Crude oil production:

    Having established the size of the oil field, the subsequent wells drilled are called ‘development’ or ‘production’ wells. A small reservoir may be developed using one or more of the appraisal wells. A larger reservoir will require the drilling of additional production wells. Multiple production wells are often drilled from one pad to reduce land requirements and the overall infrastructure cost. The number of wells required to exploit the hydrocarbon reservoir varies with the size of the reservoir and its geology. Large oilfields can require a hundred or more wells to be drilled, whereas smaller fields may only require ten or so.

    The drilling procedure involves similar techniques to those described for exploration; however, with a larger number of wells being drilled, the level of activity obviously increases in proportion. The well sites will be occupied for longer, and support services— workforce accommodation, water supply, waste management, and other services—will correspondingly increase. As each well is drilled it has to be prepared for production before the drilling rig departs.

oil rig 

 

OIL FIELD DEVELOPMENT:

    Drilling is done to fracture and penetrate the rocky layers to reach the oil formation below the Earth’s surface. A hollow steel pipe containing the drill bit with perforations at its mouth is used for drilling. Mud fluid is pumped through the top end of the drill pipe through a hose which moves down with the pipe as the drilling progresses. The drill pipe and the hose are suspended from the crown of a pyramidal structure called a rig. depicts a typical rig for drilling operations.

Schematic diagram of an oil rig for exploration. 

A high-pressure pump is employed to pump the mud solution from the mud pit through the hose such that the cuttings at the drill bit are washed out through the mouth of the drill bit and returned to the top surface through the annular space between the drill pipe and the hole developed. Cuttings with the mud solution are collected and separated from each other. Clarified mud along with fresh mud is pumped back to the drill pipe continuously. Mud is consumed due to absorption and seepage through the pores and crevices of the layers. Monitoring of the level in the mud pit is essential to assess the consumption and generation pattern of cuttings and water.

An alarming decrease in the level indicates leakage through the layers due to seepage in crevices or channels. While an increase in the level indicates ingress of underground water. Samples of drill cuttings are useful for surveying and assessing the direction of the drilling operation as they carry valuable information about the layers and formation. Continuous well logging is then carried out using a modern system of data acquisition and analysis. After drilling to a depth of 30–40 ft, a steel pipe is introduced into the hole to protect the wall of the hole formed.

     This is called the casing string, which is then cemented to the wall of the hole by pumping a fast-setting cement solution (usually Portland cement without sand) to the annular space between the pipe casing and the wall of the hole. This casing helps prevent the caving of the wall and seepage of water from the layers.

     An additional drill pipe is then joined of sufficient strength to withstand the various static and dynamic stresses for the increasing dead weight of piping, torsional stresses due to rotation, for upward and downward movement, abrasion from sand, fluid friction from mud fluid with cutting and corrosion, etc. The drilling operation is then continued and an additional casing pipe of a reduced diameter from the previous one is inserted and cemented at strategically located positions (for easy recovery of casing pipes after the well life is exhausted) until the target depth is reached. The final casing diameter may reach as small as 5–8 in.

At this stage, the top of the well (well-head) along with the casing hanger is fitted with the necessary piping and collection headers. A pipe riser is inserted in the well to lift the oil and is connected to the well-head piping and valves. The diameter and design of the pipe riser (tubing) may differ depending on the facility of the oil lifting mechanism. The well-head connection consists of a tubing header and a Christmas tree header for the collection of oil, gas, and water to the respective storage tanks.

The surface of the casing pipe at the desired target depth is punctured by bullet or missile firing by experts. The hydrostatic pressure of the mud fluid in the well hole balances the reservoir pressure, thereby preventing spouting of the well from the formation.

drilling operation in well 

WELL LOGGING:

 

Well, logging is a continuous recording process of the activities during drilling, well development, and production until the closure of the well. Thus, the record identifies the history of the well. Well, logging is carried out during the drilling operation using special probes (electrical resistivity, inductance, or magnetic resonance), physical sampling of the drilled soils and rocks, core samples, monitoring drilling fluid, etc.

