The importance of well integrity barriers evaluation in Middle East has increased dramatically in the recent years. The focus has increased to verify zonal isolation down-hole under extreme conditions. Evaluation must address a broad spectrum of downhole conditions that include various casings thicknesses, different mud weights; various types and wide variety of cement systems. The shallow aquifers corrosion problem in Middle East demands placement of a competent light or ultralight cement sheath across these aquifers having lost circulation zones. This is prime objective in order to have first line of defense against long line ecltrochmemical corrosion problem seen in all historical fields. During cementing of such surface casings; it was observed fluid column in the hole dynamically dropping or very fresh mud close to surface contained trapped air. Conventional logging techniques were not providing evaluation because acquisition was not possible under such hostile environments.
In more than 80% of cases conventional cement bond log found to have sensitivity to micro-annulus; and required pressurizing the casing. Therefore it was increased need to evaluate cement sheath behind micro-annulus. Zonal verification was also much needed in case of low acoustic impedance cement systems having contamination with mud or tail mixing with neat. Such light or ultralight cement has essentially same response as free pipe has conventional logs; therefore best evaluation was required to avoid unnecessary costly squeeze jobs or side track decisions. Moreover; a major drawback prevailed in obtaining confident answers in high doglegs and horizontal well section due to tool centering issues with conventional rotating heads. In view of that industry recognizes the need of wireline evaluation that will not require any expert input to obtain onsite deliverables in a reasonable turn out time while the rig is waiting on for decision. As a consequence, there is an increasing need for barriers evaluation with a single wireline technology suitable and effective to log in all such terrains.
This paper discusses the application of a new tool in addressing the above mentioned challenges. The new tool incorporates the use of electromagnetic acoustic transducers (EMAT) in Integrity eXplorer (INTeX) tool to generate guided acoustic waves in the casing and to measure them as they propagate along the casing circumference. The INTeX tool consists of an arrangement of coils and magnets in close proximity to a conductive casing. The casing then becomes an integral part of the transducer system. The acoustic excitation is achieved by driving currents through the coils, which creates eddy currents in the casing. These eddy currents, in the presence of a constant magnetic field, create the Lorenz forces that generate the acoustic waves. The EMATs are then used to measure the induced waves.
This system generates and measures the signals directly in the casing, eliminating any need for fluid coupling or physical contact and enabling operation in all fluid and gas environments. By varying the magnetic field and coil structure, different acoustic modes may be created and measured. The most valuable of the guided modes are the horizontal shear or SH waves, which cannot be generated by conventional compressional transducer systems. These waves propagate along the casing, with their particle displacement perpendicular to the wave propagation and parallel to the casing surface. SH waves respond directly to the shear modulus of the material that is directly coupled on the backside of the casing, enabling direct detection of a solid adhered to the casing. The Lamb/flexural modes are other guided waves that can be generated by the EMATs. These modes can be incorporated with the SH modes to detect a micro-annulus condition without the need for multiple passes and pressure applied to the casing. EMAT sensors are incorporated into a pad system in a coplanar configuration, enabling azimuthally sectored compensated attenuation measurements for the various wave types. In this paper, we look at the theoretical background physics, and field applications of INTeX tool.
Joint inversion of PP and PS reflection data has been hindered by the difficult task of registration or correlation of PP and PS events. It can perhaps be achieved by registering the events during inversion but the resulting algorithm is generally computationally intensive. In this paper, we propose a stochastic inversion of PP and PS data which have been registered to the same PP time scale using a new interval velocity analysis technique. The prestack PP and PS wave joint stochastic inversion is achieved by using the PP and PS wave angle gathers using a very fast simulated annealing (VFSA) algorithm. The objective function attempts to match both PP and PS data; the starting models are drawn from fractional Gaussian distribution constructed from interpolated well logs. The proposed method has been applied to synthetic and real data; the inverted results from synthetic data inversion compare very well with model data, and inverted results for real data inversion are consistent with seismic data and log data. These also show that the proposed method has a higher accuracy for estimating rock physics parameters while it circumvents the horizon registration problem in the data interpretation. We also estimate uncertainty in our estimated results from multiple VFSA derived models.
