Stem Test (DST) can be carried out in cased hole or open hole for collecting samples and acquiring data by flowing the well to surface. A set of tools are used in downhole test string and equipment are rigged up on surface to safely flow and flare on surface.
After the initial flow for well cleanup, the flow is redirected to a test separator where formation fluids are separated into oil, gas and water phases. Any debris, sand or other material are removed for measurements and collection of samples after stabilized flow is achieved.
Travelling Block
Travelling Block is a mechanical arrangement of pulleys. Drilling line passes over these pulleys and also over the pulleys of crown block mounted on top of the derrick or mast. The travelling block moves up and down as the drilling line winds or unwinds on Drawworks drum making it possible to hoist or lower loads as part of drilling operations.
Top Drive
Top Drive or Power Swivel is swivel that hangs on to the hook of the travelling block. It has the passage way for the mud to flow from mud system through the drill string. Top drive also has powerful motors to rotate the drill string and bit. Top drive replaces, kelly, kelly bushing and rotating master bushing.
Semi-Submersible Rig
A semi-submersible rig is a floating offshore drilling unit. As the name suggests, a semisubmersible rig is partially submerged in water. The rig floats on hollow steel cross-sections called "Pontoons." Some semi-submersible rigs have propulsion systems that allow them to navigate drilling sites under their own power, while others require a second vessel to tow them to the location. Most rigs use several anchors that help maintain the structure's orientation. Computers control the tension on each anchor chain to correct for drift. Since the rig doesn't float on the water surface, it provides a more stable drilling platform than a drillship. Semisubmersible rigs are used for drilling wells in 1,000 feet to 3,500 feet of water depth.
SCBA
Self Contained Breathing Apparatus (SCBA) is a portable breathing apparatus used by the rig crew for protection in H2S environment. SCBA provides effective face seal and oxygen supply for 30 minutes to avoid inhaling poisonous gas. It provides unrestricted movement for carrying out emergency search and rescue operations. More physical work consumes more air and hence SCBAs are equipped with an alarm when a quarter air supply remains in the equipment to have enough escape time.
Rig Building
Rig Building is the process of erecting a drilling or workover rig on a new land location. Once the operations on a location are completed, the mast is lowered and dismantled from specific joints. All the rig equipment are disconnected and transported by road to new location. At the new location, all the pieces of mast are joined. Once all accessory equipment are connected, the mast is raised as part of the process to get the rig ready for operations.
MODU
MODU is an acronym for "Mobile Offshore Drilling Unit". It is a broad term used for all types of offshore rigs. MODUs are broadly classified as Bottom Supported or Floating units. Bottom supported units are either Submersibles or Jack Ups. Floating Units are either Semisubmersibles of Drill ships.