CASE HISTORY OF USING UNIFORM K POTASSIUM FORMATE BASED BARITE-FREE MUD SYSTEM DURING THE DRILLING OF A HORIZONTAL WELL TO АС PAY ZONE OF THE YUZHO-PRIOBSKOYE FIELD

CASE HISTORY OF USING UNIFORM K POTASSIUM FORMATE BASED BARITE-FREE MUD SYSTEM DURING THE DRILLING OF A HORIZONTAL WELL TO АС PAY ZONE OF THE YUZHO-PRIOBSKOYE FIELD

In the beginning of 2015, a demand for changing the drilling mud system with consideration for more advanced well design has arisen at the fields of Gazpromneft-Khantos. Since 2005, directional wells were drilled with fresh water encapsulating drilling fluid systems based on acrylic polymer at this field. These systems were entirely suitable for failure-free drilling of S-shaped wells with a step-out of less than 2,000 m. The well design becoming more advanced and the number of horizontal wells and wells with a pilot hole increasing required modification of the drilling fluid formulations. 


Based on the case record of using different drilling fluid formulations in the region, the engineers of Gazpromneft-Khantos decided to use potassium chloride muds. The basic difference between these systems and fresh-water based muds is the presence of non-organic inhibitor in potassium chloride fluids. The potassium chloride drilling fluid systems have a number of significant advantages as compared to acrylic polymer based systems: particularly these are high stability of solids content and more balanced rheology. Nevertheless, the potassium chloride drilling fluid system used on the well pad of the South Priobskoye field did not resolve the well stability problems in full, since this well pad is located in the area of the highest stratigraphic uncertainty due to the interbedding of poorly cemented rock with dissimilar physical and chemical properties, and the presence of a tectonic fault which significantly adds to the cost of well drilling on this pad. Most of the problems during the drilling are encountered in the intervals comprising the shales of medium catagenetic maturity, which are extremely sensitive to hydrodynamic fluctuations caused by hydraulic flow of the circulation fluid and require specific technological approach.


Thus, in the first part of 2017, Gazpromneft-Khantos implemented a project of drilling a horizontal well to AC pay zone at the South Priobskoye field. In the course of this project, the engineers of Gazpromneft-Khantos together with their peers of Gazpromneft R&D Center implemented a program of engineering measures which has ensured successful drilling of the well. The project development included the selection of optimum solutions for the entire drilling program: the selection of the most suitable BHA and the optimum formulations and parameters of drilling muds, and optimization of the drilling process based on geomechanic modeling and hydraulics analysis. At the initial phase of the drilling fluids program for the horizontal drilling to АС pay zone of the South Priobskoye field, the engineers have completed detailed analysis of the processes with respect to earlier drilled wells in order to identify the key geological and technical risks which may be encountered during the drilling.


Following the results of this analysis, the primary technical challenges specific to the drilling of horizontal wells to AC pay zone of the South Priobskoye field were identified to be as follows:

 - Determining the optimum density of the drilling mud while drilling the tangent section and the liner section

- Preventing lost circulation due to high equivalent circulating density (ECD) while drilling the tangent (ESP) section

- Maintaining the wellbore stability in the interval of the tangent section and the horizontal liner section, and failure-free running of the liner conveying casing string with the liner modified for multistage hydraulic fracturing job.


Assuring the wellbore stability throughout the tangent section and the liner takes on particular importance in the projects where the wells are drilled to AC pay zone of the South Priobskoye field. It is generally known that the greatest challenges with the wellbore stability arise when steering through unstable formations at inclination angles of greater than 60 deg. and with dogleg variations. When designing the circulation program in such environment, a wide range of technical tasks must be addressed in order to drill the well without a failure.


The requirements that should be observed when selecting the drilling mud include:

- High environmental compatibility of the drilling mud system

- Zero discharge philosophy when drilling the liner conveying casing and liner intervals

- Close-loop drilling system

- Maintaining wellbore stability throughout the entire interval

- Tolerance of the drilling mud system to aggressive formation fluids.


On the basis of their objectives and the analysis completed for the wells drilled in previous years, and with consideration for all specific geological features existing in the liner conveying casing interval, the engineers of AKROS Oilfield Services Company have come up with barite-free UNIFORM K drilling mud system based on potassium formate. In addition to having increased inhibiting properties and complete absence of chloride ion, this mud system can be used with specific gravity to 1.65 g/cm3 without using high concentrations of Barite inert weighting material, which minimizes the effect of adverse hydrodynamic fluctuations of the circulation fluid on poorly cemented wellbore walls in the annulus. Before being used at the field of Gazpromneft-Khantos, barite-free UNIFORM K drilling mud system based on potassium formate was studied in the laboratory of Gazpromneft R&D Center, and recommended for field use.


Table 1 lists the basic parameters of drilling muds based on binary salts (potassium chloride /sodium) versus the potassium formate drilling mud. The circulation program, especially for the liner conveying casing interval, was prepared with the analysis of the system’s rheological profile at different temperatures, filtration properties, compatibility with potassium and sodium salts, and interaction with aggressive cement. The results of this analysis were used to determine optimum limits of the drilling mud parameters, chemicals concentration, mud treatment method and procedure for replacing the mud in the wellbore with a different type of mud. The mud parameters were maintained within pre-defined range while drilling the well. This permitted minimization of the risk of complications related to unstable behavior of the wellbore walls and adverse external effect of aggressive formation environment.


