Semester Project in Reservoir Simulation

Technical University of Crete

School of Mineral Resources Engineering

Postgraduate program in Petroleum Engineering

Reservoir Simulation

Instructor: Ch.Chatzichristos

SEMESTER PROJECT:RESERVOIR SIMULATION

Team

Konstantinos- Dionysios Pandis

Konstantinos Voumvourakis

Chania

June 2015

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Table of Contents History Matching ........................................................................................................... 1

Transmissibility.............................................................................................................. 2

Reservoir description and characteristics....................................................................... 3

Different trials .............................................................................................................................................. 10

Trial 1 ....................................................................................................................................................... 10

Trial 2 ....................................................................................................................................................... 11

Trial 3 ....................................................................................................................................................... 12

Trial 4 ....................................................................................................................................................... 13

Trial 5 ....................................................................................................................................................... 14

Trial 6 ....................................................................................................................................................... 15

Trial 7 ....................................................................................................................................................... 16

Trial 8 ....................................................................................................................................................... 17

Trial 9 ....................................................................................................................................................... 18

Trial 10 ..................................................................................................................................................... 19

Results .......................................................................................................................... 20

1

History Matching

History matching is the process of building one or more sets of numerical models (representing a reservoir)

which account for observed, measured data.History matching is about evaluating the reservoir model with

respect to the observed dynamic data, and how well the attempt of matching is done. A simulation model

represents an oil and gas reservoir in its size, shape, and physical characteristics. Its main intent is

numerically to duplicate reservoir performance by incorporating physical parameters that dictate subsurface

flow in porous media.

The first step of history matching is to set the objectives of this process. Then the method to use in the

history match is determined, which is dictated by the objectives of the history match, the company resources

available for history match, and the deadlines and data availability. Then the historical production data we

are interested to be matched need to be determined, and the criteria to be used to describe a successful

match. Then the reservoir data that can be adjusted during the history match and the confidence range for

these data is also determined. The data chosen should be those that are the least accurately known to the

field but that have the most significant impact on reservoir performance. The next step is to run the

simulation model with the best available input data.

So in essence, one can draw various inferences from a properly constructed reservoir model. It can be used

to evaluate or confirm static conditions, such as the original oil in place (OOIP), and dynamic issues, such as

well deliverability and production decline.

But the model must be verified because one cannot observe, measure, and test every aspect of a hydrocarbon

reservoir. At best, one measures an extremely small portion of the reservoir, and many measurements may

be erroneous or contradictory. Models are history matched so that under historical production constraints the

model behaves similar to actual wells.

The assumption is that once the model reacts under historical constraints, as did the actual wells, then it will

behave the same as the actual wells under future constraints. But this is often incorrect, and misused models

are common. One should not use modeling results that contradict common reservoir engineering principles.

Therefore, a good history match does not necessarily guarantee a good model. What does is the total

package, consisting of the construction, the history match, and most importantly, reasonable

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projections.Ultimately, the challenge is to incorporate the information in dynamic data in all reservoir

modeling, and to consistently span the uncertainty in predictions.

Transmissibility

The transmissibility between two adjacent blocks of the grid, measures how easily fluids can flow between

them. For a two-phase flow, the transmissibility of water at the interface of two blocks is given by this

formula:

Tw =(kA

h) av * (

Krw

𝜇𝑤 𝐵𝑤)av

This quantity consists of two parts, each of which is an average between blocks: the single-phase part

(𝐤𝐀

𝐡) 𝐚𝐯 and the two phase part (

𝐊𝐫𝐰

𝝁𝒘 𝑩𝒘)𝐚𝐯 . Moreover, A is the cross-sectional area of the interface.

The single-phase part will be a harmonic average between blocks. On the other hand for the two phase part

two different averages are used. An upstream weighting will be used for the average relative permeability

and an arithmetic average between blocks will be used for the viscosity and formation volume factor.

In order to enforce a no flow condition without changing the physical conditions of the reservoir simply set

Tw=To=0.

In eclipse simulator the keyword that was used is MULT, one for each direction; MULTX, MULTY, and

MULTZ. The keyword should be followed by one non-negative real number for every grid block in the

current input box. The values specified act as multipliers on the transmissibility’s calculated by the program

for the +X face of each grid block. Thus, a value of MULTX specified for block (I, J, K) multiplies the

transmissibility between blocks (I, J, K) and (I+1, J, K). Grid blocks are ordered with the X axis index

cycling fastest, followed by the Y and Z axis indices. Any non-neighbor connections generated due to faults

have transmissibility that reflects the MULTX, MULTY, MULTZ values. If MULTX is entered in the GRID

section, then it applies only to the transmissibility calculated by the program using the information in the

GRID section. Whenever the MULTX MULTY, MULTZ values are not specified when the end of the

GRID section is reached, the transmissibility default value is 1.0.

