Fracture-cavity Reservoirs Research Articles

Dipole acoustic remote detection (ARD) logging and its application in fracture-cave reservoir

Abstract Oil and gas from deep reservoirs are mostly enriched in the fracture-cave reservoirs. For this reason, fine evaluation of large-scale fracture-cave and effective detection of small scale fracture-cave are the key issues to be solved in the exploration of fracture-cave type carbonate reservoirs. The acoustic remote detection logging technology is based on acoustic reflection method to continuously image the formation around the well. The cross dipole array sonic data can be processed to image geological anomalies within a range of about 20 meters outside the well. Dipole S wave remote detection has the remarkable advantages of low emission frequency and larger investigation depth, but it has 180 degrees’ azimuthal ambiguity. This study is based on previously developed far field dipole S wave remote detection tool, finite difference is performed on formation models with and without are obtained for representative fracture-cave formations and remote detection processing and interpretation software is updated. These are applied on a well in the northwestern area. Through the comprehensive analysis of remote logging, conventional logging, electrical imaging logging and seismic data of the fracture-cave geological anomalies, a fine picture of the morphology and development of the geological anomalies from the near wellbore to the far end is formed

Research advances on the mechanisms of reservoir formation and hydrocarbon accumulation and the oil and gas development methods of deep and ultra-deep marine carbonates

Based on the new data of drilling, seismic, logging, test and experiments, the key scientific problems in reservoir formation, hydrocarbon accumulation and efficient oil and gas development methods of deep and ultra-deep marine carbonate strata in the central and western superimposed basin in China have been continuously studied. (1) The fault-controlled carbonate reservoir and the ancient dolomite reservoir are two important types of reservoirs in the deep and ultra-deep marine carbonates. According to the formation origin, the large-scale fault-controlled reservoir can be further divided into three types: fracture-cavity reservoir formed by tectonic rupture, fault and fluid-controlled reservoir, and shoal and mound reservoir modified by fault and fluid. The Sinian microbial dolomites are developed in the aragonite-dolomite sea. The predominant mound-shoal facies, early dolomitization and dissolution, acidic fluid environment, anhydrite capping and overpressure are the key factors for the formation and preservation of high-quality dolomite reservoirs. (2) The organic-rich shale of the marine carbonate strata in the superimposed basins of central and western China are mainly developed in the sedimentary environments of deep-water shelf of passive continental margin and carbonate ramp. The tectonic-thermal system is the important factor controlling the hydrocarbon phase in deep and ultra-deep reservoirs, and the reformed dynamic field controls oil and gas accumulation and distribution in deep and ultra-deep marine carbonates. (3) During the development of high-sulfur gas fields such as Puguang, sulfur precipitation blocks the wellbore. The application of sulfur solvent combined with coiled tubing has a significant effect on removing sulfur blockage. The integrated technology of dual-medium modeling and numerical simulation based on sedimentary simulation can accurately characterize the spatial distribution and changes of the water invasion front. Afterward, water control strategies for the entire life cycle of gas wells are proposed, including flow rate management, water drainage and plugging. (4) In the development of ultra-deep fault-controlled fractured-cavity reservoirs, well production declines rapidly due to the permeability reduction, which is a consequence of reservoir stress-sensitivity. The rapid phase change in condensate gas reservoir and pressure decline significantly affect the recovery of condensate oil. Innovative development methods such as gravity drive through water and natural gas injection, and natural gas drive through top injection and bottom production for ultra-deep fault-controlled condensate gas reservoirs are proposed. By adopting the hierarchical geological modeling and the fluid-solid-thermal coupled numerical simulation, the accuracy of producing performance prediction in oil and gas reservoirs has been effectively improved.

