Pressure detrending in harmonic pulse test interpretation: When, why and how

In reservoir engineering, one of the main sources of information for the characterization of reservoir and well parameters is well testing. An alternative to the standard drawdown/buildup test is Harmonic Pulse Testing (HPT) because it can provide well performance and reservoir behavior monitoring without having to interrupt field production, which is appealing from an economic standpoint. Recorded pressure analysis is performed in the frequency domain by adopting a derivative approach similar to conventional well testing. To this end, pressure and rate data must be decomposed into harmonic components. Test interpretability can be significantly improved if pressure data are detrended prior to interpretation, filtering out non periodic events such as discontinuous production from neighboring wells and flow regime variations that did not respect the designed test periodicity. Therefore, detrending offers the possibility of overcoming the limitation of HPT applicability due to the difficulty of imposing a regularly pulsing rate for the whole test duration (typically lasting several days). This makes HPT attractive for well performance monitoring, especially in gas reservoirs converted to underground gas storage. In this paper, different detrending methodologies are discussed and applied to synthetic and real data. Results show that, if a proper detrending strategy is adopted, information provided by HPT interpretation can be maximized and/or improved

Application of A* algorithm for tortuosity and effective porosity estimation of 2D rock images

Characterization and understanding of fluid flow phenomena in un-derground porous media at the micro and macro scales is fundamental in reser-voir engineering for the definition of the optimal reservoir exploitation strategy. Laboratory analyses on rock cores provide fundamental macroscale parameters such as porosity, absolute and relative permeability and capillary pressure curves. In turn, macroscale parameters as well as flow behavior, are strongly af-fected by the micro geometrical features of the rock, such as pore structure, tor-tuosity and pore size distribution. Therefore, a thorough comprehension of sin-gle and multiphase flow phenomena requires analyses, observations and charac-terization at the micro scale. In this paper we focus on the analysis of a 2D bina-ry image of a real rock thin section to characterize the pore network geometry and to estimate tortuosity, effective porosity and pore size distribution. To this end, a geometrical analysis of the pore structure, based on the identification and characterization of the set of the shortest geometrical pathways between inlets and outlets pairs, is implemented. The geometrical analysis is based on the A* path-finding algorithm derived from graph theory. The results provided by the geometrical analysis are validated against hydrodynamic numerical simulation via the Lattice Boltzmann Method (LBM), which is well suited for simulating fluid flow at the pore-scale in complex geometries. The selected rock for this analysis is Berea sandstone, which is recognized as a standard rock for various applications such as core analysis and flooding experiment. Results show that the path-finding approach provides reasonable and reliable estimates of tortuos-ity and can be successfully applied for analyzing the distribution of effective pore radius, as well as for estimating the effective porosity

Harmonic pulse testing for gas well deliverability assessment

Harmonic Pulse Testing was introduced in the early 1970’s as a special case of pulse testing. It is characterized by a periodic variation of production/injection rate. Subsequent developments proved that it could provide the same information as a conventional well test (permeability and skin, heterogeneity) in addition to those given by a pulse test (areal connectivity within the reservoir) if proper interpretation models were adopted. Consequently, it can be considered as a promising methodology to test a well during ongoing field operations without stopping production and thus it is very attractive for monitoring well performance, especially of gas storage wells. Initially applied to oil wells, Harmonic Pulse Testing has recently been extended to gas wells for which the assumption of Darcy flow regime is no longer valid because of inertial phenomena and/or turbulence. Harmonic Pulse Testing for gas wells comprises three or more consecutive sequences of pulses characterized by increasing average rate, similar to a Flow After Flow test. The interpretation of a single-well Harmonic Pulse test is based on the derivative approach in the frequency domain to obtain kh and the skin components (mechanical skin and D factor). The possibility of assessing well deliverability from a multi-sequence pulse test was analysed in the research work presented in this paper. Different Pulse test configurations were considered and compared with the well-established Flow After Flow test in terms of deliverability estimate. To this end synthetic well test data were generated and sensitivity to test design, well parameters and reservoir interference were carried out. Results show that multi-sequence pulse tests can be used to obtain the well deliverability of a gas well with the advantage that both the tested well and the neighboring wells needn’t be shut-in prior to or during the test

