Glover, Paul W. J. (University of Leeds) | Lorinczi, Piroska (University of Leeds) | Al-Zainaldin, Saud (University of Leeds) | Al-Ramadhan, Hassan (University of Leeds) | Sinan, Saddam (University of Leeds) | Daniel, George (University of Leeds)
New reservoirs are increasingly more heterogeneous and more anisotropic. Unfortunately, conventional reservoir modelling has a resolution of only about 50 m, which means it cannot be used to model heterogeneous and anisotropic reservoirs effectively when such reservoirs exhibit significant inter-well variability at scales less than 50 m. This paper describes a new fractal approach to the modelling and simulation of heterogeneous and anisotropic reservoirs. This approach includes data at all scales such that it can represent the heterogeneity of the reservoir correctly at each scale.
Three-dimensional Advanced Fractal Reservoir Models (AFRMs) can be generated easily with the appropriate code. This paper will show: (i) how 3D AFRMs can be generated and normalised to represent key petrophysical parameters, (ii) how these models can be used to calculate permeability, synthetic poro-perm cross-plots, water saturation maps and relative permeability curves, (iii) the effect of altering controlled heterogeneity and anisotropy of generic models on fluid production parameters, and (iv) how AFRMs which have been conditioned to represent real reservoirs provide a much better simulated production parameters than the current best technology.
Results of generic modelling and simulation with AFRMs show how total hydrocarbon production, hydrocarbon production rate, water cut and the time to water breakthrough all depend strongly both on heterogeneity and anisotropy. The results also show that in heterogeneous reservoirs, the best production data is obtained from placing both injectors and producers in the most permeable areas of the reservoir – a result which is at variance with common practice. Modelling with different degrees and directions of anisotropy shows how critical hydrocarbon production data depends on the direction of the anisotropy, and how that changes over the lifetime of the reservoir.
We have developed a method of fractal interpolation to condition AFRMs to real reservoirs across a wide scale range. Comparison of the hydrocarbon production characteristics of such an approach to a conventional krigging shows a remarkable improvement in the modelling of hydrocarbon production when AFRMs are used; with AFRMs in moderate and high heterogeneity reservoirs returning values always within 5% of the reference case, while the conventional approach often resulted in systematic underestimations of production rate by over 70%.
The porosity and permeability of binary mixtures of spherical grains were modelled theoretically and studied empirically against such variables as grain-size, grain-size ratio, grain volume fraction and grain packing. The results confirmed that binary mixing of different-sized grains always results in a porosity loss. The degree of porosity loss was found to be a function of the grain-size ratio. Consequently, the mixture with the highest grain-size ratio of 3 dropped to the lowest minimum porosity of 0.3116 while the mixture with the minimum grain-size ratio of 1.5 experienced the highest minimum porosity of 0.3716. The observed porosities could not be described by some of the existing porosity models including the ideal and fractional packing models due to the assumptions of ideal packing and no-mixing respectively underlying these models. Thus, a corrected fine packing (or replacement) model wasdeveloped during this research to incorporate the grain-size ratio effect on porosity. Together with the interstitiation model, the corrected replacement model gave the best fit to the observed porosities. The mixtures’ permeabilities could not be modelled by the grain-size/porosity-dependent permeability models because these models tend to mimic the trend of the representative porosity used. The weighted geometric/harmonic mean permeability models (weighted by volume fraction) described the observed permeabilities best.
Presentation Date: Monday, October 15, 2018
Start Time: 1:50:00 PM
Location: 202A (Anaheim Convention Center)
Presentation Type: Oral
ABSTRACT: Rock fractures have a crucial role in geomechanics affecting rock behavior. Seismic waveforms carry information about the medium through which it has propagated. Extracting information from waveforms can lead to conclusions about the heterogeneity and anisotropy of the medium and an estimation of density and mechanical stiffness of fractures. The stress field in a fractured medium is also important as it controls the closure of the fracture and hence the fracture stiffness. Using seismic waveforms, we can make indirect conclusions about the stress state of the medium. Models and experiments in a medium with discrete parallel fractures have led to confidence that the models can accurately reproduce wave interaction with fractures. But what about waveforms for waves propagating through a more complex fracture network? In this work we use a DFN tool to create a fracture network and pass seismic waves through the medium recording velocity waveforms. From this we reach conclusions on how the fracture networks are affecting the seismic waveforms with implications for real-world problems. We extend this work to include the local stress field which alters fracture stiffness along fractures establishing how these changes affect seismic waveforms compared to the initial uniform stress model.
