NZ571278A - Method for optimising the production of a cluster of wells - Google Patents

Abstract

Disclosed is a method for optimising production of a cluster of wells of which well effluent streams are commingled and separated in a fluid separation assembly into at least partly separated streams of crude oil, gas and/or other fluids. The method comprises the steps of: a) Performing a well test on each of the wells during which production from the tested well is varied and one or more individual well production variables are monitored. b) Deriving from data obtained by the well tests an estimation model for each well relating to the variation of the flow pattern of effluents produced by the tested well and of the monitored well production variables. c) Putting the well cluster in normal commingled oil and/or gas production. d) Monitoring during step c) a dynamic fluid flow pattern of the at least partly separated streams of crude oil, gas and/or other fluids by means of flow meters arranged in the at least partly separated streams of crude oil, gas and/or other fluids downstream of the fluid separation assembly. e) Monitoring during step clone or more well production variables relating to characteristics of the multiphase flow streams produced by the individual wells. f) Repeatedly estimating a dynamic commingled fluid flow pattern of the cluster of wells on the basis of the estimation models according to step b) and production variables monitored in accordance with step e). g) Performing a dynamic reconciliation process, wherein during a selected reconciliation period: -it is assumed that the estimated dynamic commingled fluid flow pattern according to step f) is an accumulation of said individual well production estimation models that are multiplied by unknown weight coefficients; -the unknown weight coefficients are estimated by iteratively varying each weight coefficient until the estimated dynamic commingled fluid flow pattern substantially matches with the monitored dynamic fluid flow pattern; and -best estimates of production flow are provided for the selected reconciliation period, and using the individual well reconciliation factors with the estimation models to estimate production from each well for a next reconciliation period. h) Defining an operational optimisation target consisting of a target to be optimised regarding production of one or more wells and/or the cluster of wells. i) Adjusting production of well effluents of the well cluster such that the optimisation target is approached. j) Steps g and i are repeated intermittently.

Description

- ! - Received at IPONZ on 27 June 2011 METHOD FOR OPTIMISING THE PRODUCTION OF A CLUSTER OF WELLS BACKGROUND OF THE INVENTION The invention relates to a method for optimising the production of a hydrocarbon production system comprising a cluster of hydrocarbon production wells and an 5 associated fluid separation assembly.

Typically, fluid streams produced by individual wells of a well cluster are commingled into multiphase streams in one or more production manifold (header) conduits and routed via a fluid separation assembly (comprising one or 10 more bulk separators and/or production separators} into fluid outlet conduits for transportation and sales of at least nominally separated streams of liquids,, gas and/or other fluids.

A problem associated with management of fluid flow at 15 the outlets of the bulk or production separator is that this fluid flow stems from the commingled flux from all the wells of the cluster and does not provide information about the composition and flux of fluids produced by the individual wells. Consequently, the individual flux of 20 fluids produced by the individual wells cannot customarily be tracked accurately in real time or instantaneously. The inability to track the individual well productions, together with the variability of well production and properties over time, leads immediately to 25 the inability to fully characterize the wells for the purposes of predicting flows if various well adjustments are made. Further, the production from the wells often interact due to limited capacity in the manifold and the separator to handle the full potential productions from 30 the wells. As an example, over-production of gas in one Received at IPONZ on 27 June 2011 well may reduce the total oil production in the cluster of wells.

A further problem with monitoring and controlling the production of a hydrocarbon production well is that such 5 a well may produce a mixture of crude oil, gas, water and condensates and that the production may contain irregular slugs of crude oil, water, solids and/or condensates. Multiphase flowmeters are often too expensive, have too restricted an operating envelop and are too complex to 10 install on individual well flowlines to allow individual oil, water and gas components of the well production to be measured continuously in real time, particularly as the well multiphase flow characteristic changes significantly over the life of the well. These multiphase 15 flowmeters also require calibration at start up and/or from time to time thereafter. Consequently, in the vast plurality of cases, the production of fluids by the individual wells is not customarily measured directly accurately continuously, or in real time. 20 International patent application W003/046485 discloses a production metering and well testing system, wherein the accumulated production of wells of an entire field is measured downstream of a bulk separator in which the produced fractions of crude oil, water, natural gas, 25 solids and/or condensates are separated and the flux and composition of the produced crude oil and/or other fractions can be accurately monitored. This accurate measurement of the accumulated production of wells of an entire field is made simultaneously, and compared, with 30 less accurate measurements upstream multiphase flow measurements that are taken simultaneously at each individual well.

Applicant's International patent application PCT/EP2005/055680, filed on 1 November 2005, "Method and 35 system for determining the contributions of individual Received at IPONZ on 27 June 2011 wells to the production of a cluster of wells" (PCT publication WO 2006/048418) discloses a method and system named and hereafter referred to as "Production Universe Real Time Monitoring" (PU RTM). The PU RTM method allows 5 accurate real time estimation of the contributions of individual wells to the total commingled production of a cluster of crude oil, gas and/or other fluid production wells, based on well models derived from well test data and updated regularly using commingled production dynamic 10 data. The PU RTM method also does not require the deployment of multiphase meters at each monitored well.

