3D Full-Field and Pad Geomechanics Models Aid Shale Gas Field Development in China
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The oilfield-development plan (ODP) for a shale gas field at the southern edge of the Sichuan Basin in China started in early 2014. The first wells drilled in the field and its adjacent blocks experienced significant challenges, such as severe mud losses, stuck tools, losses in the hole, high treating pressure, and unexpected screenout. Because an accurate understanding of geomechanics and its roles at various scales is vital, 3D full-field and pad geomechanics models were developed for achieving efficiency and effectiveness in implementing the ODP.
The Ordovician-Silurian Wufeng-Longmaxi hot shale is an emerging shale gas play in China.
Currently, the major exploration and development activities of the play are in the Sichuan Basin and its adjacent areas. For shale gas development in the Sichuan Basin and its adjacent areas, using the megascale, high-density, and continuous and regular pad drilling as is used in North America is difficult because surface and subsurface conditions are significantly different from those of the well-known North American shale plays.
The strong environmental and social constraints that typify the Sichuan Basin and surrounding area are shown in Fig. 1 above. Drilling pads for shale gas developments are commonly located in narrow valleys that are often home to farmland and residential villages with dense populations. Differences of elevation among neighboring pads can be from several hundred meters to more than 1000 m. To speed up development of marine shale gas in the Sichuan Basin and its adjacent areas with a minimized learning curve, a geoscience-to-production integration of research, engineering, and operation with its associated research and development, methodologies, and work flows must be applied. This geoscience-to-production integration aims to optimize both efficiency and effectiveness dynamically at single-well, pad, and field scales with systematic and continuous optimization of technologies and solutions and the accumulation of knowledge and experience to enhance well productivity.
One shale gas field, which is the study area of this paper, is in the mountainous area at the southern edge of the Sichuan Basin. The first wells drilled in this field and its adjacent blocks experienced significant geomechanics-associated challenges, such as severe mud losses, tools or drillingpipe sticking, losses in the hole, high treating pressure while hydraulic fracturing, and unexpected screenout of fracturing stages. Having reliable yet evolving understanding of geomechanics and its role at various scales during the progress of the ODP is vital. This paper describes the development of 3D full-field geomechanics models and high-resolution 3D pad geomechanics models and their engineering applications.
A topographic and geological survey of the study field and its adjacent blocks suggests that this area may have been affected significantly by four major tectonic movements, which resulted in extreme structural complexity and natural-fracture systems.
Dense faults, which can be seen in seismic data, are just some of the structural complexities. Numerous subseismic structures, or microfolds, and rich multiscale natural fractures complicate the structure further.
To achieve reasonable geomechanics models with increasing accuracy and reliability, this study takes an approach that includes the following procedures:
- An extensive characterization of mechanical properties was conducted by the evaluation of cores and well logs and integration with seismic data.
- Multiscale geomechanics modeling was conducted at the full-field, pad, and single-well scales with different resolutions and accuracies.
- Live modeling was performed with iterative updating and validation.
- Systematic quality control was performed with all available data continuously and promptly.
The geomechanics-modeling efforts began with high-resolution structural, geological, reservoir-property, and multiscale natural-fracture models.
A live modeling approach was applied for Earth modeling. Three-dimensional full-field, high-resolution pad geomechanics modeling was part of the Earth modeling. Multiscale models for the full field, a pad, and single wells were used for drilling, completion, and production applications as development progressed. This paper focuses on the geomechanics modeling.
One very important parameter for geomechanics modeling is the pore pressure of this shale formation. In this study, logs from acoustic measurements were used to estimate pore pressure in shale. A 3D sonic velocity model, built using a velocity verticalization process for horizontal wells, supplies crucial information for the creation of a pore-pressure model. In the studied field, anisotropic sonic measurements were acquired for four pilot wells before the first version of the geomechanics modeling was conducted. More acoustic logs were measured from logging-while-drilling sonic tools in horizontal wells. Although only monopole sonic was measured, compressional slowness, especially its measurement in the horizontal section of wells, revealed the horizontal compression characteristic of the formation and could be used in building the 3D sonic velocity model.
