Top Of Reservoir Research Articles

Productivity Formulas for a Partially Penetrating Vertical Well in a Circular Cylinder Drainage Volume

Taking a partially penetrating vertical well as a uniform line sink in three‐dimensional space, by developing necessary mathematical analysis, this paper presents steady state productivity formulas for an off‐center partially penetrating vertical well in a circular cylinder drainage volume with constant pressure at outer boundary. This paper also gives formulas for calculating the pseudo‐skin factor due to partial penetration. If top and bottom reservoir boundaries are impermeable, the radius of the cylindrical system and off‐center distance appears in the productivity formulas. If the reservoir has a gas cap or bottom water, the effects of the radius and off‐center distance on productivity can be ignored. It is concluded that, for a partially penetrating vertical well, different productivity equations should be used under different reservoir boundary conditions.

Limitation of fault-sealing and its control on hydrocarbon accumulation—An example from the Laoyemiao Oilfield of the Nanpu Sag

Based on previous studies on the internal structures of fault belts, the fault belts in the Laoyemiao Oilfield of the Nanpu Sag can be divided into three units, a crushed zone, an upper induced fracture zone and a lower induced fracture zone according to the log response characteristics. The upper induced fracture zone is characterized by the development of pervasive fractures and has a poor sealing or non-sealing capability. It therefore can act as pathways for hydrocarbon migration. The lower induced fracture zone consists of fewer fractures and has limited sealing capability. The crushed zone has a good sealing capability comparable to mudstone and can thus prevent lateral migration of fluid. Through physical modeling and comparing laboratory data with calculated data of oil column heights of traps sealed by faults, it is concluded that the fault-sealing capability for oil and gas is limited. When the oil column height reaches a threshold, oil will spill over from the top of reservoir along the lower induced fracture zone under the action of buoyancy, and the size of reservoir will remain unchanged. Analysis of the formation mechanisms of the fault-sealed reservoirs in the Nanpu Sag indicated that the charging sequence of oil and gas in the reservoir was from lower formation to upper formation, with the fault playing an important role in oil and gas accumulation. The hydrocarbon potential in reverse fault-sealed traps is much better than that in the consequent fault-sealed traps. The reverse fault-sealed traps are favorable and preferred exploration targets.

Palaeo‐climate controlled diagenesis of the Westphalian C & D fluvial sandstones in the Campine Basin (north‐east Belgium)

AbstractThe Westphalian C and D fluvial sandstones in the Campine Basin (north‐east Belgium) are potential reservoirs for the sequestration of CO2 and interesting analogues of the hydrocarbon reservoirs in the Southern North Sea. Although these sandstones were deposited in a relatively short period of time, their reservoir properties and mineralogical compositions are very different. A petrographic study complemented with stable isotope analyses, fluid inclusion microthermometry and X‐ray diffraction analyses of the clay fractions of the sandstones, which were sampled from deep boreholes (>1000 m) in the Campine Basin, revealed that these differences are related mainly to the climate at the time of deposition. The most important eogenetic processes affecting the Westphalian sandstones were the generation of a pseudomatrix by physical compaction of Al‐silicates and lithic fragments that were strongly altered by extensive meteoric leaching, kaolinitization of unstable silicates and precipitation of siderite. These processes had a detrimental influence on the reservoir properties of Westphalian C sandstones, but their impact on the Westphalian D sandstones was minimal. The difference is assumed to be related to the climate at the time of deposition, which changed from tropical humid in the Westphalian C to semi‐arid/arid during the Late Westphalian D. Both the Westphalian C and D sandstones were affected by similar mesogenetic processes. Mesogenetic quartz cementation resulted from chemical compaction and illitization of kaolinite, K‐feldspar and smectitic clays. Illitization of kaolinite was controlled by the available quantities of co‐existing kaolinite and K‐feldspar and mainly affected the Westphalian D sandstones. Illitization of K‐feldspar was controlled by the K‐feldspar content. It had a much larger impact on the reservoir properties of the Westphalian D as, in these sandstones, K‐feldspar was less affected by eogenetic alteration. The illitization of smectitic clays resulted in illite, quartz and ankerite cementation in both reservoirs. This process had a more important impact on the Westphalian C reservoir, since cementation here also resulted from smectite to illite conversion in the interbedded and underlying shales. The effect of mesogenetic alterations on the reservoir properties was much less drastic than the impact of eodiagenesis. Mesogenetic alterations do exert a significant control on the properties of the Westphalian D. The vast impact of eodiagenesis on the Westphalian C sandstones made them less susceptible to mesogenetic alteration. The effect of telogenetic processes on the porosity and permeability of the Westphalian sandstones was small and restricted to the top reservoir intervals that directly underlie the Cimmerian Unconformity. No significant telogenetic alterations related to the Variscan Unconformity were observed.

