Time-lapse Reservoir Monitoring Research Articles

New Concept Ocean-Bottom Multiphysics (OBMP) Nodes for Reservoir Monitoring

Marine-controlled source electromagnetics (CSEM) have been extensively applied to various exploration scenarios worldwide. However, its perceived value and cost relative to seismic and the scarcity of realistic case studies have limited the industry’s interest in time-lapse reservoir-monitoring (4D) applications. A feasible way to make demand for CSEM for 4D-monitoring programs would be to increase the value of information and reduce survey costs by performing joint operations where seismic and CSEM data are acquired during the same survey and at equivalent spatial densities. To this end, we propose a new multiphysics ocean-bottom nodes (OBN) concept and show the industry that CSEM can be a cost efficient and effective integrators to 4D seismic projects. To this end, we conducted a feasibility study demonstrating that horizontal magnetic field components have the required sensitivities and can be used instead of horizontal electric field components in mapping the 3D resistivity distribution and 4D fluid change responses in a given reservoir. This makes engineering a new OBN class simpler and cheaper, as various miniaturized magnetic field sensors are available off-the-shelf or readily working along with packaging and coupling solutions.

Time-lapse difference inversion based on the modified reflectivity method with differentiable hyper-Laplacian blocky constraint

Time-lapse (TL) seismic has great potential in monitoring and interpreting time-varying variations in reservoir fluid properties during hydrocarbon exploitation. Obtaining the disparities of reservoir elastic parameters by inversion is essential for TL reservoir monitoring. Conventional TL inversion is carried out by stages without coupling processing and leads to inaccurate results. We directly use the differences in seismic data responses from different vintages, namely, difference inversion, to improve the results’ credibility. It may reduce the deviations of the subtraction of base and monitor inversions in traditional methods. Moreover, most existing studies involving prestack inversion methods use the Zoeppritz equation or its approximants, which failed to consider the wave-propagation effects. We have developed a new TL difference inversion based on the modified reflectivity method (MRM), in which the internal multiples and transmission losses are taken into consideration to fine-tune the characterization of the seismic wave propagating underground. The new method is modified on the basis of the reflectivity method making it feasible in TL difference inversion, and it is derived from the Bayesian theorem. For further delineating the boundaries between layers, the differentiable hyper-Laplacian blocky constraint (DHLBC) is introduced into the prior information of a Bayesian framework, which heightens the sparseness in the vertical gradients of the inversion results and leads to sharp interlayer boundaries of the difference parameters. Synthetic and field data experiments demonstrate that our TL difference inversion method has clear advantages in accuracy and resolution compared with the Zoeppritz method and MRM without DHLBC.

Time-lapse image registration by high-resolution time-shift scan

Seismic image registration is crucial for the joint interpretation of multivintage seismic images in time-lapse reservoir monitoring. Time-shift analysis is a commonly used method to estimate the warping function by creating a time-shift map, where the energy of each time-shift point in the 3D map indicates the probability of a correct registration. We have adopted a new method to obtain a high-resolution time-shift analysis spectrum, which can help with manual and automatic picking. The time-shift scan map is obtained by trying different local shifts and calculating the local similarity attributes between the shifted and reference images. We adopt a high-resolution calculation of the time-shift scan map by applying the nonstationary model constraint in solving the local similarity attributes. The nonstationary model constraint ensures the time-shift scan map to be smooth in all physical dimensions, for example, time, local shift, and space. In addition, it permits variable smoothing strength across the whole volume, which enables the high resolution of the calculated time-shift scan map. We use an automatic-picking algorithm to demonstrate the accuracy of the high-resolution time-shift scan map and its positive influence on the time-lapse image registration. Synthetic (2D) and real (3D) time-lapse seismic images are used for demonstrating the better registration performance of the proposed method.

