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Development of a New Reservoir-Friendly Drilling Fluid for Higher Gas Production

Abdullah Al-Yami, Vikrant Wagle*, Mohammed Jubran, Marwan Al-Mulhim

Saudi Aramco, Dhahran, Saudi Arabia

Corresponding Author:
Vikrant Wagle
Petroleum Scientist, Saudi Aramco, Dhahran, Saudi Arabia.
Tel: +966138624902
E-mail: vikrant.wagle@aramco.com

Received date: 29/09/2018 Accepted date: 16/10/2018 Published date: 22/10/2018

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Abstract

Drilling gas reservoir requires high mud density to balance the reservoir pressure. To formulate such fluids, calcium carbonate (CaCO3) was used because of its high acid solubility. However, due to the high concentration of CaCO3 required for high density drilling fluid, sticking might occur which might result in fishing and/or sidetracks operations. To minimize sticking problems, barite (BaSO4) is added with CaCO3 to reduce the amount of solids needed to formulate the drilling fluid. However, barite can cause potential damage because it does not dissolve in commonly used acids. Drilling fluids were developed at a wide range of densities using CaCl2 salt with Manganese Tetroxide (Mn3O4). No similar formulations were developed before to the best of the authors’ knowledge. The properties of small particle size (D50=1 microns), spherical shape and high specific gravity (4.9 g/cm3) of Mn3O4 make it good weighting material to reduce solids loading and settling compared to CaCO3 (2.78 g/cm3 and D50=10 microns) and BaSO4 (4.20 g/cm3 and D50=20 microns). The objective of this study is to show the lab work involved in designing water-based drilling fluids using CaCl2 / Mn3O4. The experimental work in this paper involved rheological properties, thermal stability, API and HT/HP filtration. The data generated from this study showed that Lignite and Vinyl amide/ vinyl sulfonate copolymer are recommended to provide good rheological stability and filtration control for CaCl2/Mn3O4 drilling fluid. Polyanionic cellulose polymer and starch can used to formulate KCl/Mn3O4 drilling fluid with good properties at 300°F.

Keywords

Density Functional Theory, Time- Dependent Density Functional Theory, HOMO, LUMO, Scharber, Non-Covalent Interactions, Benzothiadiazole.

Introduction

The use of drilling fluids is an inevitable need in oil and gas drilling operations. Selection of the most suitable drilling fluid leads to maximize productivity and minimize cost without harming the environment and field equipment [1]. Choosing the right drilling mud is generally evaluated based on two factors:

1) Achieving well technical requirements, and 2) satisfying the environmental regulations [2]. Moreover, the chemistry of drilling fluids is a top consideration since it directly controls the drilling performance, and most importantly the extent of formation damage across the pay zone.

Formation damage mitigation across producing reservoir is essential. Drilling fluid design optimization is the primary approach to reach borehole stability and, minimize well expenditure while improving production rates. Previous studies show that formation damage could occur due to the following: (1) extent of solid invasion, (2) clays swelling, (3) emulsion between oil-based mud and formation water [3], (4) bacteria effect [4], (5) and fine migration of clay particles [5]. Mud invasion solids and filtrate are capable of reducing the permeability of the formation, especially when the mud particles have smaller diameter than pore throats, which lead to further migration of mud particles into the formation [6]. One of the characterizations of suitable drilling fluids is forming a very thin mud cake which it can form very fast and which is easy to remove.

In order to optimize the drilling and well operations, choosing the most effective weighting material in such a water-based mud formulation is required. Barite is considered one of the most popular weighting materials since it has high specific gravity, low cost and less environmental effect [7]. However, it has considerable disadvantages, where it is prone to severe sagging and insoluble in acids, which means that the barite removal process is difficult and expensive [8].

Another well-known weighting material is potassium formate. There are many advantages of potassium formate as well. It can dissolve in water, form high-density fluid [9], create very thin filter cake, stabilize shale, and minimize the potential of differential sticking. On the other hand, potassium formate can cause corrosion since it is reactive with CO2 forming more formic acid and precipitation of potassium chloride which damages the formation by plugging the pores [10].

