Sand control Introduction

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Sand production from oil and gas reservoirs will occur when the stresses in the formation rock exceed the mechanical strength of the rock material. This can happen in both unconsolidated and clastic reservoirs due to a combination of: high drawdown, depletion, changes in near wellbore fluid composition and cyclic loading of the wellbore.

A Review of sand control 

1 The extent of the sand control problem

This section outlines:

1.the need for sand exclusion and,

2.the loss in productivity resulting from sand control methods. It further stresses the economic implications of sand control in terms of increased capital and operating costs.

1.1 The need for sand control

Sand production can cause a variety of problems with numerous technical, operational, environmental and economic implications. For example, sand exclusion, be it remedial or preventative may be required to:

1.ensure the integrity of the production system and minimise facility downtime resulting from equipment failures (e.g. artificial lift).

2.avoid sand failure which may lead to:

-formation impairment,

-downhole communication,

-buckling or collapse of production casing with the possible loss of the well.

1.2 Productivity impairment

Having evolved from practices inherited from the water well industry at the turn of the century, gravel packing is still recognised throughout the industry as the most effective means of primary sand control. However since the early days remarkably little has changed except, perhaps our definition of a "successful" gravel pack. Although it is true to say that we can now successfully reduce sand from our production streams to tolerable levels for most applications, many "successful" wells continue to suffer severe productivity loss due to the installation of sand control.

High impairment levels are clearly visible after:

1.well killing operations and,

2.initial gravel packing.

2 The total cost of sand control

In financial terms the magnitude of the problem is best illustrated by considering that the average reported completion efficiency from gravel packed wells is seldom greater than 40% (and often significantly lower).

This however is only part of the problem. Severe zone impairment can lead to the requirement to drill more wells, build more locations, install bigger platforms and lay more flowlines with obvious effect on unit technical costs (UTC), project cash flow and the environment.

Less tangible, but potentially equally important factors should also be considered. For example improved completion practices may lead to:

1.higher system pressures and deferment of lift facilities hence reduced capital exposure

2.lower abandonment pressures, hence higher recovery factors.

Again, all with a clear impact on development costs.

An example such as the above, in light of total Group operated production levels through sand excluded zones, highlights:

1.the scope and incentive for improvement, and

2.the need to clarify the "sand control process", allowing those involved in field development planning to fully recognise the implications of sand production, sand exclusion and handling.

B Planning sand control

Historically, sand control was seen as a sub-surface problem best kept downhole. As a result sand exclusion was routinely installed wherever a sand production problem was thought to exist, then or at some future point in time. Awareness of the cost and consequences of sand control was limited and mainly restricted to the Production Technology discipline. However as a result of increasing pressure on operating and capital costs and the consequent growing concern about the level of damage caused by sand exclusion attention has shifted from the purely precautionary exclusion approach to one in which the costs and risks of sand exclusion were balanced with the benefits of delayed or, altogether omitted sand control. The associated 'risk analysis' approach has resulted in many other disciplines becoming involved in the sand control decision making 'process'.

1 Phases of sand control

1.1 Sand control in the exploration phase

Exploration wells are primarily drilled to test the occurrence and producibility of hydrocarbons in new play. Additionally, wells of this nature also present the first opportunity to describe reservoir characteristics, drive mechanisms and to highlight other major reservoir uncertainties. Such opportunities however, should not only be used to gather fundamental reservoir and geological data but also to obtain an early assessment of potential (sand) production problems.

Prior to designing an exploration well, it is important to conduct a limited investigation into the regional history of sand failure. Obtaining an early impression of potential completion requirements is helpful in outlining initial completion and well test designs. This is particularly important in remote areas where pre-planning by nature is necessary, as this may involve the provision of specific services such as; tubing conveyed perforating, perforation washing, gravel packing etc.