Optimization of deep acid fracturing parameters for Shunbei carbonate dissolved fault reservoirs

Abstract When encountering favorable reservoirs that have not been drilled along the wellbore trajectory of Shunbei carbonate fracture-cavity reservoirs, deep acid fracturing technology is required to communicate with favorable reservoirs and achieve production. Currently, there is no optimized method for designing deep acid fracturing parameters for Shunbei fracture-cavity reservoirs, which results in problems such as failure to communicate with favorable reservoirs after deep acid fracturing or rapid decline in production after communication. To address these issues, this paper establishes a geological feature model for fracture-cavity reservoirs based on the characteristics of Shunbei fracture-cavity reservoirs and proposes a deep acid fracturing fracture parameter optimization approach with production capacity as the goal. Using a multi-field coupled acid fracturing model that can reflect the large leakage characteristics of fracture-cavity reservoirs, the proposed fracture parameters are used to simulate and optimize the scale of pre-flushing liquid and acid liquid. The simulation results show that the desired increase in production can be achieved when the fracture diversion capacity is 30D·cm and the penetration ratio is between 0.1 and 0.3. Furthermore, this paper recommends injecting 450~900 m3 of pre-flushing liquid to communicate with favorable reservoirs (0.1≤De≤0.2), followed by the injection of 900 m3 of cross-linked acid and 450 m3 of gelled acid to construct the fracture diversion capacity through acid corrosion.

Differential Characteristics of Conjugate Strike-Slip Faults and Their Controls on Fracture-Cave Reservoirs in the Halahatang Area of the Northern Tarim Basin, NW China

The X-type strike-slip fault system and weathering crust karst fracture-cave and channel reservoirs were developed in the Halahatang area of the northern Tarim Basin. However, the relationship between the reservoir and the strike-slip fault remains controversial. Based on the core data, and taking an NE-striking strike-slip fault as an example, this paper dissects the karst reservoir from wells along the strike-slip fault damage zone and analyzes the control of scales, properties, and segmentation styles of strike-slip faults on karst reservoirs. The results show that (1) the scale of the strike-slip fault controls the distribution of the reservoir—the wider the fault damage zone, the wider the fracture-cave reservoirs; (2) the transtensional segments of the strike-slip fault are more likely to produce karstification, and the buried-hill area and the interbedded area are controlled by different hydrodynamic conditions to form different types of karst reservoirs; (3) six different parts of the strike-slip fault are conducive to the formation scale of fault fracture zones. This research provides new insight into recognizing karst reservoirs within strike-slip fault damage zones, which can be further applied to predict karst reservoirs controlled by strike-slip faults.

Mechanism, mode, and prediction of karst caves collapse in the deep marine carbonate fracture-cavity reservoir

Tahe oilfield in the Tarim Basin of China is a typical deep marine carbonate fracture-cavity type reservoir. Large paleokarst caves are the important storage space types in this reservoir. Drilling data reveals that most of the caves have varying degrees of collapsed breccia filling. A better understanding of cave collapse mechanism, modes, and distribution is crucial for highly efficient production. In this study, large karst caves in the Ordovician of Tahe oilfield are taken as research objects, and several types of typical cave conceptual models are established based on similarity theory. Physical tests and numerical simulation methods are used to reveal the collapse mechanism under different depths, shapes, filling types, and tectonic stresses of different periods. Collapse modes are established based on the revealed mechanism, morphology, and damage zone of simulated karst caves. Seismic prediction methods of different types of collapsing caves are explored according to their reflection characteristics. The results reveal that compaction and shearing damage are the main reasons for cave collapsing. The roof and surrounding rock of the cave sink in during the collapse process, with a shear-slip damage zone of 2–3 times larger than the cave diameter. The sand/mud-filled cave is shown to be more stable than the water-filled cave, and the unfilled cave is the least stable. Tectonic stresses acting in the early Hercynian period are the primary reason for the current collapsed caves. Two cave collapse modes are established, type Ⅰ being mainly affected by overlying gravity and type Ⅱ mainly affected by shear stress. There are local differences in seismic reflection characteristics between these two types. The prediction of the two types of collapsed karst caves can be realized by combining the seismic attribute of colored inversion, frequency division energy, GR inversion and coherent energy gradient, etc. The comprehensive prediction results of collapsed karst caves in Tahe oilfield show a good correspondence with oil well productivity.

