The Shunbei area in Tarim Basin develops fault-controlled fracture-cave reservoirs, and good results have been achieved in exploration and development of the main fault zones. In addition to the main fault zones, there are numerous small- to medium-sized faults in the area, which are more abundant, widely distributed, and smaller in scale. Due to the target depth (>8 000 m) and scale, it is difficult for small- to medium-sized faults and their controlled fracture-cave bodies to get clear seismic responses, so to identify and describe them is hard. Based on seismic data interpretation, spectral extension and strong reflection separation techniques were applied to enhance the kinetic information in the seismic data, effectively highlighting the seismic reflection characteristics of the small- to medium-sized fault-controlled fracture-cave bodies. Sensitive attributes were selected depending on the characteristics of different types of reservoirs. The multi-scale coherence of curvelet is found to be sensitive to small- to medium-sized faults, and the attributes such as disorderliness and frequency-division energy can be used to effectively identify fault zones and fracture-cave bodies. Small- to medium-sized fault-controlled fracture-cave bodies were successfully predicted and described by integrating the attributes reflecting different information. This technique was applied in the Shunbei area, which effectively guided well deployment, facilitating the oil and gas development.
The T3X2 gas reservoir in the Xinchang structural belt, Sichuan Basin, suffers a lot of challenges such as widespread water production, complex water production characteristics, unclear water invasion patterns, and a lack of effective water control strategies, affecting the stable production. Based on geological data and production performance from the gas reservoir, and using the theories/techniques of gas reservoir engineering, orthogonal experiments and development practices, the water production characteristics of the gas reservoir were analyzed, typical water invasion patterns were clarified, and water control strategies for gas wells were proposed. The results show that the T3X2 gas reservoir has 5 types of water production which can be identified by plotting charts. The water invasion patterns are classified into: rapid water channeling along fractures and slow water advancing. The degree of fracture development and the scale of fracturing treatments are the key factors influencing water invasion pattern. For the pattern of rapid water channeling along fractures, controlling pressure difference and balancing water drainage are critical, while for the pattern of slow water advancing, rationalizing production system and localized water blocking are recommended.
Seismic data from piedmont areas typically suffers from low signal-to-noise ratio (SNR), making it challenging to pick residual velocity fields and causing difficulties in iterative convergence of the depth domain velocity field to its optimal value. All these factors impede accurate migration and imaging of the seismic data from piedmont areas. By using the interpolation techniques in five-dimensional data regularization, the data from the original common midpoint (CMP) gathers prior to migration were reconstructed. By altering observation system, the bin attributes were enhanced to improve the SNR of the seismic data for iterative inversion of the pre-stack depth migration (PSDM) velocity field. To ensure the fidelity of the migrated data, the high-SNR CMP gathers obtained from data interpolation were used solely as inputs for the iterative inversion of the depth domain velocity field, while the original CMP gathers were preserved for the final PSDM imaging. This method enables fast and accurate iterative convergence of the depth domain velocity field. The actual application demonstrates that the method is highly feasible, and yields accurate final migrated velocity field through iterative updates and well-aligned reflections of migrated seismic profiles. This method provides a valuable reference for PSDM velocity modeling in the piedmont areas.
In the middle-late stage of waterflood development in low-permeability sandstone reservoirs in the eastern margin of the Junggar Basin, the development performance deteriorates, and the water cut increases, necessitating new effective development techniques. Different fluids were selected for oil displacement efficiency experiments. Using long cores and under simulated formation conditions, oil displacement experiments were performed for waterflooding and gas flooding with N?, CH?, and CO? after waterflooding till achieving the current recovery efficiency of the reservoir. Nuclear magnetic resonance scanning and oil-containing pore size inversion were conducted on cores before and after injection of different fluids. The results show that CO? flooding can increase the recovery factor by 21.58%. The fluids rank as CO?, CH?, H?O, and N? in a descending order of oil displacement efficiency and producing degree. N? flooding primarily recovers oil from larger pores, with the lower limit of pore size being 170.9 nm. CH? flooding primarily mobilizes oil from medium to large pores, with the lower limit of pore size being 48.7 nm. CO? flooding can extract oil from pores of all sizes, with the lower limit of pore size being 27.8 nm, the lowest level among the processes tested. A CO? flooding pilot test zone was established in the oilfield. After CO? injection, the liquid production increased, the water cut decreased, and the oil production improved, suggesting good field application.
