Massive sand production from unconsolidated sandstone can occur as soon as the well is brought on stream. This may quickly lead to unmanageable problems in which case sand control methods will be required to continue production.

In the more consolidated reservoirs, sand production may be restricted to short bursts as the well is being beaned up, followed by long periods of relatively sand free production.

In many cases it is not certain that sand production will become a problem and because wells often are more productive when sand control has not been installed, unnecessary installation should be avoided. Conversely, sand influx in wells and surface facilities can cause a variety of problems which may have a severe financial, safety or environmental impact on production operations.The economic penalties of sand control require therefore a careful assessment of the risks and consequences of sand production.

Given the sand production potential of a particular field or well, this article presents a framework for answering the following questions:

·Can sand production be avoided or tolerated ?

·When is sand control warranted or necessary ?

·Which sand control method should be selected ?

1 Sand tolerant production systems

Sand influx into producing wells and surface facilities can cause a variety of problems with potentially serious safety, environmental and/or financial consequences.

In general the level of sand production which causes a problem will vary with the location, the well and production facility design and local conditions.

The effects of sand on production operations and the potential safety, financial and environmental consequences will influence whether sand production limits are to be set and at which level.

It is the purpose of this chapter to discuss how to define the amount of sand that can be tolerated in a production system and what level of operational support is required; in other words how to formulate a sand production philosophy. 

1.1 Defining the sand production philosophy

The sand production philosophy for a particular field and production system must be formulated after careful evaluation of a number of interacting factors which range from the well and surface facilities design to the operating procedures for the production system.

In general, sand influx cannot be tolerated whenever the overall safety of production operations is jeopardized or when uneconomic measures need to be implemented in order to sustain production.

The optimal integration of the different aspects will be a function of economic, operational, safety and environmental considerations. Close cooperation between the Petroleum Engineering, Engineering and Production Operations department is essential when debating this issue because of the variety of subjects that need to be addressed:

1.Prevention of sand production: In certain cases, sand production can be prevented or minimised by limiting the applied drawdown, by a suitable selection of the perforated intervals and by implementing adequate operating practices.

2.Well and surface facilities design: Most of the problems that can be experienced stem from the abrasive effect of sand, possibly aggravated by a corrosive environment, or from sand accumulations in the wellbore and surface facilities. These two aspects are discussed further below. If during a development, sand deposition and/or erosion rates are likely to present a potential system constraint suitable measures with respect to well design - material/component selection, sand detection (shut down systems) and monitoring should be addressed.

3.The availability of production operations support will be a function of the field location, its remoteness and the planned manning level. Clearly it is essential that potential sand production problems are documented in the field development plan to highlight to process engineers to build sand handling capability into detailed process design. In principle if sand control is deferred, facilities should be designed to handle sand which will necessitate the early involvement of Production Operations staff, to assist in the development of a specific operations philosophy over the life of the venture. This will provide a list of preferred options on key operational and facility requirements, for example: unmanned versus manned, local versus remote, manual versus automatic etc. More generally this will provide guidance on various issues, whilst giving due regard to factors such as local legislation, and company policy.

4.A monitoring programme is necessary to allow early detection of significant sand production and trigger prompt remedial action, hence optimise production. Erosion of equipment at critical locations needs to be carefully monitored. A preventive maintenance programme is required to ensure critical items of the production system are functioning (e.g. safety valves) and to allow early removal of sand accumulations. This aspect is covered in Section 24.

5.Handling and disposal of produced sand may become a problem especially if disposal in an environmentally acceptable manner causes problems. If continuous sand production is experienced, dedicated sand separation and disposal facilities may be required. In general the preferred option is online sand removal systems to minimise intervention, carry over and protect down stream equipment. Cleaning sand prior to disposal is normally undertaken by specialised service companies unless sand cleaning is incorporated into the process.

6.Finally the potential risks that can be incurred because of sand production need to be carefully evaluated. Equipment failures may result in safety and environmental hazards while unscheduled production interruptions may have unfavourable economic consequences such as failing to meet contractual deliveries. The above issues form the basis of a sand production philosophy, which is incorporated into a more general operations philosophy and development plan to achieve the prime objective: optimise cash flow whilst safeguarding the technical integrity of the facilities. The appropriate involvement of all technical disciplines is considered a prerequisite in this process.

