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Suarez-Rivera, Roberto (W. D. Von Gonten Laboratories) | Panse, Rohit (W. D. Von Gonten Laboratories) | Sovizi, Javad (Baker Hughes) | Dontsov, Egor (ResFrac Corporation) | LaReau, Heather (BP America Production Company, BPx Energy Inc.) | Suter, Kirke (BP America Production Company, BPx Energy Inc.) | Blose, Matthew (BP America Production Company, BPx Energy Inc.) | Hailu, Thomas (BP America Production Company, BPx Energy Inc.) | Koontz, Kyle (BP America Production Company, BPx Energy Inc.)
Abstract Predicting fracture behavior is important for well placement design and for optimizing multi-well development production. This requires the use of fracturing models that are calibrated to represent field measurements. However, because hydraulic fracture models include complex physics and uncertainties and have many variables defining these, the problem of calibrating modeling results with field responses is ill-posed. There are more model variables than can be changed than field observations to constrain these. It is always possible to find a calibrated model that reproduces the field data. However, the model is not unique and multiple matching solutions exist. The objective and scope of this work is to define a workflow for constraining these solutions and obtaining a more representative model for forecasting and optimization. We used field data from a multi-pad project in the Delaware play, with actual pump schedules, frac sequence, and time delays as used in the field, for all stages and all wells. We constructed a hydraulic fracturing model using high-confidence rock properties data and calibrated the model to field stimulation treatment data varying the two model variables with highest uncertainty: tectonic strain and average leak-off coefficient, while keeping all other model variables fixed. By reducing the number of adjusting model variables for calibration, we significantly lower the potential for over-fitting. Using an ultra-fast hydraulic fracturing simulator, we solved a global optimization problem to minimize the mismatch between the ISIPs and treatment pressures measured in the field and simulated by the model, for all the stages and all wells. This workflow helps us match the dominant ISIP trends in the field data and delivers higher confidence predictions in the regional stress. However, the uncertainty in the fracture geometry is still large. We also compared these results with traditional workflows that rely on selecting representative stages for calibration to field data. Results show that our workflow defines a better global optimum that best represents the behavior of all stages on all wells, and allows us to provide higher-confidence predictions of fracturing results for subsequent pads. We then used this higher confidence model to conduct sensitivity analysis for improving the well placement in subsequent pads and compared the results of the model predictions with the actual pad results.
In the design of underground tunnels and caverns located in weak rocks, grouting plays an essential role. Grouting is a complex process demanding high degree of skill and knowledge in understanding of grout flow and requirements for certain geological conditions. This paper discusses a design of grouting in underground caverns with an aim to improve the stability of the structure and to reduce water inflow into the cavern through sealing of fractures. Estimation of grouting efficacy becomes an important part of any grouting since there is high degree of uncertainties involved in the interconnectivity and condition of fractures inside the rockmass. Therefore, two different trial grouting programs are devised, and their results are discussed in order to arrive at a robust grouting scheme. The systematic approach of both programs involves pre and post grouting investigations. One approach is based on multi stage grouting with efficacy check through Water Penetration Tests (WPT), Borehole Televiewer test and Seismic Refraction Test. Second approach is based on single stage grouting with efficacy check using WPT, Core logging and cross hole velocity tests. The analytical estimates of grout intake and penetration length are compared with actual grout intake in both trial programs. An understanding of the grout behavior is developed to arrive at a final grouting scheme for the given geological condition. This work shows the importance of state of art in the development of a grouting process based on geological and hydrogeological conditions and can be helpful in grouting design for underground tunnels and storage caverns
It is well seen nowadays that due to decrease in land surface availability for mobility and storage, new tunnels and caverns are continuously being constructed all over the world. However, it may not be always possible to construct such underground facilities in a competent rock strata. Therefore, in weak rock strata, grouting becomes handy to have a safe excavation by reducing water inflow and improving the strength properties of the surrounding material. Grouting technique can be adopted for excavation through soil and rock strata; however this paper discusses grouting in rock strata only. Grouting is a complex process involving various dynamics in understanding the grout flow and requirements for a given geological conditions. The basic requirements for any grouting includes (i) decision on starting mix, (ii) decision on grouting pressure, (iii) spacing between grout holes, and (iv) methodology of grouting. All these basic requirements are discussed in the paper. The sealing of grout fractures in underground structures with the grout is an art as it is not possible to visualize the flow of grout inside the rocks. However, it cannot be a blind game given to those with high intuitive experience, which demands careful observations and analytical understanding of the grout behavior grout in the rockmass. As quoted by Houlsby , “Grouting can nowadays be classed as an engineered process, it has a pecularity that there is more art to it than in most engineering works”.