Differential fault-fluid alterations and reservoir properties in ultra-deep carbonates in the Tarim Basin, NW China

The development of high-quality carbonate hydrocarbon reservoirs in the ultra-deep (7000–10000 m) Lower Paleozoic strata of the Tarim Basin was substantially related to tectonic evolution, strike-slip faults, and diagenetic fluids. Detailed studies of the influences of tectonic evolution and strike-slip faults on the properties of diagenetic fluids and the development mechanisms of ultradeep carbonate reservoirs under the control of both faults and fluids are required to identify high quality reservoirs. In this study, end-member geochemical indicators of meteoric water, hydrothermal fluid, and formation fluid were constructed based on typical diagenetic mineral and geochemical data obtained from representative wells. Meteoric karst associated with strong tectonic uplift was the main diagenetic event along strike-slip faults in the Tahe area. The Shunbei area was generally affected by buried formation water, and locally by weak meteoric water or hydrothermal fluids. The Tazhong, Shunnan, and Gucheng areas were predominantly affected by strong hydrothermal fluids along strike-slip faults associated with strong volcanic activity, while some wells, such as TZ12, reveal the influence of thermochemical sulfate reduction (TSR). Differences in fault fluid types and properties resulted in differential development of ultra-deep carbonate reservoirs among the different areas. In the Tahe area, meteoric water alteration along faults formed karst fracture-cavity reservoirs. In the Shunbei area, fault-cavity carbonate reservoirs were formed by strong strike-slip faults, whereas fluid alteration was weak. Hydrothermal dissolution reservoir in the Tazhong area, hydrothermal silicification reservoir in the Shunnan area, and hydrothermal dolomitization reservoir in the Gucheng area are developed, respectively.

The spatial and geological characteristics of fault- and paleokarst-controlled carbonate-hosted reservoirs in the Tabei Uplift, Tarim Basin, China

Seismic interpretation and characterization of Middle Ordovician carbonates of the northern Tarim Basin in China reveal a series of deep-seated, sub-vertical conjugate strike-slip faults, together with sets of apparently layer-bound fractures striking parallel or orthogonal to the faults. Detailed stratigraphic analysis, well logging response interpretations (including formation microscanner images), coupled with core sample and thin section observations highlight vertical and lateral partitioning of fracturing and dissolution processes. Fracturing and dissolution development are most intense in grain-supported host rocks (grainstones, packstones) deposited in relatively shallow water conditions. Reservoir pore spaces vary systematically from north (Tahe) to south (Shunbei), which can be attributed to their proximity to a major regional angular unconformity with overlying Upper Devonian to Carboniferous sequences. Larger-scale dissolved fracture-cavity reservoirs are developed in the northern Tahe area due to the combined effects of faulting, surface karstification, and river system development close to the base Carboniferous erosion surface. Farther south, where the rocks lie farther from the paleoerosion surface, reservoir space is characterized by smaller, more structurally controlled open cavities bounded by fault slip surfaces, breccias, and open fractures. The observed interactions between paleokarstification intensity, tectonic controls, and host rock lithological layering—and their control over the observed reservoir complexity—are likely to occur in carbonate reservoirs worldwide.

Multi-mode flow simulation of fracture-cavity reservoirs and predicting oil accumulation based on hydro-mechanical-damage coupling model

Fracture-cavity carbonate reservoirs exhibit significant heterogeneity with diverse flow modes, including porous media seepage and free flow, within fractures and cavities. This complexity is further compounded by tectonic stress. Traditional oil reservoir seepage theories often struggle to depict these fluid flow characteristics accurately. This study employs a hydraulic-mechanical-damage coupling model to conduct numerical simulations of multi-mode fluid flow within fracture-cavity reservoirs. This approach elucidates fluid flow mechanisms influenced by multi-field coupling and predicts areas favorable for oil accumulation based on actual geological models. The results show that (1) while the secondary fractures developed in the penetrating-type fracture-cavity body result in the highest oil migration efficiency and initial production, the production from this body type decreases rapidly in the later stage. Secondary fractures in the sandwich-type and side-type cavity bodies primarily offer storage, resulting in lower initial production but a slower production decline. (2) In the S1 stress state, secondary fractures primarily connect fracture-cavity bodies, whereas, in the S2 stress state, they mainly contribute to oil accumulation. (3) Secondary fractures function as efficient conduits for oil migration, and their distribution is influenced by the presence of fault zones and cavities. Consequently, the intersection of cavities and fault zones with secondary fractures leads to the formation of favorable oil accumulation areas.

