PROVEN DRILLING-FLUID ENGINEERING SOLUTIONS IN HORIZONTAL DRILLING IN BAZHENOV FORMATION, PALYANOV AREA OF KRASNOLENINSKOYE FIELD

PROVEN DRILLING-FLUID ENGINEERING SOLUTIONS IN HORIZONTAL DRILLING IN BAZHENOV FORMATION, PALYANOV AREA OF KRASNOLENINSKOYE FIELD

GazpromNeft-Khantos LLC completed drilling of two horizontal wells in Bazhenov formation during the second half of 2015 in Palyanov area of Krasnoleninskoye field. Specialists of both GazpromNeft-Khantos LLC and GazpromNeft R&D Center jointly had introduced a number of technical actions that finally ensured the successful drilling of wells.  The project development included selection of optimal solutions for the entire drilling process, incorporating the selection of optimal BHAs, drilling fluid parameters and formulations, and drilling process optimization, involving modeling of geomechanics and hydraulic calculations.  This article provides an overview of solutions and actions related to drilling fluids as proposed jointly by the specialists of Gazpromneft-Khantos LLC, Gazpromneft-R&D Center and AKROS LLC that allowed solving a number of complex technical issues arising from horizontal drilling.


Commencing the development of a drilling fluid program for drilling the horizontal wells at the Palyanov field, a detailed analysis of the regional experience of drilling such wells and determining the main technological risks during drilling was carried out.


Based on results of the analysis, the following main technological challenges specific for drilling horizontal wells to the Bazhenov formations were identified:




  • Determination of the optimal drilling fluid density while drilling the tangent section;
  • Preventing the drilling fluid loss caused by high value of equivalent circulation fluid density (ECFD) while drilling the tangent section;
  • Ensuring the stability of the tangent section and accident-free trip of the production liner equipped for multi-stage hydraulic fracturing.


Drilling wells to the Bazhenov formations in the KhMAO fields is associated with a number of technical problems, primarily caused by the need to provide borehole stability. This task becomes particularly important in projects where the development of Bazhenov formations is planned by construction of horizontal wells. It is generally known that the greatest difficulties in providing the borehole wall stability arise when it is necessary to drill the borehole through unstable rocks at inclination angles of more than 30 degrees. Under such conditions, when developing the well cleaning program, it is necessary to solve a number of technical problems to ensure trouble-free drilling.


The following is required when choosing a drilling fluid: 


  • Choose the fluid formulation that ensures the borehole wall stability across the entire length of interval drilling;
  • Choose the optimum fluid density values based on the data on past drilling the wells in the region and modeling of geomechanics;
  • Analyze the loss circulation probability during drilling and provide the measures to prevent and eliminate them, based on the chosen density, design rheological parameters of the drilling fluid and geomechanical modeling data.


Two low-angle boreholes were drilled in the Palyanov area of the Krasnoleninskoye field to the Bazhenov formation from the end of 2014 until mid-2015. Using the experience gained with the use of various drilling fluid formulations, the Gazpromneft-Khantos OJSC's specialists proposed measures to modify the formulation of inhibited PRIMOSOL potassium chloride-based drilling fluid. For example, when the formulation was modified by treatment with acrylic polymers, the changes introduced to the formulation allowed increasing the technical and economic indicators of drilling and provided stability of parameters under contamination with formation fluids.


Table 1. Formulation and parameters of fresh-water based and inhibited drilling fluids

Formulation - content of the main reagents

 

SYNTEX fresh-water based encapsulating drilling fluid

Polymeric potassium chloride PRIMOSOL drilling fluid before optimization

Function

Components

kg (L)/m3

Components

kg (L)/m3

Rheology modifier

Partially hydrolyzed polyacrylamide

1.5-2.5

Xanthan gum biopolymer

1-1.5

Bentonite

20-30

Filtration control additive

Sodium polyacrylate

0.2-0.3

Polyanionic cellulose

2-4

Plugging agent

Calcium carbonate

60-80

Calcium carbonate

60-80

Inhibitor

 

 

Potassium chloride

30-40

Drilling fluid parameters

Density, g/cm3

1.15-1.20

1.15-1.20

Viscosimeter readings

 

 

At 600 rpm

38-47

51-53

At 300 rpm

24-30

34-37

At 200 rpm

14-21

23-27

At 100 rpm

7-12

15-18

At 6 rpm

4-7

6-7

At 3 rpm

3-4

4-4

Static shear stress 10 sec

2-3

5-6

Static shear stress 10 min

4-9

16-20

Water loss, mL/30 min

7

5-6

рН

7.5-8

9.5-10

CEC (МВТ), kg/m3

55-80

14-35


Choice of drilling fluid density became an important element of measures to improve the borehole wall stability during the construction of the tangent section in the Palyanov area of the Krasnoleninskoye field. Based on results of the geomechanical studies carried out in the Palyanov area, the Gazprom Neft-Khantos specialists suggested to raise the drilling fluid density that allowed significant increasing of the borehole stability, ensuring trouble-free drilling and running the casing without complications. This example once again confirms the need to analyze the borehole wall stability using the geomechanical modeling as a basis for construction of a well with complex profile. Table 2 shows main components of the drilling fluid formulation and parameters that were maintained during drilling.

