BIPI Test Somaruga, Gazzera, Wouterlood

8/3/2019 BIPI Test Somaruga, Gazzera, Wouterlood

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APPLICATION OF BIPI TESTS EMPLOYING NATURAL ANDARTIFICIAL TRACERS IN WATERFLOODED OIL FIELDS OF THE

NEUQUINA BASIN (ARGENTINE)

C. Somaruga1, C. Gazzera2 and C. Wouterlood2

1Engineering Department. Comahue University. Buenos Aires 1400, (8300) Neuqun. Argentina.2Petrolera Perez Companc SA. Area Entre Lomas. CC 306 - Q8300BGC. Neuqun. Argentina.

Abstract. A methodology to determine the origin and distribution of bottomwater in an oil producing well influenced by several injectors inwaterflooding projects is presented. This methodology is complementary of interwell tracer tests and salinity records analysis and wasdenominated BIPI (brief interruptions of producing-injecting wells) test. It requires the closing of a well (injector or producer) during a shorttime and its later re-opening. The original flow pattern in the surroundings of the tested producing well is affected while the well is closed. Sothe water coming from a dominant injector can move forward invading areas of a layer that have been previously occupied by other waters.Once the well is re-opened, if there is a tracer concentration contrast between the waters, a concentration change is registered.The concentration change was measured and also its recurrence in waters produced during interwell tracer tests and during waterfloodingsthat are employing different injection water from the formation one. Also it was determined the original flow distribution in the surroundingof the producing well by means of a simple balance of tracer mass.It is presented and analyze here the results from tests that were performed in an oil field of the Neuquina Basin. They allowed to visualizingthe current condition of the bottomwater flow as well as to make predictions on future performance.

INTRODUCTION

The petroleum extraction process is performed in several consecutive stages, ordinarily denominated primary,secondary and tertiary recovery. The primary happens when the oil is surgent or is extracted by pumping. Thesecondary require the injection of water in certain wells (injectors) forcing the displacement of the petroleumtoward the rest of the wells (producers) (Figs.1 and 2). The tertiary recovery constitutes a variant of the secondarythat consists on the combined action of the water and other agents as polymers and/or surfactants intensificatingthe laundry action and displacement of the remainder petroleum.

When one of these methods loses efficiency is indispensable to reconsider it or to attempt other techniquesmore sophisticated and expensive. To guarantee the effectiveness of a restate or of a new production technique itis essential to have reliable information about the actual state of the reservoir. For example during the petroleumdisplacement by injected water, it is frequent to find hydraulic shortcuts among wells communicated through high

permeability channels. This retards the petroleum displacement in areas of lower permeability. The identificationof this problem, allows its correction, through the modification of the injection pattern (for example, by the

conversion of the two wells into injectors). In other occasions the shortcuts correction is achieved injectingpolymers. In order to detect and evaluate these unbalances in the water injection, the reservoir engineers analyzethe information coming from interference tests, interwell tracer tests and historical records (water fraction andsalinity). The last two, allow to explore the distribution of water injection, with the following limitations. In thiswork we try to improve their predictive capabilities by means of the application of the proposed methodology.

Figures 1. Displacement of petroleum by water.


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Figure 2. Usual disposition of injectors and producers

1. Interwell tracer tests

The interwell tracer tests constitute undoubtedly the more effective diagnose tool due to its easiness to study theinjected water distribution and the evaluation of the reservoir parameters, yielding good sensibility atheterogeneity1,2,3. An interwell tracer test requires the injection of an easily identifiable and generally solublesubstance in the injection water. Transported together with the water towards the producing wells, also connectedhydraulically with the injector, will allow drawing tracer concentration records.The limitation of these tests resides in that frequently does not recover the whole injected tracer mass, so it isimpossible to discriminate among the internal and external contributions of the mesh. In conventional interwelltracer tests each producing well can receive water from different injector wells that eventually can be external tothe analyzed mesh (Fig.3).