Numerical study of hydraulic fractures propagation in deep fracture-cavity reservoir based on continuous damage theory

Natural fractures and cavities are the primary spaces for oil and gas accumulation in fracture-cavity carbonate reservoirs. Establishing the connection between these spaces and the wellbore through hydraulic fracturing treatment is important for oil and gas extraction from such reservoirs. Due to the discontinuity and heterogeneity of the existing natural fracture-cavity system, anticipating the viability of hydraulic fracturing treatment is troublesome. A new method to simulate the hydraulic fracturing propagation in fracture-cavity reservoirs is proposed based on the continuous damage theory. The method considers the random spatial distribution of fractures and cavities and can simulate the arbitrary expansion of hydraulic fractures in the three-dimensional direction. Based on this method, the influence of different geological and engineering factors on the propagation patterns of hydraulic fractures in the fracture-cavity reservoirs is investigated. It is found that the increase of reservoir burial depth significantly limits the propagation ranges of hydraulic fractures. The propagation modes of hydraulic fractures encountering natural fractures change with increasing burial depth, undergoing a transition from “penetrate and deflect” to ”defect” and then to ”penetrate”. The reduction of horizontal stress difference increases the complexity of hydraulic fractures, but it is not conducive for hydraulic fractures to connect more natural fractures and cavities. The increase in fracturing pump rate is significantly beneficial for hydraulic fractures to connect more natural fractures and cavities. The viscosity of fracturing fluid has a significant impact on the morphology of hydraulic fracture propagation, which undergoes a transition from simple to complex, and then to simple with the change of the fracturing fluid viscosity from low to high. either too high or too low viscosity of the fracturing fluid is not conducive to the connection of more natural fractures and cavities by hydraulic fractures. The obtained conclusions can provide a reference for the design of hydraulic fracturing treatment for fracture-cavity carbonate reservoirs.

A novel fracture-cavity reservoir outcrop geological knowledge base construction method considering parameter collection and processing, mutual transformation of data-knowledge, application and update

This study endeavors to formulate a comprehensive methodology for establishing a Geological Knowledge Base (GKB) tailored to fracture-cavity reservoir outcrops within the North Tarim Basin. The acquisition of quantitative geological parameters was accomplished through diverse means such as outcrop observations, thin section studies, unmanned aerial vehicle scanning, and high-resolution cameras. Subsequently, a three-dimensional digital outcrop model was generated, and the parameters were standardized. An assessment of traditional geological knowledge was conducted to delineate the knowledge framework, content, and system of the GKB. The basic parameter knowledge was extracted using multiscale fine characterization techniques, including core statistics, field observations, and microscopic thin section analysis. Key mechanism knowledge was identified by integrating trace elements from filling, isotope geochemical tests, and water-rock simulation experiments. Significant representational knowledge was then extracted by employing various methods such as multiple linear regression, neural network technology, and discriminant classification. Subsequently, an analogy study was performed on the karst fracture-cavity system (KFCS) in both outcrop and underground reservoir settings. The results underscored several key findings: (1) Utilization of a diverse range of techniques, including outcrop observations, core statistics, unmanned aerial vehicle scanning, high-resolution cameras, thin section analysis, and electron scanning imaging, enabled the acquisition and standardization of data. This facilitated effective management and integration of geological parameter data from multiple sources and scales. (2) The GKB for fracture-cavity reservoir outcrops, encompassing basic parameter knowledge, key mechanism knowledge, and significant representational knowledge, provides robust data support and systematic geological insights for the intricate and in-depth examination of the genetic mechanisms of fracture-cavity reservoirs. (3) The developmental characteristics of fracture-cavities in karst outcrops offer effective, efficient, and accurate guidance for fracture-cavity research in underground karst reservoirs. The outlined construction method of the outcrop geological knowledge base is applicable to various fracture-cavity reservoirs in different layers and regions worldwide.

