4C SEISMIC TECHNICS: A FEASIBILITY STUDY FOR FRACTURE ZONE INSIDE THE GRANITE-BASEMENT RESERVOIR IN THE VIỆT NAM SEA

HOÀNG VIỆT BÁCH

Domestic Exploration Department, PetroVietnam Exploration-Production Corporation (PVEP)

Abstract: Almost current oil and gas reserves in Việt Nam come from fractured granite-basement reservoir. However, none has been well studied due to the poor solution of the conventional-reflection seismic data below the granite-basement top. This problem can be solved if new geophysical technics are applied for the basement reservoir areas.

Eivind et al. [1] pointed out that “4C seismic method” has been known as a new geophysical technic with latest seismic equipment, such as the I-O’s VectorSeis Ocean cable (VSO) including four (4) sensors components. This method can be carried out in the obstructed areas, the too shallow and/or too deep-water environments, where the conventional reflection seismic technics could not be carried out. 4C seismic data is normally used to study in detail on the petroleum revervoir and/or fracture zones containing oil and gas in the reservoir rocks. S-wave, the 4C seismic data, is popular for fracture study in the oil and gas industry during 1880s - 1990s. However, it seems to be not well considered in Việt Nam because of the very widespread and popular application of P-wave data in the petroleum exploration and production. S-wave velocity and density information is crucial for hydrocarbon detection because they help in the discrimination of pore filling fluids [4]. This paper intends to say that the 4C seismic method is one of the leading geophysical technics, and it can be applied for fracture study in the granite-basement reservoir in the Việt Nam Sea.

I. INTRODUCTION

During the Late Oligocene to Early Miocene, an inversion occurred in the Cửu Long Basin changing the stress field polarity of NE-SW extension to the NW-SE compression yielding the excellent fractured-basement reservoir in Việt Nam nowadays. Although many carried out studies for that type of fractured-basement reservoirs, the results of these studies were not helpul in petroleum exploration and production on the basement targets. The existing studies of fracture property inside the basement structures did not convince people due to their poor resolution of the conventional 2D/3D seismic data below the basement top.

The above mentioned challenge requires to find out a significant technics to map the fracture inside the granite-basement structure. The 4C seismic technic is aimed to fill the weekness and develop the strength of Ocean Bottom Cable (OBC) methods [1]. In this new geophysical method, 4C seismic data including S-wave data are received by the I-O’s VectorSeis Ocean (VSO) cable. The data is processed and interpreted to analyze the fracture properties (i.e. fracture trend, fracture density, etc.) inside the granite-basement highs. Based on that, the model of fractured-basement reservoir will be study in more detail and accurately to constrain the petroleum exploration and production, and fractured-reservoir management.

II. S-WAVE, A FEASIBILITY DISCRIMINATION OF FRACTURE/PORE FILLING FLUIDS

Normally, the S-wave velocity (Vs) is about a half of P-wave one (Vp). That relationship varies with different lithologic characteristics, so it is used to characterize the lithology [8]. However, unlikely the P-wave, the S-wave is sensitive with fracture/pore filling fluids because it can not prograde in the fluid environment [8]. In fact, S-wave has been also proved that it is more sensitive than P-wave in the petroleum-reservoir description. The Vs response is much better than Vp one for the same petroleum-reservoir interval, and top and base of the reservoir is therefore highlightened by the Vs data.

Therefore, S-wave velocity and density is very useful in petroleum exploration, because it can be used to describe fracture and/or pore rock filling fluids. In the anisotropic medium, such as fractured zone, the values of S-wave velocity vary with prograding trends [9]. The fast velocity (Vs-fast) is measured when S-wave prograding in parallel with fracture trend, and the slow velocity (Vs-slow) is measured when the wave prograding in perpendicular with trend of fracture. Based on that, the fracture trend and density can be analyzed from the Vs-fast and Vs-slow attribute-maps.

1. 4C seismic method

4C seismic technic is one of the OBC methods developed in recent years with the ocean bottom cable that is consistent with the too shallow, too deep and/or the obstructed areas, where the conventional reflection method could not be carried out. This 4C method uses the ocean bottom cable, I-O’s VectorSeis Ocean (VSO) including four components sensors: one hydrophone, one vertical geophone and two horizontal geophones (one is parallel with the VSO cable and the other is normal with the cable) [1]. Seismically, Possion’s ratio can be calculated from the 4C seismic data. Therefore, the lithology and liquid saturation should be estimated from the Poisson’s Ratio since that is impossible with the conventional 2D/3D data [1]. In addition, the fracture properties should be studied by 4C seismic data since S-wave has a feasibility discrimination of fracture filling fluids.

Having the latest equipment, the 4C seismic method therefore has higher cost than that of the conventional reflection methods. The 4C method is normally applied for prospects and/or field areas where they were discovered by other geophysical methods. This new geophysical technic can be used to study more in detail the structures and also save money in the petroleum exploration and production. The method is summarized as below [6].

2. 4C seimic vessel

Based on the size of the survey, one or more seismic vessels can be used. Single 4C seismic vessel is normally used in a small to medium survey, while two or more vessels may be used for a bigger survey.

