Case Study: Flow Assurance Issues Resolved by Elimination of OBM Effects

Asphaltene precipitation model examines commingled flow in deepwater Gulf of Mexico

Challenge: Eliminate oil-base mud (OBM) contamination in fluid samples, which affected flowassurance assessments during a high-riskdeepwater exploration operation.

Solution: Rely on Schlumberger to estimate flow assurance issues related to asphaltene and then establish asphaltene precipitation regimes from the uncontaminated black oil samples.

Result: Enabled making critical decisions on field development planning, sequential production, and continuous chemical injection to control solids precipitation.

Contaminated fluid samples impede field development

In this high-risk deepwater prospect, contamination of the crude oil with oil-base drilling fluid affected both fluid properties (bubblepoint pressure) and asphaltene stability, creating a very unstable fluid. One particular aspect of miscible OBM contamination is that it artificially depresses the asphaltene onset pressure, and consequently poses a significant risk of underdesigning asphaltene mitigation for deepwater applications. Also, the addition of OBM products decreased the gas/oil ratio (GOR) from 1,130 to 850 scf/bbl, reducing the amount of natural asphaltene precipitants in the live oil. Because of its global expertise in managing reservoir fluids, phase behavior, and modeling, Schlumberger was selected to conduct the measurements and build a thermodynamic asphaltene model that would predict reservoir fluid depletion and resolve the issue of gas condensate and black oil commingling.

Gas condensate and black oil commingle model

Reservoir oil has the propensity to form asphaltene particulates during primary depletion. The asphaltene formation was further aggravated by decreasing temperature and pressure conditions, both of which were factors in this case. Additional aggravation of the asphaltene formation occurred because it was titrated with a gas condensate; an increase in the mixture GOR raised the formation propensity.

Schlumberger developed an OBM contamination-free fluid model that estimated the flow assurance issues related to asphaltene, and then established asphaltene precipitation regimes for the uncontaminated black oil. Computations were developed and a mathematical model was created based on experimental data on commingled flow. The model’s predictions showed that an increase in the asphaltene onset pressure and bubblepoint pressure of the reservoir fluid would result if the samples were decontaminated. Operating guidelines were also established.

Field development planning enhanced

Prior assessment of downhole commingling had a significant impact on completion design, and hence, production strategy. In the new model the black oil showed a propensity for asphaltene precipitation, which was aggravated by a factor of five when commingled with gas condensate. Consequently, sequential production was initiated with downhole chemical injection for the black oil production phase.

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