The identification of the development stages of the karstic fracture–cavity carbonate reservoirs: a case study on the lungu oilfield, Tarim basin

The challenge of identifying connectivity within karstic fracture-cavity reservoirs significantly impedes the efficient development of oil and gas resources, primarily due to our limited understanding of the developmental stages and distribution of these unique reservoirs. In this study, we employ a layered interpretation method to predict the distribution of various developmental stages within karstic reservoirs. In our approach, we first leverage our knowledge that karstic formations are primarily shaped by the water table. We reconstruct paleomorphological data and select representative wells in a chronological order from old to new. This process enables us to determine the karst base level and divide the karstic layers in individual wells. Subsequently, we identify small-scale karstic reservoirs and proceed to select isochronous sedimentary interfaces. We then flatten seismic events and map the karstic layers from wells to their corresponding seismic sections, a technique known as well-to-seismic calibration. Lastly, we provide accurate interpretations of the karstic layers, extracting root-mean-square amplitudes to predict the distribution of each reservoir. To validate the accuracy and efficiency of our method, we applied it to fracture-cavity reservoirs in the Lungu oilfield (LGX) of the Tarim Basin. Our results demonstrate the successful identification of 22 fracture-cavity reservoirs through an analysis of the distribution and connectivity of karstic reservoirs. This outcome serves as evidence that our method can significantly enhance reservoir production in terms of both efficiency and accuracy.

Segmentation and lateral growth of intracratonic strike-slip faults in the northern Tarim Basin, NW China: influences on Ordovician fault-controlled carbonate reservoirs

Intracratonic strike-slip faults have been recognized as a major factor controlling the formation of fracture-cave carbonate reservoirs in deep buried basins, yet which properties and how the strike-slip faults influence reservoir distribution and their connectivity are still ambiguous. This uncertainty significantly restricts hydrocarbon exploration and development, such as in the Fuman oilfield, northern Tarim Basin, NW China. Using a high-resolution 3D seismic reflection survey and borehole data, we investigated the geometry and kinematic evolution of the FI17 fault zone in the Fuman oilfield. This fault zone is characterized by a single fault zone, pop-up or pull-apart structures, right-stepping en echelon normal faults, and much smaller displacement (<30 m) normal fault arrays from bottom to top. The FI17 fault zone consists of four genetic segments, including the extensional strike-slip duplex, Riedel left-lateral shear, right-stepping horsetail splay, and horizontal slip segments in map view. In particular, the formation of the ∼18 km Riedel shear zone is characterized by the growth and linkage of segmented shear faults (synthetic and secondary synthetic shears). We observed that the large-scale fault-controlled fracture-cave reservoirs are distributed in positions with wider fault zones, which are characterized by overlapping of neighboring secondary shear faults. Furthermore, the reservoir width examined in this study is natural logarithmic correlated (positively) to the fault zone width. The reservoirs linked by the same shear faults show better internal connectivity. The spatial coherence between fault geometry and reservoir features indicates that segmentation and lateral growth of intracratonic strike-slip faults controls the occurrence of fracture-cave reservoirs, which may provide support for reservoir prediction in the Fuman oilfield and other deeply buried fault-controlled carbonate reservoirs in general.

Characteristics and genetic mechanisms of fault-controlled ultra-deep carbonate reservoirs: A case study of Ordovician reservoirs in the Tabei paleo-uplift, Tarim Basin, western China

To reveal the effect of faults in the deep and ultra-deep oil and gas reservoirs, based on the comprehensive analysis of drilling logs and the core and seismic data, the development and fault-controlled regularity of the reservoirs in the high and low inner part of the Ordovician carbonate of the Tabei paleo-uplift have been discussed. The results show that large fracture-cave reservoirs are developed in the Tabei paleo-uplift, and as main reservoir, the cave-type reservoirs usually appeared as lumps or patches in the north of the pinch-out line of O3s, and gradually transitioned to isolated dots to the south. Due to the difference in control effect of faults, the degree of reservoir development differs greatly from north to south. The effective reservoir thicknesses are relatively stable and the porosity are relatively high in north, which are mainly controlled by karst and fractures, and the effect of faults on reservoir is not obvious. To the south, the reservoirs are gradually banded along the fault, and the control effect of large strike-slip faults are enhanced. This indicates that the strike-slip faults are more important to the development of reservoir in the low inner part. This difference is caused by the differences in fault controlling factors during different evolution periods of the paleo-uplift.