1.2 Methods of minimising sand production

Some sandstone reservoirs are competent enough to initially allow sand free production but changing producing conditions may induce sand production in a later stage of the life of the field. When the sandstone is of such marginal competence, sand production may be minimised by one or a combination of the following:

·Suitable selection of the perforated horizons,

·Limiting the drawdown,

·Controlling the reservoir pressure depletion,

·Implementation of specific operating guidelines.

These practices are commonly referred to as "passive sand control methods" as they usually do not require additional effort during the completion operation in comparison with a conventional completion. Their applicability is however limited as they only qualify for a certain type of reservoir, are not always practical and are frequently uneconomic because of reduced deliverability.

Based on empirical and rock mechanical considerations, initial sand failure is related to the collapse of perforation tunnels and occurs when the effective stress acting on the near wellbore region exceeds a critical value, which is a function of rock strength. Although the conditions leading to initial sand failure are relatively well understood, the likelihood of massive sand failure is very difficult to predict because of the phenomenon of post failure stabilisation. This may be caused by structural sand arches which form in the vicinity of a perforation, effectively stopping sand production. Transient (bean up rate) and cyclic loading (production/shut in cycles) effects will have a significant impact on both initial failure and post failure stabilisation.

Sand production may be prevented to a certain extent by either limiting the effective stress acting on the reservoir and/or by selecting those zones in the reservoir which will be strong enough to support the near wellbore stress during production. Depletion plays an important role as it increases the effective stress acting on the reservoir rock. Massive sand failure is also often triggered by a change in the wellstream composition such as the onset of water production.

1.2.1 Drawdown limitation

The first action to take when a well starts to produce sand at high levels is to reduce the drawdown by reducing the production rate. In view of the often erratic nature of sand sampling methods, operators generally insist on a confirmation of the high sand reading before beaning the well back. This should however be done quickly to avoid a catastrophic sand failure. Experience shows that in some cases there is a critical flowrate below which little or no sand production is experienced. This is probably the oldest form of sand control, typified by "beaning down", but is not always successful and is frequently uneconomic.

1.2.2 Selective perforation

The concept of avoiding weaker reservoir material to avoid sand production is well known. In practice, the selection of the perforated intervals is based on the value of the sonic wave transit time, as given by the sonic log or on a log-derived formation strength index.

1.2.3 Control of pressure depletion

Pressure maintenance can serve to limit the maximum effective stress to which the formation is subjected. In practice, pressure maintenance schemes are only implemented in view of ultimate recovery considerations. However, the economic assessment of pressure maintenance schemes should include the possible savings by not having to install sand exclusion and the consequent benefit of higher well productivity.

1.3 Erosion of production equipment by sand

Erosion of production equipment can be caused by cavitations of produced fluids or impingement of liquid droplets and solid particles. It is difficult to accurately predict erosion rates because of the number and complexity of the variables involved i.e.:

·Sand particle characteristics (size distribution, hardness, density, angularity...)

·Sand concentration

·Mechanical properties of tubulars and facilities

·Fluid properties (viscosity, density, liquid to gas ratio...)

·Flow parameters (velocities, flow regime...)

·Pipework geometry (local turbulence effects...)

In a corrosive environment, the material loss process can be significantly accelerated by the combined chemical and mechanical action of the produced fluids and solids.

It is commonly believed that an abrasive cannot wear a material harder than itself. Hardness is a material's resistance to cutting, scratching or indentation.  

·The Mohs scale rates hardness on the basis of what mineral will scratch another mineral. It is used mainly for minerals and is seldom applied to metals.

·The Knoop and Brinell scale relate a material's hardness to the volume of material displaced by an indenter of specific geometry (Brinell: sphere, Knoop: pyramid).

·The Rockwell scale relates a material's hardness to the penetration depth that a specific size sphere will make under a specific load.