Oil low to a lateral well in the presence of bottom water

Exact solutions are obtained for a number of two-dimensional problems of steady-state fluid flow to a lateral hole in a reservoir with a quiescent bottom fluid of higher density or with a fluid of lower density at the reservoir top

Widespread strombolian eruptions of mid-ocean ridge basalt

Glassy lava fragments were collected in pushcores or using a small suction-sampler from over 450 sites along the Juan de Fuca Ridge, Blanco Transform Fault, Gorda Ridge, northern East Pacific Rise, southern East Pacific Rise, Fiji back-arc basin, and near-ridge seamounts in the Vance, President Jackson, Taney, and a seamount off southern California. The samples consist of angular glass fragments, limu o Pele, Pele's hair, and other fluidal fragments formed during pyroclastic eruptions. Since many of the sites are deeper than the critical point of seawater, fragmentation cannot be hydrovolcanic and caused by expansion of seawater to steam. The glass fragments have a wide range of MORB compositions, ranging from fractionated to primitive and from depleted to enriched. Enriched magmas, which have higher volatile contents, may form more abundant pyroclasts than depleted magmas. Eruptions with high effusion rates produce sheet flows and abundant pyroclasts whereas those with low effusion rates produce pillow ridges and few pyroclasts. This relation suggests that high effusion and conduit rise rates are coupled to high magmatic gas contents. The eruptions are mainly effusive with a minor strombolian bubble burst component. We propose that the gas phase is an added component of variable amounts of magmatic foam from the top of the magma reservoir. As the mixture of resident magma and foam rises in the conduit, the larger bubbles in the foam rise more quickly and sweep up the smaller bubbles nucleating and growing from the resident magma. On eruption, the process of bubble coalescence is more complete for the slower rising, gas-poor lavas that erupt as pillow lavas whereas the limu o Pele associated with sheet flow eruptions commonly contain several percent vesicles that avoided coalescence during ascent. The spatter erupted at the vent is quench granulated in seawater above the vent, reducing the pyroclast grainsize. The granulated spatter and limu o Pele fragments are then entrained in a rising plume of seawater heated by the eruption, which disperses them to distances as great as 5 km from the vent.

Value of information of seismic amplitude and CSEM resistivity

We propose a method for computing the value of information in petroleum exploration, a field in which decisions regarding seismic or electromagnetic data acquisition and processing are critical. We estimate the monetary value, in a certain context, of a seismic amplitude or electromagnetic-resistivity data set before purchasing the data. The method is novel in the way we incorporate spatial dependence to solve large-scale, real-world problems by integrating the decision-theoretical concept of value of information with rock physics and statistics. The method is based on a statistical model for saturation and porosity on a lattice along the top reservoir. Our model treats these variables as spatially correlated. The porosity and saturation are tied to the seismic and electromagnetic data via nonlinear rock-physics relations. We efficiently approximate the posterior distribution for the reservoir variables in a Bayesian model by fitting a Gaussian at the posterior mode for transformed versions of saturation and porosity. The value of information is estimated based on the prior and posterior distributions, the possible revenues from the reservoir, and the cost of drilling wells. We illustrate the method with three examples.

Changes in seal capacity of fractured claystone caprocks induced by dissolved and gaseous CO2 seepage

Claystone caprocks are often the ultimate seal for CO2 underground storage when residual CO2 gas reaches the reservoir top due to buoyancy. Permeability changes of a fractured claystone due to seepage of CO2‐enriched brine and water vapor‐saturated CO2 gas are investigated. Results show that brine flow induces a large porosity increase (up to 50%) in the vicinity of the fracture due to dissolution of calcite and quartz, while permeability remains unchanged. Conversely, cyclic flows of CO2‐brine and CO2‐gas increase the fracture aperture abruptly after each gas flow period, producing a progressive decrease of the caprock seal capacity. Aperture increase is controlled by decohesion of the clay framework within a micrometer‐scale‐thick layer induced by CO2‐gas acidification. Results show that hydraulic aperture increases linearly with duration of the preceding CO2‐brine flow period, emphasizing the kinetic control of the quartz grains dissolution during the brine flow periods.

