That some strategic breakthroughs in natural-gas exploration have been made recently in the Upper Sinian Dengying Formation, Moxi-Gaoshiti area, central Sichuan Basin, urges geologists to fix on deep, ancient, and organic-rich shale domain. To figure out both geochemical characteristics and sedimentary environment in source rocks of the Upper Sinian Doushantuo Formation will be momentous for deep to ultra-deep oil and gas exploration in the whole basin. Therefore, taken typical outcrop individually from Qingping town of Mianzhu city, Yangpingguan town of Ningqiang county, Xiaoyangba village of Zhenba county, and Gaoyan town of Chengkou county in northern basin as examples, such rocks were discussed from aspects of geochemical characteristics and redox conditions during source generation, as well as the effect of both paleoproductivity and hydrothermal activity on organic-matter accumulation by means of kerogen’s microscopic examination, trace-element analysis and elemental-ratio test. Additionally, an extension model was investigated for the source rocks of Doushantuo Formation. Results show that (i) these source rocks whose extension is conditioned by sedimentary facies are featured by the thickness ranging from 8 m to 493 m, the total organic carbon (TOC) content from 0.71% to 2.60%, and the carbon-isotope value between -25.5‰ and -36.2‰ in kerogen presenting some characteristics of sapropelic-humic hybrid; (ii) apparently influenced by tectonic-sedimentary differentiation, the rocks exhibit strong heterogeneity and great difference in planar extension. Among which, waterbody deposited in basinal facies is formed in anoxic setting with notable hydrothermal impact and the average TOC of 2.6%, indicating these source rocks in high quality; and (iii) the accumulated organic matter in Doushantuo Formation of northern basin is mainly controlled by redox conditions, and partially contributed to hydrothermal activity, but the paleoproductivity effect is ill-defined.
The Middle Permian Maokou Formation has recently emerged as one of vital reservoirs for natural-gas exploitation in Longnvsi area. However, Maokou reservoirs are characterized by strong heterogeneity resulting in uncertain forming mechanisms, which may retard their efficient exploration and development. Thus, the depositional and petrological characteristics, and reservoir space and types were analyzed for this formation after core observation, thin-section identification, and logging-data analysis. Moreover, reservoir-forming mechanisms and the main geological process controlling reservoir evolution were discussed to create an evolution model. Results show that (i) Maokou reservoirs in the study area are dominantly developed with fine- to medium- crystalline dolomite in the lower part of the second member of Maokou Formation (Maokou 2 Member). Generally, unreal images of residual grain are visible in the dolomite. And such reservoirs belong to low porosity and ultra-low permeability ones; (ii) their space consists of intercrystalline pores, intercrystalline dissolution pores, needle-like pores, contiguous fractures and vugs, and fractures. And they can be classified into three types, such as fractured-vuggy, vuggy, and porous reservoirs, with the first two as the predominant; (iii) dolomite reservoirs are mostly extended into grain-beach bodies. And not only fragmental shoal but bioclastic beach facies belts exhibit remarkably higher porosity than that in inter-beach sea and platform-flat facies belts; and (iv) initial pores are stemmed from meteoric freshwater of penecontemporaneous dissolution. Dolomitization preserves reservoir space. Early hydrothermal fluid invasion along faults induces hydraulic fractures to generate saddle-shaped dolomite filling. Accordingly, the created model is in line with the fragmental shoal as the basis, the penecontemporaneous dissolution for forming reservoirs, the dolomitization for reservoir preservation, and the hydrothermal alteration for making reservoirs better. In conclusion, strong heterogeneity in these Maokou reservoirs is jointly conditioned by sedimentary facies belts and multiepisodic diagenesis, with the dissolution attributing to reservoir basis, the expanded space due to the alteration, and beach bodies’ structure and fault system as the major controls on the development of quality reservoirs.