Figure 3. Water contributions to the producing well P1 from inside (I1) and outside (I2, I3, I4) injectors

If the total tracer mass injected in the well I1 (AI) is recovered at a given time, it indicates that there is no water

pushing oil inside the mesh. In this case, the distribution of injected water can be determined by directproportionality from the amount of tracer recovered in each producer well. Neglecting irreversible diffusioneffects, the mass of tracer recovered in a producer well P (Arp) is

=t

prp dtCqA0

(1)


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where qp is the flow of water produced by the well, C the tracer concentration and t the time of the test. The waterflow rate coming from the injector of the mesh (qpi) will be

I

I

rp

pi qA

Aq = (2)

where qI is the injector flow rate. Finally the water flow rate in the well " P " from the exterior of the mesh (qpe) is

pippe qqq = (3)

Unfortunately in most cases it is observed that only a fraction of the tracer mass injected appears in the producingwells at reasonable times2,3,4. The rest can be travelling very slowly in other layers toward the producing wells orother sectors. Therefore, a partial tracer recovery can underestimate the water flow contributed by the injector ofthe mesh (Eq.2) and overestimate the external water flow (Eq.3). Therefore the streamlines of the layers will not

be correctly definite, leading to wrong simulation and interpretation. If were possible to prolong the testindefinitely it could be very probable that a fraction of the flow supposedly contributing from outside of the meshwould result in a contribution of the main injector of the mesh.

The methodology proposed in this work procure to determine the fraction of internal and external flows,without the necessity of waiting for the unlikely total tracer recovery. Also it procures to identify each externalinjector contribution.

2. Salinity analysis

The analysis of salinity records has been and is being a very usual practice among the reservoir engineers,allowing the evolution control of secondary recovery projects characterized by the present saline contrast amongthe injection water (of salinity Sali) and formation water (of salinity Salf). The arrival of injection water, generallyless saline that of formation water, reduces the salinity of the water in a producing well in proportional form tothe contributed flow rate (Fig.4), resulting a salinity value Sal. A saline mass balance in the producing wellleads to the equation:

qSalSal

SalSalq

if

f

A

=

1 (4)

Figures 4. Clean water arrival at the producer.

The Eq.4 allows to determine the injection water contribution to the producing well but it doesn't allow to knowif that contribution is originated in one or several injectors, also it does not allow to identify which is which of


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those injectors and evaluate its particular contributions. The proposed methodology tries to give answer to thesequeries.

BRIEF INTERRUPTIONS PRODUCTION-INJECTION (BIPI) TEST

1. Objectives and methodology

The aim is to evaluate the water flow coming from a particular injector in the surroundings of a producing well

that has had breaktrougth of injection water and that is being influenced by two or more injectors. To achievethis we perturb the flow by shuting-in the well and evaluating the modifications observed in the tracer and/orsalinity records.

2. Case A: Shut-in of producing wells

The proposed methodology requires in this first variant, the shut-in of the producing well at time t s and its laterre-opening at tr. The original water flow pattern in the surroundings of the well will be affected while the

producing well remains closed. The closed time (tr-ts) should be brief so that the distortion is not prolonged inexcess, but the sufficiently long as to cause a representative perturbation.

2.1. Salinity records evolutionIt is expected that after the shut-in of the producer, the contribution of injection water from the dominant injectorgo ahead in the layer invading sectors that have been occupied by formation water before (Fig.5a).

Figures 5a. Flow pattern after the shut-in of the producing well .

Once the producing well is reopening a salinity reduction should be recorded, because the sector (of the layer 1)influenced by the injector B, has been already invaded by injection water coming from the injector A (Fig.5b).The measure of the salinity change allows evaluate the injector B contribution that goes through the layer 1, inopposed direction. Denominating Sali to the salinity of the injection water and Salf to the formation water, wecan write the salinity previous to the shut-in (Sal) as:

q

SalqSalqSalqSal

ffBiA 211 ++

= (5a)

being:

211qqqq BA ++= (5b)


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Once the shut-in time of the producing well is completed and reopened, there will be a final salinity (Sal'):

q

SalqSalqSalqSal

fiBiA 211

++

= (5c)

Figure 5b. Hypothetical salinity record evolution for the case of Fig.5a.

The resolution of the Eqs. 5a, 5b and 5c will allow to evaluate each contribution of the injectors A and B that gotoward the producer, through the layer 1. If there are others injectors that contribute with formation water,through the layer 1, their contribution would be part of q1B.

