WO2019081948A1

WO2019081948A1 - Well design and associated well structures

Info

Publication number: WO2019081948A1
Application number: PCT/GB2018/053110
Authority: WO
Inventors: Michael Byrne; Lesmana DJAYAPERTAPA; Ken Watson
Original assignee: Lloyd's Register Group Limited
Priority date: 2017-10-27
Filing date: 2018-10-26
Publication date: 2019-05-02
Also published as: GB201717738D0; EP3701119A1; EP3701119B1; EP3701119B8

Abstract

There is described methods for well design, including well construction, associated well structures and well completion arrangements for use in a well. The method for designing a well may include providing a computational model of a well comprising a section of wellbore, surrounding formation, and a well completion arrangement positioned between the wellbore and surrounding formation. That well completion arrangement may define fluid paths between the surrounding formation and the wellbore, wherein the fluid paths provide an effective inflow area from the surrounding formation to the wellbore. The method may then comprise a. calculating fluid production from the surrounding formation, through the well completion arrangement, to the wellbore; and b. varying the effective inflow area of the well completion arrangement; and repeating steps a. and b. so as to determine a profile of well performance for that computational model of the well based on a range of effective inflow areas of the well completion arrangement, and determining a well design having a particular effective inflow area based on the determined profile.

Description

Well Design and Associated Well Structures

Technical Field Described examples relate to methods of well design, including well construction, associated well structures and, in particular but not exclusively, well completion arrangements for use in a well.

Background

In the oil and gas exploration and production industry, efforts are made to design well infrastructure that optimally delivers on well performance, while taking into consideration the well objectives (e.g. the type of fluids being produced), the subterranean formation, etc. Typically, industry standards or engineering biases dictate the particular type of well infrastructures that are used when completing a well in a particular type of formation. However, beyond the insight provided by analytical models that are used in well design, there remains limited real world understanding of the specific manner in which those wells function, over and above merely observing the well performance upon completion and recursively correlating that performance with an analytical model.

There is a continuing desire and need to improve well performance and completion arrangements in order to improve and optimise well design. This background serves only to set a scene to allow a skilled reader to better appreciate the following description. Therefore, none of the above discussion should necessarily be taken as an acknowledgement that that discussion is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the invention may or may not address one or more of the background issues.

Summary

There are described methods of well design, including well construction, associated well structures and, in particular, new and improved well completion arrangements that may help improve well performance and help optimise well design. In some examples, there is described a method for well design, e.g. optimal well design. The method may comprise providing a computational model of a well. That computational model may comprise a section of wellbore and surrounding formation.

In some cases, the model may comprise a well completion arrangement positioned between the wellbore and surrounding formation. Such a well completion arrangement may define fluid paths between the surrounding formation and the wellbore. Those fluid paths may provide an effective inflow area between the surrounding formation and the wellbore. The method may comprise calculating fluid flow between the surrounding formation and the wellbore, e.g. via the well completion arrangement. In some examples (e.g. production examples) this may include calculating fluid flow from the surrounding formation and the wellbore. In other examples (e.g. injection examples) this may include calculating fluid flow from the wellbore to the surrounding formation.

The method may further comprise varying the effective inflow area of the well completion arrangement. The method may then comprise repeating the step of "calculating" and "varying", accordingly, which may permit determination of a profile of well performance for that computational model of the well, e.g. based on a range of effective inflow areas of the well completion arrangement. From that determined profile, a well design (e.g. optimal well design) may be provided (e.g. having a particular effective inflow area).

The method may comprise repeating the steps of calculating and varying so as to provide a profile of well performance across the range of effective inflow areas of the well completion arrangement, and whereby the effective inflow area of the desired well design (e.g. optimal well design) relates to a region associated with a point of inflection of the profile. The effective inflow area of the optimal well design may provide for maximum fluid production/injection for that modelled wellbore and surrounding formation. The determined effective inflow area may be between a lower inflow area threshold, for example below which the inflow area corresponds to that of well completion arrangements comprising inflow control devices for control of fluids, and an upper inflow area threshold, for example above which the inflow area corresponds to that of well completion arrangements configured as standalone screens.

The profile of well performance may be provided in terms of productivity index.

