Unknown Rate History Calculation from Down-hole Transient Pressure Data Using Wavelet Transform

Abstract

In the oilfield, rate metering is less satisfying compared with pressure measurement. Detailed rate history for individual well is not always available. The common practice is that few of the wells are measured together through manifolds and daily, monthly rate or total cumulative production is measured with surface separation equipments. Rate allocation algorithm is utilized to assign production to each well, but the results are inaccurate due to low measurement frequency. For well testing, another problem is that the rate measured from the surface is very often inconsistent with the pressure measured from down-hole. These problems together make it a very challenging task for engineer to interpret transient pressure from the well down-hole gauges. This article introduced wavelet transform method to calculate unknown rate history from the down-hole transient pressure data. The Haar wavelet was chosen to process transient pressure data, where flow events can be identified with the amplitude of pressure transform. For the reservoir with constant reservoir-well parameters, the amplitude of pressure transforms is proportional to the change of rate. Based on this discovery, an algorithm for calculating unknown rate history from cumulative production has been developed. As this method is independent of the developed reservoir model, reservoir heterogeneity, skin, and wellbore storage have no effect on this method. For other complex reservoirs with variable reservoir-well parameters, such as gas reservoir and multiphase reservoir, this algorithm can also be applied resulting in small errors. In practice, the developed method can identify the offset of the flowing rate and the pressure, so synchronization of the rate and pressure can be achieved, which greatly increased the confidence of the pressure data analysis. Both theory and case studies were tested to demonstrate the robustness of this method in practice.