kèo nhà cái 188bet Quick pre-stack seismic inversion to predict reservoir properties at a gas and condensate field in Nam Con Son basin

Summary

This paper introduces a new approach to quickly predict reservoir quality along a particular directional well-path, to justify kèo nhà cái 188bet optimisation process for well location. kèo nhà cái 188bet workflow employs a simulated annealing algorithm to convert available seismic data at well locations into some petrophysical properties that indicate reservoir quality such as porosity and water saturation. kèo nhà cái 188bet inversion process is constrained by several seismic traces that are previously extracted along kèo nhà cái 188bet well-path, which help significantly reduce kèo nhà cái 188bet calculation cost. kèo nhà cái 188bet results were successfully applied in two development wells drilled into gas bearing reservoirs in Nam Con Son basin, offshore Vietnam.

Key words: Reservoir quality, seismic inversion, rock physics model, Vshale, porosity, water saturation, Nam Con Son basin.

1. Introduction

Seismic inversion (post- and pre-stack) is widely used in reservoir characterisation  [1  - 4]. kèo nhà cái 188bet basic idea is to relate changes in rock properties due to hydrocarbon  introduction into kèo nhà cái 188bet formation to kèo nhà cái 188bet changes in pre-stack seismic amplitude via a mathematical equation, which is also called a rock physics model. kèo nhà cái 188bet model is then used to convert rock properties (porosity, saturation, shale content) into some common seismic properties (velocity, density, Poisson ratio) [4], which are further used to predict other properties via cross-plot or some mathematical approaches such as neural network or multi-attribute cross-plots [5 - 7].

A gas and condensate field in Nam Con Son basin is recently developed to produce hydrocarbon from Miocene reservoirs. Geophysical studies show strong AVO responses at reservoir levels, which indicate hydrocarbon saturated sands.

kèo nhà cái 188bet fluid replacement modelling using Gassmann fluid substitution equation showed significant reduction of offset amplitude when gas replaces water within kèo nhà cái 188bet reservoir bodies (Figure 1).

Figure 1. Fluid replacement modelling shows improved AVO anomalies of gas replacing water

Intense in-house qualitative studies  [8] have been carried out to understand kèo nhà cái 188bet  reservoirs, including AVO classes, fluid factors indicator, and kèo nhà cái 188bet AB fluid factor (ABFF) to help improve kèo nhà cái 188bet interpretation and well target optimisation. In Figure 2, kèo nhà cái 188bet ABFF attribute successfully highlights kèo nhà cái 188bet fluid existence in reservoirs A1 and F1. 

Figure 2. ABFF attribute highlights fluid potential of reservoirs A1 and F1

Table 1. Input petrophysical parameters for kèo nhà cái 188bet rock physics model

Figure 3. Excellent correlations in QC plot of kèo nhà cái 188bet input rock physics model

Figure 4. Inversion flow chart

Seismic   inversion   is   practically    employed  to quantitatively predict reservoir properties at candidate well locations.  kèo nhà cái 188bet  common  approach  is performed on kèo nhà cái 188bet trace by trace basis, which is suitable for nearly vertical well-path  as  a  single  run could provide sufficient information to kèo nhà cái 188bet whole well-path. However, since most wells in kèo nhà cái 188bet field are deviated, it requires many seismic traces  to cover kèo nhà cái 188bet well profiles, consequently increasing kèo nhà cái 188bet calculation cost. A new approach is proposed to quickly estimate kèo nhà cái 188bet interested properties and help save kèo nhà cái 188bet decision making time.

2. Method

kèo nhà cái 188bet approach employs Wyllie’s linear approximation equation [9] as a rock physics model to connect kèo nhà cái 188bet petro-physical properties - porosity

(PHI), water saturation (Sw) and kèo nhà cái 188bet shale content (Vsh) - to geophysical parameters such as velocity (P-wave Vp and S-wave Vs) and density. kèo nhà cái 188bet latter set of parameters is then used to convert into synthetic responses using kèo nhà cái 188bet re-written three-term Aki- Richard approximation by Wiggins et al. [10]. Below is a set of Wyllie approximation equation used in our method:

Where:

Vpst, VpSh, Vpw, VpHC are P-wave velocities of pure sandstone framework, pure shale, water and hydrocarbon;

 Dn , Dn , Dn , Dn are densities of pure sandstone framework, pure shale, water and hydrocarbon.

kèo nhà cái 188bet parameters used at this field are listed as in Table 1. Figure 3 shows kèo nhà cái 188bet comparison between kèo nhà cái 188bet estimated Vp, Vs and density curves (red line) with kèo nhà cái 188bet actual recorded log (green line). Excellent matches between corresponding curves validate kèo nhà cái 188bet above rock physics model.

kèo nhà cái 188bet simulated annealing algorithm [3, 11] is used to invert kèo nhà cái 188bet pre-stack seismic data. This global optimisation process takes into account a low frequency model and perturbs it to find kèo nhà cái 188bet best fit model that satisfies kèo nhà cái 188bet seismic responses. kèo nhà cái 188bet outputs are a best fit model and many other fit models that can be used for uncertainty analyses. In this study, we use kèo nhà cái 188bet best fit model for final prediction values. Figure 4 summarises kèo nhà cái 188bet general workflow of kèo nhà cái 188bet inversion algorithm.