Sonic logs indicate that the Wufeng-Longmaxi shale formation is overpressurized.
Considering the limitation of using acoustic measurements for pore-pressure prediction in the Wufeng-Longmaxi hot shale formation, which has complex overpressure mechanisms, multiple methods were used for quality control. These efforts were based on thorough analysis of different data resources, including mud pressures and mud-gas logs, drilling events, prefracturing injection tests, instantaneous shut-in pressures, flowback data, and well shut-in pressures.
In this study, real-time drilling data for all drilled wells were referenced to help identify the changes in pore pressure in response to drilling mud weight. Together with total-gas observations, key evidence was supplied for the interpretation of the relationship between mud weight and pore pressure.
Finally, an advanced finite-element simulator was used to compute 3D stress distribution and multiscale natural-fracture models. The large model (80×80-m cells) covers the full field, and the pad model (20×20-m cells) covers a 15- to 20-km2 area. All have 0.5-m vertical resolution of the targeted sweet section to capture vertical heterogeneities measured from logs. Large-scale parallel-computing technology was used to perform the massive geomechanical modeling. The models were calibrated or constrained by all available data.
Computed stress models match the highly compressive background and current understanding of the dominant tectonic movements of the Sichuan Basin. They are sufficient to reveal orientations, magnitudes, anisotropies, and heterogeneities of in-situ stresses. Large variations of in-situ stresses can be quantified among pads and wells and along laterals. Such variations correspond to or align with changes in texture and composition at various scales. The full-field model was used to optimize pad and well locations and well trajectories and to assess geological integrity, resources in place, and instability of natural fractures. The full-field and high-resolution pad models were used for near-wellbore-stability analysis, real-time drilling management, and hydraulic-fracturing design and monitoring.
A critical drilling issue in the study field was the severe mud loss in some wells. Considering initial learnings from offset fields and with the first drilled pad being in the region of highest pore pressure, a mud weight as high as 2.2 g/cm3 was selected to control drilling risks in high-pressure zones. This introduced a series of drilling issues—in particular, severe mud losses near faults and fractures and tool failures in wells. Simulations of wellbore stabilities and a safe mud window that integrated the 3D pore-pressure-prediction model and geomechanics stress results enabled incorporating optimized mud weight into well planning and drilling development.
In this study, the 3D geomechanics models were widely used in engineered completions such as staging, perforation clustering, and pumping schedules; real-time monitoring and adjustments of hydraulic fracturing; engineered flowback and testing design; post-fracturing review and analysis of hydraulic-fracturing operations; and hydraulic-fracturing-effectiveness analysis and fracturing-design optimization calibrated by production data.
Three-dimensional geomechanics models for the full field and for well pads, with different scales and resolutions, were established in a shale gas field. The live-model method was used to improve the accuracy and reliability of those models continuously by using all available data sources during field development. The computed stress models match the highly compressive background and current understanding of the dominant tectonic movements of the Sichuan Basin. They are sufficient to reveal orientations, magnitudes, anisotropies, and heterogeneities of in-situ stresses. Large variations of in-situ stresses can be quantified among pads and wells and along laterals. Such variations align with changes in texture and composition at various scales.
At the same time, 3D geomechanics models were used for different applications at different scales, from the full field to a single well. The full-field model was used to optimize pad and well locations and well trajectories and to assess geological integrity, resources in place, and instability of natural fractures. The high-resolution pad models were used for near-wellbore-stability analysis, real-time drilling management, and hydraulic-fracturing design and monitoring. The approach was determined to be capable of being integrated effectively into the drilling-and-completion process. This is the first time such multiscale 3D geomechanics models have been built for China’s shale gas development.
3D Full-Field and Pad Geomechanics Models Aid Shale Gas Field Development in China
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