Pressure transient analysis a tool for reservoir ... · The goal of pressure transient testing is to ... approximately constant while the well pressure is ... Pressure Transient Analysis-

ONGC, Office of the Chief Reservoir Field Services,6th Floor, Quadrant-1, NBP Green Height

BKC, Bandra (East), [email protected]

Pressure transient analysis a tool for reservoir Assessment-A case study from the field of AssamAsset

*J.P.Srivastava, Aditya Kumar, office of Chief-RFS, ONGC MumbaiP.Mitra, B.Jena, B.Bhardwaj, RFS, Assam Asset, Nazira

Keywords

PI=Productivity Index, SBHP=Static Bottom Hole Pressure, FBHP= Flowing Bottom Hole Pressure, k=Reservoir permeability, kh=Capacity, S= skin around wellbore, Pi = initial reservoir pressure, h = effectiveReservoir thickness, LTR=Late time Region, ROI= Return of investment, PTA= Pressure transient Analysis

SummaryReservoir assessment is a collection of processes fordescribing and quantifying spatial variations in rockand fluid properties associated with reservoir. Giventhe right planning, technology and implementation,well testing can provide vital information related toreservoir pressure, distance to boundaries, arealextent, fluid properties, permeability, flow rates,drawdown pressures, formation heterogeneities,production capacity, formation damage, productivityindex and more. Analysing pressure Transient datagives the most useful information regarding reservoircharacteristics and reservoir boundaries/discontinuities. Detailed Reservoir studies duringthe early appraisal stage to determine the reservoir’sdescription provides valuable information resulting inbetter economic decisions. The reservoir parametersevaluated from PTA are very useful in updatinggeological model.

Role of pressure transient studies in reservoirassessment is described in present paper with casestudy of well A#4 of Assam Asset. Interpretation ofReservoir Limit Test data of A#4 indicates thatpressure is still in decreasing trend after 96 hours offlow. At LTR, indication of faults was observed inpressure derivative signature. Distance to the nearestno-flow boundary is 318 ft while the farther being at1770 ft. by analyzing through Radial homogenous-constant compressibility -Intersecting faults - Anyangle model. This is also in conformity with thegeological and structural set up of the study area.

IntroductionAs with any business, the goal is to maximize theROI. In the oil industry, the investment costs forexploration and production are massive. Answeringquestions about a reservoir’s productivity andperformance over time allows operators to makedecisions on how much to invest, where to place

wells, how to complete the wells, how to enhance andsustain production, what type of surface facilities willbe needed over time, and how to minimizeenvironmental, safety, and economic risks.

The reservoir analysis provides industry professionalswith guidance as to which well test method will offerthe best results in terms of the type of well test toperform, what the duration and flow rate will be, andestimations on the expected outcomes in order tobetter describe the reservoir. The goal was tounderstand the reservoir characteristics, the durationfor different well tests, and pressure, flow rate, andfluid production properties during the well test.Economic success or failure can depend on properreservoir assessment.

Long-term test or extended well test (EWT) – Thistest is important for both the exploration andappraisal phase to provide information aboutreservoir extent and continuity. The objectives are tofind the reservoir boundaries and understand thelong-term flow rates to design test facilities andperform economic analysis of the field. Reservoircompartmentalization is one of the most significantfactors to decide if field development is economic. Along-term test is the best tool to provide thisinformation. The reliance on seismic interpretations,electric logs, and PTA has formed the basis for theappraisal of a discovery along with appraisal drilling.The three components of the classic well testingproblem are flow rate, pressure and the formation.During a well test, the reservoir is subjected to aknown and controllable flow rate. Reservoir responseis measured as pressure versus time. The goal is thento assess the reservoir properties.

Types of Pressure Transient TestVarious types of pressure transient test are carried outin hydrocarbon industry for realistic assessment of

11th Biennial International Conference & Exposition

Pressure Transient Analysis- A Tool for Reservoir Assessment

reservoir for deciding the exploitation strategy .Some of them are;• Pressure Build up test• Pressure draw down / Reservoir Limit Test (RLT)• Pressure fall off test• Interference test/ Pulse test• Two rate flow test

The following generic analysis procedure is used totest and evaluate wells after completion:.1. Stabilize the well’s rate for some time after well

completion and estimate the well productivityindex based on estimates of reservoirparameters.

2. Establish a well-reservoir model for rate-timeprediction (based on Step 1) and tune the modelby history matching the observed data.

3. Design and conduct a pressure buildup test/RLTbased on the parameters estimated from theprevious two steps

4. Interpret the pressure test data and confirm themodel established by the available rate-timedata via an iterative process.

Interpretation MethodThe goal of pressure transient testing is to determinereservoir and well properties in the well drainage areaso that the well performance can be predicted. Thepressure transient response can take on severalparticular flow regime early radial flow, early linearflow, late radial flow and boundary –affected flow.Results that can be obtained from well testinganalysis are a function of the range and the quality ofpressure and rate data available, and of the approachused for the analysis.

