Abstract

Ninety-eight per cent of the total cumulative production from the Cabimas Field has originated from Miocene reservoirs. The difference in density between Miocene oil and water is very small. In addition, this crude oil has a tendency to wet the surfaces of the reservoir rock. Under these conditions, a water-in-oil emulsion, and thus higher viscosities are favoured; this in turn impairs the well production capacity. Three wells were selected (R-A1, R-A2, and R-A3) for the treatment of hidden emulsions with the use of a wetting agent in an organic solvent system.

Another factor that was thought to impair the well production capacity was plugging of the Miocene slotted-liner gravel-packed completions by organic solids which are present. For considering this problem separately, two further wells (R-B1 and R-B2) were treated only with an organic solvent system.

Treatment preparation and pump operation (between 250 and 400 bbl/well) for all five wells were completed in a single week. All of these field operations were simple and were carried out with no operational or safety problems.

Thus, the use of the wetting agent proved to be a successful method for breaking hidden emulsions. Two of the wells treated in this manner, R-A2 and R-A3, respond with an increase in the fluid production rate. An exception was well R-A1; this negative result was due to the existence of a thief zone that fails to receive enough treatment fluid. Wells treated with organic solvents only did not show any change in production rate.

Introduction

During the last 80 years, more than 360 MMBls have been produced in the Cabimas field. Ninety-eight per cent of the total oil has accrued from Miocene Reservoirs. More than 900 wells have been drilled to this target, which has proved to be highly prolific and successful: "La Rosa formation" [1]. This formation is subdivided into three units: "La Rosa Superior", "La Rosa Intermedio", and "La Rosa Inferior", which is also called "Santa Bárbara" Member and is the most important unit.

In the central, northern, and north-eastern areas of the Cabimas field, the Icotea formation (Oligocene Age) is also present. Where this formation is in direct contact with the Santa Bárbara Member, they are considered together as a single reservoir unit. In this area, the production from most of the Miocene wells has originated jointly from both of these two members (Santa Bárbara and Icotea). On the average, these formations have the following properties: Age: Miocene; formation: Sta. Barbara/Icotea; depth: 700 - 2600 ft; thickness: 80 - 120 ft; porosity: 20 - 25 per cent; permeability: 100 - 1000 md; pressure: 100 - 500 lb/in2. The main reservoir drive mechanisms have changed during time (as a function of the pressure depletion) from liquid expansion (undersaturated oil) to the current solution gas expansion. Regarding pressure maintenance, different trials have been done in the past (firefloods, waterflood, steam soak, steam drive and gas injection). However, they have failed for technical and/or economical reasons. Currently no secondary pressure support is used on Miocene reservoirs [2].

In the early seventies, SHELL performed a sidetrack campaign in more than 60 Miocene wells. Of these, 58 were completed with slotted liners and gravel-packed. These wells have been the best producers in Miocene reservoirs. However, this completion technique is currently not feasible economically.

The crude oil from the Miocene is highly aromatic and therefore has a tendency to wet the surfaces of the reservoir rock. Since the difference in density between the oil and water is so small, water-in-oil emulsions, and therefore higher viscosities, are favoured. The mobilisation of ultra-fine particles results in additional stabilisation of the emulsion and thus causes a noticeable impairment in flow behaviour. Hence, washing of the Miocene slotted-liner gravel-packed completions and avoiding the production of emulsion are the main targets of this study.

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