From apex to shoreline: fluvio-deltaic architecture for the Holocene Rhine–Meuse delta, the Netherlands

Despite extensive research on alluvial architecture, there is still a pressing need for data from modern fluvio-deltaic environments. Previous research in the fluvial-dominated proximal and central Rhine–Meuse delta (the Netherlands) has yielded clear spatial trends in alluvial architecture. In this paper, we include the backwater length to establish architectural trends from apex to shoreline. Channel-belt sand body width / thickness ratios and interconnectedness were determined, and the proportions of fluvial channel-belt deposits, fluvial overbank deposits, organics and intertidal deposits were calculated for the complete fluvio-deltaic wedge based on high-resolution geological cross sections. It was found that the average width / thickness ratio of channel-belt sand bodies in the proximal delta is 5 times higher than in the distal delta. Other down-valley trends include an 80 % decrease in the channel deposit proportion (CDP) and a near-constant proportion of overbank deposits. Additionally, interconnectedness in the proximal delta is 3 times higher than in the distal delta. Based on the Rhine–Meuse dataset, we propose a linear empirical function to model the spatial variability of CDP. It is argued that this relationship is driven by four key factors: channel lateral-migration rate, channel-belt longevity, creation of accommodation space and inherited floodplain width. Additionally, it is established that the sensitivity of CDP to changes in the ratio between channel-belt sand body width and floodplain width (normalized channel-belt sand body width) varies spatially and is greatest in the central and distal delta. Furthermore, the proportion of fluvial channel-belt sands is generally an appropriate proxy for the total sand content of fluvio-deltaic successions, although its suitability as a total sand indicator rapidly fades in the distal delta. Characteristics of the backwater zone of the Rhine–Meuse delta are (1) sand body width / thickness ratios that are lower as a consequence of channel narrowing (not deepening), (2) a rapid increase and then a drop in the organic proportion, (3) an increase in the total sand proportion towards the shoreline, and (4) a drop in the connectedness ratio. For this paper, unique high-resolution quantitative data and spatial trends of the alluvial architecture are presented for an entire delta, providing data that can be used to further improve existing fluvial stratigraphy models.

Sedimentary characteristics and sand body distributions in the Lower Permian He 8 member, Ordos Basin, China

The Lower Permian He 8 Member (P1h8) in the Ordos Basin is a typical producing zone of tight lithologic gas reservoirs. Analyses of sedimentary characteristics, electrofacies, and sand-body distributions of P1h8, conducted on modern fluvial deposits, outcrops, cores, and well logs, revealed that braided rivers that developed in the Lower P1h8 and Upper P1h8 are characterized by meandering river. Within these fluvial deposits, the procedure consists of analyzing high-resolution sequence stratigraphy and sedimentary dynamics defined from calibrated logging curve signatures and depositional studies. According to modern and ancient fluvial deposits, we have developed a process-based sedimentary conceptual model for interpreting and predicting the distribution and geometries of sand bodies in braided and meandering deposits. The main sand body of the braided river system was bars and channel fill deposits. The braided river sand bodies are distributed over multiple vertical superimpositions and overlapping horizontal connections. The meandering river sand bodies are mainly point-bar deposits, which are bead-shaped and exhibit scattered development in the vertical direction. This comparison indicates that there were significant differences between braided and meandering deposystems. The sand bodies in the Lower P1h8 were multidirectionally connected and primarily distributed in a stacked pattern. In contrast, the sand bodies in the Upper P1h8 were distributed in an isolated manner, and fine grains (mud and silt) were deposited between the sand bodies with poor connectivity. We interpreted the fluvial deposits that control the distributions of the sand body of the He8 Member in the eastern Sulige gas field and constructed a corresponding prediction model of a braided-meandering reservoir. This model will promote understanding of the extent of fluvial deposits and sand-body distribution of P1h8, thus elucidating hydrocarbon-bearing sand units of the Ordos Basin for future exploration.

