Feasibility study of hydraulic fracture geometry evaluation method using time-lapse borehole measurements of low-frequency electric field

Makarov Alexander I.; Vasilevsky Alexander N.; Eltsov Igor N.; Dyatlov Gleb V.; Dashevsky Yuliy A.

УДК 550.370

ОЦЕНКА ВОЗМОЖНОСТЕЙ МЕТОДА ОПРЕДЕЛЕНИЯ ГЛУБИНЫ ТРЕЩИНЫ ГИДРОРАЗРЫВА ПЛАСТА С ИСПОЛЬЗОВАНИЕМ МНОГОКРАТНЫХ ИЗМЕРЕНИЙ ЭЛЕКТРИЧЕСКОГО (НИЗКОЧАСТОТНОГО) ПОЛЯ

Александр Игоревич Макаров

Baker Hughes, Новосибирский технологический центр, 630128, Россия, г. Новосибирск,

ул. Кутателадзе, 4а, кандидат технических наук, научный сотрудник, тел. (383)332-94-43, e-mail: [email protected]

Александр Николаевич Василевский

Baker Hughes, Новосибирский технологический центр, 630128, Россия, ул. Кутателадзе, 4а, научный сотрудник, тел.

e-mail: [email protected]

Игорь Николаевич Ельцов

Институт нефтегазовой геологии и геофизики им. А. А. Трофимука СО РАН, 630090, Россия, г. Новосибирск, пр. Академика Коптюга, 3, доктор технических наук, зам. директора по научной работе, тел. (383)330-75-55, e-mail: [email protected]

Глеб Владимирович Дятлов

Baker Hughes, Новосибирский технологический центр, 630128, Россия, г. Новосибирск, ул. Кутателадзе, 4а, кандидат физико-математических наук, научный сотрудник, тел. (383)332-94-43, e-mail: [email protected]

Юлий Александрович Дашевский

Baker Hughes, Новосибирский технологический центр, 630128, Россия, г. Новосибирск, ул. Кутателадзе, 4а, доктор физико-математических наук, директор, e-mail: [email protected]

Разработка новых технологий для оценки геометрических характеристик трещин гидравлического разрыва пласта с помощью геофизических методов является актуальной задачей. В данной работе предлагается метод определения геометрии трещины с помощью измерений электрического поля в скважинах. Заполняя пространство трещины жидкостью с высокой электрической проводимостью и наблюдая за вариацией электрического поля в процессе гидравлического разрыва, мы можем определить глубину трещины, решая обратную задачу. Для определения возможностей предложенного метода мы проводим анализ уровня сигнала и его чувствительности к вариации длины трещины.

Ключевые слова: гидравлический разрыв пласта, электрическое поле, жидкость с высоким удельным электрическим сопротивлением.

FEASIBILITY STUDY OF HYDRAULIC FRACTURE GEOMETRY EVALUATION METHOD USING TIME-LAPSE BOREHOLE MEASUREMENTS OF LOW-FREQUENCY ELECTRIC FIELD

Alexander I. Makarov

Baker Hughes, Novosibirsk Technology Center, 630128, Russia, Novosibirsk, Kutateladze Str. 4a, Ph. D., Scientist, tel. (383)332-94-43, e-mail: [email protected]

г. Новосибирск, (383)332-94-43,

Alexander N. Vasilevsky

Baker Hughes, Novosibirsk Technology Center, 630128, Russia, Novosibirsk, Kutateladze Str. 4a, Scientist, tel. (383)332-94-43, e-mail: [email protected]

Igor N. Eltsov

Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Russia, 630090, Novosibirsk, Koptyug Prospect 3, Doctor of Science, Science deputy director, tel. (383)330-75-55, e-mail: [email protected]

Gleb V. Dyatlov

Baker Hughes, Novosibirsk Technology Center, 630128, Russia, Novosibirsk, Kutateladze Str. 4a, Ph. D., Scientist, tel. (383)332-94-43, e-mail: [email protected]

Yuliy A. Dashevsky

Baker Hughes, Novosibirsk Technology Center, 630128, Russia, Novosibirsk, Kutateladze Str. 4a, Professor, Director NTC, tel. (383)332-94-43, e-mail: [email protected]

The investigation of possible techniques for evaluation of the geometry of hydraulic fractures by geophysical methods is a topical problem. We propose to evaluate the hydraulic fracture geometry by carrying out measurements of the electric field in boreholes. Filling the fracture with a conductive fluid, energizing by direct or low-frequency electric current source and looking at the variation of the electric field in borehole during the hydraulic fracture process, we can evaluate the fracture geometry through solution of the inverse problem. For estimation of capabilities of suggested method we analyze the level of the signal and its sensitivity to the variation of the fracture length.

Key words: Hydraulic fracturing, electric field, high-conductive fluid.

Hydraulic fracturing of a reservoir is a recognized process for improving well productivity. This operation is performed by injecting a fracturing fluid into a wellbore penetrating a formation at a pressure sufficient to create a fracture. The fracture geometry may be obtained from the measurements of the rock deformation caused by the growing fracture. This analysis is usually limited to data from indirect measurements (temperature, pressure, etc.). However, this data cannot be considered a reliable fracture geometry evaluation because of wellbore effects (fluid density, fluid friction, etc.). Thus, the investigation of possible techniques for evaluation of the fracture geometry properties by geophysical methods is a topical problem [2].

Our approach is based on resistivity contrast between fracture channel and formation. The objective of the study is to propose a technical solution and describe its capabilities for determining the geometry of hydraulic fractures. The main method for investigation is physical analysis of the electric field and mathematical modeling of the electric field in 3D realistic earth models. Resolution and sensitivity analysis of the computed signals are performed to formulate the requirements to the accuracy of the measured signal.