A Theoretical and Numerical Investigation of the Pseudoskin Factor

Abstract Partial completion of wells to inhibit gas and/or water coning in producing wells has been a common practice in the petroleum. industry for many years. This procedure, however, causes an additional pressure drop or pseudoskin that reduces the well productivity. In formations that are producing with partial completion, the precise quantification of this additional pressure drop is of paramount importance for completely identifying the existence. of formation damage. Many authors have proposed mathematical solutions that can be used to estimate the pseudoskin factor caused by restricted entry. In this work, a comprehensive investigation of the available models listed in the literature used for the estimation of pseudoskin due to restricted entry was performed. Selected models were implemented in a methodology and computer program. Then, a sensitivity analysis focusing on producing wells was carried out. The effect of the height of the producing interval, height rom the. middle portion of the producing interval to the bottom of the reservoir, reservoir thickness, horizontal and vertical permeabilities and wellbore radius were investigated. Furthermore, the extension of the methodology for the determination of mechanical skin in injectivity and fall-off tests executed in injector wells is described. At the end of this paper, an example highlights the range of applicability and potential benefits of the proposed methodology. Introduction It is very frequent that wells are completed over only a fraction of their productive zone in order to delay water and/or gas coning. A well completed in this manner is referred to as a partially penetrated or restricted-entry well. Two questions of practical importance in this scenario are:what is the productivity loss caused by the partial penetration; and,should the well be stimulated? The answer to these questions requires an estimation of the pseudoskin factor caused by the partial penetration and an estimation of the mechanical (real) skin caused by formation damage or stimulation. In this work, a comprehensive investigation of the available models listed in the literature used for the estimation of pseudoskin due to restricted entry was performed. A methodology and computer code was established that, if given the relative position of the perforated interval with respect to the producing zone and the values of vertical and horizontal permeability, the pseudoskin (dimensionless pressure drop) due to the restricted entry can be computed. Five selected formulas were studied, including Streltsova-Adams(1), Kuchuk and Kirwan(2), Papatzacos(3), Yeh and Reynolds(4) and Odeh(5). The assumptions, simplifications and range of validity associated with each formula are presented. All the pseudoskin formulas used in this work assume the single phase of a fluid with small compressibility and constant viscosity. The reservoir is considered homogeneous and isotropic, with constant thickness and infinite extension. The reservoir top and base are considered impermeable. The gravitational effects are considered negligible. Considering all these assumptions, the total skin obtained from a well test interpretation is given by: Equation 1 Available in Full Paper

Up-regulation of Nicotinic Receptors by Nicotine Varies with Receptor Subtype

Recent evidence suggests that in addition to alpha4beta2 and alpha3-containing nicotinic receptors, alpha6-containing receptors are present in midbrain dopaminergic neurons and involved in the nicotine reward pathway. Using heterologous expression, we found that alpha6beta2, like alpha3beta2 and alpha4beta2 receptors, formed high affinity epibatidine binding complexes that are pentameric, trafficked to the cell surface, and produced acetylcholine-evoked currents. Chronic nicotine exposure up-regulated alpha6beta2 receptors with differences in up-regulation time course and concentration dependence compared with alpha4beta2 receptors, the predominant high affinity nicotine binding site in brain. The alpha6beta2 receptor up-regulation required higher nicotine concentrations than for alpha4beta2 but lower than for alpha3beta2 receptors. The alpha6beta2 up-regulation occurred 10-fold faster than for alpha4beta2 and slightly faster than for alpha3beta2. Our data suggest that nicotinic receptor up-regulation is subtype-specific such that alpha6-containing receptors up-regulate in response to transient, high nicotine exposures, whereas sustained, low nicotine exposures up-regulate alpha4beta2 receptors.

Revealing the reservoir: Integrating seismic survey design, acquisition, processing and inversion to optimize reservoir characterisation.