Feasibility of time-lapse gravity and gravity gradiometry monitoring for steam-assisted gravity drainage reservoirs

There is presently an increased need to monitor production efficiency as heavy oil reservoirs become more economically viable. We present a feasibility study of monitoring steam-assisted gravity drainage (SAGD) reservoirs using time-lapse gravimetry and gravity gradiometry. Even though time-lapse seismic has historically shown great success for SAGD monitoring, the gravimetry and gravity gradiometry methods offer a low-cost interseismic alternative that can complement the seismic method, increase the survey frequency, and decrease the cost of monitoring. In addition, both gravity-based methods are directly sensitive to the density changes that occur as a result of the replacement of heavy oil by steam. Advances in technologies have made both methods viable candidates for consideration in time-lapse reservoir monitoring, and we have numerically evaluated their potential application in monitoring SAGD production. The results indicate that SAGD production should produce a strong anomaly for both methods at typical SAGD reservoir depths. However, the level of detail for steam-chamber geometries and separations that can be recovered from the gravimetry and gravity gradiometry data is site dependent. Gravity gradiometry shows improved monitoring ability, such as better recovery of nonuniform steam movement due to reservoir heterogeneity, at shallower production reservoirs. Gravimetry has the ability to detect SAGD steam-chamber growth to greater depths than does gravity gradiometry, although with decreasing resolution of the expanding steam chambers.

Joint inversion approaches for geophysical electromagnetic and elastic full-waveform data

We present joint inversion approaches for integrating controlled source electromagnetic data and seismic full-waveform data for geophysical applications. The first approach is the joint petrophysical inversion carried out by reconstructing petrophysical parameters such as porosity and saturations instead of the usual geophysical parameters such as resistivity, seismic velocities and mass density. This approach utilizes the strong correlation between the electromagnetic and seismic parameters through the petrophysical relationships. Another approach that does not require the a priori petrophysical correlation is the joint structural inversion method. In this approach, the inversion is carried out by employing a regularization function for enforcing the structural similarity between the resistivity and the seismic velocities and the mass density. In this method, we employ the cross-gradient function, which has been shown on many occasions to be quite effective. By using a time-lapse reservoir monitoring example, we show that both joint inversion approaches produce results that are superior to those obtained by disjointed inversions. Hence, these joint inversions have great potential to be the next-generation tools for reservoir characterization and monitoring.

Exploiting the airwave for time-lapse reservoir monitoring with CSEM on land

In the application of controlled source electromagnetics for reservoir monitoring on land, repeatability errors in the source will mask the time-lapse signal due to hydrocarbon production when recording surface data close to the source. We demonstrate that at larger distances, the airwave will still provide sufficient illumination of the target. The primary airwave diffuses downward into the earth and then is scattered back to the surface. The time-lapse difference of its recorded signal reveals the outline on the surface of the resistivity changes in a hydrocarbon reservoir under production. However, repeatability errors in the primary airwave can destroy the signal-to-noise ratio of the time-lapse data. We present a simple and effective method to remove the primary airwave from the data, which we call partial airwave removal. For a homogeneous half space and a delta-function type of source, the surface expression of the airwave does not depend on frequency. For this reason, the primary airwave can be subtracted from the data using recordings at two frequencies, one low enough with a skin depth of the order of the reservoir depth that is sensitive to the reservoir, the other high enough to only sense the near surface. The method does not affect secondary airwave components created by signals that have propagated through the earth and returned to the surface. We show that the method provides a direct indicator of production-related time-lapse changes in the reservoir. We illustrate this for several models, including a general 3D heterogeneous model and one with strong surface topography, for situations where survey repeatability errors are large.

Description of and Results from a Novel Borehole Gravity Gradiometer

SummaryGravitec Downhole Instruments Ltd. is in the final stages of adapting its gravity gradiometer technology for deployment in the borehole environment for use in exploration as well as for time-lapse reservoir monitoring. The project, named Scorpius, is in cooperation with partners Kenda Capital and QinetiQ Ltd.Laboratory testing of the sensor has produced gravity gradient measurements. Work has also been done to forward model the effects of gravity gradient measurements in the borehole environment.