In some examples, the modelled well completion arrangement may comprise a gravel pack, modelled to have an effective inflow area provided by that gravel pack. The modelled well completion arrangement may comprise a plurality of inflow ports, defined through a well completion tubular or housing surrounding a base pipe, or the like. In some cases, the effective inflow area may be based on the number and size of the inflow ports. For examples, increasing the effective inflow area may be achieved by increasing the number of inflow ports, or the like.

In some examples, at least one effective inflow area used in the range of effective inflow areas (i.e. when modelled) may be selected, such as initially selected, based on the diameter of the well completion tubular, and or the wellbore diameter. The range of effective inflow areas may be selected based on one or more of: rate of desired fluid production, well length, or formation properties. The computational model may comprise a plurality of well completion arrangements, axially aligned and connected lengthwise.

The effective inflow area of each well completion arrangement may be individually varied in order to determine the well design, e.g. optimal well design. In some examples, the well design may comprise at least a first well completion arrangement having a first effective inflow area and a second well completion arrangement having a second effective inflow area. The second effective inflow area may be different from the first effective inflow area. In some examples, the number and/or size of inflow ports may be different between first and second well completion arrangements.

The computational model may comprise multiple well completion arrangements being modelled such that fluid is permitted to flow along the multiple well completion arrangements in an annulus provided between the modelled wellbore and surrounding formation. In other similar words, the model may comprise sections without annulus packers, or the like. The method may comprise associating a particular viscous resistance with the well completion arrangement and/or surrounding formation in order to calculate fluid production/injection, e.g. between the surrounding formation, via the well completion arrangement, to the wellbore. In some examples, the computational model of the well is a computational fluid dynamic (CFD) model.

There is also provided a well design. Such a well design may have been determined using the method described. Such a well design may be stored in a computer file (e.g. non-transitory medium or carrier), or may be provided in hardcopy format. The well design may be an optimal well design (e.g. having a particular effective inflow area based on a determined profile).

There is also provided use of such a well design, e.g. use of an optimal well design, for the design of a well. Such a designed well may comprise well completion arrangements having effective inflow areas.

There is also provided a method of well construction, e.g. optimal well construction. Such a method may comprise determining or using a well design (e.g. optimal well design having a well completion arrangement of a particular effective inflow area based on a profile of well performance, where the profile of well performance may have been determined from expected fluid production/injection based on a range of effective inflow areas. The method may further comprise completing a section of well, e.g. by positioning the well completion arrangement between wellbore and surrounding formation.

There is also provided a well comprising a section of wellbore, well completion arrangement and surrounding formation. The well may be configured such that fluids are communicated between the surrounding formation, via the well completion arrangement, and the wellbore. When producing, fluids may be communicated from the surrounding formation to the wellbore. When injecting, fluid may be communicated from the wellbore to the formation. In any event, the well completion arrangement ay serve to define fluid paths between the surrounding formation and the wellbore, those fluid paths providing an effective inflow area between the surrounding formation and the wellbore. The effective inflow areas may be provided so as to optimise well performance. In some examples, the effective inflow area of the well completion arrangement may provide maximum fluid production for that wellbore and surrounding formation. The well completion arrangement may comprise a gravel pack, autopack, or the like, having an effective inflow area provided by that pack. The well completion arrangement may comprise a well completion tubular or the like, having a plurality of inflow ports defined therethrough. The effective inflow area of the well completion arrangement may be based on the number and size of the inflow ports. The number and/or size of inflow ports may be based on the diameter of the well completion tubular, and/or the wellbore. For example, the greater the diameter, the greater the effective inflow area designed. The well may comprise a plurality of well completion arrangements, which may be axially aligned and connected lengthwise. The well may comprise a first well completion arrangement having a first effective inflow area and a second well completion arrangement having a second effective inflow area. The second effective inflow area may be different from the first effective inflow area. The multiple well completion arrangements may be provided such that fluid is permitted to flow along the multiple well completion arrangements in an annulus provided between the wellbore and surrounding formation. There is further provided well completion arrangements. Such completion arrangements may have an effective inflow area usable with the well described. Such completion arrangements may be specifically confirmed to optimise well performance (e.g. provide a particular PI).