Inverting for the subsurface velocity distribution by refraction traveltime tomography is a well-accepted imaging method by both the exploration and earthquake seismology communities. A significant drawback, however, is that the recorded traces become noisier with increasing offset from the source position, and so prevents accurate picking of traveltimes in far-offset traces. To enhance the signal-to-noise ratio of the far-offset traces, we present the theory of super-virtual refraction interferometry where the signal-to-noise ratio (SNR) of far-offset head-wave arrivals can be theoretically increased by a factor proportional to
The main difficulty with iterative waveform inversion using a gradient optimization method is that it tends to get stuck in local minima associated within the waveform misfit function. This is because the waveform misfit function is highly nonlinear with respect to changes in the velocity model. To reduce this nonlinearity, we present a reflection traveltime tomography method based on the wave equation which enjoys a more quasi-linear relationship between the model and the data. A local crosscorrelation of the windowed downgoing direct wave and the upgoing reflection wave at the image point yields the lag time that maximizes the correlation. This lag time represents the reflection traveltime residual that is back-projected into the earth model to update the velocity in the same way as wave-equation transmission traveltime inversion. No travel-time picking is needed and no high-frequency approximation is assumed. The mathematical derivation and the numerical examples are presented to partly demonstrate its efficiency and robustness.
We apply seismic interferometry to traffic noise to retrieve both body waves and surface waves. Our preferred algorithm in the presence of highly variable and strong additive random noise uses cross-coherence, which uses normalization by the spectral amplitude of each of the traces, rather than cross-correlation or deconvolution. This normalization suppresses the influence of additive noise and overcomes problems resulting from amplitude variations among input traces. By using only the phase information and ignoring amplitude information, the method effectively removes the source signature from the extracted response and yields a stable structural reconstruction even in the presence of strong noise. This algorithm is particularly effective where the relative amplitude among the original traces is highly variable from trace to trace. We use the extracted reflected shear waves from the traffic noise data to construct a stacked and migrated image, and use the extracted surface waves (Love waves) to estimate the shear velocity as a function of depth. This shear-velocity profile agrees well with the interval velocity obtained from normal moveout of reflected shear waves constructed by seismic interferometry. These results are useful for a wide range of situations applicable in both geophysics and civil engineering.
In this paper we derive a stable second order wave equation for reverse time migration in arbitrarily heterogeneous 3D orthorhombic media (ORT) with vertical and tilted symmetry axis. The formulation is equivalent to the first order elastic wave equation, thus it is physically stable and is a natural extension of the wave equations in VTI and TTI media. We compute the reverse time migration impulse responses for tilted orthorhombic media and demonstrate that our method provides stable and high quality RTM images in more general anisotropy media.
The theory of super-virtual refraction interferometry was recently developed to enhance the signal-to-noise ratio (SNR) of far-offset traces in refraction surveys. This enhancement of SNR is proportional to
Frequency analysis of surface waves propagation is an efficient tool to retrieve the vertical shear-wave velocity profile. Surface waves result from the interaction of elastic body waves with a free surface. They are commonly generated by impulsive active sources but are also present in ambient vibration. Here we compare the results of interferometry estimation of Rayleigh-wave dispersion from ambient vibration sources by using both cross-correlation methods and conventional surface wave methods. The crucial step in both methods lies in the dispersion image computation that conditions the reliability of the dispersion curve, and therefore the inversion results. We have found that the dispersion characteristics of Rayleigh waves are better defined when the cross-correlation method is used. This better definition is probably due to the relative independence of the cross-correlation output from the source function.
The analysis of microtremors has been claimed to be useful as a possible direct hydrocarbon indicator, but so far only surface measurements were presented to support the claim. However, recent results were obtained while processing microseismic data from an oil-producing reservoir, which so far show no significant correlation in the average polarization between the surface and downhole data.
In this paper, a numerical study using the distinct element method was initiated to validate understanding the relationship between fracture damage, fluid pressure and seismic velocity changes in a naturally fractured reservoir. The fracture damage zone and fluid permeable zone were successfully correlated with the time-lapse velocity changes extracted from the numerical model. The model offers the unique ability to examine directly the microprocesses leading to observed velocity changes. Validated models could be extended to quantitatively calibrate velocities required for microseismic locations over time, and predict the fracture propagation and fluid migration within a field-scale engineered reservoir.