a) Repair natural damage within the well b) Zone Transfer
c) Stimulation
d) Convert well from production to injection
a) Repair Natural Damage within the Well
The term natural damage refers to damage in the reservoir rock or the fluids within it. Examples of this natural damage include near-wellbore formation damage, sand production, excessive gas
28 | P a g e production, and excessive water production. These types of damage and their causes are described in the following sections.
This can be further categorized into:
(i) Near-Wellbore Formation Damage (ii) Sand Production
(iii) Excessive Gas Production
(iv) Excess Water Production (Coning) water shut off by various methods.
(i) Near-Wellbore Formation Damage
During the producing life of a well, the permeability of the producing formation near the
wellbore is reduced, affecting production rates. One reason for this near wellbore damage is that components of the reservoir rock react with the well fluid. Examples of formation damage include:
• Swelling of fine formation clays within the reservoir rock pore spaces.
• Blocked pore throats due to the migration of fine particles through the formation toward the wellbore.
• Emulsion blockage caused by the mixing of two normally separate (immiscible) fluids such as completion brine and crude oil. The result is a highly viscous mixture that reduces the relative permeability of the producing formation.
• Reduction of pore throat size due to the precipitation of scale—such as calcium
carbonate or calcium sulfate—from reservoir fluids as a result of temperature or pressure reduction.
29 | P a g e (ii) Sand Production
Since many oil reservoirs are located in sand beds, sand production is a naturally occurring problem. As sand moves through the reservoir and the production string, it may plug perforations, safety valves, tubing, and surface equipment. It may also erode Christmas tree components. The rate of sand production can further increase due to formation breakdown, poor production practices, poor completions, and equipment failure. A common industry technique for controlling sand production is called gravel packing.
Sized gravel particles are packed in the annulus outside a specially designed gravel-pack screen or slotted liner. Formation sand is then restricted from entering the completion. Gravel packing can be done in a cased hole or an open hole.(as shown in the fig. below) Various screen types are used for these procedures: pre-packed screens, gravel-pack screens, or simply screen assemblies.
Fig(3.3) Gravel packing
30 | P a g e (iii) Excessive Gas Production
In certain reservoirs, the gas associated with the oil serves as a major driving energy for oil production. The most common types of gas drives are solution-gas drives and gas-cap drives. In solution-gas drives, dissolved gas in the oil helps propel the oil to the surface. Eventually, some of this gas separates out of solution and becomes trapped above the oil, forming a gas cap. The energy in the gas cap then assists in propelling the oil. In some wells, the gas cap is already present when the well is completed. In either case, the gas in the cap may ―cone‖ downward toward the perforations and be produced along with the oil. Coning robs the reservoir of drive energy and lowers production rates.
To control this separation during the early stages of production, the crew controls the pressure at which the well fluids are produced from the reservoir. Maintaining a certain pressure on the well helps keep the gas in solution with the oil. As the well fluids are produced, however, this
separation is more and more difficult to maintain and a remedial workover may become
necessary. This type of workover involves cementing the existing perforations and perforating a different zone to allow oil from below the oil-gas contact point to flow to the surface.
Fig(3.4) Gas Coning
Excessive gas production in oil wells
31 | P a g e (iv) Excess Water Production (Coning)
In waterdrive reservoirs, the energy propelling the oil or gas comes from the expansion of vast quantities of water. Water is generally considered incompressible, but it will compress and expand somewhat. Considering the enormous quantities of water present in a producing
formation, this small expansion represents a significant amount of energy, which aids in driving the fluids through the reservoir to the surface. In this type of drive, the water tends to be drawn upward in the shape of a cone and eventually will reach the perforations (as shown in the fig.
below).