2.2. Tracer records evolutionThe analysis of tracer records perturbed by the shut-in of the producing well, presents two variants, because thetracer can arrive to the well from a dominant injector or from one not dominant. If it arrives from a dominantinjector, for example the injector A of Fig.5a, once carried out the shut-in and later reopening of the producingwell, an increase of tracer concentration should be observed, resulting the value:

Cq

qC

A

B

+=

1

11 (6)

where C is the concentration previous to the shut-in, and C is the concentration later to be reopened theproducing well. As in the case of salinity analysis, the Eq.6 determine if in the layer with breaktrougth ofinjection water there are contributions coming from other external injector.

However if the tracer arrives from a non-dominant injector, the situation is more complex. The shut-in of theproducing well can facilitate the water without tracer coming from a dominant injector, generate a clean areaaround the producing well. After that, the original flow pattern is reestablished by opening the producer. Inconsequence the clean area will be recorded as a concentration valley. Later, an edge will appear, indicating thearrival of the tracer front that had been left back during the previous stage (Fig.6). The analysis of occurrencetimes for the mentioned events (well shut-in, re-opening and tracer edge detection) can give information aboutthe original flow distribution, but is not clear how to make it in a quantitative way.


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Figure 6. Hypothetical concentration curves for tracer in setback case.

3. Case B: Shut-in of injecting wells.

With exemption of the last case presented, the analysis of the perturbation that take place by the shut-in and re-opening of the producing wells would allow to evaluate the water contribution from two injectors through a layer,

but it could not be identified who is responsible of each contribution. With the aim of covering this lack itintended to analyze the perturbations generated by the shut-in of injectors.

3.1 Salinity records evolutionTwo variants can arise depending on closing an injector that forces the arrival of injection water or closing aninjector that forces the arrival of formation water in the tested producer.

Case I: shut-in of an injector that forces the arrival of injection water in the producer. In relation to the case ofFig.4, the shut-in of the injector A can allow the advance of the formation water (forced by the injector B) on thesector occupied by injection water before. Meanwhile, the salinity should increase up to a saturation value (Fig.7)reaching it when the formation water finishes its invasion in the sector. Finally, the reopening of the injector Ashould reestablish the original salinity, after a certain time. Because of the salinity has reached the formationwater value, it will not be possible to evaluate in this case the flow q1B.

Case II: shut-in of injector that forces the arrival of formation water at the producer. If the closed injector is theone that contribute with formation water (injector B in Fig.4), a salinity reduction should be recorded, reaching astable value Sal' similar to that of Eq.5c. The original salinity will be reestablished after its reopening (Fig.8).Besides, in this case, the Eqs. 5a, 5b and 5c allows the evaluation of the flow rate q1B.

3.2. Tracer records evolutionThe tracer records analysis is simpler than that of salinity because the injectors that are sought to be close arecontributing with water without tracer. Therefore their clausure will cause salinity concentration increase

proportional to the water flow that are contributing to the producer (Fig.9). For the case of Fig.4, consideringtracer arrival from the injector A through the layer 1, and shut-in of injector B, the balance of tracer mass in the

producing well lead again to Ec.6.


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Figures 7. Hypothetical salinity record perturbation caused by the shut-in and reopening of an injector that forcethe arrival of injection water at the producer

Figures 8. Hypothetical salinity record perturbation caused by the shut-in and reopening of an injector that

forces the arrival of formation water at the producer.


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Figures 9. Hypothetical tracer concentration record perturbed by the shut-in of an injector that contributes

water without tracer.

3.3. Optimal shut-in timeSpecial attention should be paid after the injector shut-in, to check that a stable value of salinity (Sal) or tracerconcentration (C ) was reached. This would guarantee that a new flow pattern was established in thesurroundings of the producing well. Obviously the optimal shut-in time will depend on the distance among thewells as well as on parameters of the rock (permeability, layer thickness, porosity) and of the fluids (viscosity,compressibility).