SummaryThe escalating cost of drilling wells has meant that first class seismic data quality is essential for delineating reservoirs and identifying new well locations in complex geology. This paper describes an integrated workflow which insured that a new 3D seismic dataset acquired off the North West coast of Australia was designed, acquired, processed and simultaneously inverted to produce enhanced sub-surface image quality over a producing field.A desire for enhanced production required the geological understanding of the oil field to be improved. The poor quality of existing seismic data made it impossible to accurately identify the top and base of the prime volume carrying reservoir. A new seismic survey was designed with the specific requirement of improving the interpretability of the reservoir away from the wells. Both legacy seismic data and well data were used to aid in the design of the optimal seismic acquisition parameters. This along with single sensor technology produced excellent field data which were further enhanced by a high-end processing flow and inversion. In particular, advanced noise and multiple attenuation techniques have revealed the top reservoir and other previously unseen geological structures. Simultaneous AVO inversion was used to produce rock property volumes of Acoustic Impedance and Poisson’s Ratio which demonstrated an extremely high correlation with the well logs further proving the quality of the seismic.

Closure Confidence: How Big is That Field? A Case Study

SummaryIn many cases, the areal extent of closure is one of the biggest uncertainties in determining the in-place hydrocarbon volumes of a field. In this paper, I use the technique of cross-validation to rank depth maps over one particular field. The depth maps are generated by massive numbers of samples from a wide range of possible velocity models. Cross-validation drops out each well in turn, and predicts the top reservoir depth from the missing wells. The RMS prediction error is a measure of the quality of the velocity model. The best 100 velocity models (those with the lowest RMS) give 100 best depth maps. I assume that the best depth maps are equally probable because each has a low crossvalidation RMS value. An automatic spillpoint-finding algorithm is then used to find the area of closure in each of the best maps. Based on these maps, statistics are computed about the likely size of a field, and the uncertainty thereof. I conclude that there is a chance that the field size is significantly different to that given by the initial deterministic “best technical” depth maps.

Continuous active-source seismic monitoring of C O2 injection in a brine aquifer

Continuous crosswell seismic monitoring of a small-scale [Formula: see text] injection was accomplished with the development of a novel tubing-deployed piezoelectric borehole source. This piezotube source was deployed on the [Formula: see text] injection tubing, near the top of the saline aquifer reservoir at [Formula: see text] depth, and allowed acquisition of crosswell recordings at [Formula: see text] intervals during the multiday injection. The change in traveltime recorded at various depths in a nearby observation well allowed hour-by-hour monitoring of the growing [Formula: see text] plume via the induced seismic velocity change. Traveltime changes of [Formula: see text] (up to 8%) were observed, with no change seen at control sensors placed above the reservoir. The traveltime measurements indicate that the [Formula: see text] plume reached the top of the reservoir sand before reaching the observation well, where regular fluid sampling was occuring during the injection, thus providing information about the in situ buoyancy of [Formula: see text].

Monitoring overburden layer changes and fault movements from time-lapse seismic data on the Valhall Field

SUMMARY A method developed by Roste et al., which discriminates between layer thickness and velocity changes, is tested on pre-stack time-lapse seismic ocean bottom cable (OBC) data from the Valhall Field. A key parameter in this discrimination process is the dilation factor, α, which is the relative velocity change divided by the relative thickness change within a given layer. The high quality and good repeatability of the OBC data enables us to estimate α with a reasonable accuracy for α-values between 0 and −5. For α-values below −5, complementary information, like for instance geomechanics is required. For the top reservoir horizon we estimate a maximum subsidence of 0.50 m ± 0.29 m and a corresponding velocity decrease for the sequence from the seabed to the top reservoir of 2.0 m s−1± 0.40 m s−1. Time-lapse distortion patterns with characteristic time-shift versus offset signatures are observed. The positions and vertical extents of the distortion zones are determined from ray path considerations and modelling. The distortion zones correlate with buried faults, indicating that a (time-lapse) distortion zone might be produced by a localized slip in a fault zone. We present an extended method which allows for vertical (in addition to lateral) variations in the relative thickness and velocity changes. This method can be viewed as a simplified version of time-lapse tomography, but involving fewer unknown parameters, giving more stability to the estimated changes in thickness and velocity. Using this technique, we are able to estimate α for positions with localized time-lapse distortions.

Side lobes of wavelets impact identification of thin sand bodies

Recently, we found that side lobes of wavelets have a large impact on the identification of thin sand reservoirs when studying some gas fields in a basin in Northwest China. Reflections from the top of the H Formation, in which there are gas-bearing thin sand bodies, have the main wavelet lobe between two weak peak side lobes. The lower one always mixes with another peak reflected from the top of a thin sand reservoir. That makes it difficult to identify the sand reservoir. In order to solve this, many forward models were set up using typical well logs. 2D synthetic profiles were produced using Ricker wavelets to study the relationships between the effects of wavelet side lobes and thin sand position and frequency and between amplitude and the thin sand body. We developed the following conclusions: First, it is easier to identify thin sands in a shallower position. Second, a good way to tell sand body reflections from side lobes is by comparing profiles with different frequency windows. Third, it is helpful and effective to describe sand extent using amplitude attributes.