According to Moh's definition, a softer material cannot scratch a harder material. This may be the case for particles impinging on a surface. However in the more complex phenomenon of grinding abrasion (i.e. the removal of material by particles sliding, rolling or crushing between two surfaces), both the harder and the softer material show loss. As the material approaches the hardness of the abrasive, the wear rate drops but does not totally disappear.

Many studies have been conducted to better understand and combat equipment erosion by sand. Results of these studies and field experience show that erosional damage due to sand can be prevented provided flow velocities and sand rates are kept below a critical level and the geometry of the facilities is optimised.

Whenever high erosion rates are expected, flow velocities should be limited by increasing the tubing or flowline diameter or limiting the flowrate. This is particularly true for gas or high GOR oil wells where high velocities due to gas expansion can be encountered.

The general correlations available in the literature to determine erosion/corrosion critical velocities (e.g. API RP 14 E [822]) are generally not adequate in a sandy environment. This is because they either do not account explicitly for sand production or they only warn to reduce velocities when sand production is expected.

The tolerance of subsurface equipment to sand may become a limiting factor, especially if artificial lift is required. For example, downhole pumps which rely on a fast sliding motion in contact with the produced fluids, or on high velocity moving parts are most susceptible to sand ingress. Gas lift systems, jet pumps or low speed pumps ("beam or Moineau" type) are the most tolerant to sand production. 

1.4 Sand transport

If sand production is expected, flow velocities should be high enough to transport sand to the point where it can be separated from the produced fluids. A balance has to be found in sizing tubulars and flowlines as they are usually designed to minimize pressure rather than optimise transport velocities.

The minimum fluid velocity at which sand will be transported is a function of many variables. Such as:

·Sand particle size, shape and density,

·Fluid density and viscosity,

·Flow characteristics.

Sand settling in the wellbore can also be prevented by emphasizing simple completions (i.e minimum number of packers and accessories), a smooth flow path and minimum deviation. Some allowance for settling of initial sand bursts can be provided by large ratholes. Efficient removal of sand fill from the wellbore by wireline or coiled tubing should also be catered for. The removal of sand accumulation by e.g. coiled tubing operations is more difficult with increasing deviation , depth and completion bore.

The frequency of pigging operations should be adapted to prevent the build-up of sand accumulations in the surface flowlines.

Sand settling in the surface facilities is a function of the residence time, fluid properties and local velocities. The design of separators, pressure vessels and all potential sand traps should cater for the easy removal of sand fill, by providing drain points and jetting systems where sand can be flushed out. Additional features such as man ways, hatches, flush and drain lines should be built in all major tanks and vessels, where most of the produced sand will settle due to the long residence times.

Sand will inevitably cause production interruptions due to the sand accumulations in various parts of the production system. The monitoring and shut down system should be tailored for this situation.

1.5 Sand production and cavity growth

Experience shows that in some fields, hydrocarbons can be produced together with continuous, manageable sand production. This suggests either the "sloughing" of fluidised formation or the creation of cavities in the reservoir in direct communication with the wellbore. In general, a gradual increase of the well productivity is often noticed.

Large cavities may be undesirable for several reasons. Firstly, this may leave the production casing unsupported, cause loss of zonal isolation and in extreme cases buckling and loss of the well may result. Secondly, a remedial sand control job will be more difficult to carry out in the presence of a large cavity behind the casing. Specific guidelines on maximum allowable cavity size do not exist as such and need to be derived from local field experience.

Two possible downhole situations have been considered. In the first option, a cavity has been created in an intact formation, with the cavity continuously being enlarged. This model agrees with the observed gradual improvement in productivity. With an average perforated interval and formation thickness of 4 and 12 meters respectively, in the case of a spherical cavity, an average daily sand production of 50 kgs will enlarge the 3.4 meter sphere radius by 0.15 mm i.e. about one sand grain layer is being produced per day.

The second possibility is a formation that sloughs or creeps around the borehole, continuously squeezing sand through the perforations. This process has been observed in recent experimental work, as a result of dilatant rock behaviour (i.e. porosity increasing significantly during the failure process). Dilation would presumably increase the permeability and could also explain the observed increased productivity.

A combination of the two phenomena is also possible i.e. having a cavity of a certain size in conjunction with a zone of weakened and dilated rock material.

