A New Analysis Method of DST Pressure History Using Flowrate Deconvolution In Laplace Space

Abstract Conventional analysis method of DST buildup period uses Agwarl's equivalent time handling variable rate. Assumption of the method is semi-log radial flow for homogenous reservoir. However for heterogeneous reservoir (double porosity, double permeability) the method will cause pressure derivative curve distortion and the pressure derivation curve cannot match type curve of conventional constant rate. This paper presents a new analysis method of DST pressure history using flow rate deconvolution in Laplace space. Through wellbore storage effect computing history rate and determined pressure response by applying Laplace deconvolution method, we can estimate parameters in Laplace space. Through simulated data and oil field data, the numerical results obtained are stable and not sensitive to noise. The established method provides a new way to understand DST pressure history in homogenous and heterogeneous reservoirs. Introduction DST is widely adopted due to its fast speed, attaining a lot of information and low cost. Particular interpretation methods of DST such as flow period analysis (Ramey2,3,4 slug test analysis, Peres's5 pressure integrated method), buildup period analysis (Peres's6 pressure deconvolution, modified Horner method) and pressure history analysis are special for homogenous formation. On the other hand, the analysis theory and interpretation methods of conventional drawdown and buildup test have become mature. If DST buildup period data are converted into equivalent conventional drawdown data, conventional test analysis theory and interpretation method can be used in DST. At the present time, using variable rate superposition (Agwarl's1 equivalent time) method can convert DST buildup data into equivalent conventional drawdown data. This method comes from variable rate superposition of semi-log radial flow behavior and it is only suitable for homogenous reservoir. According to simulated data analysis of DST, obtained data through variable rate superposition processing do not match type curve of conventional constant rate for naturally fractured reservoir. Therefore, this conventional method can not correctly estimate parameters. In this paper, according to DST full pressure history characteristic and throughout wellbore storage to calculate history rate, we can get pressure response of reservoir for constant rate and directly estimate parameters in Laplace space by numerical Laplace transformation. Application of simulated and oilfield data show that the new method is more accurate than Agwarl's method and can effectively determine estimated error of initial formation pressure. Effect of Variable Rate Superposition Time on Pressure Derivative Curve DST test simulator can be used to generate "flow and buildup", history pressure data to calculate flow period and buildup period rate. According to variable rate superposition time handling buildup period data, we can attain double-log diagnose curve and pressure derivative curve of standard drawdown response. Comparing the actual analysis curve with the standard curve, it is shown that the curve match is very good between the analysis curve of variable rate superposition time and the standard curve for infinite acting homogenous reservoir. But, if using average rate superposition time Δ te = tf × Δ t / (tf + Δ t), pressure derivative of analysis curve shows up in the late period and forms assumption phenomenon with sealing boundary.