The “Better Business” Publication Serving the Exploration / Drilling / Production Industry

JUNE 2012

Artificial LiftTechnology

Sandstone ReservoirsIn Historic

Lawrence FieldShowing Favorable

ResponseTo Polymer EOR

Flooding

Reproduced for NETL/DOE

with permission from

The American Oil & Gas Reporter

www.aogr.com

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Illinois Basin Applications

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By Sinisha “Jay” Jikich, John Grube, Beverly Seyler, Nathan Webb and James Damico

CHAMPAIGN, IL.–The Lawrence Field in the Illinois Basin has produced in excessof 410 million barrels of oil since 1906 through primary and secondary waterfloodtechniques. Today, the field is producing at less than 2 percent oil cut, and like manymature fields in the United States, is approaching its economic limit, even though thereis much remaining recoverable oil.Reservoir characterization research at the Illinois State Geological Survey (ISGS) and

the National Energy Technology Laboratory under a grant from the U.S. Department ofEnergy is supporting pilot and expansion areas for implementing alkaline-surfactant-polymer (ASP) floods in two distinct sandstone reservoirs in the Lawrence Field.As operator of the field, Rex Energy Corporation has committed resources to

implement ASP floods in the field, and the research conducted by the ISGS is intendedto optimize the opportunity for success of this project. Rex Energy owns and operates21.2 square miles in the Lawrence Field, and its properties account for 85 percent ofthe field’s total gross production.

Demonstrating Potential Of ASP EOR Technology

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Using ASP core flood test results, it isestimated that 130 million barrels of theoriginal 1 billion barrels of oil in placecan be recovered using ASP in reservoirsin the Lawrence Field. ASP flooding isan untested technology in the IllinoisBasin, and this project presents an ex-ceptional opportunity to perform and doc-ument field testing of this unconventionalrecovery technique on two of the mostprolific reservoirs in Illinois.Similar reservoirs in the Lawrence Field

increased production from 1 percent to 20percent oil cuts with more than 450,000barrels of oil produced from a 25-acrepilot during an earlier surfactant-polymerflood conducted by Marathon Oil Company.ASP technology may be a more cost-ef-fective technology for improving recoveryfrom mature and shallow oil fields in theIllinois Basin than the earlier tests.

ASP Technology

The alkaline-surfactant-polymer tech-nology recovers incremental oil by co-injecting interfacial tension reductionagents (alkali and surfactant) with a mo-bility control agent that improves floodsweep efficiency (polymer). Alkali (NaOHor Na2CO3) and surfactant alters the rel-ative permeability curve through threeprimary mechanisms: interfacial tensionreduction, oil solubilization, and wettabilityalteration.Adding alkali converts acids in the oil

to soaps and makes more favorable con-ditions for surfactants. Interfacial tensionreductions in excess of 10,000-fold havebeen observed when alkali and surfactant

are blended, while either agent alone had,at best, 10- to 20-fold interfacial tensionreductions. Alkali will reduce surfactantand polymer adsorption when co-injected.Polymer and surfactant affect each other’sadsorption as well as interacting on afluid-fluid basis.Most of the unrecovered oil in the field,

located in Lawrence County, Il. (Figure1), is contained in Pennsylvanian-ageBridgeport sandstones and Mississippian-age Cypress sandstones. These reservoirsare highly complex and compartmentalized.Detailed reservoir characterization, includ-ing the development of 3-D geologic andgeocellular models of target areas in thefield, is needed to identify areas with thebest potential to recover remaining reserves,including unswept and bypassed oil.This four-year project was designed to

compile, interpret and analyze the data re-quired to conduct reservoir characterizationfor the Bridgeport and Cypress sandstonesin pilot areas as well as expansion areasfor broad fieldwide implementation of ASPflooding in the Lawrence Field. Geologicand geocellular modeling needed for reser-voir characterization and reservoir simulationwere completed as prerequisites for de-signing efficient ASP flood patterns.Characterizing the complex reservoir

geology that identifies the geologic con-ditions that will optimize oil recoveriesfor expanding the ASP flood from small,limited pilots in the Bridgeport and Cy-press sandstones to the remainder ofLawrence Field is the primary objectiveof this project. It will permit the evaluationof the oil recovery efficiency from Bridge-port and Cypress sandstone reservoirs