Ueda, Kenji (INPEX Corporation) | Kuroda, Shintaro (INPEX Corporation) | Rodriguez-Herrera, Adrian (Schlumberger) | Garcia-Teijeiro, Xavier (Schlumberger) | Bearinger, Doug (Nexen Energy ULC) | Virues, Claudio J. (Nexen Energy ULC) | Tokunaga, Hiroyuki (INPEX Corporation) | Makimura, Dai (Schlumberger) | Lehmann, Jurgen (Nexen Energy ULC) | Petr, Christopher (Nexen Energy ULC) | Tsusaka, Kimikazu (INPEX Corporation) | Shimamoto, Tatsuo (INPEX Corporation)
Abstract A design of hydraulic fracturing in variably-stressed zones is one of key components for an effective multi-zone, multi-horizontal well pad treatment. In the recent literature, optimum completion strategies catering for stimulation-induced in-situ stress changes are discussed, however, only few of these focus on vertical stress changes and its impact on multi-zone fracture geometries. In this paper, we present an approach to design contained hydraulic fractures in a high stress layers by studying the role of vertical stress shadowing on actual field data. In modeling hydraulic fractures with pseudo-3D models, if fracture simulations are initiated in high stress zones, "artificially" unbounded height growth results in very limited lateral propagation. On the other hand, 3D hydraulic fracturing models are too computationally expensive to optimize large design jobs, for example, in multi-horizontal well pads. In this paper, we employ a Stacked Height Growth Model, whereby fractures are also discretized vertically yet retain the numerical formulation pseudo-3D models. Coupling with finite element stress solvers then allows to identify vertical stress changes in the vicinity of induced hydraulic fractures and to understand the interference between hydraulic fracture sequences and their respective microseismic signatures. Considering a potential combination of fracturing sequences, it was revealed that stress perturbations from the neighboring well hydraulic fractures initiating from low stress layers can be used to increase stress within the same zone and also potentially reduce stresses in higher-stress layers above and below. By modeling and calibrating an actual multi-zone, multi-horizontal stimulation job, we elaborate on the benefits of increasing stress barriers before fracturing in higher-stress layer to avoid the chances of re-fracturing from high stress zones. Regarding hydraulic fracture geometries, we explain our results by analyzing actual microseismic observations with respect to simulated stress patterns after stimulation. We explore the notion of deliberately ordering hydraulic fracture to manage vertical interference and create more contained fractures in a multi-zone horizontal well pad. Fracturing in a higher-stress zone will naturally divert the energy into low stress, potentially unproductive zones. In an effort to manage this phenomenon, this paper presents one of the few data-rich case studies on multi-zone, multi-well engineered stimulation design. The approach shown in this paper can be a helpful reference to understand fracture height growth in the presence of both vertical and horizontal stress shadowing.
Abstract Production logs from horizontal wells in shale reservoirs indicate that more than 30% of the perforation clusters do not contribute to production. One major reason is recognized as the stress shadow effect which impedes the propagation of the interior fractures within a single fracture stage. Although limited entry perforations have been successfully introduced in horizontal wells to counteract this completion inefficiency, the complex mechanisms involved have not been fully understood. In this paper, a fully integrated workflow that incorporates fracture propagation, reservoir flow and wellbore hydraulics has been developed to evaluate the efficiency of limited entry perforations during multiple simultaneous fracture propagation. Darcy–Weisbach and classic orifice flow equations are adopted to describe the wellbore and perforation friction. The coupled reservoir and geomechanics model are solved by finite element code while a cohesive zone model, which accounts for the significant non-linear effects near fracture tip over the conventional linear elastic fracture mechanics, is used to simulate the fracturing process. During the stimulation of multiple fractures, uneven fluid distribution will be observed once the fractures begin to interfere with each other. Meantime, the difference in perforation pressure loss due to uneven fluid rates will counteract the stress shadow effects and balance fluid distribution. Thus, a larger perforation friction coefficient is favorable but it also causes higher pumping pressure. A novel proppant model is proposed to represent both stress- and time-dependent fracture conductivity change due to proppant degradation in subsequent long-term production. Production simulation results demonstrate that deliberate deployment of limited entry technique can significantly increase production but this benefit is reduced with increased cluster spacing. Sensitivity study indicates that better well performance could be obtained by reducing number of shots in each cluster and increasing number of clusters in each stage. Non-uniform perforation shots distribution is proven to be an effective means to counteract the stress shadow effects while the cluster length is unchanged. Simulation results also indicate how the heterogeneity in reservoir properties affects the performance of limited entry perforations. The proposed workflow has the advantage to integrate fracturing and production simulation in the same grid system and evaluate performance of different stimulation strategies. The comparison studies can provide critical insights to the application of engineered limited entry.
Summary Microseismic analysis is used to determine event locations, event strength, stress and energy release, and relative fracture lengths. Additionally, source mechanisms and their associated failure mechanisms are calculated for high-quality events using Seismic Moment Tensor Inversion. These microseismic observables are used together with the detailed hydraulic fracture treatment data to identify several dynamic changes in the growth of the discrete fracture network. In this study, we identify a preferred fracture set that is consistent with the regional stress and easily activated throughout the treatment program. A secondary fracture set is also observed but is only temporarily activated by increases in proppant mesh size. Abrupt changes in event rate, fracture plane orientation, source mechanism, and proximity to the treatment zone are linked to changes in the treatment program, such as mesh size, providing insight into the role and effectiveness of both fluid and proppant on extending the connected discrete fracture network.