Changes in Annulus Pressure, Increase Oil Production Carlos Brunings, Gabriel Becerra, Jose Marcano, PDVSA, Richard Marquez, INPELUZ, Armando Riviere, Leonardo Mena, Jose Porto, Alberto Pereira, ATS

ABSTRACTOil Crude production in open annulus is common in Oil Industry, due to environmental regulations, many Oil Companies decided to attach the gas line to the production line in a way that gas is not vented to atmosphere. Gas pressure in annulus is a mean to evaluate optimization conditions in each well detecting a maximum point where crude mobility is favored over water or gas near the well bore (in each well three or four points should be detected), these phenomenon is accounted for in the theories of MOBILITY, CAPILLARITY and HISTERISYS, evaluated in wells in Venezuela for the last ten years.

The Annulus Pressurization is a noninvasive method for crude production for wells with gas and water production. Software incorporated into a control valve system that closes the annulus and detects the pressures increments, choosing the optimal point by means of mathematical calculations, is the preferred method to achieve maximum crude production with minimum water yield. At maximum production point, this equipment called SOE, proceeds to discharge gas increments into the production line by controlled closing and openings of a controlled valve, at the same time the Water Cut Monitor (essential part of SOE) gives information to a computer controller (hardware), in order to modulate automatic openings and closes of annulus gas production, to control efficiently the well parameters with valuable increase of crude production. On February 6, 2015 SOE test began in oil well NZZ-196 of Petrolera Indovenezolana, daily monitoring of the well by owner and officially five (5) well tests in 6 months were performed obtaining an increment in crude yield and

decrease in water cut. SOE equipment has proven to be effective by itself, operating through an expert system to reach optimum production points with an increase of 12% in crude production. Using SOE, amount of water yield to surface was reduced which means additional savings and good amount of crude oil to increase the gains in operation.

1. INTRODUCTION Annulus pressure control in well casings was developed in Venezuela since seventeen years ago. Over 70 of these devices have been installed and about a 50% successful rate on improving production has been obtained, (1). Some of the improvements (1/3 of the wells) are due to increasing pump efficiency owing to the reduction of gas entrained into the pump when the pressure of casing is being controlled by an automated pressure control valve set at an optimum value; it causes a variation of the dynamic fluids reducing the amount of free gas in the annular space and pump The other 2/3 of the wells occurred because of water cut reduction; See table 1. One of the explanations to describe this phenomenon is by the imbibitions and drainage process. During the production of wells before annular pressure control, the displacement of the oil (wetting phase) by water (non wetting phase) is of the drainage type. The water moves through the largest porous channels of the rock, and oil through the smallest ones. During the bottom hole pressurization process (Pwf increases by properly managing the annular pressure) water tends to recede in the porous media close to the well, therefore, the process of displacement is reversed from drainage to imbibition (Figure 1) This reduces the Krw, and the same time occurs a µo reduction by the partial entry of free gas into solution until the oil is saturated at the new pressure around the well; these two phenomena contribute to the reduction of the Krw*µo product, and therefore to the reduction of water cuts. (1), in order to demonstrate this

process a mathematical model and lab test is being developed at Inpeluz.

2.THEORETICAL MODEL

Fluid flow of two immiscible fluids trough porous media can be described by Darcy´s law, and a mathematical correlation can be obtained that combines: pressure, permeability and fluid rate. This mathematical expression is developed for a cylindrical core as shown, by which oil and water is injected through it, in order to determine relative permeability and irreducible water saturation.

oeP and

weP represents inlet pressure for

oil and water osP and

wsP defines outlet

pressure at the outlet for both fluids. Fluid rates for water and oil are represented by

oq and wq , respectively. oµ and wµ represents viscosity of oil and water . Effective permeabilities are also

represented by ok and wk , respectively.

This mathematical expression differs from the ones used in the lab for determining inlet pressure in cores when using triaxial cells:

oe

o

w

o

we P

qP

=

µµ1

556.0

Applying Buckley-Leverett, for oil flow of

can be rewritten as follows:

−

−

+

=

oe

os

we

ws

oe

we

w

o

o

w

o

PP

PP

PP

kk

f

1

1

1

1

µµ

Now of depends on inlet and outlet

pressure for each phase. If we consider that

outlet pressure for water is cero ( 0=wsP ),

then permeability relationships as a function of pressure can be written as:

o

ooe

os

we

oe

o

w

ro

rw

o

w

f

f

P

P

P

P

k

k

k

k )1(1

−

−

=

=

µµ

This mathematical expression combines permeability as a function of outlet oil

pressure ( 0=osP ) and water saturation.

This new condition allows a definition of a family of curves as shown in Figure 2

Meanwhile the institute of oil research (Inpeluz) of Zulia University in Venezuela is modifying its laboratory equipment, in order to obtain from some core samples of different reservoirs, a family of curves of relative permeability as a function of pressure. This lab tests are being done at the present time of writing this paper.