In support of the drilling operation, the main contribution from the production technologist is two fold : completion and well test design. Briefly, he shares the responsibility for ensuring that production test objectives are fully met. This will normally involve: string design, selection of perforation method, provision of appropriate forms of artificial lift, stimulation design etc. If sand production is anticipated then the provision of sand production monitoring services may be considered, and in the event of a well test being interupted prematurely due to excessive sand production, a contingency sand exclusion plan should be available.

Data requirements specific to sand failure prediction and the design of a suitable sand control method should, if possible, be fully addressed at the exploration phase.

For example:

·Gathering samples of cuttings and core material during the drilling and evaluation phase of the well.

·Open hole logging requirements.

·Formation strength tests.

·Well testing, including destructive testing, and sand production monitoring.

It is stressed that in many cases this requires the justification to, for example: plugback, sidetrack and core the reservoir to evaluate the necessary rock mechanical parameters. It may also require the destructive testing of a well which is essentially capable of producing hydrocarbons. Production technologists should therefore, in the early stages, be fully prepared to justify the additional expenditure involved. This step is crucial, but is often overlooked, perhaps suggesting why in many existing developments sand production problems are identified late.

In summary, the exploration phase should be viewed as an opportunity to identify the risk of unmanageable sand production. It should also provide a list of unanswered questions, which will form the basic list of data requirements to be followed up during prospect appraisal. Without a proper well test this will not be possible, production technology input into exploration well design is therefore essential.

1.2 Sand control in the appraisal phase

The final decision on sand control cannot be made until after projected economic profiles for the available options are compared. This process commences at the end of field appraisal, but, can only be achieved with the provision of the basic parameters for all physical and subsequent economic models. Following on from an exploration success, the appraisal phase, as previously discussed, concentrates on clarifying our understanding of the reservoir, well performance and the filling of key data gaps.

a. Sand failure prediction

The delineation of reservoir size and properties is the prime objective of drilling and testing appraisal wells. Additionally, when assessing the risk of sand failure, it should be noted that a sand failure prediction study is particularly important in marginal cases where the reservoir material is neither completely unconsolidated nor fully competent. In such cases the value of a prediction study should not be underestimated, and in the context of optimising notional development plans is considered crucial.

Complete and reliable data gathering at the earliest possible moment is a highly profitable investment to support early decision making. Maps, petrophysical logs, well test results, fluid property data, core measurements (including rock mechanical data) are all used to understand and model the sub-surface. Specific data requirements for sand failure prediction studies include data acquired from openhole logs, core material and field tests.

Principally during the definition of a project there is significant scope for optimisation and cost reduction. The importance of early data in optimising field development plans cannot therefore be over stressed, as following on from the definition phase, projects become resource intensive, as detailed design, materials procurement and construction stages commence.

Maps, petrophysical logs, well test results, fluid property data, core measurements (including rock mechanical data) are all used to understand and model the sub-surface. Specific data requirements for sand failure prediction studies include data acquired from openhole logs, core material and field tests.

For example:

1.log derived data - including porosity, permeability, etc,

2.lab derived data - from cores including UCS and TWC strengths, mineralogy, fluid compatibility, grain size distribution,

3.well test data - sand influx, and in-situ stress testing.

The acquisition, storage and maintenance of relevant data in accordance with the project common data base is the responsibility of the field production technologist.

Once the basic data is collected an assessment of the risk of sand failure can be made via semi-empirical rock mechanical and other geophysical (and reservoir performance) models leading to well failure scenarios. However, it should be stressed that quantification of event probabilities and assessment of risk is a complex problem.

Although it is perhaps unrealistic to expect a sand failure prediction scenario to predict individual well failure under a given set of conditions within, say a specific year, such a scenario will give a better impression of likely ranges, allowing breakeven type calculations to be used in early decision making.

b. Sand exclusion scenarios

The appraisal campaign should also be used as an opportunity to make early evaluations of various sand control methods, including:

1.passive techniques i.e. selective perforating,

2.screens only,

3.gravel packing,

4.chemical consolidation,

5.horizontal wells, etc.