The effect of pore pressure depletion and injection cycles on ultrasonic velocity and quality factor in a quartz sandstone

Passing seismic waves generate transient pore-pressure changes that influence the velocity and attenuation characteristics of porous rocks. Compressional ultrasonic wave velocities [Formula: see text] and quality factors [Formula: see text] in a quartz sandstone were measured under cycled pore pressure and uniaxial strain conditions during a laboratory simulated injection and depletion process. The objectives were to study the influence of cyclical loading on the acoustic characteristics of a reservoir sandstone and to evaluate the potential to estimate pore-fluid pressure from acoustic measurements. The values of [Formula: see text] and [Formula: see text] were confirmed to increase with effective stress increase, but it was also observed that [Formula: see text] and [Formula: see text] increased with increasing pore pressure at constant effective stress. The effective stress coefficient [Formula: see text] was found to be less thanone and dependent on the pore pressure, confining stress, and load. At low pore pressures, [Formula: see text] approached one and reduced nonlinearly at high pore pressures. The change in [Formula: see text] and [Formula: see text] with respect to pore pressure was more pronounced at low versus high pore pressures. However, the [Formula: see text] variation with pore pressure followed a three-parameter exponential rise to a maximum limit whereas [Formula: see text] had no clear limit and followed a two-parameter exponential growth. Axial strain measurements during the pore-pressure depletion and injection cycles indicated progressive viscoelastic deformation in the rock. This resulted in an increased influence on [Formula: see text] and [Formula: see text] with increasing pore-pressure cycling. The value [Formula: see text] was more sensitive in responding to the loading cycle and changes in pore pressures than [Formula: see text]; thus, [Formula: see text] may be a better indicator for time-lapse reservoir monitoring than [Formula: see text]. However, under the experimental conditions, [Formula: see text] was unstable and difficult to measure at low effective stress.

4D PLANNING AND EXECUTION STRATEGIES FOR AUSTRALIAN RESERVOIR MONITORING PROJECTS

Time-lapse (4D) reservoir monitoring is in its infancy in Australia, but is on the verge of becoming a mainstream pursuit. We describe the 4D seismic acquisition and processing strategies that have been developed and proven elsewhere in the world, and customise those strategies for Australasian applications.We demonstrate how a multidisciplinary pursuit of real-time acquisition and processing Quality Control (QC) is an integral component of any 4D project. The acquisition and processing geophysicists must be able to understand all the factors contributing to the 4D seismic signal as they happen. Such an understanding can only arise through rigorous project QC and management using interactive visualisation technology. In turn, the production geologists and reservoir engineers will then receive 4D seismic products that can be robustly and confidently used for the construction of accurate reservoir models and the pursuit of reliable reservoir simulations and forecasts.

Physical model for the downhole orbital vibrator (DOV) - II. Rotary correlation wavelets and time-lapse seismics