As a result, water is produced, bypassing a portion of the oil reserves. Typically the first attempt to control coning involves reducing the production rate, but when this fails, a remedial workover may be needed to plug the perforations below the oil water contact zone and produce from above the watered-out zone. In many cases, however, the water eventually covers the entire producing interval and a workover is performed to totally abandon that zone and, if possible, produce from another zone. Fig(3.5) Water coning
32 | P a g e Now, if a well is producing water more than expected rate, then the possible sources of
undesirable water have to be identified. The following are the primary reasons for water contribution:
perforation in aquifer- there are often water bearing zones near gas or oil bearing zones and if some perforation are made into a water zone, then this may contribute to water production.
Water coning
Encroaching oil/water contact: in a producing zone with time the water moves upward to displace the oil towards the completed well bore.
Channel behind casing can be caused due to poor or damaged cementation behind casing.
Water may flow from one zone to another through these channels behind casing.
Water shutoff by cement squeezing: In this, the present producing interval is squeezed and plugged off by cement. The hole is then cleared to the desired oil zone interval by drilling and perforated. Subsequently the well is activated and re-completed.
Water shutoff by polymer: Poly acryl amide polymers are injected at the oil-water interface zone which absorb on to the rock matrix and remain there as a ―film‖ that attracts water.
Therefore, all water than passes near this ―film‖ is slowed by attraction to the polymer. However oil and gas is repulsed by the polymer and flow through the centre of the pores. In a sense, the polymer film creates a frictional force for the water to overcome, but it tends to lubricate the flow of oil and gas through the pores of formation. The procedure is as follows:
Subdue well by workover fluid and pull out production string.
Internal wire catcher trip and scrapper trip for hole cleaning.
Squeeze off the producing interval by cement
Cement drill and clear hole and scrap hole and perforate interval in oil-water interface zone.
Pump in polymer and seal the interval by cement squeezing.
Drill cement and clear hole till the hole producing interval, scrap and perforate Activate the well and carry out reservoir study
33 | P a g e Complete the well by running in completion string.
In case of bio-polymer water shut-off job, polymer is pumped into the producing interval followed by crude oil for well bore saturation.
b) Zone Transfer
One of the most common reasons for a workover is to recomplete a well from one zone to another. Recompletion involves changing the zone from which the hydrocarbons are produced.
Many wells are drilled to intentionally penetrate many zones, but only one zone at a time is produced. In some wells, lower zones are produced first. When depleted, they are recompleted (isolated) so that another zone farther up can be produced. As shown in the figure below:
Fig (3.6a) Zone Transfer
34 | P a g e In some cases, higher zones are produced first and then recompleted to shift production to lower zones; as shown in the figure below:
Fig (3.6b) Zone Transfer
In some recompletions from a lower zone to a higher zone, the workover crew places a cement plug, bridge plug, or Wireline set plug to isolate the abandoned zone. This helps ensure that the old perforation is adequately sealed. In a recompletion from a higher to a lower zone where a plug is not used to isolate the zone, several squeeze cement jobs may be required to isolate the upper zones and seal the old perforations
The schematic diagram for the same is given on the next page :
35 | P a g e Fig (3.6c) Zone Transfer
Zonal isolation
In most wells, an extra rat hole (a space below the perforations) is drilled below the lowest production zone. A rat hole provides clearance to run logging tools, collect produced formation material, or allow tubing-conveyed perforating guns (TCPs) to fall below the perforations. In some cases, bridge plugs or Wireline plugs cannot be recovered from the wellbore, so the rathole provides a space for disposing of these plugs below the lowest-producing level where they will not affect production.
c) Stimilation
Production in a damaged or low-producing zone can be increased by one or more techniques.
36 | P a g e The name of some known techniques that are used in the petroleum industry are as follow:
(i) Acid Stimulation
Matrix acidizing is a stimulation technique involving injection of acid into the formation rock at pressures below the level at which the rock will fracture. This technique dissolves away damage caused by drilling, completion, and workover or well-killing fluids as well as by precipitation of deposits from produced water. It is also used to etch new channels or pathways for hydrocarbons near the wellbore.
