True tri-axial test system was deployed for fracturing simulation of coal outcrops to investigate the initiation and propagation of hydraulic fractures in vertical and directional wells. The ...influences of in-situ stress and cleats on non-planar propagation of hydraulic fractures in directional wells under different relative azimuths were analyzed. The test results show that the general propagation pattern of hydraulic fractures is jointly controlled by azimuth, cleats and in-situ stress. As the relative azimuth increases, the hydraulic fractures become more complicated in geometry and subject to increasing pumping pressure and propagation pressure. If the hydraulic fractures are initiated along a direction skewed with wellbore, the effect of cleats would alter the extension path and appear distortion of hydraulic fractures, inducing more complicated fracture geometry near the wellbore, with many fractures at the initiation point. Compared with vertical wells where I-shaped or X-shaped hydraulic fractures are formed, directional wells often have twisty propagation of dominant fractures near the wellbore and presence of multi-level fractures, which impede the further extension of hydraulic fractures in coal seams.
Affected by beddings and natural fractures, fracture geometry in the vertical plane is complex in shale formation, which differs from a simple fracture in homogeneous sandstone reservoirs. However, ...the propagation mechanism of a hydraulic fracture in the vertical plane has not been well understood. In this paper, a true tri-axial pressure machine was deployed for shale horizontal well fracturing simulation experiments of shale outcrops. The effects of multiple factors on hydraulic fracture vertical propagation were studied. The results revealed that hydraulic fracture initiation and propagation displayed four basic patterns in the vertical plane of laminated shale formation. A hydraulic fracture would cross the beddings under the high vertical stress difference between a vertical stress and horizontal minimum stress of 12 MPa, while a hydraulic fracture propagates along the beddings under a low vertical stress difference of 3 MPa. Four kinds of fracture geometry, including a single main fracture, a nonplanar fracture, a complex fracture, and a complex fracture network, were observed due to the combined effects of flow rate and viscosity. Due to the influence of binding strength (or cementing strength) on the fracture communication effects between a hydraulic fracture and the beddings, the opening region of the beddings takes the shape of an ellipse.
Heterogeneity analysis of conventional data, such as geophysical log data, has been still limited to the application of near-wellbore zone, which makes it difficult to optimize the hydraulic ...fracturing design and may render suboptimal performance. However, the fluctuation of multi-stage pumping data, manifesting nonlinear behavior of physical properties with shale reservoir during hydraulic fractures propagation stage, is usually ignored. In this study, the empirical mode decomposition technique (EMDT) was introduced and applied to the multi-stage pumping data to determine the respective Intrinsic Mode Functions (IMF). By using a relationship between the IMF number and its mean wavenumber, the heterogeneity index associated with far-wellbore shale reservoir was determined. The results indicate that the heterogeneity index from multi-stage pumping data is good coincided with the effective stimulation reservoir volume (ESRV) obtained from micro-seismic events. Not only that, but it also reveals that there is a strong correlation of heterogeneity index, IMF number, ESRV, and degree of heterogeneity within shale reservoir. This work has demonstrated that heterogeneity index analysis combined with EMDT has been significantly important and essential to quantify the degree of heterogeneity within far-wellbore shale reservoir from multi-stage pumping data, which contributes to optimizing the hydraulic fracturing design and improving good optimal performance.
A mathematical model of a drillstring-riser system is developed to analyze the system dynamics when drilling in deep water. For the proposed model, a riser and a drillstring are combined as a ...pipe-in-pipe structure, along which the interactions between the riser and the drillstring are introduced by a series of spring-friction units. Numerical simulations of the developed mathematical model are carried out by Abaqus in which the finite element method is adopted. Comparing with the riser deflection calculated by previously published models which exclude the influences of the drillstring-riser interactions, the maximal riser deflection obtained by using the proposed drillstring-riser system is relatively small. Namely, the drillstring-riser interactions restrict the riser deflection. This finding indicates that, in a given safety margin of the riser deflection, the drillstring-riser system has stronger capacity of anti-deflection. Additionally, the influence of sea depth is investigated; the dynamic response of a drillstring-riser system drilling in deep water is obviously different from the corresponding condition drilling in shallow water. In order to maintain the drillstring-riser system work in a safe condition, the operations, such as thickening the riser wall, enhancing the control of the top drift, removing a part of buoyancy modules, decreasing WOB, and increasing the top tension of the riser, are suggested to be applied.
A fluid-solid-chemistry coupling model is built considering fluid flow and ion transmission induced by shale-drilling fluid system electrochemical potential osmosis, nonlinearity of flow and solute ...diffusion in shale-drilling fluid system, and solid deformation resulted from fluid flow and ion transmission. The model is used to compute the time-varying pore pressure, stress and damage index in formations around the wellbore of a well. The result shows that the pore pressure and stress are lower without consideration of the diffusion potential induced by ion selective character of shale membrane. The linear model overestimates the pore pressure and stress fields around the sidewall. The damage index reveals that collapse first occurs in formations near the wellbore, the caving pressure increases and changes obviously with time, the fracture pressure reduces but doesn't change significantly with time, and the drilling fluid density window narrows down with time.
