Well Performance Evaluation of Carbonate Reservoirs After a Novel Hybrid Volume Stimulation Treatment

A large proportion of gas and oil resources are trapped in carbonate reservoirs. Efficient development of these formations is crucial for world energy supply. Recently, a novel hybrid volume stimulation (HVS) technique has been proposed and enhanced carbonate reservoir production in the Bohai Bay Basin and the Ordos Basin of China (Cai et al., 2015; Chu, 2017). This technique involves three stages, including pad-fluid fracturing (primary fracture and fracture branch initiation), massive acid fracturing (acid etching and connection of natural and induced fractures), and proppant injection (conductivity maintenance). Compared with conventional acid fracturing, HVS generates a more complex fracture system by taking the advantage of both hydraulic fracturing and acid fracturing, mitigating high-temperature effects, and increasing the acid penetration distance. Currently, no existing models can predict the pressure and rate behavior of wells after HVS treatments due to the complex fracture geometry and the complicated flow pattern. This study presents a multi-region linear flow model to facilitate evaluating well performance of carbonate reservoirs after HVS and obtaining a better understanding of key factors that control well responses. The model incorporates the fundamental characteristics of the complex fracture system generated by HVS. The primary hydraulic fracture is characterized by two flow regions. One is for the propped primary fracture segment (region 1), while the other represents the unpropped but acid-etched primary fracture tip (region 2). The region adjacent to the primary fracture (region 3) denotes acid-etched fracture branches. Because the acid usually cannot fully penetrate the hydraulic-fracturing-induced branches, the fractal theory is employed to depict the properties of the small fracture branches beyond the acid-etched sections. Finally, the unstimulated reservoir is described by a dual-porosity region (region 4) with vug and matrix systems. Specifically, triple-porosity region 3 contains two possible flow scenarios: one is from vugs to matrices, to fracture branches, and to the primary fracture, while the other is from vugs to matrices, and to the primary fracture. Two weighting factors are utilized to describe the proportion of reservoir volume that is involved in the two fluid flow scenarios. These flow regions are coupled through flux and pressure continuity conditions. The degenerated form of this model is verified against a published analytical model. A good agreement has been achieved between the results of the two models. Analysis results show that four flow regimes can be identified in the log-log type curve. Compared with classical type curves of fractured wells, there is a distinctive fracture-branch-affected transient regime in the pressure derivative curve with a slope between one-half and unity. The HVS generated complex fracture system enhances well productivity from the inter-porosity flow regime to the late fracture-branch-affected transient regime. The impacts of various fracture and reservoir properties on pressure and rate behavior are also documented.

Three-Dimensional CT Mapping of Intra-articular Calcaneal Fractures

Background: Intra-articular calcaneal fracture is a challenge for surgeons, which must be understood to provide optimal treatment. The aim of this study was to define the distribution of the primary fracture line and the secondary fracture line of intra-articular calcaneal fractures. Methods: All X-rays and CT scans of intra-articular calcaneal fractures were collected from January 2014 to July 2020. According to the classification of Essex-Lopresti, these fractures were divided into tongue-fracture group and compression-fracture group. Construct 3D models of intra-articular calcaneal fractures in all patients, and record the location of all fracture lines, which were marked and integrated on the 3D model of intact calcaneus after virtual reduction. Heat mapping were created based on the occurrence frequency of fracture lines. Results: A total of 171 patients with intra-articular calcaneal fractures were included in this study, 4 of whom were bilateral. There were 87 cases in the tongue-fracture group, 37 cases (42.5%) involved 4 anterior articular surface, 16 cases (18.4%) involved middle articular surface, and 52 cases (59.8%) involved calcanecuboid articular surface. There were 88 cases in the compression-fracture group, including 43 cases (48.9%) involving anterior articular surface, 21 cases (23.9%) involving middle articular surface, and 63 cases (71.6%) involving calcanecuboid articular surface. Conclusion: The distribution of the primary fracture line and the secondary fracture line of intra-articular calcaneal fractures has a certain rule and correlation. Whether in tongue-fracture group or compression-fracture group, the fracture line is most often involves the calcanecuboid articular surface, followed by anterior articular surface, at least involves middlearticular surface. This study provides a theoretical basis for further exploration of calcaneal injury mechanism, construction of biomechanical model, and choice of surgical approach.

