Integrated Study of a Heavy Oil Reservoir in the Orinoco Belt: A Field Case Simulation
Authors: Rodriguez H.A. , Vaca P. , Gonzalez O. , de Mirabal M.C. 
Published: December 2007

This paper presents a field case study of a reservoir in the Orinoco Oil Belt, Venezuela. The reservoir is a heavy oil accumulation of 10 degrees API gravity, with about 2.5 billion of STOIP. It has been produced for about 12 years by more than 170 wells, 25% of them horizontal. Due to high oil viscosity, almost all vertical wells have been producing under cyclic steam stimulation; no steam has been used in horizontal wells. A special formulation for modelling the so called "foamy oil" behavior has been used, supported by laboratory tests and theoretical considerations. Additionally, a control volume finite element (CVFE) grid was used to construct a simulation model, which was history matched and used for production forecasting and economical evaluation.

Results and Observations:

A reservoir model was made based on geological, geophysical and petrophysical interpretations, using a computer platform for each discipline. Conventional statistical and analytical reservoir engineering has been done. The definition of two pseudo-components (dead crude oil and solution gas), derived from the fluids molecular composition at reservoir level, and the characteristics of the fluid at surface were used.

The relative permeability curves for the water-oil and liquid-gas systems were derived from displacement tests.

The production mechanisms identified for this reservoir are solution gas drive, a weak aquifer activity, and the application of steam processes in the area. No evidence of compaction/subsidence has been detected, however, a recovery factor of 3% has been associated with it at an abandonment pressure of 220 psi. Average reservoir has been smoothly declining at an approximately rate of 15 psi/year.

Based on laboratory tests (PVT and displacement), conventional analyses, and simulations efforts, the foamy nature of the crude oil as a production mechanism has been determined to be of high impact. Experimental results indicated a primary recovery factor up to 14%, mostly associated to oil foaminess.

Well-known state of the art numerical simulation techniques have been applied to analyze possible acting production mechanisms in this area and their impact on the recovery factor. This approach and field experiences were incorporated for the construction of a three-dimensional compositional simulation model. Gridding was generated applying CVFE method. The use of the CVFE allows: grid generation unaffected by grid orientation, modelling the individual reservoir layer boundaries, and direct refinement in selected regions, Fig 1. After a history match was obtained, Fig 2, predictions were performed for 20 your period, in which a 12–15% recovery was achieved, 5–6% of it due to foamy oil effect, Fig 3. Predictions for three scenarios were evaluated at five different well rate constraint conditions, variation of horizontal and vertical well number, as well as the maximum oil rate constraint for horizontal and vertical wells.

Prediction evaluations indicated two reasonable cold scenario cases in which new 18 horizontal wells were added. One of them is where conditions for the wells are: horizontals at maximum oil rate of 1100 B/D and verticals at maximum oil rate of 300 B/D and the other scenario is where the exploitation is done using only horizontal wells at maximum oil rate constraint of 1100 B/D. After predictions, an economical evaluation was done to define the best cold exploitation scenario for three different artificial lift method: rod pump (RP), progressive cavity pump (PCP), electrosumersible pump (ESP). Economical evaluation results did not show significant difference among artificial lift methods evaluated and the best scenario was that where horizontal and vertical wells were produced with an oil rate constraint.

Conclusions

The main primary production mechanism of this reservoir is dominated by solution gas drive and it is associated to foamy oil generation, providing pressure maintenance and allowing a high well productivity.