Previous studies and lab experiments show that manganese tetroxide (Mn3O4) is a better weighting agent and represents an excellent alternative to barite and potassium formate. Manganese tetroxide retains relatively high specific gravity and small particle size, which reduces particle settling and sagging in high mud weight formulations. Manganese tetroxide particles have a spherical shape of (4 μm) in size and specific gravity of (4.8 g/cm3) which leads to lower contact in particle-to-particle interactions [11,12]. Field studies show that when Manganese tetroxide is used in oil-based drilling fluids, there was less formation damage and high amount of productivity. Al-Yami designed KCl water-based drilling fluid utilizing manganese tetraoxide and CaCO3. The CaCO3 materials were used to improve the filtration control. The aim of this study is to discuss lab work that was performed to design water-based drill-in fluids using CaCl2/Mn3O4 at 95 pcf and 300ºF.

Experimental Studies

Materials

All additives used in this study are conventional additives used in drilling fluids such as Polyanionic cellulose polymer, starch, and Lignite.

Starch is used as filtration control additive, which is the primary component of the seeds of cereal grains or tubers such as rice, wheat, corn, and potato. It has a formula of (C6H10O5H2O) n. Starch is degraded at temperatures of maximum 250ºF with concentrations of 2 to 10 Ib/bbl.

Xanthan is a polysaccharide water soluble polymer produced by bacterial action. XC-polymer has excellent suspension properties and is used in concentration of 0.2 to 2 Ib/bbl.

Polyanionic cellulose polymer can be used to control filtrion and build up viscosty. It can be used in fresh water, sea water and salt saturated fluids in concentration of 0.2 to 5 lb/bbl.

Hydrated lime Ca(OH)2 is made by adding calcium oxide to water. It is used in high calcium content drilling fluids to increase the pH in concentration of 0.5 to 20 Ib/bbl.

For potassium chloride drilling fluids, KOH is used to increase the pH in concentration of 0.5 to 3 lb/bbl. Lignite is used to provide filtration control and enhance rheological stability at high temperature upto 400ºF in water based fluids. It is notably effective in high density mud. Typical dosages are 2 to 6 lb/bbl depending on the degree of filtration control desired. In addition to that, it shows good filtrate control and rheological stability for high density fluids.

Vinyl amide/vinyl sulfonate copolymer are terpolymers of N-methyl-N-vinylacetamide, monomer acrylamide and vinyl sulfonate monomer 2-acrylamide-2-methyl-1-propanesulfonic acid used for high temperature environment. It is used as fluid loss additive for water based fluids.

Vinyl amide/Acrylic Polymer is a low viscosity synthetic polymer to enhance filtration control in water based fluids which can work in high calcium chloride concentrations up to 100,000 ppm. The polymer can be used in brines containing Na, K+, and Mg++ concentration up to saturation and still shows stable performance in a wide range of pH environment.

CaCO3 is a good acid soluble weighting material for productive formation since it can be removed by treatment with hydrochloric acid. It can be used to formulate drilling fluids with a maximum density of 12 lb/gal due to its low specific gravity (2.6 to 2.8).

Mn3O4 is manufactured from heating manganese dioxide in air at 1000°C. Their properties of higher specific gravity (4.8), spherical shape, and solubility in different types of acids make them more attractive weighting material compared to CaCO3 (2.78 g/cm3) and BaSO4 (4.25 g/cm3).

CaCl2 can be used in combination with gypsum (calcium sulfate) when drilling evaporates sections. If the drilling fluid is not saturated, it will be washed out (Table 1). In this formulation HEC (hydroxyl ethyl cellulose) is used to build up viscosity. HEC polymers are nonionic derivatives of the cellulose polymer modified to impart water solubility to the cellulose molecule. The nonionic substitution in HEC polymers makes them very tolerant to high salt environments, including divalent calcium and magnesium. However, HEC Polymers alone are considered nonthixotropic. XC polymer functions quite well in CaCl2 fluids as long as the polymer is properly sheared in the initial mix. XC-polymer is one of a very few polymers which will build gel structure. This, therefore, makes XC-polymer the key ingredient when solids suspension is required (Table 2).