kèo nhà cái 188bet low frequency starting  models  are  generated  by interpolating logs from available wells nearby, and applying a low frequency filter to retain kèo nhà cái 188bet low frequency content. Figure 4 demonstrates kèo nhà cái 188bet inversion flow chart template.

kèo nhà cái 188bet inversion is expected to output three parameters. In order to remove kèo nhà cái 188bet non-uniqueness in kèo nhà cái 188bet solution, kèo nhà cái 188bet input seismic data has to be of multiple traces. Therefore, pre-stack data is required.

kèo nhà cái 188bet common pre-stack inversion involves comparing synthetic to real angle gather traces to update kèo nhà cái 188bet input to satisfy kèo nhà cái 188bet real data [4, 12]. However, this is very time- consuming considering many angle traces involved in kèo nhà cái 188bet calculation process at each location. For this approach, to reduce kèo nhà cái 188bet computing cost, we convert kèo nhà cái 188bet NMO corrected offset volume into intercept, gradient and curvature volumes using an input stacking velocity cube [4]. kèo nhà cái 188bet intercept corresponds to seismic response of a zero offset trace, kèo nhà cái 188bet gradient and curvature reflect how amplitude changes with offset.

Due to kèo nhà cái 188bet limitation of data acquisition parameter, kèo nhà cái 188bet pre-stack volume is limited to 30 degree angle offset for usable quality. With this limited angle range, kèo nhà cái 188bet effect of kèo nhà cái 188bet curvature  term  on  kèo nhà cái 188bet  overall  inversion  results  is minimal.  Meanwhile,  kèo nhà cái 188bet  final  stack  volume,  which  is a summation product of kèo nhà cái 188bet intercept, gradient and curvature, is still showing a strong effect on kèo nhà cái 188bet inversion results. Therefore, we use kèo nhà cái 188bet full-stack volume along  with kèo nhà cái 188bet intercept and gradient volumes for inversion constraint. To perform kèo nhà cái 188bet inversion at deviated wells, we first extract those parameters along kèo nhà cái 188bet well-paths and input them into kèo nhà cái 188bet algorithm.

Three synthetic traces created by using kèo nhà cái 188bet equation of Wiggins et al. [10] are compared against kèo nhà cái 188bet corresponding traces from real data to control kèo nhà cái 188bet inversion algorithm. Below Wiggins’ equations are used to generate synthetic traces for error calculation.

   Where:

3. Results

We successfully applied kèo nhà cái 188bet method to a drilled  nearly vertical exploration well (well-X) for quality control and on two other development locations (wells P1 and  P2) with very close results from final LWD petrophysical analyses.

kèo nhà cái 188bet nearly vertical exploration well (well-X) indicated gas bearing for two target reservoirs: a gas saturated sand A1 of 15mTVD thickness with 26% porosity, 20% average water saturation and 5% shale volume (Vsh); a massive gas and condensate saturated sand F1 of 36mTVD thickness with 14% porosity, and 46.9% average water saturation. In Figure 5, kèo nhà cái 188bet inverted results (dark blue) have excellent matches in stack, intercept and gradient traces, while kèo nhà cái 188bet porosity, Sw and Vshale, fit very well with actual curves calculated from well logs (red). kèo nhà cái 188bet estimated Vshale and porosity curves even give accurate prediction of kèo nhà cái 188bet layering from kèo nhà cái 188bet changes in values. This encourages us to apply kèo nhà cái 188bet approach to predict at other deviation well locations.

At kèo nhà cái 188bet first development location, well-P1 targeting both A1 and F1, maximum 33o deviated profile, kèo nhà cái 188bet synthetic  traces  match  excellently  with  kèo nhà cái 188bet   input traces (Figure 6). kèo nhà cái 188bet inversion predicted very well kèo nhà cái 188bet properties, with almost perfect match of kèo nhà cái 188bet logs of A1; excellent matches of porosity and Vshale and average match of kèo nhà cái 188bet water saturation of kèo nhà cái 188bet F1. With kèo nhà cái 188bet latter reservoir, kèo nhà cái 188bet  algorithm  also  predicted  layering  via  fluctuations  of porosity and Vshale curves. kèo nhà cái 188bet actual drilling results confirm this point.

On kèo nhà cái 188bet second development well-P2, targeting reservoir  F1  with  maximum   42o   deviated   profile, kèo nhà cái 188bet inversion predicted reservoir existence at target locations, with high correlations with actual seismic traces. However, kèo nhà cái 188bet estimated properties are poorer than kèo nhà cái 188bet actual well results of kèo nhà cái 188bet F1 interval (Figure 7). This suggested kèo nhà cái 188bet heterogeneity of F1 properties from well-X location, such as changes in signatures of Vshale and Sw (much lower readings). Since kèo nhà cái 188bet prediction is dependent of kèo nhà cái 188bet input model, which is controlled by kèo nhà cái 188bet well-X properties, kèo nhà cái 188bet error is reasonable.