To figure out the main controls on the enrichment and high yield of tight gas in the fourth member of the Upper Triassic Xujiahe Formation (Xujiahe 4 Member) in Jianyang area, reservoir-forming conditions were discussed for tight gas in this member through the analysis of source-rock distribution, evaluation on reservoirs’ physical properties, fault interpretation, and identification of source-reservoir assembly. Results show that (i) dominated by dark mud shale in the upper Xujiahe 3 and Xujiahe 5 members, source rocks in the study area are characterized by the total organic carbon content ranging from 0.32% to 71.40% and the hydrocarbon-generating intensity of (7-60)×108 m3/km2. And the assembly represents two features of lower source and upper reservoir as well as side source and side reservoir; (ii) for Xujiahe 4 reservoirs, as tight ones with extra-low porosity and permeability, the porosity changes from 6% to 8% and the permeability from 0.01 mD to 1 mD, respectively. And the mercury-injection curves reflect the median pore-throat radius of 1.08 μm, also indicating fine-throat porous reservoirs; (iii) Xujiahe 4 gas reservoirs whose production exceeds 20×104 m3/d in some high-yield wells and the absolute open flow achieves 50×104 m3/d belong to lithological trap gas reservoirs. There exists the positive correlation between fracture growth and fault throw vs. the distance to fault surface; and (iv) quality reservoirs are mostly extended in fracture zones associated with fourth-order faults (the ratio of fault throw to the distance to fault surface greater than 0.08), where the fracture openness has the angle smaller than 30° with the geostress direction. It is concluded that the superimposed zone of both predominant source below slope break belts and the fourth-order fault-controlled fractures is served as an optimal exploration target for Xujiahe gas reservoirs in Jianyang area. These above findings may provide theoretical foundation for fine exploration on tight gas reservoirs in the foreland slope of Sichuan Basin and thus allow for more rational exploration deployment.
Most large-sized bioreef or bioherm present somewhat mound-shaped architectures in seismic reflection. However, for low ramp lime-mud mound reservoirs of the Callovian-Oxfordian carbonate in northern marginal of Amu Darya Basin, such architectures are seismically not only indistinct but difficult to be identified due to small size. Thus, on the basis of core analysis and thin-section identification, and coupled with reflection characteristics of both target and the overlying plastic-deformed evaporates, a semi-quantitative standard was established for the lime-mud mound reservoirs to identify their spatial extension in accordance with seismic data with effect. Results show that (i) mound reservoirs are dominated by clotted limestone, siliceous sponge framework limestone, and bioclastic micritic limestone, indicating them deposited in the poor hydrodynamic setting; (ii) after the deposition of lower carbonate ramp, the generated paleogeomorphology may bring about the overlying evaporates varied in thickness. This variation causes the deformation of “oxeye pattern” when the evaporates are tectonically compressed in later stage; and (iii) the lime-mud mound reservoirs are mainly extended into a certain zone where the evaporates without “oxeye pattern” are overlapped to seismic chaos in the target with the attribute value exceeding 0.4. In conclusion, the lime-mud mound is continuously developed in A1, A2 and A4 wells, and southwest regions, but poorly around A3 well.
Both hole collapse and complexities easily happen in shale gas drilling due to exclusive bedding structure in shale, further to bring about incomplete logging data, like density logs. Therefore, to accurately predict logs just serves as one way to improve the prediction accuracy for shale gas reservoirs especially in certain areas with complex well conditions or with poor logging quality. Adopted some quality training samples from logging data in appraisal wells in a shale gas block of Sichuan Basin, a bi-directional long short-term memory (Bi-LSTM) neutral network model based on attention mechanism (Attention+Bi-LSTM model), which the existing density logs were conducted as label samples for deep learning, was built and trained to complete logging data. Moreover, conventional logs of non-radioactive sources in well correlation with density logs were selected, and attention mechanisms were incorporated into the Attention+Bi-LSTM model to reinforce characteristic learning. Results show that the density data predicted from acoustic, natural gamma ray, gamma ray without uranium and resistivity logging, and uranium element content, have an average correlation coefficient with actual density up to 0.94, also an increase of 13.6% than before. It is concluded that, with better flexibility in logs prediction in shale gas blocks, the Attention+Bi-LSTM model can effectively complete logging data, reduce radioactive-source risks as needed, and save time and costs. This built method is worth popularizing.
