Shear wave anisotropy has been widely used for characterizing the factures in a reservoir. Most of previous approaches to modelling dispersive effects on anisotropy assume the fractured medium being fully saturated. Here we address this issue by calculating elastic constants of a partially saturated rock with aligned fractures. The model is based on an anisotropic squirt theory with consideration of the influence from relative permeability and patchy saturation, which could lead to velocity dispersion occurring at a frequency lower than that in the single fluid case. Such partial saturation effects could significantly affect the variation of frequency-dependent shear wave splitting with water saturation.
Presentation Date: Monday, October 15, 2018
Start Time: 1:50:00 PM
Location: 202A (Anaheim Convention Center)
Presentation Type: Oral
Various models have been proposed to link partial gas saturation to seismic attenuation and dispersion, suggesting that the reflection coefficient should be frequency-dependent in many cases of practical importance. Previous approaches to studying this phenomenon have typically been limited to single interface models. Here we propose a modelling technique which allows us to incorporate frequency-dependent reflectivity into convolutional modelling. With this modelling framework, seismic data can be synthesized from well logs of velocity, density, porosity and water saturation. This forward modelling could act as a basis for inversion schemes aimed at recovering gas saturation variations with depth. We present a Bayesian inversion scheme for a simple 3-layer case and a particular rock physics model, and show that with appropriate prior geological information, the technique could potentially estimate gas saturation and layer thickness despite the occurrence of interfering reflections.
Presentation Date: Monday, October 17, 2016
Start Time: 3:45:00 PM
Presentation Type: ORAL
Commodity prices went through a dramatic shift from 2014 through the first quarter of 2016. In this paper, change in activity levels, stimulation techniques and the resulting impact on production throughout the Permian Basin are reviewed. This includes but is not limited to activity levels, drilled but uncompleted activity, fracture treatment fluid systems, proppant types, and proppant volumes.
The authors utilize public data sources to correlate activity level including drilled but uncompleted wells in the Permian basin to commodity price by location and target zone. Trends on fracture treatment style and downhole consumables are then correlated to activity and commodity price. The production analysis shows the impact of changing completion methodology on well performance and economics.
The number of wells completed in the Permian Basin has declined by 77% from October 2014 through January 2016. Completion trends during this time period include increasing proppant intensity along with a transition from Hybrid and Crosslink treatments to Slickwater, which grew from 36% to 59% of the completion market, while average proppant per lateral foot in the Permian Basin increased by 14% over the same time period.
The authors conclude that commodity price pressure did cause a slowdown in activity along with a transition in completion styles. However, during this period of time, short term production rates did not suffer as expected.
The activity, completion and production trends reviewed in this paper will be useful to engineers and managers for operator, well service provider and financial analysts planning and/or evaluating oil and gas opportunities in the Permian Basin at suppressed commodity prices.
Shale is strongly anisotropic in its elastic property characterization, and can usually be described by transverse isotropy with a vertical axis of symmetry (VTI). When fractures are present, shale is likely to exhibit orthorhombic symmetry. Understanding natural fractures in shale is a key step for seismic characterization of shale reservoir. Rüger equation builds up the relationship between seismic reflectivity and polar and azimuthal angles of seismic wave propagation (Rüger, 1998), which allows physical properties of fractures to be linked with the azimuthal variation of seismic reflectivity. The purpose of this study is to assess the applicability of this approach in fracture characterization to cases in which significant orthorhombic anisotropy is present since our work demonstrates that azimuthal variations of seismic reflectivity are not only controlled by fracture system but also by mineralogical composition when orthorhombic symmetry is present.
Fractures play an important role in the seismic characterization of shale reservoir. The use of seismic anisotropy for fracture detection has gained increasing acceptance in a variety of settings (Liu & Martinez, 2013). A simple approach to fracture detection involves the assumption firstly of transverse isotropy with a horizontal axis of symmetry (HTI) and secondly of “weak anisotropy” (Thomsen, 1986). Approximations to the reflection coefficients as a function of polar and azimuthal angles for this case, amongst others, were given by Rüger (1998). Application of the Rüger equation allows an “anisotropic gradient” to be mapped (Downton & Russel, 2011), and this attribute can be interpreted as an indicator of fracture density.
The aim of this paper is to assess the applicability of this approach to cases in which significant orthorhombic anisotropy is present. Our approach is rock physics model based, building on the work of Wu et al. (2012) who linked variations in VTI anisotropy to shale composition. Fractures are introduced to the model using the method of Schoenberg and Helbig (1997).