•A dynamic model of an offshore drilling tube system with pipe-in-pipe structure is developed. The interactions between the inner and outer pipes are verified to restrict the tube ...deflection.•According to the main effect analyses of optimization parameters, decreasing the top drift of riser, increasing the top tension of riser, and increasing the hanging load of drillstring are demonstrated as the three most significant ways to promote the system stability.•NCGA, Monte Carlo method and six-sigma method are combined as a new multi-objective optimization strategy, which can secure all the suggested designs are optimal and reliable even under perturbations of the design parameters.•Comparing with the single-objective optimization designs, the global optimization degrees of the multi-objective optimal designs are higher. Hence, the multi-objective optimization strategy is suggested for the designs of offshore drilling tube systems in field practices.
A complete dynamic model of an offshore drilling tube system with pipe-in-pipe structure is developed in this paper. Specifically, the riser and well are connected at well head to constitute an outer pipe, within which, the drillstring stretching from the drilling platform to downhole is viewed as an inner pipe. The interactions between the inner and outer pipes are described by a series of spring-friction units along the pipe-in-pipe structure. Comparing with the previously published models which mainly focus on the drilling riser, the pipe-in-pipe structure is applied in this new model; moreover, the tube system under the mud line is also considered as an extension of the tube system submerged in the sea. The developed dynamic model is simulated using the finite element (FE) method in Abaqus. Under the same ocean environmental loads, the maximal lateral deflection for the pipe-in-pipe structure is less than that only considering the drilling riser. This finding indicates that, for an actual offshore drilling tube system with the pipe-in-pipe structure, it has stronger capacity of maintaining reliability under heavy ocean environmental loads. Based on the newly developed dynamic model, multi-objective optimization design of the offshore drilling tube system is conducted in Isight. A new flow path of the optimization is designed. Specifically, six-sigma method is adopted to drive genetic algorithm to run the multi-objective optimization, and simultaneously drive Monte Carlo method to analyze the reliability of the obtained optimal solution. Comparing with a series of single-objective optimization designs, the global optimization degree of the obtained multi-objective optimal design is verified as the best.
Fracability evaluation is one of the great challenges during the shale gas development, which has become a hotspot for the exploration of unconventional oil and gas. Although much research work was ...conducted and some evaluation models were developed, most of them could not be applied to practical engineering operations for deep shale gas development. In this paper, a multi-level sub-system was developed based on the analytic hierarchy process, and then, a catastrophe evaluation model, avoiding over-subjective in defining the weighting coefficient of factors, was proposed to evaluate deep shale reservoirs’ fracability by using the catastrophe theory. Finally, the model was verified by using engineering data in five deep shale gas wells. The analytic results showed that the calculated results of the catastrophe evaluation model are in good agreement with the facts compared with the analytic hierarchy process. Moreover, the general expression for the potential function and its normalization formula of the common catastrophe model with arbitrary dimensions could also be derived recursively to meet practical applications. Therefore, the catastrophe evaluation model was simple and suitable for the wide application, which provides new sights for the pre-fracturing prediction and post-fracturing evaluation during deep shale reservoirs development.
Difficulties in fracture extension and proppants adding were two key factors that affected the stimulated volume of deep shale reservoir. To clarify the mechanism of hydraulic fracture (HF) ...propagation and proppants migration, several groups of large-scale true tri-axial fracturing tests with an ingenious method of sand adding were performed utilizing deep Longmaxi shale outcrops from southeastern Sichuan. The interaction behavior between vertical HF and bedding plane (BP) was discussed. The results showed that due to the low bonding strength, the BPs could be easily activated even under high vertical stress difference coefficient. Based on the different influenced degrees of BPs, four types of HF geometries in the vertical direction were observed, that is, transverse HF, horizontal HF, step-shaped HF with fissure opening, and multilateral step-shaped HF network. The migration distance of proppants was limited and almost all the proppants were distributed around the wellbore, thus inducing the formation of multiple secondary HFs. When there are natural fractures (NFs) with large aperture around the wellbore, proppants almost entirely distributed along the NFs. The main factor, causing the difficulty of fracture extension and proppants migration, was the complex HF geometry characterized by the multi-branched fractures with slippage of BPs.
•The law of fracture propagation and proppants migration of deep shale outcrops were investigated.•An ingenious method of sand adding was proposed and the effects of sand adding fracturing on fracture propagation were analyzed.•Four types of fracture geometries for deep shale in the vertical direction were summarized.•Three kinds of response characteristics of pressure curve for hydraulic propagation were summed up.
Abstract
PDC bit design, especially the design of the crown and cutters of PDC bit, is a relatively complex analysis process, which involves many factors such as the back-dip angle, cutting depth, ...cutting force, cutting power and other factors of PDC cutters. At present, the traditional design methods of PDC cutters distribution are mostly based on analysis of rock breaking efficiency of single cutter, which can only reflect the working state of the single cutter under the set parameters, and cannot fully reflect the differences of different positional cutters during the cutting process. In order to solve this problem, this article proposes a dynamic method through target formation lithology characteristics. The working parameters of all the PDC cutters are analyzed in detail basing on equal wear theory and bottom hole covering theory. The working parameters include static parameters (cutting position, back dip angle, cutting depth, effective cutting area.) and dynamic parameters (cutting force, cutting power, cutting torque and cutting energy etc.). The energy required by all the PDC cutters to break rock of per volume during drilling process is taken as the target constraint condition for iterative analysis. This design method can intuitively and effectively analyse the working parameters of cutters which located bit core, nose, shoulder and gauge area, and it provides a good reference for dynamic balance design of PDC bit.
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