With the introduction of the pressure derivativeanalysis in 1983 and the development of complexinterpretation models that are able to account fordetailed geological features, well test analysis hasbecome a powerful tool for reservoir assessment. Anew milestone has been reached by the introductionof the deconvolution. The deconvolution processconverts any variable rate pressure record into anequivalent constant rate drawdown response withduration equal to the total duration of the pressure

record. Thus more data available for theinterpretation than the original data set, where onlyperiods at constant rate are analyzed. Consequently, itis possible to see boundaries in the deconvolution, aconsiderable advantage compared to conventionalanalysis, where boundaries are not seen and must beinferred. This has a significant impact on ability tocertify reserves.

Case StudyThe well A#4 was drilled as an exploratory well withtarget depth of 3375 m in the North-Western part ofStudied area to exploit the hydrocarbon potential ofBCS pay sand. Object-III was tested in pay sand-4and its flowed @ 51 m3/d of oil with negligiblewater cut (0.2%) through 6mm bean on activation.

The well was correlated with A#1, A#2, A#3, A#7and it was observed that present well A#4 was almostat the same level w.r.t. A#3 and structurallyshallower compared to A#2 and A#1 at LCM+PaySand-4 level. At BCS well A#4 was shallowerw.r.t.A#3.

Figure-1 Sand Relief Map of Pay Sand-4

Structure contour/ sand relief map on top ofLCM/Pay Sand-4, indicate that A#4 is located on a



separate Fault Block. The prolific producer of oil &gas in Pay Sand-4 has opened a new vista forexploring further hydrocarbon potential in thenorthern part of studied area considered to be a newpool. In view of above result, geoscientific data needto be reviewed and re modeling of entrapmentcondition in Pay Sand-4 may be attempted for furtherexploration.

Figure-2 Structure Contour Map on LCM Top

A comprehensive reservoir study plan was made witha view to get a clearer picture of the Reservoircharacteristics. Identifying and mappingfractures/faults networks would help in understandingthe reservoir fluid movements and improve reservoirdevelopment plans.

Three Bean Study:Multi-bean study was conducted by continuousrecording of flowing bottom hole pressure with thehelp of sub-surface gauge through three beans inascending order along with stabilized flow rate

measurements at surface. The results of three beenstudy is given in Table-1

Beansize

Depth ofmeasurement

SBHP(KSC)

FBHP(KSC)

Delta P(KSC)

Oilproduction

(m3/d)

PI(M3/d/ksc)

248.11

5 2570 m. 229.6 18.51 36.461.96

6 2570 m. 224.3 23.81 50.9 2.137 2570 m. 209.4 38.71 66.33 1.71

THREE BEAN STUDY RESULTS OF A#4

Table-1 Three Bean Study

Indicator diagram has been made by plotting oil ratev/s drawdown to determine productivity index.(Figure-3)

Figure-3 Indicator Diagram A#4

It is observed that, the curve of indicator diagram is astraight line obeying Darcy’s law. It indicates theproduction rate is directly proportional to the pressuredrawdown with single phase flow. Reservoir pressureis under-saturated and well is flowing abovesaturation pressure. PVT analysis also confirm thefluid is undersaturated under reservoir conditions.From the indicator diagram it is inferred that theaverage PI of the well is around 1.82 M3/d/ksc.

Determination of optimum bean sizeTo determine the optimum bean, bean size v/sproduction test data was plotted. It is observed thatproductivity index is maximum with 6mm bean. Alsothere is not much variation in water cut with changein bean size; in fact water cut through 6 mm bean isnegligible. And, there is significant decrease inProductivity Index when bean is changed from 6 to 7mm.



Figure-3A, Determination of optimum Bean size.

Reservoir Limit TestThe fundamental objectives of drawdown testing areto obtain the average permeability, k, of the reservoirrock within the drainage area of the well, and toassess the degree of damage or stimulation induced inthe vicinity of the wellbore through drilling andcompletion practices. Other objectives are todetermine the pore volume and to detect reservoirinhomogeneities within the drainage area of the well.A drawdown test is one in which the rate is heldapproximately constant while the well pressure ismeasured. Ideally, the well pressure should bemeasured as near to the perforations as possible. Thewell pressure generally falls over time. The rate atwhich the pressure changes depends on reservoir andfluid properties, reservoir boundaries, and drivemechanisms. Thus, the pressure response can be usedto estimate these parameters. Thus, Reservoir LimitTest has become the most commonly practiced welltesting method to assess the reservoir. ReservoirLimit Test was conducted by continuous recording ofpressure transient data by Sub-surface Gauge for 96hrs. of well flowing through 6mm bean. The testoverview is given in Figuure-5&6.The pressuretransient analysis was done through the well testsoftware KAPPA-Saphir and well test software-Pansystem: Version 2012. The formation and reservoirfluid properties used for RLT analysis are given intable-2

During the present analysis, two different modelswere used for interpreting the data generated from the96 hours Reservoir Limit Test. it is observed that

pressure is still in decreasing trend after 96 hours offlow. At LTR, indication of faults was observed inpressure derivative signature.