ABSTRACT: The majority of open-pit mineral workings are established in hydrogeological environments in which unsaturated drainage or saturated groundwater flow occurs predominantly via discrete fracture networks. Stress relaxation resulting from open-pit mineral extraction can lead to a change in host rock fracture network configuration and fracture hydraulic properties, with the potential to change local hydrogeological characteristics and groundwater flow regimes. Research being undertaken at the University of Leeds is applying a DFN approach to investigate the hydrogeological significance of such effects in relation to methodologies for impact assessment at mineral sites. The paper presents a summary of the research approach and preliminary results. A discrete finite element approach to geomechanical modelling has been undertaken with simulation of DFN evolution in response to lithostatic unloading for a range of pre-existing discontinuity configurations, lithological types and variations in in-situ stress regimes. Preliminary modelling results have provided improved understanding of the vertical and lateral extent of potential DFN response for a range of excavation profiles. Research results will be used to define conditions under which open-pit mineral extraction could lead to hydrogeologically significant change in fracture flow drainage characteristics at a scale relevant to hydrogeological impact assessment for new and existing mineral workings.
ABSTRACT: The growing global population is leading to reduced space and a need for more resources. This is causing engineered structures to be designed within rock masses at greater depths, and subjected to significant thermo-mechanical loading. Numerous hydro-thermo-mechanical in-situ experiments, including block tests and heated plate load tests have demonstrated the effects of temperature on discontinuity mechanics at a large scale. In this study we propose two methodologies for the multi-stage testing of discontinuity shear strength at incremental temperatures under triaxial conditions. The two methodologies result in different thermomechanical behavior of the specimens. If deformation of the specimen is constrained during heating, no change in residual shear strength of the discontinuity is seen, however, if the specimen is unloaded and free to deform under thermal loading, it displays reduced shear strength upon reloading. This preliminary data has potential implications for the design of engineered structures in these elevated thermo-mechanical environments.
Advancements in engineering capability have led to structures being designed within rock masses at greater depths, where they are expected to withstand not just greater stresses, but also elevated temperatures. The thermal loading of a rock mass can occur due to the geothermal gradient in the cases of deep tunneling, mining and geothermal heat production, or due to the heat generation from high-level radioactive waste in a geological disposal facility.
Rock masses are heterogeneous and discontinuous. Under applied stresses, the mechanical behavior and strength of a rock mass is commonly controlled by the behavior and strength of the discontinuities (Hoek, 1983). Discontinuities vary widely in terms of their origin (joints, bedding, foliation, faults, shear zones etc.) and associated physical characteristics. Characterizing their mechanical properties under different conditions is therefore paramount to understanding the behavior of a rock mass under these conditions. Numerous hydrothermo-mechanical in-situ experiments, including block tests and heated plate load tests have explored the effects of temperature on discontinuity mechanics at a large scale and shown modifications in the mechanical behavior of discontinuities at these elevated conditions (Cramer and Kim, 1986; Hardin et al., 1981; Zimmerman et al., 1985). However there have been no small scale studies to understand the mechanics of individual discontinuities under these loading conditions.
Keogh, William (University of Leeds) | Boakye, Gifty Oppong (University of Leeds) | Neville, Anne (University of Leeds) | Charpentier, Thibaut (University of Leeds) | Olsen, John Helge (Statoil ASA) | Eroini, Violette (Statoil ASA) | Nielsen, Frank MØller (Statoil ASA) | Ellingsen, Jon (ConocoPhillips) | Bache, Oeystein (ConocoPhillips) | Baraka-Lokmane, Salima (TOTAL) | Bourdelet, Etienne (TOTAL)
Deposition of inorganic mineral scale on downhole completion equipment contributes to significant downtime and loss of production within the oil and gas industry. High temperature/high pressure (HT/HP) fields have reported build-up of lead sulfide (PbS) scale as a consequence of reservoir souring; and the resultant reaction between dissociated sulfide anions from hydrogen sulfide (H2S) and heavy metal cations. In this work, laboratory apparatus enabled simulation of scale precipitation under turbulent emulsion-forming multiphase conditions, with behavior of PbS particles at the oil/water interface and subsequent adhesion onto anti-fouling surfaces measured at a range of polymer concentrations. Introduction of polymer sulfide inhibitor (PSI) product to the formation brine at concentrations of 500mg/L reduced overall PbS deposition whilst addition of 5000mg/L further reduced scale crystallisation but resulted in complete emulsification of the light oil phase. The tendency of soluble polymers to act as surfactants led to increased stabilisation of the formed oil in water (o/w) emulsion with heightened PSI concentration. Optical microscope, gravimetric and rheological measurements explained depositional behaviour; whereby enhanced o/w emulsion viscosity and stability due to amphiphilic polymer adsorption onto both PbS scale and oil droplet interfaces resulted in uniform deposition upon all surfaces.