There is presently no method available to measure the true shape of a cavity behind a production casing. Current production logging tools can only measure the height of a cavity. Hence only an account of the cumulative amount of sand produced can give an idea of the total equivalent cavity volume, assuming the formation does not have a 'creeping' behaviour. Keeping an accurate historical record of produced sand is therefore important to monitor global cavity growth in terms of volume and shape. Cavity growth limits may be inferred from the following considerations:

·Prevention of zonal isolation problems i.e. to prevent the cavity from growing too close to a reservoir fluid interface or an adjacent zone which needs to be needs to be kept separated.

·The larger the cavity, the more difficult control of a remedial sand control job will become, if required. However no specific guidelines can be given on this topic.

·Buckling/collapse of unsupported casing due to axial compressive loads, resulting from formation compaction or temperature effects. Of these two effects, normally only formation compaction should be of concern at reservoir level. It is important to note that in the case of reservoir compaction, a length change will be imposed on the casing string, independently of the presence of a cavity. It is difficult to predict the effect of a cavity on the stability of the casing under those conditions.

1.6 Allowable sand production levels, field examples

The concept of a unique sand production cut-off level that can be applied to a wide variety of wells and production facilities under differing producing conditions does not exist. The adoption of a rigid arbitrary rule is likely to lead to premature remedial operations or unwarranted production restrictions.

The values quoted below should be seen as locally defined "rule of thumb" limits which may be beneficial as a flag to ensure surveillance is maintained where necessary or to signal that further action is required. In other words, they do not reflect a physical limit for sand erosion or transport phenomena and should be treated with caution to avoid production deferment.

Operators should critically review their allowable sand production limits. 

2 Framework for a sand control strategy

Sand control is obviously required whenever evidence exists that unmanageable sand production will occur initially or soon after a well is put on stream. This is typically the case for shallow, unconsolidated sandstones. The remaining problem will then be to select the most suitable sand control method.

In many cases however, it is not certain when sand production will become a problem. For the more consolidated sandstones where only indications of possible sand influx exist, a common approach is to base the decision for sand control on a simple rock strength indicator such as formation depth or the measured sonic wave transit time. It may also be policy for certain fields to apply sand control indiscriminately as an insurance against any sand production.

Decisions based on these simple approaches may result in higher initial completion costs and deferred production. The economic penalty of sand exclusion should be a strong incentive for a more critical approach.

2.1 Objectives to be considered

When considering a sand control strategy, two objectives must be pursued:

·Minimize sand production

·Maximize hydrocarbon production

Some degree of compromise between these objectives will be required since prevention of the movement of sand is generally incompatible with the unrestricted flow of fluids. An "engineered" sand control strategy should integrate technical, operational, economic and safety considerations in order to identify the need for sand control and define the optimal timing if required. A framework to support the decision making process is presented in this section.

There should be no preconceived ideas on which method will be used if sand control proves to be required. The identification of the most appropriate sand control method usually requires a much deeper and detailed level of investigation than is required to define the optimum sand control strategy. It is important at this stage to avoid getting involved in too much detail so as to keep a clear overview of the problem and an open mind to alternatives.

In other words, the requirement for sand control is a strategic issue that needs to be addressed when preparing the field development plan. However both this requirement and the selected method should be constantly reviewed in the light of the experience gained when actually developing the field.

2.2 Why and when sand control

Steps to assist the decision making process:

1: What is the sand production philosophy?

It should first be established whether the local circumstances dictate a "no sand" philosophy. The consequences and risks involved by not implementing sand control may be unacceptable when:

·The overall safety of production operations is jeopardized (e.g. offshore locations, in particular manned platforms; high GOR or high rate gas wells),

·Adequate monitoring and/or intervention is not possible (e.g. unmanned or remote locations),

·Remedial sand exclusion treatments are not operationally or economically attractive (e.g. subsea wells),

·Artificial lift requirements do not tolerate sand production.