FIGURE 2Map of ASP Bridgeport Pilot and Middagh Lease ASP Expansion Area

Bridgeport B SandtoneNet Thickness

Section 32 T4N, R12w

0’

30’

15’

250’

125’

0’

Bridgeport Channel SandtoneNet Thickness

Section 5 T3N, R12w

FIGURE 1Lawrence Field and Other Illinois Basin Oil Fields

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using ASP technology. Additional objec-tives of this project include assessing theapplicability of ASP flood technology tosimilar reservoirs in the Illinois Basinand other U.S. reservoirs.

Reservoir Characterization

The geologic and geocellular modelingneeded for reservoir characterization andreservoir simulation were completed asprerequisites to designing efficient ASPflood patterns. This work identified thereservoir characteristics that were requiredfor developing an effective strategy forexpanding from pilot areas to full-fieldimplementation of ASP flooding.A major task involved compiling data-

bases pertinent to completing reservoircharacterization of pilot and expansionareas, and is now completed with the ex-ception of newly generated data and newlyavailable data. Detailed digital cross-sectionswere completed for each of the pilots;available core were described, digitallyphotographed and interpreted. Facies weredescribed and identified on electric logs.Petrographic analyses, including thin

sections, scanning electron microscopicand X-ray diffraction analyses of selectedsamples were used for reservoir descrip-tions. Reservoir sandstone compartments

have been mapped. Cross-sections con-taining interpretations of sequence strati-graphic concepts were constructed. Thisinformation was incorporated into 3-Ddigital geologic models and 3-D digitalgeocellular models of the two pilot ar-eas.Another important task consisted of

constructing fieldwide cross-sections andcontoured structure maps to assist in iden-tifying expansion areas requiring detailedreservoir characterization ahead of imple-menting ASP flooding beyond the pilotareas. Detailed reservoir characterizationwas upscaled from the pilot areas to incor-porate more than six square miles. Everyeffort was made to study in detail thoseareas best suited to Rex Energy’s strategyfor broader application of ASP flooding.A final task is assessing ASP flood oppor-tunities in large mature fields in the IllinoisBasin as well as nationwide.Data from the 25 largest Cypress and

Pennsylvanian fields in Illinois have beencollected and formatted into tables to assessASP flood technology in this region. Theparameters needed for ASP floods havebeen assembled for several of the largerfields in Illinois, including the LoudenField in Fayette County, a 385 million-barrel field. The success of ASP flooding

in Pennsylvanian Bridgeport B sandstonereservoirs in pilot areas of the LawrenceField suggests that ASP technology can besuccessfully applied to similar reservoirsthroughout the Illinois Basin.

Major Differences

This article highlights the results fromSection 32 T4N R12W and Section 5T3N R12W of Pennsylvanian-age Bridge-port B sandstones, and changes in reservoircharacter. Bridgeport B correlative sand-stones in Section 5 T3N R12W were thesite of an ASP pilot as well as an earliersuccessful Maraflood project. The Bridge-port B sandstone in the Middagh Leaseof Section 32 T4N R12W is an activeASP flood. Figure 2 shows a map of theBridgeport ASP pilot in Section 5 andthe ASP expansion area in the MiddaghLease in Section 32.As the cross-sections show, there are

major differences in the reservoir char-acteristics of the Bridgeport sandstonesbetween these two ASP flood areas. Thesedifferences are illustrated in the two end-point wells in the A-A’ cross-section inFigure 3. The Bridgeport B sandstonehas been informally subdivided into threesubunits (B1, B2 and B3) in Section 32T4N R12W and the north-central portionof Section 5 T3N R12W. The well on theleft is the Griggs No. 109 in Section 32and on the right is the Robins No. MG-8in Section 5.Bridgeport sandstones in Section 32