It is an object of the present invention to provide a method and system to optimise production of a cluster of wells on the basis of an estimation of the contributions 15 of individual wells to the production of the cluster of wells, tailored to the particular constraints and requirements of the oil and gas production environment; and/or to at least provide the public with a useful choice. The wells in the cluster may differ in terms of 20 nature and flux of its effluents, and/or mode of operation, stimulation and/or manipulation. The wells may also produce from multiple subsurface zones or branches. The wellheads of the wells in the cluster may be located on land or offshore, above the surface of the 25 sea or on the seabed. The method according to the invention may be used to generate one or more optimisation models, taking into account only significantly relevant well and production system characteristics and effects.

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated 35 otherwise, reference to such external documents is not to be construed as an admission that such documents, or such Received at IPONZ on 27 June 2011 sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.

SUMMARY OF THE INVENTION 5 In accordance with the invention there is provided a method for optimising production of a cluster of wells of which well effluent streams are commingled and separated in a fluid separation assembly into at least partly separated streams of crude oil, gas and/or other fluids, 10 the method comprising: a) performing a well test on each of the wells during which production from the tested well is varied and one or more individual well production variables are monitored; b) deriving from data obtained by the well tests an estimation model for each well relating to the variation of the flow pattern of effluents produced by the tested well and of the monitored well production variables; c) putting the wells in normal commingled oil and/or 20 gas production; d) monitoring during step c) a dynamic fluid flow pattern of the at least partly separated streams of crude oil, gas and/or other fluids by means of flow meters arranged in the at least partly separated streams of crude oil, gas and/or other fluids downstream of the fluid separation assembly; e) monitoring during step c) one or more well production variables relating to characteristics of the multiphase flow streams produced by the individual wells; f) repeatedly estimating a dynamic commingled fluid flow pattern of the cluster of wells on the basis of the estimation models according to step b) and production variables monitored in accordance with step e); g) performing a dynamic reconciliation process, 35 wherein during a selected reconciliation period: Received at IPONZ on 27 June 2011 -it is assumed that the estimated dynamic commingled fluid flow pattern according to step f) is an accumulation of said individual well production estimation models that are multiplied by unknown weight 5 coefficients; -the unknown weight coefficients are estimated by iteratively varying each weight coefficient until the estimated dynamic commingled fluid flow pattern substantially matches with the monitored dynamic fluid 10 flow pattern; and -best estimates of production flow are provided for the selected reconciliation period, and using the individual well reconciliation factors with the estimation models to estimate production from each well for a next 15 reconciliation period; h) defining an operational optimisation target consisting of a target to be optimised regarding production of one or more wells and/or the cluster of wells; i) adjusting production of well effluents of the well cluster such that the optimisation target is approached; and j) steps g and i are repeated intermittently. Optionally, the method according to the invention may 25 further comprise the steps of: identifying for at least one of the wells in the cluster one or more numerical manipulated variables that can be manipulated directly to vary the production of the well, and thereafter deriving from data, obtained by the 30 well tests and/or during normal commingled production, and/or the estimation model, a prediction model relating the well manipulated variables to the variation of the flux or the flow pattern and/or other characteristics of the produced well effluents. Wells without identified 35 manipulated variables will have prediction models that Received at IPONZ on 27 June 2011 are constant numbers equal to the nominal estimated productions of wells; summing the prediction models of all wells of the well cluster to provide an overall commingled production 5 prediction model; adjusting production of well effluents by means of the well manipulated variables as guided by the individual well prediction models and the overall commingled production prediction model to achieve the 10 said optimisation target.

Optionally, the method according to the invention may yet further comprise the steps of: measuring an interaction pressure(s), such as a pressure(s) within one or more production manifolds in 15 the flowlines that are connected to the wellheads of the wells of the cluster of wells, in which manifolds which the flux from a plurality of well flowlines is commingled, the variation of which, as total well productions vary, indicates and links interactions 20 between effluent streams from various wells; obtaining dynamic data relating the variations of interaction pressure(s) to the measured variables of the wells, from normal commingled production and / or during periods of production upsets and / or by performing a 25 series of well interaction tests during which the interaction pressure is varied; obtaining from dynamic data relating the variations of interaction pressure(s) to the measured variables of the wells well prediction models relating the variations 30 of well manipulated variable(s) and interaction pressure(s) to the production of the wells; obtaining dynamic data relating the variations of interaction pressure(s) to the total commingled production, from periods of normal commingled production 35 and / or during periods of production upsets and / or by performing a series of tests during which the interaction Received at IPONZ on 27 June 2011 pressure is varied, and thereafter one or more manifold interaction models relating the variation of the interaction pressure(s) with the total commingled production flows flowing through the manifolds; - combining the well prediction models to the interaction pressure models to obtain an overall commingled production prediction model.

Optionally the method according to the invention may further comprise the step of periodically repeating the 10 optimisation method by aligning the prediction models with the current flows so that the aligned prediction models reflect the current flows as estimated by the dynamic reconciliation process.

The optimisation target may be a revenue function 15 relating accumulated or averaged combined and / or individual well production to actual net or gross or incremental monetary revenue, optionally including associated production costs.

The optimisation target may be required to be 20 achieved while obeying production constraints, consisting of bounds on the manipulated variables and / or the individual well productions, and / or well production quantities, including measurements, and / or that of groups of wells and / or on the interaction pressure(s), 25 and / or on the commingled total productions.

The method according to the invention may further comprise the step of performing an optimisation using any of a plurality of numerical optimization algorithms over the manipulated variables based on the operational 30 optimisation target, optionally with constraints, and well and/or overall commingled production prediction models to yield a set of optimised manipulated variables that achieve the operational optimisation target.