There is further provided a program product or computer file configured to provide the method described. In some examples, there is provided a method for optimal well design. The method may comprise:

providing a computational model of a well, the computational model comprising a section of wellbore, surrounding formation, and a well completion arrangement positioned between the wellbore and surrounding formation, the well completion arrangement defining fluid paths between the surrounding formation and the wellbore, wherein the fluid paths provide an effective inflow area between the surrounding formation to the wellbore; and then

a. calculating fluid flow between the surrounding formation and the wellbore, via the well completion arrangement; and

b. varying the effective inflow area of the well completion arrangement; and repeating steps a. and b. so as to determine a profile of well performance for that computational model of the well based on a range of effective inflow areas of the well completion arrangement, and

determining an optimal well design having a particular effective inflow area based on the determined profile. There is also provided a method for determining a well length, e.g. an optimal well length. The method may comprise providing a computational model of a well have a section of wellbore and surrounding formation, and calculating fluid production/injection between the surrounding formation to the wellbore for a particular length of well. The method then may comprise varying the length of the well, and determining an optimal well length based on calculated fluid production/injection, or at least confirming that further well extensions would not improve performance, for example. The method may comprise determining a profile for different lengths of well, for example. In some examples, there is provided a computer program product or computer file configured to at least partially (or fully) implement the apparatus and methods as described above. In some examples, there is also provided a carrier medium (e.g. non- transient carrier) comprising or encoding the computer program product or computer file. The program or file may be non-transitory. In some examples, there is also provided processing apparatus when programmed with the computer program product described. Some of the above examples may implement certain functionality by means of software, but also that functionality could equally be implemented solely in hardware (for example by means of one or more ASICs (application specific integrated circuit) or Field Programmable Gate Arrays (FPGAs)), or indeed by a mix of hardware and software (e.g. firmware). As such, the scope of the disclosure should not be interpreted as being limited only to being implemented in software or hardware.

The invention includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. As will be appreciated, features associated with particular recited embodiments relating to apparatus may be equally appropriate as features of embodiments relating specifically to methods of operation or use, and vice versa.

It will be appreciated that one or more embodiments/aspects may be useful in well design, including well construction, and may help improve well performance and optimise well design and well completion arrangements.

The above summary is intended to be merely exemplary and non-limiting.

Brief Description of the Figures

A description is now given, by way of example only, with reference to the accompanying drawings, in which :-

Figure 1 shows an example of a well having a wellbore and well completion arrangements;

Figure 2 shows profiles of well performance;

Figure 3 shows a flow diagram of a method for designing a well; and

Figure 4 shows an example of a well having different well completion arrangements.

Description of Specific Embodiments

Figure 1 shows a simplified exemplary representation of a well generally identified by 10 extending through a subterranean formation 20 (e.g. a hydrocarbon-bearing formation). Here, a wellbore 30 (e.g. comprising production tubing) permits the production of fluids to surface 40 in a known manner. In addition, and as is shown in Figure 1 , the well 10 has been designed to have multiple well completion arrangements 100, which are positioned between the wellbore 30 and surrounding formation 20, as will be further described. It will be appreciated that such well completion arrangements 100 may be provided in sections that, when completed, are connected together along some or all of the length of the well 10.

While the well 10 of Figure 1 may be considered to show a deviated or horizontal well configuration, it will be appreciated that in other examples the apparatus and methods described herein may be used in vertical, or near-vertical, sections of a well 10, and indeed in mother bores and/or laterals, fishbones, etc., as may be expected. Further, the examples described herein may be used across a range of completion arrangements, including dual or multi-completions, as will be appreciated by a skilled reader. Further still, the examples described here have been given with reference to oil and gas production, but of course in other examples, the apparatus and methods may be used with other fluids being produced and/or injected. A skilled reader will readily be able to implement those various alternative embodiments accordingly. Typically, during the design phase of such a well 10, consideration is given to the type of well completion arrangements 100 used along the length of the well 10. Often, such well completion arrangements 100 are selected based on desired well performance and expected conditions at the formation 20. Therefore, where the formation 20 may be predominantly sandstone, or the like, screens (e.g. standalone screens, "SAS") may be installed in order to filter, and so mitigate the risk of, sands being produced in the event of fines being liberated at the formation. Other sand control methods include gravel packs, autopacks (e.g. where an annulus is filled with formation sand), or the like. Sand, or the like, that is produced can of course be harmful to well infrastructure and processing equipment, and affect long term well performance. Ideally, where methods are required in order to control sand production, then there is a desire to minimise any restriction to flow from the formation 20 to the wellbore 30. In other words, where sand control may be necessary, it nevertheless is understood that the minimum restriction to flow, while meeting sand control requirements, would be preferable. In doing so, the performance of the well can be maximised, and the restriction - although necessary for sand control - affects as little as possible the overall well performance. Simplified representations of standalone screens 100a used for sand control are shown in Figure 1 , by way of an example. Again, in other examples, dual or multi-completions may equally be used, depending on circumstance.