Productivity equations for an off-center partially penetrating vertical well in an anisotropic reservoir

This paper presents steady state and pseudo-steady state productivity equations for an off-center partially penetrating vertical well in an anisotropic reservoir, using a uniform line sink model. This paper also gives equations for calculating pseudo skin factor due to partial penetration. Compared with the equations for partially penetrating wells in the literature, the new equations, which are based on three dimensional models, are simple and accurate. If the well is in steady state with a circular cylinder drainage volume, and if the top and bottom reservoir boundaries are impermeable, the radius of the cylindrical system and off-center distance appear in the productivity equations. If the reservoir has a gas cap or bottom water, the effects of the radius and off-center distance on productivity can be ignored. A new equation is provided to calculate the productivity of a partially penetrating vertical well arbitrarily located in the anisotropic reservoir with a box-shaped drainage volume under pseudo-steady state. A convenient expression to calculate the Dietz shape factor is also presented. It is concluded that, for a partially penetrating vertical well, different productivity equations should be used under different reservoir boundary conditions.

«Alle schrijnwerkers van dit land zouden aan deze eischen niet kunnen voldoen»

On 8 April 1933, Professor Henry van de Velde was officially commissioned to design the new library of the Rijksuniversiteit Ghent, together with buildings for the Higher Institute for Art History and Classical Studies (HIKO), and the Veterinary and Pharmaceutical Institute, the latter of which was quickly replaced by an Institute for Minerology and Geology. Only the library and the HIKO were actually built on the Blandijnberg – the highest point in the city – on the location of the De Vreesebeluik, once a model neighbourhood for workers. Van de Velde’s design for the Faculty of Letters and Philosophy was also never realised. Henry van de Velde designed a high-rise functioning as a book depot, 64 meters high, his ‘fourth’ tower of Ghent, and a symbol of knowledge. The concrete construction was developed together with his colleagues Gustave Magnel and Jean-Norbert Cloquet. Van de Velde took great care with the belvedere, the highest storey, which was first conceived as a museum of books, but later became a sober reception room for important guests. He himself selected several large interior-decorating firms to carry out the work. Malcorps from Brussels did the interior: the finishing with black marble, ceruse oak and wenge, a plastered ceiling, a parquet floor, the metal radiator covers painted black. At the end of 1939, the interior was almost complete. During the Second World War, the tower was occupied by the German soldiers. They built a look-out at the top of the water reservoir. Later came an anti-aircraft post, which was blown up on the Germans’ retreat, resulting in damage to the reservoir. The belvedere, only completed in 1950, gradually deteriorated because of the damage. The construction of the engine room for the new lift in 1967 destroyed the harmony of the space. In 1971, the belvedere underwent not a restoration but a ‘renovation’, during which Henry Van de Velde’s designs for the ceiling and the floor were not respected. This unfortunate intervention suffered rapid deterioration due to poor maintenance. Other parts of the complex were also poorly maintained, adapted and ‘renovated’. In 2003, under the impulse of PROJECT2, the urgent need for a complete restoration of the whole building complex was recognised. On 16 September 2005, the board of the university took the crucial decision to borrow 30 million euros for the restoration and renovation. Within a few years, the Boekentoren can once again become a symbol of the Ghent University

A new model for assessing trap integrity and oil preservation risks associated with postrift fault reactivation in the Timor Sea

Based on comparisons between structural histories and the distribution of current and paleo-oil accumulations, it is proposed that the partitioning of postrift strain between faults in relation to trap geometry was critical in determining oil preservation during Neogene fault reactivation in the Timor Sea. Most of the trap-bounding faults in the region have been reactivated; however, the distribution of postrift displacements is heterogeneous and depends heavily on rift-phase fault size, location, and interaction with nearby faults. Preferential localization of postrift strain onto larger faults in the population resulted in the partial protection of some fault-bound traps with favorable geometries, but promoted breaching of others. Oil columns tend to be preserved where the crest of the trap is bound by a fault segment that has accommodated relatively low postrift displacements (less than about 60 m [196 ft]) during reactivation, typically where smaller rift faults are overlapped by larger rift faults. Complete loss of oil column is generally observed where the crest of the trap is bound by a typically large fault with high postrift displacements (greater than about 60 m [196 ft]). Where faults with high postrift displacements are located downdip of the trap crest, hydrocarbon columns are preserved down to the depth of the intersection between this fault and the top reservoir horizon. A simple trap integrity model based on these observations was found to be largely consistent with a database of 69 drilled traps in the region. The mechanisms and models discussed in this study are likely to apply to other petroleum systems where fault reactivation represents a risk to hydrocarbon preservation.