Evolution of H2S gas within oil reservoirs can occur through both microbiological and geochemical means; a consequence of the activity of Sulfate Reducing Bacteria (SRB) and chemical reactions resulting from seawater injection, respectively 1. The dissociation of H2S to its constituent anions in water can be seen in equations 1 and 2, with Pb2+ cations within produced water reacting readily with sulfide based species to form PbS (galena), as seen in equation 3.
Flow assurance complications in oil and gas production associated with deposition of sulfide scales are becoming increasingly frequent in high temperature/high pressure (HT/HP) fields. This paper builds on previous work 2, 3, where adhesion of PbS nanoparticles onto surfaces was found to be the overwhelmingly dominant deposition mechanism; with fouling behavior contingent on formation of an oil in water (o/w) Pickering emulsion and wettability of the foreign surface in a multiphase system. As such, oil wetted hydrophobic fluoropolymer surfaces were found to significantly limit the adhesion of PbS scale 2. Whilst the scaling mitigation potential of both anti-fouling surfaces and inhibitors has been investigated extensively on an individual basis 4-7, this work is the first on their combined efficacy in sulfide forming multiphase processes. As operators become increasingly intent on applying anti-fouling coatings onto downhole equipment to prevent the deposition and build-up of scales, understanding the synergy (or lack thereof) between chemical and surface mitigation techniques is critical.
Many petrophysical properties of tight rocks, such as permeability and electrical resistivity, are very stress sensitive. However, most mercury-injection measurements are made using an instrument that does not apply a confining pressure to the samples. Here we further explore the implications of the use and analysis of data from mercury-injection porosimetry or mercury-injection capillary pressure measurements (MICP). Two particular aspects will be discussed. First, the effective stress acting on samples analyzed using standard MICP instruments is described. Second, results are presented from a new mercury-injection porosimeter that is capable of injecting mercury at up to 60,000 psi into 1- or 1.5-in. core plugs while keeping a constant net stress up to 15,000 psi. This new instrument allows monitoring of the electrical conductivity across the core during the test so that an accurate threshold pressure can be determined.
Although no external confining pressure is applied (unconfined) when using the standard MICP instrument, this doesn’t mean that the measurements can be considered as unstressed. Instead, the sample is under isostatic compression by the mercury until it enters the pore space of the sample. As an approximation, the stress that the mercury places on the sample is equal to its threshold pressure. Thus, the permeability calculated from standard MICP data is equivalent to that measured at its threshold pressure. Not all the samples have the same stress dependency, thus comparing measured permeabilities at a single stress with values calculated from standard MICP data, corresponding at different threshold pressures, can lead to erroneous correlations. Therefore, the estimation of permeabilities from standard MICP data can be flawed and uncertain unless the stress effect is included.
Results obtained from the new mercury-injection system porosimeter under net stress, are radically different from those obtained from standard MICP instruments, such as the Autopore IV. In particular, the measurements at reservoir conditions produce threshold pressures that are three times higher and pore-throat sizes that are one-third of those measured by the standard MICP instrument. The results clearly indicate that calculating capillary-height functions, sealing capacity etc. from the standard instrument can lead to large errors that can have significant impact on subsurface characterization.
In this work we describe a method for noise suppression that exploits the correlated nature of noise in time and space in attempt to transform the recorded noise into white, Gaussian noise. The application of this technique on microseismic data allows for the imaging of events at SNRs previously not possible. It is further shown to be highly robust when handling unexpected changes in the noise's behaviour. Introduction The separation of signal and noise has long been studied in seismology due to the detrimental effects of noise in seismic imaging and inversion (Wang et al.,2008; Maxwell, 2010; Forghani-Arani et al., 2012). This is of particular importance to surface microseismic monitoring where events are often below the noise level. Numerous studies have shown that noise is correlated in time and space (Normark, 2011, Schilke et al.,2014) as well as containing a mixture of stationary and non-stationary aspects (Birnie et al., 2015). An efficient noise suppression algorithm must take into account both the spatiotemporal and mixed-stationarity aspects of the noise.