Attention should focus on quantifying:

·The effect of sand control options on well potential and ultimate recovery.

·Reservoir management considerations including the number of zones to be completed, water or gas breakthrough and selectivity.

·Expected levels of sand production in connection with artificial lift selection, process design and eg. allowable tubing velocity.

·Reliability and maintainability.

·The initial cost and ease of installation.

When these issues have been addressed and well performance is understood, integrated field performance models incorporating production offtake scenarios can be assessed.

It is incumbent on the field production technologists to fully programme these activities and justify resource (CTR) requirements prior to obtaining expenditure authorisation and initiating a data gathering programme.

A comprehensive and timely sand failure prediction study will allow a quantitative assessment of the likelihood of sand failure. Furthermore the effect of sand control on development options should be fully assessed and be apparent at the end of the appraisal phase. However, the difficulties associated with the quantification of uncertainties should be made clear to the various disciplines involved in project design.

1.3 Operations philosophy and process design

In the past downhole methods have proven reliable in "eliminating" sand from our production streams. Consequently, sand handling was not an important criterion for process design or operational considerations within the industry. This is reflected by the conspicuous lack of design guidelines in this area.

Poor productivity associated with traditional forms of primary sand control has led a number of operators to advocate acceptance of the risk of sand failure in completion design. This strategy, presents a number of potential problems for process design and production operations:

·Erosion/corrosion of surface pipe work and facilities.

·On-line/batch sand separation and disposal facilities.

·Pigging requirements.

·Uncertainty over the timing of artificial lift.

·The need for reliable, cost effective remedial sand exclusion techniques to be used, in the event of a well failure.

·The need for sand production monitoring and detection equipment.

It is therefore important to address such issues in conjunction with sub-surface development options early in the development life cycle. Detailed process engineering design should begin only when off take policies, well requirements and production philosophies have been agreed. Associated cash flows for the preferred development options can be compared, and selected on the basis of UTC and associated risk.

1.4 Development plans

The overriding aim early in a field's appraisal is to establish the optimum production plan by evaluating a number of development options. This results in a conceptual field development plan, the purpose of which is to translate sub-surface uncertainty into fit-for-purpose infrastructure and production facilities. This should be an unambiguous, live document subject to regular review.

Before any final decision is taken during formulation of a sand production philosophy (e.g. defining tolerable levels of sand production) a series of important inter-related issues have to be considered, and incorporated as project sensitivities.

For example:

1.Are the risks associated with delaying the installation of sand control acceptable ?

This of course will vary from one area to another, but is particularly important:

-If at any stage the safety or integrity of the production system could be compromised.

-When considering unmanned installations.

-In fields where the cost of remedial action is high (e.g. subsea wells).

2.Where there is risk of failure, will numerous wells fail within a similar time frame, with the loss of field potential ?

Clearly, such questions can only be answered by an integrated team approach aimed at reaching a balanced solution. For example, if the option not to install sand control is selected, contingency sand exclusion measures should be readily available - and their suitability regularly reviewed.

Once sand exclusion requirements have been identified and agreed, production technologists are faced with the problem of sand exclusion selection and its impact on well design and development philosophy. This exercise will encompass a whole series of technical investigations ranging from: reservoir drilling fluid selection to material and equipment specification. Such studies conducted in conjunction with drilling (completion) engineering and service companies, should result in completion requirements and procedures outlined in the field development plan and supporting documentation.

1.5 Installation of sand control

When the final decision on whether or not to install sand control is made, relatively detailed completion designs will be compiled based on the field development plan. These will then form the basis for

1.completion service tender exercises,

2.material and equipment procurement and

3.support well proposals and detailed completion programmes.

It is stressed that this also applies in cases where a "lets wait and see" strategy is adopted and initially sand control is omitted. Such wells should be designed with total well functionality in mind, allowing cost effective remedial operations to be performed, if required, without jeopardising project cash flow. This may require the acquisition of contingency measures to be left "on the shelf".