Ocean Pearl is a typical example of 4C seismic vessels. With 108 m in length, the vessel has been equipped to handle 12×6000 m VSO cables and dual source arrays with volumes in excess of 4000 cu. in. With a heli-deck and 200 days endurance this vessel has been designed and rigged to provide the highest quality 4C data worldwide with unprecedented operational efficiency.

3. C-wave Source Design

In response to increasing demand for orthogonal geometry (cross-spread) surveys, where radial symmetry of source output is required, omni-directional sources, such at that illustrated below, can be designed to meet specific client requirements.

Figure 1. Paleocene fracture zone (in oval) analyzed from Vs-fast and S-wave percentage maps having E-W trend (70-90^o) and NW-SE (110-130^o) (after [10]).

4. Cable and source deployment

Ocean bottom cable deployment and recovery is accomplished hands-free using a combination of linear engines and winches. Coupling the superior station keeping capabilities of the M/v Bourbon’s Dynamic Positioning-2 system with the winch/linear engine control systems ensures that the VSO cables are deployed and recovered safely, rapidly and accurately at all times.

5. Cable/Sensors

The VSO system utilises solid-state accelerometer sensors whose performance has been extensively proven onshore. The Gulf of Mexico operates with 6 cables, each 6000 m long, with 4C sensor nodes spaced every 25 m. Each sensor node contains a 3-component MEMS (Micro Electro-Mechanical Systems) accelerometer (VectorSeis) and a hydrophone in addition to the cable telemetry electronics. Between the sensor modules, there is a steel armoured cable with sufficient strength to carry the tension during deployment and recovery in water depth up to 2000 m. This cable design permits the cable to be dragged into position where water bottom conditions allow. Dragging the cable in this way improves sensor coupling to the seafloor. An I/O patented in-line stress decoupling system avoids the compromise in 3C vector fidelity which usually results from such tensioned deployment.

6. Recording buoy

Each 6000 m cable is connected to a radio-controlled remote recording buoy. Data recording to dual Raid discs in each buoy is controlled and QC’d using a long range 900 MHz radio system, whilst geophysical QC is accomplished using a broader bandwidth 2.4 GHz link.

7. 4C seismic data.

Figure 2. 4C seismic data (PS) in the right hand side showing clearly different top of reservoir with that of the P-wave data from the conventional reflected seismic method in Alba field, North Sea. A development was successfully designed and drilled based on the 4C seismic data (after [3]).

Figure 3. Fracture zone along the major NW-SE trending fault analyzed from the Vs-fast and Percent S-wave anisotropy maps (after Gaiser and Vandok, 2003).

III. FEASIBILITY APPLICATION OF THE 4C SEISMIC TECHNIC FOR STUDYING FRACTURE ZONE INSIDE THE GRANITE-BASEMENT RESERVOIR IN THE VIỆT NAM SEA

The feasibility study of the 4C seismic method for the fracture properties was discussed above, and it may be concluded that it is suitable to apply in the study on the fracture zone containing oil and gas. We need to analyze whether this method can point out fracture trend and density in the granite basement reservoir in Việt Nam or not.

Most of petroleum reserves in Việt Nam are situated in the fractured granite-basement structures, such as Sư Tử Trắng (White Lion), Bạch Hổ (White Tiger), Rạng Đông Fields. This is a very special petroleum trap in Việt Nam and the world (Fig. 4), in which fractures and/or minor faults play a very impotant petroleum-bearing role. However, properties of the fracture, such as fracture trend, fracture density, dip, etc., were still not identified due to the poor solution of the conventional P-wave data below the basement top. Currently, not including FMI data, none of new geophysical method is applied to study fracture zone inside the granite-basement structure.

Since an application of new geophysical technics is needed for fracture study inside the granite-basement reservoirs; the 4C seismic method may be a good choice. The 4C seismic data will be processed and interpreted to map the imaging of fractures and/or minor faults inside the granite structures. Therefore, the model of fractured granite-basement reservoir can be studied more detail and accurately. That may reduce risks in petroleum exploration and production and also gives aids in the fractured-reservoir management.

Figure 4. Model of the fractured granite basement reservoir in the Cửu Long Basin
(after [5]).

IV. CONCLUSIONS

The study on S-wave is still not well considered in the petroleum exploration and production in Việt Nam, whilst it can provide a very feasibility discrimination of petroleum reservoirs and/or farcture trend and density in the granite basement-structures.

The 4c seismic method is a new geophysical technic with latest equipment. It can be operated in the obstructed area, in the two shallow- and too deep-water environment where the conventional reflection seismic methods could not be carried out. The 4C seismic technic is used to study petroleum reservoirs by measuring 4C data including S-wave using the VSO cable with four sensors components.

Almost petroleum reserves of Việt Nam is trapped in the fractured granite-basement reservoirs. However, the study on the inside of the basement structure, such as the distribution of fracture and/or minor faults, is limited because of the poor solution of seismic data below the basement top. This can be concluded that the use of the new 4C seismic technic is needed for studying fracture properties inside the granite-basement reservoir. It also provides a good aid in the fractured-reservoir management. Based on that, the exploration and well development can be designed more accurately and successfully.

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