Optionally, the production of well effluents of the 35 wells may be varied by adjusting the opening of a production choke valve at the wellhead of the wells or in Received at IPONZ on 27 June 2011 flowlines connected to the wells, or of a flow control valve in a lift gas injection system of the wells, or by other means of stimulating or restricting the production of the wells.

Optionally, the production of well effluents of the wells may be varied by adjusting the interaction pressure(s) of the production system by means of rerouting well production through parallel production manifold conduits that are connected between upstream and 10 downstream manifolds, or by adjusting the pressure of the fluid separation assembly or assemblies.

Required adjustments predicted by the method according to the invention to achieve the optimisation targets may be automatically transmitted to the wells and 15 the production system, or alternatively, after validation by a human operator.

One or more of the estimation and/or prediction models may optionally be generated in part or in full from theoretical and/or empirical physical and/or 20 mechanical and/or chemical characterization of the wells and/or the production system.

The optimization target can be adjusted in reaction to and/or in anticipation of changes to the production requirements and/or costs and/or revenues and/or 25 production infrastructure and/or state of the wells and/or the state of the production facilities; and optionally followed up by the conduct of the optimization process, the results of which are implemented and/or used for analysis and planning and/or recorded for future 30 action.

The method and system outlined herein is further applicable to the case where the optimisation target is achieved by optional means of temporary close in of production in one or more wells of the well cluster, or 35 the initiation of production of wells of the well cluster that were initially not in production.

Received at IPONZ on 27 June 2011 One or more of the estimation and/or prediction models may optionally be compared and/or evaluated against theoretical and/or empirical physical and/or mechanical and/or chemical characterization of the wells 5 and/or the production system; for the purposes of troubleshooting and/or diagnosis and/or for improving the models and/or for analysis leading to longer time horizon production management and optimization activities.

The methods of this invention apply also when one or 10 more of the wells from the cluster of wells are periodically, or intermittently, operated, or are operated from time to time, and the production or associated quantities to be optimised, and optionally, constrained, are evaluated, for example averaged, over 15 fixed periods of time larger than that characteristic of the periodicity or intermittent operation.

The methods of this invention apply also when one or more of the wells from the cluster of wells are periodically, or intermittently, operated, or are 20 operated from time to time, and the duration of its operation, as a proportion of a fixed period of time, is taken as a manipulated variable for the well.

The methods of this invention apply additionally to an optimization target defined on wells in the well 25 cluster with two or more subsurface zones. In this case, "zone production estimation models" and "zone production prediction models" are generated in addition to the "well production estimation models" and "well production prediction models".

The method according to the invention allows the characterization of the behaviour of wells individually and within the context of the overall production facility as a function of variables that can be freely manipulated at the wells and also for the overall facility. .The 35 characterization of the wells and their interactions with the facility allows directly the accurate real time Received at IPONZ on 27 June 2011 prediction and optimisation of well production within the context of the production facility. The method according to the invention may include consideration of constraints on the production, arising from both the interactions 5 between wells due to the limitations on the facilities, as well as externally imposed constraints. The method according to the invention is also referred to as "Production Universe Real Time Optimisation" (PU RTO).

The "PU RTO" method according to the invention has 10 several advantages over prior art methods, for example, as outlined in PU RTM described in International patent application PCT/EP2005/055680 (PCT publication WO 2006/048418}. In particular, the "PU RTO" method according to the invention may be used to derive various 15 well and production system characteristics from simple well and production testing at the well and production facility alone, enabling easier model maintenance and dispensing with measurements and quantities not continuously measured, but nevertheless unpredictably 20 variable over periods of time in a production environment, such as piping surface roughness, reservoir pressure-volume-temperature fluid characteristics and composition, equipment and well performance curves, and similar. In other words, "PU RTO" is "data driven". 25 Specifically, the "overall well and production system model" of the commingled well production system may be constructed without preconceptions as to its underlying physical nature other than the use basic fundamental topological and physical relations, and purely from 30 measured data.

The method according to the present invention may be used to provide characterization of the combined well and production system that will be of benefit additionally for offline analysis and planning activities. In other 35 words it is another benefit of the present invention that it may provide a method and system to link real time Received at IPONZ on 27 June 2011 current well production characteristics to the offline analysis and modelling to support wider, weekly, monthly planning and optimisation of the potential of an integrated production system comprising of multiple 5 cluster of wells and associated production facilities.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described by way of example in more detail with reference to the accompanying drawings in which: FIG. 1 schematically shows a production system in which a multiphase fluid mixture comprising crude oil, water, natural gas and/or other fluids is produced by a cluster of multiple wells of which two are represented and transported via multiphase fluid transport pipelines 15 to a bulk separator; FIG. 2 schematically shows how the "Aligned Well Production Prediction Models" and "Aligned Well and Overall Production Prediction Model" is generated from a set of well and production system testing data; and 20 FIG. 3 schematically shows how the "Well Operational Production Optimisation" problem and the "Overall Facility Operational Production Optimisation" problem are formulated to solve for the "Optimised Setpoints" for the wells selected for individual well optimisation, and the 25 "Optimised Setpoints" for the other wells and the overall production facility.