Characteristics of Chaotic Weak Reflection in Carbonate Fracture-Cavity Reservoirs: A Case Study of Ordovician Reservoirs in Block 10 of the Tahe Oilfield, Tarim Basin, China.

The identification of carbonate reservoirs is a critical task in oil and gas exploration. Chaotic weak reflection is an important type of reflection characteristic in carbonate reservoirs. Widespread distribution of chaotic weak reflection has been observed in the Ordovician Yijianfang Formation in the Tahe area. However, there is still a lack of systematic research on the distribution, characteristics, reservoir space, and reservoir types of chaotic weak reflection in the Tahe area. To address this issue, this study conducted a comprehensive analysis of multiscale data and found that chaotic weak reflections are distributed within 100 ms below the top interface of the Middle Ordovician. Seismic profiles exhibit a "string of pearls" with a "tail" or "pearl-like" widening feature. On well logs, all three porosity curves exhibit localized peaks, with increases in acoustic (AC) and compensated neutron (CNL) and decreases in density (DEN), while the deep and shallow lateral resistivity (RD, RS) curves show positive amplitude differences. In the FMI, they are represented by dark sinusoidal curves and dark small patches. Based on these response characteristics, two types of reservoir spaces are identified for chaotic weak reflections: high-angle vertical fractures and caves with heights less than 7 m, which can form independent reservoirs or fracture-cavity complexes. Three types of reservoirs are distinguished: fracture, cavity, and cavity-fracture types.

An Overview of the Differential Carbonate Reservoir Characteristic and Exploitation Challenge in the Tarim Basin (NW China)

The largest marine carbonate oilfield and gas condensate field in China have been found in the Ordovician limestones in the central Tarim Basin. They are defined as large “layered” reef-shoal and karstic reservoirs. However, low and/or unstable oil/gas production has been a big challenge for effective exploitation in ultra-deep (>6000 m) reservoirs for more than 20 years. Together with the static and dynamic reservoir data, we have a review of the unconventional characteristics of the oil/gas fields in that: (1) the large area tight matrix reservoir (porosity less than 5%, permeability less than 0.2 mD) superimposed with localized fracture-cave reservoir (porosity > 5%, permeability > 2 mD); (2) complicated fluid distribution and unstable production without uniform oil/gas/water interface in an oil/gas field; (3) about 30% wells in fractured reservoirs support more than 80% production; (4) high production decline rate is over 20% per year with low recovery ratio. These data suggest that the “sweet spot” of the fractured reservoir rather than the matrix reservoir is the major drilling target for ultra-deep reservoir development. In the ultra-deep pre-Mesozoic reservoirs, further advances in horizontal drilling and large multiple fracturing techniques are needed for the economic exploitation of the matrix reservoirs, and seismic quantitative descriptions and horizontal drilling techniques across the fault zones are needed for oil/gas efficient development from the deeply fractured reservoirs.

Comprehensive Experimental Design of Polymer Crosslinked Gel Plugging for Drilling

In order to further improve the comprehensive experimental skills of undergraduates and penetrate the concept of green chemistry education, a comprehensive experiment was designed to prepare polymer crosslinked gel plugging agent with polymer as the main agent and loofah pulp as the toughening material, and evaluate its effectiveness in simulating downhole leakage layer. This experiment applies organic/inorganic compound chemistry knowledge to the field of oilfield chemistry, and combines experimental skills with field work (TaHe Oilfield) to improve students' professional skills and oilfield field adaptability. TaHe Oilfield is the main position for the construction of tens of millions of tons of Northwest Oil Field. During the "Fourteenth Five-Year Plan" period, the large caves were basically under full control. How to develop small and medium-sized fractures and improve the use of reserves and realize the stable production of Tahe Oilfield is facing greater challenges. When adopting the "one well multiple control" method to develop small and medium-sized fractures and cavities, the key technical problem is the low success rate of plugging in carbonate fracture-cavity reservoirs, which requires plugging operations. Through this experiment, students can understand the mechanism of plugging agent in the leakage channel, and master the preparation and performance evaluation methods of gel plugging agent. Let students understand the site construction technology.