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Reservoir description and characteristics

The reservoir that will be examined in this project is consisted of 6 different producers; P-A1H, P-A2AH, P-

A39A, P-A17, P-A35& P-ABRH, and 4 different injectors; I-A38, I-A5H, I-H2 & I-INJEC. It was decided

that history matching will be done for the 4 out of 6 producers, excluding producer P-ABRH and producer

P-A35 because they contribute the least in the total reservoir production. The other producers are far more

important in terms of oil production.

A description of the initial reservoir will follow as well as all the different graphs obtained from the data of

the original case that will be history matched.

Figure 1 illustrates the saturation of oil in the reservoir, after the injection of water. The darker red the area,

the more oil it has.

Figure 1: Original reservoir fluid flow.

In the next figure (figure 2), in which the reservoir is colorless and without gridlines, a sense of the

complexity of the reservoir is evident. It is clear that not all the wells are vertical, and that some of them are

horizontal wells, which in turn makes more difficult to appreciate and evaluate the behavioral conditions that

apply and affect the reservoir under flowing fluids. The horizontal wells are P-A1H, P-A2AH, P-A17,

whereas P-A39A is a vertical one and reaches the area right next to P-A1H. So apart from the variability in

the geology structure of the reservoir, that is another significant reason which strongly influences the

behavior of the reservoir. Injection wells I-A5H and I-H2 are vertical; whereas injector I-A38 is deviating

from the vertical axis until it reaches the area next to I-H2.

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Figure 2: Reservoir wells.

The production oil rate for each well is presented in the next graph. This graph is an indication of the

significance of each well with respect to the oil production. It can be noticed that the production order, from

high to low, is: well P-A1H, well P-A2AH, well P-A17, well P-A39A, and finally well P-A35.

Figure 3: Well oil production rate for the production wells.

It is obvious that not all the wells produce simultaneously and certain wells act in such way that assists the

operation of others. At first the P-A1H production well starts to operate until the 2500 days of production.

This well contributes the most in cumulative oil production of the reservoir. The production well P-A2AH

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starts operating from the first day of production and has also a significant contribution in the cumulative oil

production of the reservoir. This well stops producing at about 1800 day of production and then operates for

a few days more until 2200 days from the initiation of the production has been reached. The production well

P-A17 starts production at about 600 days and operates until 3150 days of operations. When the above well

stops production P-A35 starts to produce but contributes the least in cumulative oil production and for that

reason it is excluded from history matching. The production well (deviated producer) P-A39A starts

production few days before the P-A1H shuts down in order to extract the oil from the near P-A1H region.

The injection water Rate for each well is presented in the graph below (figure 4).

Figure 4: Well Water Injection rates, Initial.

Likewise, from the graph above it is obvious that the injectors (I-A38, I-A5H, I-H2 & I-INJEC) do not

operate all at the same time, just like the producers. First the I-A5H well is operating and then it stops and

injector I-A38 starts injecting until the end of the production period. The two injectors above are positioned

at close positions and can be treated as if there was a single injector. I-H2 well starts injecting only after the

first 1000 days of production and operates until about 2200 days of production. The contribution of the

above injector is poor in comparison with the I-A5H and I-A38 injectors. I-INJEC injector does not operate

at all in the given time period. In the following figures the original case that needs to be history matched is

presented in order to identify the deviation from the history of production and to compare with the optimum

result at the end of the history match process.

The figures that follow give a clear view of the starting point of the history match process. In all of them, the

plots are a comparison of the original starting situation of the reservoir, to the data on which we are

supposed to do the history match. Ultimately, the graphs for the optimum case will be compared to the

following graphs.

Figure 5 illustrates the well oil production total; the data of the history are compared to the data of the initial

case. The discrepancy of the production for each well is crystal clear, and it is this difference the simulation

model that is to be constructed will have to eliminate, to the highest possible degree.

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Figure 5: Well oil production total; History vs Initial

Figure 6 illustrates the well water cut; the data of the history are compared to the data of the initial case.