Furthermore, laboratory tests of UNIFORM K potassium formate based mud permitted several focus areas for process adaptability of the system to be identified. For instance, the lubricating additive was completely excluded from the formulation, and approach to рН maintenance and chemical impurities (CO32 and HCO3) control was changed. Modifications made to the mud formulation allowed stop using the acrylic polymers, which subsequently during the drilling of the tangent section has become a prerequisite for converting the formate-based UNIFORM K mud to MAX-FLOW drilling-in mud without any downtime. The avoidance of acrylic polymers had absolutely no impact on inhibiting properties of the drilling mud and resistance of the system to aggressive formation chemicals, the evidence to which was the stability of chemical composition of mud filtrate (P(f)/M(f) throughout the entire time of the liner conveying casing interval drilling.


                Table 1. The Formulations and Parameters of PRIMOSOL and UNIFORM K Inhibited Drilling Muds Based on Potassium Chloride and Potassium Formate used when Drilling the Tangent Sections on the Well Pad of the Yuzhno-Priobskoye Field 

Formulation – The Content of Basic Chemicals

 

PIMOSOL potassium chloride polymer mud

UNIFORM K potassium formate based polymer mud

Function

Components

kg(l)/mм3

Components

kg(l)/mм3

Viscosifiers

Xanthane gum biopolymer

 

1.5-2.5

 

 

Xanthane gum biopolymer

 

2.37

Filtration control additive

Polyanionic cellulose

7.2-16.4

Polyanionic cellulose, starch

2-4/7.3

Sealant

Calcium carbonate

60-80

Calcium carbonate

90-120

Inhibitor

Potassium chloride

114-205

Potassium chloride (base)

1.35

Weighting material

Sodium chloride

20

 

 

рН control agent

Caustic /potassium carbonate / soda ash

2-4/0/0

 

1.84/7/9.1

Drilling Muds Parameters

Density, g/cm3

1.28-1.50

1.28-1.45

Fann Viscosimeter Readings

Rheology measurement temperature, °C

49

49

600 rpm

44-93

57-97

300 rpm

31-58

39-66

200 rpm

22-44

31-54

100 rpm

15-31

21-39

6 rpm

6-14

4-13

3 rpm

5-13

3-10

PV cP

13-35

18-31

YP lb/100ft2

18-23

21-35

СНС 10 s

5-14

4-9

СНС 10 s

7-48

5-18

Water loss, ml/30 min

4.9-6.2

4-6

Cl-, mg/l

44 000 - 140 000

0

К+ Content

29 630 – 32 386

250 000 – 105 000

рН

9,5-8

9,5-11

МВТ, kg/m3

0-54

0-28

P(f)/M(f)

0,15/0,3 – 0,18/1,58

1,4/0,031 – 3,14/0,083

P(m)

0,2-0,3

4-2

Lubricant in solution %

3 - 16

0


Figures 1 and 2. Comparative Analysis of Filtrate of Drilling Muds used in the Tangent Sections of Wells Drilled on the Well Pad of the Yuzhno- Priobskoye Field

                                              

An important part of the wellbore stability program when drilling the tangent hole is the determination of correct density of the drilling mud, the change of the mud inhibiting base and its chemical stabilization by combined treatment of potassium carbonate, soda ash and caustic. Using geomechanics studies that were conducted by Gazpromneft-Khantos in the region as the groundwork, in addition to increasing the mud density to 1.45 g/cm3, the mud engineering team has recommended to increase the inhibiting properties of the system and avoid using high concentration of the inert solids (marble chips and barite), replacing them with dissolved solids (potassium formate dissolved to 1.35 g/cm3), which allowed stabilization of the wellbore by decreasing hydrodynamic fluctuations and activity of water fluid in the mud system, and hence trouble-free drilling and running of the liner conveying casing down to the total depth.


Furthermore, when drilling the well with barite-free UNIFORM K mud system based on potassium formate, the company used E.P. Lubricity Tester fixed laboratory equipment for the measurements of the actual friction factor. This facilitated comparative study to evaluate the effect of the lubricating additive concentration on the friction factor in various types of drilling fluids with different solids concentration. The results of this study infer that the percent concentration of the lubricant has no effect on the friction factor reduction, and it can only be changed by using a lubricant with high-quality friction additives, which form strong lubricating film on wearing surfaces. 


Figure 3. Friction Factor Change vs. Percent Concentration of Lubricating Additive in Various Types of Drilling Muds

  

It was established during the study that in conventional types of drilling muds both for drilling-in and tangent hole drilling, the friction factor lies in the range between 19.2 and 23.5 (lb/in), and the lubricant concentration ranges from 3 to 5%, and in some cases reaches 20% vol., but in spite of that for conventional drilling fluid systems the friction factor remains in the said range. As far as the friction factor of barite-free UNIFORM K potassium formate based drilling system is concerned, during the drilling of the well with zero concentration of lubricant the friction factor was recorded to vary in the range from 15.2 to 18.1 (lb/in). This comparative evaluation of conventional muds and barite-free UNIFORM K potassium formate based drilling system indicates high performance of the base. The absence of lubricating additives in the system makes it environmentally friendly, which is quite important when drilling wells in water conservation zones and offshore projects.


