New Approaches for Permanent Downhole Gauge (PDG) Data Processing

Abstract

One of the major issues in processing permanent downhole gauge (PDG) data is that too many transients exist over a reasonable time period, say 6 months. A formula was proposed to predict the transients that may be detected or missed. Reasonable prediction was achieved via the formula.

Noise usually exists in data recorded by PDGs. Denoising is thus one of the most important steps in PDG data processing. In order to denoise the data, the data noise level must be estimated beforehand. Unfortunately, the data noise level is typically case-dependent, and therefore, it is impossible to identify a universal value for the level that may be used for all the application scenarios.

One appropriate approach to estimate the noise level is to first best fit the data, subtract the predicted pressure response from recorded values, and then calculate the noise level based on the difference. We propose to apply nonlinear regression via the Polytope method (Gill, Murray, and Wright, 1981) for best-fitting PDG data to determine the noise level. It is found that the new approach is superior to the least square error (LSE) linear regression as used by Khong (2001), because the bottom-hole wellbore pressure response in a well should be treated as a nonlinear function of time over the majority of the well production/injection/shut-in period. Unless a very small range of the data (say 2 h) is considered, a linear pressure response with time is not anticipated. Furthermore, with nonlinear regression through the Polytope approach, there is no strong restriction in data quantity and data density, hence, automatic detection of the data noise level can be implemented.

Keywords:

• permanent downhole gauge (PDG)
• data processing
• transient identification
• denoise
• wavelet
• polytope method

ACKNOWLEDGMENT

We would like to take this opportunity to express our sincere thanks to ChevronTexaco EPTC management for their permission to publish this pa per. Thanks also go to Mr. Kenneth Ferguson for nice comments and suggestions.