One exception to using solely sand screens or the like, of course, is in circumstances in which, due to the nature of the formation 20 and well structure, it may be expected that unwanted fluids could be produced during the normal production (e.g. gas-coning and/or water-coning at the heel of the well 10). In those circumstances, a well 10 can be designed such that well completion arrangements 100 may be used that comprise a plurality of inflow control devices or valves (ICDs and/or ICVs), which operate to control and restrict fluid being produced from the surrounding formation 20. Such well completion arrangements 100 comprising ICDs can be set at surface for a particular pressure drop across the devices at the time of running the well completion arrangement 100, or indeed can act autonomously so as to be open or closed when certain conditions are observed. Well completion arrangements 100 having ICVs can also be controllable, e.g. from surface, in the event of an observed production of unwanted fluids, or to control fluid conditions within the wellbore, i.e. choke inflow at certain sections of the well. In any event, the function of such ICD/ICV arrangements is generally to control and restrict the flow of unwanted fluid into the wellbore 30. Figure 1 shows a simplified example of such well completion arrangements 100b, which would comprise a plurality of ICDs/ICVs (not shown for ease). In order to provide zonal isolation, packers 1 10 or the like may be required in order to isolate the annulus provided between the formation 20 and the well completion arrangements 100b from that of axially adjacent annulus (i.e. and so provide zonal control).

Where ICD/ICV completions 100b are not required, the conventional well design wisdom dictates that minimal or no restriction to the flow be positioned between the formation 20 and the wellbore 30 so as not to unduly affect well performance. It will be appreciated that well performance may be measured in terms of a productivity index (PI). Such well performance (e.g. PI) may be considered to be a measure of the volume of fluid produced for each unit of pressure that acts (e.g. that is employed) to encourage fluid flow to surface 40.

A typical process for a competent well engineer when designing such a well 10 includes initially considering the well objectives, such as productivity index, drainage requirements, fluids being produced, likely operations performed (e.g. acid injection, fracturing, etc.). This provides some constraints on the type of infrastructure that may be used in the well 10.

Subsequently, based on formation 20 data (e.g. rock type, permeability, porosity, etc.) the options for well completion arrangements can be considered. These may include, for example, cased or perforated arrangements, open hole arrangements, or arrangements used from sand control, e.g. screens and/or gravel packs. The specific type of well completion arrangements 100 that may be used can be down-selected based on the desired well objectives. A simplified analysis or 1 D nodal analysis (e.g. using Darcy laws) is then implemented, based on the formation data available in order to determine inflow potential at the wellbore 30. Based on that analysis, the various sections of well completion arrangement 100 (e.g. 100a, 100b) can be selected together with filter arrangements (e.g. filter meshes, wires, screens, or the like), required valve arrangements, packer positioning and type (e.g. swellables), etc., as may be required. An approximation of actual "real world" well performance can then be provided by making analytical adjustments and offsets to the modelled well performance based on, for example, expected skin factor (e.g. an approximation of the "real world" damage that may limit the ability for the formation 20 to produce fluids - such damage to the formation 20 having taken place during drilling, completing, etc.).

A simplified approximation of the well performance for a given well completion arrangement 100a, 100b (or indeed section of well 10 having multiple well completion arrangements 100) is shown by the solid-line profile 200 in Figure 2. In Figure 2, well performance (e.g. PI) is shown on the vertical y-axis, while overall restriction to flow from the formation 20 to the wellbore 30 through well completion arrangements 100 is shown on the horizontal x-axis.

