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Abstract

ChevronTexaco has 13 wells producing offshore Africa via electric submersible pumps (ESPs) that experienced problems primarily due to the transient nature of the multiphase inflow through the sinusoidal well profiles. A study completed in 2002 used a transient multiphase simulator to model the sinusoidal sections of each producer and calculate the conditions that the ESPs could see over the life of the well for various drawdown conditions and speeds. This paper will focus on how the results from the simulator were validated and used to reduce operational downtime and improve future ESP completion designs. This paper will also demonstrate how severe sinusoidal well profiles impact production operations.

Introduction

The Banzala Field, located in Block 0, offshore Cabinda, Angola has thirteen wells that have been producing since 2000 with ESPs. In 2002 it was observed that the declining reservoir pressures and rising GORs were adversely effecting production. The ESPs would frequently shut down due to low amperage and because this was an unmanned platform, the associated downtime was significant. The engineering staff associated with this property wanted assistance in modifying the equipment or well designs to get optimal performance from the ESPs.

All these wells were drilled as sinusoidal horizontals or multilaterals. Due to the presence of a shallow gas hazard, it was necessary to place the well jacket on the flank of the Banzala structure, necessitating the need for world record high angle drilling to intersect the reservoir.¹ Figures 1 - 6 are some examples of typical well profiles. As the reservoir inflow began to decline, so did the fluid velocities in the horizontal sections. As as result, gas bubbles started to coalesce and accumulate in the high spots within the undulating wellbore. Accumulation continued until the pocket of gas bridged the hole diameter, eventually being forced out as a slug and this process continued to cycle. It was assumed that when the larger gas slugs reached the ESPs, the amperage would drop below the minimum allowable set in the surface control panel and the power to the ESP would be shut off.

Traditionally, studies such as this would begin by using a standard nodal analysis program to evaluate the inflow. The issue with these programs is that they are static models and the parameters that needed to be evaluated for this project were dynamic in nature (i.e size and frequency of gas slugs). At the time of this study, there did not appear to be any dynamic multiphase functions available in commercially available nodal analysis software. However, ChevronTexaco uses a transient multiphase simulator for analyzing flow behavior in risers and flowlines on most of its deepwater and subsea developments. Being that these sinusoidal well profiles bore a similar resemblance to a flowline profile, it was decided to use this program to model fluid flow behavior in these wellbores.

The principle aspects of work performed during the study included using a transient multiphase simulator (OLGA) to model the sinusoidal/horizontal hole section of each producer, calculate the current conditions (e.g. frequency and volume of gas slugs), then perform a sensitivity study to determine what might be done to alleviate any slugging problems found. The results from the transient analysis were then used to develop optimal ESP and completion designs for existing and future wells.

Transient Multiphase Flow Simulations

Transient multiphase flow simulations were conducted on each wellbore to determine the potential for slugging in each wellbore. This is distinct from traditional, steady-state simulation methods in that instead of merely predicting the flow regime within the wellbore, the simulation results were intended to indicate the approximate frequency and size of liquid slugs and gas bubbles. The OLGA simulator was used to perform this modeling. Developed by a SINTEF/IFE joint project in 1984-1992, OLGA is a mechanistic model that was developed from a large volume of data obtained on large scale flow loops by SINTEF. Today the model is an industry standard for transient pipe flow simulation.