Figure 5. Inverted dark blue curves having excellent matches with input seismic traces and actual logs (red curves) at well- X 

Figure 6. Inverted dark blue curves with excellent matches with input seismic traces and actual logs (red curves) at well- P1

Figure 7. Inverted dark blue curves with having excellent matches with input seismic traces but average matches with actual logs (red curves) at well- P2

4. Conclusion

kèo nhà cái 188bet introduced method offers a quick-look prediction of reservoir quality along a proposed deviated well path by directly inverting kèo nhà cái 188bet seismic data into reservoir properties. It requires a starting model acting as an initial guess, guiding kèo nhà cái 188bet algorithm to kèo nhà cái 188bet optimal result that satisfies seismic traces extracted along kèo nhà cái 188bet well-bore. kèo nhà cái 188bet starting model can be generated from interpreted petrophysical parameters of a nearby well.

This method is proven to be reliable at well locations within homogeneous area. However, for highly heterogeneous reservoirs, kèo nhà cái 188bet method will not well predict kèo nhà cái 188bet properties unless proper guidance of kèo nhà cái 188bet geological heterogeneity is given to kèo nhà cái 188bet input model.

At kèo nhà cái 188bet current stage, kèo nhà cái 188bet algorithm is working along a particular well-path. In case there are enough  controlling  wells,  kèo nhà cái 188bet  algorithm   could  be used for 3D volumes for spatial mapping of reservoir properties.

Acknowledgement

We thank kèo nhà cái 188bet Vietnam Oil and Gas Group and Bien Dong POC for allowing us to use kèo nhà cái 188bet data and publish kèo nhà cái 188bet results.

Reference
1. Albert Tarantola. Inversion of seismic eflection data in kèo nhà cái 188bet acoustic approximation. Geophysics. 1984; 49(8): p. 1259 - 1266.

2. Brian Russell, Dan Hampson. A comparison of post-stack seismic inversion methods. SEG Technical   Program   Expanded   Abstracts.   1991: p. 876 - 878.

3. Mrinal K.Sen, Paul L.Stoffa. Global optimization methods in geophysical inversion. Elsevier Science Publications, Netherlands. 1995.

4. Daniel P.Hampson, Brian H.Russell, Brad Bankhead. Simultaneous inversion of pre-stack seismic data. SEG Technical Program Expanded Abstracts. 2005: p. 1633 - 1637.

 5. David M.Dolberg, Jan Helgesen, Tore Hakon Hanssen, Ingrid Magnus, Girish Saigal, Bengt  K.Pedersen. Porosity prediction from seismic inversion, Lavrans field, Halten Terrace, Norway. kèo nhà cái 188bet Leading Edge. 2000: p. 392 - 399.

6. Joseph Christian Adam Frank Valenti. Porosity prediction from seismic data using multi-attribute transformations, N Sand, Auger field, Gulf of Mexico. kèo nhà cái 188bet Pennsylvania State University, College of Earth and Mineral Sciences. 2009.

7. Josimar Silva, Gorka Garcia, Viviane Farroco, Elita de Abreu, Andrea Damasceno. Joint estimation of reservoir saturation and porosity from seismic inversion using stochastic rock physics simulation and Bayesian inversion. 12th International Congress of kèo nhà cái 188bet Brazilian Geophysical Society & EXPOGEF, Rio de Janeiro, Brazil. 15 - 18 August, 2011: p. 1327 - 1330.

8. Han N.Tran, Guy Peterson, Nam H.Tran, Lam Q.Nguyen, Hai M.Hoang. Well location optimization using seismic AVO analysis in turbidite reservoir gas sands, Block 05-2, Nam Con Son basin, offshore Vietnam. PVEP Technical Forum “Challenging Reservoirs in Vietnam”, Ho Chi Minh city. 29 - 30 May, 2013.

 9. M.R.J.Wyllie, A.R.Gregory, L.W.Gardner. Elastic wave velocities in heterogeneous and porous media. Geophysics. 1956; 21(1): p. 41 - 70.

10. Ralph Wiggins, Geogre S.Kenny, Carrol D.McClure. A method for determining and displaying kèo nhà cái 188bet shear-velocity reflectivities of a geologic formation. European Patent Application 0113944. 1983.

11. William L.Goffe, Gary D.Ferrier, John Rogers. Global optimization of statistical functions with simulated annealing. Journal of Econometrics. 1994; 60(1, 2): p. 65 - 99.

12. James L.Simmons, Milo M.Backus. Waveform- based AVO inversion and AVO prediction-error: Geophysics. 1996; 61(6): p. 1575 - 1588.

13. Steven R.Rutherford, Robert H.Williams. Amplitude-versus-offset variations ingassands. Geophysics. 1989; 54(6): p. 680 - 688.

14. Ravi P.Srivastava, Mrinal K.Sen. Stochastic inversion of pre-stack seismic data using fractal-based initial models. Geophysics. 2010; 75(3): p. 47 - 59.