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Zhou, Jian (State key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Sinopec Research Institute of Petroleum Engineering) | Zeng, Yijin (State key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Sinopec Research Institute of Petroleum Engineering) | Jiang, Tingxue (State key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Sinopec Research Institute of Petroleum Engineering) | Zhang, Baoping (State key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Sinopec Research Institute of Petroleum Engineering) | Shen, Boheng (Missouri University of Science and Technology) | Zhou, Jun (State key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Sinopec Research Institute of Petroleum Engineering)
ABSTRACT: The multi-staged fracturing is becoming one of the key strategies for the shale gas development worldwide. The impact of stress shadow and natural fractures on fracture could cause unexpected fracture geometry in this case. The staged fracture initiation and fracture geometry during fracturing in shale are investigated through a series of tri-axial fracturing experiments. The shale blocks made by fresh shale outcrop were tested with a varied of perforation intervals as 80mm, 120mm, 160mm, 200mm, respectively. The testing results demonstrated that the multi-fracture interference occurred and it caused complex unbalanced facture geometry when the notch interval was 80mm and 120mm. By real-time diagnosis with microseismic(MS) data, we found that due to the double effect of stress shadow and natural fracture, the second fracture tends to be much shorter compared with the first fully developed fracture. Meanwhile, the direction of the second fracture tends to be a diverging fracture. However, the obvious effect of stress shadow was not found in our tests when the notch interval was 160mm and 200mm. At last, a simple mode for effective stimulation of reservoir volume (ESRV) calculation based on MS data was introduced to compare individual ESRV for different fracture geometries.
With the development of shale gas in the last decades, horizontal multi-stages hydraulic fracturing have more and more become valuable technique for stimulation of shale reservoirs. In naturally fractured shale reservoirs, the widely held assumption that the hydraulic fracture is an ideal, simple, straight, bi-wing, but planar feature is untenable because of natural fractures, faults, bedding planes and stress contrasts. In this kind of shale reservoirs due to interaction with natural fractures or frictional interface, the fracture may propagate asymmetrically or in multiple strands or segments.
The presence of natural fractures alters the way the induced fracture propagates through the rock. The early studies (Zoback, 1977; Daneshy, 1974; Lamont and Jessen, 1963; Blanton, 1982) have shown that the propagating fracture crosses the natural fracture, turns into the natural fracture, or in some cases, turns into the natural fracture for a short distance, then breaks out again to propagate in a mechanically more favorable direction, depending primarily on the orientation of the natural fracture relative to stress field. A fracture interaction criterion to predict whether the induced fracture causes a shear slippage on the natural fracture plane leading to arrest of the propagating fracture or dilates the natural fracture causing excessive leak-off was proposed based on mineback experiments (Warpinski and Teufel, 1987). A simple criterion for crossing was proposed by applying a first order analysis of the stresses near a mode I fracture impinging on a frictional interface oriented normal to the growing fracture (Renshaw and Pollard, 1995). According to their work, crossing will occur if the magnitude of the compression acting perpendicular to the frictional interface is sufficient to prevent slip along the interface at the moment when the stress ahead of the fracture tip is sufficient to initiate a fracture on the opposite side of the interface. Scaled laboratory experiments and numerical tests proved that high flow rate or viscosity yields fluid- driven fractures, while low flow rate just opens an existing fracture network (Beugelsdijk and de Pater, 2000, 2005). Laboratory scale tests also found that interaction of a hydraulic fracture with a natural fracture depended heavily on the stress state, inclination of the natural fracture with respect to the hydraulic fracture, and the strength of the natural fracture (Zhou et al., 2010 and Ingraham et al., 2016). For the case of natural fractures in shale are mineralized, researchers embedded planar glass discontinuities into a cast hydrostone block as proxies for cemented natural fractures and used these blocks to perform tests to examine the effects of cemented natural fractures on hydraulic fracture propagation. Their results show that obliquely embedded fractures are more likely to divert a fluid-driven hydraulic fracture than those occurring orthogonally to the induced fracture path (Olson et al., 2012).
Traditionally, survey merging is achieved by applying the optimal bulk time shift, phase rotation and a time-varying amplitude scalar obtained to match two or more different surveys. With an increase in demand for seismic reservoir characterization, the amplitude-related seismic attributes are of greater importance. Therefore amplitude-spectrum matching is critical to survey merge processing. In contrast, the use of Fourier Transform in amplitude spectrum matching is often taken for granted, by ignoring the requirement for stationarity of the seismic signal under investigation. Our study presented here shows that ignoring the non-stationarity requirement imposed by the theory of Fourier Transform could be problematic in practice. Using the Gabor transform, a solution to this problem is illustrated by application to real data examples from offshore northwest shelf of Western Australia.
Deepwater field developments in the Gulf of Mexico typically consider Spar, Tension Leg Platform and Semisubmersible hull forms as potential candidates for floating facilities. Since 2005, field development studies for floating systems have had to consider more severe environmental conditions, including increased wind and wave criteria released in API Bulletin 2INTMET in 2007, increased or more prevalent loop/eddy current events and longer wave periods. These changes have quantifiable impacts to Spar hull and mooring design that are evaluated in this paper. In addition to the design challenges presented by the environment, operator functional requirements (e.g. hull-supported top-tensioned risers), robustness requirements (e.g. minimum air gap in survival conditions) and execution plan considerations (e.g. hull dry transport constraints) also have a quantitative impact on the Spar configuration.
This paper presents a summary of the recent design challenges affecting Gulf of Mexico Spar design and uses global performance analysis to evaluate different options to update the Spar configuration to effectively satisfy the design challenges. Design solutions that produce acceptable global performance results are further evaluated to quantify the potential benefit to delivery cost and schedule based on overall hull weight. Based on the analysis results, recommendations are made regarding the best solution to meet the identified post-Katrina design challenges. Results indicate that the optimum number of heave plates depends on the top-tensioned riser support system. The effect of overall hull length (a typical execution plan constraint) on overall weight (and therefore cost) for a given payload is identified. The technical solutions and recommendations are applicable to all future field developments that are considering a Spar hull concept to support floating facilities.
The offshore oil industry is characterized by more challenging developments, increasing costs, and ever-increasing focus on safety and survivability. The technical conclusions and recommendations from the work discussed in this paper will assist operators that choose the Spar concept in developing more cost effective designs that retain the required robustness and survivability for safe, reliable operations.