Water bbl 0.85   Density pcf 77  
Defoamer gal 0.01   Plastic viscosity cp 16-24 24
CaCl2 (78%) lb 112   Yield point lb/100 ft2 24-26 26
XC-polymer lb 0.5 1 10 sec gel lb/100 ft2 04-Jun 6
HEC lb 0.5 1 10 min gel lb/100 ft2 Dec-14 14
Starch lb 3 6 Filtrate ml/30 min 04-May 5
Lime lb 0.15 1 pH   09-Oct 10
CaSO4.2H2O lb 5 6 Chlorides mg/l 1,60,000  
CaCO3 ‘fine’ 1b 9          
CaCO3 1b 16          
‘Medium’              

Table 1. CaCl2 used to drill evaporates sections.

Formulation and Material concentration (One barrel) Average fluid properties

Density Freshwater pcf bbl 75      
    0.89 Density lb/ft3 75
           
Defoamer gal 0.01 Plastic Viscosity cp 16-24
ALAP
XC-Polymer lb 0.50 – 1.0 Yield Point lb/100 ft2 24-26
Starch lb 4.0 – 6.0 10 sec gel lb/100 ft2 04-Jun
Lime lb 0.25 – 0.50 10 min gel lb/100 ft2 08-Dec
CaCl2 (78%) lb 8 Filtrate ml/30 min 04-May
CaCO3 “fine” 1b 99 Cake thickness 32nd in API 01-Feb
      pH   9 – 10
      Chloride mg/l × 1000 12

Table 2: CaCl2 used to drill a reservoir section.

Procedure

Many attempts and tests were done to formulate Mn3O4 water based drilling fluids formulations in order to achieve the final formulations suitable for deep drilling. The tests needed are:

1. Rheological properties

2. API and HP/HT standard filtration, and 3-Thermal stability

Intensive lab work was done using API RP 13B-1 procedure to design CaCl2/Mn3O4 water based drill-in fluids at a density of 95 pcf at 300°F.

Results and Discussion

The presence of oxygen in drilling fluids can accelerate corrosion rates and degradation of water-soluble polymers. An oxygen scavenger can be used to remove the oxygen. Sodium sulfite is an example of oxygen scavenger as shown in Equation 1:

O2 + 2Na2SO3 → 2Na2SO4 (1)

The drilling fluid properties including PV, YP, Filtration (API and HT/HP), filter cake thickness, and drill-in fluid pH were measured. To evaluate thermal stability, the drill-in fluid was aged for 16 hours at 300°F and the properties mentioned earlier were measured again.

Polyanionic cellulose polymer was used to formulate Mn3O4/CaCl2 drilling fluid. Good rheology properties were obtained but having controlled fluid loss was the concern (Table 3). High concentration of CaCl2 was used so calcium bromide was added to minimize recrystallization.

Formulation
Additive   Concentration
Water   0.815 bbl
XC-Polymer   1.0 ppb
Polyanionic Cellulose Polymer   2.0 ppb
Lime   0.5 ppb
CaBr2   0.0255 bbl
CaCl2 (78%)   235.54 ppb
Mn3O4   69.549 ppb
Results
Parameter Before Hot Rolling After Hot Rolling
600 rpm 71 62
300 rpm 47 40
200 rpm 36 30
100 rpm 24 21
6 rpm 7 9
3 rpm 5 7
10 sec gel, lb/100 ft2 4 7
10 min gel, lb/100 ft2 12 10
Plastic viscosity, cp 24 22
Yield point, lb/100 ft2 23 18
HTHP filtration, ml/30 min No Control No Control

Table 3. Formulation utilizing XC polymer and PAC-R and CaBr.

CaCl2 concetration was reduced and Mn3O4 amount was increased to investigate the compatibility of Polyanionic cellulose polymer with Mn3O4/CaCl2 drilling fluid (Table 4). Starch and CaCO3 fine and medium were added to improve the fluid loss control. However, no control was observed even before hot rolling.