The importance of this investigation deals on the fact that permeability can be represented as a function of the pressure variable , especially at the outlet of the core sample which represents in the reservoir Pwf or flowing bottom hole pressure.

−

−

=

we

ws

oe

os

oe

o

w

ow

ow

we

PP

PP

Pqk

kqP

1

11

µµ

3. EQUIPMENT DESCRIPTION

This equipment is self-contained (hardware/software) and diagnoses the behavior of the parameters of the artificial lift system and determines the optimal parameters of operation of the lifting system (speed, casing pressure, etc.) to obtain the maximum production of the well. See Figure 3

The software consists of expert systems and diagnostic patterns that control favorably irruption of water or gas and improve oil production, this is performed trough: analysis of electrical parameters coming from the variable frequency drive (vfd), interpretation of down hole well pressure, surface temperature of the fluid and wellhead pressure, in order to obtain the optimal casing pressure.

The equipment monitors the operational conditions of the wells and the software decides if it has to actuate over the control valve installed at the casing or over the speed of the motor. See Figure 4

Components

A. Field computer. The Remote Control Unit (RCU) receives information from: vfd, motor, torque, frequency, velocity of pump, and can control the frequency and speed of the surface drive.

The RCU is programmed to gather information from the drive and operate the artificial lift system based on the analysis of the Expert System. It records and sends operational and historical data to a SCADA , DCS , etc. It communicates with automation devices trough 4-20 mA, 1-5 Volts and Modbus RTU protocol, interfaces with RS-232, RS-485 or Ethernet.

B. Software It is programmed in Language C/CC+; the software was developed using Artificial Intelligent techniques, such as: Expert Systems, neural network, and diffusive logic. The expert system tries to

increase the input pump pressure by closing the annular control valve, and releasing the casing pressure when it is needed, in order to maintain an optimal point. Maintaining this optimal point is essential for optimizing the well production. The software also analyzes and process information gathered from: the instrumentation installed at the wellhead, VFD parameters, different pattern recognition of pump cards, at the same time, the program must avoid a significant decrease of the dynamic fluid level to evade down hole pump pump damage (Figure 5).

C. Surface Instruments (Figure 6)

• Fluid temperature device

• Pressure sensor at the tubing

• Control valve at the casing

• Water cut monitor in the flow line,

the brand is Sentech WCM,

patented Secap technology, based

on an electronic oscillator that is

influenced by the dielectric constant

of the media surrounding it. (Figure

7).

• Batteries to provide energy for valve actuation and avoid harmonics and noise from vfd

4. Results for well NZZ-196

Well NZZ-196 completed in the H sand of

the mixed company (ONGC and PDVSA)

Indo Venezolana is located in eastern

Venezuela (Figure 8), was selected for

evaluation of SOE equipment because it

had a stable production of 730 BFPD and a

water cut that varied between 24-36% in

2014, (method of collecting is by one

sampling of water cut at the wellhead), °API

of the oil is 15 , a GOR around 200, a

progressive cavity pump was installed on

July 2012 (Figure 9) . On February 5 2015,

a well test was performed with a portable

well test unit consisting of a separator and

tank, water cut measurements were taken,

and to get an average, several well samples

were collected and the result was 32%, with

a total fluid of 623 BFPD and net oil of 440

BOPD at 200 RPM, this velocity remained

the same through the whole evaluation till

October 2015. This well test was taken as

the initial well test to compare against the

following tests with SOE equipment. After

that the well was pressurized manually with

the equipment, without the use of the water

cut monitor, which was not available at the

time, and the calibration of the expert

system was based on electrical and fluid

level parameters as can be seen in table 2.

A well test was performed on February 7 at

a casing pressure of 250 psi and tested 690

BFPD, 490 BOPD with an average water

cut of 33%.

Also from this table 2, it can be concluded

that fluid level over the pump increased

from 502 feet to 723 feet, improving pump

efficiency, since less gas is liberated, about

88 psi aditional of Pump intake pressure,

from 208 psi to 288 psi approximately

(Figure 10)

Fluctuating water cut measurements were

reported during the months between March,

April and May since the optimal casing

pressure point was not obtained as the

water cut monitor was damaged during

installation and was under repair, the

calculation for optimal point was still manual

(around 420 psi). A well test performed 12

of April by a multiphase test meter( Agar)

with an electronic water cut measurement

system, which is the most reliable, since it

calculate water cut every 10 minutes during

the test, Agar utilize a microwave

transmitter (2.45 Giga hertz) to measure

bulk dielectric properties of the flow stream,

regardless of salinity, density or viscosity

and the high frequency signal will maintain

accuracy in the presence of process

coatings.