We compute synthetic reflectivity with the full orthorhombic model before inverting parameters from this data with the Rüger equation. We demonstrate a number of possible ambiguities in interpretation of the Rüger parameters; in particular the anisotropic gradient can be sensitive to composition as well as fracture properties. We believe that anisotropic rock physics modeling must have an important role to play in guiding interpretation in such cases.
In this paper, we study frequency-dependent PP and PS reflections from the interface of a two-layer model. The lower medium is considered to have frequency-dependent elastic properties. Numerical modeling has been carried out to understand how two reservoir properties - porosity and water saturation, affect the frequency-dependent behavior of PP and PS reflections. We find that porosity has an effect on the magnitude of PP and PS reflections, and also affects the frequency-dependence of reflections to a certain extent for full water saturation. Water saturation significantly influences the frequency-dependence of the PP reflection. For partial saturation where there is a phase reversal, the phase reverses gradually without the amplitude becoming zero, which is different from the elastic case. We also find that the frequency-dependent PS reflections are as inconspicuous as PP reflections for partial saturation, with relatively high frequency-dependence at large incident angle.
Mesoscopic fluid flow in porous media is considered to be an important mechanism that gives rise to seismic velocity dispersion and attenuation. Velocity dispersion leads to a frequency-dependent AVO response, which can be modeled by considering attenuation during seismic wave propagation. Chapman et al. (2006) studied fluid induced velocity dispersion and attenuation using frequency-dependent rock physics theory. Innanen (2012) derived frequency-dependent PP, PS and SS reflection coefficients by incorporating a nearly constant Q into the Zoeppritz equations. An additional cause of frequency-dependent reflections is the tuning effect from thin layers. Quintal and Schmalholz (2009) studied the combined effect of tuning and attenuation due to fluid flow, based on Biot’s theory.
Velocity dispersion can also cause an observable seismic wave signature in field data. Wu et al. (2015) displayed field data from the Vienna basin, in which strong attenuation due to wave-induced fluid flow generated both a frequency-dependent amplitude and gradual phase reversal effect. This new phenomenon was modeled with the Chapman et al. (2002) squirt model.
Amalokwu, Kelvin (National Oceanography Centre Southampton and University of Southampton) | Best, Angus I. (National Oceanography Centre Southampton) | Chapman, Mark (University of Edinburgh) | Minshull, Tim (University of Southampton) | Li, Xiang-Yang (British Geological Survey)
P-waves are known to be sensitive to the presence of partial gas saturation and to the presence of aligned fractures (anisotropy) in rocks. Studies combining the effect of multiphase saturation and aligned fractures are limited even though such conditions are common in the subsurface. Using octagonal-shaped synthetic sandstones with aligned penny-shaped fractures, we conducted laboratory ultrasonic experiments to investigate the combined effects of water saturation and aligned fractures on P-wave anisotropy. The fractured samples were designed to simulate the effect of fractures in the Earth according to theoretical models (Hudson style models). Our results show interesting sensitivity of P-wave anisotropy to the presence of fractures. At high water saturation values, the stiffening effect from the fluid is greater at 0° to the fracture normal than at 90° to the fracture normal, causing a decrease in the P-wave anisotropy parameter “ε”. Combining a frequency dependent fractured rock model and a frequency-dependent partial saturation model, we can qualitatively explain our experimental data.
Aligned fractures and partial gas saturation are common in hydrocarbon reservoirs, and both phenomena strongly affect seismic wave properties, the former causing seismic anisotropy. Fracture characterisation and saturating fluid discrimination are important goals for seismic exploration. Most commercial seismic surveys use P-waves to remotely characterise the subsurface. This has led to extensive studies and the wide use of P-waves for fracture characterisation (in the form of P-wave anisotropy) and for identification of partial gas saturation. P-wave anisotropy is used for fracture characterisation but is also known to be sensitive to the fracture fill (dry, water or oil) (Bakulin et al., 2000, Chapman et al., 2003). Therefore it is important to understand these effects for improved reservoir characterisation during exploration production and monitoring.
Partial gas saturation and aligned fractures in rocks are both known to cause dispersion as a result of wave-induced fluid flow (WIFF) (see Müller et al., 2010). Although still an actively developing field, both effects have been extensively studied separately. However, relatively little is known about the combined effect of multiphase saturation and aligned fractures on P-wave anisotropy. Saturation and fracture properties are usually inferred from seismic data using theoretical models and there is an interest in validating, calibrating or extending these theoretical models (Batzle et al., 2006).