Formulation
Additive Concentration
Water 269.5 bbl
XC-Polymer 1.75 ppb
Polyanionic Cellulose Polymer 3 ppb
starch 6 ppb
lime 0.5 ppb
CaCl2 (78%) 101 ppb
Mn3O4 110 ppb
CaCO3 (Fine) 10 ppb
CaCO3 (Medium) 15 ppb
Sodium Sulfite 0.5 ppb
Results
Parameter Before Hot Rolling
600 rpm 137
300 rpm 86.5
200 rpm 62.3
100 rpm 37.6
6 rpm 4.1
3 rpm 3.7
10 sec gel, lb/100 ft2 2.3
10 min gel, lb/100 ft2 2.6
Plastic viscosity, cp 45.5
Yield point, lb/100 ft2 35.1
HTHP filtration, ml/30 min No Control

Table 4. Formulation utilizing XC polymer and PAC-R and Starch and CaCO3 fine and CaCO3 medium.

Vinylamide/Acrylic Polymer was used to replace starch to control the fluid loss (Table 5). Good rheology properties were obtained but no fluid loss control was achieved even after adding more solids (CaCO3 and Mn3O4) and reducing CaCl2 salt concentration. Vinylamide/Acrylic Polymer was not compatible with Mn3O4/CaCl2 drilling fluid.

Formulation
Additive Concentration Concentration
water 294 bbl 289.8 bbl
XC polymer 1.75 ppb 1.75 ppb
Vinylamide/Acrylic 6 ppb 6 ppb
Polymer    
Polyanionic Cellulose Polymer 7 ppb 7 ppb
CaCl2 (78%) 70 ppb 64 ppb
Mn3O4 50 ppb 60 ppb
CaCO3 (Fine) 23 ppb 25 ppb
CaCO3 (Medium) 15 ppb 17 ppb
Sodium Sulfite 2 ppb 2 ppb
Lime 0.5 ppb 0.5 ppb
Results
Parameter Before Hot Rolling Before Hot Rolling
600 rpm 44.5 75.3
300 rpm 31.5 49.7
200 rpm 24.5 42.2
100 rpm 18.4 33.1
6 rpm 8.4 18.2
3 rpm 7.8 13.5
10 sec gel, lb/100 ft2 7.8 13.2
10 min gel, lb/100 ft2 7.9 14.6
Plastic viscosity, cp    
Yield point, lb/100 ft2    
HTHP filtration,    
ml/30 min No Control No Control

Table 5. Formulation utilizing Vinylamide/Acrylic Polymer.

Lignite was used in Mn3O4/CaCl2 h drilling fluid in attempt to control fluid loss. Manganese Tetraoxide was used only in the formula without CaCO3 to evaluate its filtration performance in combination with lignite. XC- polymer, Polyanionic cellulose polymer were used to evaluate their rheological performance. Exposure of drilling fluids to temperature might change the fluid’s rheology and filtration. Good fluid loss control was observed before and after hot rolling at 300°F and 300 psi for 16 hours (Table 6).

Formulation
Additive   Concentration
Water   0.815 bbl
XC-Polymer   1.75 ppb
Lignite   4.0 ppb
Lime   0.5 ppb
CaBr2   0.0255 bbl
CaCl2 (78%)   235.54 ppb
Mn3O4   69.549 ppb
Results
Parameter Before Hot Rolling After Hot Rolling
600 rpm 64 77
300 rpm 44 47
200 rpm 35 36
100 rpm 25 25
6 rpm 9 7
3 rpm 7 5
10 sec gel, lb/100 ft2 7 5
10 min gel, lb/100 ft2 12 8
Plastic viscosity, cp 20 30
Yield point, lb/100 ft2 24 17
HTHP filtration, ml/30 min 25 ml 26 ml

Table 6. Formulation utilizing XC polymer and Resinex and CaBr.

Lignite was used again but with lower concentration of CaCl2 salt and with higher concentration of Mn3O4 and only 5 lb/bbl of CaCO3 (Table 7). An earlier study by Al-Yami at el., showed that 5 lb/bbl of CaCO3 with Mn3O4 can provide acceptable fluid loss control and thin filter cake when combined with good fluid loss additives (Figure 1). Good rheology and fluid loss control before and after hot rolling was observed from this formulation.

chemistry-concentration

Figure 1. Effect of CaCO3 concentration on properties of Mn3O4 drilling fluid.