Logging interpretation indicates low consistency in laumontite-containing sandstone reservoirs of the Middle Jurassic Shaximiao Formation, central Sichuan Basin. Thus, combined with logging response characteristics, both qualitative logging identification and quantitative evaluation were performed on laumontite and its content by means of thin-section analysis, and scanning electron microscopy (SEM) and X-ray diffraction (XRD) experiments. Results show that (i) such reservoirs with laumontite in the first member of Shaximiao Formation (Shaximiao 1 Member) in Zitong block of central basin are characterized by low density and gamma ray, and high resistivity in their logging response, which are greatly different from those of laumontite-free reservoirs in Tianfu block of central basin; (ii) in the sandstone with laumontite, the density log-derived porosity ranges in 0.5%-1.2% higher than the acoustic log-derived porosity. On the basis of an apparent porosity model of density-acoustic slowness, the created method makes the quantitative content evaluation possible, with the consistency between model calculations and core analysis exceeding 85%; (iii) laumontite cementation increases the blocking rate in pore and throat by 23%-35% and reduces the preservation rate in primary pores by 18%-22%, respectively, resulting in sandstone reservoirs with remarkably poorer physical properties in Zitong block than those in Tianfu block; and (iv) such combinations as low gamma ray and density, and high resistivity in laumontite-containing sandstone can serve as an effective mark to distinguish Shaximiao reservoirs in the study area. It is concluded that laumontite diagenesis is the main control on strong heterogeneity in Shaximiao reservoirs. And the created logging evaluation model offers a new option for calculating parameters in similar reservoirs with complex lithology, being conducive to accuracy increase in reservoir interpretation.
Young’s modulus and Poisson’s ratio are the key parameters that need to be considered in fracturing stimulation of deep shale gas reservoirs. To accurately predict the two parameters, a rock mechanics parameter prediction model was established for the deep shale gas reservoirs of the Upper Ordovician Wufeng Formation and the first member of the Lower Silurian Longmaxi Formation (hereafter referred to as Long 1 Member) in the Lintanchang area, southeastern Sichuan Basin, based on the results of triaxial compressive strength test, and through the Gaussian Process Regression (GPR) method. The Young’s modulus and Poisson’s ratio obtained from this model were quantitatively evaluated. The research results indicate that, (i) due to the influence of internal stress weak surfaces, the stress-strain curves of deep shale samples exhibit more obvious post-peak fluctuations as temperature and pressure increase; (ii) with relatively short training time and fast prediction speed, the GPR model can mitigate the impact of “vertical anisotropy and transverse isotropy” in shale reservoirs，resulting in all residual distributions exhibiting an approximately symmetric isosceles triangluar pattern; both the prediction accuracy of rock mechanics parameters (Young’s modulus and Poisson’s ratio) and the confidence level of GPR model exceed 90%, achieving significantly improved prediction precision; and (iii) the predicted curves of single-well rock mechanics parameters (Young’s modulus and Poisson’s ratio) fit well with the results of rock mechanics test, truly reflecting the rock mechanics properties of deep shale reservoirs in the Wufeng Formation-Long 1 Member. It is concluded that the bottom of layer ③ and layer ② in the Wufeng Formation-Long 1 Member have strong brittleness characteristics and favorable engineering stimulation conditions, making them the primary intervals for future development of deep shale gas in this area.
To investigate the variation laws and differences in target areas of the stress sensitivity of the abnormally high pressure gas reservoirs in the Neogene Miocene Huangliu Formation, Dongfang (DF) area, Yinggehai Basin, three typical target areas (DF13-A, DF13-B and DF13-C) were selected for evaluation experiment on the permeability stress sensitivity of cores with bound water under high temperature and high pressure (HTHP). Moreover, the reservoir characteristics were systematically studied through various experimental means involving rock mechanics, scanning electron microscope (SEM), mercury intrusion porosimetry, and nuclear magnetic resonance (NMR), so as to further identify the differences and controlling factors of the stress sensitivity. The results are obtained in four aspects. First, physical properties are the main factors controlling the permeability stress sensitivity of the HTHP gas reservoirs in the area. The better the physical properties, the weaker the stress sensitivity. Second, Even under the same physical property conditions, there are still significant differences in stress sensitivity (the strongest in DF13-C, followed by the one in DF13-B, and the weakest in DF13-A). This is believed to be related to the different compositions of clay minerals and internal pore structures of reservoirs in the target areas. Third, there exists water sensitivity and velocity sensitivity damages in the illite/smectite (I/S) mixed layers, under the same clay mineral content, the high proportion of I/S mixed layers and the pore structure complexity will aggravate the permeability loss rate under the action of stress, allowing the reservoirs to exhibit strong stress sensitivity. Fourth, the influence of pore structure on stress sensitivity is primarily due to the differing contributions of pore throats at various scales to permeability and the extent to which they are affected by stress. Specifically, large pore throats contribute significantly to permeability and undergo greater changes under stress compared to small pore throats. As a result, the permeability of large pore throats declines substantially. It is concluded that the stress sensitivity of the abnormally high pressure gas reservoirs in the DF area is mainly related to physical properties, clay mineral compositions, and pore structures.
