Numerical Simulation Of Two Horizontal Wells Drilled In A Heavy Oil Reservoir Located In Maracaibo Lake, Venezuela
Authors: Fernandez A. B.G. , Zerpa A. L.B. 
Published: December 2007

Horizontal well technology is being applied in Venezuela as a new method for improving oil production in mature heavy oil reservoirs. The National Oil holding company, PDVSA, has used horizontal re-entry technology to enhance the productivity of old vertical wells in these reservoirs.

In 1995 two horizontal wells, a re-entry and a new well, were drilled in the Bachaquero-1 heavy oil field, located in the Maracaibo lake in the western part of Venezuela. This reservoir, at 2500' TVDSS, is characterized by low gravity 10.6 API oil. with viscosity of 650 cP at initial reservoir conditions (950 psia and 140 F). Reservoir permeability averages 1700 md. The reservoir has been produced since 1969, and the average pressure has now declined by approximately 50% in some areas, leaving little remaining energy to aid production.

This paper presents the numerical simulation of these two horizontal wells. The main objective of the numerical simulation was to history match the horizontal well behavior during cold production, and to investigate the feasibility of introducing a cyclic steam injection scheme to enhance productivity.

The simulation was performed using a pseudo-compositional, fully implicit 3D thermal simulator. Results presented include the history match of the two horizontal wells and nearby vertical wells, sensitivity analysis to well length and other parameters to aid future well design, and investigation of several steam injection schemes. Wellbore discretisation was used to represent the horizontal well, as this method has been found to be superior for calculation of friction effects and wellbore oil viscosity increase due to gas breakout and cooling, for heavy oil reservoirs.

Simulation shows interference effects between the horizontal well and nearby vertical wells to be significant, with the horizontal well productivity dropping nearly 50%. This effect is also seen in the production results. The optimum parameters for the steam cycle scheme were also predicted, and these results successfully applied in field operations.

Introduction

Oil production in the Venezuelan Bolivar Coast region started in the 1950's, and many reservoirs located in this area are now considered mature. Although significant crude volumes still remain underground, production has been declining for some time. Horizontal well technology has been successfully used as a means of stemming this decline. Bachaquero-1 is a heavy oil reservoir of the region which has produced only about 4% of the oil in place (figure 1).

Due to low reservoir pressures and high crude viscosities, by 1994 conventional vertical wells could not maintain commercial "cold" production rates. A cyclic steam injection scheme was applied to reduce the high crude viscosity, and to increase the oil production of the wells.

In 1995 two horizontal wells (a re-entry and a new well LL-125 and LL-3343), were drilled to investigate the use of this technology to increase oil production. However, the "cold" production rates of these wells were little better than the cold production from a vertical well. It was decided to investigate the use of a cyclic steam injection scheme to enhance productivity. Prior to applying this technology, it was necessary to design the optimum operating conditions for the horizontal well scheme, and define the likely enhancement of productivity for the well.

This paper presents the results of the history match of the cold production for the 2 horizontal wells and nearby vertical wells, the design of the optimum cyclic steam injection scheme (injection rate, time, steam quality, soak time and oil rate), and a sensitivity analysis to define optimum well length and wellbore diameter for future horizontal wells.

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