The NJH block of the Santanghu oilfield features sandstone reservoirs containing medium-viscosity oil, with crude oil viscosity of 20.8 mPa·s. The reservoir is at medium water cut stage, with a predicted waterflood recovery factor of 22.70%, leaving a limited potential for further enhanced oil recovery. To figure out an applicable enhanced oil recovery (EOR) technique, laboratory experiments and field test were conducted on CO2 near-miscible flooding for medium-viscosity oil to understand the mass transfer patterns and EOR mechanisms of this technique, thereby determining its feasibility. The research results show that the front of the CO2 flooding mainly plays a swelling effect, and the rear exerts a stronger extraction effect than the front. Reducing the viscosity and improving the remaining oil displacement efficiency are the main stimulation mechanisms. The viscosity of surface crude oil reduced by 55%, the content of C2-C15 components increased by 18.3%, and the displacement efficiency improved by 4.6 times. Permeability ratio is found to be the primary factor influencing swept volume, with a permeability ratio of 6, leading to a recovery factor of only 13.84% in low-permeability layers. During the field test, the cumulative injected gas volume is 2.66×104 t, cumulative oil production is 0.78×104 t, and oil exchange ratio is 0.29, confirming a promising application of CO2 near-miscible flooding for medium-viscosity oil.
In the Zhijing-Ansai area of the Ordos Basin, the reservoirs in the 8th and 9th members of the Upper Triassic Yanchang formation (Chang 8 and Chang 9 members) are tight, with complex pore structure and unclear vertical distribution of effective reservoirs. A method for evaluating the effectiveness of the tight sandstone reservoirs was proposed based on nuclear magnetic resonance (NMR) logging data and mercury injection data of rock samples. For wells with NMR logging data, the proportions of macropores, mesopores, and micropores can be directly obtained from the NMR data, and an NMR logging-based three-pore component index can be constructed. For wells without NMR logging data, the pore throat radius index can be established by using the relationship between NMR transverse relaxation time and pore throat radius. Both the NMR logging-based three-pore component index and the pore throat radius index can quantitatively characterize the pore structure of tight sandstone reservoirs. Integrating with analysis of formation test data, it is found that single-well liquid production index per meter is positively correlated with pore structure. Thus, an effectiveness evaluation standard for tight sandstone reservoirs was established. The application of this standard to the Chang 8 and Chang 9 members in the Zhijing-Ansai area demonstrates excellent results, with significantly improved accuracy of well logging interpretation.
The low-viscosity reservoirs in the Tuha basin exhibit rapid decline of oil relatively permeability in the initial development stage and slower decline in the late development stage with high water-cut. This paper presents a new mathematical model of oil/water relative permeability. The new model simplifies the determination of parameters and offers a high fitting accuracy. It can describe the oil relative permeability curve and the convex water relative permeability curve, and also the common X-shaped oil/water two-phase relative permeability curve. For convenient application, the new model was configured with a corresponding water flooding analytical method, and then compared with the Gao’s simplified model of oil/water relative permeability. In this way, the linear relationship between the average water saturation of oil layer and the water saturation at the outlet end was further validated. By directly substituting this relationship into the fractional flow equation, a new generalized water cut variation pattern was derived. The actual application of this model shows good results, making it a valuable reference for similar reservoirs.
Based on the configuration theory and the analytic hierarchy process (AHP), the configuration shapes of the combined conventional mercury intrusion-constant rate mercury intrusion (CMI-CRMI) curves were classified, and a universal three-segment configuration pattern for the curves was established. The indicative significance of this pattern to the pore-throat systems and their wetting hysteresis was clarified. The results show that the combined CMI-CRMI curves consist of three configuration segments: a, b, and c, which are interconnected but exhibit distinct shapes. The segment a displays an overlapping shape, indicative of a macro-pore-throat system, where the combined CMI-CRMI curve shows no wetting hysteresis. The segment b demonstrates a separated shape and can be subdivided into subsegments b1 and b2. Subsegment b1 indicates a meso-pore-throat system, where the CRMI intrusion curve shows no wetting hysteresis, but the CMI curve does. Subsegment b2 also indicates a meso-pore-throat system, where the combined CMI-CRMI curve shows wetting hysteresis. The segment c exhibits an overlapping shape, representing a micro-pore-throat system, where both the CMI and CRMI curves exhibit equal wetting hysteresis. The deformation of the mercury meniscus during CMI is concentrated in the segments b and c, while the deformation of the mercury meniscus during CRMI is concentrated in the segments b2 and c. Subsegment b1 in both the CMI and CRMI curves can be used for contact angle correction. This three-segment configuration pattern of the combined CMI-CRMI curves provides a significant guidance for segmental contact angle correction and pore-throat distribution characterization.