Outside the Box: Innovative Application of Diversion as a Replacement for Bridge Plug

Effective stage-to-stage isolation is typically accomplished by setting a bridge plug in a properly cemented casing between stages. This isolation plays a vital role in a horizontal well multistage fracturing completion. Failure of isolation not only impacts the well productivity but also wastes fracturing materials. The challenges isolation failure poses for stimulation effectiveness include both detection and remediation. First, there has been historically no reliable and cost-effective solution to detect stage-to-stage isolation onsite. One may only start to realize this problem when inconsistent production is observed. Second, existing remedial actions are seldom satisfying in case of an isolation failure. Most commonly, a new plug is set to replace the failed one. However, because the perforation clusters of an unstimulated stage may create irregularities in well inside diameter (ID) (e.g., casing deformation or burr), there is a risk that the plug will be damaged or become stuck when it passes the perforation area. Also, when the plug passes a perforation cluster, the perforations start to take in the pump-down fluid, which can increase the difficulty of the pump-down job. A novel remedial action uses high-frequency pressure monitoring (HFPM) and diversion to solve both challenges. The stage isolation integrity is evaluated in quasi-real time by analyzing the water hammer after the pump shutdown. In the case of a plug failure, large-particle fracture diversion materials and techniques can establish temporary wellbore isolation through a quick and simple delivery process. To close the cycle, the effect of the diversion can be evaluated by HFPM, which can reveal the fluid entry point of the treatment fluid after diversion. The technique was applied to two cases in Ordos basin in which wellbore isolation failure interrupted the operation. The problem identification, development of the solution workflow, and observation from treatment analysis are discussed. In both cases, the stage-to-stage isolation was recovered, and the drilled sand body was successfully stimulated without involving costly and time-consuming well intervention. The stimulation operation of the entire well was successfully resumed in a timely manner.

Study on Stability of Tunnel Surrounding Rock and Precipitation Disaster Mitigation in Flowing Sand Body

Flowing sand is a special surrounding rock encountered by tunnel construction. Due to the looseness and low viscosity of the flowing sand, after excavation, the sand body is easy to flow along the open surface. In addition, the water seepage also causes tunnel instability. Considering the characteristics of water seepage, how to improve the stability of flowing sand bodies and prevent the instability of surrounding rocks has become a difficult problem. In this paper, a parametric experiment on the surrounding rock taken from the project site was carried out, and then, a numerical simulation of the flowing sand body was conducted to study the precipitation construction method and stability of the flowing sand body. Other than that, the tunnel face vacuum dewatering, vertical vacuum dewatering at the top of the tunnel, and the vacuum dewatering technology of the gravity well in poor geological section were systematically analyzed in our research. A radial vacuum enclosed precipitation process for the face of the tunnel was proposed, which effectively solved the problem concerning continuous seepage of water in the front. Through numerical simulation and field experiments, the basis for determining the precipitation parameters of the tunnel face was obtained, while aiming at the top position of the tunnel, a vertical vacuum negative pressure precipitation method of intercepting the top seepage water and the water supply behind the top of the tunnel was proposed. For the bottom of the tunnel, setting gravity wells on the side walls for the purpose of preventing seepage at the bottom was put forward. The application of these methods in the project ensured the safety of construction and improved the construction schedule. After the completion of the dewatering construction, the method of inserting plywood into the small pipe was adopted to avoid the collapse of the dry sand. Then, to solve the problem of borehole collapse in flowing sand bodies, pipe feeding was introduced, thus further enhancing the precipitation effect. Furthermore, in view of the problem that the dewatering hole in the flowing sand body is easy to collapse, resulting in the failure of 60% of the dewatering hole and the sand body is extracted from the dewatering pipe, causing the risk of the cavity at the top of the tunnel, a method of pipe following is presented to avoid the damage of geotextile caused by directly inserting the dewatering pipe and further improve the dewatering effect. All the above processes together form an omnidirectional three-dimensional negative pressure precipitation method that considers the special sand body flow and water seepage of unfavorable geology and that has been proved to enhance the stability of surrounding rock in practice.

The Impact of Plane Heterogeneity on Steam Flooding Development in Heavy Oil Reservoirs

