Abstract

The Petroleum Development of Oman, PDO has been considering adapting new technologies to ensure that strategic resources are optimally explored, developed and produced during the life cycle of their fields. Among the various options available to achieve these objectives, Underbalanced Drilling technology is considered to evaluate the benefits over traditional drilling and completion methodologies in terms of reservoir performance.

Accordingly, an Underbalanced Drilling reservoir candidate screening process was initiated to examine the suitability of the technology for seven gas reservoirs in PDO.

The screening process began by taking basic reservoir and drilling data and processing it through expert system software. The software rated the candidacy of the reservoir to help determine the likelihood of achieving success with underbalanced vs overbalanced drilling and completion techniques, to identify the best potential underbalanced candidates from the given portfolio and proceed with a detailed analysis.

The expert system score for underbalanced candidacy is calculated by considering different modules. Each module includes a comparison of the key differentiators between overbalanced and UB techniques that impact the UB candidacy of a reservoir. To make these comparisons, the expert system assumes a well is drilled overbalanced and assesses the potential effect on reservoir performance and then estimates the potential benefit of underbalanced drilled wells with minimal damage.

In this paper, the results of the screening process for the gas reservoirs examined are presented and discussed. The study confirmed that candidate selection is a critical first step before initiating any underbalanced drilling project. It also explains why when correct candidates are chosen, UBD significantly improves productivity compared with conventional overbalanced drilling practices.

The UBD Screening Process

This paper investigates the reservoir suitability of underbalanced drilling using an Expert System screening process that was developed in conjunction with leading industry consultants to increase success in candidate selection and to evaluate the benefits of underbalanced drilling over traditional drilling and completion technology in terms of reservoir performance.

Table 1 presents the input data required for each of the seven reservoirs studied in this Phase of the project. All data input was reviewed for completeness and consistency. Figure 1 illustrates a generic schematic of the expert system screening process.

The Expert System Phase I Process Methodology

The 'quick-look' screening study is the first step in the process. The phase I score for underbalanced candidacy is calculated by considering different modules. Each module includes a comparison of the key differentiators between overbalanced and UB techniques that impact the UB candidacy of a reservoir. To make these comparisons, the system assumes a well is drilled overbalanced and assesses the potential effect of drilling the well overbalanced on reservoir performance and then estimates the potential benefit of underbalanced drilled wells with minimal damage.

The input parameters are run through various modules containing algorithms and logic rules that asses underbalanced candidacy based on the following premises:

  1. Likelihood and severity of formation damage,

  2. The overall impact of damage on productivity and recovery,

  3. The ability to successfully treat this damage with conventional stimulation techniques and finally,

  4. The relative economics of underbalanced drilling vs. conventional drilling, completion and stimulation techniques.

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