DETAILED DESCRIPTION OF PREFERED EMBODIMENTS OF THE INVENTION In the description in this specification reference 30 may be made to subject matter which is not within the scope of the claims of the current application. That subject matter should be readily identifiable by a person skilled in the art and may assist in putting into practice the invention as defined in the claims of this 35 application.

Received at IPONZ on 27 June 2011 _ 12 - The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting statements in this specification and claims which include the "comprising", other features besides 5 the features prefaced by this term in each statement can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in similar manner.

Reference is made to FIG. 1. FIG. 1 depicts a simple embodiment of a production system comprising a cluster of 10 wells of which effluents are commingled at a production manifold and routed to a production separator. Well 1 is shown in detail, and may be taken as representative of the other wells in the cluster. The other wells in the cluster may however differ in terms of nature and flux of 15 its effluents, and / or mode of operation / stimulation / manipulation.

Well 1 comprises a well casing 3 secured in a borehole in the underground formation 4 and production tubing 5 extending from surface to the underground 20 formation. The well 1 further includes a wellhead 10 provided with monitoring equipment for making well measurements, typically for measuring Tubing Head Pressure (THP) 13 and Flowline Pressure(FLP) 14. Optionally, there may be downhole monitoring equipment 25 for making subsurface measurements, for example Downhole Tubing Pressure(DHP) 18, and/or subsurface and/or surface tubing and/or flowline differential pressure meters, for example wet gas meters (not shown). The wells may also produce from multiple subsurface zones or 30 branches. The wellheads of the wells in the cluster may be located on land or offshore, above the surface of the sea or on the seabed.

The well 1 , like well 2, and some but not necessarily all of the other wells in the cluster, will also have 35 some means of adjusting production, such as: a production control choke 11 or a fixed bean choke (not shown) and/or Received at IPONZ on 27 June 2011 a lift-gas injection control system 12 or downhole interval control valves (not shown), which control the production from one or more inflow regions of the well. Numerical "manipulated variables" are associated with 5 each of these means of adjusting production.

The production system further includes a plurality of well production flow lines 20, extending from the wellheads 10 to a production manifold 21, a production pipeline 23 and a means of separating the commingled 10 multiphase flow, in this case a production separator 25.

Production manifold pressure measurement 22 and production separator pressure measurement 26 will often be available on the production manifold and the production separator as shown. There will be some means 15 of regulating the level of the production separator, and optionally its pressure or the pressure difference between the separator its the single-phase outlets. For simplicity a pressure control loop 27 is show in FIG. 1. Typically, the production manifold pressure measurement 20 22 {alternatively the production separator pressure measurement 26) will be used as "interaction pressure", the variation of which as the well production rates are varied, is an indicator of the degree of interaction between the wells.

The production separator 25 is provided with outlets for water, oil and gas 35, 36 and 37 respectively. Each outlet 35, 36 or 37 is provided with flow metering devices, 45, 46 and 47 respectively. Optionally, the water and oil outlets can be combined. The production 30 separator pressure may optionally be controlled by regulating the gas flow from 37, thereby affecting the manifold pressure 26 and the flowline pressure 14 and thus the production of the individual wells.

The well measurements comprising at least data from 35 13 and optionally from 14, 18, liftgas injection rate from 12, position of production choke 11, and other Received at IPONZ on 27 June 2011 measurements as available, are continuously transmitted to the "Production Data Acquisition and Control System" 50. Similarly, the commingled production measurements 45, 46, 47 are continuously transmitted to the 5 "Production Data Acquisition and Control System" 50. The typical data transmission paths are illustrated as 14a and 45a. The data in 50 is stored and is then subsequently available for non-real time data retrieval for data analysis and model construction as outlined in 10 this patent. The data in the "Production Data Acquisition and Control System" is also accessed by "PU RTM" in real time for use in conjunction with "well production estimation models" for the continuous real time estimation of individual well productions. Some 15 well production rate controls will also be adjustable from "Production Data Acquisition and Control System" 50 for remotely adjusting and optimising the well production, for example, the production choke opening or the liftgas injection rate, and the signal line for lift-20 gas injection rate control is shown as 12a.

An associated well testing facility may optionally and is preferably to be available for the individual testing and characterization of the wells. In the absence of a well testing facility, testing for well 25 model construction may be conducted utilizing measurements 45, 46, 47 from the production separator.

Reference is now made to FIG. 2, which provides one preferred embodiment of the "data driven" modelling process for this invention. The intent is to generate 30 sustainably useful models fit for the purpose of the invention, taking into account only significantly relevant well and production system characteristics and effects. The procedure leading to the generation of "Aligned Well Production Prediction Models" and "Aligned 35 Well and Overall Production Prediction Model" for a Received at IPONZ on 27 June 2011 cluster of n wells indexed for which one "manipulated variable" for each well has been identified, and for which a dedicated well test facility are available, is described as follows: - a series of well tests are conducted during which production from each well is varied for well characterization by changing the "manipulated variable" of the well. The well test data 60 is used to generate a "well production estimation model" 61 in the form >*,■ =fi(unvi), valid for a range of v;- within a set wherein the vector is the oil, water and gas production of well i, ui is the vector of measurements at well i , and v; is the manipulated variable at well / . The procedure for constructing "well production 15 estimation model" using dedicated well test facilities is as previously outlined in "PU RTM". In "PU RTM" distinction is not made between the measured variables u\ and the manipulated variable v,-, (which is also measured) but the distinction is required for this invention; 20 - a series of "well and production facilities interaction tests" are then conducted to obtain "well and production facilities interaction test data" 62, or alternatively, data can also be derived from "production data" 63 which contains a record of dynamic events during 25 normal production, such as when major gas producing wells are closed in. a "well production manipulation and interaction model" 65 is then constructed relating the variations of "manipulated variable" of the well and the "interaction 30 pressure" to the measurements at the well flowline. The relation between the "manipulated variable" and the measurements at the well flowline may be derived from well test data 60, for example "DDWTs", or from "production data" 63 by adjusting the manipulated Received at IPONZ on 27 June 2011 variable during normal well production, and recording the reactions of the measurements at the well flowline. Similarly, the relation between "interaction pressure" and the measurements at the well flowline is established 5 from "well and production facilities interaction tests" data 62 or from "production data" 63 recording dynamic events during normal production.