SUMMARY The downhole orbital vibrator (DOV) acts as a rotating acoustic point force coupled to the borehole fluid. Operated as a Vibroseis source modulated over rotational frequencies 50- 350 Hz, the DOV can generate crosswell seismic signals at well separations up to 800 m in oilfield sediments. Source-wavelet stability and crosswell seismic traveltime resolution are estimated from an ensemble of 13 000 wavelets recorded at a crosswell seismic facility with 300 m source-sensor separation. DOV auto-correlation source wavelets Si cross-correlated against the ensemble mean wavelet Smn give correlation coefficients γ i = SiS mn having mean γ mn ≈ 99.97 per cent and standard deviation γ rms ≈ 0.03 per cent. S wavelets give observed traveltimes τ i with normalized standard deviation �τ rms/τ mn ≈ 0.03 per cent. This level of seismic monitoring source stability and crosswell traveltime resolution offers considerable promise for accurate, cost-effective time-lapse seismic imaging of active geofluid reservoirs. Application of stable DOV waveform production to time-lapse seismic imaging is, how- ever, affected by the dual-wavelet nature of rotary motion cross-correlations. In general, DOV cross-correlation signals mix time-symmetric (even) wavelets χ cc(t) ∼ cos(ω(t)t) � cos(ω(t)t) ∼ χ cc(−t) and time-antisymmetric (odd) wavelets χ cs(t) ∼ cos(ω(t)t) � sin(ω(t)t) ∼ − χ cs(−t), wheredenotes correlation, ω(t) describes the modulation of rotational frequency, and time-reversal t ↔− t equates to interchanging clockwise/counter-clockwise (cw/ccw )D OV rotations. Cross-correlating sensor motion along source-sensor axis x with source motion along axis g mixes wavelet symmetries as χ g(t) ∼ cosφχ cc(t) + sinφχ cs(t) where cos φ = g · x and angle φ orients the DOV relative to the source-sensor axis. DOV orientation uncertainty �φ affects the sensor wavelet apparent traveltime as �τ ≈ 1.3(�φ/2π )T min, T min the period of peak modulation frequency. Since source orientation un- certainty can generate apparent traveltime uncertainty as large as 1-2 ms, time-lapse seismics cannot effectively ignore DOV orientation. Traveltime resolution can be improved to order 0.1 ms without knowing DOV orientation if traveltimes are computed using even/odd wavelets composed by summing/differencing cw/ccw wavelets. Prospects for time-lapse resolution im- prove if observers can orient the DOV. A DOV equipped with a collar of rotation-phase point detectors surrounding the rotating mass permits observer selection of the monitor sensor, hence controlling effective source orientation. DOV orientation can be guided by an on-board biaxial tiltmeter (for borehole tilt δ along an axis making angle φ with DOV sensor orientation, biaxial tilts T 1 and T 2 fix φ as T 1 =− cos φtan δ and T 2 = sin φtan δ). Numerical simulation of full-sensitivity time-lapse reservoir monitoring suggests it is feasible to resolve migrating oil/water substitution volumes of characteristic dimension 20 m with crosswell transmission data over ≈600-800 m offsets, or in backscatter data at offsets ≈100-200 m using a DOV and sensor array operating in a single well.

Case histories of high-accuracy land gravity gradient measurements

The concept of using gravity gradient measurements for gaining knowledge of the subterranean zone is not new. Instruments have been built and field measurements have been conducted over the past 100 years. Improvements in both sensor performance and survey procedures have resulted in the present capability of making high accuracy (sub-Eotvos) measurements in real-world areas of interest. A summary of recent field activity using Lockheed Martin gradiometer systems will be presented in this paper. Recent improvements have resulted in a land-based gravity gradiometer system capable for use in a broad range of land applications, including void and tunnel detection, void characterization, and time-lapse reservoir monitoring.

Integrated time-lapse reservoir monitoring and characterization of the Cervia Field: a case study

Seismic time-lapse reservoir monitoring (4D) was used to assess the performance of a gas field (Cervia), located offshore Italy, in the central part ofthe gas-producing area of the Adriatic Sea. The chief objective of this work was the characterization of the field producing intervals by applying an innovative methodology based on a high level of integration of seismic data with borehole data. The work, carried out on two ‘legacy’ 3D seismic surveys and all available borehole data, consisted of three main steps. Characterization of the physical properties of the main reservoir, by extensive measurement and analysis of the core samples. This was in order to implement a petroacoustic model for calibration of seismic acoustic impedance volumes to reservoir description parameters. In addition all borehole data were employed during the implementation, calibration and testing of the petroacoustic model. Integrated time-lapse processing of the two seismic volumes to achieve high repeatability. Seismic processing parameters were determined independently and objectively for each of the two surveys using well data. Spectral equalization of wavelets in the seismic traces was performed to further improve repeatability. Integrated Multidimensional Petroacoustic Calibration (IMPAC) of the two seismic volumes. Here the microscopic properties of the reservoir rocks, obtained from the petroacoustic model outlined in step 1 above, were used, together with log data, to derive the main reservoir description parameters (i.e. lithology, porosity, saturation and pressure). This approach produced good repeatability between the two processed seismic volumes. In addition, time-lapse analysis of the acoustic impedance volumes, calibrated to reservoir description parameters, enabled the visualization of fluid movement within the main reservoirs. This, in turn, led to the identification of areas where the gas may only have been partially depleted. These significant results will provide an important contribution for an improved and more efficient gas production from the field.