Figure-4 Pressure Transient Test Overview

Figure-4 History Plot (Model match with actualhistory)

Depth of Measurement, (m) 2570

Pay Thickness, h (ft.) 10.14

Oil Rate , (M3/D) 50.9

Gas Rate, (M3/D) 3300

Average Porosity (%) 0.17

Oil Viscosity at Reservoir Condition,(cp) 3.3587

Oil Formation Volume Factor, (Bo), V/V 1.5845

Total Compressibility (Ct), (Psi-1) 7.667X10-6

Bubble Point Pressure, (Kg/cm 2) 84.37API Gravity 28.03

Table-2

Table-2, Formation & Reservoir Fluid Parameters

This was also confirmed from geological map. Thetwo models selected for interpretation of data are:Radial Homogeneous - Intersecting Faults – Constant



Compressibility and Radial homogenous parallelfault. (Figure-5). It is observed that there is anincreasing trend in derivative plot. Reservoir ishaving a radial homogenous, single phase flow,constant compressibility with an intersecting faulteffect at a distance of 318 ft and 1770ft. The result ofinterpreted data is given in table-3.

Resulted Paramete rs PANVR-2012

Saphir NL Semilog Conve tionalAnalys is

Estimated Permeability (md) 376 460 454

Capacity (kh) 3822 4660 4612

Skin Factor (S) -2.16 -1.25 -1.13

Estimated Initial ReservoirPressure (ps i)

3405 3405 3405

Distance to boundary (L1) 517.97 318 328

Distance to boundary (L2) 2274.31 1770

Shape Factor (CA) 22.31 22.17

Transmiss ibility Kh/µo (md-ft./cp) 1138.17 1387.73 1373.43

TABLE-3

The comparison analysis of all three interpretationmethods reveals that there is not much difference inparameters like the Permeability (k), Skin factor (S),and estimated reservoir pressure and presence offault. But, considering the Geological and structuralset up of the study area along with the subsurfaceposition of the well presumes the intersecting Faultsat 800 angle – Constant Compressibility model to bebest fit. As shape factor of 22.17 also corresponds towell inside right angle triangle.

Figure-5 Log-Log Plot (Saphir)

Figure-5 Log-Log Plot (PAN System)

0

50

100

150

200

250

300

350

400

450

500

0.010.1110100

ΔP

, psi

a

Shut in Time Δt, Hours

Semilog Plot, Well A#4 Reservoir PropertiesCt= 7.67 x 10-6 psirw = 0.354 ft.h = 10.14 ft.ø = 0.17 %

Oil PropertiesBo = 1.584 RB/STBµo = 3.358 cp

Production ParametersQo = 320 STB/D

ResultsK = 454.90S= (-) 1.13Nearest Boundary= 328.19 Ft.

tx=7.0 hr

Figure-6 Conventional Semilog Analysis

Figure-7 Semi-Log Analysis



This plot shows how far the pressure would havediffused into the reservoir for the duration of the test.

Figure-8, 2-D Geometry Plot

Figure-8, 2-D Geometry Plot (Angle 800)

ConclusionResults that can be obtained from well testinganalysis are a function of the range and the quality ofpressure and rate data available, and of the approachused for their analysis. With the advancement intechnology in acquiring reservoir data using precisionQuartz and strain electronic gauges and advancedtransient software, it is significant to interpretcomplex reservoir parameters which can be most usefull in Reservoir description or what was expectedfrom the well test interpretation.

Interpretation of Pressure Transient contributes to theimprovement of the Geological understanding andmodel.

Identifying and mapping faults networks would helpin understanding the reservoir fluid movements andimprove reservoir development plans.

The interpretation of pressure transient data indicatesthat, best suited model is radial homogenous withboundary condition of intersecting faults at rightangle at a distance of 318 ft and 1770ft respectively.Geological and structural set up of the study areaalong with the subsurface position of the wellindicates the intersecting Faults

The structure contour/sand relief map on top of PaySand-4 also indicates that northern boundary fault isNW-SW treading, while the southern boundary isdefined by two intersecting fault.

References Inhouse Report No.

AA/NZR/SST/RFS/PTA/CHAO/2013-14.Entitled “ Initial Reservoir Studies of theExploratory Well A#4.

From straight line to deconvolution: theevolution of state of art in well test analysis. ByA.C.Gringarten, Imperial college London.

Conventional Semilog analysis technique byProf. D. Tiab, Oklahoma.

AcknowledgementsThe authors express their thanks to ONGCmanagement for allowing presenting this work in theinternational conference. The authors are furtherthankful to M/S Reservoil for providing Saphir NLfor analysis of Pressure transient data.


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Pressure transient analysis a tool for reservoir ... · The goal of pressure transient testing is to ... approximately constant while the well pressure is ... Pressure Transient Analysis-