Hydrochloric acid (HCl) is used to treat limestone, dolomite, and other carbonate type rocks, while hydrofluoric acid (HFl) is used in sandstone reservoirs. A mixture of HCl and HFl called
―mud acid‖ is used to dissolve damaging clay deposits. Damage from waxes or asphaltenes in produced oil can be treated with organic solvents.
(ii) Hydraulic Fracturing
In some wells it is necessary to intentionally fracture a formation to provide a deeper flow path for oil and gas into the wellbore. Fracture (―frac‖) fluids include oil, water, acid, emulsions, foams, or combinations of these. The frac fluids are pumped down hole under high pressure at a high rate of flow to fracture the formation. These frac fluids include finely grained particles called prop pants. Proppants are made from sand particles of a controlled size or sintered bauxite (aluminum ore). The proppant remains in the fracture to help hold the fracture open after pump pressure is bled off. An acid fracture job (often called ―acid frac‖) involves pumping a gelled acid at a pressure above the formation fracture limit. The gel creates a fracture, and the acid
37 | P a g e etches the rock surfaces, creating an irregular pattern. No proppant is used in an acid frac. When the earth‘s forces cause the fracture to close, the uneven surface of the frac faces will not match and a new conduit for oil and gas is formed.
(iii) Steam Injection
Steam is one type of stimulation technique for increasing production in zones of high-viscosity oil. Steam is injected into the formation to improve the oil‘s flow properties. High-temperature equipment and appropriate workover procedures are required when steam injection is used to stimulate production.
(iv) Waterflood Injection and CO2 Injection
Waterflood injection and CO2 injection fall into the category of secondary recovery or Enhanced oil recovery (EOR). Waterflood is a method used to increase production from an existing reservoir by injecting water into the reservoir to displace the oil. Generally, reservoirs that are geologically bounded on at least three sides are better candidates for water flooding, since the water is trapped in place and not free to migrate out. The water generally used is produced formation water from a nearby source. CO2 injection (or ―CO2 flood‖) is a process by which carbon dioxide gas is injected\ into the reservoir to replenish drive energy and recover additional oil that would have otherwise been left in the reservoir. CO2 is often present in certain gas reservoirs in conjunction with hydrocarbon gas. Gas processing plants separate the CO2 from the hydrocarbon gas and send it to pipelines for transport to the field for injection. CO2 injection has been used for years in certain mature oilfields such as the Permian Basin in the southern United States.
38 | P a g e d) Convert well from production to injection
Workovers are done to convert producing wells to injection wells. In this type of workover converting a producing well configured for continuous or intermittent gas lift. As shown in the figure given on the right hand side.
Fig(3.7) Converting a producing well configured for a continuous gas lift
Using wireline tools, the gas-lift valves are retrieved from their receptacles, or side-pocket mandrels, in the completion and replaced with special regulators that control the amount of gas injected into a particular zone in the reservoir. Typical injected gases include carbon dioxide schematic diagram for Sucker Rod Pump is given on the next page:
39 | P a g e Fig (3.8) Sucker Rod Pump
Workover tasks for wells with artificial-lift operations may include:
• For rod pump: Repair or replace the pump on the end of the sucker rod string. Damage may result from wear, fouling with sand, or pressure locking. This workover would involve using a rod pulling unit to retrieve the rod string from inside the production tubing. In some cases, the reciprocating motion of the rods abrades and eventually cuts through the production tubing. In this situation you must pull both the rod string and the production string.
•For hydraulic pump: Retrieve the pump through the tubing for repairs or replacement. In some instances, the tubing must be cleaned out first as scale or paraffin buildup may prevent the pump from passing through it.
40 | P a g e
• For ESP: Retrieve and repair or replace faulty ESPs and associated motor electrical cable.
Fig (3.9) Electrical Submersible Pump
• For gas lift: Using Wireline, retrieve and repair or replace gas-lift valves that have lost their functionality. (Damaged gas-lift valves may allow gas to pass straight through the valve with no restriction because the internal precharge has been lost or because the elastic parts, called bellows, have lost their resilience.)
41 | P a g e