188bet vn 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 188bet vn optimisation process for well location. 188bet vn 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. 188bet vn inversion process is constrained by several seismic traces that are previously extracted along 188bet vn well-path, which help significantly reduce 188bet vn calculation cost. 188bet vn 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]. 188bet vn basic idea is to relate changes in rock properties due to hydrocarbon  introduction into 188bet vn formation to 188bet vn changes in pre-stack seismic amplitude via a mathematical equation, which is also called a rock physics model. 188bet vn 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.

188bet vn fluid replacement modelling using Gassmann fluid substitution equation showed significant reduction of offset amplitude when gas replaces water within 188bet vn 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 188bet vn  reservoirs, including AVO classes, fluid factors indicator, and 188bet vn AB fluid factor (ABFF) to help improve 188bet vn interpretation and well target optimisation. In Figure 2, 188bet vn ABFF attribute successfully highlights 188bet vn 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 188bet vn rock physics model

Figure 3. Excellent correlations in QC plot of 188bet vn input rock physics model

Figure 4. Inversion flow chart

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

2. Method

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

(PHI), water saturation (Sw) and 188bet vn shale content (Vsh) - to geophysical parameters such as velocity (P-wave Vp and S-wave Vs) and density. 188bet vn latter set of parameters is then used to convert into synthetic responses using 188bet vn 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.

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

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

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

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

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

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

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

   Where:

3. Results

We successfully applied 188bet vn 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.

188bet vn 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, 188bet vn inverted results (dark blue) have excellent matches in stack, intercept and gradient traces, while 188bet vn porosity, Sw and Vshale, fit very well with actual curves calculated from well logs (red). 188bet vn estimated Vshale and porosity curves even give accurate prediction of 188bet vn layering from 188bet vn changes in values. This encourages us to apply 188bet vn approach to predict at other deviation well locations.

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

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

188bet vn introduced method offers a quick-look prediction of reservoir quality along a proposed deviated well path by directly inverting 188bet vn seismic data into reservoir properties. It requires a starting model acting as an initial guess, guiding 188bet vn algorithm to 188bet vn optimal result that satisfies seismic traces extracted along 188bet vn well-bore. 188bet vn 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, 188bet vn method will not well predict 188bet vn properties unless proper guidance of 188bet vn geological heterogeneity is given to 188bet vn input model.

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

Acknowledgement

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

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