Case study

D.W. Boardman, SPE, Intevep S.A.

September 1997, Rio de Janeiro fifth petroleum Engineering Conference and exhibition.

“Design considerations for Multi-lateral wells”

This case study describes a general method for the modelling of multi-lateral well performance, considering the influences of branch inflow performance and completion pressure drops (especially for heavy oil reservoirs), and stating the importance of some parameters such as completion diameter, well trajectory and artificial lift positioning (whenever it is necessary), also taking into account the economics as an essential factor in differentiating between different design approaches for a multi-lateral well.

Designing a multi-lateral well adds a sort of complexity because of considering the well branch interactions in addition to the normal consideration in conventional horizontal or vertical wells. Well branch interactions depend on branch inflow performance and completion performance from sand face to the junction points.

Prediction of well performance

A technique made by Gallivan has been used for the modelling that considers completion pressure drop, inflow performance and artificial lift performance and positioning.

In this study coupling branch inflow performance from a numerical simulator was used with vertical flow performance tables created for each section in the completion using a network pressure solver. This technique can be used for any type of well either multi-lateral or conventional horizontal or vertical well, however for accurate design data for well trajectory and completion type will be required or put onto consideration as sensitivity parameters. Generally the completion factors are an essential factor in multi-lateral well design. To include viscosity effect on cooling of fluid in completion in this case study analytical model of coupled vertical lift enthalpy balance was used.

Modelling Simulation

In this case there is a reservoir consists of two units of sand K1(upper) and L1(lower) with different permeability values Fig. The reservoir pressure in meantime is 760 Psi and oil gravity is 11 API. Because of some missing PVT data, there was uncertainty in determining fluid properties.

The field had some successful horizontal wells, but it was required to investigate the usage of multi-lateral wells as a development approach. Using a simulation model, it was possible to predict the productivity and to optimise the design of dual lateral well each lateral in one of the sand units Fig.

In this case it was a heavy oil reservoir (11 API), so it was necessary to include artificial lift. It was found that beam pumps were the most suitable and efficient because of sanding and economic considerations, with a fixed intake pressure of 50 Psi.

Design considerations

The main design consideration is an economic comparison between dual branch multi lateral well performance and that for a conventional horizontal well or even adding a herringbone lateral to the dual lateral well becoming a multi branch well. At this time a base case of dual lateral well was the most suitable but needs further optimisations concerning artificial lift position, well trajectory and completion diameter.

Positioning of Artificial lift

Completion pressure drops are dependent on liquid head, so it is useful to lower the pump for the artificial lift as possible to increase well performance. Two design options were available for dual lateral well using medium (build angle 15/100 ft) and short radius (build angle 30/100 ft) technology Fig. From the cumulative production curves Fig it is observed that lower branch in the medium radius well stops flowing after 1 year because of no fluid lifting. This was because of 46% reduction in recovery, although the upper lateral had also reduced drawdown. That means briefly that the short radius lower lateral cannot flow at low flow rates in the 7” completion.

Diameter of Completion

Considering the lower lateral, comparison on the pressure drop was made for different completion tubulars (7”, 4.5”, and 3.5”) at flow rate of 100 stb/d. Using a 3.5” instead of 7” would reduce the pressure loss in completion and increase the drawdown. That proves that completion diameter has a direct effect on well performance and is important to be considered in the design of multi lateral well.

Well GOR

Increasing GOR and reducing reservoir pressure is useful for the vertical lift. Comparison between different GOR with different completion sizes Fig shows that the best design is for 3.5” completion with GOR of 160 scf/stb, however by increasing the GOR to 400 scf/stb then 4.5” completion becomes slightly better. It is so useful in some cases that the flow rate and GOR of the lateral branches can be tuned according to the current situation along the life of the well.

Economics

In this study economic classification based on well cost per discounted barrel of oil was used. table shows estimations for well costs. It is observed that short radius dual lateral well is the most economic using the 10% discount factor. Also using discount factor of 15% short radius dual lateral well is the most economic even if with just 4%better than two horizontal wells.

Conclusion

Some of the most important parameters to consider when designing a multi lateral well are completion diameter, artificial lift positioning and well trajectory especially for heavy oil reservoirs.

As investigated previously, it was observed that using smaller liner instead of the conventional 7” completion in the lower lateral increased well performance.

The use of short radius technology with dual lateral well reduces well cost, and it is more economic than two individual horizontal wells.

Adding a lateral in the same sand increase the whole productivity, but it reduces the profit because of the branch interference effect, however making changes to fluid viscosity and increasing the viscosity above 3000 cp can overcome these effects and makethe additional lateral profitable.

References

Appendix