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A major oil and gas operator in China’s Bohai Bay needed to assess the field’s hydrocarbon distribution, which required obtaining formation fluid samples in a poorly consolidated and extra-heavy oil- bearing sandstone. The Reservoir Characterization eXplorer™ (RCX™) service from Baker Hughes, a GE company (BHGE), was able to provide the required samples, even in the heaviest recorded oil viscosity ever sampled by BHGE.

Traditionally, formations such as those found in Bohai Bay pose serious obstacles to wireline formation testing and sampling operations, including sand influx issues and probe seal loss. Although the RCX service has a strong track record in collecting highly viscous samples in unconsolidated formations, sampling this extra-heavy oil in a friable clastic reservoir with mobilities ranging from 4.6 to 16.1 mD/cP presented a new challenge.

A customized operational procedure was developed using the RCX service configured with a large-face probe and the SampleView™ downhole fluid-analysis modulestrong> from BHGE to analyze the downhole fluids in real time. The procedure incorporated an oil sampling technique known as selective-slug sampling. This involved multiple opening and closing cycles of the downhole sample tank to ensure selective (oil) slugs were progressively collected. Controlled pump-outs were employed to improve hydrocarbon breakthrough while maintaining the sample in single-phase condition for laboratory analysis.

The SampleGuard™ Geomechanics Integrated Reservoir Sampling servicestrong> also from BHGE, ensured an efficient, successful sampling operation. The service optimized the pump-outs while minimizing the mechanical failure of the rock under drawdown conditions that typically cause sand influx from unconsolidated formations.

Challenges & Results
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Challenges

  • Samples were needed from high-viscosity, extra-heavy oil in a low-mobility reservoir
  • Shallow, unconsolidated clastic formation presented a risk of sand production and probe seal loss

Results

  • Eight high-quality samples were collected in the heaviest viscosity ever sampled by BHGE: 8° API extra-heavy oil with a viscosity of 48,210 cP
  • Samples were successfully obtained from two depth stations in 22 hours
  • Customized procedure from BHGE minimized sampling time and risk of mechanical failure