In the application of reservoir lithology identification, the efficiency, accuracy and effective information integration ability of machine learning algorithm have been fully verified, especially in unconventional reservoirs with strong heterogeneity such as shale. Based on the optimal selection of parameters such as natural gamma, T2 geometric mean, structural index, skeleton density index, density, and deep lateral resistivity, and using a random forest algorithm combined with recursive feature elimination (RF-RFE), major lithologies of the shale reservoirs in the Middle Permian Lucaogou formation in the Junggar basin were identified. Lithology prediction was conducted on the same dataset using conventional RF and support vector machine (SVM) algorithms, and the results were compared with those obtained from thin-section identifications. It is found that RF-RFE yields better results with only half of the logging parameters, and the parameters defined by optimal selection help reduce the algorithm’s running time. Thus, the use of RF-RFE algorithm can realize optimal selection of characteristic logging parameters, more accurate identification of shale lithology, and reduction of running time. The algorithm provides a new approach for complex lithology identification and multi-parameter selection.
There are widespread alluvial fans with extremely thick gravel deposits in the surface of the piedmont zones in western China. These fans exhibit strong azimuth anisotropy. The tomographic inversion velocity differs greatly from the vertical velocity, severely impacting the accuracy of static correction value of 3D seismic data and the imaging of shallow-to-medium layer in pre-stack depth migration. Based on the HTI medium theory, a method for determining the symmetry axis and constructing an anisotropic model through azimuthal tomography fitting using first-arrival forward modeling was proposed. First, based on anisotropy, the first-arrival time from both micro-logging forward modeling and actual data is examined and compared to identify the symmetry axis characteristics. Then, elliptical fitting is performed on the azimuthal first-arrival tomography inversion model to derive the initial fast and slow velocities and the symmetry axis orientation. Next, by using 2D VTI anisotropic forward modeling and tomography inversion, the correction coefficients of the velocity in the direction of the symmetry axis are obtained. Finally, an anisotropic velocity model is established for the piedmont gravel zone.
Barrier/interlayer is a key factor that significantly affects fluid flow and controls the distribution of oil and water, and it serves as a crucial evidence for understanding the distribution of remaining oil. Barrier/interlayer in deep strata is difficult to identify due to the high coring cost, large depth error in logging data, low resolution of conventional logging curves, and ambiguous signals from thin interbeds. Through core analysis of key wells, the logging curves sensitive to barrier/interlayer and their response characteristics were identified. By employing wavelet decomposition and reconstruction, the conventional sensitive logging curves were processed with high resolution, which reduced the smoothing effect of adjacent layers and highlighted the logging response characteristics of thin layer interfaces, making thin layer identification resolution enhanced by nearly 100%. By integrating the vector pattern of formation dip and pseudo-imaging characteristics of barrier/interlayer, a method for identifying and dividing deep barrier/interlayer was established. Actual applications demonstrate that this method allows for precise identification of barrier/interlayer, with a much higher capability than conventional methods. This method yields an accuracy of layer correlation between wells increased by 38%, elucidating the issue of inclined oil-water contact and providing remaining oil distribution.
Considering the varying lithofacies and lithology of the proximal glutenites in the Dongying sag，a three-dimensional geological model of the glutenites was established for wide-azimuth seismic forward modeling. Using the simulated data cube，and through azimuthal stacking of gathers in OVT-domain，the effects of azimuth variation on parameters such as seismic travel time and amplitude were analyzed，and the relationships between azimuth/amplitude and favorable reservoirs were established. The results show that the variation in the sedimentary direction of the glutenites causes azimuth differences in seismic wave propagation，leading to azimuthal anisotropy in seismic reflections. The data cube obtained from azimuthal stacking at the azimuth perpendicular to the sedimentary boundaries is more sensitive to the responses of the top and internal boundaries of the glutenite，with stronger amplitudes. It more effectively reveals the contacts between glutenites of different periods，thereby facilitating the accurate identification of glutenite and fine prediction of favorable reservoir distribution. Wide-azimuth OVT-domain seismic data are proved effective in glutenite prediction，and have been successfully applied in predicting glutenite reservoirs in the steep slope zone of the northern Dongying sag，with the prediction results in good agreement with actual drilling results.