commonly show a stacking of up to threelenses, while the sandstones in Section 5are characteristically thick and blocky chan-nel fill. Core-measured permeability isplotted in red on the right side of each logwith a scale of 0-600 milliDarcies. Averagecore-measured permeability in Section 5sandstone is 314 mD, compared withSection 32, where the average permeabilityis 113 mD. Recognizing and understandingchanges in permeability and porositythroughout the reservoir is important indelineating flow units, understanding chan-neling of reservoir fluids, and determiningremaining recoverable oil.Understanding characteristics related to

the depositional environment and examiningthe clay mineralogy as well as taking aclose look at the petrography of the sand-stone can go a long way toward explainingthese pronounced differences in perme-ability. These wells are about 1.25 milesapart, but it is common to see this rapidchange over only a few hundred feet, as isshown in cross-section B-B’ in Figure 4.

FIGURE 3End-Point Wells at A in Section 32 T4N R12W

And A’ in Section 5 T3N R12W

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The north-to-south cross-section B-B’ shows channel fill sandstone and non-channel fill tidally influenced sandstonedeposits in the Bridgeport A and B inter-vals from Section 32 T4N R12W to Sec-tion 5 T3N R12W. This cross-sectionconnects the 1980s Maraflood pilot anda recently completed ASP pilot in thechannel fill sandstone in Section 5 withthe tidally influenced sandstone depositof the Bridgeport B in Section 32, whereASP flood development is in progress.

Depositional EnvironmentSandstones in Section 5 T3N R12W

initially were deposited in a fluvial channel,but continuous transgression led to achange in the environment to tide-influ-enced coastal and estuarine. Thick sand-stones in Section 5 were deposited initiallyin an incised channel with the lowererosive contact, tabular cross-bedding andindistinct zones being part of the fluvialfacies. Sea level transgression occurredduring sedimentation within the incisedchannel, resulting in a change in faciesfrom fluvial to coastal marine-estuarinesandstone deposition. The thick sandstonebody in Section 5 becomes more ripplebedded with flaser and lenticular beddingnear the top, indicating the transition fromfluvial facies to coastal/estuarine facies.The abundance of replaced plant ma-

terial and spores in the lower fluvialfacies of the Section 5 sandstone may in-dicate that the muddy area surroundingthe estuary was a lycopod swamp. As thesea level continued to rise, the incisedchannel widened upward until the estuaryfinally overran the banks of the channeland embayed the entire area of interest.The thinner sandstone reservoirs in

the Bridgeport B interval of Section 32T4N R12W were deposited in a tidallyinfluenced coastal/estuarine environment.These sandstones are finer-grained andmore compacted, and therefore, less porousand permeable than the channel fill reser-voir sandstones in Section 5 T3N R12W.Tidal flat, intertidal flat, tidal sand barsand tidal estuarine deposits are located inSection 32. Tidal indicators in the sandstonein Section 32 include ripple bedding,flaser and lenticular bedding, tidal rhyth-mites and tidal couplets. Some burrowingtrace fossils also can be observed.Many of these features are also ob-

served in the uppermost portions of thechannel fill sequence in Section 5. Severalstill stand and channel fill episodes areobserved in Bridgeport units in the areaof interest. The complexity of this depo-sitional system illustrates the need for

detailed mapping of individual sandstonereservoirs, and also explains the high de-gree of variability in reservoir character-istics and geometries over a small area.Understanding this complexity is necessaryfor successful EOR development.

Reservoir MappingAn isopach map of the net sandstone

of the Bridgeport B in Section 32 T4NR12W and the thick correlative Bridgeportchannel sandstones in Section 5T 3NR12W are shown Figure 2. As illustratedin the B-B’ cross-section in Figure 4, theBridgeport B in Section 32 occupies thesame stratigraphic position as the thicksandstone in Section 5. In Section 32T4N R12W, the Bridgeport Sandstone ismapped with a two-foot contour intervalwith the warmer colors representingthicker sandstone (Figure 2). In Section5 T3N R12W, the Bridgeport Sandstoneis mapped using a 10-foot contour intervalwith the warmer colors representingthicker sandstone.Reservoir sandstone in the northern

