Abstract

During the initial production from several wells at the oilfield C in Southern Pannonian basin - Croatia, the oil production was significantly restricted due to occurrence of emulsion in swabbed produced fluid. Laboratory analyses of produced fluid samples indicated a stable water-in-oil emulsion. The laboratory investigation was carried out in order to study the mechanism of possible generation of emulsion downhole as a result of mixing the mud filtrate and formation water with crude oil in near wellbore zone. Numerous variables were tested to evaluate their implications to emulsion formation and its stability, as well as a better characterization of emulsion properties. The findings of this study suggest the inevitable downhole generation of hard to break w/o emulsion in the case of mixing the particular crude oil with mud filtrate and/or formation brine.

Introduction

During drilling a mud filtrate penetrates the pore space, causing, in the most cases, the formation damage around the wellbore. One of the forms of such permeability impairments is the formation of emulsion, mixing the crude oil with mud filtrate. From the moment when flow from reservoir starts, sooner or latter, concurrently with oil the formation water will be produced. In both cases, or in their combination, when brine and oil are mixed, the emulsions can occur.

Many authors during the several decades of research on natural generated oil emulsions approached this problem studying most often the factors that affect their stability, conditions of tendency for emulsion forming, or emulsion breaking. Generally, all them agree that this technical problem is so complex, characterized with numerous variables which requires almost individual approach to each particular case, applying widely accepted analytical methodology.

Case History

After well completion, a payzone at depth 2746-2728 m (BHST=145 C) was perforated. Instead of oil flow production, after unloading, a very viscous bituminous like material was obtained at surface by swabbing. The continuous swabbing brought the well on sporadic production of oil and water with decreased flow rate. Production logging indicated high skin effect in near wellbore zone.

The payzone was drilled using polymeric water base mud (composition shown in Table 1).

The samples of produced fluid were collected and analyzed in laboratory in order to find out the cause of formation damage and flow rate restrictions. Preliminary laboratory analyses of produced swabbed fluid indicated the stable emulsion of brine and oil. Using deemulsifier the aqueous phase was separated from oil and analyzed. Measurement showed relatively high pH value (8.9) and low salt content (3 g NaCl/L). These data suggested that aqueous phase, found in emulsion, was mud filtrate or formation brine diluted with mud filtrate.

Experimental

The experiments were carried on in order to simulate the possible conditions of emulsion generation downhole and characterize and explain this process.

Materials Used. Tests were conducted using the samples of (a) crude oil and its distillation fractions, (b) synthetic formation water, (c) mud filtrate, (d) emulsions generated in laboratory.

The composition and base properties of crude oil (sample from pay zone) are shown in Table 2. For obtaining the mud filtrate the polymeric water base mud (Table 1) with the same composition as used during drilling the well was prepared. The mud filtrate properties are shown in Table 3. For formulating the synthetic formation water, the sample of original water from the well was analyzed first. Than, based on these data, the synthetic formation water with similar composition and properties was prepared (data shown in Table 4).

Emulsions and Emulsification Procedure. Emulsification tests were carried on in laboratory using various content of aqueous phase (synthetic formation water and/or mud filtrate) mixed with crude oil or one of its distillation fractions (Table 5). For emulsification a standard homogenizer with propeller rotor was used, agitating (400 rpm) for two hours at 82 C.

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