It will be appreciated that the overall restriction to flow (or pressure drop) provided by well completion arrangements 100a, 100b may be approximated by an "effective inflow area". That is to say that the well completion arrangement 100a, 100b may be considered to define fluid paths (e.g. a plurality of fluid paths) between the surrounding formation 20 and the wellbore 30, and that those cumulative fluid paths provide an overall effective inflow area, or otherwise permeability, from the surrounding formation 20 to the wellbore 30. It will be appreciated that an effective inflow area may be approximated for various different types of well completion arrangements (e.g. from ICD/ICV completions to gravel packs, standalone screens, hydraulically enhanced sand screens, or the like). In some examples, the effective inflow area may be considered to be an aggregate of the collective inflow areas of ports or nozzles, or the like, at the completion. Those ports/nozzles may be uniform in size such that increasing number of ports/nozzles increases the effective inflow area.

As is shown in Figure 2, the expected well performance remains low when using restricted completion including ICDs/ICVs. This is generally considered to be an acceptable trade off. On one hand, such completion 100 may restrict the immediate maximum well performance, but on the other hand it does permit control of fluids. As the effective inflow area increases (i.e. along the x-axis) the well performance improves until essentially plateauing at a maximum well performance. It is expected that open hole arrangements, and to some extent very open sand screens, operate at this well performance. Therefore, conventional wisdom dictates that, where possible, the restriction to flow from the formation 20 to the wellbore 30 should be as minimal as possible so as not to unduly affect well performance.

However, contrary to received wisdom, the inventors have postulated that the "known" profile 200 shown in Figure 2 is not an accurate representation of the restriction/performance profile as occurs in a "real world" environment. As will further be described below, in many cases it is proposed that reducing the restriction (e.g. increasing the "openness") of well completion arrangements 100a, such as sand screens, can in fact be detrimental to well performance.

Figure 2 shows a further example of a profile 300 of well performance (shown in broken lines) as predicted and observed by the inventors. Here, the expected well performance remains low when using restricted completion including ICDs/ICVs, as before. However, unlike the previous assumptions, as the effective inflow area continues to increase, beyond that used in typical ICD/ICV completions 100b, the well performance potentially improves beyond that expected by the "standard" profile 200. However, as the effective inflow area continues to increase, a point of inflection is reached (i.e. a maxima 310), after which further effective inflow area has the unexpected effect of reducing well performance. Without being bound by theory, it is believed that excessive flow entering the wellbore 30 from the formation, via minimally restricted flow paths, may in fact cause undue turbulence in the wellbore which can cause poor pressure and flow performance. As such, it has been proffered that the implementation of standalone screens or other similar well completion arrangements 100a may, in fact, be non-optimal, and may provide poorer well performance than would otherwise be obtained by a particular formation 20 (as is shown by the profile 300 shown in Figure 2).

The profile 300 shown in Figure 2 provides an exemplary approximation of well performance and effective inflow area when negative effects of the interaction of fluid flow into the wellbore 30 are taken into account. The result of which suggests that an optimal well design can be obtained in which, rather than implementing standalone screens or the like, alternatively well completion arrangements 100a are configured to have a particular effective inflow area based on the determined profile 300. That effective inflow area may be greater than that of ICD/ICV completion, but less than that of standalone screens, or other such sand control completion arrangements 100a. In light of the above, an alternative methodology for well design has been developed by the inventors, which can be used to optimise that well design. One example of a method 400 is shown in Figure 3, which is implemented computationally. Typically, and following the example in Figure 3, the design of such a well 10 may initially include the development of a computational model 410. Here, the model of the well 10 may comprise the section of wellbore 30, surrounding formation 20, and a well completion arrangement 100 positioned between the wellbore 30 and surrounding formation 20. As explained above, the well completion arrangement 100 can define fluid paths between the surrounding formation 20 and the wellbore 30, whether that be in the form of inflow areas defined in well completion tubulars, and/or defined by the flow paths in gravel packs, autopacks, or the like. In any event, the fluid paths can be considered to provide an effective inflow area (e.g. permeability) from the surrounding formation 20 to the wellbore 30.

It will be appreciated that the model may be a partial section model, a 2D model, or indeed a 3D representation of a section of the well 10. In this example, the model may be implemented using computational fluid dynamics. In other cases, alternative numerical analysis may be used. In some cases, however, the model may approximate components, such as the well completion arrangement 100, by associating a viscous resistance with those components, as described by the inventor's UK and US granted Patents GB251541 1 , GB2474275 and US8849637. As such, a single fluid CFD model that is able to model fluid flow from formation 20 through to wellbore 30 may be implemented.