half of Section 32 trends east-to-west,and while the sandstone in the southernhalf of the section takes on a triangularshape, the overall trend is predominatelyeast-to-west. This triangular BridgeportB region is bounded by two thick sand-stone bodies that trend into Section 5T3N R12W from the northeast and north-west, and converge in the southern partof the section. Thicknesses of these blockysandstones range up to »170 feet.These thick sandstone bodies that appear

correlative to the Bridgeport B are actually

channel fill deposits that are related to asea level drop and erosion of the BridgeportB sequence, following deposition of theBridgeport B and are, therefore, youngerreservoir strata than the Bridgeport Breservoirs. This sequence stratigraphiccomplexity is prevalent throughout thePennsylvanian rock section in the LawrenceField, and is a reservoir characteristiccomponent that must be understood foreffective ASP implementation. A cross-section grid was constructed

in the eastern part of Section 5 T3N R12Wusing all available wells with core-measuredporosity and permeability values. Most ofthe core-measured porosity and permeabilityvalues are from thick channel fill sandstones.This is the same area as the 1980sMaraflood pilot and the recently completedASP pilot. The cross-section D-D’ inFigure 5 shows core permeability valuesinterpolated between the wells, indicatingthe presence of flow units in the sandstoneswith higher permeability and porosity.Warmer colors in the right section are

areas of higher permeability (600 mD)and cool colors are areas of lower perme-ability. These cross-sections show somewell-to-well continuity of very high per-meability and porosity values, indicatinga strong likelihood of channelized flow inthis reservoir unit. It is difficult to correlatepreferential flow units within the channelfacies reservoirs with standard mappingtechniques using the older-style spontaneouspotential (SP)/electric logs that are princi-pally available throughout Lawrence Field.Recent log suites and core information

FIGURE 4Cross-Section B-B’ Showing Relationship of

Pennsylvanian-Age Bridgeport B Across Sections 32 and 5

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have greatly enhanced the ability to de-lineate reservoir characteristics that arenecessary to increase the likelihood forsuccessful ASP EOR applications.

EOR Target

The Bridgeport B sandstones in Section32 are being targeted because these reser-voirs are more compartmentalized thanthe thick channel fill sandstones in Section5. These compartmentalized reservoirsare being used in an early phase assessmentof ASP flood design and development ina confined area that results in the mosteffective flood recovery. The success fromthese Rex Energy-confined reservoir flooddesigns will be implemented in larger-scale floods, such as the 350-acre Deltaregion in the southern half of Section 32. After developing the geological model,

a geocellular model was constructed usingthe geological data and reservoir geome-tries as a foundation. This is used to con-firm the geologic reservoir model and isrequired for effective reservoir simulationmodeling. The 3-D modeling images arealso a valuable visualization tool for eval-uating reservoir characteristics such ascompartmentalization, porosity, perme-ability and residual oil saturation. A com-prehensive geologic reservoir character-ization assessment is essential to developan effective reservoir simulation model.A simulation model that most precisely

portrays the reservoir will increase accu-racy of pore volume calculations usedfor chemical input estimations and oilrecovery projections, and can be usedfor effective flood design. The finalized

geocellular model, located in the northeastquarter of Section 32 and containing per-meability, porosity and oil saturation,was then transferred to Rex Energy foruse in simulation modeling. The resultsof the simulation will guide future de-velopment of the field for ASP purposes.Figure 6 is a plan view of the Bridge-

port B model, showing the outline of thereservoir. The model is 200 acres locatedin the northeast portion of Section 32T4N R12W. This porosity model thatdisplays an area where porosity is ≥16percent. The red box outlines the locationof Rex Energy’s ongoing Middagh ASP

flood. Locations of older wells with SPdata used to develop the model are shownas blue lines and dots, and modern porositylogs are labeled with black numbers.