Characteristic Law of Borehole Deformation Induced by the Temperature Change in the Surrounding Rock of Deep Coalbed Methane Well

The borehole stability of the coalbed methane (CBM) well has always been vital in deep CBM exploration and development. The borehole instability of the deep CBM well is due to many complicated reasons. The change in the surrounding rock temperature is an important and easily overlooked factor among many reasons. In this research, we used methods that include experiment and numerical simulation to study the characteristic law of the borehole deformation induced by the changes in the surrounding rock temperature of deep CBM well. The experimental results of the stress–strain curves of five sets of experiments show that when the experimental temperature rises from 40 °C to 100 °C, the average stress when coal samples are broken gradually decreases from 81.09 MPa to 72.71 MPa. The proportion of plastic deformation in the entire deformation stage gradually increases from 7.8% to 25.7%. Moreover, the characteristics that some key mechanical parameters of coal samples change with the experimental temperature are fitted, and results show that as the experimental temperature rises from 40 °C to 100 °C, the compressive strength, elastic modulus, and main crack length of coal samples show a gradually decreasing trend. By contrast, the Pois-son's ratio and primary fracture angle show a gradually increasing trend. Moreover, the relativity of the linear equations obtained by fitting is all close to 1, which can accurately reflect the corresponding change trend. Numerical simulation results show that a high temperature of the surrounding rock of the deep CBM well results in a high range of stress concentration on the coal seam borehole and high deformation.

Experimental Investigation on Cyclic Huff-n-Puff with Surfactants Based on Complex Fracture Networks in Water-Wet Oil Reservoirs with Extralow Permeability

The injection from a well to other wells can be difficult in extralow-permeability oil reservoirs. In order to address this issue, a method of cyclic huff-n-puff with surfactants based on complex fracture networks for a single horizontal well was proposed and then investigated in terms of the effects of injection and fracture parameters on the oil recovery in water-wet extralow-permeability models. Firstly, the interfacial tension (IFT) and contact angle with different surfactant concentrations were measured to determine the basic properties of the surfactants. Then, the experiments of huff-n-puff with surfactants at different threshold injection pressures and soaking time were carried out to determine the oil increasing effects and analyze the pore-scale (micropores, mesopores, and macropores) mechanisms by combining the technology of nuclear magnetic resonance (NMR), which showed that the recovery increased with threshold injection pressure mostly in mesopores and macropores, while that increased with soaking time mostly in micropores. Eventually, the experiments of cyclic huff-n-puff based on different fracture distributions were conducted in six plate-fractured models to investigate the effects of surfactants, primary fracture, and secondary fracture on each cycle of huff-n-puff. Cyclic huff-n-puff with surfactants assisted by complex fracture networks including both primary and secondary fractures would bring to a higher oil recovery. However, other methods should be taken after several cycles of huff-n-puff due to the rapid reduction of oil recovery of each cycle. The findings for the proposed method should provide a meaningful guide to the development of extralow-permeability oil reservoirs.

Variation characteristics of coal-rock mechanical properties under varying temperature conditions for Shanxi Linfen coalbed methane well in China

In the actual exploitation process of coalbed methane (CBM), as the fluid in the wellbore continues to circulate, the surrounding rock of the CBM well will continuously exchange heat with the fluid in the wellbore, resulting in continuous changes in the temperature of the surrounding rock itself. Linfen, Shanxi is the main exploitation area for CBM in China. This paper aims further to improve the exploitation efficiency of CBM in this area and conducts experimental research on the change characteristics of coal-rock mechanical properties under varying temperature conditions. The experimental results show that under constant pressure conditions, the higher the temperature, the lower the stress value when the coal-rock breaks. In the process of reaching peak strength, the higher the temperature, the higher the proportion of coal-rock plastic deformation in its entire deformation stage. The compressive strength, elastic modulus, and main crack length of coal-rock will decrease with temperature. The Poisson's ratio and primary fracture angle will increase with the increase of experimental temperature.