Quantitative Interpretation Model of Interwell Tracer for Fracture-Cavity Reservoir Based on Fracture-Cavity Configuration

The fracture-cavity combination structure between wells in fracture-cavity reservoirs is complex and changeable. Reliably identifying and quantitatively characterizing the fracture-cavity combination structure between wells has become an important prerequisite for flow channel adjustment in fracture-cavity reservoirs after water channeling and flooding. Aiming at the problems that it is difficult for the existing carving technology to characterize the flow characteristics of the injected fluid in the interwell fracture-cavity composite structure during the production process, and it is difficult for the existing interwell tracer proxy model to consider the specific fracture-cavity composite structure, this paper proposes a quantitative interpretation model for interwell tracers in fracture-cavity reservoirs with different architectures. Taking the Tahe fracture-cavity reservoir as the object, the matching relationship between the interwell fracture-cavity structure and the tracer curve was analyzed, and the tracer curve characteristics of five types of fracture-cavity structures were clarified. Considering the basic idea of tracing, a unified quantitative interpretation model of tracers under different fracture-cavity configurations based on branched flow channels and karst caves was deduced and established, and the input parameters required to apply the model, the parameters obtained directly by fitting, and further expandable calculated parameters were clarified. The interpretation model was used to fit, quantitatively interpret, and verify the reliability of the tracer curves of three wells in group TK411 of fracture-cavity unit S48 in the fourth area of Tahe Oilfield. The results show that the tracer curve fitting effect of each well was good, and the average relative error between the total flow rate explained by the tracer and the daily water production during the tracer monitoring period in the mine was only 3.02%, which effectively shows that the applicability and reliability of the quantitative interpretation model are established. The research results provide an effective way to apply tracer data in deep mining while improving the quantitative characterization ability of interwell tracer monitoring in fracture-cavity reservoirs.

Migration of paleo-uplift in southwestern Tarim Basin and its implications for orogenesis and reservoir development, NW China

Based on well horizon calibration, the typical seismic profiles in southwestern Tarim Basin were interpreted systematically, regional geological sections were established, and the regional denudation thickness of each tectonic period was restored. On this basis, the plane morphology maps of ancient structures of the Cambrian pre-salt dolomites in different periods were compiled, and the spatial distribution, development, evolution and migration of paleo-uplift in the late Early Paleozoic were analyzed. In the late Early Paleozoic, there existed a unified regional paleo-uplift widely distributed in southwestern Tarim Basin, which is called the southwestern Tarim plaeo-uplift. The “Tarim SW paleo-uplift” and “Hetian paleo-uplift” proposed in previous literatures are not independent, but the result of the spatio-temporal migration and evolution of the southwestern Tarim paleo-uplift identified in this paper. The southwestern Tarim paleo-uplift emerged at the end of Middle Ordovician, and took its initial shape with increased amplitude in the Late Ordovician. During the Silurian, the southwestern Tarim paleo-uplift rose steadily and expanded rapidly to the east, incorporating Pishan—Hetian and other areas, with the structural high locating in the Pishan—Hetian area. During the Devonian, the southwestern Tarim paleo-uplift began to shrink gradually, to a limited range in the Pishan—Hetian area in the southern part of the basin. During the Carboniferous, the southwestern Tarim paleo-uplift became an underwater uplift, that is, the paleo-uplift gradually died out. The southwestern Tarim paleo-uplift belongs to the forebulge of the southwestern Tarim foreland basin in the late Early Paleozoic, and its formation and evolution are related to the early Paleozoic orogeny of the West Kunlun orogenic belt in the south of the Tarim Basin. The migration of the southwestern Tarim paleo-uplift from the northwestern part of the southwestern Tarim Basin to the Pishan—Hetian area indicates the early Paleozoic orogenic process of the West Kunlun orogenic belt, which started in the western section at the end of Middle Ordovician and extended from west to east in a “scissor” style. The migration and evolution of the southwestern Tarim paleo-uplift controlled the development of unconformities at the end of Middle Ordovician, the end of Late Ordovician, and the end of Middle Devonian, and the spatial distribution of dissolved fracture-cave reservoirs in weathered crust below the unconformities in the southwest of Tarim Basin. The migration of the structural high of the southwestern Tarim paleo-uplift also played an important role in controlling the development of dissolved fracture-cave reservoirs in weathered crust.

