Previous production drilling in the Bekok field has demonstrated two major challenges to a successful campaign. Firstly, substantial borehole instability can be encountered when drilling deviated wells through the Terengganu shale (K shale). The K shale was previously thought to be a reactive shale and that the problem originated from fluid-shale interactions. Secondly, invert emulsion oil based mud had previously been utilised on earlier production drilling but substantial productivity impairment was experienced against the K sands.

A study of three synthetic based mud systems was carried out in return permeability tests on K sand cores. One emulsifier system was identified as demonstrating significantly less impairment. This system when applied, allowed for good productivity in the K reservoir.

Exploration wells drilled with potassium chloride polymer muds showed low potassium depletion levels, suggesting that the shales were not reactive. XRD/XRF analysis of side wall cores from offset fields indicated that the shales consisted dominantly of small surface area clays, Table 1. This mineralogy and the low salinities of the shales suggested that the shales might be relatively weak.

Table 1.
SELIGI SHALE MINERALOGY ANALYSIS
(Courtesy of Esso Production Malaysia Inc.)
WellDepth (ft)FormationSurface Area m2/gIllite %Kaolinite %Chlorite %Montmorrilonite %
Seligi 3 3820 – 3840 172 43.2 17.9 11.9 1.4 
4440 – 4470 199     
4650 – 4680 179 21.8 34.3 21.1 0.8 
5790 – 5810 111     
5830 – 5850 178 39.0    
5870 – 5890 168 40.4 23.2 13.9 not detected 
Seligi 4 3670 – 3700 145 39.0 12.2 12.2 0.6 
4510 – 4540 207 51.2 17.1 14 0.7 
4960 – 4990 217     
5780 – 5800 168     
6060 – 6075 178 35.3 14.4 9.8 5.9 
6620 – 6640 176     
Seligi 9 3937 – 3970 134 45.6 14.2 11.4 not detected 
4692 – 4725 195     
5020 – 5053 178 42.1 22.2 42.1 not detected 
5151 – 5184 195     
6168 – 6201 171 48.4 16.7 14.3 not detected 
6267 – 6300 129     
SELIGI SHALE MINERALOGY ANALYSIS
(Courtesy of Esso Production Malaysia Inc.)
WellDepth (ft)FormationSurface Area m2/gIllite %Kaolinite %Chlorite %Montmorrilonite %
Seligi 3 3820 – 3840 172 43.2 17.9 11.9 1.4 
4440 – 4470 199     
4650 – 4680 179 21.8 34.3 21.1 0.8 
5790 – 5810 111     
5830 – 5850 178 39.0    
5870 – 5890 168 40.4 23.2 13.9 not detected 
Seligi 4 3670 – 3700 145 39.0 12.2 12.2 0.6 
4510 – 4540 207 51.2 17.1 14 0.7 
4960 – 4990 217     
5780 – 5800 168     
6060 – 6075 178 35.3 14.4 9.8 5.9 
6620 – 6640 176     
Seligi 9 3937 – 3970 134 45.6 14.2 11.4 not detected 
4692 – 4725 195     
5020 – 5053 178 42.1 22.2 42.1 not detected 
5151 – 5184 195     
6168 – 6201 171 48.4 16.7 14.3 not detected 
6267 – 6300 129     

A study of the Malay Basin shales using extended leak off tests, electric log data and tests on shale core provided a geomechanics model that gave minimal wellbore instability. Field experience further defined the model. The application of required mud weight support to balance induced stress resulted in greatly enhanced wellbore stability.

Bekok B-12 well was the first well in the Bekok field revisit to pass through the K shale to penetrate the K reservoir and as such is a test well for the studies made prior to drilling operations.

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