A vertical well with a rectangular hydraulic fracture filled with a high-conductive fluid with proppant materials is considered. It is placed in a homogeneous formation with resistivity of 100 Ohmm. The length of the fracture wing (L) is in the

range of 10 to 200 m, the W fracture width is 30 m, the fracture channel D is 0.6 cm and the fracture productive channel resistivity 0.05 Ohm-m. The electric field sensitivity to the fracture wing length was investigated for a set of various measurement setups. We present the voltage difference |UmnI between two points M and N at the distance 10 m. We suppose the observation point £ is placed in the middle of the MN line. To compute the electric current leakage from a cased tube, a special algorithm [1] was applied.

a b c

Fig. 1. Examples of measurement configurations for fracture wing length tracking in the presence of a cased and uncased borehole

Examples of measurement configurations are shown in the Fig. 1. The first configuration (Fig. 1.a) includes an uncased well with fractured interval and the point electric current source in the middle of the fracture. In the second configuration (Fig. 1.b), the cased well is fractured across the well direction (along the y-axis). The receivers were placed in the non-cased neighboring well. The third setup (Fig. 1.c) utilizes the electric current source grounded in the cased-neighbor well that is 100-m

n

away from the fractured non-cased well. The casing tube conductivity is 107 S/m and its length (H) is 1000 m, and the current source is in the middle of the tube. The measurement points are along the z-axis in the non-cased fractured well.

To describe quantitatively the capability of acquisition schemes for fracture

E'

geometry estimation (described above), the sensitivity nLl of the measured signal E; is introduced to the fracture wing length L:

E( _ d\nEj

ain l

_ ae^al

_ Ei' L ,

where i is the direction of the electric field component (x, y, z), and AE; is the electric signal difference caused by a variation of the fracture wing length AL. To analyze the applicability of the proposed method, both the voltage signal level and the electric field sensitivity to fracture depth L was analyzed. To demonstrate the electric field method of measurement, level ^ is equal to 0.1.

For the measurement setup of Fig. 1.a, nEz and |UmnI is plotted versus the fracture length (Fig. 2). The signal level decreases as the measurement point (£)

moves away from the fracture along the borehole. The sensitivity of the signal to the fracture wing length L takes values greater than 0.1 when L is between 10 and 80 m. The better/higher sensitivity is attained when the measurement point is as close to the fracture edge as possible.

0 50 100 150 200 0 50 100 150 200

Fracture wing length L, m Fracture wing length L, m

_a__b_

Fig. 2. Sensitivity of the electric field to the fracture wing length and voltage in the receiver line MN at the various observation points £.

£

Fig. 3 shows the plot of nLy and |UmnI versus the fracture length for the second setup (Fig. 1.b). In the presence of the casing tube, the voltage in MN line is reduced

by 100 times in comparison to the signal level for the first setup, but it is still

£

measurable by the current apparatus. The sensitivity nLy values are greater than 0.1

when L is between 10 and 120 m. The more distant the receiver point, the less the

£

voltage level at the receiver and the lower the nLy.

>

s

o un

0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

| { = 20m |

1 f" 40 m 60 m

1 <

f =

f = 80 m

50 100 150 200

Fracture wing length L, m

I Z = I

| Z = 40m~|

50 100 150 200

Fracture wing length L, m

b

a

Fig. 3. Sensitivity of the electric field to the fracture wing length and voltage in the receiver line MN at the various observation points £

For the third measurement setup (Fig. 1.c), n£z and |Umn| is plotted versus the fracture length (Fig. 4). In this setup, the signal sensitivity to the L takes values greater than 0.1 when L is between 25 and 130 m (for £ = 6 m). If the observation point (£) is 60 m, the L is between 10 and 200 m. The better sensitivity is attained when £ is 60 m. | UMN\ for £ = 60 m is greater than 80 ^V at fracture wing length L = 100 - 200 m. This signal level can be measured by current apparatus with sufficient accuracy. To track the fracture growth to the range L = 0 - 200 m, the following

conditions are recommended: an optimal sensor placement for electric voltage |UMN| measurement at points £ = 6 - 60 m; the direct current source is grounded in the cased well parallel to the fractured well with a distance of 100 m between them.

Fig. 4. Sensitivity of the electric field to the fracture wing length and voltage in receiver line MN at the various observation points

Based on the feasibility study (presented above), the hydraulic fracture geometry was evaluated using time-lapse measurements of the electric field in boreholes. The suggested method comprises an injection of highly conductive fluid and energizing the fracture with a direct (low-frequency) current. During the process, time-lapse measurements of the electric field are performed before and after the fracturing process. Changes in the electric field while fracturing are acquired in active or/and observation wells with electrode arrays. The acquired electrical data are inverted for the fracture geometry.

REFERENCES

1. Dashevsky Yu., Surodina I.V. et al. Forward and Inverse Problems of Geoelectrics in the Methods of Piles Foundation Control without Damaging // Industrial Mathematics Siberian Journal. - 2005. - Vol. 7. - № 2 (22).

2. Kim J., Um E.S., Moridis G.J. Fracture Propagation, Fluid Flow, and Geomechanics of Water-Based Hydraulic Fracturing in Shale Gas Systems and Electromagnetic Geophysical Monitoring of Fluid Migration // SPE Hydraulic Fracturing Technology Conference. The Woodlands, Texas, USA, 4-6 February 2014. - SPE 168578.

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