·The consequences of deferred production due to an unplanned shut down are unacceptable. Production availability problems can become very acute for gas fields which occasionally have to be produced at high output levels in response to peak demand. Under those conditions, massive sand failure may occur because of the higher drawdown and consequent higher stress levels on the formation. There may also be little spare capacity to sort out any problems occurring in the surface facilities. In general, the level of sand that can be tolerated in the production system will be a function of operational, safety, environmental and economic

2: Can the wells be initially produced to their target level with acceptable sand rates?

Having established the sand production philosophy, a dedicated production test should establish whether sand control is required initially or not. Dedicated tests are the most diagnostic of the initial sand production potential of a well. They consist of producing at gradually increasing rates until the maximum desired rate is reached or massive sand failure is experienced.

3: Can massive sand production occur during the lifetime of the reservoir?

For friable sandstone reservoirs which can be initially produced without sand problems, the prediction of massive sand production is a crucial issue. There are currently no models available that can accurately forecast this event. It is possible however to assess initial sand failure, i.e. the conditions that will lead to the initial collapse of perforation tunnels. This event may be followed by post failure stabilisation and lead to periods of relative sand free production at specific producing conditions. Other factors such as a change in the composition of the produced fluids (i.e. increasing water cut) often contribute to the onset of continuous sand production. Therefore even if the initial sand failure criteria can be established, considerable uncertainty over the onset of massive sand failure is likely to remain. The confidence level will depend on the availability of field "calibration" data points, in other words sand failure events.

A sand prediction study should aim at assessing the producing conditions likely to lead to massive sand production. Together with a depletion forecast, the point in time when a well will be at risk of massively producing sand can then be estimated. Dedicated "sand influx" production tests can be carried out to simulate future operating conditions and to provide a more reliable insight in the risk of future massive sand failure. This topic is covered in more detail in Section 10.

4: When should sand control be implemented?

For reservoirs which can be initially produced sand free or with tolerable sand levels and for which the risk of sand failure as a function of time has been established, the decision when to implement sand control should depend on an economic evaluation. The merit of deferring sand exclusion will be a function of the following:

·Advantages:

-Higher well productivity

-Lower initial capital costs (well completion)

·Disadvantages:

-Increased operating costs

-Higher capital costs (surface facilities)

-Deferred production if massive sand failure occurs

-More difficult to install sand control completion when sand failure occurs. Experience shows that all sand control methods cause various degrees of productivity impairment. This will result in output restrictions if the drawdown cannot be increased to compensate for the productivity loss. Production availability assumptions should explicitly take into account impairment effects due to sand exclusion. If sand control is deferred, the production system may have to be upgraded for sand tolerance resulting in higher initial capital expenditure. The higher operating costs will be a reflection of the additional production operations support required.

5: Other considerations Flexibility in reservoir management: Installation of sand control equipment may limit the ability to carry out remedial treatments such as shutting off depleted and gas or water producing zones. Complexity of well completion: This may become an important consideration in the case of multiple completions. Some sand control methods have a tendency to congest the wellbore with a lot of hardware. This may limit the access to adjacent zones or limit the number of zones that can be completed in the same well. It will also be more difficult to workover these wells. Well history: Is it a new well or a workover? In case of a remedial treatment, the prime issue may become the economic viability of a sand control job. Also the chances of successful remedial sand control will be lower if the well has already produced much sand.

2.3 What if initial sand control is not justified

If initial sand control is not justified, it must be ensured that production operations will not be jeopardized when sand production is experienced. The following points need to be addressed:

·Design of the production system (well and surface facilities) for the expected sand production levels

·Operating practices to prevent sand production

·Monitoring and operating procedures for the production system

·Review of planned subsurface operations for their possible influence on sand production (e.g. acid stimulation, additional perforation).

·Sand prediction studies in the light of future production experience and development plans. This may require laboratory studies or dedicated production tests.

·Contingency plans for the event that massive sand production occurs in one or more wells. For instance when failure is related to the onset of water production, many wells may fail over a short period of time.

3 Selection of a sand control method

Once the decision to install sand exclusion has been taken, the problem of selecting a sand control method remains. This is not always an easy task as in many cases several methods can be used.