Figure 6: Well water cut; History vs Initial

Figure 7 illustrates the field oil production total; the data of the history are compared to the data of the initial

case. The deviation in between the respective lines of the history data and the initial’s case data is

significant. The simulation model should also accomplish to eliminate as much as possible this disparity.

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Figure 7: Field oil production total; History vs. Initial

Figure 8 illustrates the field oil production rate; the data of the history are compared to the data of the initial

case.

Figure 8: Field oil production rate; History vs Initial.

Figure 9 illustrates the well oil production rate for wells P-A2AH, and P-A17; the data of the history are

compared to the data of the initial case.

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Figure 9: Well oil production rate (1); History vs Initial.

Figure 10 illustrates the well oil production rate for wells P-A39A, and P-A1H; the data of the history are

compared to the data of the initial case.

Figure 10: Well oil production rate (2); History vs Initial.

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Process for History Matching

In order to perform the history matching technique the use of different boxes was selected in order to alter

the direction of flow and achieve the result that is desirable. In every box that was created transmissibility

multipliers in x- , y- and z- directions were used in order to alter the transmissibility of specific grid blocks.

For the proper directions of the flow to be identified the use of flowviz 2014.1 was crucial.

The table below presents the various different boxes installed and the transmissibility multipliers applied at

each direction in the reservoir in order to perform the history matching adjustments.

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Figure 11 illustrates the relative position of the boxes in the reservoir that were implemented throughout the

history matching. In many cases the area that a box has been assigned to influence, gets into the area that

some other box influences as well. The depth of the boxes that were installed does not have the same depth;

so this is an aspect that needs to be taken into consideration. However, when there is a conflict of

transmissibility assigned at a certain area, then the a multiplication of both transmissibility factors take

place.

Figure 11: Boxes implemented in reservoir space.

Different trials

The graph that is presented below for all the different trials is the one of well oil production total; history vs.

trial. The reason for that is because the main concern during the process of history matching was the well oil

production total for the four production wells that was selected to be history matched, since it is a critical

indicator of the production process.

Trial 1

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Figure 12: Well oil production total; History vs. Trial 1.

Two of the boxes (Box1, Box5) used aimed in increasing the flow in a certain area one box aims to block an

area (Box3) near the injectors (I-A5H, I-A38) and two other blocks to reduce partially the transmissibility of

the sections (Box2,Box4). After this trial the quantities that P-A1 and P-A2 produced deviate even more

from the original case. It is obvious that those adjustments have the opposite result for the history match that

is attempted.

Trial 2


In this attempt only Box1 is used with increased transmissibility (3 times higher) in every direction, in order

to identify the effect of this block in the four wells that are selected for history matching. This trial

maximizes even more the deviation of P-A1 and P-A2 production wells but results in a good history match

for well P-A17.

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Trial 3


In this trial three boxes (Box2, Box4, and Box5) where used in order to increase the transmissibility of the

areas that were placed, and on the other hand, three other wells (Box4, Box6, and Box7) target in decreasing

them. The aim is to increase the flow of water coming from the injectors, and therefore, increase in this

manner the secondary recovery of the reservoir. Results indicate that P-A2 production is decreased and tend

to be history matched and P-A17 achieves history matching. P-A39 also has a smaller deviation from its

original value. Even though this trial provides some encouraging results with respect to 3 out of 4 wells, it

fails to history match the most important one that is the main producer, well P-A1.

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Trial 4


The target of this attempt is to try to obtain better results for producer P-A1 and at the same time identify the

ways that is influenced. For that reason, five boxes are used; two of them (Box2, Box5) are constructed in

such order, that they will impose a tendency to increase the transmissibility of the region placed, and three

others (Box4, Box7, and Box8) that have a counter operation, which is to reduce the transmissibility of the

respective areas. One of the aspects of this attempt is to partially block the interconnection between P-A1

and P-A2 drainage areas, and another aspect is to reduce the quantity of oil that P-A1 produces. The latter is

achieved by reducing the transmissibility in the y-direction and at the same time the x and z directions are

significantly increased. The main goal of improving the P-A1 producer is achieved; however the other three

producers now present a significant deviation.

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Trial 5


Two of the boxes used increase the transmissibility (Box5, Box2) of the area that they are extended and the

other four (Box3, Box4, Box6, Box7) act towards reducing it or try to change the flow direction. The aim in

this attempt is to change the direction of the flow of the injection wells. Producer P-A17 is history matched

but the other three deviate from the production history.