Overview of global 4D seismic implementation strategy

For many Shell-operated fields around the world, time-lapse reservoir monitoring (4D) is now an integral part of field management and some 25 dedicated 4D surveys were acquired for this purpose by the end of 2002. This widespread application is the result of a focused implementation effort aimed at global deployment to maximize the value extracted from the surveys in terms of saved costs, increased production, increased recovery and improved HSE management, where effective implementation is achieved through a combination of global operatorship and technology capability. Apart from ensuring global deployment, there is the challenge to extend the range of 4D applicability. To achieve this the project has three application portfolios: today's‘ proven’ 4D technology portfolio, which is about monitoring fluid movements in thick clastic oil reservoirs offshore. Results come largely from comparing reservoir simulator output with difference maps derived from repeat 4D streamer surveys; a‘ stretch’ portfolio where the technology is applied to gas reservoirs, land data, stacked reservoirs, carbonate fields and to the monitoring of pressure changes; a‘ tomorrow's technology’ portfolio, which has the potential to increase the application base even further. The new technologies are about the use of permanent arrays, downhole acquisition, passive listening and ‘smart fields’ where semi-continuous 4D monitoring provides eyes and ears between the wells. More and more value is realized as 4D becomes fully integrated with subsurface work flows and modelling tools. Benefits from 4D technology for individual fields can be in the tens or hundreds of millions of dollars. A large fraction of these come from 4D surprises, illustrating that we tend to underestimate our uncertainties and suggesting different approaches to field management.

Spatially limited tomographic inversion for time-lapse oil reservoir monitoring

In this paper, we develop an improved technique for using seismictravel time tomography for time-lapse reservoir fluid monitoring. A model of a real reservoir is used to simulate the tomographic response to fluid injection for enhanced oil recovery (EOR).In order to evaluate the applicability of seismic tomography to fluid-injection EOR monitoring, a numerical experiment was carried out. As ordinary tomography inversion failed to obtain good results, an inversion technique was developed under the assumption that ‘a velocity change occurs mainly in the reservoir’ during the EOR process.A velocity estimation method for direct imaging of the injected fluid front was also developed. The results of this estimation improve the applicability of seismic tomography for monitoring of fluid-injection EOR.

Spatially limited tomographic inversion for time-lapse oil reservoir monitoring

SUMMARY In this paper, we develop an improved technique for using seismic travel time tomography for time-lapse reservoir fluid monitoring. A model of a real reservoir is used to simulate the tomographic response to fluid injection for enhanced oil recovery (EOR). In order to evaluate the applicability of seismic tomography to fluid-injection EOR monitoring, a numerical experiment was carried out. As ordinary tomography inversion failed to obtain good results, an inversion technique was developed under the assumption that ‘a velocity change occurs mainly in the reservoir’ during the EOR process. A velocity estimation method for direct imaging of the injected fluid front was also developed. The results of this estimation improve the applicability of seismic tomography for monitoring of fluid-injection EOR.

4-D time lapse reservoir monitoring of Nelson Field, Central North Sea: Successful use of an integrated rock physics model to predict and track reservoir production