Steam flooding is proven to be an effective method to improve the development effect of heavy oil reservoirs. And steam flooding is the most common oil recovery technology for heavy oil reservoirs in China. However, because of the various reservoir physical properties, bring great challenges to successful steam flooding development. According to the previous research and development practice, we know that reservoir heterogeneity has a great influence on the development effect of water flooding. Due to the heterogeneity of reservoirs, the development of different injection-production well patterns will be affected. However, it is uncertain whether reservoir heterogeneity has an impact on steam flooding development effect. In order to clarify the above scientific issues, we take Xinjiang steam flooding oilfield as the research object to carry out relevant research. According to the reservoir distribution characteristics of Xinjiang Oilfield, three conceptual heterogeneity models representing permeability, thickness, and geometric plane heterogeneity are firstly proposed. Then, mathematic models with different plane heterogeneity of reservoir sand were built. Based on the mathematic model, initial conditions, boundary condition, and geological parameters of conceptual models, different steam flooding patterns were studied by applying numerical calculation. It is found that heterogeneity is an important geological factor affecting the development of steam flooding of heavy oil reservoir. And the results showed that cumulative oil production was different of different flood pattern at the same production condition. It can be concluded that the development effect of steam flooding of heavy reservoirs is strongly influenced by flood pattern. In order to improve development effectiveness of steam flooding of heavy oil reservoirs, flood pattern should be optimized. For each type of plane heterogeneity reservoir, a reasonable flood pattern was proposed. For plane heterogeneity of permeability, thickness, and geometry form, under the conditions of that as the producer was deployed in high permeability, thick, wide sand body and injector was deployed in low permeability, thin, narrow sand body, the recovery of steam flooding in heavy oil reservoir was better. Finally, how the three types of plane heterogeneity influence steam flooding of heavy reservoirs was discussed by adopting a sensitivity analysis method. The results show that the influence of permeability heterogeneity is the largest, thickness heterogeneity is the second, and geometric heterogeneity is the least. This conclusion can help us improve the development of this reservoir. And also, the findings of this study can help for better understanding of properly deployed well pattern and how to effective develop the heavy oil reservoirs of strong plane heterogeneity for other heavy oil reservoirs.

Discrete Element Simulation Analysis of Damage and Failure of Hydrate-Bearing Sediments

It is of great significance to study the damage and failure law of hydrate-bearing sediments for for exploration and development, as well as for warning secondary disasters such as tsunami and earthquake. The discrete element modeling and simulation method has the advantages of low cost, strong repeatability and accurate response to the microstructure of samples, therefore , the discrete element method is used to simulate and analyze the damage and failure law of hydrate-bearing sediments in this paper. First, a triaxial undrained shear teat model of hydrate-bearing sediments is established. by discrete element simulation software; Then, the effects of different influencing factors on the fracture characteristics of hydrate hydrate-bearing sediments is studied; Finally, the effects of different factors on the initiation stress and damage stress of hydrate sediments are analyzed, and the damage law of hydrate-bearing sediments is obtained. The results show that: (1) The cementation of hydrate particles is greater than its bearing effect, thus the lithology of sediments changes from loose sand body to brittle rock with the increase of hydrate saturation, resulting in the corresponding change of fracture characteristics from loose sand body failure to brittle rock failure. (2) With the increase of hydrate saturation, the initiation stress of sediment shows a step-by-step increase law, and the damage stress is positively correlated with the shear modulus. (3) The heterogeneity of hydrate distribution is closely related to the failure mode of sediments. With the increase of hydrate distribution heterogeneity, the initiation stress displays an increase-decrease-increase pattern while the damage stress shows a law of increasing and then decreasing, which has the high sensitivity to heterogeneity of hydrate distribution. The research results have certain theoretical reference significance for the exploitation practice of natural gas hydrate.

Fine Depiction of the Single Sand Body and Connectivity Unit of a Deltaic Front Underwater Distributary Channel: Taking the Third Member of the Dongying Formation in the Cha71 Fault Block of the Chaheji Oilfield as an Example

By taking the third member of the Dongying Formation in the Cha71 fault block of the Chaheji oilfield as an example, the single sand body of the deltaic front underwater distributary channel is meticulously depicted by using the data of well logging and performance production. Portrays the vertical separation model, total lateral separation type, vertical type, lateral superposition type, 4 types of single sand body vertical superimposed and bay type, bank contact between docking, instead of four kinds of single sand body lateral contact type, and summarizes its logging facies identification. The quantitative prediction model of the single sand body was established, the characteristics of single sand body plane distribution were summarized, and the identification of the oil-water layer and the lower limit of reservoir effective thickness were studied. And we got the conclusion that based on the fine characterization of the single sand body and the lower limit standard of effective reservoir thickness, the distribution range of the effective reservoir and connecting unit is determined. Finally, the connectivity of the connecting unit is verified by dynamic data.