美国Silica公司开发了Micro Stim支撑剂生产线，这种新型的微型材料系列有两种尺寸，包括Micro Stim（200目/ 75μm）和Micro Stim Plus（325目/ 44μm）。 作为参考，Micro Stim支撑剂具有与人的头发相同的中值粒径（MPD / D50），而Micro Stim Plus支撑剂具有其一半的中值粒径，其粒度分布达到纳米粒子范围。
这些微支撑剂通过渗透进裂缝，从而促进那些未利用的次生裂缝网络的改造，支撑裂缝并为提高产量提供通道，已经有人使用这些微支撑剂作为储层改造设计的先导产品，Micro Stim已经成为美国各地完成该设计的主要支撑剂。例如，2017年12月，该公司提供了900万磅的Micro Stim支撑剂用于Marcellus页岩的压裂工作，它被用作主要的支撑剂。
由于粒径较小，Micro Stim支撑剂不仅可以进入较小宽度的裂缝，而且沉降速度也会有所改善，因为沉降速度随颗粒尺寸的减小而成比例降低。实验室测试表明，标准100目支撑剂的沉降速度比Micro Stim快3倍，比普通自来水中的Micro Stim Plus支撑剂快14倍。由于沉降速度显著降低，这些微支撑剂具有较强的运移能力。流动性能的加强使得支撑剂可以在低粘度流体和低体积流量下保持悬浮。改进的悬浮性能使得微支撑剂可以通过不规则和复杂几何形状的次生裂缝并远离井眼。由于可以接近裂缝尖端并增加裂缝有效长度因此可以获得更多的支撑面积。
Micro Stim Plus支撑剂是由优质高硅矿石（99.5％以上的二氧化硅）精心研磨而成的硅石粉，尺寸符合规格。近年来，石英砂已在许多井、地层和地区作为先导支撑剂应用。高纯度二氧化硅将促进流体的化学惰性，降低储藏损害（例如粘土膨胀）。颗粒不是球形的，但是具有与常规二氧化硅支撑剂相当的硬度（7莫氏硬度）。其他微米/纳米尺寸的颗粒不具备这样的硬度，这是一个明显的优势。
Microproppants Unlock Potential Of Secondary Fractures
Proppant selection has long been considered one of the critical success factors in well stimulation. Over the years, proppants have ranged in size from 12/20 mesh and 16/30 mesh all the way down to much finer 100 mesh, which is very common. As the industry has seen the demand for proppant continue to move to finer mesh sizes, U.S. Silica decided to investigate what products might be available that are smaller than the 100-mesh product offered today and could easily be utilized in fracture stimulation design to access an existing or induced fracture network that remains untapped.
As the oil and gas industry learns more about unconventional shale fracturing, it appears that an extensive naturally occurring secondary fracture network of induced and natural fissures is present in many shale wells. These secondary fracture networks have been estimated to take as much as 90% of the total liquid volume pumped during a hydraulic fracturing stimulation. However, just because the fracturing fluid goes into these secondary fractures doesn’t mean the proppant does. These secondary fracture widths are estimated to be in the low micron range (50 μm). Even the smallest of conventional proppants used today would be too large to access this fracture network and provide stimulated production pathways to the main fracture.
If the proppant cannot go into the secondary natural fractures due to its size, then the secondary fracture network will most likely close and constrict upon pressure drawdown, leaving a majority of the stimulated reservoir volume (SRV) ineffective or wasted during the production phase. To take full advantage of the SRV and access the full potential that lies in the secondary network, a smaller proppant is required to penetrate, stimulate and dilate this vast untouched reservoir volume.
U.S. Silica has developed the Micro Stim proppant product line. Available in two sizes, this new family of microproppants includes the Micro Stim (200 mesh/75 μm) and the Micro Stim Plus (325 mesh/44 μm). For reference, Micro Stim proppant has a target median particle diameter (MPD/D50) that is equivalent to the thickness of a human hair, while Micro Stim Plus proppant has half this MPD with a particle size distribution that reaches into the nanoparticle-range.
These microproppants facilitate the stimulation of the untouched and unrealized secondary fracture network by penetrating these fractures, propping them and providing conductive pathways for increased production. There have been stimulation designs using these microproppants as a lead-in product and Micro Stim as the primary proppant completed across the U.S. For example, in December 2017 the company provided 9 million pounds of Micro Stim proppant for a job in the Marcellus Shale, where it is being utilized as the primary proppant.
Settling rate, proppant transport
Due to the smaller particle size the Micro Stim proppants will not only access the smaller fracture widths but will have an improved settling velocity rate since settling rate decreases proportionally with a reduction in particle size. Laboratory testing has shown that standard 100-mesh proppant has a settling rate that is three times faster than Micro Stim and 14 times faster than Micro Stim Plus proppants in ordinary tap water. Further amendments to the fluid chemistry will only enhance the inherent reduction in settling rate of these proppants, allowing further penetration in the secondary natural fracture. Because of this dramatic reduction of the settling rate, these microproppants have enhanced proppant transport characteristics. This enhanced mobility allows the proppant to stay suspended in low-viscosity fluid and at low volumetric flow rates. The improved suspension performance allows these microproppants to transverse farther away from the wellbore through the irregular and complex geometry of a secondary fracture as it approaches the fracture tips to increase effective fracture length and thus leave more surface area propped.
The Micro Stim proppant is a whole-grain product that can be transferred through typical existing transfer equipment for conventional proppants, similar to 100-mesh proppant. The capability to utilize this equipment allows immediate utilization in the field with no equipment alterations. It is selected from a high-purity silica source (more than 99.5%), which maintains high hardness and enables an inert-like behavior in fracturing fluid compatibility like all other existing silica sands. The company has multiple mine locations across the U.S. capable of producing this material to meet large volume demands with short lead times and access to all major Class 1 railroads.
The Micro Stim Plus proppant is ground silica flour refined from a quality high-silica ore (more than 99.5% silicon dioxide) and sized to specification. The ground silica has been used in recent years as a lead-in proppant in multiple wells, formations and regions. The high-purity silica content will facilitate an inert nature to fluid interaction and formation damage (e.g., clay swelling). The particles are not spherical in shape but do maintain the hardness (7 Mohs) equivalent to conventional silica proppants. This is a distinct advantage compared to other micro/nano-sized particles that do not have an equivalent hardness.
Bulk shipments require specialized equipment and engineering controls to ensure the product will not cake or bridge along with respiratory exposure reduction and mitigation for HSE considerations. Ground silica flour has been used for years in the industry as an additive for cementing and is being used within fracturing applications as well.
The company has seen many changes in the selection and utilization of proppants over the past decade and helped lead proppant innovation over that time period. The development of the Micro Stim proppant family for the stimulation of secondary natural fractures is the next evolution for proppants in the stimulation design. Microproppants could hold the key to vastly increasing effective fracture lengths and SRVs and adding significant value to the well’s financial performance and longevity.