In these and other examples, the specific geometry of the components in the well may be modelled together with the overall geometry of the well. In other words, rather than modelling the geometry of the overall well structure and then approximating the performance of components in the well (e.g. using set parameters in a model), in some examples the specific geometry (e.g. size, inflow area, orientation, etc.) of the components themselves may be provided in the model too, together with the overall length of completion, etc. This means that the model of the well may comprise both the geometry of the well at a relative macro scale, as well as the geometry of the well components at a relative micro scale.

In any event, subsequently, the well completion arrangements 100 can be selected having particular effective inflow areas for initial calculation of well performance. Depending on other well 10 conditions, the initial selection 420 of the effective inflow area may be provided by inflow ports of known sizes, e.g. commercially available ICD/nozzle arrangements (i.e. devices having known diameters given that these can be used with ease in any finally-designed completion 100). The number of inflow ports or the like may be selected, which may be defined in a number of different configurations, such as through a base pipe or completion tubular, provided by tortuous paths, axial tubes or nozzles, in a housing surrounding a base pipe, or the like, in a known manner. Initial considerations suggest that a relationship between the effective inflow area and the diameter of the well completion arrangement 100/wellbore exists, such that the maximum well performance may be achieved in many circumstances using a common ratio between inflow area and diameter. In some cases, the ratio may be of the order such that the effective inflow area is around half the diametric area of the wellbore/completion tubing. Of course, while this may be true in many cases, in some other cases other factors may prevail. Nonetheless, beginning at a ratio along these lines may assist with initial consideration of optimal effective inflow area.

After the size and number of inflow ports is selected, the fluid being produced from the surrounding formation 20, through the well completion arrangement 100, to the wellbore 30 can be calculated 430 for a given effective inflow area. This may permit calculation of well performance for that given effective inflow area, and the initial compilation of the profile 300 shown in Figure 3 for those particular well conditions.

In the event that a maxima 310 is not observed 450, the effective inflow area of the well completion arrangements 100 can be varied 420, and the fluid being produced recalculated. It will be appreciated that a range of effective inflow areas may be used in order to develop the profile 300 of well performance. From that profile 300 of well performance, an optimal well design may be determined 460 having a particular effective inflow area (or range of possible effective inflow areas) based on the determined profile 300.

It will be appreciated that both the size and number of inflow ports may be varied in order to vary the effective inflow area. Further, it will be appreciated that in other well completion arrangements 100 the effective permeability (e.g. when using gravel packs, autopacks, etc.) may be varied in order to vary the effective inflow area.

It will also be appreciated that in the example profile 300 shown in Figure 2, a maxima is shown in a curved or otherwise parabolic profile 300 of well performance across the range of effective inflow areas of the well completion arrangement 100. However, it may be that in other scenarios, further points or inflection or variances in profile 300 are observed across the range of effective inflow areas. Nonetheless, an optimal solution may be obtained, which may likely be around the point of inflection of a section of the profile 300. Of course, it will be appreciated by the skilled reader that in some cases, the well design may be restricted such that, due to external commercial restrictions or constraints, only a selection of particular well completion arrangements 100 are available, e.g. having a range of predefined inflow ports of particular number/sizes. In those examples, the optimised well design may include the most effective well completion arrangement 100, based on the modelled analysis. Such well completion arrangements 100 may have an effective inflow area tending towards the maxima (e.g. as close as possible based on the completion 100 available), or may be most cost effective based on the potential performance gains versa difference in well completion costs.

In some cases, and as is shown in Figure 2, the determined effective inflow area may be between a lower inflow area threshold 320, below which the inflow area corresponds to that of well completion arrangements comprising inflow control devices/valves for control of fluids, and a upper inflow area threshold 330, above which the inflow area corresponds to that of well completion arrangements 100 configured as standalone screens. The well design may simply specify that the well completion arrangements 100 are configured to sit within that range. In that regard, an optimal well design may be determined having a well completion arrangement of a particular effective inflow area based on a profile of well performance, where the profile of well performance has been determined from expected fluid production based on a range of effective inflow areas. The "real life" well 10 may then be completing by positioning the well completion arrangement 100 between wellbore and surrounding formation and, where necessary, modifying the effective inflow area (e.g. using ICVs).