Having evaluated the average penetration rate and the time for circulation/wiper trip and round tripping recorded during the drilling of the liner conveying casing intervals, we may definitely say that barite-free UNIFORM K potassium formate based drilling system is a versatile system and maintains the optimum technical and economic performance during the drilling of wells in complicated geologic environment where uncertainty is present.


Table 2.  Laboratory Tests for Compatibility of Barite-Free UNIFORM K Drilling Mud 

 

                       Sequence Number of the Drilling Mud System

Chemical

Concentration (g/l)

1

2

3

4

5

6

Water

ml

0

0

0

0

1000

50% solution 1

KCOOH 1.35 brine

g

1000

1000

1000

1000

0

MEX-GUM S

g

3

3

3

3

3,5

50% solution 5

MEX-PAC HV

g

1

1

1

1

1

MEX-PAC LV

g

4

4

4

4

5

MEX-CARB F/M

g

100

100

100

100

100

K2CO3

g

4

4

4

4

0

Caustic

g

to рН 9.5

Khvalynsky clay

g

50

50

50

50

50

NаCL

g

0

0

0

0

70

KCL

g

0

0

0

0

55

Cement

g

0

2

4

6

0

0

DENSITY

g/cm3

1.44

1.44

1.44

1.45

1,17

1,305

Rheology measurement temperature

°C

49

49

49

49

49

49

600

rpm

97.0

98.0

89.0

87.0

73.0

84.0

300

rpm

67.0

67.0

60.0

59.0

52.0

59.0

200

rpm

54.0

54.0

48.0

48.0

44.0

49.0

100

rpm

37.0

37.0

33.0

33.0

32.0

35.0

6

rpm

10.0

10.0

9.0

9.0

12.0

12.0

3

rpm

8.0

8.0

7.0

7.0

10.0

10.0

СНС 10 s

lbs/100ft2

9.0

9.0

8.0

8.0

11.0

11.0

СНС 10 min

lbs/100ft2

13.0

13.0

14.0

21.0

16.0

16.0

PV

cP

30.0

32.0

29.0

28.0

20.0

25.0

YP

cP

36.0

35.0

32.0

31.0

32.0

34.0

рН

12.0

12.0

12.0

12.0

9.0

10.0

Fl (API)

cm3

3.4

3.6

4.2

4.4

5.2

4.8


*The studies of compatibility of barite-free UNIFORM K mud system with aggressive cement and traditionally used NaCL and KCL salts were proved to be true in the well site environment. The controllability of physical and chemical parameters of the drilling mud stayed on a high level.


 The integrated approach to the drilling program development which includes the design of the optimum bit program and BHA, drilling practice optimization, preparation and elaboration of the circulation program, and displacement of the well when using different types of drilling muds, has allowed successful completion of the well drilling cycle with the multistage hydraulic fracturing assembly run in the hole.


The total time it took to drill the well was 38 days, being an absolute drilling record on this well pad. It is noteworthy that the drilling fluids services and all related engineering and technology solutions were provided by Russian drilling fluids company.


The experience acquired in the area of drilling fluids allows a few step-ahead solutions to be identified for the drilling of horizontal wells at the fields with tight oil reserves in Bazhenov formation and the Paleozoic reservoirs:

-  The specified density of drilling muds should not be supported by calculations based on the minimum recommended overbalance pressure alone, but backed by the drilling experience acquired in the region and geomechanic modeling. Also, the formulations should be designed with considerations for aggressive formation environment expected to be encountered while drilling through Cherkashenskaya formation. Only the analysis of these four components may validate the decision on the optimum density and base of the drilling mud, appropriate formulation, mud parameters and drilling practices.

- The use of potassium formate based UNIFORM K drilling fluid system with correctly designed density allows maintaining stable cap of the pay zone and drilling the well without failures.

- An undeniable advantage of this mud system is the increased tolerance of polymers to high temperatures, which widens the applicability of this system, while the absence of chloride ion makes it environmentally safe when used in sensitive bodies of water.


 Authors:

S.A. Cherevko, Deputy General Director, Well Drilling and Completion, GAZPROMNEFT – KHANTOS

A. Yu. Khomutov, Drilling and Cementing Fluids Manager, Drilling and Well Interventions Department, GAZPROMNEFT R&D CENTER

M.A. Syroegin, Drilling Fluids Manager, Engineering Technologies Team of Engineering Department, GAZPROMNEFT – KHANTOS

A. V. Korolyov, General Director, AKROS

S.V. Popov, Technology Service Manager, AKROS

Yu. V. Lukovkin, Technology Service Engineer, AKROS 

Source:  "Burenie i neft'", 2017, № 7-8, 48