Results obtained were 692 BFPD, 398

BOPD, 43%water cut, 238 GOR, Figure 11

represents this period

Finally during late May and early June 2015,

the new water cut monitor arrived and was

installed, also automatically the software

was getting the optimal point due to

changes in the program; this water cut was

callibrated against continuos water cut

sampling collected in two hours at different

days in June and analyzed in the laboratory

of San Tome District, and also was

compared against a well test performed by

Agar system on 14 June 2016 , following

are the results in table 3, in which there is

less than 2% difference between AGAR and

SOE´s water cut monitor.

The result in Figure 12 shows that the

Sentech System records the water cut in

real time and continuously

As mentioned a well test performed by

Agar, June 13th yielded: 756 BFPD, 518

BOPD, 386 GOR (gas oil ratio), 30% water

cut at 332 psi casing pressure as the

optimal point finally was found for controlling

water production and improving pump

efficiency. .

Further well test were performed at the

same casing pressure, resulting for August

12 th, Total fluid per day 712 (BFPD), and

491 (BOPD) barrels of oil with a water cut of

30.5% recorded from the SOE equipment,

By September 13 th, 708 BPD and 488

BOPD with the same water cut as August.

Finally well test on 13 of October, with 740

BFPD, 460 BOPD, and 35.3% water cut

from the Sentech monitor installed at the

wellhead. See table 4.

In early November 2015 the evaluation of

the equipment finished, and was

disconnected from the well, all equipment

were returned to the owner ATS

5. Conclusions

• After automatic control of casing

pressure has been established at its

optimal point, SOE has controlled in

well NZZ-196 water cut, and

improved pump efficiency,

increasing oil well production by a

12% average.

• Best water cut accuracy

measurement is obtained through

electronic monitoring, using

equipment such as Agar or Sentech,

if these equipments are not

available, then a fair water cut

measurement can be calculated by

sampling continuously or getting an

average, and lastly through a one

sample collection at the wellhead.

• Lab tests are under way in order to

correlate relative permeability of oil

and water with different pressures

and water saturation , in order to

determine the optimal flowing

pressure.

6. REFERENCES

1.- Victor Castillo 2012 internal report on

Maxiprod, Awpa presentation of Petrolera

Indovenezolana, 2014.

2.- Brunings C . et al. Oil Production

Increase through an Automated Annulus

Pressure Control System applied Extra-

heavy Oil wells in PDVSA, San Tome

District; World Heavy Oil Congress, 2009-

128

3.- Ramirez M, et al . Automated Surface

Gas Handling Through Expert Systems for

Optimization of Artificial Lift Systems, Spe

13lAAl,2013

Table 1.

NUMBER EQUIPMENT WELL YEAR BOPD (BEF.) BBOPD (AFT) ORIGIN1 MAXIPROD UM-113 1998 118 187 WATER CUT REDUCTION2 UM-78 1998 546 872 WATER CUT REDUCTION3 UM-105 1998 288 540 WATER CUT REDUCTION4 UM-114 1998 154 173 WATER CUT REDUCTION5 GG-185 2001-2002 33 263 WATER CUT REDUCTION6 MFB-440 2001-2002 142 400 WATER CUT REDUCTION7 MFB-584 2001-2002 63 960 WATER CUT REDUCTION8 MFB-432 2001-2002 384 1094 WATER CUT REDUCTION9 MFB-595 2001-2002 29 238 WATER CUT REDUCTION10 JOC-605 2004 190 570 WATER CUT REDUCTION11 MOT-35 2006 1326 2518 WATER CUT REDUCTION12 MOT-58 2006 608 896 WATER CUT REDUCTION13 MOT-59 2006 787 1100 WATER CUT REDUCTION14 LS-2880 2012 67 168 WATER CUT REDUCTION15 LS-2200 2012 98 187 WATER CUT REDUCTION16 SIAP MFB-386 2005-2006 131 170 PUMP EFFICIENCY/GAS 17 MFB-460 2005-2006 58 98 PUMP EFFICIENCY/GAS 18 MEL-170 2006 69 100 PUMP EFFICIENCY/GAS 19 11M-269 2005 1283 1506 PUMP EFFICIENCY/GAS 20 MFB-398 2008-2009 47 56 WATER CUT REDUCTION21 MFB-460 2008-2009 105 115 PUMP EFFICIENCY/GAS 22 MFB-582 2008-2009 79 118 WATER CUT REDUCTION23 MFA-70 2008-2009 17 78 PUMP EFFICIENCY/GAS 24 JOC-627 2008-2009 40 94 WATER CUT REDUCTION25 PCPOS MFB-656 2007-2008 800 900 PUMP EFFICIENCY/GAS 26 MFB-659 2007-2008 780 900 PUMP EFFICIENCY/GAS 27 MFB-670 2007-2008 270 450 PUMP EFFICIENCY/GAS 28 PDVSA MFB-669 2014 190 234 PUMP EFFICIENCY/GAS 29 AWPA NZZ-177 2010-2014 603 772 WATER CUT REDUCTION30 NZZ-217 2010-2014 688 1147 WATER CUT REDUCTION31 NZZ-246 2010-2014 100 270 WATER CUT REDUCTION32 CH-502 2014-2015 689 742 PUMP EFFICIENCY/GAS 33 CH-508 2014-2015 312 355 PUMP EFFICIENCY/GAS 34 SOE NZZ-196 2015 441 518 WATER CUT CONTROL