For example, the designed well completion arrangement may be configured for controllable inflow, e.g. comprising a plurality of inflow control valves, or the like, which may be operable to provide a variable effective inflow area. In those examples, the well completion design may specify the expected optimal effective inflow area together with a range of effective inflow areas such that, when deployed, the effective area inflow of the well completion arrangement can be controlled in order to fine-tune performance (e.g. in the event that optimal performance is observed to be at a slightly different effective inflow area than that calculated). In that regard, the methodology outlined in Figure 3 may be implemented without the need for a computer model, as such. In other words, an approximation of the optimal inflow area may be postulated, and well completion arrangements 100 deployed having the capability to vary their effective inflow areas. In use, those well completion arrangements may be modified so as to optimise performance (e.g. determining the profile 300 of well performance, experimentally).

Figure 4 shows an example of a section of well 1 5 comprising a section of wellbore 35, well completion arrangement 150 and surrounding formation 25, wherein the well 15 is configured such that fluids are produced from the surrounding formation 25, through the well completion arrangement 150, to the wellbore 35. Here, the well completion arrangement 150 defines fluid paths 152, 155 between the surrounding formation 25 and the wellbore 35, those fluid paths 152, 155 providing an effective inflow area from the surrounding formation 25 to the wellbore 15, as above. Again, the effective inflow areas 152, 155 are provided so as to optimise well performance. Here, two well completion arrangements 150a, 150b are axially aligned and connected lengthwise. As can be seen, a first well completion arrangement 150a has been provided with a first effective inflow area and a second well completion arrangement 150b has been provided with a second effective inflow area, the second effective inflow area being different from the first effective inflow area. In this example, the number and/or size of inflow ports is different between first and second well completion arrangements 150a, 150b. What is also noticeable, and is not limited to this example, is that fluid may be permitted to flow along the multiple well completion arrangements in an annulus 170 provided between the modelled wellbore and surrounding formation. In other words, no packers are required as the effective inflow areas are not intended to be used for zonal isolation. It will further be appreciated that, when implementing the above analysis, well infrastructure and methodology that, contrary to popular held belief, extending the length of a well may not provide improved well performance as inappropriate completion may limit well performance. For example, the method may comprise providing a computational model of a well have a section of wellbore and surrounding formation, and calculating fluid production from the surrounding formation to the wellbore for a particular length of well. The method then may comprise determining an optimal well length based on calculated fluid production, or at least confirming that further well extensions would not improve performance, for example. It will be appreciated that the above described methods of well design, including well construction, associated well structures and, in particular, new and improved well completion arrangements, may help improve well performance and help optimise well design. Based on the departure from previous well designs and conventional thinking on well design, particularly those provided in a sand formation, a well that is designed and constructed according to the above methodology would be readily apparent to a skilled reader. Further, the apparatus and methods may be used with other fluids being produced and/or injected (e.g. passing from wellbore to formation via completion). A skilled reader will readily be able to implement those various alternative embodiments accordingly.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the spirit and scope of the invention.

Claims

CLAIMS:

1 . A method for optimal well design, the method comprising:

providing a computational model of a well, the computational model comprising a section of wellbore, surrounding formation, and a well completion arrangement positioned between the wellbore and surrounding formation, the well completion arrangement defining fluid paths between the surrounding formation and the wellbore, wherein the fluid paths provide an effective inflow area from the surrounding formation to the wellbore; and then

a. calculating fluid production from the surrounding formation, through the well completion arrangement, to the wellbore; and

determining an optimal well design having a particular effective inflow area based on the determined profile.

2. The method according to claim 1 , wherein steps a. and b. are repeated so as to provide a profile of well performance across the range of effective inflow areas of the well completion arrangement, and wherein the effective inflow area of the optimal well design relates to a region associated with a point of inflection of the profile.

3. The method of claim 1 or 2, wherein the effective inflow area of the optimal well design provides the maximum fluid production for that modelled wellbore and surrounding formation.

4. The method according to claim 3, wherein the determined effective inflow area is between a lower inflow area threshold, below which the inflow area corresponds to that of well completion arrangements comprising inflow control devices for control of fluids, and an upper inflow area threshold, above which the inflow area corresponds to that of well completion arrangements configured as standalone screens.

5. The method according to any the claims 1 to 4, wherein the profile of well performance is provided in terms of productivity index.

