DEVELOPMENT OF OBM UNIDRIL FORMULATION FOR DRILLING AHFP WELLS IN YAMALO-NENETS AUTONOMOUS DISTRICT

DEVELOPMENT OF OBM UNIDRIL FORMULATION FOR DRILLING AHFP WELLS IN YAMALO-NENETS AUTONOMOUS DISTRICT

In summer 2016, in Yamalo-Nenets Autonomous District (YNAD), a new development of AKROS LLC was applied, namely, the UNIDRIL mud system with improved rheology. The system was developed using the integrated approach to determine optimal parameters and formulation together with NOVATEK STC. The system enabled to eliminate complications while drilling an exploration well and to deliver the target drilling results.

Drilling of exploration wells in conditions of abnormally high formation pressure (AHFP) cause risks typical of using high-density flush fluids. Normally, AHFP intervals contain a minimal safe density range between anomaly coefficients and fracturing coefficients. For designing wells in such conditions, it is important to perform all necessary hydraulic calculations at the design stage to determine maximum pressures and equivalent circulating density (ECD) resulting from drilling mud circulation. In addition to hydraulic calculations, determination of an optimal well design, and BHA to be applied, it is necessary to develop formulations of drilling muds that would ensure the hydraulics program performance taking into account potential contamination with drill cuttings, planned densities of flush fluid, and concentration of solids.

Specifics of developing drilling mud formulations and hydraulics program for drilling the AHFP wells is additionally complicated in case of using oil (hydrocarbon) based drilling muds (OBM) with specific features. In the first turn, this is a significant dependence of rheology on temperature. In low temperature conditions, during well construction in YNAO permafrost intervals, this feature can result in viscosity growing in upper intervals in case of long-term circulation stops, for example, before lowering a casing string. Rheology growth, in its turn, causes the ECD increase after the circulation is restored and the increase of circulation loss probability. However, in spite of the technical features of drilling with OBM, application of such systems is justified for drilling of exploration wells due to a higher well productivity, especially gas and gas condensate wells [1,2].


Table 1.Target rheology properties

Parameters

BoD for density of 2.05

Measured temperature, оС

70°

From/to

Gel strength 10 s

lb/100ft2

10 

15

Gel strength 10 min

lb/100ft2

 20

25

PV

cP

45

60

YP

lb/100ft2

5

10













At the end of 2015, AKROS together with NOVATEK STC performed the design work to develop the OBM UNIDRIL formulation with densities equal to 1.75 and 2.05 g/cm3 for drilling an AHFP exploration well in YNAD.

At the first stage, based on hydraulic analysis of the well design and BHA to be applied, NOVATEK STC’s specialists formulated requirements for drilling mud rheology at different temperatures for densities of 1.75 and 2.05 g/cm3. See Table 1 for Basis of Design (BoD) data.

Based on the set parameters and target values, AKROS laboratory tested several UNIDRIL system formulations with different emulsifying agents applied. Laboratory testing resulted in two formulations described in Table 2, which demonstrated rheology closest to the target values. An important condition for testing was contamination of laboratory samples with drill cuttings and measurement of rheology at several temperature values; sedimentation stability was also determined for all samples. Based on laboratory study results, two UNIDRIL formulations were approved for application in the field.


Table 2. Results of laboratory testing to optimize the formulation

MINERAL OIL

ml

600

530

ORGANIC BENTONITE

g

10

6

EMULSIFIER I

ml

22

10

EMULSIFIER II

ml

7

15

Lime

g

23

23

Water

ml

60

28

CaCl2

g

24

10

WETTING AGENT

ml

4

4

RHEOLOGY MODIFIER

g

4

4

FILTRATE CONTROL AGENT HTHP

ml

1

1

Barite

g

920

1570

CALCIUM CARBONATE

g

50

50

DRILL CUTTINGS

g

48

48

Drilling mud parameters after thermal ageing for 16 hours at 80°С

Density, g/cm3

1,75

2,20

Rheology at temperature:

°С

50

80

50,00

80

600 rpm

100

69

182

92

300 rpm

52

31

94

48

200 rpm

35

22

65

34

100 rpm

19

13

35

19

6 rpm

3

4

5

4

3 rpm

3

3

4

4

Gel strength 10 s

lb/100ft2

5

7

5

8

Gel strength 10 min

lb/100ft2

13

15

18

19

Plastic viscosity

cP

47

26

89

43

YP

lb/100ft2

3

5

6

4

ES, volts

V

>2000

>2000


Application of the developed formulation in summer 2016 fully confirmed the laboratory results and enabled to get improved rheology values and optimized dependencies of rheology profile on temperature. See diagram 1 for actual measurements of OBM parameters while drilling in the AHFP interval.

 

The staged approach to development of the UNIDRIL OBM formulation for drilling the AHFP exploration well in YNAD enabled:


  •  implementation of complete engineering solutions for well flushing, from hydraulics program development and laboratory testing to application of the formulation in the field;

  • Strict compliance of engineering recommendations enabled to eliminate losses of expensive OBM flush fluid while drilling and lowering of strings in the AHFP intervals;

  • Considering the obtained results, this approach can be recommended for staged modeling of drilling mud systems for development of flushing programs and well construction designs, especially for cases with OBM to be applied.

 

  

AUTHORS:

Grigoriev M. S., head of monitoring of drilling center NOVATEK-NTC LLC

Sidorov D. N., deputy chief of monitoring of drilling center NOVATEK-NTC LLC

Vlasov E.N., drilling fluids expert NOVATEK-NTC LLC

Korolev A. V., general director AKROS LLC

Ryabtsev P. L., technical director AKROS LLC

Source: "Burenie i Neft"№3 (2017), 46.