Traditional proppants used in hydraulic fracturing have strongly water-wet surfaces, which retain water within the proppant pack and reduce the relative permeability to hydrocarbons. Oil-wet proppants have been evaluated, but pose significant operational challenges and have yielded poor recovery of treating fluids in the laboratory suggesting unsatisfactory hydrocarbon production if applied in the field. This paper describes the development and testing of a new proppant designed to exhibit a neutrally-wet surface. The modified surface does not have a preferential affinity for oil, gas or water, and therefore will not promote the preferential entrapment of any phase within the proppant pack. This concept yields significant advantages under multiphase flow conditions.
Laboratory results are presented showing dramatic improvement in the multiphase flow performance of the neutrally wet proppant. Pressure losses due to the simultaneous flow of water, gas and liquid hydrocarbons are significantly reduced leading to an improvement in relative permeability to hydrocarbon production. The surface treatment is resistant to abrasion, can withstand both acidic and basic environments, and can be used at reservoir temperatures up to 400° F. Testing and field application have also confirmed the compatibility with common fracturing fluid systems.
The neutral wettability proppant is expected improve the recovery of frac fluids, achieving longer effective fracture lengths through superior clean-up of the fracture. In addition to improved frac lengths, production rates are further increased by improved effective conductivity under realistic multiphase conditions. Field results will be presented which illustrate the positive results.
This proppant technology and the results described in this paper should be useful for completions, production and reservoir engineers dealing with hydraulically fractured wells, particularly in oil and condensate-rich reservoirs that are particularly challenged by multiphase flow. The implementation of this technology should improve completion effectiveness and well economics.
Amalokwu, Kelvin (National Oceanography Centre) | Best, Angus I. (National Oceanography Centre) | Sothcott, Jeremy (National Oceanography Centre) | Chapman, Mark (University of Edinburgh) | Minshull, Tim (University of Southampton) | Li, Xiang-Yang (British Geological Survey)
Water saturation effects on wave velocity and attenuation in porous rock with aligned fractures. Summary Elastic wave velocity and attenuation are known to be sensitive to the presence of aligned fractures (anisotropy) and to partial gas saturation but their combined effects are poorly understood. Using synthetic, silica-cemented, sandstones with aligned penny-shaped voids to simulate the effect of fractures in the Earth according to theoretical models, we conducted laboratory ultrasonic experiments to investigate the effects aligned fractures would have on velocity and attenuation under partial saturation conditions. Our results for the non-fractured rock agree with published data, but the fractured rock exhibits significantly different behavior, pertinent to fluid saturation estimation from remote seismic measurements in fractured rocks. Introduction Properties of crustal rocks are usually obtained remotely using seismic methods.
Summary Azimuthal amplitude-versus-offset (AVOZ) data can contain abundant information on fracture properties, lithology and fluid saturation, but procedures for interpretation in terms of such properties remain relatively poorly developed. This paper develops recently developed rock physics model based inverse schemes with a particular emphasis on accounting for the frequency-dependence of anisotropy, which is believed to be important in fractured reservoirs. Frequency-dependent anisotropy is expected to be influenced by fluid mobility, and determining this parameter from seismic data would be advantageous. The method is applied to a range of models, and it is shown that under relevant conditions it is theoretically possible to obtain fluid mobility information from AVOZ. Introduction Azimuthal variations of AVO data (AVOZ) have been widely used as a method to detect fractures for some time.
Guo, Zhiqi (Jilin University) | Zhang, Feng (China University of Petroleum) | Li, Xiangyang (China University of Petroleum) | Ren, Yingrui (University of Edinburgh) | Chapman, Mark (University of Edinburgh) | Shen, Ye (CNOOC Research Institute)
The presence of vertical aligned fractures in the Bakken Formation results in azimuthal seismic AVO responses. In this paper, we design a realistic geologic model of the fracture zone in the Bakken Formation, and investigate corresponding azimuthal seismic AVO responses. In the model, based on results from formation images, we assume that fractures and cracks cut through all seven major geologic units in the Bakken Formation with a specific ratio of fracture intensity about 3:3:2:1:2:1:1. Corresponding anisotropy parameters are calculated based on Hudson's theory. We then employ the reflectivity method to generate elastic seismograms. AVO curves picked for four interested geologic units indicate azimuthal variations in amplitudes especially at far offset. The decrease in amplitude at far offset corresponds to the increase in azimuth, and the AVO gradient also presents a linear variation with azimuth. However, because picking of the exact amplitudes is difficult due to interference and the presence of overlying fractures, it may be appropriate to treat the formation as an integrated part and investigate azimuth variations in RMS amplitudes. Comparison results indicate that the stacked RMS amplitudes act as a better indicator of the variations in crack density and fluid saturation than the corresponding AVO gradient.