The Middle Permian Maokou Formation gas reservoirs in the Longnvsi block of the central Sichuan Basin are characterized by thin reservoir thickness, strong heterogeneity, low porosity and permeability in matrix, making it difficult to optimize favorable zones, which severely impedes the efficient development of the reservoirs. To address the challenge, the reservoir classification characteristics and high-quality reservoir development models were clarified, by deeply analyzing the actual drilling data, distribution patterns of beaches, and presedimentary paleogeomorphy. Combined with the main controlling factors of gas well productivity, a method for optimizing favorable zones in thin reservoirs with strong heterogeneity was formed, and two types of favorable zones selected. The proposed method was verified in subsequent wells. The research results are obtained in four aspects. First, the Maokou Formation gas reservoirs are divided into fractured-vuggy, vuggy, and porous types, with the first two being high-quality reservoirs. Second, two high-quality reservoir development models, i.e. high-energy beach + paleogeomorphic slope, and high-energy beach + paleogeomorphic highland, are established. Combined with the seismic response characteristics of “double wave peak” or “lower strong complex wave” from drilled wells, it is predicted that high-quality reservoirs are mainly developed near Wells MX145 and MX039-H1, with an average thickness of 4.1 m. Third, gas well productivity is primarily controlled by high-quality reservoir type, thickness, and fracture development degree. Gas wells in well-developed fractured-vuggy reservoirs with greater thickness generally reveal higher productivity. Fourth, the favorable zones of the reservoirs can be categorized into two classes, covering a total area of 429.66 km2. Dominated by fractured-vuggy reservoirs, the Class I exhibits strong dolomitization in reservoir, high-quality reservoir thickness exceeding 4 m, stable well production no less than 20×104 m3/d, and dynamic gas reserves no less than 20×108 m3. Dominated by vuggy reservoirs, the Class II shows moderate to strong dolomitization in reservoir, high-quality reservoir thickness of 2-4 m, stable well productivity of 10×104 - 20×104 m3/d, and dynamic gas reserves less than 20×108 m3. The proposed method of favorable zone optimization for the development of gas reservoirs with thin thickness and strong heterogeneity can provide ideas and references for the efficient development of similar gas reservoirs.
In the Yuanba gas field of northeastern Sichuan Basin, the Upper Permian Changxing Formation biogenic reef gas reservoirs are buried at depths ranging from 6300 to 7200 m, with reservoir temperatures between 149 and 164 ℃. The Lower Triassic Feixianguan Formation exhibits formation pressures of 118-120 MPa, with average H2S content of 4.64% and CO2 content of 5.72%. The reefs are relatively poor in reservoir connectivity, leading to incomplete gas reserve producing in some reefs. In order to improve the ultimate recovery of the gas reservoirs, sidetracking through original wellbores is employed for reservoir connection to efficiently produce the remaining reserves. Based on the analysis of engineering geological characteristics and previous drilling practices, a set of key technologies has been developed, including the casing program optimization design of sidetracked hole, low-density high-temperature-resistant acid-soluble drilling fluid, and ultra-deep slim hole trajectory control. These technologies provide effective solutions to the problems such as the co-occurrence of leakage and blowout caused by significant difference in formation pressure coefficients between the Lower Triassic Jialingjiang-Feixianguan Formations and the Upper Permian Changxing Formation, casing deformation induced by the developed gypsum salt rocks in the Jialingjiang-Feixianguan Formations, high leakage risk in low-pressure coefficient reservoirs (with a pressure coefficient of 0.6) and the difficulty of reservoir protection, and hard trajectory control of slim holes. The application of the technologies to Well YB103-1H demonstrates that, (i) the sixth spud with specialized drilling techniques for the Changxing Formation low-pressure reservoirs enables a safe well construction and also reservoir protection by reducing drilling fluid density to avoid leakage; (ii) the low-density high-temperature-resistant acid-soluble drilling fluid achieves the dual objectives of wellbore stability and reservoir protection; and (iii) the optimization of bottomhole assembly (BHA) allows for both trajectory control and sticking prevention. It is concluded that the proposed key technologies for sidetracking of ultra-deep slim holes provide not only technical guarantee for efficiently producing remaining gas reserves and enhancing ultimate recovery in this gas field, but also references for subsequent safe and efficient operations of ultra-deep slim hole sidetracking.
