Good prediction of the performance of water-alternating-gas (WAG) processes relies on the proper estimation of three-phase relative permeabilities and their hysteresis with injection cycles. This is often a lengthy and expensive procedure requiring the numerical interpretation of several WAG corefloods, usually performed at reservoir conditions. In this paper, we propose an automated procedure based on multi-experiment optimization, leading to a consistent set of three-phase hysteresis parameters for all available experimental data. We apply it to two near-miscible WAG coreflood experiments that differ in the number and length of their injection cycles (long slugs versus short slugs). Both experiments were performed at reservoir conditions on a horizontal sandstone core, with light oil from a West African field and below minimum miscibility pressure. These experiments were carried out using extensive monitoring, including material balance at standard and reservoir conditions, full compositional analysis of liquids and gas produced at standard conditions, differential pressure measurements across the core and three-phase in situ saturations using a dual energy X-ray scanner. History-matching results were obtained by coupling our optimization tool with an in-house research reservoir simulator (IHRRS), combining an advanced EoS-based equilibrium relaxation model with a three-phase relative permeability hysteresis model. Optimization was performed with the Nelder-Mead algorithm, using a relatively large number (>10) of fitting parameters to model the relative permeability functions and their hysteresis. Multiple sets of parameters were easily obtained to match each experiment individually, suggesting that the history-matching of a single experiment is poorly constrained. As expected, muti-experiment optimization led to a better constrained but more challenging problem to solve; yet all available data could be matched reasonably well with a common set of parameters and several acceptable solutions were found. By providing a more robust estimation of three-phase hysteresis parameters, the proposed method increases the reliability of our simulation-based interpretations, necessary to evaluate the stakes of a WAG project.
The interpretation of multiple water-alternating-gas (WAG) experiments carried out with different injection strategies (first injected fluid, lengths of injected slugs) is necessary to improve our understanding of WAG mechanisms and to build a reasonably predictive three-phase relative permeability model for reservoir simulation. This paper presents an extension to the well-known three-phase hysteresis model of Larsen & Skauge and its application to interpret an extensive series of WAG experiments carried out under mixed-wet conditions, on the same low permeability sandstone core, at three different levels of gas-oil interfacial tension (IFT), namely 0.04 (low), 0.15 (intermediate) and 2.7 mN.m−1(high). After presenting the main experimental results, we describe an extended formulation – which was implemented in our in-house research reservoir simulator (IHRRS) – where the conventional Land formalism for the calculation of imbibition scanning curves and gas trapping is replaced by a more general model allowing arbitrary trapping functions and accounting for the nature of the displacing fluid (oil, water, or a mixture of both). This new approach – which is thought to be necessary for accurate modeling of WAG processes in mixed-wet systems and/or when gas-water and gas-oil IFTs differ significantly – is used within a history-matching workflow to generate multiple sets of fitting parameters consistent with the experimental data. For each IFT, an experimental dataset consisting of a gasflood and two long slugs (> 1 PV) WAG corefloods starting with either gas or water was history-matched. Each individual WAG coreflood was first interpreted together with the corresponding gasflood data, and then a simultaneous history-matching of all the coreflood data was attempted. An acceptable match of the experiments could only be obtained by restricting the common set of fitting parameters and by introducing a trapping model tuned to the experimental data and accounting for a different amount of trapped gas in the presence of an oil bank (observed in some experiments). This work offers new insights on the methodology to follow and the physical aspects to be improved, such as the gas imbibition scanning curves under three-phase flow conditions, in order to build a more reliable three-phase hysteresis model, capable of predicting multiple injection strategies and applicable to different wettabilities and gas-oil IFTs.
Prediction of miscible WAG performance relies on proper calibration of thermodynamical and petrophysical models. Swelling, miscibility and stripping phenomena must be captured in the equation of state (EOS) and the oscillations of gas and water saturations require using history-dependent relative permeabilities. This paper presents data synthesis and numerical interpretation of a complete experimental program including bulk PVT measurements, horizontal coreflood experiments at reservoir conditions and a spontaneous imbibition test. Prior to flooding experiments, CO2-oil phase behavior was characterized through swelling test, slim tube, multiple contact test (MCT) and supercritical fluid extraction (SFE) to provide a reliable equation of state. Coreflood experiments were conducted using carbonate outcrop cores, carbonated water, undersaturated live oil from a Middle East field, and multiple contact miscible CO2. Produced gas and oil were experimentally separated at reservoir conditions. X-ray monitoring was carried out. The experimental data were matched using our in-house research reservoir simulator (IHRRS) and an optimization module. We used the classical and an extended version of Larsen & Skauge relative permeabilities model, coupled to our calibrated equation of state (EOS) slightly corrected with alpha-factors to impose the residual oil saturation. Spatial variations of remaining oil were observed, with a minimum achievable oil saturation of 7 %. The condensate produced by stripping and the oil produced by mechanical sweep were matched separately. Two-dimensional simulations were performed to take into account gravity segregation, evidenced by in-situ saturations measurements. We found an acceptable history-match for each three-phase experiment individually. Significant relative permeability hysteresis was observed for both gas and water phases. Some of the three-phase parameters were found dependent on saturations path and not compatible for simultaneous history-match of multiple experiments. This paper provides a robust methodology for miscible CO2 WAG experimental data acquisition and history match. The hypotheses and limitations of the relative permeability model were stated: for instance, trapped water saturation may be introduced. A new hysteresis model was developed allowing for water trapping, which improved the quality of the history match of the experimental observations.
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