Semi-analytical solution for bottomhole pressure transient analysis of a hydraulically fractured horizontal well in a fracture-cavity reservoir

This paper study the role of hydraulic fracture properties on the transient bottomhole pressure (BHP) behavior of a horizontal well producing from a tight fracture-cavity reservoir. A combination of point source function, Laplace transformation and Perturbation transformation are used to obtain BHP step by step. Through literature comparison and numerical simulation, the results of BHP have a good consistency, which indicates the proposed method is scientific and reasonable. We divide the fluid flow into five stages, namely the wellbore storage stage, the karst cave fluid flows to the fracture stage, the radial flow stage of karst cave and fracture system, the matrix fluid flows to the fracture stage and the quasi-steady state stage. We come to the conclusion that the number of fractures and fracture direction mainly affect radial flow stage. In contrast, the length of horizontal subsection and skin factor mainly affect the karst cave fluid flows to the fracture stage. The matrix fluid flows to the fracture stage is more obvious when the fracture half-length and the horizontal segment spacing of the horizontal well are small. The study believes special attention should be paid to reforming the formations at both ends of the horizontal well. The advantage of this method is to incorporate well geometry (skin factor) and hydraulic fracturing design (fracture parameters), which is useful for well test interpretation through generating a new set of type curves. What’s more, this new method has the characteristics of easy calculation. The findings of this study can help for better understanding of well test analysis in fracture-cavity reservoir. However, the limitation of this study is that it is only suitable for this situation the horizontal well does not encounter karst caves and the karst caves in the reservoir are connected to the wellbore through fractures.

Analytical model for transient pressure analysis in a horizontal well intercepting with multiple faults in karst carbonate reservoirs

In recent years, the Shunbei karst-carbonate reservoirs becomes a huge productivity oilfield, which produced over one million tonnes crude oil annually. However, there are difficulties in understanding production contribution of each fault-karst branch in reservoirs, which significantly impacts the efficient development. Multibranched fault-karst reservoirs in the Shunbei have an obvious tree-shaped geostructure, in which the natural fractures and eroded cave develop along multiple large-scale faults. The existing models for pressure transient analysis (PTA) were mainly established for fracture-cave reservoirs only with single fault, which cannot be applicable to characterize the multibranched fault-karst reservoir. To fill this gap, a novel analytical PTA model for horizontal commingled production well in the multibranched fault-karst reservoir was established to describe pressure response and identify flow regimes. First, our model includes the Darcy flow with the fluid compressibility effect in fracture region, and the large-scale vertical storage flow in cave region as well as the horizontal laminar flow in the horizontal wellbore. Then, the accuracy of this PTA model is verified by comparing it with the existing single branch fault-karst pressure model. Further, we applied the model to analyze the Shunbei oilfield case data. Last, the effect of boundary type, fluid compressibility effect, fracture physical properties, and cave spatial distribution on the pressure response are discussed in detail. The sensitivity analysis results show (a) the cave storage flow regime exhibits an obvious unit-slope-line on pressure derivative curve, at the time the skin transient flow constitutes a V-shape characteristic. (b) The number of fracture-cave branches can be directly obtained by counting the number of V-shaped appearances on the pressure derivative curve. (c) The fluid compressibility effect leads to an upward trend on the pressure and its derivative, reservoir engineers should be cautious to explain that characteristic as a closed boundary effect. (d) The cave volume and cave position control the timing of the V-shape occurring. As the cave volume increases, the linear flow regime lasts longer and the V-shaped feature becomes apparent. With the cave distance and cave depth increasing, the V-shape characteristic comes later. This work can provide technical support for accurate characterization of multibranched fault-karst reservoirs, and give a type curve analysis method for rapidly diagnosing the spatial location of each karst cavity by analyzing bottom-hole pressure.