 

 Table 2. Optimized fluid formulation and parameters when drilling a tangent borehole section to the Bazhenov formation

    Polymeric potassium chloride PRIMOSOL drilling fluid after optimization for drilling of the tangent section  

Function

Component

kg/m3

Rheology modifier

Xanthan gum biopolymer

1.5-2

Filtration control additive

Polyanionic cellulose

2-4

Plugging agent

Calcium carbonate

60-80

Inhibitor

Potassium chloride / Sodium chloride

30-40

Incapsulator /

Stabilizer

Polyacrylamide

1.5-2

Drilling fluid parameters

Density, g/cm3

1.27-1.46

Viscosimeter readings

 

At 600 rpm

41-52

At 300 rpm

31-37

At 200 rpm

23-30

At 100 rpm

13-20

At 6 rpm

6-7

At 3 rpm

4-6

Static shear stress 10 sec

5-7

Static shear stress 10 min

15-22

Water loss, mL/30 min

<6

рН

9.5-10

CEC (МВТ), kg/m3

15-30


Since in developing the borehole cleaning program the need to maintain high drilling fluid density should be considered, a lost circulation risk analysis was carried out. Based on results of the analysis carried out, the potential risks and reservoirs were identified, where fluid loss was possible due to high value of equivalent circulation fluid density (ECFD) during drilling. To exclude this type of complications, a special complex of measures for artificial increasing the hydraulic fracturing gradient (Stress Cage) was developed based on establishing the special mixture formulations based on graphite and fractionated calcium carbonate. Since this approach was not fully implemented while drilling the tangent section in the first well, minor complications of minor lost drilling fluid occurred. When drilling the second well, the planned program for using this approach has been implemented in full scale and has completely prevented the loss.

For the drilling of the interval for the production liner, the AKROS LLC's specialists proposed the UNIDRIL system. UNIDRIL is a state-of-art drilling fluid system, an inverse emulsion based on mineral oil. Before application in the field, the UNIDRIL system was studied in the Gazpromneft-R&D LLC laboratory and, based on results of the study, a positive conclusion and permission to use this drilling fluid in the field were obtained. Table 3 shows main parameters of drilling fluid and its basic formulation. In preparing the borehole cleaning program for well construction, particularly for the production liner, the rheological profile of the system at various temperatures was analyzed and optimal water-oil ratio and rheological parameters of the UNIDRIL system were determined, taking into account the recommended safe density window obtained in the geomechanical simulation. During drilling, the drilling fluid parameters were maintained within a predetermined range that allowed minimizing the risk of lost drilling fluid based on mineral oil.


 Table 3. Optimized formulation and parameters of mineral oil-based drilling fluid for horizontal drilling

    UNIDRIL mineral oil-based drilling fluid  

Function

Component

kg (L)/m3

Base

Mineral oil

550-600

Calcium chloride solution

200-250

Rheological properties

Package of emulsifiers

25-30

Organophilic bentonite

8-10

Weighting agent

Calcium carbonate

90-100

Barite

500-550

Drilling fluid parameters

Density, g/cm3

1.41-1.42

Viscosimeter readings

At a temperature of 49°C

At 600 rpm

60-73

At 300 rpm

33-43

At 200 rpm

21-33

At 100 rpm

13-22

At 6 rpm

5-9

At 3 rpm

4-8

Static shear stress 10 sec

8-10

Static shear stress 10 min

10-30

HTHP filtration test, mL/30 min

 

Oil/Water ratio

75-80/25-20

Electrostability, V

600-750



An integrated approach to development of the drilling program, including selection of the optimal bit program and BHA, optimization of drilling regimes, preparation and development of a drilling program for well construction, allowed successful completion of well construction by running the assembly for multi-stage fracturing. Drilling the well took 45 days that corresponds to the value set by the project.

The drilling fluid experience gained will make it possible to identify promising approaches for the construction of horizontal wells in fields with hard-to-recover reserves:


  • drilling fluid services and implementation of all technical solutions in this area were provided by the Russian drilling fluid company;
  • chosen drilling fluid densities should be confirmed not only by calculations based on the minimum recommended overbalance pressure, but also by results of calculating the borehole stability obtained in geomechanical modeling. Decision to choose the optimal drilling fluid density can be made only on the basis of the analysis of these constituents with selection of appropriate drilling fluid formulation and parameters.
  • based on the borehole cleaning program chosen for the implementation, it is necessary to analyze the probability of borehole stability risks, on the one hand, and fluid loss, on the other. To prevent the drilling fluid loss, it is necessary to carry out hydraulic calculations of the equivalent circulating density and determine its maximum permissible value in terms of rheological parameters of the drilling fluid, taking into account the planned rates of penetration and drilling regimes.
  • use of an "artificial increase in fracturing gradient" technology is a promising direction to prevent fluid loss during well construction in terrigenous sediments, when maintenance of high drilling fluid density is required.
  • the use of inhibited PRIMOSOL potassium chloride-based drilling fluid with the additional use of acrylic polymers and correctly chosen density, makes it possible to ensure stability of the pay reservoir cover.
  • use of a UNIDRIL mineral oil-based drilling fluid provided an efficient process of drilling the horizontal section and possibility for running the assembly for multi-stage fracturing.
AUTHORS:

Cherevko S. А., deputy General Director of well construction Gazpromneft'-Khantos LLC

Khomutov А. Y., head of drilling and cement slurries of the Department of Drilling and well intervention works Gazpromneft'-NTC LLC 

Korolev А. V., general director AKROS LLC

Ryabtsev P. L.., technical director AKROS LLC

Source: "Burenie i Neft"№3 (2016), 42.