SUCCESSFUL AUTOMATIC CASING GAS PRESSURIZATION EVALUATION

Table 2 Well parameters, SOE operating manually

Table 3 Comparison of water cut from SOE, Agar and a Laboratory

Table 4. NZZ-196 Well Test

DATE BFPD BOPD WATER CUT (%)

WATER CUT MEASUREMENT

SOE CONTROL

TEST EQUIPMENT

% OIL INCREMENT

05/02/2015 630 440 32 AVERAGE NONE TANK -07/02/2015 690 490 33 AVERAGE MANUAL PORTABLE TANK 1112/04/2015 692 398 43 AGAR MANUAL AGAR -1013/06/2015 756 518 31 AGAR AUTOMATIC AGAR 1830/07/2015 729 517 29 SENTECH/SOE AUTOMATIC SEPARATOR 1812/08/2015 712 491 30,5 SENTECH/SOE AUTOMATIC SEPARATOR 1213/09/2015 708 488 30,5 SENTECH/SOE AUTOMATIC SEPARATOR 1113/10/2015 740 460 35,3 SENTECH/SOE AUTOMATIC SEPARATOR 5

Figure 1. Hysteresis of permeability curves.

Figure 2. Relative Permeability curves vs pressure and Sw

0.01

0.1

1

10

100

1000

10000

0 0.2 0.4 0.6 0.8 1

krw/kro

Sw

RP = 2% RP = 20% RP = 50% RP = 80%

Figure 3. Surface well Schematic for automatic valve control

oe

os PPRP /=

Figure 4. Functional schematic of a well optimization system

Figure 5. Components of SOE Equipment

Figure 6. Surface Instruments

Figure 7. Water Cut Monitor

Figure 8. General data Indovenezolana, San Cristobal Field.

Prod. Acumulada petróleo (Nov. 2015): 111,3 MMBN Prod. Acumulada gas (Nov. 2015): 42,6 MMMPCN Pozos Activos (Nov. 2015): 109 Pozos Inactivos (Nov. 2015): 22 Pozos de disposición de agua 03 Res. Rem. De petróleo (Nov. 2015): 255.7 MMBN Res. Rem. De gas (Nov. 2015): 52.7 MMMPCN Unidad productora principal: FM. OFICINA N° de yacimientos oficiales: 4 Profundidad promedio: 2700 pies Rango de gravedad 14-18°API Producción Actual: 24.6 MBND Método de producción: BCP N° de estaciones de flujo 2 Superfície: 160.18 Km2

RESERVOIR OFIH SCR2, WELL NZZ-196

Figure 9. Well Completion

Figure 10. Increase in pump Efficiency

PIP: 280 psi

PWF2

PIP :200 psi

PWF1

AGUA

PETROLEO

PETROLEOAGUA

GAS

GAS

Anular

PIP: 280 psi

PWF2

PIP :200 psi

PWF1

AGUA

PETROLEO

PETROLEOAGUA

GAS

GAS

PIP: 280 psi

PWF2

PIP :200 psi

PWF1

AGUA

PETROLEO

PETROLEOAGUA

GAS

GAS

Anular

Anular

DIAGRAMA DE COMPLETACION DIAGRAMA DE COMPLETACION

Figure 11. Water cut measurements, SOE Manual mode

MUESTRAS POZO NZZ-196

42,6

22

36

26

2016

19

38,48

19

400

26 2832

36

6

1620 20

3432 30

24

34

2824

2024

14

26

20

24 2420

2429

13

40

9

42

0

510

15

2025

3035

40

4550

FECHA

28-Sep

23-Oct

30-Oct

13-Nov

26-Nov

05-Feb

05-Feb

05-Feb

05-Feb

07-Feb

07-Feb

07-Feb

07-Feb

07-Feb

07-Feb

07-Feb

07-Feb

09-Feb

09-Feb

09-Feb

09-Feb

09-Feb

09-Feb

10-Feb

12-Feb

05-Mar

12-Mar

13-Mar

07/Abr

11/04/15

27/04/15

20/05/15

02/06/15

08/06/15

09/06/15

11/06/15

11/06/15

13/06/15

14/06/15

FECHA

%Ay

S

%AyS

Figure 12. Sentech water cut measurement