Various parameters, such as porosity, permeability, and water saturation in oil, of the formation are also obtained by the resistivity probes. During the drilling operation, information about the drill bit, its movement, and direction are determined by these probes. The direction of drilling is ascertained by the dipole sharing investigation tool (DSI). Information is also gathered to release drill bits stuck in the well, monitoring the perforation operation of the casing to communicate with the formation, the properties of oil and gas in the formation, etc. At various stages of production, well probing is used to inspect the casing, the wall of the uncased well, etc., for necessary maintenance operation of the well.

OIL PRODUCTION PROCESSES:

The gas lift method employs high-pressure gas, usually, air or carbon dioxide, which is introduced into the well through the annulus, and oil is carried through the inner tubing, leading to the well-head piping. Initially, the well is filled with the mud fluid and the oil cannot move up owing to the hydrostatic head of the mud fluid. As the gas enters the annulus and piping; the density of the mud column decreases and the hydrostatic head decreases, and as a result, the mud fluid is lifted by the oil pressure. A mud–oil mixture is collected and separated on the surface tanks. When complete displacement of mud takes place from the well and from the pores of the layer near the borehole by the oil pressure, oil production starts increasing. 

 

gas lift method

A sucker rod lift well: contains a piston (or a plunger) pump lowered into the inner tubing. The piston is operated by a metallic wire or rod leading through the tubing and above the well-head and connected to a wire rope from a hanger attached with a reciprocating driving system at the base of the well-head. The piston is contained in a cylinder with non-return valves fitted at both ends.

During the upstroke of the piston, the bottom valve opens, keeping the top valve closed and, as a result, the cylinder pressure falls below the reservoir pressure, forcing oil to enter the cylinder. While during the downstroke of the piston, the upper valve opens and the bottom valve closes and oil in the cylinder is pushed up to the tubing through the upper valve. Thus, the volume of oil displaced upward in the tubing is proportional to the stroke length of the piston. When the tubing is filled with oil after repeating the reciprocating operation, oil starts flowing upward and is collected.

a sucker rod lift 

 A submersible pump well contains a centrifugal or screw pump installed in the tubing lowered into the borehole. Both the electric motor and the pump are submersed in the well bottom. Electric cable sealed in a flame-proof arrangement is lowered into the well hole through the tubing. The motor is usually kept below the pump in the tubing. Pumps are small in diameter (3–6 in), multistage centrifugal or screw pumps. Since entrainment of sand particles and gas may cause problems to the centrifugal pumps, modern wells are using high-capacity multistage screw pumps that can carry slurries, viscous oil, and even gas.

In fact, future wells will deliver more viscous oil contaminated with sand and clay materials, therefore, increasing use of submersible screw pumps will take place in modern and existing wells. Modern screw pumps with a diameter as small as 6 in with a capacity of 100 m3 or more per day and with a head of 1000 m are being used in wells. The number of stages of a pump may be more than 100 tightly fitted in a tubing. 

pumping for oil  lifting

 

The hydraulic pumping method: employs a special type of tubing that consists of two tubes. The inner tube is of a larger diameter in which the plunger or the diaphragm pump is lowered into the borehole. The plunger or the rod of the diaphragm is forced by pumping a liquid over it in a reciprocating manner. Oil is discharged through the outer pipe through its annular space and is delivered to the surface tank. This method does not require lowering any electrical cable and no wire for actuating the plunger.

A high-pressure reciprocating surface pump delivers the liquid forced up and down the plunger of the pump in the borehole in a reciprocating manner. The plunger pump can be withdrawn on the surface from the inner pipe by forcing liquid through the annular outer pipe. 

 

hydraulic pumping for oil lifting 

The rate of production from a single well may not be large. Hence, a good number of wells, varying from 100 to 1,000 wells depending on the rate of production, are drilled in the area where the formation is spread. Excitation (stimulation) of the wells by gas or water injection from the surrounding injection wells (judiciously located) is extremely necessary to increase reservoir pressure to the flowing wells.

Modern methods also employ combustion of oil in the surrounding wells to push the oil in the formation by heat effect on reducing viscosity in the porous channels of the formation. A proper temperature gradient is essential from the channels of the combustion zone to the target well. Crude oil from all these wells is collectively routed to storage and conditioning.