Subjective views and 'cultural heritage' can easily influence the selection process. For instance, in the past ten years there has been a growing tendency to rely on cased hole gravel packs. This trend should be reversed as viable alternatives exist and field experience has shown that productivity impairment can be a major drawback of cased hole gravel packs. The selection of a method for a particular well or formation should be based on the following considerations:

·Which sand control method can be applied?

The basic sand control methods are briefly discussed in this section together with their respective merits, technical limitations and applicability.

·Which method is likely to yield the best productivity or at least the target production rates?

·What is the most cost effective method?

In many cases the optimum sand control method can only be determined after carrying out comparative field tests. Also the long term performance of sand control completions may only be measured after significant production experience has been collected. For many small fields requiring only few wells for development, this makes the selection of optimal sand control measures difficult and often requires the application of regional experience and correlation. It should be noted that project economics are mainly determined by early well performance.

3.1 Basic sand control methods

Sand control methods may be classified as mechanical exclusion, chemical consolidation methods or a combination of the two.

3.1.1 Mechanical methods

These methods rely on a downhole filter which stops the formation sand while allowing fluids to flow from or into the formation.

Downhole screen - Historically this was the first method of sand control used. Different types of screens can be used e.g. slotted liners or wire wrapped screens The openings in the screen are generally sized such that the formation sand will bridge on these openings. The "prepacked screen" is a more elaborate type of screen which incorporates a consolidated gravel sheath to control the formation sand.

Gravel pack - Properly sized gravel is placed against the formation and held in place by a screen which is normally of the wire wrapped type. This gravel is sized to stop the formation sand. The gravel pack can be placed either in perforated casing (IGP- Internal Gravel Pack ) or open hole (EGP- External Gravel Pack). The open hole is generally under reamed prior to placing the gravel. This can also be done after milling a section of the casing away (MCUGP- Milled Casing Underreamed Gravel pack).

Gravel packing is the most widely used sand control method and many different systems have been developed which differ in the design of the mechanical equipment, the properties of the completion fluids used and the way the gravel is placed. 

3.1.2 Chemical consolidation

Sand consolidation is a sand control method whereby fluids containing a cementing agent are injected into the formation to provide a bond between the sand grains after curing. As the sand grains are coated and bonded together by the cementing compound, some permeability will be lost following the treatment. The successful treatment must provide the additional strength required while preserving as much formation permeability as possible. Various types of chemical consolidation systems are commercially available. Sand consolidation systems are sophisticated chemical processes which require strict adherence to quality control for successful application.

3.1.3 New developments

Some new techniques show promise and are currently being investigated and developed. Some of these methods are still in the conceptual stage while others have been field tested.

1.Selective placement: The EPOSAND formulation has been modified (Wellfix DP 2000) to be compatible with the rubber elements of a selective placement tool (e.g. Nowsco's SPT tool) which is run on coiled tubing. This will allow a greater control of the placement of the resin in the formation. This new formulation is currently being field tested.

2.Chemical shut-off: The objective of this method is to shut off a sand producing zone with a plugging chemical placed with a selective placement tool. This method is at the field trial stage.

3.Expandable screens: The main idea is to run a screen in a collapsed state which can be subsequently expanded against the borehole and effectively stop formation sand. This technique is still at the research stage.

3.2 Comparative merits of sand control methods

Downhole screens

Advantages Simple, cheap and easy to install, especially slotted pipe. Good results quoted for specific cases (horizontal wells).

Disadvantages The finer the sand, the smaller the screen openings need to be and consequently the more difficult they are to manufacture and the more flow resistance is created. Screens are very susceptible to plugging during installation or during production (e.g. wax/scale deposition), this is particularly true for prepacked screens. Screens are very susceptible to wash outs due to erosion by sand (slotted pipe and WWS).Poor experience due to plugging and reliability problems. Poor quality control and assurance for prepacked screensApplicabilityTo be considered in the following cases :In economically marginal wells. When other methods are difficult to apply (e.g. horizontal wells).For exploration well testing. Water wells.

External gravel pack

Advantages Higher deliverability than comparable IGP's.(larger wellbore radius, no perforations to pack).More forgiving than IGP in case of placement problems.