It is again stressed that the production technologist is the key player in this activity - he/she is responsible for well completion design, and by definition he/she should coordinate the sand control method selection. Furthermore, the production technologist will be involved in the selection, supervision and general performance monitoring of sand control service companies, this is not solely the responsibility of the contract holder.

1.6 Operations and (re)development

A production surveillance programme is required to allow optimisation of the production plan. Well performance monitoring is a pre-requisite with or without sand control and will continually address: bean up behaviour, flow efficiency, pressure loss, sand production, cavity growth for each completion configuration selected. This will allow new and existing procedures/standards should be challenged, in the search for more reliable, flexible and cost effective completion and process alternatives. To ensure proper feedback and evaluation of techniques employed, appropriate monitoring of long term production is essential.

With respect to the operation of surface facilities it is equally important to have a similar monitoring programme. For example, in areas where limited sand production is tolerated, such as high velocity gas applications, it is considered essential to rigourously monitor, and predict the integrity of the well, flowline and process equipment.

2 Sand failure mechanisms

Sand failure mechanisms can be subdivided into

1.compressive failure as a result of depletion and drawdown,

2.tensile failure as a result of drawdown alone, and

3.erosion.

Compressive and tensile failure are being analysed by continuum modelling. The effect of erosion, the dislodging of discrete sand grains which was not previously considered is now also being investigated.

Further studies are underway to investigate to what extent modern geo-statistical and risk analysis techniques may aid in formulating correlations between core and log derived parameters.

Tolerable sand production: In the past a tendency has grown to prevent sand production at all costs. Taking into account the penalty in lost productivity, this trend has to be reversed. In many cases, a critical review of tolerable sand levels is warranted. A theoretical approach to the definition tolerable levels of sand production in a multiphase environment is extremely complex and can-not be expected to provide definitive design guidelines. A semi-empirical approach is being pursued. Dedicated field tests are underway to assess equipment damage due to sand production in oil wells.

Gravel packing: Although very successful in controlling sand production, this relatively mature technology often causes severe losses in well productivity. Effort has concentrated on optimising the process of gravel placement into perforation tunnels with the development pre-packing procedures.

Horizontal wells: Drilling and completing horizontal wells through unconsolidated sands is now a routine operation. Companies are rapidly developing their "own" technology to take full advantage of horizontal wells. A section has been included which reviews the important completion aspects when designing horizontal completions through unconsolidated sands.

Chemical consolidation: Field experience has proven that sand consolidation is an attractive and viable alternative to gravel packing, although it presently suffers from a more restricted application range. Extending the application of chemical consolidation by concentrating research resources on "opening the applications window" to include: increased temperature range, poorer quality reservoir and longer intervals, is being pursued.

Success with chemical techniques offers tremendous scope to rationalise existing completion designs in many areas, for example, chemical consolidation in combination with "monobore" as a cost effective form of initial and remedial sand control.

Operational and facility aspects of sand handling: Increasingly Companies are prepared accept some risk of sand production in an attempt to maximise production, and reduce development costs. Examples of this are extending velocity limits in gas wells , rationalising bean-up procedures, and enhancing the sand handling capacity of production facilities. Stringent monitoring is thus becoming more important. Unfortunately industry experience with sand monitoring has generally been disappointing. Poor system reliability and high sensitivity to changing production conditions (rate, GOR etc) have led to major operational and calibration problems.

3 Roles and responsibilities

Although the overall development plan by nature dictates a multi-disciplinary approach, the prime responsibility for managing and driving the process of sand control throughout the field life rests firmly with the production technologist.

In conclusion, the practical problem facing most companies is how to keep the process of sand control active and properly managed in all the relevant engineering functions, throughout the development cycle. Establishing process related performance measurements and a documented list of controls and procedures is therefore necessary. This will ultimately lead to an assurance that sand control gets the attention it deserves through appropriate setting of objectives, targets, responsibilities and regular review.

 

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