Prove di Pozzo non Convenzionali a Basso Impatto Ambientale

Le prove di produzione eseguite su pozzi a olio o a gas consentono di caratterizzare alcuni parametri chiave per la descrizione del comportamento fluido-dinamico di pozzo e di giacimento. Le prove di produzione di tipo convenzionale vengono eseguite erogando una o più portate costanti, registrate in superficie, e misurando la corrispondente evoluzione della pressione nel tempo a fondo pozzo. Durante la fase esplorativa e di delimitazione del giacimento, specialmente in ambienti off-shore, i fluidi prodotti durante la prova vengono generalmente bruciati in fiaccola. Nell’ottica di una maggiore tutela e sicurezza dell’ambiente, una delle alternative più promettenti è rappresentata dalle prove di iniezione che consistono nell’iniettare un fluido, liquido o gassoso, in giacimento e registrare la risposta dinamica del sistema. Tali prove consentono pertanto di evitare l’erogazione di fluidi in superficie e quindi l’emissione di gas esausti in atmosfera a seguito della combustione in fiaccola nonché qualsiasi rischio di sversamento nel caso di miscele di idrocarburi liquidi o di fuga nel caso di miscele di idrocarburi gassosi. Gli studi sinora condotti hanno riguardato sia gli aspetti teorici, soprattutto allo scopo di poter progettare e interpretare correttamente questo tipo di prove, sia l’analisi di alcune applicazioni reali ai fini di validare le potenzialità delle prove di iniezione come efficace alternativa alle prove di produzione tradizionali. Lo studio fa parte del programma di ricerca del polo SEADOG (Safety & Environmental Analysis Division for Oil & Gas), nato nell’ambito della collaborazione tra il Politecnico di Torino e la Divisione DGS del MISE sul tema della sicurezza off-shore

Improved lithology prediction in channelized reservoirs by integrating stratigraphic forward modelling: towards improved model calibration in a case study of the Holocene Rhine-Meuse fluvio-deltaic system.

Stratigraphic forward modelling (SFM) provide the means to produce geologically coherent and realistic models. In this paper, we demonstrate the possibility of matching lithological variability simulated with a basin-scale advection-diffusion SFM to a data-rich real-world setting, i.e. the Holocene Rhine-Meuse fluvio-deltaic system in the Netherlands. SFM model calibration to real-world data in general has proven non-trivial. This study focuses on a novel inversion process constrained by the top surface and the sand proportion observed at specific pseudo-wells in the study area. Goodness-of-fit expressed by a new fitness function, gives the error calculated as the average of two calibration constraints. Computational efficiency was increased significantly by implementing a new optimization process in two hierarchical steps: a) optimization in terms of sediment load and discharge, which are the most influential parameters having the largest uncertainty and b) optimization with respect to the remaining uncertain parameters, these being sediment transport parameters. The calibration process described allows for the most optimal combination of achieving acceptable levels of goodness-of-fit, feasible runtimes and multiple (non-unique) solutions to obtain synthetic stratigraphic output best matching real-world datasets. By removing model realizations which are geologically unrealistic, calibrated SFM models provide a multiscale stratigraphic framework for reconstructing static models of reservoirs which are consistent with the palaeogeographic layout, basin-fill history and external drivers (e.g. sea level, sediment supply). The static reservoir models that are matched with highest certainty therefore contain the highest geological realism and may be used to improve deep subsurface reservoir or aquifer property prediction. The new methodology was applied to the well-established Holocene Rhine-Meuse dataset which allows a rigorous testing of the optimization and the calibrated SFM allows investigation of controls of the Holocene development on the sedimentary system

Graphene-Based Membrane Technology: Reaching Out to the Oil and Gas Industry

This paper presents a critical review and the state of the art of graphene porous membranes, a brand-new technology and backdrop to discuss its potential application for efficient water desalination in low salinity water injection (LSWI). LSWI technology consists in injecting designed, adequately modified, filtered water to maximize oil production. To this end, desalination technologies already available can be further optimized, for example, via graphene membranes, to achieve greater efficiency in water-oil displacement. Theoretical and experimental applications of graphene porous membranes in water desalination have shown promising results over the last 5-6 years. Needless to say, improvements are still needed before graphene porous membranes become readily available. However, the present work simply sets out to demonstrate, at least in principle, the practical potential graphene membranes would have in hydrocarbon recovery processes

Underground Hydrogen Storage Safety: Experimental Study of Hydrogen Diffusion through Caprocks

