C046 Combined Seismic Survey Based on the Use of Different Types of Seismic Waves (CT-Seismic) Conception and Case Studies

Transcription

1 C046 Combined Seismic Survey Based on the Use of Different Types of Seismic Waves (CT-Seismic) Conception and Case Studies B.Y. Meltchouk* (Geoacoustic Ltd), O.L. Kuznetsov (Institute for New Oiland-Gas Technologies), I.A. Chirkin (Institute for New Oil-and-Gas Technologies), S.I. Karnaukhov (JSC GAZPROM), R.N. Mukhametzyanov (Gazpromneft Co.), Y.A. Kuryanov (INTEGRA Group Co.), S.I. Slionkin (Slavneft-NPTS R&P Center), G.V. Kashirin (Slavneft-NPTS R&P Center), V.I. Gorelikov (JSC Gazpromneft) & S.L. Aroutunov (NTK "ANCHAR") SUMMARY Considered is a conception of combining seismic technologies (CT-seismic) based on the use of different seismic waves: reflected, scattered, and natural emission waves which are characterized by different origins and bring us diverse types of geological information. Considered is the conception of CT-seismic based on the use of Focusing Transformation of seismic wavefield introduced by Slionkin (SFT). SFT is the efficient tool allowing identify different types of seismic waves. Combined seismic survey allows materially extend the range of solutions at the stages of prospecting, exploration and production of HC resources. Presented are examples of applying CT-seismic to solving geological and geophysical problems at the exploration and production stages including an example of combing SVSL and CMP for offshore zone in Bering Sea. Presented is an example of applying CT-seismic for monitoring the production of HC to improve the oil recovery and production rate.

2 Introduction. The development of seismic exploration methods follows two main lines. The first line assumes up-dating and perfection of well known 2D and 3D seismic based on the use of mirror-reflected waves. The second line assumes development of crucially new breakthrough seismic technologies based on the use of other types of seismic waves. Among such new types are scattered and natural emission waves which are characterized by extra-low energy ( times less as compared with reflected waves). These waves enclose the important geological information. To study such low-energy waves (determine their amplitude and generation locations) the Focusing Transformation (SFT) of seismic wavefield has been introduced by Slionkin [1]. Based on the use of scattered and natural emission seismic waves and SFT-algorithms special seismic technologies: Side-View Seismic Location - SVSL [2] and Seismic Location of Emission Centers - SLEC [3] were designed which considerably extend the range of solutions provided by seismic survey. A combination of seismic technologies (CT-seismic) making use of different types of seismic waves (reflected, scattered, and emission waves) allows to solve major problems at the stages of prospecting, exploration and production of HC resources more reliably and completely. 1. Conception of CT-seismic. Physical basis of CT-seismic is the use of seismic waves of different origin: reflected, scattered and emission waves. SFT-transformation of seismic wavefield is the universal tool allowing to identify different types of waves in the observed wavefield. SFT allows focus a diverging front of seismic wave observed on the surface into a focusing point where this wave was emitted. SFT-algorithms can be applied to processing of seismic data acquired by conventional 2D and 3D observation techniques and by specialdesigned areal apertures. Focusing of reflected seismic waves allows enhance the quality and resolution of seismic sections owing to higher noise immunity and increased stacking fold in focusing points. By way of example Fig.1 presents a comparison of two sections produced by processing of the same 2D seismic observations acquired on Kuyumba oil field (East Siberia) with the stacking fold of 48 using SFT-algorithms (a) and modern CMP technique (b). a b Fig.1. Fragment of 2D seismic time section: processing based on Slionkin Focusing Transformation (a) and conventional modern CMP processing (b). As seen we have much more information throughout the target horizon in the SFT-section (a) as compared to conventional CMP section (b). This characteristic feature of SFT-processing of reflected seismic waves is of crucial importance for the purposeful identification of horizontal reflectors related to gas-water and oil-water contacts in arches and tilted strata at

3 the slopes of structures. Another advantage of SFT-transformation is the possibility of scanning the lower hemi-sphere. By shifting (vertically and horizontally) focusing points we could produce 2D and 3D-matrices of the energy of scattered waves and natural emission waves. This principle has been implemented in SVSL and SLEC seismic technologies making use of areal observation apertures. Actually CT-seismic includes the following basic technologies: CMP-2D, CMP-3D, SVSL, and SLEC. In particular areas the ANCHAR technology is included which is based on other proprietary principles of data processing in order to estimate oil saturation of formations [4]. Fig.2. Combined CMP (black-white) and SVSL (color) section where gaswater contact reflectors are shown. 2. Case Studies Examples of Applying CT-seismic. Prospecting stage. The first example deals with prospecting of targets in offshore zone of Bering Sea. Fig.2 presents a combined seismic section produced by applying SFT-algorithms to conventional marine seismic data (black-and-white section) and SVSL section of open fracturing (color section). Here in the arch of structural trap horizontal reflectors are exhibited relating to gas-water contact (GWC). SVSL distribution of open fracturing produced by processing of scattered waves is shown by color. Within the limits of the trap we see subvertical abnormally fractured zones confined only to steep flanks of the fold that is indicative of the hermetic arch of the trap. All resulting indications (structural trap, HC saturation, and hermetic seal) make reasonable drilling of a prospecting well at this site. a b c d Fig.3. Akobinsk field, Bashkirian deposits. (a) Structural maps CMP-3D; (b) ANCHAR map of HC anomalies; (c) SVSL map of fractured zones; (d) combined map showing drilling sites. The second example illustrates the use of CT-seismic in the exploration under complicated conditions of salt-dome tectonics of the Akobinsk field (Orenburg). The field is embedded