The "well production manipulation and interaction model" 65 will have the form ui = Si(vi>w) , for each of the wells i = where wgW is the common "interaction pressure" ranging over set W, ui is the vector of measurements at well i, and v,- is the manipulated variable at well z, as before.

A "well production prediction model" 66 for each well i = l,2,r..,n is then generated from the "well production estimation model" 61 and the "well production manipulation and interaction model" 65 as follows: )'i = fi (", ,V,) = ft (S, (V,, W), V,.) = A, (vf, w) , so that the function relates the vector X-, the oil, water and gas production of well i, to the w, the common "interaction pressure", and v;, the manipulated variable at well i.

Given the characterization of the well 66, it is now required to derive the dependence of Wr the "interaction 25 pressure" at the production manifold in this embodiment, to the total commingled production flows routed via the production manifold, and optionally, of variables, vw, that are manipulated at a overall facilities level which also affect the "interaction pressure", for example, the 30 production separator pressure setpoint 27. This is achieved by using the "well and production facilities interaction test" data 62 to build an "interaction pressure model" 67, W = ^(i^;vw) , where Jp is defined to be Received at IPONZ on 27 June 2011 the vector of the sum of the production of the wells routed to the production manifold, in this case y=±y, i=i There will be instances wherein data from well 5 testing or production is not available or reliable for a particular well or parts of the overall system, for example when a well is not yet been brought into production. In such cases, models based on theoretical and/or empirical physical and/or mechanical and/or 10 chemical characterization of the wells and/or the production system can be used in place of 65 or 67.

Once the "well production manipulation and interaction models" 66 and the "interaction pressure model" 67 are developed, the optimisation process is 15 implemented as per the online part of the workflow in Fig.2 and then as in Fig.3. The desired production optimisation formulations are also set out as follows: Well Optimisation 78, for selected wells, defined by an index set / which is a proper subset of 1j2,in 20 the form max i?f (y-, v;) "i subject to constraints c/,yO/i'vf"i)-0 f j =1,2,...,Ji _ where 4(y,.v,) is the objective or revenue function 7 6 for the well i to be maximized by varying vt, the manipulated variable at well /, subject to J-, constraints on ynvnui, the well production, the well manipulated variables and the measured variables, respectively, defined by the constraint inequalities ci.j{yi>vi>ui)-^ , 77.

Overall Production Optimisation, 83, for wells igl 30 (which are wells not already optimised as part of 78), in the form Received at IPONZ on 27 June 2011 max R{y,vi,uj,w,vw) V;,ie/,V|f, f subject to constraints cj(y>yi>vi>ui>wivw)-® r j-1)2,...,; where R(y,vnu;,w,vw) is the objective or revenue function, 81, to be maximized by varying v;, the manipulated variable at wells i£l , subject to constraints, 82, cj{y>ynvi>ui>w>vw)-0 on y>w>vw f the overall commingled production, the interaction pressure, and the optional vector of variables that can be manipulated at a overall facilities level, respectively, and ynvi>un i = 1,2,3,...n f ^he 10 individual well productions, well manipulated variables, and well measured variables for the wells i = l,2,3,...n .

With the wells in normal commingled production, "PU RTM" is run online to produce continuous real time estimates of production at each individual well 70. The 15 "current" "PU RTM" estimates of individual well production J/, and the "current" measurements and manipulated variable values vnw are used to align the "well production manipulation and interaction models" y. = A.(vr.,w) by defining a constant offset = ys - h^v^w) at J;ijv/sw. The "Aligned Well Production Prediction Models" 75 then have the form =^(vf>w) + ^,-. The alignment process allows the optimisation focus to be on incremental changes in the production at the well and overall production, so that if ^v/=v/-v/, Aw=w-w and Av,- = >*/ >*,-, then = 0, if Avf = 0 ancj Aw = 0. For illustrative purposes, the "Aligned Well Production Prediction Model", 75, for each well *, can be cast in the "separated difference" form Ays = AjAvj + BlAwl ^ symbols can be envisaged as either matrices or functionals operating on and Aw. Optionally, cross terms and second and Received at IPONZ on 27 June 2011 higher order terms on Av; and Aw can be inserted without loss of generality.

For each well i&I, which are wells for which a well optimization is desired, given its "Aligned Well Production Prediction Model", 75, and the objective or revenue function 76, and associated optimisation constraints 77, the well optimisation, 78, may then be conducted to solve for the optimal value of v;, 79, the manipulated variable at well i. Note that the well optimisation necessarily assumes the common well interaction variable w f which is an variable affected by the collective production of the wells and variables at the overall production system level, is unchanged by the well optimisation, or has negligible effect on the optimisation result.