Underground Hydrogen Storage (UHS) provides a large-scale and safe solution to balance the fluctuations in energy production from renewable sources and energy consumption but requires a proper and detailed characterization of the candidate reservoirs. The scope of this study was to estimate the hydrogen diffusion coefficient for real caprock samples from two natural gas storage reservoirs that are candidates for underground hydrogen storage. A significant number of adsorption/desorption tests were carried out using a Dynamic Gravimetric Vapor/Gas Sorption System. A total of 15 samples were tested at the reservoir temperature of 45 °C and using both hydrogen and methane. For each sample, two tests were performed with the same gas. Each test included four partial pressure steps of sorption alternated with desorption. After applying overshooting and buoyancy corrections, the data were then interpreted using the early time approximation of the solution to the diffusion equation. Each interpretable partial pressure step provided a value of the diffusion coefficient. In total, more than 90 estimations of the diffusion coefficient out of 120 partial pressure steps were available, allowing a thorough comparison between the diffusion of hydrogen and methane: hydrogen in the range of 1 × 10−10 m2/s to 6 × 10−8 m2/s and methane in the range of 9 × 10−10 m2/s to 2 × 10−8 m2/s. The diffusion coefficients measured on wet samples are 2 times lower compared to those measured on dry samples. Hysteresis in hydrogen adsorption/desorption was also observed

Unconventional well testing: a brief overview

The definition as well as the economic viability of the most suitable development strategy of a hydrocarbon reservoir mainly depend on the quantity and type of fluids, and on well productivity. Well testing, consisting in measuring the pressure variations induced in the reservoir during hydrocarbon production, has been used to assess fluid nature and well potential for decades, especially in exploration and appraisal scenarios. Today's industry drivers for formation evaluation methodologies demand safe, environmentally friendly, and cost effective test procedures due to more stringent environmental regulations aimed at reducing emissions of greenhouse gases and hence restricting flaring of hydrocarbons produced during a test as well as the need for reducing operating costs - even more so with current oil prices. Different methods have been proposed or resuscitated over the last years, such as wireline formation tests, closed chamber tests, production/reinjection tests, injection tests and harmonic pulse test, as viable alternatives to conventional well testing. In this paper an insight is provided on wireline formation tests, injection testing and harmonic pulse testing to illustrate how testing techniques are evolving in the oil industry. The first one is a sort of mini test of the formation. The second methodology is one of the most promising methodologies to replace traditional well testing in reservoir characterization, but it does not offer the possibility of sampling formation fluids and thus is complementary to the previous one. The third one gives the same information of a conventional test, except for initial pressure, without significantly altering field operations thus it is well suited for monitoring production and storage wells

Pressure Detrending for Harmonic Pulse Test Data Preprocessing

Harmonic Pulse testing was developed as a form of well testing that can be applied during ongoing production or injection operations. A pulsed signal is superimposed to the background pressure trend thus no interruption of well and reservoir production is required before and during the test. The pulsed pressure and rate signal analysis is performed in the frequency domain; to this end, the pressure and the rate signals need to be decomposed into harmonic components. The derivative of the harmonic components in the frequency domain can then be analyzed similarly to a conventional well test. In practice the interpretability of the derivative of the harmonic components can be significantly improved if the pressure trend to which the pressure pulses are superimposed is removed, i.e. a detrending of the pressure data is performed prior to well test interpretation. In the present paper, the results obtained after applying different detrending methodologies to pressure data recorded during pulse tests in different reservoir conditions are presented and discussed. Analyses on synthetic test data proved that polynomial detrending is effective in removing the pressure trend induced by field depletion and constant well interference but cannot deal with transient effect related to preexisting rate history or ongoing production changes. Conversely, some of the detrending algorithms based on a heuristic approach are very effective to remove both. Moreover, detrended data can be further regularized by excluding anomalous cycles from the analysis, i.e. cycles that do not respect the designed test periodicity, such as in the case of well interference and/or temporary interruption of the pressure pulses during the execution of the test. The adoption of an effective detrending strategy can considerably improve the quality of the pressure data obtained from harmonic pulse tests and thus the test interpretability. Therefore, it offers the possibility of overcoming the limitation of applicability due to the difficulty of imposing a regularly pulsing rate for the whole test duration (typically lasting several days). This makes harmonic pulse tests very attractive for well performance monitoring, as in gas storage fields

A New 3-D Numerical Model to Effectively Simulate Injection Test

Paper of SPE Europec/EAGE 200