Nelson Field, in the U.K. sector of the Central North Sea approximately 180 km east of Aberdeen in 280 ft of water, is on the Forties-Montrose ridge and forms a broad, low relief anticline with four-way dip closure (Figure 1). Figure 1. Nelson Field in relation to Forties-Montrose trend and established infrastructure. 4-D seismic data, acquired using conventional towed streamers, have been successfully applied to the Paleocene reservoirs of the North Sea. Importantly, AVO applications enhance the 4-D difference signal in this reservoir. We tested a new multivariate solution to predict shear-wave slowness against field data, and used this to establish an integrated rock physics model of the Forties Sandstone. This formed the basis for full elastic inversion of the AVO data sets. Full elastic inversion results demonstrate that quantifiable seismic differences can be detected and that they are of similar magnitude to those estimated independently from core and log data. Our study found that the predicted, simulated, drainage pattern and the actual time-lapse seismic difference signal are in good agreement. The 4-D is detecting OWC movements as small as 15–20 ft within the main sand channel fairways. 4-D seismic may provide a valuable tool to monitor reservoir drainage patterns for areas characterized by low net-to-gross sand ratios, but the method clearly works best within main sand fairways to target potential upside areas. The field was discovered in 1988. An extensive appraisal campaign by Enterprise and Shell/Esso followed. By 1990 13 appraisal wells had been drilled. The field began producing in February 1994 from eight wells. To date, 27 development wells have been drilled from a single central platform: 23 oil producers and 4 water injectors. In addition, four subsea producers exploit the southern satellite area. Plateau production was maintained during 1994–1996 at 150 000 b/d. Top Reservoir is relatively shallow at …

Integrating reservoir engineering and satellite remote sensing for (true) continuous time-lapse reservoir monitoring

The poem “The Blind Men and the Elephant” is based on an ancient Indian fable in which each man described rather differently what he sensed about an elephant. A similar situation can, and sometimes does, occur in reservoir description. Evidence shows that a reasonably thorough understanding of a reservoir is not very likely without viewing it from many angles and perspectives. This paper will show how two different perspectives of a reservoir—reservoir engineering and satellite remote sensing—can improve reservoir description and even allow (true) continuous time-lapse reservoir monitoring. The idea is simple. If subsidence occurs over an oil field, satellite remote sensing can detect the subsidence, and reservoir engineering can model the subsidence. By integrating reservoir engineering and satellite remote sensing, we can continuously monitor the hydrocarbon production. We use two examples, one from East Mesa Geothermal Field and one from Lost Hills Oil Field, to illustrate the idea. The principle of satellite synthetic aperture radar (SAR) is fairly straightforward. A microwave sensor, which can be on a spacecraft, passes a spot on the ground and takes a “shot” of it. After a certain period, the same sensor passes over the same spot and takes a second “shot” (Figure 1). By comparing the two shots with a reference elevation map, a detailed surface deformation map can be obtained for the period between the passes. Figure 1. Principles of SAR for measuring deformation (from http://www-star.stanford.edu/sar_group/). The image shows two passes of the same microwave sensor over the same location on earth. The difference between the two passes is determined by both earth topography and surface deformation. When data from a reference elevation map or a third pass is used for comparison, the topography effect can be eliminated. What remains is solely surface deformation. SAR can measure surface deformation at very high resolution (a …

Reliability analysis in dynamic systems: Implications for positioning marine seismic networks

Real‐time integrated monitoring of reliability (the impact of undetected biases in positioning observations on the position of geophysical elements) during marine seismic surveys is essential to ensure there is no significant loss in the resolution of the processed seismic image. The B-method is currently the most rigorous testing procedure for bias identification. It has been used extensively in geodetic networks, but to date it has found very little application within a dynamic environment. Here the method is expanded for use in dynamic systems—especially in marine seismic positioning networks such as those represented by a multisource, multistreamer, multivessel acquisition system. With this approach, we have found completely rigorous reliability estimates based on an integrated navigation method. We conclude that although the reliability of a system is in general a consequence of the redundancy, its geometry, and the stochastic model of the observations, these factors affect internal and external reliability in rather different ways. A key contribution is the demonstration that simple, easily understandable, and explainable statistics can be used to give a more complete description of quality than that obtained from precision measures alone. Assessing precision alone may give a false sense of security regarding the quality of the location of a seismic spread. The exploration and production industry is now making increasing use of old and new marine streamer 3-D surveys as part of 4-D time lapse reservoir monitoring projects. A more rigorous approach to positioning data quality, such as the method described in this paper, may play a material role in the avoidance of interpretable artifacts in these projects.

Time Lapse Reservoir Monitoring and Characterization - An Update: ABSTRACTS

Time Lapse Reservoir Monitoring and Characterization - An Update: ABSTRACTS