Risk Factors for Refracture following Primary Osteoporotic Vertebral Compression Fractures

Background: In the aging population, osteoporosis and related complications have become a global public health problem. Osteoporotic vertebral compression fractures are among the most common type of osteoporotic fractures and patients are at risk of secondary vertebral compression fracture. Objectives: To identify risk factors for secondary vertebral compression fracture following primary osteoporotic vertebral compression fractures. Study Design: Retrospective study. Setting: Department of Orthopedic, an affiliated hospital of a medical university. Methods: This retrospective cohort study evaluated the risk factors for secondary vertebral compression fracture in 317 consecutive patients with systematic osteoporotic vertebral compression fractures who received percutaneous vertebroplasty and kyphoplasty or conservative treatment. Patients were divided into secondary vertebral compression fracture (n = 43) and non- secondary vertebral compression fracture (n = 274) groups. We retrospectively analyzed clinical characteristics and radiographic parameters, including gender, age, body mass index, number of primary fractures, primary treatment (percutaneous vertebroplasty and kyphoplasty or conservative treatment), nonspinal fracture history before primary fracture, primary fracture at the thoracolumbar junction, steroid use, bisphosphonate therapy, and Hounsfield units value of L1. Results: Comparison between the groups showed significant differences in age (P = 0.001), nonspinal fracture history (P < 0.001), and Hounsfield units value of L1 (P < 0.001). The receiver operating characteristic curves demonstrated that the optimal thresholds for age and Hounsfield units value of L1 were 75 (sensitivity: 55.8%; specificity: 67.5%) and 50 (sensitivity: 88.3%; specificity: 67.4%), respectively. In multivariate logistic regression analysis, nonspinal fracture history (OR = 6.639, 95% CI = 1.809 – 24.371, P = 0.004) and Hounsfield units value of L1 < 50 (OR = 15.260, 95% CI = 6.957 – 33.473, P < 0.001) were independent risk factors for secondary vertebral compression fracture. Limitations: The main limitation is the retrospective nature of this study. Conclusion: Patients with low Hounsfield units value of L1 or non-spinal fracture history are an important population to target for secondary fracture prevention. Key words: Risk factor, vertebral, secondary fracture, osteoporosis

Outcomes of 173 metacarpal and phalangeal fractures treated by intramedullary headless screw fixation with a 4-year follow-up

Surgical treatment of extra-articular metacarpal and phalangeal fractures should be considered when there is instability or in patients with high demand. To overcome the issues related to the use of Kirschner wires, external fixators, and open reduction and internal fixation procedures, intramedullary headless screw fixation (IHSF) is an alternative technique to achieve primary fracture stability and early return to daily activities. We report the data of the Italian Multicentric Intra-Medullary Experience, which includes 173 cases of extra-articular unstable fractures (38 phalanges and 135 metacarpals) treated with the IHSF. After surgery, patients underwent early mobilization without splinting. The results confirm the reliability of IHSF in cases with non-articular involvement, showing a good recovery rate in terms of bone healing and range of motion. Level of evidence: IV

Experimental and Numerical Study on Proppant Transport in a Complex Fracture System

Slickwater fracturing can create complex fracture networks in shale. A uniform proppant distribution in the network is preferred. However, proppant transport mechanism in the fracture network is still uncertain, which restricts the optimization of sand addition schemes. In this study, slot flow experiments are conducted to analyze the proppant placement in the complex fracture system. Dense discrete phase method is used to track the particle trajectories to study the transport mechanism into the branch. The effects of the pumping rate, sand ratio, sand size, and branch angle and location are discussed in detail. Results demonstrate that: (1) under a low pumping rate or coarse proppant conditions, the dune development in the branch depends on the dune geometry in the primary fracture, and a high proportion of sand can transport into the branch; (2) using a high pumping rate or fine proppants is beneficial to the uniform placement in the fracture system; (3) sand ratio dominates the proppant placement in the branch and passing-intersection fraction of a primary fracture; (4) more proppants may settle in the near-inlet and large-angle branch due to the size limit. Decreasing the pumping rate can contribute to a uniform proppant distribution in the secondary fracture. This study provides some guidance for the optimization of proppant addition scheme in the slickwater fracturing in unconventional resources.