Abundant hydrocarbon resources have been discovered in the ultra-deep Ordovician carbonate strike-slip fault damage zones of the Tarim basin. However, these zones cannot be accurately characterized due to the low resolution of seismic data obtained from the ultra-deep layers, thereby restricting the efficient evaluation and target selection of the strike-slip fault-controlled hydrocarbon reservoirs. According to the seismic responses of the strike-slip fault damage zones in the Fuman oilfield, and based on the structure-oriented filtering, the eigenvalues and eigenvectors were calculated by using the structure tensor method, and the projection energy along the fault direction was enhanced by selecting appropriate time windows and stacking vertical thicknesses, which accentuates the strike-slip fault damage zones, enabling a clearer delineation of their boundaries and intensities. The results show that this method provides a clearer depiction of strike-slip fault distribution, allows for the identification of smaller-scale faults, and effectively delineates the width and intensity of ultra-deep carbonate strike-slip fault damage zones, which can be used to evaluate the development degree of the strike-slip fault damage zones. This method can be employed in trap evaluation, well placement, trajectory design, and well monitoring, which will improve drilling success rates and individual well productivity.
The Yingmai-2 area is situated on the Yingmai-2 structure in the southern part of the Yingmaili low bulge of the Tabei uplift, Tarim basin. Affected by tectonic movements and magmatic intrusion, a large number of directionally-aligned high-angle fractures were developed on the structure. The reservoirs are mainly fractured. Due to the deep burial depth and strong heterogeneity of the reservoirs in the Yingmai-2 area, predicting these fractured reservoirs using post-stack fracture prediction techniques like coherence, curvature, and ant-tracking proves challenging. Based on the pre-stack wide-azimuth offset vector tile (OVT) domain gather data from the Yingmai-2 area, well-controlled optimization processing of pre-stack OVT domain gather was implemented by employing techniques such as pre-stack denoising and anisotropy time difference correction to yield high-quality pre-stack wide-azimuth gather data. Subsequently, pre-stack facies-controlled fracture prediction was conducted under the constraints of post-stack multiple attributes that can represent fractured-vuggy reservoirs, including gradient structure tensor, amplitude curvature, and residual wave impedance. This methodology can effectively predict fractures in ultra-deep fractured-vuggy carbonate reservoirs, and offer valuable insights for the exploration and development of fractured reservoirs in the Tarim basin.
The GCD work area of the Tarim basin is covered by desert, where the acquired seismic data are superimposed with high-energy sand dune resonance noise due to the loose and thick dune structures, hampering the accurate imaging of the Ordovician marine reef-beach reservoir. Conventional imaging methods are disadvantageous due to low resolution, defocusing and other defects. On the basis of fidelity-based seismic processing, a three-step method for substantial attenuation of the sand dune resonance noise was incorporated. The 3D frequency space domain pre-stack random noise suppression technology was adopted to enhance the signal-to-noise ratio (SNR) of pre-stack preprocessing gathers. Furthermore, vertical seismic profile velocity, logging data, and interpreted horizons were combined to enable a multi-information-constrained grid tomography modeling and anisotropic reverse time migration. The results show that the sand dune resonance noise is effectively attenuated, and the SNR of pre-stack seismic data is significantly improved as compared to conventional denoising methods. The residual random seismic noise in pre-stack gathers is effectively suppressed, and the post-processing energy in velocity spectra of the gathers is more focused, facilitating precise velocity model establishment. Based on the high-density seismic imaging gathers, depth-domain model inversion where the inversion grid is gradually refined from coarse to fine was performed, and the established velocity model can accurately characterize the Ordovician marine reef-beach reservoirs. Anisotropic reverse time migration accurately relocated the complex wavefields of the Ordovician marine reef-beach reservoirs, enhancing resolution and focusing. The research results provide a support for optimizing the design of the sidetracking trajectory for Well G1 in the GCD work area, which was validated by actual drilling results. These techniques can provide a robust technical support for high-precision imaging and processing of seismic data in desert areas of the Tarim basin.