3.4. Several injectors

If several injectors contribute with water without tracer (or water formation) to a producer, in the same layerwhere the injector that was previously closed is contributing, it can be invaded the surroundings of the producerwith water without tracer or water formation (Fig.10). Indeed, the shut-in of the injector B could facilitate theadvance of the tracer on its influence area, but it would be also possible an advance of clean water coming fromthe injector C. Consequently the associated concentration jump CB = CB - CB will be smaller than to thecorresponding to the complete tracer invasion in the area influenced by injector B. The later shut-in of theinjector C would generate another pick of height CC = CC - CC. In this situation it can be considered that thewater contribution of each injector it is proportional to the concentration jump that causes their shut-in:

CB

B

CB

B

CC

C

qq

q

+

+11

1

(7)

CB

C

CB

C

CCC

qqq

+

+11

1

(8)

with

m

CB

A

CB

C

CC

q

qq +=

+

1

11

(9)

where2

CB

m

CCC

+=

(10)


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is the averaged initial concentration for the shut-in of injectors B and C.

Once calculated q1A by integration of the tracer record concentration (Eqs.1 and 2), or starting from the salinity,by application of Eq. 4, the later resolution of the Eqs. 7, 8 and 9 would allow to evaluate the flows q1B and q1C.

Figure 10. Several injectors contributing water through a same layer to a producer.

FIELD TEST

1. Description

A field test was carried out in a mesh of a secondary recovery project that is developing in Charco Bayo Field(Fig.11) in the Neuquina Basin (Argentine). The aim is to study the behavior of the salinity and tracer recordscomparatively, when the flow is perturbed by the shut-in of wells. 370 GBq (10 Ci) of Tritium has been injected(like tritiaded water) in impulsive form, in the well I88. The corresponding water injection rate is 100 m3/da with

salinity (Sali) 1.2 gr/L. The salinity of the formation water (Salf) is 96.7 gr/L.Tritium has been detected particularly in the producer P46, distant 600 meters to injector I88. The well P46

produces 30 m3/da of water with salinity (Sal) of 40 gr/L (aprox.). The continuous contribution of Tritium fromthe injector I88 toward this well, has motivated that its election, due to is possible to calculate, in precise way, thewater flow rate derived toward it, by integration of the tracer concentration record (Eqs.1 and 2). Also wassupposed that the fault is a seal that disconnect the injector I42.

2. Salinity records analysis

The proposed methodology supposes in a first stage the analysis of the salinity records perturbed by shut-insand/or reductions of injection flow rates, that occurred in the previous year to the Tritium injection. The purposeis to check the influence of the injectors on the producer P46. In a second stage it will be analyzed the salinityrecords perturbed by the shut-in of the producing well P46 in order to evaluate the injectors contributionsquantitatively. It is important to mention that the interpretations have been based on the analysis of historical

records resulting from monthly samplings. Therefore certain estimations by interpolation and extrapolation wererequired.

2.1. Shut-in of injection wellsDuring the year previous to the tracer injection, some modifications took place in the injected flow rate ininjectors I88 and I48. Specifically an abrupt increase of salinity was detected in the water produced by the wellP46 after a reduction in the water flow rate of injector I88 (Fig.12). Salinity changes were not detected as aconsequence of modifications in the injection flow rate of well I48.


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Figure 11. Sector where the field test was developed.

Figures 12. Perturbations of injection water flow rate in well I88 and salinity response in well P46.

0 1 2 3 4 5 6 7 8 9 10 11 12

0

10

20

30

40

50

60

70

80

90

100

110

120

...... salinity

___ injection flow rate

Injectionflowrate(m3/day)

Salinity(gr/L)

Time (months)

0

10

20

30

40

50

60

70

80

90

100

110

120


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These results suggest a substantial contribution of injection water from injector I88. Evidently the reducedinjection of well I88 facilitated the invasion of formation water, presumably pushed by the injector I48 toward the

producer P46.Nevertheless is important to highlight the apparent existence of a threshold in the injection flow rate of well I88.In reference to Fig.12, only values inferior to the threshold (located between 75 and 55 m3/day), would allow theinvasion of formation water from other injector. This fact prevents the utilization of Eqs. 7, 8 and 9 forevaluation of injection flow rates.

2.2. Shut-in of producing wellA significant salinity reduction was detected later of the shut-in and reopening of producer P46 (Fig.13). The

initial salinity value of 45.8 gr/L decreased to 2.15 gr/L (extrapolated). The application of Eqs. 5a, 5b and5c allowed the operator to determine the existence of one or several contributing layers with:

q1A = 15.97 m3/day of injection water from an injector. q1B = 13.73 m3/day of formation water pushed by another.

injector. q2A = q2B = 0 m3/day of water from layers without

breakthrough of injection water.