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Trial 6


In trial number six only three boxes where used. Box number 3 aims to change the water flow of the main

injectors and increase the transmissibility in two other boxes (Box1, Box2). This attempt results on the

optimization of just the P-A1 main producer without a significant effect in the rest of the producers.

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Trial 7


In this attempt six Boxes are used in order to alter the direction of the flow of the reservoir. Only Box 5 aims

to increase relative permeability of the area that is situated (close to P-A1). The increase that Box 5 provides

is only at x and y directions and reduces the vertical transmissibility of the area reducing with this way the

oil quantity of the main producer. The other Boxes (Box3, Box4, Box5, Box6, Box7) aim in blocking

interconnections of nearby wells, and in changing the direction of the flow in order to history match the four

main producers. Only P-A1 main producer is history matched at the end of the run.

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Trial 8


This attempt uses the same Boxes as the previous on (trial 7) and the only difference is in transmissibility

multipliers of Box15 which is placed right in front of the P-A2 producer. The increased transmissibility of

the X and Z direction lead to an increase in the production of P-A1 and to a reduced oil production in P-A2.

The raise in vertical transmissibility did not lead to the increase of the production of P-A2 well because the

cells that were influenced were not the ones that have a direct contact with the producer. In addition

affecting the transmissibility in z-direction changes the transmissibility in other directions and thus changes

the direction of flow. This trial optimizes the one producer but lead to a significant deviation of the total

history matching of the reservoir.

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Trial 9


Box 15 is removed because it disturbs the history match of the other producers. Box 16 is implemented in

this attempt in order to alter the relative flows in another area away from the producers and injection that

concern the reservoir. The aim of Box 16 is to reduce the quantity of oil that is produced from P-A2 and P-

A17 producers that are affected the most from the changes at this area. In this attempt a relatively satisfying

history match is achieved in three out of four production wells, which means that the final solution is close

to what has been already designed.

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Trial 10


The same boxes are used as in the previous run. The only change is in the factors affected the relative flow

of Box 5. The y- and z- direction are reduced in order for the relative difference to be in favor of x direction.

With the above adjustment a portion of the oil instead of moving towards the P-A2 producer to move

towards the direction of P-A1 well. The other two wells have small deviations from the previous case.

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Results

Hereby the optimum case chosen will be presented, by making use of graphs derived from petrel software.

Figure 22 illustrates the oil saturation of the reservoir for the optimum case, after water injection.

Figure 22: Reservoir fluid flow for the optimum selection.

In the following figure (figure 23), an annotated picture shows the boxes that were selected in the optimum

solution of the history matching process.

Figure 23: Boxes implemented in reservoir space for the optimum solution.

In order for the saturations of the fluid phases to be shown, certain graphs are presented as well. In the

figures to follow, a more detailed view of the consequences in permeability will be presented, due to the

installation of the boxes that were previously presented. In order to better illustrate the effect of the boxes,

some representative slices of the reservoir have been cut off. Those slices are chosen at the areas mostly

affected by the presence of the boxes.

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Figure 24: Fluid flow of reservoir slices through boxes 3 & 16.

Figure 24 attempts to identify saturations in specific cells of box 3 and box 16. The basic function of box 3

is to block the injection of water and alter the direction of flow towards P-A39A in order to increase the

quantity of oil that is produced from this well. Saturation of oil is increased around the area of the well;

therefore our target is accomplished. It can also be noticed that in the very last layer of the slice, the

saturation of oil has been increased and this is due to the low transmissibility in Z-direction. Box 16 aims to

reduce the flow of oil that is produced from wells P-A1 and P-A39 and this has an effect in the saturations. It

is obvious that there are a lot of boxes with increased oil saturation.

Figure 25: Fluid flow of reservoir slices through boxes 3, 5, 6 & 16.

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Figure 26: Fluid flow of reservoir slices through boxes 3, 6 & 16.

This cross section gives a better understanding of the flow of fluids in y direction due to the presence of the

three boxes in the area. Box 6 affects only the top 2 layers that are oil saturated and reduce the

transmissibility of those cells. The target is to reduce the quantity of oil that P-A39 produces. Generally at

the edges of the reservoir and at the bottom layer a quantity of oil is trapped. Moreover, it can be noticed

that below Box 4, a significant quantity of oil is trapped also. This is due to the fact that Box 4 enforces a

really low transmissibility in the cells and acts only on 6 top layers.