ABSTRACT: We examine P-wave propagation in a fractured medium using effective medium, explicit fractures, and localized effective medium representations of fracturing, to quantify their effectiveness. We model a published experiment with multiple parallel fractures. Initial models assume uniform fracture stiffness across all fractures. A methodology is presented for inverting a source from the experiment. We find that the waveforms from the three models do not match each other. For propagation parallel to the fractures, the explicit model performs best with excellent agreement with the experimental waveform. The waveform from the localized TI model is reasonably similar, matching arrival, predominant period, frequency content, but not amplitude. The TI model is a poor match with significant differences in period, amplitude and high frequency content. For perpendicular propagation, none of the models properly match the experimental waveform. All models reproduce the significant delay in arrival, but only the explicit and local TI models produce a reduction in period and frequency content mimicking the experiment. All models produce a reduction in amplitude but not to the degree of the experiment. An explicit model accounting for the effect of the non-uniform stress-field better matches the experiment, indicating similar developments are needed for the other two representations.
Fractures in rocks are significant in a number of engineering applications. For example, an assessment of rock fractures is extremely important for characterizing the effectiveness of the geological barrier to a nuclear waste disposal repository, as they can provide fluid pathways, increasing the rock permeability. By examining the full waveform from either active or passive seismic sources we can obtain information on fracturing as seismic waves interact with the fractures (e.g., Schoenberg, 1980). Many studies (e.g., Crampin, 1981, Hudson, 1981, Majer, et al., 1988, Rathore, et al., 1995, Kawai, et al., 2006, Tillotson, et al., 2011, de Figueiredo, et al., 2013, Ding, et al., 2014, Tillotson, et al., 2014) have assumed that the response of a large number of fractures, can be mapped into the overall effective behavior of the medium, by linking the elastic constants with the fracture density and orientation, leading to anisotropy in the wave velocities (effective medium modelling). An alternative approach is discrete fracture representation using displacement discontinuities. The model was introduced for seismic wave propagation by Schoenberg (1980) and has been studied by numerous authors, (e.g., Pyrak-Nolte, et al., 1990, Nakagawa, et al., 2002, Hildyard, 2007, Chichinina, et al., 2009b, Perino, et al., 2010, Fan & Sun, 2015, Shao, et al., 2015). Pyrak-Nolte et al. 1990 examined the displacement discontinuity model for elastic wave propagation through multiple parallel fractures, in an experiment using laminated steel plates to simulate natural fractures. Hildyard (2007) then showed that an explicit representation of the fractures could match the experimental recorded waveforms but only if the effect of a non-uniform loading on fracture stiffness was included. In this research we try to model this experiment on laminated steel block (Pyrak-Nolte, et al., 1990), using the numerical modelling code WAVE3D (Hildyard, et al., 1995) to demonstrate the resulting changes to seismic waveforms for both effective and discrete fracture models, in order to establish under what conditions each representation is most appropriate and how close the models are to producing real effects on waveforms.
Woodman, J. (University of Leeds) | Murphy, W. (University of Leeds) | Thomas, M. E. (University of Leeds) | Ougier-Simonin, A. (British Geological Survey) | Reeves, H. (British Geological Survey) | Berry, T. W. (Arup, Lendal Arches)
ABSTRACT: The preparation of identical synthetic samples with the same morphological and mechanical characteristics allows for repeatable and reliable testing of discontinuities under varying conditions. Studies on the behavior of replica discontinuities to date have mostly been undertaken on laboratory shearbox apparatus, allowing for the characterization of discontinuity mechanics at low stresses. This study presents a new methodology for creating representative replica discontinuities suitable for testing under triaxial conditions, allowing for characterization at elevated stresses and temperatures. The advancement of computer aided design (CAD), three dimensional (3D) scanning and 3D printing has been used to design and create 3D printed molds in acrylonitrile butadiene styrene (ABS) plastic. Synthetic materials are then cast in to the molds creating cylindrical samples with a pre-existing discontinuity of quantifiable morphology suitable for triaxial testing. Statistical and visual analyses show the morphological characteristics of the synthetic discontinuities to be highly repeatable. In addition, the mechanical behavior of different synthetic compositions in unconfined compressive strength and triaxial is compared to the behavior of natural lower strength sedimentary lithologies. The behavior of the tested synthetic materials is found to not be representative of lower strength sedimentary lithologies, with stress-strain behavior showing failure in a quasi-brittle manner.
All rock masses contain discontinuities such as fractures, faults, joints, bedding planes and shear zones. Failure through intact rock is uncommon unless stresses are extremely high. The behavior and strength of the rock mass is instead commonly controlled by the behavior and strength of the discontinuities (Hoek, 1983). Triaxial testing of discontinuities was first undertaken by (Jaeger, 1959) and was used for testing discontinuity mechanics throughout the 1960s on synthetic replica discontinuities, artificially created discontinuities and real discontinuities (Raleigh and Paterson, 1965; Byerlee, 1967; Heuze and Goodman, 1967). Raleigh and Paterson (1965) also carried out some of the earliest work of testing discontinuities at elevated temperatures.