Diagnostic and analysis of long-term transient pressure data from Permanent Down-hole Gauges (PDG)
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Permanent Down-hole Gauge (PDG) is the down-hole measuring device installed during the well completion. It can provide the continuous down-hole transient pressure in real-time. Since 1980s, PDG has been widely applied in oilfields. The wide field applications have demonstrated that the long-term pressure monitoring with PDG is useful for production optimization, reservoir description and model calibration. Analysing the long-term, noisy and large volume of PDG pressure data and extracting useful reservoir information are very challenging. Although lots of achievement has been made in PDG data processing, such as denoising, outlier removal and transient identification, analysis of long-term transient pressure from PDG is still difficult due to several challenging problems. The first problem is the dynamic changes in reservoir-well properties, which can cause the linearity assumption for pressure-transient analysis invalid, also the reservoir model needs calibration to match the field performance. The second problem is unknown or incomplete flow rate history. These problems together make it a very challenging task for engineer to interpret long-term transient pressure from PDG. This study investigates novel methods to analyse the long-term transient pressure from PDG with Wavelet Transform (WT). Firstly, a new diagnostic function named as Unit Reservoir System Response Aurc has been developed, and it can effectively diagnose the nonlinearities from PDG pressure due to the changes in reservoir-well properties. The nonlinearity diagnostic and evaluation is an important procedure before pressure analysis. Secondly, a model-independent method of reconstructing unknown rate history has been developed. This method has wide applications, considering the effects of skin, wellbore storage, reservoir heterogeneity and multiphase flow. Thirdly, based on the nonlinearity diagnostic result, sliding window technique is proposed to analyse long-term pressure with nonlinearities and update reservoir model with time-dependent reservoir properties. The synthetic cases and field data application have demonstrated that the developed methods can reveal more useful reservoir information from PDG pressure and realize the potential of PDG as the tool of reservoir management.