4 below the salt ridge. This often results in a false plan of subsalt structures. Fig.3 presents results of identifying the anticlinal trap in Bashkirian pay deposits using a combination of CMP-3D (a), ANCHAR survey of HC-saturation of formations (b), and SVSL study of fracturing of Bashkirian deposits (c). Comparison of these maps is presented in Fig.3d. Earlier well #171 was drilled (based on CMP-2D and ANCHAR) producing a limited gas inflow (at the depth ~5000m) of 60 thous. m 3 /day. SVSL study performed later suggests that well #171 has tapped the trap not optimally (in a medium fractured zone). Next well #172 was spud in a more favorable site characterized by the presence of trap, HC saturation and high fracturing. Open fracturing index, conv unit Clay content maximum Oil saturation index, conv unit minimun - Gradient vector of SE energy; - injection wells -producing wells; - suspended wells; - 1-st order recommended wells; - 2-nd order recommended wells. Fig.4. Results of survey Rostashinsk oil field by CT-seismic: (a). SVSL distribution of open fracturing; SLEC data: (b) litho-facies characterization; (c) miniblock structure of the field; (d) distribution of HC saturation of reservoir; (e) layout of new drilling sites. For exploration stage presented is an example of results produced by CMP-2D, SVSL and SLEC seismic technologies during post-exploration of northwest part of Rostashinsky oil field (Orenburg region) Fig.4. Productive horizon D3 is embedded at the depth of ~4.2 km. It is characterized by oil inflow rate of t/day in the operating eastern sector of the field. The results of SVSL (a) and SLEC (b-d) seismic survey presented as maps of distribution of open fracturing, lateral heterogeneity of oil-saturation, litho-facies characterization and miniblock structure of the pay series allow to build a detail geological model and provide optimal pattern of production wells. One of possible patterns of new drilling wells is presented in Fig.4e. A proposal was submitted to drill the first-order wells in the nodes of miniblock facets, and the second-order wells shall be drilled in the centers of the miniblocks. Next, the oil saturation was estimated for the area enlarged to the west and north in order to look for possible HC accumulations outside the limits of the field. CMP-3D seismic survey performed to the north of the Rostashinsky field suggested that the HCsaturated zones identified by SLEC survey are characterized by uplifted embedding of horizon D3, i.e., the presence of a trap. In fact this result extended the area of the field and increased HC in-situ reserves of the field. Presently an exploratory well is planned to the northeast of the field within the limits of outlined anomaly of HC saturation.

5 The production stage is illustrated by example of SLEC seismic survey [3] of Rostashinsky oil field which was put into production in SLEC observations were performed in a mode of long-term monitoring of natural seismic emission. Fig.5 presents basic directions of water flow from injection wells and oil flow from a geodynamic pump compared to a map of current potential flow rate for reservoir D3 (5a) and a correlation between production rate and relative variance of the mean energy of seismic emission waves (5b). In Fig.5a highlighted is a fragment where oil flow is blocked by water flow from a line of injection wells. This effect resulted in low flow rate in production wells located behind this line in a radial zone of oil saturation. In Fig.5b highlighted are wells candidates for IOR operations. Overall, in this field a more efficient radial-concentric pattern of inject and production wells is recommended instead of existing isometric system in the eastern part of the field. a b Blocking of oil flow by a water flow from injection wells Production rate, t/day Radial oil-saturated zone Current production potential Wells recommended for IOR Relative SE dispersion, conv unit Predicted production rate, tons/day Relative variance of seismic emission Fig.5. Layout of basic lines of water flow from injection wells and oil flow from a geodynamic pump (a) and a correlation between production rate and relative variance of the mean energy of seismic emission waves (5b). Conclusions 1. The use of CT-seismic and integrated interpretation of seismic studies based on different types of seismic waves could provide efficient solutions to many urgent problems of seismic survey at prospecting and exploration stages and give crucial information when drawing geological and dynamic fluidal models of a field. 2. Long-term passive seismic monitoring applied at a permanent basis in oil fields under production is a promising technique which will optimize and increase the recovery of HC resources. Acknowledgements Authors thank managers and geophysicists of GAZPROM, Gazpromneft, Orenburggazprom, Slavneft, and Orenburgneft companies for assistance and support. References 1. Slionkin, S.I., Kashirin, G.V., Masjukov, A. [1999] Seismic Imaging by Focusing Transform. The Second Balkan Geophysical Congress, Abstracts. 2. Kouznetsov, O.L., Faizulline, I.S., Chirkine, I.A. et al. [2001] Study of 3-D distribution of geomedium fracturing by side-view seismic location method (SVSL). Extended Abstracts of the 63-rd EAGE Conference, Amsterdam, June 11-15, 2001.Amsterdam, Kuznetsov, O.L., Chirkin, Sydykov, K.Z., et al. [2006] Applying Seismic Location of Emission Centers (SLEC) to Monitor the Production in Oil-and-gas Fields. EAGE Workshop Passive Seismic: Exploration & Monitoring Applications, December 10-13, 2006, Dubai. 4. Suntsov, A.E., Aroutunov, S.L., Mekhnin, A.M., et al. [2006] Passive Infra-Frequency Microseismic Technology Experience and Problems of Practical Use. EAGE Workshop Passive Seismic: Exploration & Monitoring Applications, December 10-13, 2006, Dubai. EAGE 69th Conference & Exhibition London, UK, June 2007