Disadvantages Not generally recommended if shale layers are present (clay smearing when underreaming). Underreamed section needs to be fairly stable to avoid mixing of sand with gravel. Poor reservoir selectivity (less of a problem for MCUGP).Underreaming is a critical phase to avoid impairment (long exposure to fluids, cuttings present...).Repair after failure can be more complex than for an IGP.Generally more expensive to install than an IGP. Depth control is an operational problem in oil-rim situations. Applicability Widely applicable, the limiting factors are: Formation related considerations (presence of shale streaks...). Reservoir management problems (selectivity, water shut-off...).Well deviation, borehole size.

Internal gravel pack

Advantages The most widely used method. Excellent reliability and flexibility, used in many different configurations. Applicable in a wide range of formation types (e.g. sand/shale sequences, sands with high clay or silt content).Simpler to work "clean" compared to EGP (no drilling/ underreaming).

Disadvantages Low productivity is often observed although theoretically the PI should be comparable to that of perforated completions.Limits the scope for selective treatment of the production interval (i.e. water shutoff). Wellbore congested with hardware.Workovers are difficult as the screen, packers and gravel usually need to be removed prior to a repair or recompletion. The placement of gravel is more difficult than for EGP due to the smaller annular space and because the gravel needs to be squeezed through the perforations. Applicability Can be used for a wide range of applications. The limiting factors are: Completion complexity in the case of multiple zones.Well deviation, borehole size. Productivity impairment problems.

Chemical consolidation

Advantages Less apparent impairment than gravel packs. Sand consolidation can simplify multiple completions (no hardware obstructing he wellbore) and reduce workover costs. Job can be carried out without rig support (through tubing or coiled tubing treatment), hence potentially cheap.

Disadvantages Accurate and efficient placement of the treatment fluids is critical for success; the treated interval is limited to some 3-4 meters although this restriction may be overcome with the use of new tools/techniques for treating long zones. Treatment may only reduce sand production to an acceptable level but not fully prevent it. Treatment effectiveness may deteriorate with time. Interval to be treated should be properly cemented. Chemicals used require care in handling. Quality management crucial to treatment success. Applicability Consolidation methods generally prefer a relatively clean formation sand (i.e. less than 20 % clay).Best applied where formation permeabilities are greater than 500 mD. Low permeability sands may be unacceptably impaired by the consolidation treatment. Formation temperatures to be within the working range of the consolidation system. Selective placement tools may help enhance flexibility and applicability of the system enabling through-tubing completions to be carried out where applicable.

3.3 Data required for the selection of a method

Selecting the appropriate sand control method requires a detailed knowledge of the subsurface conditions and well data. Local operational and cost considerations may also play an important role. The data required is listed below:

·Reservoir data:

-Depth, pressure depletion forecast and temperature.

-Formation fluids: gas, oil, water, salinity, corrosive components etc.

-Porosity, permeability, reservoir heterogeneity, presence and extent of shale barriers.

-Rock strength, mineralogy, compatibility with completion fluids.

-Distance to gas and water contacts and expected movements.

-Drainage of sub reservoirs, differential depletion.

-Requirement for remedial treatments (stimulation, water shut-offs), recompletions.

·Well data:

-Type of well (producer, injector), location (land, offshore).

-New completion or workover.

-Completion interval length, configuration (cased/open hole, wellbore size).

-Single or multiple completion, selective production required.

-Artificial lift equipment.

-Well history.

·Production data:

-Target flow rate.

-Field production history, amount of sand produced.

-Development/timing of water or gas entry.

-Impairment data for the sand control methods considered.

·Operational data:

-Allowable sand levels.

-Availability of rig/workover hoist.

-Completion costs.

-Expertise available.

4 Sand exclusion and productivity impairment

As discussed in the introduction to this manual experience shows that sand control completions often restrict the flow of fluids from the reservoir. The severity of this problem varies from case to case and may not result in immediate deferred production if the well can be beaned up to compensate for the loss of productivity. In general however, productivity impairment is a cause of great concern as it directly affects production potential, short term deliverabilities, the number of drainage points required, the timing of installation of artificial lift, abandonment pressure and ultimate recovery. Until significant impairment reductions can be achieved, well productivity considerations will play a major role in the decision to install sand control and in the selection of a sand control method.