The "interaction pressure model" 67 W = ^CP>VW) can j-,e aligned by defining constant = w-k(y,vw) f where the "current nominal value" of the commingled production measurements y is denoted by y . The "aligned interaction pressure model" is then of the form w = k{yivJ) + dw .

Further, we assume the estimates y, have been fully reconciled by "PU RTM" with the latest nominal overall commingled production measurements so that n y=Hyi j=i Again, for illustrative purposes, the "aligned interaction pressure model" can also have the "separated difference" form Aw = KAy + LAvw r where n Ay=y-y =y~Yji i=1 and Av,v = v„-^.

Received at IPONZ on 27 June 2011 - ti 2>, Now ,=i is the estimate of total commingled production from the "Aligned Well Production Prediction Models" 75. Hence the "Aligned Well + Overall Production Prediction Model" 80 in "Difference" form is then constructed by combining "Aligned Well Production Prediction Models" 75 with the "aligned interaction pressure model". From Ay=f Z yij- y=Z^ = Z A+B-+ f ZB> W?+LAv») V 1=1 / 1=1 1=1 1=1 v 1=1 / we get &y = ±AAV,+\±B^KAy+IAvJ which is an implicit form of the "Aligned Well + Overall Production Prediction Model" 80 relating the variables Aj>,Av(.,AvWf respectively, the total commingled production, the manipulated variables at the wells, and the manipulated variables at the overall production system level. For given values of Avf,Avw ^ depending on the form of the functionals ±A„±B„K,L 1=1 1=1 , the implicit form of 80 can be solved Ay by a plurality of methods. In the case where the interaction components £,b„k,l /=i are matrices of real numbers, for example, we have ;'=] A? = Z ArAv.-+ ^Z5f jiAv- i=i which is solvable for Ay given values of v,->vw, unless, for example, the operator Hi?5')* Received at IPONZ on 27 June 2011 is not invertible.

An estimate of the manipulated variables v,->vwrequired to optimise the production of the overall facility is then obtained by combining "Aligned Well + Overall 5 Production Prediction Model" 80 with the objective or revenue function 81, and associated optimisation constraints 82 to form the "Overall Facility Operational Production Optimisation" 83. The optimisation is then conducted to solve that for optimal values of the "optimised manipulated variables" vnvw, 84.

Depending on the form of 83, the optimised variables 84 may be directly computed or an automated numerical iterative optimisation procedure applied. There is a plurality of automated numerical iterative optimisation 15 approaches that are applicable depending on the form of 83. Refer for example to the textbook "Nonlinear Programming - Theory and Algorithms", 2nd Edition, 1993, by M Bazaraa, H.D. Sherali and C.M. Shetty, or more generally for various rigorous or heuristic "Global 20 Optimisation" methods, see for example, Computers and Chemical Engineering 28 (2004) 1169-1218, "Part I Retrospective on optimization", and "Part II. Future Perspective on Optimization", by L.T. Biegler, I.E. Grossmann, and references therein. For a preferred 25 embodiment where the manipulated variables are continuous variables, and 83 is defined by continuous smooth nonlinear model and revenue functions and inequality constraints, a sequential quadratic programme {SQP) with multiple starting points is used for the automated 30 numerical iterative optimisation to yield the "optimised manipulated variables".

The set of "optimised manipulated variables" is then available for further action. Optionally, the "optimised manipulated variables" are reported to the production 35 facility operators for implementation at the wells and Received at IPONZ on 27 June 2011 the facility, or alternatively, directly transmitted to the "Production Data Acquisition & Control System" 50 for automated implementation.

The computation and application of the optimised 5 manipulated variables is conducted from time to time, and is controlled by a "Optimization Initiation System" 90. Preferably, "Well Operational Production Optimisation" and the "Overall Facility Operational Production Optimisation" are initiated on a periodic basis, for 10 example once every day, and/or on demand, in anticipation of changes to the state of the philosophy of management of the wells or of the production system or of the constraints or of the optimisation target. In one embodiment, changes in gaslift availability will 15 automatically initiate an optimisation.

In a preferred embodiment of the "PU RTO" method according to the invention: All models are validated and updated as necessary using current latest and historical test data. 20 - The "PU RTM Well Production Estimation Model" is verified and updated periodically by checking against normal well testing.

The well manipulated variables are cycled periodically during normal well production to allow 25 verification and updates of the "Well Manipulation & Interaction Models".

Production data is captured during normal operations to validate and update as necessary the "Well Manipulation & Interaction Models", and the "Interaction 30 Pressure Model".

The "optimised manipulated variables" are transmitted after inspection by a human operator.

The computational optimisation step determines the wells that will either be opened up to resume the 35 individual well production, or closed in to stop the individual well production, or switched between different Received at IPONZ on 27 June 2011 production separators, addition to the "optimised manipulated variables" for the well while in production.

In this case, the manipulated variables vi>vw will include binary values 0 or 1, and various rigorous and heuristic methods are available for its solution, depending on the structure of the models and optimisation formulations used.

The prediction models of the wells and the overall production system, which reflect the reality of the well and production system, should periodically be compared and evaluated against theoretical physical and mechanical models of the wells and/or the production system, if these are available. The evaluation and comparison of the models derived from the actual well performances as per this invention against theoretical models will yield information to aid in longer time horizon production management and optimization activities.