6. The method according to any of the claims 1 to 5, wherein the modelled well completion arrangement comprises a gravel pack, modelled to have an effective inflow area provided by that gravel pack.

7. The method according to any of the claims 1 to 5, wherein the modelled well completion arrangement comprises a plurality of inflow ports, define through a well completion tubular.

8. The method according to claim 7, wherein the effective inflow area is based on the number and size of the inflow ports.

9. The method according to claim 8, wherein at least one effective inflow area used in the range of effective inflow areas is selected based on the diameter of the well completion tubular, and or the wellbore diameter.

10. The method according to any of the claims 1 to 9, wherein the range of effective inflow areas are selected based on one or more of: rate of desired fluid production, well length, or formation properties.

11 . The method according to any of the claims 1 to 10, wherein the computational model comprises a plurality of well completion arrangements, axially aligned and connected lengthwise.

12. The method according to claim 1 1 , wherein the effective inflow area of each well completion arrangement is individually varied in order to determine the optimal well design.

13. The method according to claim 12, wherein the optimal well design comprises at least a first well completion arrangement having a first effective inflow area and a second well completion arrangement having a second effective inflow area, the second effective inflow area being different from the first effective inflow area.

14. The method according to claim 13, when dependent upon claim 8, wherein the number of inflow ports is different between first and second well completion arrangements.

15. The method according to any of the claims 1 1 to 14, wherein the computational model comprising multiple well completion arrangements is modelled such that fluid is permitted to flow along the multiple well completion arrangements in an annulus provided between the modelled wellbore and surrounding formation.

16. The method according to any of the claims 1 to 15, wherein the method comprising associating a particular viscous resistance with the well completion arrangement, and a particular viscous resistance with the surrounding formation in order to calculate fluid production from the surrounding formation, through the well completion arrangement, to the wellbore.

17. The method according to any of the claims 1 to 16, wherein the computational model of the well is a computational fluid dynamic (CFD) model.

18. An optimal well design having a particular effective inflow area based on the determined profile, as provided by method of any of the claims 1 to 17.

19. Use of an optimal well design according to claim 18 for the design of a well comprising well completion arrangements having effective inflow areas.

20. A method of optimal well construction comprising,

determining an optimal well design having a well completion arrangement of a particular effective inflow area based on a profile of well performance, the profile of well performance determined from expected fluid production based on a range of effective inflow areas; and

completing a section of well by positioning the well completion arrangement between wellbore and surrounding formation.

21. A well comprising a section of wellbore, well completion arrangement and surrounding formation, wherein the well is configured such that fluids are produced from the surrounding formation, through the well completion arrangement, to the wellbore, and wherein the well completion arrangement defines fluid paths between the surrounding formation and the wellbore, those fluid paths providing an effective inflow area from the surrounding formation to the wellbore, and wherein the effective inflow areas are provided so as to optimise well performance.

22. The well according to claim 21 , wherein the effective inflow area of the well completion arrangement provides maximum fluid production for that wellbore and surrounding formation.

23. The well according to claim 21 or 22, wherein the well completion arrangement comprises a gravel pack or autopack having an effective inflow area provided by that pack.

24. The well according to claim 21 or 22, wherein the well completion arrangement comprises a well completion tubular, having a plurality of inflow ports defined therethrough.

25. The well according to claim 24, wherein the effective inflow area of the well completion arrangement is based on the number and size of the inflow ports.

26. The well according to claim 25, wherein the number and/or size of inflow ports is based on the diameter of the well completion tubular, and/or the wellbore, and wherein the greater the diameter, the greater the effective inflow area.

27. The well according to any of the claims 21 to 26, wherein the well comprises a plurality of well completion arrangements, axially aligned and connected lengthwise.

28. The well according to claim 27, wherein the well comprises a first well completion arrangement having a first effective inflow area and a second well completion arrangement having a second effective inflow area, the second effective inflow area being different from the first effective inflow area.

29. The well according to any of the claims 21 to 28, wherein the multiple well completion arrangements are provided such that fluid is permitted to flow along the multiple well completion arrangements in an annulus provided between the wellbore and surrounding formation.

30. Well completion arrangement having an effective inflow area usable with the well of any of the claims 21 to 29.

31 . A computer program product or computer file configured to provide the method of any of the claims 1 to 17.

32. A method for optimal well design, the method comprising: