Drilling Reservoirs in East Siberia: Oil-in-Water Emulsion

By A. Kharitonov, R. Burdukovsky, S. Pogorelova, E. Tikhonov, Rodin S., Kostarev N., Usenko A., Khampiev E., March 28, 2013

As formations get depleted, the flow drives change, and the conventional mud technologies (muds creating excessive overbalance) can become less appropriate. Oil-in-water emulsion technologies have reached well beyond conventional limits, now being a transition point in our industry. Ensuring safe, not abnormal overbalance, the oil-in-water technology still allows the use of the standard drilling equipment with maximum performance. The oil-in-mud emulsion technology in highly depleted formations reduces the risk of differential sticking, thus significantly decreasing the risks of losses leading to usage of expensive special tools, such as: LWD, MWD and APWD. The purpose of the oil-in-mud emulsion technology is not to eliminate conventional muds. The latter are still applicable in formations with high and normal pressures and even in partially depleted formations. In cases when the formation pressure conditions make application of conventional muds impossible, the oil-in-water emulsions are the option to consider.

Introduction

Srednebotuobinskoye field is a part of Leno-Vilyuisky geocryological region situated at the junction of three major geological structures: Tungus Late Paleozoic tectonic depression, Botuobinsky Early Paleozoic elevation and Vilyuisky Mesozoic tectonic depression. Development of Vendian age Botuobinsky formation (V bt) of Srednebotuobinskoye field is the priority since it contains most of the recoverable resources of the field.

In structural and tectonic aspects, the reviewed region is located in the Eastern part of the Siberian Platform within the tectonic structure of 2nd categorythe Vilyuisky tectonic depression, which is complicated by structures of lower categories. The Srednebotuobinskoye field is associated with the namesake anticline structure of the 3rd category, located in the center of Mirninsky dome of Nepsko-Botuobinskaya tectonic uplift.

The target reservoir: Botuobinsky horizon (V bt) associated with the Nizhnebyukskaya suite (V bk1). Srednebotuobinskaya structure has a brachyanticline shape with slight elevation differential (up to 10 meters) in the dome and with more steep wings.

The Srednebotuobinskoye field is a porous sandstone (mudstones and siltstones have subordinate importance) reservoir at 1,890-1,930 meters TVD. It is characterized by low formation pressure (with pressure gradient of 0.0073 mPa/m), formation temperature of 14 С, and average permeability of 550-730 mD (max up to 5,000 mD as per core investigations and logging data). The field is developed by directional drilling of wells with 600700-meter laterals. The main problem encountered during drilling of lateral holes was mud losses resulting in failure to reach targets. The challenge was to eliminate the problem. After extensive laboratory analyses of cores and cutting samples a specially designed DRIL-N® drilling fluid was offered for application in the reservoir.

The Challenge to be Solved and Analysis of Wells Drilled with BARADRIL-N® Drilling Fluid Before Implementing the New Mud System

Production drilling started in April, 2009. Eight directional wells were drilled, with average lateral sections of 600-700 meters, using Baroid’s mud. At the 142 mm section, the BARADRIL-N® drilling fluid was used. The recipe and mud properties are given in Tables 1 and 2.

 

Table 1. Recipe of BARADRIL-N® Drilling Fluid Additive

Chemicals / Реагенты

Concentration, kg/m3 / Концентрация, кг/м3

Function / Функция

Caustic soda / Каустическая сода

1.5

pH agent / Регулятор рН

Modified Starch

14

Filtration control / Контроль фильтрации

Biopolymer

3

Viscosifier / Загуститель

KCl

30

Salt / Соль

CaCO3 50

50

Colmatant / Кольматант

High Performance Lubricant

15

Lubricant / Смазка

Biocide

0.5

Bactericide / Бактерицид

 

Table 2. Properties of BARADRIL-N® Drilling Fluid Additive

Parameter / Параметр

Value / Значение

MW, SG / Плотность, г/см3

1.03-1.05

Funnel viscosity, sec/l / Условная вязкость, сек/л

42-48

Plastic viscosity, cPs / Пластическая вязкость, сПз

16-18

Yield point, API, lbs/100 ft2 / ДНС АНИ, фунт/100фут.2

18-22

Water loss API, ml/30 min / Фильтрация АНИ, мл/30 мин

4-5

рН

7-9

Gels , 10 s, lbs/100 ft2 / СНС 10 сек, фунт/100 футов2

 5-10

Gels , 10 min, lbs/100 ft2 / СНС 10 мин, фунт/100 футов2

 9-12

CL-, mg/l / CL-, мг /л

15 000-170 00

Ca++, mg/l / Ca++, мг/л

 <400

MBT, kg/m3 / MBT, кг/м3

<7

Cake, mm / Корка, мм

≤ 1

Sand, % vol. / Песок, % об.

 ≤ 1

 

At the project initial stage in 2009 the main problems related to BARADRIL-N drilling fluid (γ=1.02–1.05 SG) during drilling of the 142 mm section were as follows:

- Continuous mud losses with rates up to catastrophic;

- Downtime due to elimination of mud losses;

- Time spent to make fresh mud.

The hydraulic calculations demonstrated (see Fig. 1) that with MW of 1.02 SG the ECD at TD is 1.13 SG (equal to 0.0110 mPa/m gradient). This value is by 55 percent more than formation pressure. Baroid’s proprietary DFG™ 4.3 software was used for calculations. The average time for liner job was 22 days, with four-five days spent to eliminate mud losses. The analysis proved the necessity to use mud with lower MW to reduce overbalance. Drilling of six out of eight lateral holes was complicated by losses of various ratesfrom moderate to catastrophic. The average loss volume per well was 320m³.

 

Proposed Solution and Justification for the Use of BARADRIL-N® Drilling Fluid and XP-07™ Enhancer

This review and subsequent proposal aimed at the system to increase the reservoir exposure and to reduce time losses and costs during wells construction at Srednebotuobinskoye field was based on the customer’s TUNGD LLC desire to optimize processes of lateral drilling. Drilling in the reservoir with minimum overbalance and with optimized selection of colmatant and inhibitor helps to minimize the reservoir damage and to save the production properties of the formation. To achieve this goal, Baroid offers BARADRIL-N drilling fluid and XP-07 enhancer. The BARADRIL-N XP-07 system is an oil-in-water emulsion, with 10-40 percent oil content, depending on required MW. The MW range is 0.95 SG and higher. The BARADRIL-N XP-07 system was designed for drilling of wells with abnormally low formation pressures and long lateral sections. It has all properties of WBM.

It was successfully used in Russia for number of major customers.

 

Analysis of Cores and Cuttings’ Lab Testing Results, and Selection of the Best Mud Recipe

The Baroid laboratory designed a recipe for BARADRIL-N ХР-07 mud (Table 3) for the group project for Srednebotuobinskoye field. Mud properties are given in Table 4.

 

Table 3. Recipe of BARADRIL-N XP-07 Drilling Fluid

Chemicals / Реагенты

Concentration, kg/m3 Концентрация, кг/м3

Function / Функция

Water / Вода

740

Base fluid / Основная жидкость

Mineral Oil

225

Mineral oil / Минеральное масло

Oil-in-Water Emulsifier

8

Oil-in-Water Emulsifier / Эмульгатор

Caustic soda /  Каустическая сода

2.5

pH control / Регулятор рН

Modified Starch

10

Filtration control / Контроль фильтрации

Biopolymer

2

Viscosifier / Загуститель

KCl

20

Salt / Соль

CaCO3 50

50

LSM / Кольматант

Proprietary Oil-in-Water Lubricant

8

Oil-in-Water / Lubricant Смазка

Biocide

0.5

Bactericide / Бактерицид

 

Table 4. Properties of BARADRIL-N ХР-07 Drilling Fluid

Parameter / Параметр

Value / Значение

MW, SG / Плотность, г/см3

0.96-0.98

Funnel viscosity, sec/l / Условная вязкость, сек/л

42-48

Plastic viscosity, cPs / Пластическая вязкость, сПз

As low as possible
Как можно ниже

Yield point, API, lbs/100 ft2 / ДНС АНИ, фунт/100 фут.2

18-22

Water loss API, ml/30 min / Фильтрация АНИ, мл/30 мин

4-5

рН

7-9

Gels, lbs/100 ft2 / СНС 10 сек, фунт/100 фут.2

 5-6

Gels, 10 min, lbs/100 ft2 / СНС 10 мин, фунт/100 фут.2

 7-9

CL, mg/l / CL-, мг /л

12 000

Ca++, mg/l / Ca++, мг/л

 <200

MBT, kg/m3 / MBT, кг/м3

<7

Cake, mm / Корка, мм

≤ 1

Sand, % vol. / Песок, % об.

 ≤ 1

 

Achieved Results

Application of BARADRIL-N XP-07 drilling fluid at Srednebotuobinskoye field started in 2011. This system was used for drilling of eight 142 mm holes, with average length of 670 meters. The overbalance was reduced by 23 percent, which helped improve the hole condition. Mud losses were reduced by 81 percent, compared to biopolymer mud. No time was spent to eliminate losses.

The main component of the XP-07 mud system is (alkanoic mixture СН3-(СН2)n-СН, where n is 9 to 14) with specific weight of 0.74 SG. This oil contains max. 0.05 percent of aromatic hydrocarbons. With addition of small amount of multipurpose emulsifier, the oil generates stable oil-in-water emulsion. Due to low weight of oil, only 20-25 percent of ХР-07 is required to obtain the designed MW. This system is resistant to biodegradation, aggressive environments and remains stable for a few months if treated with bactericide. This can allow for saving and usage of remaining mud on subsequent wells. It does not take any additional equipment to store and transport. The friction factor is 0.012lower than that of conventional WBM. This helps reduce time for BHA orientation during directional drilling (BHA is easily steerable with more efficient sliding). Water loss is low2.5 ml/30 min (API). This helps to generate a thin and firm filter cake to help prevent sticking of BHA.

The hydraulic calculations demonstrated (see Fig. 1) that with MW of 0.96 SG the EMW at TD is 1.07 SG (less than that of BARADRIL-N fluid by 0.06 SG). Baroid’s proprietary DFG™ 4.3 software was used for calculations. (Fig. 2).

In 2012, eight lateral holes were drilled, with average length of 670 meters. Considering lessons of the previous year, the company managed to optimize the recipe and treatment process, and that helped reducing the use of LSM sealing material by 7.7 times. Furthermore, mud losses caused by overbalance were brought down nearly to zero. The cost of  one drilled meter was reduced by 25 percent, compared to 2011. In 2011, the cost of  one meter drilled with BARADRIL-N XP-07 fluid was 1.88 times higher than that of WBM; in 2012, this value was 1.4. The average drilling time was reduced by 45 percent. The data are presented in charts (Fig. 3, 4, 5).