Received at IPONZ on 27 June 2011

Claims (23)

WHAT WE CLAIM IS:

1. A method for optimising production of a cluster of wells of which well effluent streams are commingled and separated in a fluid separation assembly into at least partly separated streams of crude oil, gas and/or other 5 fluids, the method comprising: a) performing a well test on each of the wells during which production from the tested well is varied and one or more individual well production variables are monitored; 10 b) deriving from data obtained by the well tests an estimation model for each well relating to the variation of the flow pattern of effluents produced by the tested well and of the monitored well production variables; c) putting the well cluster in normal commingled oil 15 and/or gas production; d) monitoring during step c) a dynamic fluid flow pattern of the at least partly separated streams of crude oil, gas and/or other fluids by means of flow meters arranged in the at least partly separated streams of 20 crude oil, gas and/or other fluids downstream of the fluid separation assembly; e) monitoring during step cjone or more well production variables relating to characteristics of the multiphase flow streams produced by the individual wells; 25 f) repeatedly estimating a dynamic commingled fluid flow pattern of the cluster of wells on the basis of the estimation models according to step b) and production variables monitored in accordance with step e); g) performing a dynamic reconciliation process, 30 wherein during a selected reconciliation period: -it is assumed that the estimated dynamic commingled fluid flow pattern according to step f) is an Received at IPONZ on 27 June 2011 - 25 - accumulation of said individual well production estimation models that are multiplied by unknown weight coefficients; -the unknown weight coefficients are estimated by 5 iteratively varying each weight coefficient until the estimated dynamic commingled fluid flow pattern substantially matches with the monitored dynamic fluid flow pattern; and -best estimates of production flow are provided for the 10 selected reconciliation period, and using the individual well reconciliation factors with the estimation models to estimate production from each well for a next reconciliation period; h) defining an operational optimisation target 15 consisting of a target to be optimised regarding production of one or more wells and/or the cluster of wells; i) adjusting production of well effluents of the well cluster such that the optimisation target is approached; 20 and j) steps g and i are repeated intermittently.

2. The method of claim 1, further comprising the steps of: - identifying for at least one of the wells in the 25 cluster one or more numerical manipulated well production variables that can be manipulated directly to vary the production of the well, and thereafter deriving from data, obtained by the well tests and/or during normal commingled production, and/or the estimation model, a 30 prediction model relating the well manipulated variables to the variation of the flux or the flow pattern and/or other characteristics of the produced well effluents, wherein any wells without identified manipulated well production variables will have prediction models that are 35 constant numbers equal to a nominal estimated production of the wells; Received at IPONZ on 27 June 2011 - 26 - summing the prediction models of all wells of the well cluster to provide an overall commingled production prediction model; adjusting production of well effluents by means of 5 the well manipulated variables as guided by the individual well prediction models and the overall commingled production prediction model to achieve the said optimisation target.

3. The method of claim 2, further comprising the steps 10 of: - measuring one or more interaction pressures, the variation of which interaction pressure, as total well productions vary, indicates and links interactions between effluent streams from various wells; 15 - obtaining dynamic data relating the variations of interaction pressure(s) to the measured variables of the wells, from normal commingled production and/or during periods of production upsets and/or by performing a series of well interaction tests during which the 20 interaction pressure is varied; - obtaining from dynamic data relating the variations of interaction pressure(s) to the measured variables of the wells well prediction models relating the variations of well manipulated variable(s) and interaction pressure(s) 25 to the production of the wells; - obtaining dynamic data relating to the variations of one or more interaction pressures to the total commingled production, from periods of normal commingled production and/or during periods of production upsets and/or by 30 performing a series of tests during which the interaction pressure is varied, and thereafter one or more manifold interaction models relating the variation of one or more interaction pressures with the total commingled production flows flowing through the manifolds; Received at IPONZ on 27 June 2011 - 27 - - combining the well prediction models to the interaction pressure models to obtain an overall commingled production prediction model.

4. The method of claim 3, wherein the interaction 5 pressue is a pressure within one or more production manifolds in well flowlines that are connected to the wellheads of the wells of the cluster of wells, in which manifolds the flux from a plurality of well flowlines is commingled. 10

5. The method of any one of claims 2-4, wherein the method further comprises periodically repeating the optimisation method by aligning the prediction models with the current flows so that the aligned prediction models reflect the current 15 flows as estimated by the dynamic reconciliation process.

6. The method of any one of claims 1-5, wherein the optimisation target is a revenue function relating accumulated or averaged combined and/or individual well production to actual net or gross or incremental monetary 20 revenue, optionally including associated production costs.

7. The method of claim 6, wherein the optimisation target is to be achieved while obeying production constraints, consisting of bounds on the manipulated 25 variables, and/or the individual well productions, and/or well production quantities, including measurements, and/or that of groups of wells and/or on one or more interaction pressures, and/or on the commingled total production of the well cluster. 30 8. The method of claim 6 or 7, further comprising the step of performing an optimisation using any of a plurality of numerical optimisation algorithms over the manipulated variables based on the operational optimisation target, optionally with constraints, and the 35 well and/or overall commingled production prediction

8.Received at IPONZ on 27 June 2011 - 28 - models to yield a set of optimised manipulated variables that achieve the operational optimisation target.