Fault-controlled carbonate reservoirs are highly heterogeneous, with interweaving development of pores, fractures, and vugs of various sizes. For this kind of reservoirs, the dynamic reserves calculated using conventional material balance methods may be larger than the static reserves. By incorporating water-oil ratio and considering rock compressibility coefficients for different pore-fracture-vug media, a comprehensive compressibility coefficient suitable for the fault-controlled reservoirs was derived. On this basis, a new flow material balance equation was established for the fault-karst reservoir, and its accuracy and applicability were verified using numerical simulation. The research results show that the dynamic reserves calculated by the new equation have an error of only 0.1099% with the static reserves obtained from numerical simulation, confirming the new equation’s reliability and accuracy. In the Halahatang area, the relative error between the dynamic reserves calculated using the new equation and the static reserves derived from geological modeling for multiple wells ranged from -4.82% to -0.15%, which is significantly lower than that calculated using the conventional material balance equation. The results obtained from the new equation are closer to actual conditions, making it more suitable for calculating the reserves of the fault-controlled carbonate reservoirs in the Halahatang area.
In order to study the fracability evaluation method for low-permeability tight reservoirs, experiments were conducted on six core samples from Well Y301 and Well Y3 in the Yongjin oilfield, Shawan sag, Junggar basin, and the parameters such as rock mineral composition, porosity, stress-strain curves, P-wave velocity, and S-wave velocity were obtained. The experiment results agreed well with logging data, and an empirical rock mechanical model was established for the study area. Meanwhile, based on the equivalent medium model, a new model considering mineral composition and pore structure characteristics was developed for calculating rock brittleness index. Then, a method for constructing the rock mechanical parameter profile of low-permeability tight reservoirs based on logging data was established and applied in Well Y301. The application results show that the Qigu formation in Well Y301 has good fracability, which lays a foundation for the comprehensive evaluation of fracability of tight sandstone reservoirs.
Continental shale reservoirs are characterized by low porosity, ultra-low permeability, high clay mineral content, rapid mineral composition variation, and strong formation heterogeneity. Therefore, the water saturation calculated with Archie formula or conventional mathematical statistical models often introduces large errors. To improve the calculation accuracy of water saturation in continental shale reservoirs, taking the shale from Lower Jurassic Lianggaoshan formation in the Fuxing block of southeastern Sichuan basin as an example, the limitations of existing methods for calculating water saturation were analyzed, and the feasibility of applying the composite wave impedance reconstructed from the combination of P wave and S wave in array acoustic logging and logging density to calculate water saturation was demonstrated. Based on this analysis, a method for calculating water saturation in continental shale reservoirs was proposed. This method considers the influence of rock minerals and effectively avoids the limitations of electrical logging and non-electrical logging, and finally improving applicability. The application of this method has yielded favorable results in multiple wells in the shale reservoirs of Lianggaoshan formation, southeastern Sichuan basin, with calculated water saturation closely matching those from core analysis, and absolute errors ranging from 1.3% to 2.2%, meeting the requirements for well logging evaluation.
Improving the resolution of seismic data processing is an effective means for predicting thin reservoirs. The primary objective of high-resolution processing is to effectively recover high- and low-frequency information of seismic data, broaden frequency bandwidth, and maintain signal-to-noise ratio and fidelity of seismic data. Using the high-resolution processing technology for restoring weak signals through harmonic decomposition, and based on compressed wavelet transform, the high- and low-frequency weak seismic signals were restored according to harmonic components. Firstly, the seismic signals within effective frequency bands were decomposed into various baseband signals by using the compressed wavelet transform. Then, the high-order and low-order harmonics of each baseband signal were calculated and added to the wavelet transform coefficients. Finally, inverse wavelet transform was performed to restore the high- and low-frequency weak signals. In this process, only the baseband signals within the effective frequency band are estimated, which helps to maintain the signal-to-noise ratio. The wavelet transform coefficients of seismic signals are consistent with the stratum reflection coefficients, verifying that the technology has high fidelity and good relative amplitude preservation. The actual application of the high-resolution processing technology shows that it can maintain the signal-to-noise ratio and significantly widen the seismic bandwidth, resulting in clearer seismic profile breakpoints, higher resolution, and better identification of thin reservoirs of about 40 m thick at the depth below 6 000 m.
Conventional pre-stack depth migration imaging techniques separate the migration imaging and static correction processing. In the processing of seismic data from complex mountainous areas, due to the factors such as complex surface condition, drastic lateral velocity variation near the surface, and exposure of high-velocity interval, the static correction based on the assumption of surface consistency may cause wave field distortion. This distortion leads to big errors in calculation of travel time, affecting the effects of depth migration imaging. To solve this problem, a full-depth modeling and imaging technique based on true surface migration was proposed. This technique starts with velocity modeling and travel time calculation from the surface elevation, and addresses static correction implicitly in the migration imaging process. It has been satisfactorily applied in the processing of complex mountainous data.