From the analysis performed in 2.1 is clear that

q1A = q88 = 15.97 m3/d. and q1B = q48 = 13.73 m3/d.

Figures 13. Perturbation of salinity record of well P46 caused by its shut-in and reopening.

3. Tracer record analysis

3.1. Shut-in of the producing wellThe experimental tracer record obtained in the well P46 is shown in Fig.14. The well P46 was closed at day 601when the tracer concentration was 0.043 uCi/L. Three days later the well was reopened, registering 0.081 uCi/L.Thus, from Eq.6:

893.1

88

4888 =+

=q

qq

C

C(11)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

0

10

20

30

40

50

Extrapolated curve(exponential)

45.85 10-3

2.15 10-3

salinity10-3(

ppm)

time (months)


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The valued of q88 obtained by integration of the tracer concentration record of Fig.14 was q88 = 12.14 m3/day, thusthe estimation of the total flow rate is

q88 + q48 = 1.893 x 12.14 m3/day = 22.98 m3/day

The continuous tracer production in well P46 announces that the water flow rate q88 obtained by tracer recordintegration, will increase respect to the calculated at the day 1436.

Figure 14. Tracer concentration record in producer well P46.

4. DiscussionFrom this results we can conclude that:

The analysis of salinity records of the well P46 perturbed by shut-in of injectors, indicates that the dominantinjector is the I88. Nevertheless the apparent lack of linearity between q and C disables the utilization ofEqs. 7, 8 and 9 for injector flow rate evaluations. The analysis of salinity records of the well P46 perturbed by its own shut-in indicates that the well I88 iscontributing 15.97 m3/d. The rest (13.73 m3/d) can come from the I48. By integration of Tritium concentration record has been determinated at this time, that injector I88 iscontributing 12.14 m3/d, although this value can be increased with a bigger recovery tracer. The analysis of tracer concentration record of the producing well P46 perturbed by it own shut-in, indicate thatapproximately 53% of the produced water comes from the injector I88 (15.90 m 3/d), in agree with the valueobtained from salinity analysis.

It is expected that the difference among the value obtained by integration of tracer concentration records andthat obtained from the analysis of the records perturbation caused by its shut-in (equivalent at 3.76 m3/d) willdecrease while its continue recording tracer. From the point of view of the oilfield productivity, the results indicate that the sector swept by the injector I88has been totally flooded, coming formation water from injector I48 or other external to the mesh. This suggeststhe possibility of future arrivals of petroleum banks from this sector.

CONCLUSIONS


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The field results obtained from the analysis of salinity records agree with those obtained by the analysis of tracerrecord. This would enable the use of the proposed methodology, like a tool to deepen the knowledge about the originand distribution of the water at a producing well, whenever is possible to differentiate via salinity or artificialtracers. It is particularly important in the case of salinity analysis because their control and record is a current

practice among the reservoirs engineers, to which can be easily added the analysis of salinity records perturbedby shut-in of wells. In the case of interwell tracer tests the proposed methodology is able to evaluate water flow rates from injector

wells, without necessity of awaiting the unlikely total recovery of the tracer mass injected. This contributes to acorrect definition of streamlines and better simulation and interpretation of the tests. Nevertheless the results, it is indispensable the realization of other tests with strictly controlled shut-ins ofinjectors and producers. Also it should be carried out dense samplings of the produced water, trying to cover thewhole interval of time. It would avoid it the necessity of estimate for interpolation and extrapolation.

REFERENCES

1.Abbaszadeh-Dehghani M. and Brigham W. Tracer Testing for Reservoirs Description". Journal of Petroleum Technology, N5, 519-527(1987).2. Hutchins, R. and Dovan, H. Aqueous Tracers for Oilfield Applications". SPE paper 21049 (1991).3. Lichtenberger G. J. Field Applications of Interwell Tracers for Reservoir Characterization of Enhanced Oil Recovery Pilot reas". SPE

paper 21652 (1991).

4. Somaruga C. and Crapiste, G. New G. Methodology in Conventional Interwell Tracer for Evaluation of Secondary Recovery Process".SPE paper 68889 (2000).

BIPI Test Somaruga, Gazzera, Wouterlood

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