In order to efficiently develop the deep shale gas resources in the Sichuan Basin and solve the engineering complexities such as lost circulation, pipe sticking, and wellbore collapse caused by complex geological conditions during drilling process, two wells, A and B, in the Weidong 204 wellblock are taken as research objects. The factors that may induce drilling complexities are analyzed, including the developed formation fractures, weak formation support, and abnormal formation pressure coefficients. Some techniques are adopted: the key parts of tools that are prone to fatigue and damage are inspected and the bottomhole assembly (BHA) is optimized to avoid tool damage; for the issue of fish disposal, it is recommended to maintain a certain distance to flush the sand on the fish head and closely observe changes in suspending weight, pump pressure, and torque parameters; drilling fluid performance is optimized and water loss is controlled to prevent the formation of false mud cakes; in case of lost circulation, the drilling tool should be promptly lifted and the vibrating screen and liquid level be observed closely to quickly take control measures. The research results indicate that, (i) inspection of key parts and correct use of tools can effectively avoid damage and greatly improve durability and safety; (ii) flushing and parameter observation during fish disposal are helpful for timely response to and rapid handling of downhole complexities; (iii) optimization of drilling fluid performance is crucial for maintaining wellbore stability and providing a guarantee for safe drilling; and (iv) in case of lost circulation, timely lifting of drilling tool and strictly monitoring of vibrating screen and liquid level provide strong support for quickly taking control measures and avoiding well control risks. It is concluded that the handling techniques adopted for the two wells provide effective technical means and preventive measures against downhole complexities during subsequent fast shale gas well drilling in the study area, thus facilitating efficient development of deep shale gas resources in the Sichuan Basin.
To address the challenge of accurately determining shut-in drill pipe pressure (SIDPP) under complex well conditions such as a layer with simultaneous overflow and lost circulation, a research method integrating theory with practice was adopted. By analyzing key factors affecting SIDPP determination and overflow treatment cases from Wells MX145, ST1, and HS101 in the Sichuan Basin, this study discussed the methods and experiences for determining SIDPP under complex well conditions. The following research results are obtained. (i) For wells that are shut due to overflow in the formation with narrow drilling fluid density window, the post-shut-in pressure buildup in wellbore easily leads to lost circulation, making the conventional pressure buildup method fail in determining the SIDPP. In this case, the circulation method is recommended; or, the possible range of formation pressure for the overflow well is estimated by analyzing the drill pipe/casing pressure curves, combined with the data from offset wells. (ii) When the pressure buildup method is used to obtain SIDPP, the time to read the pressure is a key consideration, that is, the drill pipe/casing pressures at the time of pump shut-down, rather than the value before pump shut-down, should be recorded. (iii) For long open-hole sections with multiple formation pressure coefficients, the SIDPP may not truly reflect the current formation pressure, and the complex well conditions such as lost circulation-to-overflow transition can be mitigated or prevented through pressure-bearing capacity test, preemptive formation energy release, and managed pressure drilling techniques. The study concludes that SIDPP determination is somewhat challenging under complex well conditions, but accurate calculation of the static liquid level height within the wellbore and proper interpretation of wellhead pressure variation enable the derivation of relatively accurate SIDPP value, providing a reliable basis for well control and kill operations.
Information technology is expected to be introduced into the efficient development of unconventional hydrocarbon resources, and the sharing and analysis of professional data in shale gas geology-engineering integrated researches. Therefore, an integrated shale gas geology-engineering data analysis platform (hereinafter referred to as the integrated platform) has been constructed by using the data cube model together with the Business Intelligence (BI) tool for data management. The research results show that, (i) a data collection area is established to serve as a data source, enabling the integrated platform to achieve the unified collection and storage of various professional data on shale gas; (ii) a data asset area is built to provide a professional data warehouse encompassing the exploration and development data model (EPDM) and the business wide table, which helps obtain high-quality data as the foundation for data cube construction; (iii) a series of data cubes that support multi-scenario applications are constructed to unify the indicator coverage, providing data support for scenario applications; (iv) a core indicator dashboard is designed to visually display various key indicators, by which several reusable data dashboard components and specific functional modules can be formed to provide support for scenario applications; and (v) professional application scenarios involving drilling, fracturing, flowback, and production performances, etc., are established to form business subject domains, thus creating a comprehensive research and production management system. It is concluded that the integrated platform can conveniently and visually display multi-dimensional data, rapidly respond to the user’s query requirement, and keep data closed while dimensional switching. The research findings provide not only a technical support to shale gas geology-engineering integrated studies but also a reference for construction of similar BI application platforms.