Figure 27: Fluid flow of reservoir slices through boxes 3, 6.

As already mentioned box 3 reduces the transmissibility in the top two layers and blocks the flow through

those cells. As far as the influence of those two boxes in P-A39 well is concerned, Box 3 aims in reducing

oil production and box 6 aims in increasing it using secondary recovery technique.

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Figure 28: Fluid flow of reservoir slices through boxes 4 & 5, passing over producer PA-1H.

In figure 28, it can be easily noticed the effect of the boxes in oil saturation of the reservoir. Box 4 (which

acts on 6 layers) enforces a really low vertical transmissibility. Consequently, there exists some oil that is

left behind in between layers and does not to flow towards the producing wells. The flow of water is

significantly promoted due to the presence of Box 5 which has altered the direction of flow. Finally producer

PA-1H receives a significant quantity of oil due to water injection applied in the area.

Figure 29: Fluid flow of reservoir slices through boxes 5 & 7, passing over producers P-A1H and P-

A2AH.

The above picture indicates the flow of fluids toward P-A1H and P-A2AH wells, which are the two main

producers of the reservoir. The increased transmissibility of box 5 affects the saturation of the cells that are

shown above. Increased oil saturation can be distinct in the grid cells next to P-A1 (transmissibility in x-

direction is promoted the most). The increased transmissibility in the z-direction of box 5 transfers a great

quantity of water towards P-A2AH and increases secondary recovery of oil. In the cell of Box 7 a reduced

flow of water can be shown due to reduced transmissibility in this area. Oil saturation is greater in the cells

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next to P-A17. From the above figure the P-A39A well is shown and the area next to P-A1H which is oil

saturated. This well operates in order to produce the oil that P-A1H cannot reach.

Figure 30: Well oil production total; History vs. Optimum.

Finally for the optimization of the history matching a really small adjustment is performed in the relative

permeability of the z- direction; in order for the main producer (P-A1) to be sufficiently history matched.

This attempt is considered to be the optimum one because the four main producers of the reservoir match at

the same time the history of the production to a tolerable degree. After the selection of the optimum trial is

made several plots are presented to support this selection.

In figure 31 the water cut of the optimum scenario and the water cut of the history of production are

presented in order for a comparison to be made. The water cut of the final attempt demonstrate a deviation in

certain dates but the general trend is the same. This deviation occurs due to the fact that the history matching

process was aiming in the history match of WOPT and not to those of the water cut of the four wells. On the

other hand when comparing the original case before history matching, a significant improvement is obvious

and the greater deviation for the water cut is for producer P-A39. Producer P-A39 has the least contribution

in oil production and deviates to a greater extent at the WOPT diagram after the history match procedure.

That is the reason why the deviation in water cut is greater for this well, compared to the other three

producers.

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Figure 31: Well water cut; History vs. Optimum.

In figure 32 the cumulative production for the reservoir is presented in comparison with the history of

production. Until 3400 days of production the two cumulative productions present an almost perfect match.

After that and until the end of the production period the cumulative oil production starts deviating from the

history data. This is due to the fact that there is not a perfect match between the four production wells that

are used and moreover because producer P-A35 is excluded from history matching but is taken into account

in the cumulative oil production of the reservoir. The P-A35 producer starts operating at the day 3200 and

after a few days the FOPT starts deviating more intensely.

Figure 32: Field oil production total; History vs. Optimum.

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Figure 33: Field oil production rate; History vs. Optimum.

Finally, for the identification of the quality of the history mach process the field oil production rate is

presented in comparison with the history rate. In general, until day 2800 deviations are not significant and

this is evidence that history matching quality is adequate. After the 2800 day and especially after the 3200

day of production the deviation becomes greater. This result is for similar reasons with FOPT

overproduction indications and deviation of the water cut in P-A39 producer. The main reason is the lack of

P-A35 producer form history matching procedure (operates at the end of production) and the overestimation

of his production well until the end of the production period.

To conclude, the quality of the history matching that was performed during this project is regarded as

adequate. From the three graphs that were presented above the best fit as far as the history match is

concerned, is in FOPT of the reservoir. On the other hand well water cut and field oil production rate for the

reservoir present a deviation between history and optimum trial especially in the last days of production. In

general, the simulation model that was built could in fact provide realistic future prediction scenarios that

will contribute valuable information for the prospective production of this particular reservoir.

Documents

Semester Project in Reservoir Simulation