Field Results

Rex Energy is reporting a positive re-sponse on its Middagh ASP pilot projectin the Pennsylvanian Bridgeport B reservoir.Oil response in the 15-acre flood has con-tinued to show an increase in the averageoil cut from 1 to 12 percent. Total patternoil production increased from 16 barrelsa day to stabilize within a range of 65-75bbl/d in the last three months of 2011.Peak oil production rose to 100-plus bbl/d,with some individual wells in the pilotshowing oil cuts greater than 20 percent.A second 58-acre pilot (Perkins-Smith)

adjacent to and likely in communicationwith the Middagh pilot has been initiated.Preliminary brine injection is in progresswith plans to initiate ASP injection bymid-2012. The response is expected bymid-2013 with peak recovery anticipatedby late 2013. Rex Energy is projectingfull-scale expansion, with the next stepof development being a 351-acre projectscheduled to begin in mid-2013. Prelim-inary development has been initiated inthis Delta Unit area located in the southernhalf of section 32 T4N R12W.Reservoir characterization has a critical

role in developing a complex chemicalEOR process such as ASP, especially inhighly compartmentalized reservoirs thatextend laterally from several acres to tens

FIGURE 6Plan View of Bridgeport B Model with Outline of the Reservoir

X (km)

X (km)

Y (km

)

Y (km

)

636.80

636.60

636.40

636.20

636.00

635.80

636.80

636.60

636.40

636.20

636.00

635.80

1,063.20 1,064.801,064.601,064.401,064.201,064.001,063.801,063.601,063.40

1,063.20 1,064.801,064.601,064.401,064.201,064.001,063.801,063.601,063.40

Porosity

0.24

0.22

0.20

0.18

0.16

0.14

0.12

0.10

N/A

W E

N

S

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FIGURE 5Core Permeability Cross-Section D-D’

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of acres in size. A synergistic approach be-tween reservoir characterization and formationevaluation, coupled with reservoir modeling

and simulation, reservoir management, wellmanagement and corresponding surface fa-cilities, is the only approach to assure success

in chemical flooding involving a high degreeof technology such as ASP. r

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SINISHA “JAY” JIKICH is a projectmanager at the National Energy Tech-nology Laboratory in Morgantown, W.V.,where he manages EOR-focused projects.He is also an adjunct faculty in thechemical and petroleum engineering de-partment at the University of Pittsburgh.Jikich has 30 years of experience inEOR, geological sequestration, reservoircharacterization and simulation, and isthe recipient of two Fulbright grants tolecture in the petroleum engineering de-partments in Hungary and Serbia. Heholds an M.S. in polymer physics fromthe University of Bucharest and an M.S.and Ph.D. in petroleum engineeringfrom the University of Wyoming.

JOHN GRUBE has 35 years of ex-perience in petroleum geology research,and exploration and development, in-cluding 13 years serving in the industryin Denver and 22 years at the Illinois

State Geological Survey, specializing instratigraphy of cratonic basins, deposi-tional environments, lithofacies modeling,play analyses, core, sample and outcropanalyses, field studies, reservoir geometryand heterogeneity, and enhanced oil re-covery. Grube holds a M.Sc. in geologyfrom the Colorado School of Mines.

BEVERLY SEYLER has 30 yearsof experience in petroleum geology re-search, reservoir characterization anddepositional environments. She was headof the oil and gas section at the IllinoisState Geological Survey for 12 years, re-tiring in 2010. Specialties include depo-sitional environments, sedimentology, litho-facies modeling, petrography, field studies,and Paleozoic stratigraphy of the IllinoisBasin. Seyler holds an M.S. in geologyand a M.A. in geography from the StateUniversity of New York-Fredonia and theState University of New York-Buffalo.

NATHAN WEBB is a geologic spe-cialist at the Illinois State GeologicalSurvey. He has been with the surveysince completing a master’s in glacialgeology and geomorphology at the Uni-versity of Illinois at Urbana-Champaignin 2009. Webb’s work involves strati-graphic and sedimentological studies ofpetroleum reservoirs in the Illinois Basin.

JAMES DAMICO has been an as-sistant geologist at the Illinois State Ge-ological Survey since 2006. His focus ison using geostatistical methods to char-acterize heterogeneity in reservoirs andbuilding 3-D computer simulations ofreservoir architecture using stochasticmethods. Damico holds a B.S. in earthsciences from Purdue University and anM.S. in geology from Wright State Uni-versity.

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