Many different mechanisms may contribute to restrict well productivities and these are not always fully understood. This is particularly true for sand control completions and many studies are currently ongoing to better understand and combat this phenomenon.

The purpose of this chapter is to offer a broad overview of how impairment of sand control completions may occur and how impairment and productivity of sand control completions can be modelled.

4.1 Mechanisms of impairment

The following factors may contribute to impairment of productivity through sand control completions:

·Formation damage.

·Limited penetration of the pay zone.

·Flow restrictions introduced by some items of the sand control completion.

Formation damage can be defined as a reduction of the original permeability of the reservoir rock near the wellbore. Any field operation may be a source of formation damage at all phases of the life of a well i.e. during drilling, completion, workover, production and stimulation operations.

Drilling and cementing through the reservoir generally creates a zone flushed with mud or cement filtrate in the immediate vicinity of the wellbore. The natural permeability of this zone may be affected due to the interaction of the invading fluids with the reservoir rock (e.g. particle invasion, mobilisation or clay swelling). In order not to restrict the flow of hydrocarbons from the reservoir into the wellbore, this damaged zone has to be bypassed (by perforating or fracturing) or must be removed (by underreaming or perforation washing).

The basic goal of excluding the movement of sand is generally incompatible with the unrestricted flow of fluids. The factors contributing to impaired productivity of sand control completions are reviewed below. The objective of this discussion is to emphasize that sand control completions inherently carry a potential for productivity impairment and that great care needs to be exercised to minimise impairment during the design and execution of a sand control job.

4.1.1 Chemical consolidation

The permeability of the treated zone around each perforation will inevitably be affected because the formation sand grains in the near wellbore area are coated with the cementing compound. Also the reservoir might be partially penetrated because the total length of a consolidated interval is limited to ensure proper placement of the treatment, resulting in an additional pressure drop in the near wellbore region. Other factors which influence the performance of a chemically consolidated interval are the perforation pattern (density and length), the effectiveness of the perforation operation and the placement of the treatment fluids.

In some cases however, the productivity of chemically consolidated zones is seen to improve with time. It is speculated that this "clean-up" effect is due to the limited failure of the consolidated zone.

4.1.2 Gravel pack

In general, productivity impairment of a gravel pack completion may be a consequence of the following factors:

·Mixing of sand and gravel at the formation interface when gravel packing or when producing the well.

·Gravel off specifications.

·Generation of fines in the gravel during transport and placement operations which subsequently reduce the reservoir and gravel pack permeability.

·Inadequate gravel selection due to poor sand sampling.

·Incompatibility of completion fluids with the formation.

The following factors, specific to either internal or external gravel packs, may also contribute to impairment.

·External gravel pack: In open hole gravel packs, the effective wellbore diameter is enlarged by underreaming. Provided there is a good permeability contrast between the formation sand and the gravel pack, the productivity of the zone should benefit from the increased wellbore radius (radial flow). Considerable care must be taken to ensure that underreaming does not itself induce significant formation damage. Dedicated underreaming fluids are used but a filter cake and a flushed zone will still be created because this operation is always carried out under overbalance conditions. It may be difficult to completely remove this filter cake prior to gravel packing.

·Internal gravel pack: In cased hole gravel packs, the perforation tunnels ideally are fully packed with gravel. Provided there is a good permeability contrast between the formation sand and the gravel, the flow through the perforation tunnels should not be significantly impeded. In practice, the well needs to be killed after perforating to retrieve the guns and to run the gravel pack equipment.

This can lead to formation damage if the kill fluid is not properly formulated or if loss control material is subsequently left behind in the perforations. Also, it is difficult to effectively pack all the perforations with gravel. Mixing of formation sand and gravel may then occur in those perforations which are not tightly packed, resulting in very high resistance to fluid flow.

4.2 Prevention of impairment

Removal of impairment is usually difficult and costly and the basic approach should be the prevention of productivity impairment. To achieve this objective, it is necessary to view the entire process of drilling, completion and production as a whole. A mishap at any stage of the well development may eventually result in productivity impairment.