Understanding fault facies improves reservoir modelling

R E S E R V O I R M A N A G E M E N T

By introducing the concept of fault facies, a new research project aims toarrive at a more realistic way of representing faults and fault properties inthe subsurface. The result is expected to improve our understanding ofreservoir behaviour and thus improved recovery.

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Halfdan Carstens

Most petroleum reservoirs includefaults. However, present reservoirmodelling methods encounter

serious problems when trying to capturethe fact that faults represent three dimen-sional rock bodies with complex internalarchitectures and petrophysical propertiesthat are intrinsically linked to the forcesthat acted on the rock during fault move-ment.

Faulted reservoirs are more complex todrill, they are more difficult to produce andthe behaviour of fluid flow is hard to pre-dict. "Not understanding fault impact onreservoirs has a multi-million dollar pricetag," claims Dr. Jan Tveranger at the Centrefor Integrated Petroleum Research (CIPR)in Bergen, Norway.

A research project has therefore beeninitiated with the aim of improving thegeological understanding of fault facies.The project is currently in the process ofadapting existing modeliing software to tohandle fault rock volumes for reservoirmodelling purposes.

Fault complexityFaults are displacements of rock layers in

the stratigraphic succession. On the surfa-ce, faults are identified by inspection ofrock sequences by experienced field geo-logists, while in the subsurface; faults areidentified from seismic data, well logs orcore samples by a multitude of experts ongeology, geophysics and petrophysics.

The usual and simple way of demonstra-ting the presence of a fault is by drawing aline on a map or on a cross-section (e.g.seismic lines), or by illustrating it as a planein a 3D volume. The entire asset team will

be familiar with this approach. Regrettably,this way of illustrating faults does notreflect reality. Faults are much more com-plex as a result of their genesis.

"A fault is not a line or a plane. A faultrepresents a three-dimensional rock body.Moreover, the petrophysical properties arechanging as a function of strain that origi-nates during fault movement, meaningthat the impact of a fault is not restricted tothe fault itself, but extends into the adja-cent rock volume. A faulted reservoir willtherefore exhibit different production cha-racteristics compared to the same unfaul-ted rock," says Tveranger who acts as coor-dinator for the research project.

Modelling the reservoir"Over the last ten years, three-dimensio-

nal modelling using "ultrafast" computershas become the standard way of compi-ling, manipulating and presenting geologi-cal, geophysical and engineering data. Ithas proven an invaluable tool for arrivingat a better understanding of reservoirdynamics and the effect of sedimentaryarchitectures and property distributionson flow. As a result, there are a number ofsoftware packages allowing users to buildsophisticated 3D geological models thatserve as input to fluid-flow simulators,"explains Tveranger.

However, there is one major built-indrawback with these tools that relates tohow faults are treated. "Traditionally 3Dreservoir modelling software tools empha-size the impact of sedimentological featu-res over structural ones for two reasons:First, most geologists involved in reservoirmodelling are sedimentologists, andsecond, reservoir simulators were not origi-nally designed to include faults.The produ-

A fault causes both displacement and modification of the petrophysical porperties of the surrounding hostrock. As can be appreciated from comparing a conventional fault model (left) with a conceptual fault faciesmodel (right); the conventional approach only captures half the story.

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cers of modelling tools have consequentlynot seen the need for a complicated faultzone design for geological 3D grids, andtectonic features are still modelled in avery simplified way: as planes and curvedsurfaces. This is in spite of the fact that thecapabilities of modelling programs haveimproved significantly over the years," saysTveranger.

The need for capturing effects causedby tectonic deformation has been recogni-sed long ago. Norwegian oil companiespioneered the fault modelling effort in theearly 90’s resulting in a number of speciali-sed applications for modelling fault pro-perties. However, these tools have notbecome an integral part of the standardmodelling workflow. "In practice, mostreservoir engineers still claim their right toapply history matching rather than geolo-gical data," says Tveranger.

Standard reservoir modelling tools andmethods largely ignore the volumetricaspects of fault impact and implementfaults as surfaces along which displace-ment and peremeability reduction take

place. The 3D volumetric architecture offaults as well as changes to petrophysicalproperties of the rocks surrounding faultsremain unadressed.

It follows that current 3D modellingtechniques and tools fail to incorporate thefull impact of tectonic deformation onhost-rock petrophysical properties andthus its effect on fluid flow in faulted reser-voirs," says Tveranger.

The project"Considering that most petroleum reser-

voirs include faults, we think that the pre-sent way of including tectonic features inreservoir modelling does not adequatelyhelp us understand reservoir behaviour,"says Tveranger. CIPR has therefore launc-hed a project in cooperation with Roxar (aleading software company supplyingreservoir-modelling tools), the NorwegianComputing Centre and the departments ofEarth Science and Mathematics at the Uni-versity of Bergen, with the aim of predic-ting spatial variability of petrophysical pro-perties in sedimentary rock-volumes affec-

ted by fault zones.Fault Facies (FF) is a proposed new 3D

modelling concept that addresses the lackof realism in existing methods of represen-ting faults in petroleum reservoir models.

The three-year project has a budget ofroughly 3 million dollars and will involveexperts on sedimentology, structural geo-logy, rock mechanics, seismic interpretati-

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Terminology used to describe the deformation zone elements or fault facies.

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The conventional method of describing faults as simple planes, or curves incross-section, is evidently misleading.

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Fault Facies - definitionThe concept of sedimentary facies has

shown itself to be both useful and easy toimplement in reservoir modelling. It ser-ves to describe depositional reservoirarchitectures and property distributions.

A similar approach to describing rocksaffected by tectonic deformation is intro-duced by the Fault Facies project. A faultfacies is thus informally defined as anyfeature or body of rock with propertiesderived from tectonic deformation.