Additive   Concentration
Water   0.756 bbl
XC-Polymer   1.5 ppb
Lignite   6 ppb
Lime   0.5 ppb
CaCl2 (78%)   202.9 ppb
Mn3O4   72.326 ppb
CaCO3 (Fine)   5 ppb
Results
Parameter Before Hot Rolling After Hot Rolling
600 rpm 80 48
300 rpm 53 30
200 rpm 42 22
100 rpm 30 14
6 rpm 13 3
3 rpm 10 2
10 sec gel, lb/100 ft2 10 2
10 min gel, lb/100 ft2 12 7
Plastic viscosity, cp 27 18
Yield point, lb/100 ft2 26 12
HTHP filtration, ml/30 min 32 ml 32 ml

Table 7. Formulation utilizing XC polymer and Resinex Formulation.

Finally, Vinyl amide/vinyl sulfonate copolymer was tested without CaCO3 and high concentration of CaCl2 (Table 8). The best drilling fluids properties were obtained when using Vinyl amide/vinyl sulfonate copolymer in the Mn3O4/CaCl2 drilling fluid even after hot rolling for 300°F and 300 psi for 16 hours.

Additive Formulation Concentration
Water   0.815 bbl
XC-Polymer   1.75 ppb
Vinyl amide/vinyl sulfonate copolymer   2.0 ppb
Lime   0.5 ppb
CaBr2   0.0255 bbl
CaCl2 (78%)   235.54 ppb
Mn3O4   69.549 ppb
Results
Parameter Before Hot Rolling After Hot Rolling
600 rpm   123
300 rpm   76
200 rpm   57
100 rpm   33
6 rpm   6
3 rpm   3
10 sec gel, lb/100 ft2   4
10 min gel, lb/100 ft2    
Plastic viscosity, cp 20 47
Yield point, lb/100 ft2 25 29
HTHP filtration,
ml/30 min 14 ml 12 ml

Table 8. Formulation utilizing XC polymer and Therma Check and CaBr.

Al-Yami showed that Polyanionic cellulose polymer and starch are better than Lignite and Vinyl amide/vinyl sulfonate copolymer polymers in providing good rheological stability and filtration control for KCl/Mn3O4 drilling fluid. In this study, we showed that Lignite and Vinyl amide/vinyl sulfonate copolymer are better than Polyanionic cellulose polymer and starch in providing good rheological stability and filtration control for CaCl2/Mn3O4 drilling fluid.

Conclusions

In this study, Mn3O4/CaCl2 drilling fluid (95 pcf) was designed and tested. Based on the testing results, the following conclusions can be drawn:

1. Oxygen scavenger was added to Polyanionic cellulose polymer to extend its stability and provide good rheological properties. However, the filtration control was not good.

2. Adding starch and CaCO3 and Polyanionic cellulose polymer did not solve the fluid loss control problem.

3. Vinylamide/Acrylic Polymer was not compatible with Mn3O4/CaCl2 drilling fluid. No fluid loss control was achieved even after reducing CaCl2 concentration.

4. Lignite and Vinyl amide/vinyl sulfonate copolymer are better than Polyanionic cellulose polymer and starch in providing good rheological stability and filtration control for CaCl2/Mn3O4 drilling fluid.

5. Polyanionic cellulose polymer and starch are better than Lignite and Vinyl amide/vinyl sulfonate copolymer in providing good rheological stability and filtration control for KCl/Mn3O4 drilling fluid.

6. The use of small concentration of CaCO3 (5 lb/bbl) with Mn3O4 (203 lb/bbl) improved the filtration and reduced the filter cake compared to using Mn3O4 alone.

Nomenclature

BHCT=bottomhole circulating temperature,

°F BHST=bottomhole static temperature, °F

PV=plastic viscosity, cp

YP=yield point, lb/100 ft2

Si Metric Conversion Factors

In × 2.54*        E-02=m

(°F-32)/1.8*        E+00=°C

ft × 3.048*        E-01=m

gal × 3.785 412        E-03=m3

lbm × 4.535 924        E×01=kg

psi × 6.894 757        E-03=Mpa

lbm/gal × 1.198 26        E-01=S.G

bbl × 1.58987        E-0=m3

*Conversion factor is exact.

References