9. The method of any one of the claims 1-8, wherein the production of well effluents of the wells is varied by 5 adjusting the opening of a production choke valve at the wellhead of the wells or in flowlines connected to the wells, or of a flow control valve in a lift gas injection system of the wells, or by other means of stimulating or restricting the production of the wells.

10.10. The method of claim 9, wherein the production of the wells is stimulated or restricted by any means of reversible and controlled closing in and opening up of a well, a setpoint of a control loop at the well with the production choke valve as an actuator, a setpoint of a 15 control loop on the well gas-lift injection rate or pressure, well gas-lift injection off duration, well gas-lift injection on duration, well gas-lift injection rate, a setpoint of a control loop on the well jet pump hydraulic fluid injection line, well electrical 20 submersible pump (ESP) speed, well rod pump motor speed, well rod pump off duration, and/or well downhole interval control valve opening.

11. The method of any one of the claims 1-10, wherein the production of well effluents of the wells is varied by 25 adjusting one or more interaction pressures of the production system by means of rerouting well production through parallel production manifold conduits that are connected between upstream and downstream manifolds, and/or by adjusting the pressure of the fluid separation 30 assembly and/or by adjusting the valves for the routing of well effluents to one or more manifolds that commingle production or that route the commingled production to one or more production separators, and/or by adjusting speed of a compressor in an outlet conduit of the fluid 35 separation assembly. Received at IPONZ on 27 June 2011 - 29 -

12. The method of any one of the claims 1-11, wherein required adjustments predicted to achieve the optimisation targets are automatically transmitted to the wells and the production system, optionally, after 5 validation by a human operator.

13. The method of any one of the claims 1-12, wherein one or more of the estimation and/or prediction models may optionally be. generated in part or in full from theoretical and/or empirical physical and/or mechanical 10 and/or chemical characterization of the wells and/or the production system.

14. The method of any one of the claims 1-13, wherein the optimisation target is adjusted in reaction to and/or in anticipation of changes to the production requirements 15 and/or costs and/or revenues and/or production infrastructure and/or state of the wells and/or the state of the production facilities; and optionally followed up by the conduct of the optimisation process, the results of which are implemented and/or used for analysis and 20 planning and/or recorded for future action.

15. The method of any one of the claims 1-14, wherein the optimisation target is achieved by optional means of temporary close in of production in one or more wells of the well cluster, or the initiation of production of 25 wells of the well cluster that were initially not in production.

16. The method of claim 2 or 3, whereby one or more of the estimation and/or prediction models may optionally be compared and/or evaluated against theoretical and/or 30 empirical physical and/or mechanical and/or chemical characterization of the wells and/or the production system; for the purposes of troubleshooting and/or diagnosis and/or for improving the models and/or for analysis leading to longer time horizon production 35 management and optimisation activities. Received at IPONZ on 27 June 2011 - 30 -

17. The method of any one of the claims 1-15, wherein one or more of the wells from the cluster of wells are periodically or intermittently operated and the production or associated quantities to be optimised and 5 optionally constrained are evaluated, over fixed periods of time larger than that characteristic of the periodicity or intermittent operation.

18. The method of claim 17, wherein the production or associated quantities to be optimised and optionally 10 constrained are averaged over fixed periods of time larger than that characteristic of the periodicity or intermittent operation.

19. The method of claim 17, wherein one or more of the wells from the cluster of wells are periodically or 15 intermittently operated and the duration of its operation, as a proportion of a fixed period of time, is taken as a manipulated variable for the well.

20. The method of any one of claims 1-15, wherein the method is applied additionally to an optimisation target 20 defined on wells in the well cluster with two or more subsurface inflow zones, in which case, "zone production estimation models" and "zone production prediction models" are generated in addition to the "well production estimation models" and "well production prediction 25 models".

21. The method of Claim 1, whereby the well production variables include one or more of the following variables: well tubing head pressure, well flowline pressure, well tubing head temperature, well flowline temperature, 30 differential pressures across well production choke valve, differential pressures across any differential pressure producer,including a wet gas venturi, on the well flowline, flow meters,nominally suitable only for single phase flow, which are suitable to be used to 35 provide an input to the well estimation models even if the well has multiphase flow, well production choke valve Received at IPONZ on 27 June 2011 31 opening state or position, opening state or position of any means of reversible and controlled closing in and opening up of the well, well lift-gas injection rate, well jet pump hydraulic fluid injection rate, well production casing pressure, well electrical submersible pump (ESP) speed, well ESP pump intake pressure, well ESP down hole pump discharge pressure, well ESP down hole venturi differential pressure, well ESP power consumption, well ESP motor phase current, well rod pump motor power input, well rod pump motor speed, well rod pump stroke displacement, well rod pump load cell, beam pump gear box shaft position, well rod pump differential speed / motor/gear box slip, well downhole tubing pressure, well downhole annulus pressure, well downhole tubing or annulus temperature ,or various derivations thereof monitored by distributed temperature sensors, well downhole interval control valve opening, amplitude of a selection of sound frequencies from one or more sound sensors mounted on well flowline, propagation delay of correlated sound patterns at a selection of frequencies from two or more sound sensors mounted in an upstream-downstream direction on a well flowline.

22. The method of claim 21, wherein the well production variables are pressure and/or other fluid flow characteristics of the individual well effluent streams.

23. A method, as defined in claim 1, substantially as herein described with reference to any example thereof and with or without reference to any of the accompanying figures. AGENTS FOR THE