Lithologi, burial history, diagenesis andthe stress field govern fault facies.

A fault is a displacement ofrock layers resulting in amodification of the petrop-hysical properties in avolume of host rock.Structures that appear as asingle fault from a seismicsection will typically com-prise several faults withmore complicated geome-tries and tectonically inducedpetrophysical heterogeneities. It isevident from this sketch that the impactof a fault is not restricted to the fault itself. Rather, it extends into theadjacent rock volume. The scale of impact depends on tectonic setting,fault throw, lithology and burial history including diagenesis.

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on, reservoir simulation, programming,mathematics and geostatistics.

"The ultimate goal is to employ faultfacies as an integral part of future 3Dmodelling of hydrocarbon reservoirs in theoil industry. This will allow users to captureand predict the effect of faults and othertectonic features on hydrocarbon flow inthree dimensions with significantly higheraccuracy and realism than is presently pos-sible."

The aim of the proposed project is toarrive at a model that includes more realis-tic way of representing faults and fault pro-perties. This will ultimately allow tectoni-cally modified rock bodies to be modelledwith the same kind of precision and flexibi-lity as sedimentary architectures aremodelled at present, and allow a betterunderstanding of fluid flow dynamics infaulted reservoirs.

The project intends to publish resultscontinuously throughout the project peri-

Fault within the Suez graben, Sinai, showing downfaulted sandstones and Eocene pre-rift carbonates in the hanging block. The damage zone caused by faulting is clearly evident.

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CIPRThe Centre of Integrated Petroleum

Research (CIPR) at the University ofBergen intends to increase recoveryfrom existing oil reservoirs by expan-ding the understanding of multiphaseflow in porous media. Geology, chemis-try, mathematics and physics arecombined in order to establish newreservoir models for quicker and moreaccurate simulations.

CIPR’s partners comprise the Institu-tes of Geoscience, Mathematics, Phy-sics, Chemistry and Microbiology at theUniversity of Bergen. 150 persons workfull or part time at CIPR, totalling about50 man-years each year. The Centreattempts to meet industry’s need forqualified candidates by educating 200master and 100 Ph. D. students the

next 10 years. Sixty percent of CIPR’s budget is

covered by the oil industry, and the newstatus as a Centre of Excellence ensuresan annual funding of approximately twomillion dollars from the NorwegianResearch Council.

"If we manage to increase oil recove-ry by only one part per thousand, theinvestment in CIPR will be covered,"says centre leader Arne Skauge, who isa petroleum engineer with long experi-ence from the oil industry, research andeducation. "We want to contribute toincreased recovery in existing oil fields,as well as to make new, economicallymarginal discoveries profitable, hesays."

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od in refereed national and internationaljournals. Considering the number of PhDand researchers participating in the pro-ject, 35-50 papers are expected to bepublished from project start-up to twoyears after its termination.

The project addresses a fundamentalaspect of how geological information isincluded in models. As these models serveas standard tools for many aspects ofexploration and hydrocarbon production,results of the proposed project are expec-ted to add to both improved recovery(improved reservoir simulation models)and exploration (mapping of faults andfractures as well as prediction of reservoirproperties).

Improved recovery

Several oil and gas fields that are cur-rently being produced have experiencedsignificant problems with fault related fea-tures in the reservoirs. Current reservoirmodelling tools are not able to realisticallycapture the impact of tectonic deformati-on of fluid flow.This is a serious obstacle tooptimising production strategies and reco-very.The project aims to provide an impro-ved methodology and tools to solve thisproblem.

Also, relevant to exploration, prospectshave been deemed too risky to drill, as sea-ling properties of faults within these pro-spects could not be established with suffi-cient reliability.

"It is a stated aim of the project to deve-lop these tools and methods within theframework of existing standard modellingapplication in order to ensure a speedypractical implementation. The net resultwill be improved understanding of reser-voir behaviour and thus improved recove-ry," concludes Jan Tveranger.

Reservoir modelling Three-dimensional digital modelling

of reservoirs has become a standard inthe petroleum industry. The purpose ofsuch modelling is to describe the geo-metry and the petrophysical properties ofthe reservoir in order to calculate thevolume of oil and gas present, and topredict how the reservoir performsduring production.

Reservoir modelling has thus becomea necessity to determine if it is economi-cally feasible to produce the reservoir,and- if so –how to produce the fluids toget the most out of the reservoir.

Reservoir models are built by combi-ning information from seismic and welldata. The seismic data primarily givesinput to understand the geometry, whilethe well data is essential to what kind ofproperties should be assigned to thereservoir rock. In between the wells, ithas become common practice to fill thegap by applying field analogues. Geolo-gists look for analogues that have com-parable depositional environments to thereservoir.

Modelling requires that the reservoirbe divided into blocks or cells. Such cellsare typically 50x50x1 m. A reservoirmodel will thus consist of tens of thou-sands of cells that each have an assignedvalue as to fluids, porosity and permea-

bility. Altogether these cells constitute a3D grid.

The reservoir model is built in twosteps. First, a geological model is builtbased on the knowledge gathered fromseismic data, well data and field analo-gues. The next step is to transfer the datato a flow simulator that will give know-ledge on how the fluids flow through thereservoir. The results are used to deter-mine where both production and injecti-on well should be placed and how theproduction and injection should be regu-lated to get optimal production and maxi-mum recovery.

Digital model of a reservoir.

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Field analogues are important in reservoir modelling for interpolating detailed petrophysical propertiesbetween wells. The Ainsa Basin in the Pyrenees is widely used for several depositional facies, includingdeep-water sediments.

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"Current 3D modellingtools fail to incorporatethe full impact of tecto-

nic deformation onhost-rock petrophysicalproperties and hence on

fluid flow in faultedreservoirs.