Estimation of layer thickness and velocity changes using 4D prestack seismic data

In some hydrocarbon reservoirs, severe compaction of the reservoir rocks is observed. This compaction is caused by production and is often associated with stretching and arching of the overburden rocks. Time-lapse seismic data can be used to monitor these processes. Since compaction and stretching cause changes in layer thickness as well as seismic velocities, it is crucial to develop methods to distinguish between the two effects. We introduce a new method based on detailed analysis of time-lapse prestack seismic data. The equations are derived assuming that the entire model consists of only one single layer with no vertical velocity variations. The method incorporates lateral variations in (relative) velocity changes by utilizing zero-offset and offset-dependent time shifts. To test the method, we design a 2D synthetic model that undergoes severe reservoir compaction as well as stretching of the overburden rocks. Finally, we utilize the method to analyze a real 2D prestack time-lapse seismic line from the Valhall field, acquired in 1992 and 2002. For a horizon at a depth of around [Formula: see text], which is near the top reservoir horizon, a subsidence of [Formula: see text] and a velocity decrease of [Formula: see text] for the sequence from the sea surface to the top reservoir horizon are estimated. By assuming that the base of the reservoir remains constant in depth, a reservoir compaction of 3.6% (corresponding to a subsidence of the top reservoir horizon of [Formula: see text]) and a corresponding reservoir velocity increase of 6.7% (corresponding to a velocity increase of [Formula: see text]) are estimated.

Converted wave imaging of the Toba Caldera, Indonesia

Receiver functions calculated from data recorded by the 1995 PASSCAL Toba Seismic Experiment in northern Sumatra, Indonesia and the permanent station PSI reveal the presence of a low seismic velocity zone at between 8 and 14 km depth, which is interpreted to be the top of a magma reservoir. The Moho under the Toba caldera lies near 30 km with a localized thickened region that reaches 39 km beneath the eastern edge of the caldera with an abrupt shallowing to the northeast. This thickened crust may be where hot gabbroic material underplates the base of the crust to serve as the heat source for shallower volcanism. Alternatively, the thickened crust and Moho offset may be due to localized crustal thickening when this region was situated in a sharp restraining bend of a now extinct segment of the Sumatran fault.

Anisotropic geometrical-spreading correction for wide-azimuth P-wave reflections

Compensation for geometrical spreading along a raypath is one of the key steps in AVO (amplitude-variation-with-offset) analysis, in particular, for wide-azimuth surveys. Here, we propose an efficient methodology to correct long-spread, wide-azimuth reflection data for geometrical spreading in stratified azimuthally anisotropic media. The P-wave geometrical-spreading factor is expressed through the reflection traveltime described by a nonhyperbolic moveout equation that has the same form as in VTI (transversely isotropic with a vertical symmetry axis) media. The adapted VTI equation is parameterized by the normal-moveout (NMO) ellipse and the azimuthally varying anellipticity parameter [Formula: see text]. To estimate the moveout parameters, we apply a 3D nonhyperbolic semblance algorithm of Vasconcelos and Tsvankin that operates simultaneously with traces at all offsets andazimuths. The estimated moveout parameters are used as the input in our geometrical-spreading computation. Numerical tests for models composed of orthorhombic layers with strong, depth-varying velocity anisotropy confirm the high accuracy of our travetime-fitting procedure and, therefore, of the geometrical-spreading correction. Because our algorithm is based entirely on the kinematics of reflection arrivals, it can be incorporated readily into the processing flow of azimuthal AVO analysis. In combination with the nonhyperbolic moveout inversion, we apply our method to wide-azimuth P-wave data collected at the Weyburn field in Canada. The geometrical-spreading factor for the reflection from the top of the fractured reservoir is clearly influenced by azimuthal anisotropy in the overburden, which should cause distortions in the azimuthal AVO attributes. This case study confirms that the azimuthal variation of the geometrical-spreading factor often is comparable to or exceeds that of the reflection coefficient.